3 - Centre National de Recherches Météorologiques - Météo France

Atmosphereand environmentstudiesIn 2007, several developments were carried out concerninghydrometeorology, oceanography, nivology, reduced visibility ormodelisation of atmospheric chemistry... Some notable progresswere made on discharge forecasts thanks to SIM improvements,on fog understanding, … Moreover, air quality ensemble forecastand desert dusts forecasts were two major innovations of the last12 months.Hydro-meteorologySea-air coupled processesduring heavy precipitating eventsFor the strongly precipitating events that frequentlyoccur over the Mediterranean basinduring the autumn, the Mediterranean Seais an important source of heat and moisture.The sea can also sometimes contribute toamplify the flood due to the sea-level riseand the generation of a strong swell, generatedby the strong winds that accompany theheavy rainfall.To study the impact on the oceanic mixedlayer of heavy precipitation and strong seawinds observed in these situations, as wellas the feedbacks on the atmosphericevents, a full 2-way ocean-atmosphere cou-pling has been designed between theatmospheric model MESO-NH, through itssurface scheme SURFEX, and an oceanic 1Dmodel (Gaspar et al.,1990). Applied to threeheavy precipitation events (Aude 12-13November 1999, Gard 8-9 September 2002and Hérault 3 December 2003) at a 2.4 kmhorizontal resolution, the coupled systemreproduces fine scale oceanic responses. Asignificant deepening and cooling of theoceanic mixed layer are thus found underthe strong winds associated with the precipitatingsystems. An impact of the precipitationis also shown through a decrease of thesalinity induced by fresh water inputs andthe building-up of inner mixed layers relativelylittle salted. Compared with a simpleone-way forcing, the 2-way interactive couplingtends to moderate the two boundarylayers responses, without modifying thehigh-resolution and short range forecast ofheavy precipitating events.This study constitutes a first step in finescaleair-sea coupled modelling and producesoutcomes for a future campaign fieldon the hydrological cycle of the Mediterranean(HyMeX). 1Improvement of the hydrological transfersin the surface model ISBAA good representation of hydrological transfersis necessary for a good simulation ofrunoff and water transfers to the rivers. Oneoption, amongst others, to improve the ISBAsurface scheme consists in introducing adiminution of the saturated hydrologicalconductivity with depth. This parametergoverns the speed of the water transfers inthe soil, its diminution being linked to thecompactation of the soil with depth.After some very encouraging applications atthe scale of the Rhône basin and at globalscale, this modification has been tested inthe application SAFRAN-ISBA-MODCOU atthe French scale. After several one-dimensionaltests, the calibration of correspondingparameters has been defined. Usingthe observed discharge observations atsome 200 stations, the parameters werecalibrated to reproduce as faithfully as possiblethe observed discharge.The validation showed that the improvementwas significant over most stations(increase of the Nash criterion). The impacton the root reservoir wetness is almost neutral,while the humidity of the sub-root reservoiris increased, allowing a betterrepresentation of floods and low flows. Thisnew calibration improves the overall qualityof the SAFRAN-ISBA-MODCOU runs. It will beused either in the framework of dischargeforecast or on impact studies of climatechange. 232 . Research and development: annual report 2007