Theoretical Explanation for the Relationship between Radar Reflectivity and Vertical Velocity Fluctuations in Convective Clouds

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Description

Understanding the statistical properties of vertical motion is important for the validation and development of cumulus and microphysics parameterizations. However, observing in-cloud vertical velocities in deep convection is challenging. The dynamical and microphysical characteristics of convection are known to be related and radar reflectivity is a widely available source of microphysics data. This study evaluates whether these dynamical-microphysical relationships can be used to estimate statistical properties of vertical velocity using reflectivity. To this end, a WRF simulation is conducted near Darwin, Australia on 18 January 2006. The simulation has a 500-m horizontal resolution, 60 vertical levels, and circulation and reflectivity data is written out every minute. Model results suggest that statistical properties of vertical velocity can be estimated from the conservation properties of hydrometeors that constitute radar reflectivity within convective cells. Our method is also evaluated using reflectivity and vertical velocity observations from a radar wind profiler in Darwin, Australia during the 2005-2006 and 2006-2007 wet seasons. The reflectivity from the radar wind profiler is found to skillfully estimate vertical velocity variability near the melting layer in stratiform precipitation regions. However, reflectivity from the radar wind profiler does not skillfully estimate vertical velocity variability in deep convective cells. This lack of skill is likely due to inability for the profiler to account for the strong horizontal advection often associated with convective cells. Our results motivate new observation strategies for scanning radars that will collect targeted observations to improve our understanding of the interactions between dynamics and microphysics in deep convective updrafts.