Sensitivity of torrential rain events to the sea surface temperature based on high‐resolution numerical forecasts

[1] Torrential rains often occur in the western Mediterranean region during the fall season when the Mediterranean Sea is still warm. The Mediterranean Sea acts in moistening and warming the low level of the atmosphere. Then, the southerly to easterly flow that prevails before and during torrential rainfall events, transports the conditionally unstable air toward the coasts where the convection can develop often triggered by a flow interaction with orography. This study examines the sensitivity to the sea surface temperature (SST) of very short range (18–24 hours) high-resolution (2.4 km) forecasts of heavy precipitation events. Three torrential rainfall events were selected as representative of extreme rainfall events that occurred over southern France: two cases of quasi-stationary mesoscale convective systems and one other case characterized by a slow moving frontal perturbation. For each case, a number of runs is performed with the MESO-NH research model using several SST fields differing in their resolution or/and their average value over the Mediterranean basin. Results show that an increase (a decrease) of SST by several degrees, on average, intensifies (weakens) the convection. The convection can even be stopped with strong decrease of SST. Impacts on the stationary behavior of the systems have also been pointed out. A more detailed SST field influences the mesoscale patterns of the sea surface heat fluxes but have almost no significant effect on the convection and the low-level jets forecast. Eventually, the SST has been allowed to evolve during the runs through the action of the air-sea interface fluxes, resulting in local effects such as significant cooling of the SST beneath the low-level jet but almost no impact on the very short range forecasts of heavy precipitation.

[1]  V. Ducrocq,et al.  Storm-Scale Numerical Rainfall Prediction for Five Precipitating Events over France: On the Importance of the Initial Humidity Field , 2002 .

[2]  J. Bao,et al.  Numerical Simulations of Air-Sea Interaction under High Wind Conditions Using a Coupled Model: A Study of Hurricane Development , 2000 .

[3]  V. Caselles,et al.  Torrential precipitations on the Spanish east coast: The role of the Mediterranean sea surface temperature , 1995 .

[4]  D. Rowell.  The Impact of Mediterranean SSTs on the Sahelian Rainfall Season , 2003 .

[5]  S. Levitus.  Climatological Atlas of the World Ocean , 1982 .

[6]  S. Planton,et al.  Atmospheric boundary layer response to sea surface temperatures during the SEMAPHORE experiment , 1998 .

[7]  Richard C. J. Somerville,et al.  On the use of a coordinate transformation for the solution of the Navier-Stokes equations , 1975 .

[8]  E. Bazile,et al.  The 12–13 November 1999 flash flood in southern France , 2001 .

[9]  SST data improve modeling of North American monsoon rainfall , 2003 .

[10]  David J. Stensrud,et al.  Numerical diagnosis of a small, quasi‐tropical cyclone over the western Mediterranean: Dynamical vs. boundary factors , 2003 .

[11]  V. Ducrocq,et al.  Les précipitations intenses et les inondations des 12 et 13 novembre 1999 sur le sud de la France , 2003 .

[12]  Véronique Ducrocq,et al.  Initialization of a fine‐scale model for convective‐system prediction: A case study , 2000 .

[13]  Hervé Giordani,et al.  Sensitivity of Cyclogenesis to Sea Surface Temperature in the Northwestern Atlantic , 2001 .

[14]  M. Millán,et al.  Torrential Rains on the Spanish Mediterranean Coast: Modeling the Effects of the Sea Surface Temperature , 2001 .

[15]  E. Bazile,et al.  A mass‐flux convection scheme for regional and global models , 2001 .

[16]  S. Planton,et al.  Modeling and Analysis of Ageostrophic Circulation over the Azores Oceanic Front during the SEMAPHORE Experiment , 2000 .

[17]  R. Romero,et al.  Numerical simulation of an extreme rainfall event in Catalonia: Role of orography and evaporation from the sea , 1997 .

[18]  I. Ginis,et al.  Real-Case Simulations of Hurricane-Ocean Interaction Using A High-Resolution Coupled Model: Effects on Hurricane Intensity , 2000 .

[19]  S. Planton,et al.  Toward a Better Determination of Turbulent Air–Sea Fluxes from Several Experiments , 2003 .

[20]  Véronique Ducrocq,et al.  L’événement des 8-9 septembre 2002 : situation météorologique et simulation a mésoéchelle , 2004 .

[21]  Charles A. Doswell,et al.  A Diagnostic Study of Three Heavy Precipitation Episodes in the Western Mediterranean Region , 1998 .

[22]  Véronique Ducrocq,et al.  The Meso-NH Atmospheric Simulation System. Part I: adiabatic formulation and control simulations , 1997 .

[23]  William G. Pichel,et al.  Comparative performance of AVHRR‐based multichannel sea surface temperatures , 1985 .

[24]  Romualdo Romero,et al.  Numerical study of the October 2000 torrential precipitation event over eastern Spain: analysis of the synoptic-scale stationarity , 2002 .

[25]  Florence Sevault,et al.  Transient climate change scenario simulation of the Mediterranean Sea for the twenty-first century using a high-resolution ocean circulation model , 2006 .

[26]  Kerry A. Emanuel,et al.  The Ocean’s Effect on the Intensity of Tropical Cyclones: Results from a Simple Coupled Atmosphere–Ocean Model , 1999 .

[27]  Philippe Gaspar,et al.  Modeling the Seasonal Cycle of the Upper Ocean , 1988 .

[28]  H. Andrieu,et al.  The Catastrophic Flash-Flood Event of 8–9 September 2002 in the Gard Region, France: A First Case Study for the Cévennes–Vivarais Mediterranean Hydrometeorological Observatory , 2005 .