Satellite Inference of Thermals and Cloud Base Updraft Speeds Based on Retrieved Surface and Cloud Base Temperatures

Abstract:

Updraft speeds of thermals have always been difficult to measure, despite the significant role they play in transporting pollutants and in cloud formation and precipitation. In this study, updraft speeds measured by Doppler lidar are found to be correlated with the observed planetary boundary layer (PBL) and surface properties in the buoyancy-driven PBL over the Southern Great Plains (SGP) site operated by the U.S. Department of Energy’s Atmospheric Radiation Program (ARM). Based on the found relationships, two approaches are proposed to estimate both maximum (W_max ) and cloud base (Wb ) updraft speeds. The required input data are PBL height, 10-m horizontal wind speed, wind shear, surface skin temperature and 2-m air temperature. The application for remote sensing of updraft speeds in cloud-topped PBL from space was tested by using satellite-retrieved surface and cloud base temperature in combination with European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis data. Validation against lidar-measured updraft speeds indicates the feasibility of retrieving W_max (root-mean-square error, RMSE, is 0.32 m/s) and W_b (RMSE is 0.42 m/s) for global coverage. This information is essential to advance the understanding of aerosol-cloud interactions. This method does not work for stable or mechanically-driven PBL.