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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Investigation of moisture anomalies over Mediterranean Sea using precipitable water estimated from MSG SEVIRI data

Liliana Velea (ISAC-CNR , (permanent: NMA Bucharest, Romania)); Gian Luigi Liberti

Within the frame of EU sponsored project MOD&OBS (www.windeng.net/ModObs/ModObs_Home_page.pdf) aimed to improve the capability to model Marine Boundary Layer (MBL), the potential added value of MSG SEVIRI observations due to increased spatial resolution (coastal studies), temporal sampling (diurnal signals) and pseudo sounding capabilities (2 H2O + 1 CO2 channels) is explored. Aim of the study is to find a MSG-SEVIRI based observable related to the moisture component of the latent heat turbulent flux, focusing on the Mediterranean Sea. The analysis of temporal and spatial structures of such observable, together with wind estimation on the areas of interest, is expected to contain valuable information about processes at the air-sea interface.
The approach presented here relies on the analysis of Total Precipitable Water vapor (TPW) derived from MSG SEVIRI data. We estimated TPW using a Split-Window Differences algorithm developed at Spanish Meteorological Center. The SST data used is a standard product of OSI-SAF (Ocean and Sea Ice Satellite Application Facility), available over the Atlantic with a spatial resolution of 0.1 degree and a temporal sampling of 3 h. The preliminary results presented are based on the analysis of about 14 weeks of processed observations, sampling different seasons and including some cases corresponding to measurement campaigns in the Mediterranean (e.g. June 2007- Ligurian Air-Sea Interaction Experiment.)
The results are analyzed using two measures: the ratio TPW/SST as an indicator of degree of decoupling between atmosphere and sea surface and the frequency of occurrence of anomalies in near-surface relative humidity (RH), derived from thermodynamically-based hypothesis (Stephens, 1990) to quantify the intensity of these events. The analysis of high resolution spatial and temporal variations of these parameters show consistent large scale features (e.g. the patterns of low TPW/SST ratio corresponding to known upwelling areas). Small scale features, like for example the area of systematically low RH, showing diurnal variations, in the Eastern Mediterranean, for a 3-days period in June 2007, are also seen. Investigations on the connection with other parameters (e.g. surface winds) are ongoing.

Stephens, G.L., 1990: 'On the relationship between Water Vapor over the Oceans and Sea Surface Temperature', J.Climate, 3, 634-645