Upper Tropospheric Divergence in Tropical Convective Systems from Meteosat-8 (Formerly P1.38)

The upper level wind field divergence in tropical convective cloud systems is directly inferred from satellite tracked Atmospheric Motion Vectors (AMVs) using successive images at 15 minute intervals in the water vapor channel (6.2 µm) of Meteosat-8. The use of radiances in the strongly absorbing 6.2 µm channel confines the feature tracking to the upper troposphere, enabling the derivation of consistent wind fields. A Lagrangian approach following the convective cloud systems throughout a day shows a pronounced diurnal cycle of the divergence with maximum values of 450 * 10-6 s-1. A comparison with the ECMWF forecast system shows that the forecast does not depict the observed spatial structure and does not reach the maximum values of observed divergence at the scale of tropical cloud systems. Divergence averages over the study area of about 1300 * 1400 km2 agree within a factor of two.