WAter vapor LIdar NEtwork ASsimilation – WaLiNeAs

To fill the observational gap imparing more accurate HPE predictions in southern France, we aim at implementing an integrated prediction tool, coupling network measurements of WV profiles, and a numerical weather prediction model to precisely estimate the amount, timing, and location of rainfall associated with HPEs in southern France. The Water vapor Lidar Network Assimilation (WaLiNeAs) project will deploy a network of 6 autonomous Raman WV lidars around the Western Mediterranean to provide measurements with high vertical resolution and accuracy to be assimilated in the French Application of Research to Operations at Mesoscale (AROME-France) model, using a four-dimensional ensemble-variational approach with 15-min updates. This integrated prediction tool is expected to enhance the model capability for kilometer-scale prediction of HPEs over southern France up to 48 h in advance. The field campaign is scheduled to start early September 2022, to cover the period most propitious to heavy precipitation events in southern France. The Raman WV lidar network will be operated by a consortium of French, German, Italian, and Spanish research groups. This project will lead to recommendations on the lidar data processing for future operational exploitation in NWP systems.

The proposed WV lidar network has been designed to account for all relevant WV sources and transport patterns known to contribute to the generation of HPEs in southern France. This network will aim at demonstrating the benefit of the assimilation of vertically resolved WV data in the operational 1.3-km grid size AROME-France NWP system which enables ensemble-variational data assimilation for kilometer-scale prediction of heavy precipitation over southeastern France. The ensemble variational data assimilation system available since 2022 will produce hourly analyses and will be operated with a rapid update cycle of assimilation of new observations
at least each 15 min.

The WaLiNeAs project is a unique, innovative initiative that will allow for assimilating lidar-derived WV profiles in the lower troposphere with hectometer-scale vertical resolution in near real-time conditions for a continuous period of at least 3 consecutive months. The benefit of WaLiNeAs to the academic and operational communities is dual: advance knowledge of the processes at play in the life cycle of HPEs and enhance the predictability of HPEs in southern France at scales relevant for meteorological studies. Both aspects are dealt with in the framework of WaLiNeAs.

For near-real-time assimilation purpose, the 6 Raman lidars will deliver 15 min averaged profiles with vertical resolution of 100 m and a targeted root-mean-square error of 0.4 g kg-1 in the first 2-3 km, day and night. In addition, the statistical uncertainties of these profiles as well as the atmospheric variance will be provided. The WV profile acquisition, processing, and transmission sequence is anticipated to span over 25 min.

The WaLiNeAs camapign will be conducted in the fall of 2022. Nevertheless, we provide below de description of what as been done to date (end August 2022).

1- Positioning of the stations of the water vapor lidar network in the western Mediterranean:
Discussions within the consortium led to the repositioning of some of the lidar deployment sites compared to what was initially planned in the proposal, and a modification compared to the description of the network in the publication of Flamant et al. (2021). In particular, the modifications were proposed based on a recent analysis done by NRCM on 2 HPEs from 2021.

2- Development of the two LSCE Raman Water Vapor lidars:
The two lidars, HORUS-1 and HORUS-2 have been integrated at the LSCE at the beginning of July 2022, and are being tested. Jérémy Lagarrigue, SupOptician and IE recruited at LSCE for 12 months in 2021-2022 and financed by the project, is actively involved in the development of the two lidar systems. He will also participate in the implementation of the LSCE lidars (HORUS-1, HORUS-2 and WALI) of the WaLiNeAs network in the initial phase of the field campaign in the fall of 2022.

3- Harmonization of water vapor data files for all WaLiNeAs lidars:
Following the writing of the Data Management Plan, the consortium agreed to define a common format for writing water vapor mixing ratio profile data from each of the 6 lidar systems participating in WaLiNeAs. This single format NetCDF file will be produced 4 times per hour and made available to Météo-France via a dedicated ftp site set up at Météo-France. It is essential to have valid observations every 15 minutes to be assimilated in the 4DEnvar Arome as it is the case for other observations such as radars, SEVIRI on MSG, ground GPS or surface stations. A sample file will be made available to the CNRM team before the beginning of the campaign, at the time of recruitment of the post-doc who will work on the assimilation of lidar observations in Arome-France.

4- Near-real time lidar data montoring chain
The candidate recruited on June 1st and financed by the project, Guillaume Thomas, is already trained to start working on the subject. The post-doc will have to implement the real-time data monitoring chain, including a step to transform NetCDF files into data usable by the AROME tools. He has already built a prototype of such a chain which is already running in real time and which currently uses LIDAR pseudo-observations (made by perturbing the AROME draft). This chain includes a correction taking into account the attenuation of the lidar beam by gases and aerosols and goes as far as providing daily graphs of the monitoring statistics.

The progresses in the utilization of water vapor lidar data in near-real time assimilation in NWP models will enhance the forecasters capability anticipate HPEs in kilometric-scale NWPS outputs. Results of the project will be presented to the forecasters of the met services involved in HyMeX in France, Italy and Spain, as well as to the students of Meteorology School in Toulouse (ENM) and the participants in the MedLab initiative. Results will also be presented to the students of the Master2 “Ocean-Atmosphere et Surfaces Continentales” (Univ. P. Sabatier, Toulouse) and of the Master1 «Atmosphere Ocean Continent« (Univ. P. Sabatier, Toulouse) and to the Master “Ocean, Atmosphere, Climat et Observations Spatiales” (Sorbonne Université). The videos that will be made from numerical simulations will be an excellent teaching tool. In addition, several master theses will be proposed to these students during the course of the project in the different domains of the project.

This project will contribute significantly to the scientific objectives of CES04 « Innovations scientifiques et technologiques pour accompagner la transition écologique » through the development of all-weather, unattended, rugged and operational Raman lidar systems for smart monitoring of the environment, and WV in particular. In the WaLiNeAs project aims at developing the test bed of an integrated prediction tool, coupling network measurements of WV profiles and a weather forecast model to precisely estimate precipitable water upstream of an event 24 to 48 hours in advance in Southern France. This project is highly innovative and will lay the foundation for a future integrated warning tool aiming to prevent natural hazards associated with heavy precipitation events as often experienced along the Mediterranean coastline. Once the proof of concept is validated in the framework of the WaLiNeAs project, similar integrated tools may be applied in other parts of the World to avoid similar natural hazards. The project will also contribute to the science objectives of the CES01 « Milieux et biodiversité : Terre fluide et solide », i.e. enhancement of our understanding of processes related to the life cycle of convection and the characterization of the critical zone in Southern France and other regions impacted by heavy precipitation events. It will lead to recommendations on the lidar data processing for future operational exploitation in NWP systems. Overall, the project will contribute to increase the accuracy of forecasts of quantitative precipitation in order to satisfy the societal demands in terms of amount, timing, and basin-specific locations of rainfall and flash flooding.

C. Flamant, P. Chazette, O. Caumont, P. Di Girolamo, A. Behrendt, M. Sicard, J. Totems, D. Lange, N. Fourrié, P. Brousseau, C. Augros, A. Baron, M. Cacciani, A. Comerón, B. De Rosa, V. Ducrocq, P. Genau, L. Labatut, C. Muñoz-Porcar, A. Rodríguez-Gómez, D. Summa, R. Thundathil and V. Wulfmeyer, 2021: A network of water vapor Raman lidars for improving heavy precipitation forecasting in southern France – Introducing the WaLiNeAs initiative, Bull. Atmos. Sci. Tech., 2, 10, doi.org/10.1007/s42865-021-00037-6

Extreme heavy precipitation events (HPEs) pose a threat to human life but remain difficult to predict. Considerable efforts to improve the skill of the forecasts for such severe events have been made in recent years and significant progress has been realized through the development of convection-permitting numerical weather prediction systems (NWPS). However, our ability to predict such high-impact events remains limited because of the lack of adequate high frequency, high resolution water vapor (WV) observations in the low troposphere (below 3 km). HPEs occurring in small and steep watersheds are responsible for the triggering of flash floods with a sudden and often violent onset and rapid rising time, typically from 1 to 6 h following the causative rainfall.

We aim to implement an integrated prediction tool, coupling network measurements of WV profiles and a numerical weather prediction model to precisely estimate the amount, timing and location of rainfall associated with HPEs in southern France (struck by ~7 HPEs per year during the fall).

The proposed WaLiNeAs project is a unique, innovative initiative that will for the first time ever allow assimilating high vertical resolution lidar-derived WV profiles in the first 3 km of the troposphere. The benefit of WaLiNeAs to the academic and operational communities is dual: advance knowledge of the complex dynamical and dynamical processes controlling the life cycle of HPEs and enhance the predictability of HPEs in southern France at scales relevant for meteorological studies. Both aspects are dealt with in the framework of WaLiNeAs.

A network of 5 autonomous Raman WV lidars will be deployed in the Western Mediterranean to provide measurements with high vertical resolution and accuracy, closing critical gaps in lower troposphere WV observations by current operational networks and satellites. Near real-time processing and ensemble assimilation of the WV data in the French operational Application of Research to Operations at MEsoscale (AROME) model, using a 4DEnVar approach with 15 min updates, is expected to enhance the model capability for kilometer-scale prediction of HPEs over southern France 48 hours in advance.

The field campaign is scheduled to start early September 2022, to cover the period most propitious to heavy precipitation events in southern France. The Raman WV lidar network will be operated by a consortium of French, German and Italian research groups. Lidar data will be made available to Météo-France shortly after being acquired up to 96 times per day.

Besides demonstrating the potential of WV lidar data assimilation in a near real-time operational context, an ancillary objective of the project is also to show that Raman lidars can be left to operate continuously almost unattended for a period of at least 3 months. It is a prerequisite in the perspective of future deployment of operational Raman lidar systems meant to fulfil the observational gaps in WV in the lower troposphere of the current operational observation networks and satellites. This project will lead to recommendations on the lidar data processing for future operational exploitation in NWPS.

This project will contribute significantly to the scientific objectives of CES04 « Innovations scientifiques et technologiques pour accompagner la transition écologique » through the development of all-weather, unattended, continuous operation of Raman lidar systems for smart monitoring of the environment, and WV in particular. This project is highly innovative and will lay the foundation for a future integrated warning tool aiming to prevent natural hazards associated with heavy precipitation events as often experienced along the Mediterranean coastline. Once the proof of concept is validated in the framework of the WaLiNeAs project, similar integrated tools may be applied in other parts of the World to avoid similar natural hazards.