Alexandre Nicolas

Sebastián Bouzat

Marcelo N Kuperman

Influence of selfish and polite behaviours on a pedestrian evacuation through a narrow exit: A quantitative characterisation

Keywords: Keyworks pedestrian, evacuation, behaviour, experimental

We study the influence of selfish vs. polite behaviours on the dynamics of a pedestrian evacuation through a narrow exit. To this end, experiments involving about 80 participants with distinct prescribed behaviours are performed; reinjection of participants into the setup allowed us to improve the statistics. Notwithstanding the fluctuations in the instantaneous flow rate, we find that a stationary regime is almost immediately reached. The average flow rate increases monotonically with the fraction c s of vying (selfish) pedestrians, which corresponds to a "faster-is-faster" effect in our experimental conditions; it is also positively correlated with the average density of pedestrians in front of the door, up to nearly close-packing. At large c s , the flow displays marked intermittency, with bursts of quasi-simultaneous escapes. In addition to these findings, we wonder whether the effect of cooperation is specific to systems of intelligent beings, or whether it can be reproduced by a purely mechanical surrogate. To this purpose, we consider a bidimensional granular flow through an orifice in which some grains are made "cooperative" by repulsive magnetic interactions which impede their mutual collisions.

Introduction

In Boris Vian's Froth on the Daydream, the main character witnesses a massive collision on an ice skating rink; no sooner have the skaters tumbled into a deadly pileup than housekeepers enter the rink and sweep away their corpses while singing. In real life, crowd disasters are not taken so light-heartedly; recent examples during the emergency evacuation of a nightclub in Bucharest (October 2015) [START_REF] Gellet | Dozens killed in fireworks accident at bucharest nightclub[END_REF] or the massive stampede in Mina during the Muslim Hajj (September 2015) [START_REF] Sarah Almukhtar | How the hajj stampede unfolded[END_REF], which resulted in thousands of casualties, demonstrate how tragic these events can be. The issue of evacuation in public facilities and, more generally, questions related to pedestrian flows are therefore of paramount importance.

It is needless to say, though, that the description or modelling of the dynamics of crowds is, at the very least, challenging. Since first-principles approaches are hopeless in any conceivable future, models have to rely on a combination of assumptions and empirical observations. Fortunately, the success of rather simple physical models in describing increasingly complex entities (complex fluids, bacteria [START_REF] Marchetti | Hydrodynamics of soft active matter[END_REF], birds and fish [START_REF] Gautrais | Deciphering interactions in moving animal groups[END_REF]) bolsters the desire for similarly successful models for pedestrian crowds. But how detailed, or "realistic", need these models be to appropriately describe the flow of pedestrians in a given situation? The numerous existing models and simulation software [START_REF] Gwynne | A review of the methodologies used in evacuation modelling[END_REF][START_REF] Bellomo | On the modeling of traffic and crowds: A survey of models, speculations, and perspectives[END_REF] vary greatly as regards their sophistication, from minimalistic cellular automata [START_REF] Burstedde | Simulation of pedestrian dynamics using a two-dimensional cellular automaton[END_REF] to continuous agent-based models incorporating a large number of parameters. Even if we explicitly bypass the psychological intricacy of the human mind and take for granted the pedestrians' choice of a target point and his or her behaviour, the dynamics that result from these choices, and how simple or complex they are, remain mostly unknown.

The flow of pedestrians at a constriction, e.g., the disordered egress through a narrow exit, seems to be largely governed by physical constraints and is therefore worth studying from a mechanical perspective (not to mention its importance for safety science). Still, one can expect the attitude of pedestrians to play an important role in the dynamics.

Our objectives are two-fold. First, we wish to characterise the influence of prescribed polite (cooperative) and selfish (vying) behaviours, regardless of their psychological origins, on the evacuation process of a heterogeneous crowd. Second, we investigate whether the observed characteristics are tied to the inherent complexity of intelligent beings or whether they can be reproduced in a simple granular system. To this end, repulsive magnetic interactions are introduced between some grains to mimic the avoidance of physical contact presumably associated with polite behaviour.

We have chosen to divide the presentation of our work as follows. In this contribution, we shall expose the underpinning of the granular analogy and its "behavioural extension", present our original setups in detail and expose the global flow properties of pedestrians and grains. The microscopic characterisation of the flows and a more thorough analysis of the granular analogy shall be published separately, in a companion paper [START_REF] Nicolas | Evacuation through a narrow doorway: effect of the pedestrians' behaviours on the global flow and microscopic dynamics[END_REF].

Underpinning of the work

In the last decade, large-scale experiments of pedestrian flows through bottlenecks have been conducted. Within the framework of a German collaborative project, the flow of voluntary pedestrians under normal (cooperative) conditions was studied at the abrupt narrowing of a corridor, from 4 m to a variable width w ∈ [0.8m, 1.2m] [START_REF] Seyfried | Enhanced empirical data for the fundamental diagram and the flow through bottlenecks[END_REF]. Contrary to perhaps less exhaustive earlier measurements [START_REF] Hoogendoorn | Pedestrian behavior at bottlenecks[END_REF], the flow was observed to increase monotonically with w, instead of undergoing a stepwise growth. For w 0.9m, a "zipper effect" is visible, whereby two lanes form in the narrower corridor and entrances tend to be grouped in pairs, one pedestrian on either lane, with a short headway to avoid physical contact; this, claim the authors, leads to anticorrelated time headways ∆t between successive passages. More recently, a Spanish-Argentine collaboration performed evacuation drills through a narrow (69cm-wide) door with almost 100 participants and revealed robust statistical features. In each realisation, a given degree of competitiveness was imposed on the whole crowd, from low (i.e.,, avoidance of all contacts) to high (mild pushes) [START_REF] Garcimartín | Experimental evidence of the ¨faster is slower¨effect[END_REF][START_REF] Pastor | Experimental proof of faster-is-slower in multi-particle systems flowing through bottlenecks[END_REF]. These experiments provided evidence for the so called "faster-is-slower" effect, whereby a crowd made of individuals eager to move faster actually needs longer to evacuate than a less competitive crowd, due to the formation of clogs. Furthermore, the distributions of time lapses ∆t between successive egresses were found to be heavy-tailed and well described by power laws at large ∆t, viz.

p(∆t) ∼ ∆t -α .

(1)

These features, along with a couple of others, appear to be universal to flows of particles through narrow bottlenecks, as they have also been reported for sheep entering a barn, granular hopper flows, and colloids flowing through an orifice [START_REF] Zuriguel | Clogging transition of many-particle systems flowing through bottlenecks[END_REF][START_REF] Garcimartín | Flow and clogging of a sheep herd passing through a bottleneck[END_REF]. This analogy between pedestrian evacuations and granular flows through a bottleneck suggests that the dynamics of pedestrians in such situations are governed by the following few key mechanisms: Excluded volume. Clogging occurs in granular hopper flows because of the formation of arches bridging the gap between walls at the outlet; these arches resist pressure thanks to the solid contacts between the grains that constitute them [START_REF] To | Jamming of granular flow in a two-dimensional hopper[END_REF]. For orifices smaller than 3 or 4 particle diameters in two dimensions (2D), permanent clogs almost surely occur and the flow halts [START_REF] To | Jamming of granular flow in a two-dimensional hopper[END_REF][START_REF] Janda | Unjamming a granular hopper by vibration[END_REF]. To et al. rationalised the probability of permanent clogging in 2D by calculating the probability of formation of a convex arch between the walls. In addition, contrary to the case of liquids, anisotropic force chains develop in the granular system, so that the height of the granular column is not transmitted down to the bottom; most strikingly, the pressure measured at the outlet is virtually independent of the height of the granular layer; this is the so called Janssen effect [START_REF] Janssen | Versuche über getreidedruck in silozellen[END_REF].

Fluctuations or vibrations. Arch-like structures are also observed in pedestrian flows through bottlenecks, but they lead to non-persistent (although potentially long) clogs, responsible for the existence of relatively large time lapses ∆t between successive egresses and intermittency [START_REF] Pastor | Experimental proof of faster-is-slower in multi-particle systems flowing through bottlenecks[END_REF]. In granular flows, too, arches can be broken provided that the system is vibrated strongly enough [START_REF] Janda | Unjamming a granular hopper by vibration[END_REF]. Therefore, in a model, such fluctuations should be present in order to avoid permanent clogs; they are generally described by a noise term.

As the orifice size D 0 is increased, at a given small vibration amplitude, the importance of vibrations is reduced as the flow transits from a regime of intermittent flow dominated by the time needed for vibrations to break clogs, to a regime of continuous flow. The latter regime is the usual range of applicability of Beverloo's relation between the flow rate and D 0 [START_REF] Beverloo | The flow of granular solids through orifices[END_REF][START_REF] Janda | Unjamming a granular hopper by vibration[END_REF]. A parameter controlling the transition between these regimes is Φ, the fraction of time during which the system flows [START_REF] Zuriguel | Clogging transition of many-particle systems flowing through bottlenecks[END_REF]. Note that, according to Eq. 1, Φ → 0 if α < 2. For pedestrian flows, as long as the door is wide enough for agents to go through, Φ > 0.

Frictional contacts vs. no contacts. In competitive evacuations, people will be in contact. Conversely, under normal conditions collisions are avoided and contacts are rare. One may reasonably think that this lack of contacts will strongly affect the flow [START_REF] Lumay | Flow of magnetic repelling grains in a two-dimensional silo[END_REF]. Indeed, in granular systems, simply replacing frictional grains with frictionless ones reduces the probability of clogging, insofar as arches become less stable [START_REF] To | Jamming of granular flow in a two-dimensional hopper[END_REF]. For slighlty deformable frictionless particles (surfactant-coated emulsion droplets), clogging only occurs at very narrow outlets, only slightly larger than the particle diameter [START_REF] Hong | Jamming is difficult in frictionless 2d hoppers[END_REF]. To go beyond frictionless contacts and really investigate systems with no particle contacts, Lumay et al. recently studied the 2D hopper flow of repulsive magnetic discs [START_REF] Lumay | Flow of magnetic repelling grains in a two-dimensional silo[END_REF]. It is noteworthy that even without contacts clogs were observed for aperture sizes significantly larger than the particle diameter, through the formation of arches held by magnetic repulsions. But, in the absence of a vibrating device in the setup, the authors rather focused on wider apertures. Thus, the following question is still open: does the preclusion of contacts between grains in a hopper flow suffice to reproduce the change from a competitive evacuation through a narrow door to a cooperative one?

Overall, the aforementioned experiments deal with relatively homogeneous crowds or systems. Here, in order to better illuminate the effect of individual behaviours, we shall consider heterogeneous systems.

Description of the setups

Pedestrian evacuation experiments

Setup

We performed evacuation experiments involving more than 80 voluntary participants on the premises of Centro Atómico Bariloche (CAB), Argentina. The participants were students and researchers, aged 20 to 55 for the greatest part, with a woman/man ratio of about 1:3. They were asked to evacuate a delimited space through a 72 cm-wide door, with the specific instructions exposed below. The geometry of the setup is sketched in Fig. 1.

In light of its potential risks, the experiment was validated beforehand for ethical issues and prepared in collaboration with the Safety and Hygiene group of CAB. The doors and the walls were protected with paddings and the whole evacuation process was supervised by 3 staff members who could stop it at any moment by blowing a whistle. No incident whatsoever was reported during or after the drill.

This experimental setup is directly inspired from [START_REF] Garcimartín | Experimental evidence of the ¨faster is slower¨effect[END_REF][START_REF] Pastor | Experimental proof of faster-is-slower in multi-particle systems flowing through bottlenecks[END_REF], but some significant changes were introduced. All participants were instructed to "head for the door purposefully (con ganas), but without running, pushing or hitting others", but in each evacuation drill, a fraction c s of participants was randomly selected to behave selfishly while the rest should behave politely. Selfish participants were told to wear a red headscarf to be recognisable on the videos and allowed to "elbow their way through the crowd, with mild contacts but no violence whatsoever". Their polite counterparts, on the other hand, were to "avoid any contact and try to keep their distance".

Moreover, an innovative trick was devised to curb finite-size effects: egressing participants were reinjected into the room, as if there were periodic boundary conditions. To maintain homogeneity in the system and limit the clustering of, say, fast participants, two reinjection circuits were set up, a short one and a longer one, and the evacuees were directed to either one alternatively.

These experiments were briefly described in (China)

Data analysis

The flow at the exit was filmed by two video cameras (acquisition rate: 60Hz) placed just above the door while a third camera provided a more global view.

To study the dynamics at the individual scale, time frames of escapes were constructed, following Refs. [START_REF] Garcimartín | Experimental evidence of the ¨faster is slower¨effect[END_REF][START_REF] Pastor | Experimental proof of faster-is-slower in multi-particle systems flowing through bottlenecks[END_REF], by extracting a line of pixels just past the door from every frame of the video and stitching these lines together: an example of such a time frame is shown in Fig. 3. The individual escape times were collected semi-manually by clicking on each pedestrian, with the help of a home-made computer routine. The reproducibility of the experimental results was tested by performing two runs in nominally identical conditions, but at different times and with different individuals as selfish agents; the results were in fair agreement (7% difference in the global flow rate, similar -though not identical -distributions of time lapses between escapes, data not shown).

Two-dimensional granular hopper flow

Setup

To investigate the analogy between pedestrian and granular flows, we also conducted experiments on constricted flows of non-magnetic (green) and magnetic (red) granular discs (see Fig. 2). The discs are about 1 mm-thick and confined between a vibrated chipwood panel at the bottom and a plastic sheet. Thin plastic bars placed between the plates delimit a 2D funnel-like region, with an aperture of variable width w =3, 4 or 5cm and an opening angle of 58 • .

Both types of discs consist of a circular plastic cap of 13mm of diameter, mounted on a narrower bronze washer for the non-magnetic ones, or a narrower neodyme magnet otherwise. Their total weights are 0.545g ± 0.01g and 0.582g ± 0.01g, respectively. The chipwood panel was carefully polished and varnished to reduce the friction coefficient and make it less heterogeneous. At rest, i.e., without vibrations, the static friction coefficient was about µ 0.32 = tan (18 • ). The chipwood panel was inclined by an angle roughly equal to the friction angle, 18 • , or perhaps slightly smaller, but the vibrations produced by an unbalanced motor attached to the panel allowed sliding. At the beginning of each experiment, the discs were inserted from the top of the setup, in a relatively homogeneous fashion.

In the following, the aperture width w and the relative fraction of magnetic discs in the system are varied; all other parameters are kept constant.

Data analysis

The granular flow was filmed with a 60Hz camera placed above the aperture. The analysis is very similar to that performed for the pedestrian experiments (see Fig. 3). 1: List of the performed evacuation experiments, with the prescribed fraction of selfish agents c s , the effective fraction c s , the average flow rate J and the density ρ in front of the exit. Note that, for J, a confidence interval of 2σ √ n on either side is given, where σ is the standard deviation of j(t) averaged over δt = 7 s and n the number of (independent) sampling points. † In these runs, participants were asked to "head for the door", and not to "head for the door purposefully".

Experimental results for the pedestrian evacuation

Let us start with the study of the pedestrian flow. We recall that the participants are prescribed either a polite or a selfish behaviour. Here, we present results with c s =0%, 10%, 60%, 90%, and 100% of selfish participants. Generally, we only have data for one run for each c s , but, thanks to the recirculation, each run comprises about 200 to 300 passages through the door.

General observations

As expected, no collisions or contacts are observed with crowds of polite agents (c s = 0%). On the contrary, cooperative behaviour is witnessed in front of the exit, with, e.g., sometimes one participant waving to a neighbour to go ahead. As the fraction c s of selfish agents increases, more and more contacts and soft collisions occur, either on purpose or because of the pressure exerted by the neighbours. At the same time, the density of participants in front of the exit rises markedly. At high c s , in regions of low density, vying agents walk visibly faster on average than their polite counterparts and some align in files of 2-3 selfish agents.

Since selfish agents reach the door faster, they pass more times through the door in each run. Therefore, the prescribed concentration c s of selfish agents should be replaced by an effective concentration c s , corresponding to the number of passages. Table 1 gives the correspondence between the nominal and effective concentrations.

"Instantaneous" flow rates and the question of stationarity

To get rid of the dependence on the initial position and of the finite crowd size effects at the end of the evacuation, it is preferable to collect data in the stationary state. To ascertain stationarity, we compute the time-dependent flow rate j(t), averaged over a sliding time window δt, an example of which is plotted in Fig. 4(left). Clearly, for relatively small δt, of order 1s, the flow rate fluctuates considerably. (Centre) Dependence of the average flow rate J on the pedestrian density ρ in front of the exit. The blue downwards triangles correspond to a set of evacuation drills in which the participants were just asked to "head for the door" (see footnote in Table 1).

(Right) Complementary cumulated distribution P (τ > ∆t) for the granular flow with w = 4 cm; the data have been cumulated over two to three experiments for each fraction of non-magnetic (green) discs, as indicated in the legend.

Averaging over larger time windows, e.g., δt = 7 s, tends to wash out these fluctuations, even though the curve remains noisy. Nevertheless, beyond this noise, the data clearly point to the almost immediate attainment of a steady state (for j(t)), that persists until (nearly) the end of the experiment. This contrasts with the bottleneck flow experiments conducted by Seyfried et al. [START_REF] Seyfried | New insights into pedestrian flow through bottlenecks[END_REF], which involved up to 60 participants and did not quite reach a steady state. However, in their case, the crowd was initially positioned 3 meters away from the bottleneck and no recirculation contrivance was used. Taking a closer look at our time series j(t), we realise that, in the experiment with 10% of selfish agents in particular, the flow rate decreases moderately 30 to 40 seconds before the end of the evacuation (which lasted 190 s overall). In the videos, we notice that most selfish agents have already escaped by this time, hence an excess of polite pedestrians, the last of whom display no sign of hurry whatsoever.

Average flow rate

Turning to the average flow rate J, we notice that it increases monotonically with c s (see Table 1): the more selfish agents there are, the faster the evacuation. As a corollary to this statement, no "faster-isslower" effect is observed, unlike in [START_REF] Garcimartín | Experimental evidence of the ¨faster is slower¨effect[END_REF][START_REF] Pastor | Experimental proof of faster-is-slower in multi-particle systems flowing through bottlenecks[END_REF], where long jams at the door were found to delay significantly the evacuation for the most competitive crowd. This discrepancy is not necessarily a contradiction. Instead, we suspect the precise instructions given to the participants to have mattered: for safety reasons, we insisted repeatedly that the participants should not to push each other (although soft contacts were allowed). It follows that we also observed some jams at the exit for selfish crowds, but they never lasted long, because there was not enough mechanical pressure to stabilise them. We shall come back to this issue in the light of the results for the granular system.

Let us now put in perspective the values we obtained for J, which range from 1.26 to 2.37 s -1 . Using a door of almost identical width (69 cm instead of 72 cm), Pastor et al. measured global flow rates † J = 2.63 s -1 , 2.56 s -1 and 2.43 s -1 for homogeneous crowds of moderate, medium, and high competitiveness, respectively [START_REF] Pastor | Experimental proof of faster-is-slower in multi-particle systems flowing through bottlenecks[END_REF]. The fact that these values lie at the upper range of our range of flow rates (corresponding to c s = 100%) supports the idea that the three situations considered in [START_REF] Pastor | Experimental proof of faster-is-slower in multi-particle systems flowing through bottlenecks[END_REF] were more competitive than ours, hence at higher pressure. On the other hand, Seyfried et al. measured specific capacities J s (defined as the average flow rate per meter of door width) of about 1.9 s -1 • m -1 in their setup, with a value relatively independent of the bottleneck width w in the range [0.9m, 1.2m] (in fact, J s was found to decrease slightly for smaller w) [START_REF] Seyfried | New insights into pedestrian flow through bottlenecks[END_REF]. For w = 0.72 m, J s 1.9 s -1 • m -1 yields a flow rate slightly smaller than 1.37 s -1 , which corresponds to the case of our polite crowds (c s = 0% or 18%). Noticing that Seyfried et al.'s experiments were performed under normal conditions, with no contact between the participants, we conclude that our results are perfectly consistent with theirs.

Density in front of the door

On the basis of a comprehensive comparison with the literature, Seyfried et al. claimed that the main parameter controlling the flow rate J was the density ρ in front of the door, while the other parameters such as the position of the bottleneck or its length were of minor importance. In spite of their considerable divergences, most models in the literature, whether empirical or not, predict a quasi-linear increase of J at low densities, followed by a peak and a decrease at high densities, due to hampered motion.

Does this apply to our experiments? We compute the average density ρ in a ≈ 0.5m 2 zone in front of the exit and find values ranging from 4 to 10 αm -2 , approximately, as listed in Table 1. Here, α is a numerical factor close to unity that is mainly due to the optical distortion of the images; we should also mention that our measurement method may slightly overestimate the higher range of densities. Going beyond these caveats, we observe a monotonic increase of J with the density ρ. This holds even for the highest density, ρ = 9.5 αm -2 at c s = 100%, where people are closely packed in front of the door, whereas the models reviewed in [START_REF] Seyfried | New insights into pedestrian flow through bottlenecks[END_REF] predict a decreasing J (ρ) curve in this range of densities.

Whether the prescribed behaviours affect the flow rate exclusively indirectly, via the density at the door, or also directly, can only be settled by a more local analysis of our experiments [START_REF] Nicolas | Evacuation through a narrow doorway: effect of the pedestrians' behaviours on the global flow and microscopic dynamics[END_REF]. Nevertheless, we would like to remark that at very high densities the body resistance limits the crowd's compressibility. Accordingly, an increase of mechanical pressure, e.g., due to people pushing at the back, will only moderately raise the local density, but it is expected to generate significantly longer clogging events. 4.5. Is a small fraction of selfish people beneficial for the evacuation?

At the 2010 Love Parade in Duisburg, "pushers" were recruited to push the crowd entering the festival area in order to enhance the inflow [START_REF] Helbing | Crowd disasters as systemic failures: analysis of the love parade disaster[END_REF]. As a side question, we wonder whether selfish agents, when they are scarce, act in this way and accelerate the flow of the lagging polite crowd. Comparing the flows with c s = 0% and 18%, we find that the vying behaviour of some is inefficient for the rest of the crowd: although their presence increases the global flow rate J, the specific flow rate of polite agents is reduced with respect to the situation without selfish agents. But more extensive data at lower values of c s would be required to reach a robust conclusion.

Distribution of time lapses between escapes

In Fig. 5, we plot the distributions of time lapses ∆t between successive escapes through the door for c s = 0% and 90%. For the polite crowd, the distribution is peaked at a value of ∆t slightly below 1 s. This peak is shifted to lower value for c s = 90% and, above all, the distribution broadens and the probability of very small values ∆t ≈ 0 is strongly enhanced. This feature turns out to be characteristic of flows with a significant fraction c s of selfish agents and reflects bursts of quasi-simultaneous escapes through the door.

Experimental results for the granular flow

General observations; (non)-stationarity

Having analysed the global statistics of pedestrian evacuations through a narrow door, we turn to the granular flow. In systems of only magnetic discs (c s = 0%, with c s now referring to the fraction of non-magnetic grains), the density increases gradually, but quite visibly, with the depth: at the bottom, the discs are much closer to each other than at the top, but still non-touching. This separation gets larger as the experiment proceeds and the weight of the granular layer decreases. In other words, there is no Janssen effect. It also follows that the flow is non-stationary at c s = 0%: the flow rate j(t) slows down with time (data not shown). Besides, no persistent clog is observed, even for the narrowest door, w = 3 cm. We ascribe this difference with respect to Lumay et al.'s recent experiment with magnetic discs [START_REF] Lumay | Flow of magnetic repelling grains in a two-dimensional silo[END_REF] to the absence of vibrations in their setup.

On the other hand, in systems with a significant fraction of non-magnetic discs (c s = 60%, 100%), persistent clogs occur for w = 3 cm, which have to be broken manually. For larger doors, the vibrations suffice to unclog the system. Besides, there is not so prominent a decrease of the flow rate j(t) with time and a quasi-stationary regime seems to be reached for c s = 100% (as expected in a non-vibrated system), even though fluctuations blur the picture.

Average flow rates

For the narrow door, w = 3 cm, magnetic grains flow faster overall than neutral ones, which undergo severe clogs. It is tempting to draw a parallel with the pedestrian evacuation experiments of [START_REF] Pastor | Experimental proof of faster-is-slower in multi-particle systems flowing through bottlenecks[END_REF], in which the collision-prone competitive crowd evacuated more slowly than the less competitive one. We should however remark that, here, the isolated neutral discs do not slide faster than their magnetic counterparts.

The situation differs for w = 4 cm. In this case, the global flow rate J is larger for the system which exhibits contacts and collision, i.e., here, the non-magnetic one (J = 6.2 s -1 at c s = 100% vs. J = 4.7 s -1 at c s = 0%). This echoes our observations in the pedestrian experiments. Nevertheless, contrary to these, the flow rate J for an intermediate fraction of non-magnetic discs, c s 60%, is by far larger (J ≈ 11 s -1 ) than that at c s = 100%; the non-stationarity of the flow at c s = 0% makes another difference. As in the pedestrian experiment, the more negative slope of P (τ > ∆t) at small ∆t indicates that there are more quasi-simultaneous egresses at large c s than at c s = 0%, where grains are separated. On the other hand, the softening of the slope at large ∆t reveals the presence of long clogs at large c s , which are indeed observed in the videos. This is in line with the stronger flow intermittency in the presence of many vying pedestrians.

Time lapses

Discussion and outlook

We have studied the influence of polite vs. selfish behaviours on the evacuation of pedestrians through a narrow door. We have observed a monotonic increase of the global flow rate with the fraction c s of selfish agents, i.e., a "faster-is-faster" effect, and with the density ρ in front of the door, up to ρ ≈ 9 -10 m -2 . In parallel, evacuations at high c s display stronger intermittency and are characterised by the presence of quasi-simultaneous escapes. Under the hypothesis that the absence of mutual contacts characterise the motion of polite pedestrians, we have proposed to model them as repulsive magnetic discs in a granular hopper flow. This analogy helps us explain some features observed in the pedestrian evacuation in mechanical terms, but also suffers from some deficiency, such as the absence of stationarity in the contact-free flow.

Figure 1 :

 1 Figure 1: (Left) Sketch of the experimental geometry, with a = 7.3 m, b = 3.5 m, and c = 4.3 m. (Right) snapshot of an evacuation.

Figure 2 :

 2 Figure 2: (Left) Sketch of the experimental geometry and (right) snapshot of an experiment. The chipwood panel is inclined by about 18 • and vibrates.

Figure 3 :

 3 Figure 3: Time frames (zoom) for a granular experiment (top) and a pedestrian evacuation experiment (bottom). The vertical axis represents the position along the direction of the door and the horizontal coordinate is time. c s Effective fraction c s Flow rate J (s -1 ) Density ρ (≈ m -2) 0% 0% 1.26 ± 0.07 3.70 10% 18% 1.39 ± 0.09 4.49 60% 71% 2.20 ± 0.09 7.63 90% 90% 2.36 ± 0.15 8.26 100% 100% 2.41 ± 0.17 8.98 0% † 0% † 1.01 ± 0.05 2.69 30% † 46% † 1.41 ± 0.08 4.94 60% † 68% † 1.71 ± 0.12 6.04

Figure 4 :

 4 Figure4: (Left) Time-dependent flow rate j(t) in an experiment involving c s = 90% of selfish pedestrians. The thin lines are the data averaged over δt = 1 s, whereas the thick lines are for δt = 7 s. (Centre) Dependence of the average flow rate J on the pedestrian density ρ in front of the exit. The blue downwards triangles correspond to a set of evacuation drills in which the participants were just asked to "head for the door" (see footnote in Table1). (Right) Complementary cumulated distribution P (τ > ∆t) for the granular flow with w = 4 cm; the data have been cumulated over two to three experiments for each fraction of non-magnetic (green) discs, as indicated in the legend.

Figure 5 :

 5 Figure 5: Histograms of time lapses ∆t between successive escapes in the experiment with c s = 0% of selfish agents (left) and c s = 90% (right).

Figure 4 (

 4 Figure 4(right) presents the complementary cumulated distribution P (τ > ∆t) of time lapses between successive egresses.As in the pedestrian experiment, the more negative slope of P (τ > ∆t) at small ∆t indicates that there are more quasi-simultaneous egresses at large c s than at c s = 0%, where grains are separated. On the other hand, the softening of the slope at large ∆t reveals the presence of long clogs at large c s , which are indeed observed in the videos. This is in line with the stronger flow intermittency in the presence of many vying pedestrians.

† Note that J = 1 ∆t , where ∆t is the average time lapse between sucessive escapes in an experiment.

Acknowledgements. We are grateful to the Grupo de Higiene y Seguridad of the CAB for their help in devising the pedestrian experiment and to all the voluntary participants.


Yuriy Kuleshov
email: y.kuleshov@bom.gov.au

Suelynn Choy

Erjiang Frank Fu

Fabrice Chane-Ming

Yuei-An Liou

Alexander G Pavelyev

Analysis of meteorological variables in the Australasian region using ground-and space-based GPS techniques

Results of analysis of meteorological variables (temperature and moisture) in the Australasian region using the global positioning system (GPS) radio occultation (RO) and GPS ground-based observations verified with in situ radiosonde (RS) data are presented. The potential of using ground-based GPS observations for retrieving column integrated precipitable water vapour (PWV) over the Australian continent has been demonstrated using the Australian ground-based GPS reference stations network. Using data from the 15 ground-based GPS stations, the state of the atmosphere over Victoria during a significant weather event, the March 2010 Melbourne storm, has been investigated, and it has been shown that the GPS observations has potential for monitoring the movement of a weather front that has sharp moisture contrast. Temperature and moisture variability in the atmosphere over various climatic regions (the Indian and the Pacific Oceans, the Antarctic and Australia) has been examined using satellite-based GPS RO and in situ RS observations. Investigating recent atmospheric temperature trends over Antarctica, the time series of the collocated GPS RO and RS data were examined, and strong cooling in the lower stratosphere and warming through the troposphere over Antarctica has been identified, in agreement with outputs of climate models. With further expansion of the Global Navigation Satellite Systems (GNSS) system, it is expected that GNSS satellite-and ground-based measurements would be able to provide an order of magnitude larger amount of data which in turn could significantly advance weather forecasting services, climate monitoring and analysis in the Australasian region.

Introduction

Conventional observations of meteorological variables (air pressure, temperature and moisture) are collected daily at thousands of meteorological stations around the world, to be used for weather analysis and forecasting. Subsequent statistical analysis of the archived data over long-term period (decades and longer) allows one to derive conclusions about climate (average state of weather) based on instrumental records obtained at meteorological stations. Conventional observations are well established and archives of meteorological variables recorded at many stations worldwide go back for more than a century. Such continuity of records is crucial for climate research, detection of historical trends in meteorological variables, etc. However, conventional records are restricted to locations of meteorological stations.

In modern time, data obtained by optical, infrared, radio wave and microwave remote sensing instruments revolutionised the science of meteorology and climatology as they provide potentially global coverage and consequently improved access to areas which have limited number of meteorological stations (data sparse areas) or not covered by conventional observations at all. Remote sensing data complement conventional observations and are widely used in numerical weather prediction, for climate monitoring and analysis adding value to and improving skill of weather forecasts, the accuracy of trend analysis of meteorological variables, etc. It is of particular importance for meteorological and climatological applications in the Southern Hemisphere where weather observation stations are much less in numbers than in the Northern Hemisphere. In this study, we present results of analysis of meteorological variables over data sparse areas using the measurements obtained by ground-and space-based techniques which utilise radio signals of the global positioning system (GPS).

Atmospheric water vapour is a critical component of the greenhouse effect and plays a significant role in the global climate system. The knowledge of the long-term spatial and temporal variability of water vapour is vital for understanding climate change. Ground-based GPS has long offered the prospect of complementing meteorological observations by providing integrated vertical column of precipitable water vapour (PWV) estimates beginning with pioneering studies of [START_REF] Bevis | GPS meteorology: remote sensing of atmospheric Water vapor using the global positioning system[END_REF][START_REF] Bevis | GPS meteorology-mapping zenith wet delays onto precipitable Water[END_REF]. There have been numerous studies since, over the USA, Japan, Korea, Europe, India, Taiwan and Antarctica, which demonstrated usefulness of this methodology [START_REF] Rocken | Sensing atmospheric Water vapour with the global positioning system[END_REF][START_REF] Seko | Three dimensional distribution of water vapor estimated from tropospheric delay of GPS data in a mesoscale precipitation system of the Baiu front[END_REF]Liou and Huang, 2000;Liou et al., 2000;[START_REF] Liou | Comparison of precipitable water observations in the near tropics by GPS, microwave radiometer, and radiosondes[END_REF][START_REF] Jade | Estimates of precipitable water vapour from GPS data over the Indian subcontinent[END_REF][START_REF] Zhang | Variational assimilation of GPS precipitable water vapor and hourly rainfall observations for a meso-beta scale heavy precipitation event during the 2002 Mei-Yu season[END_REF][START_REF] Bock | Verification of NWP model analyses and radiosonde humidity data with GPS precipitable water vapor estimates during AMMA[END_REF]. One of the most valuable attributes of ground-based GPS-PWV is the ability to provide high temporal resolution and high accuracy of PWV estimates under all weather conditions at a very low cost. In this paper, we present results of deriving PWV using ground-based GPS reference stations network and investigate potential of the GPS-PWV methodology for monitoring of weather fronts.

Space-based instruments provide even wider (potentially global) coverage than regional ground-based networks, including the world ocean which covers about 70% of the Earth's surface. One of the emerging satellite remote sensing technique for obtaining atmospheric temperature and moisture records is GPS radio occultation (RO) methodology, which provides all-weather capability, long-term measurement stability, high vertical resolution and high-accuracy measurements in the middle to upper troposphere, stratosphere and ionosphere (e.g., [START_REF] Rocken | Analysis and validation of GPS/MET data in the neutral atmosphere[END_REF][START_REF] Heise | Global monitoring of tropospheric water vapor with GPS radio occultation aboard CHAMP[END_REF][START_REF] Liou | FORMOSAT-3 GPS radio occultation mission: preliminary results[END_REF]Liou et al., , 2010;;[START_REF] Pavelyev | Principle of locality and analysis of radio occultation data[END_REF][START_REF] Kumar | Impact of quasi-biennial oscillation on the inter-annual variability of the tropopause height and temperature in the tropics: a study using COSMIC/ FORMOSAT-3 observations[END_REF][START_REF] Santhi | Global morphology of convection indices observed using COSMIC GPS RO satellite measurements[END_REF][START_REF] Dhaka | Indications of a strong dynamical coupling between the polar and tropical regions during the sudden stratospheric warming event January 2009, based on COSMIS/ FORMOSAT-3 satellite temperature data[END_REF]. The high accuracy of the GPS RO methodology is of particular importance for reliable estimates of the atmospheric characteristics over the regions where conventional meteorological upper air observations from radiosondes (RS) are sparse or not available.

Evaluation of GPS RO methodology for retrieval of atmospheric profiles has been conducted in a number of studies in various regions [START_REF] Wickert | Comparison of vertical refractivity and temperature profiles from CHAMP with radiosonde measurements[END_REF][START_REF] Kuo | Comparison of GPS radio occultation soundings with radiosondes[END_REF][START_REF] Fu | An evaluation of GNSS radio occultation technology for Australian meteorology[END_REF][START_REF] Rao | Validation of the COSMIC radio occultation data over Gadanki: a tropical region[END_REF], and the high accuracy of retrieval of meteorological variables has been demonstrated. [START_REF] He | Assessment of radiosonde temperature measurements in the upper troposphere and lower stratosphere using COSMIC radio occultation data[END_REF] used GPS RO data to assess the accuracy of temperature measurements by four types of RS in the upper troposphere and low stratosphere and demonstrated a close-to-zero mean difference between GPS RO and RS data for two types of radiosonds, Vaisala-RS92 and Shanghai radiosonde systems. However, large temperature biases are shown for the MRZ and VIZ-B2 radiosonde systems relative to GSP RO, which are probably caused by diurnal radiative effects [START_REF] He | Assessment of radiosonde temperature measurements in the upper troposphere and lower stratosphere using COSMIC radio occultation data[END_REF].

This study is concerned with analysis of meteorological variables in the Australasian region (continent of Australia, Maritime continent, the Indian and the Pacific Oceans) and the Antarctic, which are much less covered by meteorological observations than the Northern Hemisphere. Specifically, the state of the atmosphere over the Australasian region and the Antarctic is investigated. The Australasian region is highly important in terms of impacts on weather and climate. The region is significant in meteorology because it is considered the most important energy source region in the entire global circulation system owing to a number of coincident factors, the most significant being geographical location and topography, both of which contribute to the development of the warmest large area of ocean on the Earth, the Tropical Warm Pool [START_REF] Graham | Decadal-scale climate variability in the tropical and North Pacific during the 1970s and 1980s: observations and model results[END_REF][START_REF] Zhang | ENSO-like interdecadal variability: 1900-1993[END_REF]. This is a region of intensive ocean-atmosphere interaction with widespread convection and environment favourable for tropical cyclone development. El Niño-Southern Oscillation (ENSO), a coupled ocean-atmosphere phenomenon in the central Pacific [START_REF] Trenberth | The definition of El Niño[END_REF], is another significant climate driver of the planet, which causes extreme weather (such as floods and droughts) in many regions of the world [START_REF] Nicholls | ENSO, drought and flooding rain in Southeast Asia. South-east Asia's Environmental Future[END_REF][START_REF] Kuleshov | Extreme weather and climate events and their impacts on island countries in the Western Pacific: cyclones, floods and droughts[END_REF]. The Indian Ocean Dipole (IOD), a phenomenon which is similar to the ENSO in the Pacific, involves an aperiodic oscillation of sea-surface temperatures and atmospheric pressure in the Indian Ocean [START_REF] Saji | A dipole mode in the tropical Indian Ocean[END_REF], and its positive phase is associated with droughts in adjacent land areas of Indonesia and Australia. Consequently, accurate knowledge about the state of the atmosphere over the Indian and the Pacific Oceans is vital for understanding the ENSO and the IOD and their impacts on climate of the Australasian and other regions of the planet. Here we present analysis of collocated GPS RO and in situ RS data over the Indian and the Pacific Oceans to demonstrate the accuracy of GPS RO retrieval of atmospheric temperature and moisture profiles in the tropics.

The climate of Antarctica is the coldest on the Earth, with the lowest near-surface temperature of -89.2 °C recorded at Vostok Station [START_REF] Budretsky | New absolute minimum of air temperature[END_REF]. However, due to a global warming, near-surface and air temperatures in Antarctica are rapidly increasing. Positive trend in near-surface temperature is well documented and significant warming of the Antarctic Peninsula was observed [START_REF] King | Recent climate variability in the vicinity of the Antarctic peninsula[END_REF][START_REF] Vaughan | Climate change: devil in the detail[END_REF]. Assessment of Antarctic near-surface temperature change has shown that significant warming extends well beyond the Antarctic Peninsula to cover most of West Antarctica. West Antarctic warming exceeds 0.1 °C per decade over the past 50 years and is strongest in winter and spring [START_REF] Steig | Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year[END_REF]. Although this is partly offset by autumn cooling in East Antarctica, the continent-wide average near-surface temperature trend is positive. Analysing a complete temperature record for Byrd Station, a linear increase in annual temperature between 1958 and 2010 by 2.4 ± 1.2 °C has been found, establishing central West Antarctica as one of the fastest-warming regions globally [START_REF] Bromwich | Central West Antarctica among the most rapidly warming regions on earth[END_REF]. In fact, significant warming extends well beyond the Antarctic Peninsula and covers most of West Antarctica [START_REF] Steig | Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year[END_REF]. However, there is no agreement on magnitude of atmospheric temperature trends over the Antarctic. Thus, reliable satellite remote sensing data are crucial for accurate estimation of atmospheric temperature variability over the vast areas of the Antarctic.

The total precipitation in Antarctica, averaged over the entire continent, is about 166 mm per year [START_REF] Vaughan | Reassessment of net surface mass balance in Antarctica[END_REF]. According to the Köppen climate classification, areas that receive less than 250 mm of precipitation per year are classified as deserts. As such, the continent of Antarctica is largely a desert, apart from coastal areas with higher rate of precipitation. Thus, the air over Antarctica is generally very dry, and the potential accuracy of air temperature estimates as derived from GPS RO observations is high as the retrieval algorithm depends on amount of moisture content [START_REF] Rocken | Analysis and validation of GPS/MET data in the neutral atmosphere[END_REF][START_REF] Kuo | Inversion and error estimation of GPS radio occultation data[END_REF]).

Here we present analysis of vertical distribution of atmospheric temperature over polar region (Antarctica) derived from GPS RO observations, to give another example of value of remote sensing data for providing essential upper air data in the Southern Hemisphere where RS network is extremely sparse.

Data and methods

To analyse the state of the atmosphere over the Australasian region, we used GPS RO data from FORMOSAT-3/COSMIC (F3C) mission. F3C is the first GPS RO mission that employs a satellite constellation for RO observations and the constellation consists of six Low Earth Orbit satellites [START_REF] Liou | FORMOSAT-3 GPS radio occultation mission: preliminary results[END_REF]Fong et al., 2008a, 2008b[START_REF] Wickert | GPS radio occultation with CHAMP, GRACE-A, SAC-C, TerraSAR-X, and FORMOSAT-3/COSMIC: brief review of results from GFZ[END_REF]. F3C was launched in 2006, and the constellation is currently operational providing a large number of daily observations. GPS RO data from F3C constellation were obtained from COSMIC web site http://www. cosmic.ucar.edu/. We analysed GPS RO temperature and moisture profiles obtained by F3C constellation over the Australasian region. Six meteorological stations located in the tropics and having high quality of RS records have been selected for comparison (Fig. 1, the selected stations are highlighted by green circles). Five years of RS data (2006-2010) have been compared with GPS RO collocated events. In this study, GPS RO collocated profiles have been selected to satisfy criteria of (i) spatial collocation to be within 300 km radius from geographic position of the meteorological site, and (ii) temporal collocation to be less than 3 h before or after the time of RS launch (spatial and temporal collocation criteria have been defined based on results of [START_REF] Zhang | An investigation of atmospheric temperature profiles in the Australian region using collocated GPS radio occultation and radiosonde data[END_REF]. The number of atmospheric profiles for RS and GPS RO events at the selected tropical sites is presented in Table 1.

Tropical air over the Australian region is generally warm and moist; however, there is a distinct seasonality. In the Southern Hemisphere, November to April is a wet season and May to October is a dry season; this is translated into significant difference in distribution of rainfall, atmospheric moisture and temperature (for detail, see [START_REF] Zillman | A Hundred Years of Science and Service[END_REF][START_REF] Kuleshov | Seasonal and latitudinal variations of lightning ground flash activity in Australia[END_REF]. In terms of seasonal rainfall, the climate of Australian tropics is classified as summer dominant, with marked wet summer and dry winter (Fig. 2). In this classification, the differences between summer and winter rainfall (Figs. 3 and4, respectively) are used to identify six major climate zones. In Fig. 2, the median annual rainfall and seasonal incidence (the ratio of the median rainfall over the period November to April to the period May to October) is used to identify these six major zones across Australia. As evident from Figs. 3 and4, in the tropics, most of the rainfall occurs in the summer months and it is associated with the monsoon. Based on this classification, we stratified the observations between wet seasons, which also corresponds to tropical cyclone (TC) seasons (November to April) and dry, non-TC seasons (May to October).

To evaluate vertical distribution of atmospheric temperature over the Antarctic, we used GPS RO data obtained from the Challenging Minisatellite Payload (CHAMP) for geoscientific research mission [START_REF] Wickert | Atmosphere sounding by GPS radio occultation: first results from CHAMP[END_REF]. CHAMP was launched in 2000, and for a decade, the mission has been operational. As long records are essential to estimate climate change accurately, CHAMP data were selected to derive temperature trends over Antarctica. We use CHAMP data processed by UCAR (Level 2 wetPrf data from May 2001 to August 2008 available online at http://www.cosmic.ucar.edu/). CHAMP GPS RO data were analysed at nine pressure levels (850,700,600,500,300,200,150, 100 and 50 hPa) and compared with RS data obtained at seven Antarctic meteorological stations.

To investigate the potential of using ground-based GPS observations for retrieving vertical column integrated PWV over the Australian continent, we utilised the data from the Australian GPS reference stations network, e.g., the State of Victoria GPSnet (Department of Environment and Primary Industries, 2013). The GPS-based methodology can be used for retrieving column integrated PWV profiles from the time-varying tropospheric zenith path delay, which can be retrieved by stochastic filtering of the GPS measurements [START_REF] Bevis | GPS meteorology: remote sensing of atmospheric Water vapor using the global positioning system[END_REF]. In this study, the GPS-PWV estimates were derived using the GPS meteorology method originally proposed by [START_REF] Bevis | GPS meteorology: remote sensing of atmospheric Water vapor using the global positioning system[END_REF][START_REF] Bevis | GPS meteorology-mapping zenith wet delays onto precipitable Water[END_REF]. GPS signals are delayed when propagating through the troposphere, and this delay is known as the zenith path delay (ZPD) or the zenith total delay (ZTD). The conversion from ZTD to PWV involves a two-step process. The ZTD is divided into a hydrostatic term (known as zenith hydrostatic delay, ZHD) caused primarily by the dry gases in the atmosphere and a wet term (known as zenith wet delay, ZWD), which is mainly contributed by water vapour contained in the atmosphere (Davis et al., 1985). ZHD can be calculated given the local surface pressure and known location of the GPS receiver. Subtracting ZHD from ZTD yields ZWD, from which PWV can be estimated given near-surface air temperature. In our study, the ground-based GPS-derived PWV are integrated vertical column water vapour.

To validate remote sensing data obtained by F3C and CHAMP space missions and the Australian GPS reference stations network, we used conventional upper air observations obtained by RS. For meteorological stations located on the Australian continent, the RS data were obtained from archive of the National Climate Centre, the Australian Bureau of Meteorology. In Fig. 1, the location of observation stations of the Australian Upper Air Network is presented. For the stations in Antarctica, RS records (monthly time series) were extracted from the HadAT2 data set of the Hadley Centre, UK Met Office [START_REF] Thorne | Revisiting radiosonde upper-air temperatures from 1958 to 2002[END_REF], available online at http://hadobs.metoffice.com/hadat/hadat2.html.

RS is the primary operational source of upper air observations, including pressure, temperature and moisture; for this reason, they are used for independent validation of GPS-PWV and GPS RO data. An RS flight ascends to 2 km in 7-8 min and reaches 5 km in about 20 min after the launch. As the temporal variability of the GPS-PWV and GPS RO in these time scales is not significant, GPS-derived values are often taken at times when RS flights are released (e.g., 11:00 UTC and 23:00 UTC). It is these values of RS data that were used for comparison with GPS-PWV and GPS RO data using the collocation criteria described above.

While coastal areas of Australia are reasonably covered by the RS network, the geographic distribution of the stations in the interior of the continent is very sparse. Similarly, vast areas of the Indian Ocean, the Pacific Ocean and the Antarctic are covered by only a few stations. On the other hand, satellite-based measurements provide global coverage and thousands of atmospheric pressure, temperature and moisture profiles could be obtained daily over remote areas with limited conventional observations. For example, F3C provides approximately 2500 daily GPS RO events globally. This is a comparable amount of atmospheric profiles to those obtained by the global RS network, which has about 2000 stations worldwide. On average, the Australasian region obtains around 300 RO events daily (Fig. 1), which is more than a number of atmospheric profiles that 38 Australian RS stations provide. Similarly, ground-based Australian GPS/GNSS reference stations network provides observations of much higher density (and frequency) than the upper air stations network (RS are typically launched only once or twice a day). For example, there are 15 GPSnet stations in Victoria within a 200-km radius from central Melbourne and only one RS station. Thus, remote sensing methodologies, which provide the high accuracy of retrieval of meteorological variables, have great potential not only to complement conventional observations but in the near future to become a superior source of data for meteorology and climate research and applications.

In the next section, we present results of the GPS ground-and spacebased atmospheric moisture and temperature retrievals validated with in situ RS data to demonstrate value of these remote sensing methodologies for detailed description of vertical structure of the atmosphere in various climatic regions (from the tropics to polar areas), potential for enhancing severe weather forecasting and deriving accurate estimates of atmospheric temperature trends.

Results

Atmospheric moisture retrieval over the Australian continent from GPS ground-based and RS data

We verified the accuracy of GPS-PWV data using RS data at two sites of the Australian Bureau of Meteorology Upper Air Network. The Airport and MOBS and 11 m between WGGA and Wagga Wagga. Comparing 1 week of GPS-PWV and RS-PWV estimates at these stations, good agreement between the GPS and the RS-PWV data has been found, with the uncertainties in the GPS-PWV of the order of 2-3 mm. This level of uncertainty agrees with previous studies [START_REF] Tregoning | Accuracy of absolute precipitable water vapor estimates from GPS observations[END_REF]Liou and Huang, 2000;Liou et al., 2000;[START_REF] Liou | Comparison of precipitable water observations in the near tropics by GPS, microwave radiometer, and radiosondes[END_REF][START_REF] Feng | GPS water vapour experimental results from observations of the Australian Regional GPS Network (ARGN)[END_REF][START_REF] Feng | GPS water vapour experimental results from observations of the Australian regional GPS Network[END_REF][START_REF] Wang | A near-global, 2-hourly data set of atmospheric precipitable water from ground-based GPS measurements[END_REF][START_REF] Thomas | Precipitable water vapor estimates from homogeneously reprocessed GPS data: an intertechnique comparison in Antarctica[END_REF], demonstrating that GPS can be used with confidence as a supplementary sensor for atmospheric water vapour to complement observations from the Bureau's upper air network.

Here we present a case study to investigate potential applicability of the GPS-PWV methodology for weather forecasting, specifically for monitoring of weather fronts. In this study, 15 ground-based GPS stations from the Victorian GPS infrastructure network, i.e. GPSnet, were used (refer to Fig. 5) to examine if the GPS observations could be used to monitor the movement of weather fronts prior to severe thunderstorm occurrences. The March 2010 Melbourne storm, which brought heavy rainfall, large hail stones, strong winds and flash flooding to the state of Victoria was selected as a case study.

The March 2010 Melbourne storms were a series of storms that passed directly over Greater Melbourne. In the days before the storm, a large mass of warm, humid air had passed down from the state of Queensland, where it had caused record flooding in southern Queensland. This warm air mass clashed with a cold and dry air of low pressure system coming from the west generating severe thunderstorms. The first storm was generated in the west of Victoria in the morning on 6 March. The storm cells then extended in a southeast direction across the state between 14:00 and 17:00 AEDT (Australian Eastern Daylight time, which is UTC +11:00 daylight saving), passing central Melbourne at around 14:40 AEDT. At the storm peak on 6 March, a 400-km band of rain and hail stretched across the state moving in a southeasterly direction.

Fig. 6 shows the GPS-PWV time series at the six GPSnet stations (their locations are highlighted by yellow circles in Fig. 5) over the 1week period, i.e., a few days prior to and after the storm passage. The PWV values range between 15 mm and 50 mm, with some distinct variability caused by severe weather processes. It appears that the time series of PWV at all stations are consistent. The fluctuations in the PWV time series can in fact be linked to the variations in water vapour content in the troposphere, which coincide with the passing of the storm cells. The PWV increases from around 20 mm to 40 mm on DOY 063 to 064 (4 to 5 March 2010), corresponding to the arrival of warm and moist pre-frontal air. The warm pre-frontal air holds more water vapour than the cold post-frontal air mass. Thus, as the warm pre-frontal air passed through Greater Melbourne from west to east, the water vapour content in the atmosphere increased steadily; and when the cold and dry post-frontal air moves through, the amount of water vapour decreased from around 40 mm back to 20 mm. Fig. 7 depicts a closer snapshot of the GPS-PWV time series at six selected GPSnet stations in the western and eastern regions of Melbourne (a buffer radius of 200 km from Melbourne CBD) on DOY 064 (5 March 2010). As the cold front moves eastward, the first station to register a large drop in PWV was BALL, followed by APOL, BACC, MOBS, ELLI, and YALL. This phenomenon occurred at slightly different times as seen in the time series presented in Fig. 7. This shows that groundbased GPS stations located in different parts of the state can remotely sense the passage of frontal air mass. This information could be utilised to monitor and predict the course/direction of the frontal air. Furthermore, the ground-based GPS technique is capable of providing continuous observations of the storm passage with high temporal resolution. If the lateral distance between stations is known, then the velocity of the frontal air passage can be estimated. The spatial distribution of the water vapour content at 2-h intervals during the cold frontal air passage is shown in Fig. 8 (direction of the front is shown by an arrow on the top left panel). It illustrates the movement of the cold dry post-frontal air mass as it passed eastward through Greater Melbourne. We estimated the velocity of the frontal air passage from the ground-based GPS observations and radar images and found that both estimates correspond well, providing the value of approximately 35 km/h.

Atmospheric temperature and moisture retrieval over the Australasian region and Antarctica estimated from GPS RO and RS data

Comparison of atmospheric temperature and moisture profiles obtained by RS and GPS RO over the Australasian region has been made over five year period from May 2006 to December 2010 at pressure levels from 1000 hPa to 30 hPa. Data from six selected stations located in the South Indian and South Pacific Oceans (Table 1 and Fig. 1; the location of the stations is highlighted by green circles) have been examined. The number of profiles was approximately evenly distributed between TC seasons (November to April) and non-TC seasons (May to October). In general, during TC seasons (Fig. 9a) good agreement between RS and GPS RO temperature profiles (differences b±0.5 °C) has been found at all stations for pressure levels from 400 to 50 hPa (at altitudes from about 6.5 km in the upper troposphere up to 25 km in the lower stratosphere, especially between 10 and 15 km). For pressure levels above 400 hPa (below 6.5 km altitudes), discrepancies ranging from 0.8 to 1.35 °C were observed mainly due to high level of moisture in the lower troposphere, which affects the accuracy of algorithms for GPS RO temperature retrieval (see [START_REF] Rocken | Analysis and validation of GPS/MET data in the neutral atmosphere[END_REF]Kuo et al., 2004 for detail). It should be noted that the distance between RS and GPS RO profiles as well as types of RS instruments could also be important contributors to these differences [START_REF] He | Assessment of radiosonde temperature measurements in the upper troposphere and lower stratosphere using COSMIC radio occultation data[END_REF]. Similar, for non-TC seasons (Fig. 9b), good agreement between RS and GPS RO atmospheric temperature profiles (b±0.5 °C) was observed at most pressure levels. Compared to TC seasons, the differences were reduced at pressure levels between 800 and 400 hPa, which we attribute to improving in the accuracy of temperature retrieval due to lower atmospheric moisture in May-October compared to TC seasons. Large differences in RS-GPS RO temperature observations of about 1-1.3 °C consistently observed at Tromelin may indicate an issue with RS data at this station; however, this investigation is out of scope for this study. Detailed comparison between daily GPS RO and RS data collocated in space (latitudes b 5°, longitudes b 2.5°) also demonstrates that in general temperature differences are b 2 °C with minimum differences in the upper troposphere at altitudes between 10 and 15 km (see example for Gillot station (20.9°S 55.5°E) located in the South Indian Ocean during strong TC season 2007/2008 in Fig. 10).

In addition to temperature, atmospheric moisture profiles were also examined. Specific humidity was derived from vertical profiles of relative humidity and partial pressure of water vapour obtained by RS and GPS RO, respectively. Differences in specific humidity for TC seasons (Fig. 11a) were b0.5 g/kg at Darwin, Townsville, Cocos Islands and Mahe for pressure levels below 850 hPa (at altitudes above 1.5 km) and at Antanarivo and Tromelin for pressure levels below 700 hPa (at altitudes above 2.5 km). Larger differences (from 1 to 2 g/kg for Darwin, Cocos Islands and Townsville) were observed at pressure levels above 900 hPa (at altitudes between the surface and about 1 km). Relative differences calculated as RS-GPS RO/GPS RO were b 15-20% above 400 hPa (below 6.5 km) and b30% below 350 hPa (above 7.5 km). Comparing atmospheric moisture profiles for TC seasons and non-TC seasons (Figs. 11a and8b, respectively), one can find that differences in values of specific humidity retrieved by RS and GPS RO were b0.5 g/kg for pressure levels below 700 hPa independently of the season with exception at Tromelin where the differences ranging from 0.5 to 1 g/kg were observed between 700 and 600 hPa pressure levels during TC period. Analysing moisture profiles, distinct difference in seasonal convective activity at Antanarivo, Darwin, Townsville and Tromelin was observed as the differences in specific humidity during non-TC and TC periods (see example for Darwin in Fig. 11c). High level of convective activity around Darwin during November to April (wet season or TC season) is well documented. On average, Darwin experiences more than 80 thunderstorms during the wet season which represents the highest thunderstorm activity in Australia [START_REF] Kuleshov | Thunderstorm and lightning climatology (Chapter 4[END_REF].

These findings are encouraging as they provide confidence that the accuracy of GPS RO-derived temperature profiles in the tropics, despite high atmospheric moisture content, is comparable with that of RSs. Thus, GPS RO data could be utilised effectively in tropical atmospheric wave studies in the upper troposphere and lower stratosphere during TC seasons in the Indian and the Pacific Oceans where RS data are available only from a few stations [START_REF] Gubenko | Identification of wave origin of temperature fluctuations and determination of the intrinsic frequency of internal gravity waves in the Earth's stratosphere derived from radio occultation data[END_REF][START_REF] Gubenko | Reconstruction of internal gravity wave parameters from radio occultation retrievals of vertical temperature profiles in the Earth's atmosphere[END_REF][START_REF] Chane-Ming | Observation and a numerical study of gravity waves during tropical cyclone Ivan[END_REF].

As for the Antarctic, six Antarctic coastal stations (Fig. 12) were selected after initial analysis of RS records based on the completeness of data records for the recent period (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) to validate the GPS RO temperature data. On average, less than 2-3% of RS data were missing at nine standard pressure levels from 850 to 50 hPa for these six stations. RS data from Amundsen-Scott station at the South Pole were also used to represent the interior areas of Antarctica, although the percentage of missing data was relatively high: 21-24% for the six standard pressure levels from 500 to 50 hPa. To enable the RS-RO comparison, GPS RO temperatures were sampled using a 500-km radius circles centred at each of the seven stations. The resulting GPS RO temperature series are referred to as the collocated RO temperature series hereafter and are compared with the corresponding in situ RS temperature series.

It was found that at all seven stations and all nine pressure levels the collocated RO temperatures are generally in good agreement with the corresponding in situ RS temperatures, in terms of both the actual values and the seasonal cycle. In the lower stratosphere to the upper troposphere (from 50 hPa to 300 hPa), the collocated RO temperatures and their corresponding RS temperatures are in such good agreement that they are hardly distinguishable from each other (see example of RS (black) and GPS RO (blue) time series in Fig. 13). Similarly, very good agreement was found at other levels from 50 hPa to 300 hPa (not shown).

To investigate recent atmospheric temperature trends over the Antarctic, the time series of the collocated RO and RS data were examined at each pressure level for the seven stations. Linear trends estimated from the collocated RO and RS temperature series are shown in Fig. 14. Strong lower-stratospheric cooling trends have been identified at all levels from 50 to 300 hPa for all Antarctic stations by both methods, with only two exceptions (at McMurdo (89,664) RS data indicate positive trend at 50 hPa while GPS RO data indicate strong cooling, and at Novolazarevskaja (89,512) RS data indicate negative temperature trend at 300 hPa while GPS RO data indicate a weak warming trend). The 7-station-mean stratospheric cooling rates at 200 and 100 hPa pressure levels are about -2.6 °C and -3.2 °C (or -2.1 °C and -3.5 °C) per decade from the collocated GPS RO (or RS) data, respectively (see the Means in Fig. 14). Once again, we note that both GPS RO and RS data are in excellent agreement for the low-stratospheric measurements at levels from 50 hPa to 300 hPa. In the troposphere (at levels below 300 hPa) both cooling and warming trends are detected at different stations depending on their geographical location; however, RO and RS data are less consistent in terms of detecting temperature trends in the lower to mid-troposphere than in the lower stratosphere. Nevertheless, for the stations in the Western Antarctica, Halley (89,022) and Novolazarevskaja (89,512), GPS RO data indicate warming trends in the upper troposphere at 500 hPa and 700 hPa pressure levels, in agreement with finding of earlier studies about rapid near-surface warming [START_REF] Vaughan | Climate change: devil in the detail[END_REF]. For interior of Antarctica, cooling at all pressure levels from 50 to 500 hPa was found (Amundsen-Scott station, 89009).

Discussion

In this report on recent progress in advancing climate studies in the Australasian region, we demonstrated that GPS satellite-and groundbased measurements are valuable source of meteorological data. We presented examples of distribution of atmospheric characteristics over the Indian and the Pacific Oceans, continents of Australia and Antarctica derived from GPS RO measurements and verified the accuracy of RO temperature and moisture retrieval with in situ RS data. Investigating recent atmospheric temperature trends over Antarctica, the time series of the collocated RO and RS data were examined and strong cooling in the lower stratosphere and warming through the troposphere over the Antarctica has been identified by both methodologies, with detected the strongest cooling trend of about -3 °C per decade at 100 hPa level. These findings demonstrate that GPS RO measurements could significantly advance climate studies for data sparse areas like the Australasian region and the Antarctic. Early study by Turner et al., 2006, through We also evaluated potential of the Australian network of GPS stations to estimate moisture content of the atmosphere (PWV) and presented a case study for monitoring a severe thunderstorm event using GPS-PWV methodology. Comparing GPS-PWV and RS-PWV estimates at two selected stations in Victoria and NSW, good agreement between the GPS and the RS-PWV data has been found, with uncertainties in the PWV estimates in the order of 2-3 mm, which is likely as radiosonde data are accurate to about 1.2 mm (e.g., [START_REF] Feng | GPS water vapour experimental results from observations of the Australian regional GPS Network[END_REF][START_REF] Wang | Systematic errors in global radiosonde precipitable water data from comparisons with ground-based GPS measurements[END_REF]. Using data from the 15 ground-based GPS stations from the Victorian GPS infrastructure network, we investigated the state of the atmosphere over Victoria during a significant weather event, the March 2010 Melbourne storm, and demonstrated that the GPS observations has potential for monitoring the movement of a weather front that has sharp moisture contrast. Using the ground-based GPS observations and weather radar images, we estimated a velocity of the frontal air passage and found that both estimates correspond well which demonstrates the potential of using GPS data for estimating the velocity of fronts.

These findings could have potentially important implications for enhancing the reliability and accuracy of severe weather forecasting services [START_REF] Benevides | On the inclusion of GPS precipitable water vapour in the nowcasting of rainfall[END_REF][START_REF] Shi | Real-time GPS precise point positioning-based precipitable water vapor estimation for rainfall monitoring and forecasting[END_REF][START_REF] Sparta | Spatial interpolation of GPS PWV and meteorological variables over the west coast of peninsular Malaysia during 2013 Klang Valley flash flood[END_REF]. Accurate estimates of time of front arrival at particular localities are essential to provide early warning to public, aviation, etc., about timing of expected thunderstorm-related hazards (lightning, hail, strong winds and flash flooding) associated with passage of weather front. Lightning activity associated with thunderstorms in SE Asia and Australia is high [START_REF] Kar | Aerosol effects on the enhancement of cloud-toground lightning over major urban areas of South Korea[END_REF][START_REF] Kuleshov | Spatial distribution and frequency of thunderstorms and lightning in Australia (Chapter 8, in book)[END_REF][START_REF] Liou | Study of cloud-to-ground lightning and precipitation and their seasonal and geographical characteristics over Taiwan[END_REF], and it is vital to accurately forecast time of thunderstorm development to provide general public with early warning. This could potentially save lives (e.g., on average, 5 to 10 people are killed by lightning in Australia annually (Lightning Protection, 2007) and significantly reduce economic damage (e.g., the total damage attributed to Sydney storm in 1999, which was the costliest natural disaster in Australian insurance history, was estimated around A$2.3 billion). It is equally important for aviation weather forecasting, with implication for improving safely of passengers and reduction of economic losses due to flights' delays or cancellations.

However, it should be noted that not all fronts have such clear moisture signature as the March 2010 Melbourne storm presented here as a case study. In general, GPS-PWV methodology may be good for monitoring dry lines as well as weather fronts with sharp moisture contrast. There is another potential problem for monitoring weather fronts using GPS-PWV, which is related to measuring vertically integrated water column using this technique rather than detecting near-surface moisture feature of a front. This means that for a strong front with less sharp moisture gradient, the frontal location may not be identifiable from the GPS-PWV alone so accurately as it was demonstrated in Section 3.1. Thus, in our future research, we will address this issue of refining GPS-PWV methodology for application to severe weather forecasting utilising additional surface-based and remote sensing data. A geographic information system [START_REF] Chrisman | Exploring Geographic Information Systems[END_REF] would be an invaluable tool for simultaneous analysis of multi-layered data sets of fields of atmospheric pressure and temperature, radar images, overlaid with GPS-PWV data and conventional meteorological observations to develop an operational system for weather front monitoring and enhancing severe weather forecasting services.

In this study, we extensively used F3C data. F3C, a constellation of six LEO satellites, is one of the most successful missions that utilised GPS radio signals for meteorology and climate application. It is in operational use since 2006. Success of the F3C mission encouraged the development of the follow up mission-FORMOSAT-7/COSMIC-2 (F7C2) constellation of 12 LEO satellites. The first launch of six satellites of the low inclination constellation is planned for 2016 and the second launch of another six satellites into the high inclination orbits-for 2018. Consequently, significant increase in temporal and spatial coverage of the Australasian region by the RO events from F3C and F7C2 constellations with multiple Global Navigation Satellite Systems (GNSS) systems (i.e. GPS, GLONASS, Galileo and QZSS) is expected.

To summarise, in the near future, with further expansion of GNSS system, it is expected that GNSS satellite-and ground-based measurements would be able to provide much larger (an order of magnitude) amount of quality atmospheric data which in turn could significantly advance studies on climate analysis and monitoring in the Australasian region.

Concluding remarks GPS RO technique has been recognised as an emerging technique for

Earth's atmospheric observation. Atmospheric profiles derived from GPS RO observations provide valuable information about the state of the atmosphere over the oceans where upper air data from conventional meteorological observations are particularly scarce. With the recent GPS modernisation and new global and regional GNSS systems in the near future, next-generation RO missions will have opportunity and capability to utilise signals (in over ten different frequencies) from more than a hundred of GNSS satellites. As a consequence, the resolution, quantity and quality of the GNSS RO observations will be improved significantly, and the data will have significant impact on various climatological and meteorological applications. Valuable information derived from GNSS RO atmospheric profiles would be utilised to improve the accuracy of weather forecasting and climate studies. Similarly, data from GNSS ground-based stations (e.g., the Australian ground-based GPS/GNSS reference stations network) will provide accurate estimates of atmospheric moisture which could have positive impact on skill of weather forecasting and in longer terms assist with improving our understanding of the regional climate processes.

Fig. 1 .

 1 Fig. 1. Location of stations of the Australian Upper Air Network (red triangles) and a sample of distribution of GPS RO events (black dots) over the Australasian region for one day. Location of six RS stations in the tropics selected for comparison with GPS RO are highlighted by green circles.

Fig. 2 .

 2 Fig. 2. Major seasonal rainfall zones of Australia.

Fig. 3 .Fig. 4 .

 34 Fig. 3. Average summer rainfall across Australia over the period 1961 to 1990.

Fig. 5 .

 5 Fig. 5. Geographical location of the 15 selected GPSnet stations within 200 km radius from Melbourne with blue lines indicating locations of the front at 12 UTC (left) and 18 UTC (right) (DOY 064, 5 March 2010), respectively.

Fig. 7 .

 7 Fig. 7. A close-up snapshot of the GPS-PWV results from six GPS stations during the passage of the weather front eastward through Greater Melbourne on DOY 64 (5 March 2010). The different coloured lines represent the different GPS stations marked in the figure on the left.

Fig. 6 .

 6 Fig. 6. Time series of the seven days GPS-PWV (mm). Passage of the pre-, post-frontal air and dissipation are marked on the time series. The different coloured lines represent the different GPS stations marked in the figure on the right.

Fig. 9 .

 9 Fig. 9. Difference in temperature retrievals between collocated RS and GPS RO profiles for six tropical stations (Antanarivo, Tromelin, Mahe, Cocos Island, Darwin and Townsville) for May 2006-December 2010 (a) during the Southern Hemisphere TC seasons (November to April) and (b) non-TC seasons (May to October).

Fig. 10 .

 10 Fig. 10. Time-altitude distribution of atmospheric temperature at Gillot station, La Réunion (20.9°S, 55.53°E) during strong tropical cyclone season in the south-west Indian Ocean (2007-2008) derived from (a) daily GPS RO observations collocated in space (latitudes b5°, longitudes b2.5°) and (b) RS at 11:00 UTC and (c) the difference.

Fig. 11 .

 11 Fig. 11. (a and b) Same as in Fig. 6 but for specific humidity. (c) Comparison of specific humidity derived from collocated RS (solid line) and GPS RO (dots) during the whole period of May 2006 to December 2010 (red), not-TC (green) and TC (blue) seasons for Darwin, Australia.

Fig. 12 .

 12 Fig. 12. Location of seven Antarctic stations used in this study to provide upper air data and 500 km radius circles centred at each station to encompass area of collocated GPS RO profiles.

Fig. 13 .

 13 Fig. 13. Comparison of atmospheric temperature time series, with trend lines superimposed, for in situ 100 hPa RS data (black) and the corresponding GPS RO data (blue) at the indicated stations.

  analysis of RS observations at nine stations across Antarctica for the period 1971-2003, demonstrated cooling rates of about -0.16 °C per decade at 100 hPa. Our findings indicate that during 2001-2008 rate of cooling at 100 hPa level was much higher, about -3.2 °C per decade. Our results are in agreement with outputs of climate models which indicate that recent stratospheric cooling and tropospheric warming are expected consequences of the observed increase in anthropogenic greenhouse gas concentrations (IPCC AR5, 2013).

Fig. 14 .

 14 Fig. 14. Comparison of linear trends of atmospheric temperature estimated from in situ RS data and the corresponding GPS RO data for the period from September 2001 to August 2008. Note that station 89009 (a high elevation station) has no RS data at 700 hPa and no bars are shown in the lower-right panel.

Table 1

 1 Geographical location of RS stations, number of RS and GPS RO atmospheric profiles and number of collocated profiles obtained from May 2006 to December 2010.

	Radiosonde station	Number of profiles
	(latitude, longitude)	RS/GPS RO profiles	Collocated profiles
	Antananarivo (-18°48′, 47°29′)	2683/1616	700
	Tromelin (-15°53′, 52°11′)	1306/1314	244
	Mahe (-4°37′, 55°27′)	2390/785	306
	Cocos Island (-12°30′, 96°50′)	1649/897	269
	Darwin (-12°25′, 130°50′)	6730/983	949
	Townsville (-19°15′, 146°49′)	5142/1546

Acknowledgments

This work was partially supported by the RMIT Foundation and the Malcolm Moore Industry Research Grant French StraDyVariUS ANR-13-BS06-0011-01 grant, and Taiwan NSC 102-2111-M-008-027 and NSC 102-2221-E-008-034 grants.