Tropical moisture enriched storm tracks over the Mediterranean and their link with intense rainfall in the Cevennes-Vivarais area during HyMeX
Résumé
During the Intensive Observation Period 15b of the first Special Observing Periods of the Hydrological Cycle in the Mediterranean Experiment (HyMeX), a variety of mesoscale convective systems (MCSs) impacted the Cevennes-Vivarais (CV) target area leading to over 100 mm of 24h accumulated rainfall on 20 and 21 October 2012. The CV area was first impacted by a V-shape MCS developing over the Cevennes mountains, then by a MCS initiating on the eastern foothills of the Pyrenees and finally by three MCSs initiating over the sea. The MCSs initiated and propagated along a well-defined storm track ahead of an approaching upper-level trough, as observed with the 15 min resolution Spinning Enhanced Visible and Infrared Imager. The storm track was characterized by strong southeasterly winds over the Mediterranean and high integrated water vapour content (IWVC), as derived from observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Atmospheric Infrared Sounder (AIRS). The ground-based Water vapour Raman Lidar, located in the Balearic Islands, captured the increasing moistening of the free troposphere, up to 5 km, associated with the eastward propagation of the surface low from Gibraltar to a location west of the Balearic Islands. MODIS and AIRS observations, together with Weather Research and Forecasting (WRF) model simulations, revealed the tropical origin of the high moisture content characterizing the storm track, with IWVC values on the order of ~35 kg m−2, and enhanced moisture being observed below 500 hPa. The WRF simulations also showed that the MCS initiation offshore was very likely caused by low-level wind convergence and conditionally unstable air along the storm track, between North Africa and southern France. Low-level convergence resulted from the interaction between a strong south-westerly swirling flow around the low pressure centre and an easterly low-level jet present along the southern France coastline.