Remote Sensing Of North Atlantic Storms: Synergetic Use Of Active Microwave And Optical Data

Abstract

The aim is to use Synthetic Aperture Radar (SAR) in synergy with optical data to analyse severe weather systems, e.g., Tropical Cyclones, North Atlantic Storms and Polar Lows. Radar reflectivity over the ocean depends on the roughness of the sea surface and thus mainly on the wind field. Additionally the backscatter of the radar signal is influenced by the size of hydrometeors, e.g. rain drops or snow in the atmosphere and their precipitation rate. The Medium Resolution Imaging Spectrometer (MERIS) instrument that is flying on board the ENVISAT satellite or the Moderate Resolution Imaging Spectroradiometer (MODIS) that is flying on the AUQA/TERRA satellites provide an image over a specific region every three respectively every two days. These measurements are useful to estimate cloud parameters. Synthetic Aperture Radars (SARs) are capable of imaging synoptic wind fields near the sea surface with a coverage of up to 500 km x 500 km and a resolution on a subkilometer scale. For retrieving wind speeds from SAR data a model function (CMOD 4 and 5) [1] relating the Normalized Radar Cross Section (NRCS) of the ocean surface to the local nearsurface wind speed, wind direction versus antenna look direction and incidence angle is used. As examples severe storm system are investigated. By synergetic use of ENVISAT ASAR, MERIS and MODIS data the relationship between cloud patterns with different cloud parameters, e.g. cloud top pressure, cloud optical thickness and NRCS is investigated. A high correlation between convective clouds and NRCS was detected. Downburst produced by a convective downdraft over a region of less than 1 to 10 km horizontal extension and their interactions with the sea surface are observed in SAR images. The variation of the backscattering coefficient due to rain is considered, too.