Linear relation between convective cloud base height and updrafts and application to satellite retrievals
Authors
Daniel Rosenfeld — The Hebrew University of Jerusalem
Youtong Zheng — Geophysical Fluid Dynamics Laboratory/Princeton University
Category
Vertical Velocity
Description
Variation of observed cloud base updraft with VCEIL-measuredcloud base height at (a) SGP site, (b) MAGIC campaign, and (c) SGP + MAGIC. In (c), red and blue dots stand for SGP and MAGIC, respectively.
Retrieving cloud base updraft is important because it also makes possible the retrieval of CCN from satellite. Rosenfeld et al. (2014) have shown that combining satellite retrieved cloud base drop concentrations with radar-retrieved cloud base updraft can be used for retrieving the CCN and supersaturation at cloud base. Our findings provide the basis for retrieving cloud base updraft by from space, thus allowing calculating both cloud base updrafts and CCN. This has huge importance, because the capability of retrieving both properties will allow us to disentangle the effects of aerosols and updrafts on cloud radiative effects. The inability to do so until now is the main cause for the large uncertainty in cloud-mediated aerosol forcing on the climate system (IPCC). This is made possible based on the following principles: In the 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 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%.
Reference:
Rosenfeld D., B. Fischman, Youtong Zheng, T. Goren, D. Giguzin, 2014: Combined satellite and radar retrievals of drop concentration and CCN at convective cloud base. GRL, DOI:10.1002/2014GL059453
