Quantifying the spatial and temporal response of UTH and OLR to deep convection over Tropical Africa
Upper Tropospheric Humidity (UTH) has a strong control on clear-sky Outgoing Longwave Radiation (OLR). Moisture from the boundary layer is transported to the drier upper troposphere by convective ascent in the tropics and realised in the form of deep convective clouds. The spatial and temporal response of UTH and the corresponding OLR are cause for debate. This study uses geostationary satellite imagery from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) to estimate UTH using water
... te UTH using water vapour channel radiances. Deep convection over Tropical Africa is detected using the difference between 6.2 mm and 7.3 mm brightness temperatures. The sensitivity of TOA brightness temperatures to cloud properties including cloud top height and optical depth are modelled using the Santa Barbara Disort Atmospheric Radiative Transfer model with thresholds developed using colocated matchups with CloudSat and CALIPSO cloud classifications. The most appropriate thresholds are determined using probability statistics and receiver operating characteristic curves. Deep convective clouds are tracked over their lifetime in June and December 2010 using a cloud tracking algorithm, based on an area overlap method. A general robust pattern in the UTH response emerges. A stronger response of UTH is found in the spatial domain than that over the temporal domain. UTH decreases with distance from the cloud edge, whilst a small increase is seen over the cloud lifetime. This was found to be controlled by cloud size and cloud lifetime, with larger and longer lived clouds causing a stronger perturbation in UTH. The UTH response was found to be stronger in June than in December. A strong negative correlation is found between UTH and OLR perturbations, with OLR measured using the Geostationary Earth Radiation Budget (GERB) instrument. This pattern is stronger in December than June.