LOW-LEVEL CLOUD MOTION WINDS FROM METEOSAT HIGH-RESOLUTION VISIBLE IMAGERY

Low-level wind fields over the Atlantic have been derived from clouds in Meteosat high-resolution visible images experimentally with one pro duction cycle per day over a period of more than 1 yr. The cloud motio n winds from VIS imagery (VIS-CMW) use a template size of 32 x 32 VIS pixels, corresponding to about 80 km x 80 km at the subsatellite point , which is four times better than for the corresponding IR (infrared w indow) winds (160 km x 160 km). The yield is increased through the bet ter spatial resolution of the VIS images and a better contrast between cloud and ocean surface, which effectively leads to an increase in wi nd vectors by a factor of 6. This implies a much better description of the low-level atmospheric flow by the VIS-CMW as compared to IR winds . The impact of the new VIS-CMW has been tested with a data assimilati on experiment at the European Centre for Medium-Range Weather Forecast s, and small positive improvements have been found. The mean vector rm s difference versus the verifying analysis shows improvement by up to 15% over some areas of the Atlantic Ocean. Comparisons of the short-te rm forecast using VIS cloud motion winds with independent scatteromete r surface winds confirm the small improvements.