Quantitative Analysis of the Delta-t Approach for Estimating Convective Mass Flux Using Ground Radar Observations

The upcoming NASA Earth Venture Mission named INvestigation of Convective UpdraftS (INCUS) will deploy three Ka-band radars in space with consecutive temporal sampling of 30, 90, and 120 seconds to provide the first spaceborne estimates of convective mass flux (CMF) across the global tropics. Such sampling is based on the ‘delta-t’ principle that the change in reflectivity over short time scales is related to the CMF within a convective system. While this idea is fundamentally based on the equations of the evolution of momentum and condensate in clouds and is supported by model simulations, investigations of the relationship between CMF and delta-t using observations have been scarce due to limited availability of high temporal resolution radar observations. In addition, there are other factors than CMF that can influence the change in reflectivity over short time scales such as horizontal advection and cloud microphysical processes.

This study uses ground-based radar observations to analyze the relationship between vertical velocity (w) and the change in reflectivity over different temporal and spatial scales. Specifically, radar scans at high temporal resolution that are co-located with Doppler-derived w estimates are used to examine the role of horizontal advection, changes in microphysics, and w to the derived vertical CMF using the delta-t technique. Innovative sampling using Multisensor Agile Adaptive Sampling (MAAS) during the Department of Energy (DOE) TRACER (Tracking Aerosol Convection Interactions Experiment) and National Science Foundation (NSF) ESCAPE (Experiment of Sea Breeze Convection, Aerosols, Precipitation, and Environment) field campaigns in the summer 2022 near Houston, TX provided convective core-following RHIs at sub-minute time scales thereby facilitating this analysis. The MAAS controlled two C-band radars, the DOE CSAPR-2 radar and the Colorado State University (CSU) CHIVO, which allowed for sampling of isolated convective cores located in optimal locations to derive w using multi-Doppler retrievals. Additionally, high resolution model simulations of a case study from these campaigns are compared to the ground radar estimates of delta-t, w, and convective mass flux, toward the goal of understanding how accurately changes in reflectivity are captured by the model simulations.