Linear relation between convective cloud base height and updrafts and application to satellite retrievals

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Monday, 5 January 2015
Youtong Zheng, University of Maryland, College Park, MD; and D. Rosenfeld and Z. Li

In convective planetary boundary layer (PBL), buoyancy is the dominant mechanism driving turbulence, and the growth of PBL depth. Measurements done by the DOE/ARM program at the Southern Great Plains site and onboard an ocean-going ship show that updraft speeds measured by Doppler lidar and 95-GHz cloud Radar are tightly linearly correlated with cloud base height (H_b). Based on these found relationships, a method of satellite retrieval of maximum (W_max) and cloud base (W_b) updraft speeds in cloud-topped PBL is proposed. H_b, as an input for updraft estimation, is obtained from satellite-retrieved cloud base temperature in combination with 2-m air temperature derived from European Centre for Medium-Range Weather Forecasts reanalysis. Validated by the lidar and Radar measurements, good agreements were found for the satellite retrieval of W_max with RMSE (root-mean-square error) = 0.37 m/s and MAPE (mean-absolute-percentage-error) = 16%, and W_b with RMSE = 0.37 m/s and MAPE = 27%

This study contributes to the existing body of knowledge in at least three ways. First, a linear relation between convective cloud base height and updrafts is the first time to be observed over both continent and ocean. Second, it has not been possible until now to retrieve updraft speed from satellite measurements in buoyancy-driven boundary layers with global coverage. Third, the method does a good job of retrieving convective W_cb by satellite. The 27% of MAPE (mean-absolute-percentage-error)for W_cb retrieval corresponds to an error of 7 to 13% in cloud base droplet concentration if the CCN supersaturation spectrum is known. This is very useful for aerosol-cloud interaction studies.