Abstract
Abstract A characterization of the large-scale environment associated with precipitating systems in the Mediterranean region, based mainly on NOAA-16 Advanced Microwave Sounding Unit (AMSU) observations from 2001 to 2007, is presented. Channels 5, 7, and 8 of AMSU-A are used to identify upper-level features, while a simple and tractable method, based on combinations of channels 3–5 of AMSU-B and insensitive to land–sea contrast, was used to identify precipitation. Rain occurrence is widespread over the Mediterranean in wintertime while reduced or short lived in the eastern part of the basin in summer. The location of convective precipitation shifts from mostly over land from April to August, to mostly over the sea from September to December. A composite analysis depicting large-scale conditions, for cases of either rain alone or extensive areas of deep convection, is performed for selected locations where the occurrence of intense rainfall was found to be important. In both cases, an upper-level trough is seen to the west of the target area, but for extreme rainfall the trough is narrower and has larger amplitude in all seasons. In general, these troughs are also deeper for extreme rainfall. Based on the European Centre for Medium-Range Weather Forecasts operational analyses, it was found that sea surface temperature anomalies composites for extreme rainfall are often about 1 K warmer, compared to nonconvective precipitation conditions, in the vicinity of the affected area, and the wind speed at 850 hPa is also stronger and usually coming from the sea.
Highlights
The Mediterranean region is a densely populated area under climatic and environmental stresses, concerning the availability of water resources, and is one of the most responsive regions to climate change (e.g., Hulme et al 1999; Giorgi 2006)
We use diagnostic tools based on the Advanced Microwave Sounding Unit (AMSU) data, both for detecting precipitation (AMSU-B) as well as characterizing upper-level features (AMSU-A)
This study contributes to the effort of identifying and quantifying the sources of uncertainties that affect the forecast and climate projection in the Mediterranean region, in particular for events of extreme rainfall
Summary
The Mediterranean region is a densely populated area under climatic and environmental stresses, concerning the availability of water resources, and is one of the most responsive regions to climate change (e.g., Hulme et al 1999; Giorgi 2006). Despite our knowledge of the necessary key ingredients for a strong rainfall, its predictability is limited at best to a couple of days because there is a large range of processes that play a role, as well as uncertainties of the system due to the inherent nonlinearity of the atmospheric dynamics and the several scale interactions In this sense, there has been an increased interest in identifying and quantifying the sources of uncertainties that affect the forecast and climate projection in the Mediterranean region. This work aims at contributing to this effort, to contrast the large-scale environment of systems yielding moderate or very intense precipitation To this end, we use the data provided by the Advanced Microwave Sounding Unit (AMSU) radiometer to detect moderate and heavy precipitating areas and their embedded upper-level environment. The final Section contains a summary and discussion including a comparison of our results with those of other authors, in relation to large-scale circulation patterns and weather regimes
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