Retrieving the Convective Thermals and Updraft Speeds at Cloud Base from VIIRS

Abstract:

Updraft speeds of thermals have always been difficult to measure, despite significant roles they play in transporting pollutants and in cloud formation and precipitation. To our knowledge, no attempt to date has been made to estimate updraft speed from satellite information in the boundary layer and at the cloud base. In this study, we introduce two methods of retrieving the maximum updraft (W_max) and updraft at cloud base (W_b) in the planetary boundary layer topped by convective clouds. The first method uses ground-air temperature difference to characterize the surface sensible heat flux, which is found to be correlated with updraft speeds measured by the Doppler lidar over the Southern Great Plains (SGP). Based on the relationship, we use the satellite-retrieved surface skin temperature and reanalysis surface air temperature to estimate the updrafts. The second method is based on a good linear correlation between cloud base height and updrafts, which was found over the SGP, the central Amazon, and on board a ship sailing between Honolulu and Los Angeles. We found a universal relationship for both land and ocean. The performance of these two methods of retrieving updrafts was tested against the lidar and Radar measurements with good agreements found for both methods. Compared with the first method that only works over land, the second method expands its applicability to ocean and is more accurate in retrieving W_max the with RMSE (root-mean-square error) = 0.38 m/s and MAPE (mean-absolute-percentage-error) = 19%, and W_b with RMSE = 0.34 m/s and MAPE = 21%.