Satellite Inference of Thermals and Cloud Base Updraft Speeds

Zheng, Y., University of Maryland, College Park

Vertical Velocity

Convective Processes

Zheng Y, D Rosenfeld, and Z Li. 2015. "Satellite Inference of Thermals and Cloud-Base Updraft Speeds Based on Retrieved Surface and Cloud-Base Temperatures." Journal of the Atmospheric Sciences, 72(6), 10.1175/jas-d-14-0283.1.

Validation of satellite-estimated W_cb against lidar-measured updrafts at the Southern Great Plains site

Validation of satellite-estimated W_cb against lidar-measured updrafts at the Southern Great Plains site

Thermals transfer and distribute heat, moisture, momentum, and pollutant materials from the surface layer to the upper part of the convective mixed layer. Thermals also play a central role in the formation of convective clouds and precipitation. Cloud base updrafts govern water vapor supersaturation there, determining the concentration of cloud condensation nuclei (CCN) activated into cloud droplets. Because the combination of cloud base updraft speed and CCN concentration determines cloud properties, assessing aerosol effects cannot be done without first disentangling them from updrafts. So, in addition to being a key variable dictating cloud development, the retrieval of updraft velocities is critical to advancing our understanding of aerosol-cloud-mediated effects. Although a handful of air-borne and ground-based approaches of measuring updrafts exist, such measurements are extremely rare. The goal of this study is to demonstrate the feasibility of satellite retrieval of thermals and cloud base updraft speeds, which has been considered impossible to do from space until now.

A novel method of National Polar-orbiting Partnership/Visible Infrared Imaging Radiometer Suite satellite retrievals has been proposed and tested for the speed of the thermals at the height where they reach their maximum (W_max) and cloud base updraft speeds (W_cb) in the convective planetary boundary layer. This method is based on retrievals of surface and near-surface air temperatures, and surface wind speed. These variables determine the heat flux that propels the thermals to form clouds when reaching the condensation level. This retrieval method has been validated against the Doppler lidar and in situ meteorological measurements at the Southern Great Plains site.

Applying the method to National Polar-orbiting Partnership/Visible Infrared Imaging Radiometer Suite data along with European Centre for Medium Range Weather reanalysis data, has retrieved the W_max and W_cb and validated them against the lidar-measured updrafts. The validation shows good agreements with root-mean-square error of 0.32 and 0.42 m/s for W_max and W_cb, respectively. The mean absolute percentage error for W_cb is 24 percent.

This method contributes to the existing body of knowledge in at least two ways. First, it has not been possible until now to retrieve updraft speed from satellite measurements in buoyancy-driven boundary layers. Second, the method does a good job of retrieving convective W_cb by satellite. The mean absolute percentage error of the retrieved W_cb is 24 percent. This corresponds to an error of 7 to 11 percent in cloud base droplet concentration if the CCN supersaturation spectrum is known. This is a useful accuracy for aerosol-cloud interaction studies and an essential step towards disentangling the effects of dynamics and aerosols on cloud radiative effects. Thus, it moves towards reducing the uncertainty in quantifying anthropogenic climate forcing.