X-Band Polarimetric & Doppler radar observations of heavy precipitation events over the Mediterranean region (France).

Numerous heavy precipitations take place over the western Mediterranean Sea especially during summer and in some case lead to severe floods over the continent (Vaison la Romaine in 1992, Draguignan the 15-16 June 2010, Antibes 30 October-1 November 2010...). Since the Mediterranean is surrounded by mountain barriers, and strong contrasts between sea and continent exist, local forcing is important. The present work aims at examining the environmental conditions and precipitation structures of storms observed in these regions and at improving our knowledge about the life-cycle of the more intense events and processes that govern their evolution. It uses observations gathered by the X band polarimetric & Doppler weather Radar HYDRIX. This radar, owned by the LATMOS/CNRS, developed and operated by NOVIMET, collects in an operational way since 2008 reflectivity, Doppler and polarimetric data with a time step and a spatial resolution of 5 minutes and 1 km respectively. In addition to these data, derived products such as instantaneous rainfall, cumulated rain, cell displacement and hydrometeor classification are obtained in real time. It also provides Doppler information in clear air region. All these data can be used to document the low level environment and the internal dynamics, microphysics and the interaction-mechanisms (wind shear, ...) leading to high-accumulated surface rainfall. The presentation focus on the first step of this work devoted to the temporal and spatial characterisation of the precipitation that affects the Provencal and Ligurian region. A statistical and a spectral analysis are performed to construct a climatology and to document the life cycle and the dynamical-microphysical characteristics of a few representative high-impact events

Heavy rainfall in Mediterranean cyclones. Part I: contribution of deep convection and warm conveyor belt

In this study, we provide an insight to the role of deep convection (DC) and the warm conveyor belt (WCB) as leading processes to Mediterranean cyclones’ heavy rainfall. To this end, we use reanalysis da-ta, lighting and satellite observations to quantify the relative contribution of DC and the WCB to cyclone rainfall, as well as to analyse the spatial and temporal variability of these processes with respect to the cy-clone centre and life cycle. Results for the period 2005-2015 show that the relationship between cyclone rainfall and intensity has high variability and demonstrate that even intense cyclones may produce low rainfall amounts. However, when considering rainfall averages for cyclone intensity bins, a linear relationship was found. We focus on the 500 most intense tracked cyclones (responsible for about 40-50% of the total 11-year Mediterrane-an rainfall) and distinguish between the ones producing high and low rainfall amounts. DC and the WCB are found to be the main cause of rainfall for the former (producing up to 70% of cyclone rainfall), while, for the latter, DC and the WCB play a secondary role (producing up to 50% of rainfall). Further analysis showed that rainfall due to DC tends to occur close to the cyclones’ centre and to their eastern sides, while the WCBs tend to produce rainfall towards the northeast. In fact, about 30% of rainfall produced by DC overlaps with rainfall produced by WCBs but this represents only about 8% of rainfall produced by WCBs. This suggests that a considerable percentage of DC is associated with embedded convection in WCBs. Finally, DC was found to be able to produce higher rain rates than WCBs, exceeding 50 mm in 3-hourly accumulated rainfall compared to a maximum of the order of 40 mm for WCBs. Our results demonstrate in a climatological framework the relationship between cyclone intensity and pro-cesses that lead to heavy rainfall, one of the most prominent environmental risks in the Mediterranean. Therefore, we set perspectives for a deeper analysis of the favourable atmospheric conditions that yield high impact weather

CloudSat-based assessment of GPM Microwave Imager snowfall observation capabilities

The sensitivity of Global Precipitation Measurement (GPM) Microwave Imager (GMI) high-frequency channels to snowfall at higher latitudes (around 60◦N/S) is investigated using coincident CloudSat observations. The 166 GHz channel is highlighted throughout the study due to its ice scattering sensitivity and polarization information. The analysis of three case studies evidences the important combined role of total precipitable water (TPW), supercooled cloud water,and background surface composition on the brightness temperature (TB) behavior for different snow-producing clouds. A regression tree statistical analysis applied to the entire GMI-CloudSat snowfall dataset indicates which variables influence the 166 GHz polarization difference (166∆TB)and its relation to snowfall. Critical thresholds of various parameters (sea ice concentration (SIC), TPW, ice water path (IWP)) are established for optimal snowfall detection capabilities. The 166∆TB can identify snowfall events over land and sea when critical thresholds are exceeded (TPW \u3e 3.6 kg·m−2, IWP \u3e 0.24 kg·m−2 over land, and SIC \u3e 57%, TPW \u3e 5.1 kg·m−2 over sea). The complex combined 166∆TB-TB relationship at higher latitudes and the impact of supercooled water vertical distribution are also investigated. The findings presented in this study can be exploited to improve passive microwave snowfall detection algorithms

SLALOM: An all-surface snow water path retrieval algorithm for the GPM microwave imager

This paper describes a new algorithm that is able to detect snowfall and retrieve the associated snow water path (SWP), for any surface type, using the Global Precipitation Measurement (GPM) Microwave Imager (GMI). The algorithm is tuned and evaluated against coincident observations of the Cloud Profiling Radar (CPR) onboard CloudSat. It is composed of three modules for (i) snowfall detection, (ii) supercooled droplet detection and (iii) SWP retrieval. This algorithm takes into account environmental conditions to retrieve SWP and does not rely on any surface classification scheme. The snowfall detection module is able to detect 83% of snowfall events including light SWP (down to 1 × 10−3 kg·m−2) with a false alarm ratio of 0.12. The supercooled detection module detects 97% of events, with a false alarm ratio of 0.05. The SWP estimates show a relative bias of −11%, a correlation of 0.84 and a root mean square error of 0.04 kg·m−2. Several applications of the algorithm are highlighted: Three case studies of snowfall events are investigated, and a 2-year high resolution 70°S–70°N snowfall occurrence distribution is presented. These results illustrate the high potential of this algorithm for snowfall detection and SWP retrieval using GMI

Caractérisation multi-échelles de la pluie et des processus associés dans l'Eurorégion Alpes-Méditerranée - De l'observation radar à la prévision

The aim of this thesis is to improve the understanding and forecasting of rainfall small scalevariability in the Alps-Mediterranean Euroregion. In this purpose, we used measurementsfrom an X-band, polarimetric and Doppler radar called Hydrix. This radar is located since2007 at Mont Vial (Nice, France) and continuously measures rainfall since 2009, with a 1 km 2spatial resolution and a 5 minutes temporal resolution. This database provides usefulinformation to study fine scale space-time variability of rainfall. First, the diurnal andseasonal cycles, as well as rainfall inter-annual variability, are analysed. Then, rainfall scalerelationships are spatially and temporally characterised and the processes associated to rainfallat both synoptical and local scales in the region are identified. Finally, a new nowcastingmethod suitable for a single Doppler radar is evaluated.L’ objectif de cette thèse est d’améliorer notre compréhension et la prévision desprécipitations dans l’Eurorégion Alpes-Méditerranée, une région aux caractéristiquestopographiques particulièrement complexes. Pour cela, nous avons utilisé les données d’unradar bande X, polarimétrique et Doppler appelé Hydrix. Ce radar est situé depuis 2007 sur leMont Vial dans la région des Alpes-Maritimes. Il mesure en continu les précipitations au-dessus de cette région avec une résolution spatiale de 1 km 2 et temporelle de 5 minutes. Grâceà la base de données constituée, nous avons pu étudier la variabilité spatio-temporelle à fineéchelle des précipitations. Nous avons notamment analysé les cycles diurnes et saisonniersainsi que la variabilité inter-annuelle des précipitations. La suite de ce travail a été consacré àl’identification des propriétés d’échelle des précipitations spatialement et temporellement.Ensuite, les processus responsables de la variabilité de la pluie dans la région à l’échellesynoptique ainsi qu’à l’échelle locale ont été identifiés. Pour finir la faisabilité d’une méthodede prévision du cycle de vie des précipitations à court-terme utilisant un radar Doppler a étéétudiée

Spatial and temporal variability of rainfall in the Alps-Mediterranean Euroregion

International audienceThis study describes the main patterns of rainfall distribution in the Alps-Mediterranean Euroregion using a ground radar and characterizes the associated processes using model outputs. The radar dataset spans the period 2009-2012 with fine spatial (1 km) and temporal (5 min) resolutions.The most significant rain accumulations were observed in 2009 and 2010, while the most intense extreme events occurred in 2010. Conversely, 2012 was a rather dry year. Model outputs revealed that the wind shear, the pressure and the meridional wind at low level were the three main factors explaining the rainfall variability between 2009 and 2012.At the monthly scale, the maximum of rain accumulation was observed in November along coast. Results also showed that the most intense rain rates were observed during early summer and fall in the Pre-Alps. The monthly variability was characterized by a displacement of extreme rain events from land to sea from late spring to winter. Correlation analyses showed that this displacement was essentially controlled by the CAPE (Convective Available Potential Energy).Rainfall showed a diurnal variability from April to August for land areas of the Alps-Mediterranean Euroregion. The diurnal variability was significant during the spring and summer months with maximal rain intensity between 16:00 and 18:00 UTC. The correlation of the rainfall with CAPE showed this cycle was related to atmospheric instability. A secondary peak of average rain rate was observed during the early morning and was likely triggered by land breezes.The results highlighted that rainfall characteristics are extremely diverse in terms of intensity and distribution in this relatively small region

Large and local scale environment associated to precipitation in the Alps-Mediterranean Euroregion

International audienc

X-Band polarimetric and Doppler radar observations of heavy precipitation events over the Mediterranean region

Numerous heavy precipitations take place over the western Mediterranean Sea especially during summer and in some case lead to severe floods over the continent (Vaison la Romaine 1992, Draguignan 15-16 June 2010, ...). Since the Mediterranean is surrounded by mountain barriers and strong contrasts between sea and continent exist, local forcing are important. This study aims to examine the environmental conditions and precipitation structures of storms in order to improve our knowledge about the life-cycle of intense events and processes that govern their evolution. It uses observations gathered by the X band polarimetric & Doppler weather Radar HYDRIX. This radar, developed by the LATMOS and operated by NOVIMET from the Mont Vial (mountain peak near Nice (France)), collects in an operational way since 2008 reflectivity, Doppler and polarimetric data with a time step and a spatial resolution of 5 minutes and 1 km respectively. In addition to these data, derived products such as instantaneous rainfall, cumulated rain, cell displacement and hydrometeor classification are obtained in real time. It also provides Doppler information in clear air region. All these data can be used to document the low level environment and the internal dynamics, microphysics and the interaction-mechanisms (wind shear, ...) leading to high-accumulated surface rainfall. The presentation focus on the first step of this work devoted to the temporal and spatial characterisation of the precipitation that affect the Provencal/Ligurian region. This is realized by means of a statistical analysis of these data to construct a climatology and to document the life cycle and the dynamical-microphysical characteristics of a few representative high-impact events

X band polarimetric and Doppler radar observations of precipitation variability over the mediterranean region (France)

The Mediterranean region is known to be characterized by a high spatial and temporal variability of precipitations. Precipitations fluctuations in time and space are due to a large spectrum of processes from climate dynamics to droplet formation. Because of its latitude (between 36° and 44° N) the Mediterranean region is affected by sub-tropical storm as well as mid-latitude synoptic disturbances. Hence the summer is warm and dry while being persistently affected by the Azores anticyclone and the winter is temperate and moderately humid because of the mid-latitude westerlies. Moreover since the Mediterranean is surrounded by mountain barriers, and strong contrasts between sea and continent exist, local forcing is also important. Our work aims at examining the environmental conditions and precipitation structures of storms observed in these regions and at improving our knowledge about the life-cycle of the more intense events and processes that govern their evolution. It uses observations gathered by the X band polarimetric & Doppler weather Radar HYDRIX. This radar, owned by the LATMOS/CNRS, developed and operated by NOVIMET, collects in an operational way since 2008 reflectivity, Doppler and polarimetric data with a time step and a spatial resolution of 5 minutes and 1 km respectively. In addition to these data, derived products such as instantaneous rainfall, cumulated rain, cell displacement and hydrometeor classification are obtained in real time. It also provides Doppler information in clear air region. All these data can be used to document the low level environment and the internal dynamics, microphysics and the interaction-mechanisms (wind shear, ...) leading to high-accumulated surface rainfall. The presentation will be devoted to the temporal and spatial characterisation of the precipitation that affects the 100 km radius region around Nice in a range from annual to daily scale and to the analysis of the universal scaling behavior of the rain. To document those two topics, we analysed the data with the help of various tools such as rain frequency and mean, cumulative distribution function, fourier and wavelet transform