Diameters of cumulonimbi over land and ocean and their dependence on cloud base height

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Monday, 18 January 2010
Earle R. Williams, MIT Lincoln Laboratory, Lexington, MA; and M. F. Donovan, Z. J. Luo, C. Kessinger, H. Cai, N. Rehak, D. Megenhardt, M. Steiner, J. Hawkins, R. Bankert, and D. Bou Karam

The contrast in the vigor of convection between land and ocean is now widely recognized, and is perhaps most clearly manifest in global maps of lightning activity, as in the NASA TRMM observations with the Lightning Imaging Sensor. The systematically greater updraft speeds over land present graver hazards to commercial aviation in areas of convective turbulence, icing, hail and lightning. It is frequently assumed that a contrast in Convective Available Potential Energy (CAPE) is responsible for the land-ocean contrast in cumulonimbus updraft speed, but detailed comparisons of geographical variations in CAPE fall short of the mark here. The present study is concerned with observational tests of one alternative hypothesis that updraft width in cumulonimbi scales with the cloud base height. The latter quantity is systematically greater over land and the correspondingly greater updraft width will enable a more efficient conversion of CAPE to updraft kinetic energy. A-Train observations with the CALIPSO lidar and CloudSat radar can be used to measure the widths of cumulonimbi over land and ocean with the same instruments. Cloud base heights will be determined by proximity station observations of temperature and dew point temperature. Both the Wide Field Camera on CALIPSO and the MODIS vis channel will be used to provide two-dimensional context for the clouds selected for width measurements by lidar and radar in quasi-three dimensions. Preliminary analysis with the CloudSat radar on ~2000 clouds has shown that for 10 dBZ cloud widths greater than about 5km, the clouds over land tend to be wider than those over ocean.