ERAD 2012 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY

Gap-filling, X-band radars as part of the RHYTMME program; an opportunity to retrieve

real-time, multiple-Doppler wind fields in previously inaccessible regions of southeast

France

Jeffrey Beck1 and Olivier Bousquet1 1Météo-France/ Centre National de Recherches Météorologiques, 42 Avenue de Coriolis,

Toulouse, France, 31052 Cedex, jeff.beck@meteo.fr, olivier.bousquet@meteo.fr (Dated: 1 June 2012)

1. Introduction

France’s national ARAMIS radar network contains a total of 24 radars (eight S-band and 16 C-band, Figure 1), many of which have dual-polarization technology. Together, this network covers over 90% of the country, when considering all elevations, making it one of the most dense national radar networks in the world.

Fig. 1. Theoretical current radar coverage for France showing all radars including C-, S- and X-band radars. Single and dual-polarization radars are also shown in green and purple, respectively. However, regions still exist where radar coverage is lacking (Fig. 2a), particularly in the complex terrain of southeast France, where the risk of high-precipitation events and potential flash-flooding warrant the need for an expanded network of radars. Therefore, the Risques HYdrométéorologiques en Territoires de Montagnes et MEditerranéens (RHYTMME) project was

a) b) c)

Fig. 2a,b,c. Theoretical radar coverage (in green) for southeast France showing regions well and poorly covered by the current ARAMIS radar network (a) and then with the addition of the Mt. Vial and Mt. Maurel RHYTMME radars (b), and future coverage with all RHYTMME radars (c). Note that these maps do not take potential masking effects into account (e.g., beam blockage by terrain and ground clutter).

ERAD 2012 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY

developed to mitigate coverage gaps in the French ARAMIS radar network, and to improve the ability to forecast and study localized, high-risk precipitation events (RHYTMME, 2012). At present, two RHYTMME radars are currently operational: Mt. Vial and Mt. Maurel, providing coverage for the maritime French Alps (Fig. 2b). Eventually, a total of four RHYTMME, X-band, dual-polarization radars will be deployed in the Alpine region of southeast France (Fig. 2c) to assess the improvement in radar coverage and products this new network can provide. Mt. Colombis will be the third radar to be installed (in 2012) and the final radar, Vars Mayt, in 2013. In the future, the possibility also exists to expand the network to the Northern Alps towards Grenoble, France (2014) as well as to other mountainous regions such as the Pyrenees, where other gaps in radar coverage can also be mitigated.

2. Methodology

The radars in the ARAMIS network collect radial velocity, reflectivity, and (depending on the radar) a suite of dual-polarimetric moments every fifteen minutes at elevations ranging from 0.5 to 60 degrees. A triple-PRT scheme is used (Tabary et al., 2007) allowing for an unambiguous Nyquist velocity of 60 ms-1. The numerous dual-polarimetric moments are collected and used in a fuzzy logic algorithm (Gourley et al., 2006) to separate non-precipitating from precipitating echoes. This information is then used to filter the radar data of ground clutter, second trip, and other anomalous returns, in order to prepare the data for multiple-Doppler retrieval. The reader is referred to Bousquet et al. (2008a) for more details about ARAMIS radar data pre-processing. The processing algorithm used by the ARAMIS radar network was adapted to the RHYTMME, X-band radars, and successfully implemented to allow for automated editing and filtering of erroneous and other anomalous radar data. However, noise and other errors are produced by the dual-PRF scheme used by Mt. Vial and the triple-PRT scheme used by Mt. Maurel, in part due to small ratios between PRF values (e.g., Jorgensen et al. 2000), and require a median filter and further editing to ensure that data are error free. Application of median filters to remove erroneous radial velocity data from radars using dual-PRF and triple-PRT schemes has been successfully implemented in the past (Tabary et al., 2006; Bousquet el al., 2008a; Montmerle and Faccani, 2009). Finally, The process of data interpolation and wind synthesis is carried out using the MUSCAT software suite (Bousquet and Chong 1998). Prior to any multiple-Doppler retrievals, theoretical coverage maps were generated to show the potential multiple-Doppler retrieval region with and without Mt. Vial and Mt. Maurel at specific vertical levels. Beam blockage caused by topography was ignored for this exercise; however this does not impact the results of the coverage over the majority of the domain. In addition, Mt. Maurel and Mt. Vial are located above 1500 m ASL, and do not scan at negative elevations, therefore their impact is not seen at low levels. At 2.5 km ASL, the theoretical coverage for the three ARAMIS radars is shown (Fig. 3a).

a) b) c)

Fig. 3a,b,c. Theoretical single- and multiple-Doppler coverage maps for southeast France, with the number of radars covering an area depicted by the color. Coverage is shown with only ARAMIS radars (a), while on the right, coverage is shown with the addition of Mt. Vial (b) and with both Mt. Vial and Mt. Maurel (c). Dual- and multiple-Doppler coverage is already extensive over a large portion of the domain. However, with the addition of Mt. Vial (Fig.3b) and Mt. Maurel (Fig. 3c), the coverage at 2.5 km ASL is dramatically expanded, with dual- and multiple-Doppler coverage extending well into the Mediterranean Sea, and over-determined Doppler coverage increasing over much of the region. Two case studies were chosen to assess the impact of this expanded multiple-Doppler domain. The first case occurred prior to data availability from the Mt. Maurel radar, allowing for an independent assessment of the addition of only Mt. Vial to the multiple-Doppler syntheses. The event occurred between the evening of 24/10/2011 and the early morning hours of 25/10/2011, when a strong synoptic system affected southeast France. Precipitation amounts of up to 150 mm occurred during this event along the Mediterranean coast near both Marseille and Nice as well as near the Mt. Vial radar. The second precipitation event chosen for analysis occurred on 31/1/2012 and provided the opportunity to assess the impact of Mt. Maurel in addition to Mt. Vial on the multiple-Doppler syntheses. A convectively weaker precipitation event

ERAD 2012 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY

than the first case was produced due to a low pressure system situated in southeast, where up to 30 mm of rain fell. However, the breadth of the precipitation allowed for the retrieval of multiple-Doppler winds over a large region.

3. Analysis

Initially, the synoptic system of 25/10/2011 was primarily affecting the Rhone river valley at 0130 UTC and strong reflectivity had not yet reached the extreme southeastern portions of France. Looking at an example elevation of 3 km ASL, even at 0130 UTC, coverage in previously inaccessible regions by the ARAMIS radars (Fig. 4a) is improved by the addition of Mt. Vial (Fig. 4b) adding reflectivity and some two-dimensional wind vectors.

a) b)

Fig. 4a,b. Multiple-Doppler syntheses for 25/10/2011 at 0130 UTC for ARAMIS radars only (a), and for ARAMIS radars plus Mt. Vial (b), showing black 2-D wind vectors (every 10th vector is shown), reflectivity in color (dBZ), and terrain contours in black. As the synoptic system progressed eastward, by 0400 UTC, Mt. Vial presents a clear advantage to the multiple-Doppler syntheses. The ARAMIS radars are still incapable of retrieving reflectivity in southeast France (Fig. 5a), and the network is insufficient to allow the recuperation of multiple-Doppler wind vectors. However, Mt. Vial provides additional reflectivity at 3 km ASL over a broad region, and provides the ability to retrieve 2-D wind vectors over southeast France, as well as over the Mediterranean Sea (Fig. 5b).

a) b)

Fig. 5a,b. Multiple-Doppler syntheses for 25/10/2011 at 0400 UTC for ARAMIS radars only at 3 km ASL (a), and for ARAMIS radars plus Mt. Vial (b), showing black 2-D wind vectors (every 10th vector is shown), reflectivity in color (dBZ), and terrain contours in black. Vertical black line in Figure 5b indicates location of cross-section in Figure 6.

ERAD 2012 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY

The greatly varying topography of the region significantly impacts synoptic events approaching from the west. In Figure 6, a vertical cross section from Fig. 5b at x = 175 km shows the change in terrain from sea-level into the Alps, and its effect on vertical velocity during the event of 25/10/22 at 0400 UTC. a)

b)

Fig. 6a,b. ARAMIS and Mt. Vial Multiple-Doppler synthesis (y,z vectors) for 25/10/2011 at 0400 UTC for x=175 for all vertical levels (every 5th vector shown in the horizontal), (a) vertical velocity shown in color (ms-1) and topography as a black solid line, (b) same as (a) but with reflectivity shown in color (dBZ). The location of the cross section can be found in Figure. 5b as a vertical black line. A correlation can be seen between the changes in terrain and the impact on vertical velocity though the depth of the column within the multiple-Doppler retrieval. Gaps in the ARAMIS network result in missing radar coverage in the lowest couple kilometers above ground level over the mountainous region of southeast France, showing the importance of these gap-filling radars.

During the precipitation event of 31/1/2012, Mt. Maurel radar data were added to the multiple-Doppler syntheses to retrieve the two-dimensional wind field as before. Figure 7a shows the retrieved wind field for only ARAMIS radars at 1700 UTC for 2.5 km ASL, while Figure 7b shows the benefit that both Mt. Maurel and Mt. Vial data provide to the multiple-Doppler syntheses at the same time. It is clear that with five radars in total, a definitive advantage in the recuperation of the wind field and respective reflectivity exists. Specifically, offshore wind vectors are again retrieved as before from Mt. Vial data, while the addition of Mt. Maurel provides wind field data previously inaccessible north and northwest of Mt. Vial, of which neither area is accessible using solely the ARAMIS radar network. 4. Conclusions

Results are presented from multiple-Doppler syntheses from 25/10/2011 and 31/1/2012 as two synoptic systems

traversed southeast France. Syntheses are produced using the ARAMIS network radars alone, and then including the newly-installed RHYTMME-based Mt. Vial radar for the 25/10/2011 case, and both Mt. Maurel and Mt. Vial data for the 31/1/2012 case. Coverage is shown to be limited in the mountainous region of southeast France using only the ARAMIS radars. In addition, it is shown that multiple-Doppler retrieval of three-dimensional winds over the Mediterranean (in addition to low-levels over the mountainous terrain) is impossible without the aid of the gap-filling, Mt. Vial and Mt. Maurel

ERAD 2012 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY

a) b)

Fig. 7a,b. Multiple-Doppler syntheses for 31/1/2012 at 1700 UTC for ARAMIS radars only at 2.5 km ASL (a), and for ARAMIS radars plus Mt. Vial (b), showing black 2-D wind vectors (every 10th vector is shown), reflectivity in color (dBZ), and terrain contours in black. radars. The region covered by the new Mt. Vial and Mt. Maurel radars is extremely susceptible to dangerous flash-flooding events, and the RHYTMME project, comprising the installation of four new X-band radars within the Alps of southeast France by 2013, intends to help forecast and better understand the evolution of these events. The ability to accurately predict QPE and to retrieve three-dimensional wind fields, in particular upstream of complex orography within this region are key to achieving this goal. Additional X-band RHYTMME radars will continue to expand the radar coverage over southeast France and improve the accuracy and ability to study extreme weather events. Retrieval of three-dimensional wind data offshore and adjacent to mountainous terrain, including the Mediterranean Sea, has been identified as a key component in the evaluation and improvement of numerical weather prediction for orographically induced extreme rainfall events. Therefore, the use of these data in numerical simulations represents the next step in this research. Work to assess the accuracy of the non-hydrostatic, mesoscale, AROME-WMED model will be conducted through statistical analysis of wind speed and direction at multiple levels. Retrieval of three-dimensional wind data over the Mediterranean Sea In addition, real-time, multiple-Doppler analyses will be available during the upcoming HyMeX field campaign (HyMEx, 2012).

Acknowledgments The authors wish to thank the European Union, the Provence-Alpes-Côte d’Azur Region and the French Ministry of Ecology, Energy, Sustainable Development and Sea for co-financing the RHYTMME project. Thanks are due to Pierre Tabary, Fadela Kabeche, Hassan Al-Sakka, Abdel-Amin Boumahmoud, Clotilde Augros, and Beatrice Fradon for their help with data acquisition and processing during this research.

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ERAD 2012 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY

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