Ben Aurélien

Daoud

Eric Sauquet

Guillaume Bontron

Charles Obled

Michel Lang

Daily quantitative precipitation forecasts based on the analogue method: improvements and application to a French large river basin

Keywords:

This paper presents some improvements of a probabilistic quantitative precipitation forecasting method based on analogues, formerly developed on small basins located in South-Eastern France. The scope is extended to large scale basins mainly influenced by frontal systems, considering a case study area related to the Saône river, a large basin in eastern France. For a given target situation, this method consists in searching for the most similar situations observed in a historical meteorological archive. Precipitation amounts observed during analogous situations are then collected to derive an empirical predictive distribution function, i.e. the probabilistic estimation of the precipitation amount expected for the target day. The former version of this forecasting method (Bontron, 2004) has been improved by introducing two innovative variables: temperature, that allows taking seasonal effects into account and vertical velocity, which enables a better characterization of the vertical atmospheric motion. The new algorithm is first applied in a perfect prognosis context (target situations come from a meteorological reanalysis) and then in an operational forecasting context (target situations come from weather forecasts) for a three years period. Results show that this approach yields useful forecasts, with a lower false alarm rate and improved performances from the present day D to day D+2.

Introduction

Many water-related stakeholders need quantitative precipitation forecasts as reliable as possible to anticipate discharges in river basins several hours or days ahead. For example, French operational flood forecasting services require precipitation forecasts several days ahead to anticipate flood risks [START_REF] Lacaze | Sensitivity of flood forecasting models towards the quality of future rainfalls: a case study of sensitivity analysis and assessment of the operational efficiency on the river Yonne and the river Loing[END_REF], and hydroelectricity power producers need accurate and reliable precipitation forecasts to anticipate the discharge evolution along the regulated rivers and their tributaries in order to perform their activities and to provide safety control [START_REF] Bompart | An operational hydrometeorological forecasting chain for CNR's hydroelectric production needs[END_REF].

Nowadays forecast uncertainties related to meteorological predictions and implied by modelling processes are more and more taken into account, leading to probabilistic quantitative precipitation forecasts (PQPFs) that provide ensemble forecasts. In particular, this kind of forecast enables to evaluate the risk of extreme events. At least, two approaches for producing PQPFs are commonly used: (i) regional ensemble weather forecasts based on dynamical approaches (e.g. COSMO-LEPS, [START_REF] Marsigli | The COSMO-LEPS mesoscale ensemble system: validation of the methodology and verification[END_REF]PEARP, Thirel et al., 2008;EFAS, Thielen et al., 2009;ECMWF-ENS, Miller et al., 2010), (ii) statistical approaches based on a search for analogues (e.g. [START_REF] Obled | Quantitative precipitation forecasts: a statistical adaptation of model outputs through an analogues sorting approach[END_REF][START_REF] Hamill | Probabilistic quantitative precipitation forecasts based on reforecast analogs: Theory and application[END_REF][START_REF] Messner | Probabilistic forecasts using analogs in the idealized Lorenz96 setting[END_REF][START_REF] Marty | Toward real-time daily PQPF by ananalog sorting approach: Application to flash-flood catchments[END_REF].

The principle of analogue methods (AMs) is to use deterministic meteorological outputs of a target day D as an input for a statistical search of similar past days in terms of general circulation patterns.

Similarity is measured on a set of relevant predictor variables computed by Numerical Weather Prediction (NWP) models. Finally the analogue situations selected are used to produce a sample of observed daily precipitations, which provides a probabilistic forecast for the day D.

The AMs assume a relationship between large-scale variables (predictors) and a local-scale target variable (predictand). Different predictors can be found in the literature. Table 1 presents a short description of a selection of representative models developed to predict precipitation fields under various climates. Note that this table does not include methods using combination of predictors in linear regression to estimate predictands. One can note that the number of predictors may differ from one region to another. [START_REF] Timbal | The Bureau of Meteorology Statistical Downscaling Model Graphical User Interface: user manual and software documentation[END_REF] suggest defining combinations of predictors adapted for each season and each climatic region.

The predictors can be divided into two categories. Most AM developments deal with one or more low flow atmospheric fields, i.e. descriptors of the meteorological state of the atmosphere at synoptic scale and a set of geopotential heights is usually chosen. The number of geopotential heights and their corresponding pressure level varies from one study to another: [START_REF] Altava-Ortiz | Application of the MM5 and the analogous method to heavy rainfall events. The case of 16-18th October 2003 in Catalonia ([END_REF] used three geopotential heights at 500, 850 and 1000 hPa to estimate precipitation amounts for one major heavy rainfall event in Catalonia (Spain) while [START_REF] Diomede | The use of meteorological analogues to account for LAM QPF uncertainty[END_REF] selected the geopotential at 500 hPa to estimate precipitation in Northern Italy. Following Bontron and Obled (2005), [START_REF] Horton | Spatial relationship between the atmospheric circulation and the precipitation measured in the western Swiss Alps by means of the analogue method[END_REF] and [START_REF] Chardon | Spatial Similarity and Transferability of Analog Dates for Precipitation Downscaling over France[END_REF] selected analogous situations based on geopotential heights at 500 and 1000 hPa-level in Switzerland and in France, respectively.

The second category of predictors is made of moisture related variables with among them, the largescale precipitation outputs PRCP from NWP models (e.g. [START_REF] Diomede | Calibration of Limited-Area Ensemble Precipitation Forecasts for Hydrological Predictions[END_REF][START_REF] Turco | Testing MOS precipitation downscaling for ENSEMBLES regional climate models over Spain[END_REF]. Themeβl et al. (2011) found PRCP the most important predictors for local precipitation far beyond the other variables related to the state of the atmosphere. However [START_REF] Dayon | Transferability in the future climate of a statistical downscaling method for precipitation in France[END_REF] have compared different versions of AM and pointed out large possible biases in an application to France, when PRCP is used. In Catalonia, [START_REF] Barrera | Heavy rain prediction using deterministic and probabilistic models -the flash flood cases of 11-13 October 2005 in Catalonia (NE Spain)[END_REF] added the humidity at 1000 hPa to the set of predictors suggested formerly by [START_REF] Altava-Ortiz | Application of the MM5 and the analogous method to heavy rainfall events. The case of 16-18th October 2003 in Catalonia ([END_REF]. Similarly Bliefernicht and Bardossy (2007) selected three predictors (two geopotential heights and the moisture flux at 700 hPa defined by the product of the specific humidity at 700 hPa and the westerly wind) to predict rainfall in the Rhine basin in Germany. [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF] and later [START_REF] Gibergans-Báguena | Improvement of the analog forecasting method by using local thermodynamic data[END_REF] and [START_REF] Diomede | The use of meteorological analogues to account for LAM QPF uncertainty[END_REF] tested different combinations of variables as predictors. [START_REF] Diomede | The use of meteorological analogues to account for LAM QPF uncertainty[END_REF] showed that geopotential at 500 hPa and vertical velocity at 700 hPa are the most relevant set of predictors to estimate precipitation in Northern Italy while [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF] highlighted that the best moisture related variable to be used in addition to synoptic scale variables to estimate precipitation in South-Eastern France is a combination of relative humidity and total column precipitable water. More exhaustively, [START_REF] Gibergans-Báguena | Improvement of the analog forecasting method by using local thermodynamic data[END_REF] have identified 7 variables among a list of 33 thermodynamic descriptors and the AM combining these 7 variables to geopotential fields as predictors outperforms the other tested AMs. These works show that improved analogues sorting technique can be expected by incorporating new levels of analogy (i.e. incorporating new predictors and related similarity criteria). Note that analogy between meteorological situations can be also used in a post-processing of precipitation forecasts to improve the prediction skill of NWP models (e.g. Wang and Fu, 2009).

We will focus here on the application of AMs, and more specifically on one of the reference PQPF methods already used in France in an operational context and suggested by Bontron and Obled (2005) following the work of [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF]. This former method was developed considering small basins located in South-Eastern France, subject to flash-flood events. The aim of this paper is to extend the scope of this AM, considering large French river basins under oceanic influences. This study presents then two improvements of the reference method and compares the relative performances of the latter with two alternatives, on the large French basin of the Saône river. We consider two contexts: (i) a "perfect prognosis context" where the meteorological fields of the target day D are directly built with observed data or re-analysed by a NWP model; (ii) an "operational forecast" context where the meteorological fields are provided by an operational NWP model, including forecast errors.

Section 2 presents the study area and the datasets used, and section 3 introduces the scores selected for the evaluation of probabilistic forecasts. Section 4 gives the main principles of the benchmark method for searching analogues, based on a two-step procedure, plus two alternatives adding supplementary explanatory variables, air temperature and vertical air motion. The main results of the comparison of the methods are analysed in the fifth section, considering the "perfect prognosis" and the "operational forecast" contexts. The last section draws general conclusions.

2.

Study area and data

Study area

This application of the AM focuses on the Saône River basin located in eastern France. With a response time of 12 to 24 h, the Saône River is one of the most important tributaries of the Rhône River. This basin is under oceanic influences and westerly fluxes generating rainfall when large fronts pass but it can be also affected by heavy rainfall events extending from the Mediterranean region. For this application, the total drainage area (∼ 30000 km²) is divided into three subcatchments defined by three gauging stations (Figure 1). This partition is considered in the operational forecasting system developed by the French electricity producer CNR (Compagnie Nationale du Rhône).

Meteorological archive

Three well-known re-analysis archives have originally been identified for this study: (i) the first National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis (NNR) [START_REF] Kalnay | The NCEP/NCAR 40-Year Reanalysis Project[END_REF]; (ii) the 45-Year European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA), available from 1957 to 2002 [START_REF] Uppala | The ERA-40 re-Author-produced version of the article[END_REF]; (iii) the new generation of European reanalyses now available, called ERA-Interim [START_REF] Dee | The ERA-Interim reanalysis: configuration and performance of the data assimilation system[END_REF]. Finally, based on the results of previous works (Ben [START_REF] Daoud | A flood forecasting system based on probabilistic precipitation scenarios: performance evaluation and sensitivity analysis to the re-analysis archive[END_REF], ERA was chosen.

.

Precipitation

A 39-year precipitation data set over the 1970-2008 period was extracted from the 8-km-resolution atmospheric re-analysis over France performed with the Safran gauge-based system (Quintana-Seguí et al. 2008[START_REF] Vidal | A 50-year highresolution atmospheric reanalysis over France with the Safran system[END_REF]. However the years 1970 and 1971 were only available after the optimisation phase in perfect prognosis context (section 4). These additional data were then used for the analysis of AMs performances in both real-time and perfect prognosis contexts (section 5). Basin scale precipitation data at daily time step (from 6 UTC the current day to 6 UTC the day after) were obtained by superimposing the delineation of the three sub-basins taken from [START_REF] Sauquet | Mapping mean annual river discharges: geostatistical developments for incorporating river network dependencies[END_REF] on the Safran grid. Table 2 shows the main characteristics of the Saône sub-catchments:

proportion of dry days F0, daily rainfall amounts with return periods of 5 and 10 years PJXA5 and PJXA10 given by the Gumbel distribution fitted to the annual maximum series, and maximum values Pmax observed over the period.

These statistics show that only 20% of the days are dry, a rather low value given the climatological regime of the region (semi-continental climate). That may be explained by the size of the catchment leading to the assimilation of several rain gauges among which at least one of them reported one non-zero value. This effect of spatialisation should be emphasised by the aggregation over the sub basins. Regarding heavy events, the influence of the orography is illustrated by the high values for the Doubs basin, which is characterised by the Jura mountain. However, the downstream section of the Saône basin is also characterised by high values, even higher than for the Doubs basin (Pmax).

This fact is closely related to southerly heavy rainfall events extending from the Mediterranean region, especially in autumn.

Besides, since i) climatological regimes and precipitation distributions of the sub-catchments are different and ii) precipitation amount is an asymmetric variable, a set of transformed precipitation data was built for the development step of the AM. The data were transformed following this equation:

10 PJXA P P raw = ( 1
)
Where P is the transformed value, raw P is the raw precipitation value and PJXA10 the value with return period 10 years.

Numerical scores for performance evaluation

Performance evaluation of any forecasting system relies on numerical scores to assess the accuracy of the forecasts (see [START_REF] Jolliffe | Forecast verification. A practitioner's guide in atmospheric science[END_REF] for an extended list of scores). In this study,

Author-produced version of the article published in Atmospheric Research, 169, Part A, 1, 147-159, The original publication is available at http://www.sciencedirect.com/ doi:10.1016/j.atmosres.2015.09.015 several scores are used: i) A global performance criterion on the overall forecasts (i.e. considering no rain situations as well as precipitation event). The Continuous Ranked Probability Skill Score was chosen to perform the optimisation process;

ii) End-user oriented criteria based on contingency tables (i.e considering the ability of the system to detect specific event), used to evaluate specifically the optimised AMs. Several scores from contingency tables with prescribed threshold have been used.

Continuous Ranked Probability Skill Score (CRPSS)

The CRPSS score is based on the Continuous Ranked Probability Score (CRPS; [START_REF] Brown | Admissible scoring systems for continuous distributions[END_REF][START_REF] Matheson | Scoring rules for continuous probability distributions[END_REF][START_REF] Hersbach | Decomposition of the ranked probability score for ensemble prediction systems[END_REF] which is a continuous version of the widely used Ranked Probability Score. CRPS is equivalent to the mean absolute error of a deterministic forecast.

It is especially recommended for the verification of continuous variables probabilistic forecasts when performances are evaluated for all the range of possible values (no threshold is considered).

The CRPS is defined as:

∑ ∫ = +∞ ∞ - - = N i x dx x H x F N CRPS 1 )]² ( ) ( [ 1 0 (2)
where N is the number of forecasts, 0

x is the observed value, F is the distribution function of the forecasted variable x , and 0 x H is the Heaviside function (set to 0 for values below 0 x and 1 for values above 0 x ). The minimum value of CRPS is 0, when the forecast system is perfect and there is no theoretical upper bound for CRPS. Thus, the lower the CRPS the more useful the forecast.

Furthermore, the CRPS takes into account i) the forecast resolution part, which represents the uncertainty of the probabilistic forecast, and ii) the forecast reliability part, that could be interpreted as the error between the observed value and the median value of the ) (x F distribution [START_REF] Hersbach | Decomposition of the ranked probability score for ensemble prediction systems[END_REF][START_REF] Candille | Evaluation of probabilistic prediction systems for a scalar variable[END_REF]. In this work, we followed [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF] who defined the CRPS as a score of utility, which is easily decomposed into a resolution part (Reso) and a reliability part (Reli):

[ ] [ ] dx x F Reli dx x H x F Reso Reli Reso CRPS Q x Q ∫ ∫ - = - = + = ∞ + ∞ - % 50 0 % 50 ) (

Q

is the 50% quantile of the distribution function F .

For the interpretation of the results in terms of gain, it is widespread to use a reference forecast.

Thus, the relative CRPS skill score (CRPSS) could be used instead:

) / ( 1 ref M CRPS CRPS CRPSS - = (4)
where M CRPS and ref CRPS are the scores of method M and of a reference method, respectively. In this paper the reference method will be given by the climatological distribution of the predictand x.

Other references such as persistence or random forecast may be used [START_REF] Wetterhall | Statistical precipitation downscaling in central Sweden with the analogue method[END_REF][START_REF] Diomede | The use of meteorological analogues to account for LAM QPF uncertainty[END_REF]. If CRPSS is 0, there is no improvement from method M with regard to the reference. If CRPSS is higher than 0, M performs better than the reference method. If CRPSS is 1, forecasts obtained with M are perfect (i.e. CRPS M = 0). Note that this skill score may be applied to the resolution and reliability parts of the CRPS (Eq. 3).

Several methods are compared in the following section and it is more appropriate to evaluate the gain G of performance of any improvement in AMs. When one method B is compared to one method A, G is defined by the relative skill score of the CRPS of the method B where the method A is considered as the reference method:

) / ( 1 ) / ( A B CRPS CRPS A B G - = (5)
The perfect score G is 1, whereas a null value of G means no improvement in forecast skill provided by the method B.

Scores from contingency tables

An operational stakeholder is mainly interested in a forecasting system which performances are satisfactory with regard to his expectations. For instance, irrigation decision management needs information on the probability of additional rainfall provided by short-term weather forecasts, while flood warning services need to estimate if heavy rainfall (over a specific threshold) is expected before providing flood warning alerts. In this context, additional scores are computed from contingency tables.

To build a contingency table, each forecast is considered as a forecast of the occurrence or nonoccurrence of an event, that will or will not effectively occur. In the precipitation context, an event is defined by a precipitation threshold T. Exceedance over specific thresholds T is examined using various quantiles f P taken from the empirical distribution of precipitation amounts derived from analogous days. Thus, within a period of evaluation of n days, given a precipitation threshold T, the contingency table (see table 3) is fulfilled as followed, considering for each day the observed and forecasted precipitation values o P and f P :

-N CA = number of correct alarms ( T P f > and T P o > );

-N FA = number of false alarms (

T P f > and T P o ≤ ); - N MA = number of missed alarms ( T P f ≤ and T P o > ); -N CR = number of correct rejections ( T P f ≤ and T P o ≤ ).
According to the methodology, the sum (N CA + N FA + N MA + N CR ) is the total number n of days considered in the evaluation period. If the forecast system is perfect, then it would produce only correct alarms and correct rejections, no false alarm and no missed alarm. Once the contingency table is completed, it is possible to compute a panel of scores [START_REF] Stephenson | Use of the odds ratio for diagnosing forecast skill[END_REF]. In this study, three scores were calculated:

) /( ) /( ) /( ; ) /( CR FA FA MA CA CA FA CA FA MA CA CA N N N N N N TSS N N N FAR N N N POD + - + = + = + = (6)
The Probability Of Detection (POD) measures the probability that an observed event is well forecasted by the system. POD is between 0 and 1 (1 for a perfect forecast). The False Alarm Rate (FAR) gives the probability that an expected event did not occur. FAR is between 0 and 1 (0 for a perfect forecast) but could not be calculated if no event is forecasted, what might occur for a high threshold T. Since no system is perfect, the better way is to strike a balance between a POD high enough and a FAR low enough. The True Skill Statistic (TSS), also known as Hanssen-Kuipers score, takes into account both correct alarms and false alarms. It corresponds to the difference between POD and the Probability Of False Detection, as defined by [START_REF] Wilks | Statistical Methods in Atmospheric Sciences[END_REF]. It gives an overview of forecast system skill, since it is based on all indices of the contingency table. TSS has the disadvantage to be not so useful for extreme events since it could converge to POD (N CR becoming very large). This score has a range of -1 to +1 (1 for a perfect forecast, 0 indicating no skill). [START_REF] Duband | Reconnaissance dynamique de la forme des situations météorologiques. Application à la prévision quantitative des précipitations[END_REF] initiated the development of the AM in France, which was followed by several academic works by [START_REF] Guilbaud | Prévision quantitative des précipitations journalières par une méthode statisticodynamique de recherche d'analogues[END_REF], [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF] and Ben [START_REF] Daoud | Améliorations et développements d'une méthode de prévision probabiliste des pluies par analogie. Application à la prévision hydrologique sur les grands bassins fluviaux de la Saône et de la Seine[END_REF]. This paper relates to the latter work, focussed on the case of large basin under oceanic influences, such as the Saône river basin. We will show how the introduction of additional variables enables to provide a better forecast skill.

Development of the analogue method

Author-produced version of the article published in Atmospheric Research, 169, Part A, 1, 147-159, The original publication is available at http://www.sciencedirect.com/ doi:10.1016/j.atmosres.2015.09.015

Each time a new variable is introduced, a new step of analogues selection is created in the algorithm. According to this methodology, it is necessary to define several parameters and variables: predictors, spatial domain on which the analogy is searched for, number of analogues for each successive step, and also a criterion to measure the quality of the analogy. These parameters are obtained by optimisation.

During this optimisation phase, the whole period where predictors from the meteorological archive and predictand from the hydrological archive are available was divided into two independent sub-periods: the calibration period over which the method was optimised (~25 years), and the validation period over which the results were then validated (~5 years). Splitting into calibration-validation periods was not undertaken by considering two successive sub-periods, as it could introduce a climatological bias (succession of alternating wet and dry periods, heterogeneity in the meteorological archive, stationarity issues related to climate change), but rather by removing five evenly spaced years from the whole period (1978, 1983, 1988, 1993 and 1998).

In this study, the choice of the variables was mainly guided by their meteorological meaning related to the mechanism responsible of a precipitation event. Several variables expected to be useful for precipitation forecasting were tested in the analogues selection algorithm (equivalent potential temperature as an indicator of air masses transition, potential vorticity as a predictor for lifting processes at synoptic scale, etc.).

Finally, two additional variables were introduced: temperature and vertical velocity. The main results obtained from the optimisation phase are presented in the following sub-sections after a brief description of the starting AM.

4.1

Method A: The benchmark method for searching analogues [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF] developed a version of the analogue method (called hereafter method A) adapted for small to medium-sized sub-catchments. The selection of analogous days is divided into two successive steps: the first step relies on an analogy on the general synoptic circulation based on geopotential heights fields [START_REF] Guilbaud | L'approche par analogues en prévision météorologique[END_REF], whereas the second one involves humidity predictors at local scale.

Let's consider a target day D for which meteorological forecasts are generated. Before applying the algorithm of analogues selection, the past candidate situations are pre-selected by taking seasonal effects into account. Indeed, according to [START_REF] Lorenz | Atmospheric predictability as revealed by natural occurring analogues[END_REF], factors such as vegetation, sea temperature or snowpack, depending on the period of the year (i.e. the season), should have impacts on precipitation formation processes. Consequently, the dates of candidate situations should be restricted to a temporal window around the target date, which period was optimised and fixed to 4 months by [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF].

Method A is divided in two steps (Figure 2):

-A first step of selection (L1-A) is applied in order to search for N1 situations that are analogous to the target one, based on geopotential heights at 1000 and 500 hPa-levels (Z1000 & Z500) considered at 12 UTC the day D and 00 UTC the day D+1 respectively.

The analogy criterion is the S1 score [START_REF] Teweles | Verification of prognosis charts[END_REF], which emphasises analogy in fields shape (already used by [START_REF] Guilbaud | L'approche par analogues en prévision météorologique[END_REF]. The optimised spatial domain of analogy is large enough to catch the main features that are responsible for precipitation;

Method B: Temperature as a seasonal indicator

In method A, a large sample of past situations contained in a moving temporal window of 4 months is considered as candidate situations for the preliminary step of the selection. For instance, to forecast precipitation expected for the 1st of January, candidate situations for analogy are considered between the 1st of November and the 1st of March of every year of the period covered by the reanalyses archive. This constraint on seasonality is also applied by other authors (see for example Gibergans-Baguena and Llasat, 2007;[START_REF] Diomede | The use of meteorological analogues to account for LAM QPF uncertainty[END_REF]. One main drawback is that selecting candidate situations in a limited temporal window may be too restricting, removing sometimes very informative analogues. The suggested solution to overcome this limitation is to make a pre-selection in method B based on a variable with a pronounced seasonal pattern more appropriate that the date itself. The search for such a variable leads naturally to the air temperature (Tair). Values of Tair between 925 hPa and 500 hPa were tested by creating a preliminary step (L0-B) before (L1-A). Several times within the target day D were also considered: from -12h to +42h by step of 6h (-12h is corresponding to 12 UTC the previous day of the day D).

The optimal predictor was determined through an optimisation procedure. To limit calculation time, before starting the optimisation procedure several parameters of this new analogy step were carefully fixed a priori:

the spatial domain considered for this preliminary step L0-B is reduced to the grid point nearest to the target catchment gravity centre, here the Saône River basin (5°E; 47,5°N)

since the seasonal effect should be monitored at the most local scale;

the numerical criterion that measures analogy is RMSE (another measure of distance could be used, however the S1 score that accounts for the shape of the temperature field should not be relevant a priori);

the number of preselected situations is assumed to be roughly equivalent to the number of situations automatically chosen by method A (i.e. about 3000 for a historical period of 30 years);

the subsequent steps of method A are applied without any change (L1-B=L1-A and L2-B=L2-A).

A first testing was carried out by analysing Tair at only one pressure level and one time. Figure 3 shows that Tair should be analysed in very low troposphere (925 hPa) at +36h, leading to a gain of +2.7% towards method A. The fact that temperature should be taken beyond the standard period of total rainfall, which is from +6h to +30h, was rather unexpected.

We assume that this time lag is related to the propagation of the frontal rainfall systems. Let us consider a front that defines the boundary between two air masses with contrasting thermal states. It is well-known that precipitations usually occur along the front. Searching analogy after 30h allows detecting changes in the air mass since temperature is an indirect way to follow the movement of the fronts. The link to the passage of frontal systems is partly confirmed by a further analysis discriminating wet days from dry days. For each of the two samples, the range for pressure level and time defined below is screened and distinct combinations are finally obtained for dry days and for wet days (i.e.700 hPa at +12h and 925 hPa at +36h, respectively).

A further step in the optimisation consisted in measuring the benefit of introducing several predictors related to Tair with distinct pressure levels and times. All possible combinations with different numbers of pressure levels and times (1 level and 1 time, 1 level and 2 times, 2 levels and 1 time for each level) were investigated, leading to the following: Tair should be taken at 925 hPa at +36h and at 600 hPa at +12h. Finally, the other parameters formerly chosen based on expert knowledge were revised considering this optimal set of Tair predictors:

the optimal number of preselected candidates ( 2900) is finally close to the initial number obtained through the temporal window (i.e. 3000);

performances decrease when the size of the domain increases. The elementary grid domain (only one grid point) initially considered is found optimal;

different numerical criteria for analogy were tested (mean absolute error, correlation coefficient, S1 score) and the best performance in terms of CRPSS is obtained with RMSE.

The gain of method B towards method A over the calibration period and over the validation period arises to +4.0% and +4.5%, respectively, showing that the introduction of temperature in a preliminary analogue selection step (L0-B) leads to better performance than with a moving temporal window.

.

Method C: Accounting for the vertical motion

This section aims at introducing a new predictor which accounts for vertical motions, especially frontal lifting, that may be responsible of well spatially-organised rainfall fields over large areas.

Choice of the predictor

Here several predictors extracted from ERA were tested, among them:

-Equivalent potential temperature (EPT; [START_REF] Rossby | Thermodynamics applied to air mass analysis[END_REF], frequently analysed in weather forecasting, which allows to characterise humidity and temperature gradients of the air mass.

An important gradient of EPT in low layers of the troposphere (usually at 850 hPa pressure level) is observed when a frontal zone crosses one region;

-Horizontal divergence (DIV), which fields enable the forecaster to detect convergence zones and thereafter the presence of a front when DIV is analysed in low troposphere;

-Vertical velocity (VV), which represents the vertical motion that occurs in low troposphere when two different air masses are confronted.

A correlation analysis was performed between the predictors already used in method B and the set of variables (EPT, VV and DIV), for the nearest grid point to the Saône River basin (5°E; 47,5°N).

Results showed a significant relationship between EPT at 850 hPa and geopotential height at 500 hPa, with a correlation coefficient of 0.72, which is partly explained by thermodynamic laws.

Conversely there is no obvious link between variables used in method B and DIV or VV (the coefficients of correlation range from 0 to 0.25). Based on this analysis, the two variables DIV and VV were tested in the AM.

Since the geopotential fields mostly control vertical motions, the selection of analogues with DIV or

VV was introduced after L1-B but with two options (before or after considering the moisture information, i.e. between L1-B and L2-B, or after L2-B) using prescribed parameters for this additional step. The first option provides the better result and scores obtained with VV were better than with DIV (G value maxima of +1.75% for DIV at 925 hPa against +2.24% for VV at 850 hPa, towards method A). This leads to the choice of VV to account for vertical motion processes through a selection step between L1-B and L2-B. This version of the AM is called hereafter method C, where L1-C=L1-B, L2-C is additional selection step, and L3-C=L2-B.

One should note that VV is usually considered as a good predictor for small scale convective systems such as thunderstorms. Indeed, this kind of convection, which is frequently due to topography (mountains), is associated with strong vertical velocities. However, given the rough spatial resolution of the reanalysis, and due to the poor representation of the topography, proposed VV values cannot be strictly interpreted in terms of low level orographic lifting but variability in VV results rather from the frontal lifting due to contrasting air masses.

Optimisation

As for the pre-selection with temperature, it is necessary to determine beforehand some parameters to limit calculation time during the optimisation process. Then the methodology followed for the optimisation of this new selection step is based on assumptions. The parameters that may be easily fixed, for which the impact on performances is assumed to be low, are (i) the number of analogues (set at 50) and (ii) the times of the predictor observation (set at +12h and +24h).

The analogy criterion used for the preliminary testing with VV was the RMSE score. Mean absolute error, correlation coefficient, Teweles-Wobus score were also considered and the associated analogy domain optimised for each criterion. Results showed that gains of about +2% towards method B are obtained with MAE and RMSE.

Secondly, the optimisation of the numbers of analogues retained at L1-C (predictor is geopotential height) and L2-C (predictor is VV) was carried out. At the preliminary step L0-C (predictor is temperature), there is no reason for re-optimising the number of analogues, whereas at the last step the number of 25 analogues was kept to allow comparison with the scores obtained through the method B. Finally, the optimum numbers are 170 for L1-C and 70 for L2-C, based on the additional predictor VV. The related gain G towards method B arises +4.52%, against only +2.24% before reoptimisation of these numbers. This improvement highlights the importance of the re-optimisation when new predictors are introduced.

Lastly, the predictor itself was optimised, that is the choice of the pressure level and times at which

VV should be considered. We followed the same methodology applied for temperature. A first testing was performed by screening all combinations of only one level pressure and one time (Figure 4), and for each combination, the spatial domain was optimised. Positive values of the gain G towards method B are obtained within the standard period of total rainfall, and G is maximal when VV is taken in low troposphere (850 to 700 hPa) at +18h. Thus, the pressure level was fixed to 850 hPa, and all combinations of 1 to 5 observation times among the 10 possible times were successively considered. Again, the analogy domain was optimised consistently. Figure 5a displays optimal combinations of times and Figure 5b gives the related value of gain G towards method B.

Figure 5a shows a symmetry in the repartition of the times. This result highlights that the search for analogy on vertical motions does not give priority to any particular time. Further, the gain towards method B increases with the number of times involved, arising more than +6% when VV is analysed at all times of the rainfall period: VV should be used at every time of the rainfall period. Although the result is not shown here, the optimal analogy domain is identical for all combinations but one. size with the step of analogy (Figure 6). However, a further testing for the choice of the predictor was carried out by discriminating wet days from dry days. Results showed that for wet days, the highest gain (+4% towards method B) is obtained for four times instead of five to put emphasise on wet days forecast. The following parameters of the newly introduced selection step L2-C are therefore:

-RMSE is the analogy criterion;

the predictors are VV values at 850 hPa and at +6h, +12h, +18h, and +24h;

the size of domain stands between previous step and next step (for the Saône catchment, 2.5°E to 7.5°E and 45°N to 47.5°N); -N2 = 70 analogue days are chosen among the N1 =170 ones selected according to the geopotential heights.

Considering these evolutions, the performances were then computed by applying the updated method over the calibration and validation periods. The corresponding gains towards method B over the two periods reach +5.94% and +5.91%, respectively.

Lastly, a slight improvement of the AM was obtained through the re-optimisation of the predictor of the last selection step (L3-C). Indeed, new pressure levels are available in the ERA-40 reanalyses: henceforward, two variables describing the relative humidity are taken both at 925 and 700 hPalevels instead of a single one at 850 hPa-level, and again at the times +12h and +24h. The related gain reaches +2.2% over both the calibration and the validation periods.

The new method C is organised in four steps (Figure 7): a preliminary selection step L0-C (identical to L0-B) through temperature; a second one L1-C (identical to L1-B and L1-A) through geopotential heights, a third one L2-C searching for the analogy on vertical motions, and the last one through humidity L3-C (similar to L2-B and L2-A).

Accuracy of perfect prognosis and real time forecasts

Perfect prognosis forecasts

During the development phase of the algorithm, parameters were optimised in a perfect prognosis mode over the calibration period, and improvements were confirmed over the validation period. In this section, the total length of available record was considered for searching analogues dates, i.e.

from 01/08/1970 to 30/08/2002. That means an extension of 7 years, compared to the previous section (5 years of the validation period defined for section 4, plus the 2 years 1970 and 1971 that were then available in the SAFRAN archive, see section 2.3). Since the numbers of dates selected at each step strongly depend on the length of the available learning period, all numbers were reoptimised leading to N0=3900 instead of 2900; N1=330 instead of 170; N2=90 instead of 70;

whereas N3=25 remains identical.

For each of the three methods, PQPFs were provided over the whole period of 32 years. The CRPSS was computed on the three sub-catchments, in addition to the gain G towards method A, and averaged. The score CRPS was also decomposed into a resolution part and a reliability part (see section 3.1). Following the same methodology, the gains G towards method A were obtained in terms of resolution and reliability (Table 4). The decomposition of the CRPS score shows that most improvements are related to a gain in resolution. Nevertheless, PQPFs provided by methods B and C are also more reliable than those from method A, with positive values of G. It is interesting to note that the gain of skill with method C is rather similar to the one obtained by [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF] when humidity variables (TCW and RH850) were introduced in the AM.

The TSS score was computed with each method for different probability thresholds derived from the predictive cumulative distribution. These values are defined by quantiles used to characterise the distribution: a central value (60% quantile) bounded by two values that form an asymmetric confidence interval (20% and 90% quantiles). Different events defined by precipitation thresholds T were examined: 0 mm (threshold of rain/no rain), and others corresponding to quantiles from the marginal probability distribution of non-zero rainfall amounts.

Figure 8 shows that performances depend on both the intensity of the event (precipitation threshold)

and on the predictive quantile: when the forecast value is defined by the 20% quantile of the predictive cumulative distribution, the highest skill is obtained for dry days, whereas the highest score is obtained for high precipitation thresholds (here the 80% quantile of distribution of non-zero values) when the forecast is defined by the 90% quantile. This latter result highlights the underestimation often observed with an AM; i.e. heavy rain events are anticipated using a high predictive quantile. The shapes of the curves look very similar demonstrating no major discrepancy between method performances. However the method C seems to outperform the two other methods when the forecast is defined by a small to medium predictive quantile. For the highest precipitation thresholds, performances obtained with methods B and C are equivalent. Furthermore, by comparing the score differences between methods, the source of improvement is obviously the introduction of VV, especially for medium to high predictive quantiles.

The previous results naturally lead to the following question: are the gains due to a better ability to detect events? Are they due to a decrease of false alarms? Figure 9 shows POD and FAR scores obtained considering the 60% predictive quantile, function of the precipitation threshold. Values for POD are roughly similar between the three methods, with a slight advantage for method B. By contrast, the graph of FAR scores reveals noticeable differences. A reduction of the FAR score of 5 to 10% for medium to high precipitation thresholds is obtained when vertical velocity is used as predictor, whereas the use of temperature to select candidate situations does not improve the false alarm rate (method B equivalent to method A). In conclusion, in a perfect prognosis mode, the improved skill of the method C is mainly due to a reduced false alarm risk. Given the intermediate performances of method B and the small differences in terms of FAR score, only results of methods A and C are presented thereafter.

Real time forecasts

In the previous sections, the AMs were optimised and evaluated in a perfect prognosis context (predictors are extracted from a reanalysis). During this calibration phase, predictand was defined by the transformed value of precipitation (see section 2.3) and predictors came from re-analysis . In this section, we suggest assessing the performance of the AMs in near real time conditions, i.e. the prediction scheme is kept unchanged, but predictors are now provided by a NWP model embracing meteorological uncertainties due to modelling errors and biases while the predictand is now defined by the raw value of precipitation.

Predictors extracted from forecasts available in the Meteorological Archive Retrieval System of the European Centre for Medium-Range Weather Forecasts (ECMWF) are the inputs to the AMs.

Performances are compared over the 01/10/2001-01/10/2004 period. The choice of the period was partly conditioned by the frequent changes in operational forecasting system (modifications were made in 2001 and later in 2006). A period of three years was considered long enough to reasonably allow an objective evaluation of PQPFs. The initialisation of each forecast run is at 00h UTC, and the maximum lead-time for PQPFs is eight days. An adjustment was made since temperature is not available at 600 hPa-level over the selected period. Different pressure levels close to 600 hPa-level were tested for method C in a perfect prognosis context before retaining the temperature at 700 hPa.

Since raw values of precipitation are used as predictand here, and given the differences between the climatological regimes of the three sub-catchments of the Saône basin, in this section CRPSS scores were computed for each sub-catchment separately. The CRPSS score was evaluated at each leadtime (from the current day D to the day D+7) over the 3-year period with CRPS ref given by forecasts based on climatology. The evolution of the CRPSS scores with the lead-time, obtained for the Doubs sub-catchment with methods A and C, is shown in Figure 10. Note that results are quite similar for the two other sub-catchments. Method C outperforms method A up to the lead-time D+2

and the skill of both methods decreases gradually with the lead-time, becoming close from t=D+3 up to t=D+7.

As noted in the previous section, introducing the air temperature in the preliminary selection step does not lead to substantial improvement for this basin (comparison between results from methods A and B). The higher efficiency obtained for method C for the three first lead-times (from days D to D+2) results mainly from the use of vertical velocity as additional predictor. We may suspect that starting from day D+3 the better efficiency initially shown by method C is neutralised by the inability of the ECMWF model to forecast VV for longer lead-times. The poor performances of AM predictions beyond 6 days may similarly be related to the poor reliability of the geopotential forecasts. Thus, the first step of analogue selection may not be pertinent enough, leading to irrelevant predictions at following steps. Otherwise, whatever the method the CRPSS becomes negative by day D+6, meaning that beyond, the forecast based on climatology brings more information about rainfall than the AMs. Then, with the current NWP models, forecasting precipitation by using the AM beyond six or seven days ahead may not be of interest.

Real time mode versus perfect prog mode

The drop of forecast efficiency with the lead-time is probably due to the increasing inability of the NWP model to forecast the predictors. In order to check if this assumption is correct, forecasts were run over the same period in a context of perfect prognosis, i.e. predictors come from the ECMWF analyses. Figure 11 should note another important result from Figure 11c: even though for the first lead-time the skill of reliability is higher for method C than for method A, method C becomes less reliable than method A from day D+2. Variables VV are thus less robust predictors compared to the others (geopotential heights and humidity): VV forecasts quality is sharply downgraded with the lead-time. However, the discrepancy between the two curves is tiny from day D+4, showing that the errors in other predictor forecasts become predominant.

Conclusions

The aim of this work was the adaptation of an analogue method, initially developed in the 2000s for small to medium-sized mountain catchments, to large river basins mainly influenced by frontal systems at large scale. In this study, we focused on the Saône basin located in eastern France.

Hence several tests were performed in order to improve the selection of analogues over large river basins, by proposing new predictors that are useful for a weather forecaster. Especially, different predictors that are representative of the large-scale lifting process were tested. As outcomes of this exploration, two main successive evolutions of the method were found. In the previous version of the AM [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF], seasonal effects were taken into account by considering only past dates included in the same 4-month temporal windows. This fixed criterion of pre-selection has been replaced by a preliminary selection based on temperature. Yet one should note that this novel evolution is not specific to large river basins. In contrast, the second evolution of the AM enables to characterise and localize large-scale fronts that are the main generator of rainfall over the large river basins in France. Among the predictors tested, the vertical velocity proved the most pertinent. It should be noted that given the climatology of rainfall events and the topography of the zone of interest in this study, local lifting due to orography is negligible and then not taken into account.

Here vertical velocity is only considered as a characteristic of large scale air vertical lifting. This new step of analogue selection was inserted between the two steps of the previous version of the AM.

All parameters of these new analogue selection steps (number of analogues, times and pressure levels of the predictor, spatial domain of analogy, optimization criterion) were optimised over a calibration (learning) period in a perfect prognosis context. Results of the optimisation show a substantial increase of PQPF skill, which was preserved over an independent validation period.

Thus the gain of skill is of the same order of magnitude as the one obtained by [START_REF] Bontron | A probabilistic adaptation of meteorological model outputs d'analogues. Utilisation des réanalyses NCEP-NCAR et application aux précipitations du Sud-Est de la France[END_REF] when humidity variables were introduced as predictors in the AM. This upgraded analogues sorting technique was recently applied to the whole of France [START_REF] Radanovics | Optimising predictor domains for spatially coherent precipitation downscaling[END_REF] through SANDHY "Stepwise ANalogue Downscaling for HYdrology".

An analysis was performed to deal with the study of the AM performances in depth, both in perfect conditions, where predictors are observations, and in real conditions, where predictors come from weather forecasts provided by the ECMWF model. Results show that:

-The main improvement of the overall skill is due to a lower false alarm rate, as the detection of precipitation events is not very different between the updated AM and the previous one.

-In real conditions, the overall increase in skill obtained with the new AM is perceptible only until the lead-time D+2. As a consequence, by D+3 the better efficiency due to the analogy on vertical motions is neutralised by the inability of the ECMWF model to forecast correctly the vertical velocity. Vertical velocity even downgrades PQPFs skill at longer lead-times.

-On the one hand, from a certain lead-time PQPF reliability does not depend only on the quality of VV forecasts, but also on the quality of the predictors used to select the first sample of analogues (geopotential heights). On the other hand, PQPF resolution does not depend on NWP forecasts but only on the algorithm of selection of analogues itself.

The three versions of the AM presented here are associated with different levels of complexity.

Comparing methods allow assessing the relative interest of replacing the calendar criterion with an analogy on temperature (method A versus method B) and of the introduction of a new analogy step based on vertical velocity (method C versus method B). The gain obtained with the introduction of the air temperature is less than the gain obtained with a new step. Finally, method B leads to intermediate prediction skills, between methods A and C. One should note that air temperature is certainly a more universal predictor than vertical velocity (adapted to localize large frontal systems in France) and thus that method B may perform better than method C under other climates. One should also note that method B requires less computational resources than method C, which could be an advantage for an application in an operational context.

In conclusion, there are still several ways to improve the AM: in the context of precipitation forecasting particularly, the AM needs to be adapted to the zone of interest by selecting other predictors closely linked to physical processes that are responsible of precipitation events. This can be done especially as new generations of reanalyses are still emerging, containing new predictors at higher and higher spatial resolution. In the case of French large river basins, large scale vertical motions may be exploited in the AM to improve PQPF skill. However, when a new predictor is introduced in the algorithm, one must pay attention to the capacity and reliability of the NWP models in providing forecasts for this predictor, especially at long lead-time. 2006) Catalonia (Spain) Z850, Z1000 [START_REF] Barrera | Heavy rain prediction using deterministic and probabilistic models -the flash flood cases of 11-13 October 2005 in Catalonia (NE Spain)[END_REF] Catalonia (Spain) Z500, Z850, Z1000, humidity at 1000 hPa Bliefernicht and Bardossy (2007) Western Germany Z700, Z1000, SH700, westerly wind at 700 hPa Bontron and Obled (2005) South-Eastern France Z500, Z10000, TCW, RH850 [START_REF] Horton | Spatial relationship between the atmospheric circulation and the precipitation measured in the western Swiss Alps by means of the analogue method[END_REF] Swiss Alps (Switzerland) Z500, Z1000 [START_REF] Marty | Toward real-time daily PQPF by ananalog sorting approach: Application to flash-flood catchments[END_REF] Southern France Z500, Z1000, TCW, RH850 [START_REF] Mehrotra | Evaluation of downscaled daily rainfall hindcasts over Sydney, Australia using statistical and dynamical downscaling approaches[END_REF] Sydney region (Australia) T700, T850, meridional gradient of Z850, VV500, VV850, Z850 Osca et al. ( 2013 Western France SLP, Z1000, Z500, T850, Z1000-Z500, TCW

-

  A second step (L2-A) is applied in order to search, among the N1 dates, for N2 situations that are analogous to the target one, based on a humidity variable defined by the product of relative humidity at 850 hPa-level (RH850) and the total column of water (TCW), considered both at 12 UTC the day D and 00 UTC the day D+1. The analogy criterion is the root mean square error (RMSE). The spatial domain of analogy is small, reduced to a 2x2 grid points centred on the target catchment, meaning that humidity should be considered at a local scale.Considering the study area and the archives containing predictors and precipitation, the parameters of the two steps of analogue selection (numbers N1 and N2 and the two spatial domains) were optimised over the whole period. N1 and N2 were fitted by testing different couples (N1, N2), where N1 is a multiple of 10 set between 10 and 200 and N2 a multiple of 5 set between 5 and N1. The spatial domains were optimised by increasing them successively in each direction and by retaining at each step the best domain among the four expanded domains. One can note that an extended version of this procedure was proposed by[START_REF] Radanovics | Optimising predictor domains for spatially coherent precipitation downscaling[END_REF]. In this study, the optimised numbers N1 and N2 are 70 and 25 respectively, and the optimised spatial domains are [-5°E to 15°E; 40°N to 50°N] and [5°E to 7.5°E; 45°N to 47.5°N] for geopotential analogy (step L1-A) and moisture related analogy (step L2-A), respectively. Method A has been implemented at the Laboratoire des Transferts en Hydrologie et Environnement in Grenoble since 2004. It was used in real-time by the Observatoire Hydro-météorologique Méditerranéen Cévennes Vivarais (http://www.ohmcv.fr/) to anticipate extreme rainfall events in south-eastern France until January 2015.

  displays scores obtained with methods A and C, both in a perfect prognosis context (PERF) and in real conditions (OPER), for the Doubs sub-catchment, function of the leadtime, with: a) the CRPSS scores, b) the resolution and c) the reliability parts of the CRPS score. The accuracy of the weather forecasts is quite high for the first lead-time D since for each version of the AM the curves OPER and PERF merge. Starting from the day ahead (D+1), according to the graphs a) (CRPSS) and c) (skill of reliability) the accuracy of operational forecasts decreases for each AM version, while the graph b) (skill of resolution) shows no strong tendency with the leadtime. Resolution of the PQPFs provided by the two versions of the AM is not affected by the accuracy of the NWP forecasts (Figure 11 b). Consequently, this result indicates that the resolution of the PQPFs does depend mainly on the algorithm of selection of analogues. The comparison of the curves of methods A and C shows that PQPFs from the method C have a much better resolution than those from the method A. Then the predictor VV tends to reduce forecast uncertainties. One

  , Austria) Geostrophic wind (direction and velocity) at 1000 and 500 hPa[START_REF] Teng | Assessment of an analogue downscaling method for modelling climate change impacts on runoff[END_REF] South-Eastern Australia SLP, PRCP, SH850, Surface max temperature Tmax, T850, zonal and meridional wind components at 850 hPaThemeβl et al. (2011) Austria 27 variables[START_REF] Timbal | An estimate of climate change for Western France using a statistical downscaling technique[END_REF]

Figure 1 .Figure 2 .

 12 Figure 1. Location of the Saône river basin divided into three sub-catchments.

Figure 3 .

 3 Figure 3. Gain of performance (G in %) expressed towards method A, function of the time and the pressure level of the temperature analysis. Vertical dashed lines represent the standard period of the total rainfall.

Figure 4 .

 4 Figure 4. Gain of performance (G in %) expressed towards method B, function of the time and the pressure level of the vertical velocity analysis. Vertical dashed lines represent the standard period of the total rainfall.

Figure 5 .

 5 Figure 5. a) Combination of times and the number of times and b) gain of performance G (in %) function of the number of times.

Figure 6 .

 6 Figure 6. Schema showing the fitting of the analogy domains successively applied at step 1 (L1-C -identical to L1-B), new step 2 (L2-C -step with VV), and step 3 (L3-C -identical to L2-B). Crosses represent the mesh centres (grid points).

Figure 7 .

 7 Figure 7. Schema of the methods of analogues searching.

Figure 8 .

 8 Figure 8. TSS evolution, according to precipitation thresholds defined by percentiles of the climatological distribution of non-zero, obtained in perfect prognosis with methods A, B and C. Forecast value is represented by a) the 20% quantile, b) the 60% quantile and c) the 90% quantile of the predictive distribution.

Figure 9 .

 9 Figure 9. POD (left) and FAR (right) scores evolution,according to different precipitation thresholds defined by percentiles of the climatological distribution of non-zero, obtained in perfect prognosis with methods A, B and C. Forecast value is represented by the 60% quantile of the predictive distribution.

Figure 10 .Figure 11 .

 1011 Figure 10. CRPSS score as function of the lead-time, obtained in real conditions (operational forecasts) with methods A and C for the Doubs sub-catchment. The reference forecast is the climatological areal rainfall distribution

  This domain, formed by 3 × 2 grid points, is centred on the Saône catchment. Besides, it isAuthor-produced version of the article published in Atmospheric Research, 169, Part A, 1, 147-159, The original publication is available at http://www.sciencedirect.com/ doi:10.1016/j.atmosres.2015.09.015

interesting to note that this domain sets between the domains applied for L1-C and L3-C, indicating that the way this intermediate selection is introduced is consistent with a regular decrease of domain

Table 1 .

 1 Predicators and study areas for a selection of AM applications (TCW: total column of water; RH: relative humidity, SLP: Sea Level Pressure; Z: geopotential height; T: temperature, SH: specific humidity; PRCP: Total precipitation; VV: vertical velocity; additional numbers indicate the related atmospheric level in hPa)

	As verified in the past,

http://dx.doi.org/10.1051/lhb:200501002, 2005. Bontron, G.: Prévision quantitative des precipitations: adaptation probabiliste par recherche

Table 2 .

 2 Characteristics of the Saône basin pluviometric regime (frequency F0 of 0 mm days (%), daily rainfall amounts (mm) with a return period of 5 (PJXA5) and 10 years (PJXA10), maximum amount (mm) (Pmax) observed during the 39 year period(Aug. 1970 -July 2008).

			SLP, PRCP, SH850, RH850, RH700, Surface max temperature
	Timbal et al. (2008)	Australia	Tmax, T850, zonal wind components at 850 hPa, meridional wind
			components at 700 hPa and 850 hPa
	Turco et al. (2011)	Spain	PRCP
	Wetterhall et al. (2005)	South-central Sweden	SLP
	Wu et al. (2012)	Southeastern Mediterranean	SLP;H700; T700;U850; V850
		Spain, Portugal, Southern
	Zorita and von Storch (1999)	France, Northern Morocco, Northern	SLP
		Algeria

Table 3 .

 3 Contingency table for a precipitation threshold T (N CA = number of correct alarms; N FA = number of false alarms; N MA = number of missed alarms; and N CR = number of correct rejections).

				Observation ( o P )
				P o >	T	P o ≤	T
		P f >	T	N CA	N FA
	Forecast ( f P )	P f ≤	T	N MA	N CR

Table 4 .

 4 Gains G (%) of performance towards method A obtained by applying method B and method C (total gain and gain decomposed into resolution and reliability).

		Total	Reliability	Resolution
	Method B	+3.7%	+2.7%	+4.7%
	Method C	+12.3%	+7.8%	+17.0%

Author-produced version of the article published in Atmospheric Research, 169, Part A, 1, 147-159, The original publication is available at http://www.sciencedirect.com/ doi:10.1016/j.atmosres.2015.09.015

Acknowledgements

The financial support provided by CNR and Cemagref-Irstea for the PhD research of A. Ben Daoud is gratefully acknowledged. The NCEP/NCAR and ECMWF ERA-40 re-analysis data were obtained from the NOAA web site (http://www.cdc.noaa.gov/) and from the ECMWF data server (http://data.ecmwf.int/data/d/era40_daily/), respectively. The authors acknowledge Météo-France for providing the ECMWF forecasts and the Safran data set for research purposes. The authors acknowledge the anonymous reviewers for their very helpful comments.

AM performances for precipitation forecasts will probably increase in parallel with the improvement of NWP models to more distant lead-times.