Convective Vertical Air Motion Estimates by the Spaceborne Doppler Radars of NASAs Aerosols, Clouds, Convection, and Precipitation (ACCP) Observing System

Convective clouds serve as a primary mechanism for the transfer of thermal energy, moisture, and momentum through the troposphere, significantly impacting the large-scale atmospheric circulation and local environment, and affecting the probability of subsequent cloud formation. Observations of the number and magnitude of vertical transport in shallow and deep convection over the tropical oceans are simply not available and sparsely available over land. Arguably, spaceborne radar observations are the only way to sample convective clouds over the vast tropical oceans. However, estimating the vertical air motion ( 50 ms-1) with an accuracy of 1-2 ms-1 from a platform that moves at ~ 7,500 ms-1 is a daunting task. The spaceborne Doppler radars of NASAs Aerosols, Clouds, Convection, and Precipitation (ACCP) Observing System employ the Displaced Phase Center Antenna (DPCA) technique that practically offsets the platform motion. Here, a variety of shallow and deep convective cloud scenes from numerical model outputs are used as input to an end-to-end spaceborne Doppler radar simulator. The output of the simulations (retrieved vertical air motion) is compared against those derived directly by the numerical models. First, the accuracy and resolution of the vertical air motion estimates from the ACCP radars in shallow and deep convection is discussed. Second, the impact of the radar footprint on retrieved vertical air motion parameters such as updraft area, updraft magnitude and updraft mass flux are discussed and the role of forward instrument simulators for an objective apples-to-apples comparison between the expected ACCP vertical air motion estimates and numerical model output direct estimates.