Raindrop Breakup and Coalescence Diagnosed from UAZR and KAZR Observations

Poster PDF

Description

This study uses three activities to bridge the research areas of Precipitation Remote Sensing and Precipitation Modeling. The first activity exploits Rayleigh and Mie scattering processes to retrieve vertical air motions and raindrop size distributions (DSDs) from observations made by 0.915-GHz and 35-GHz vertically pointing radars (also known as UAZR and KAZR radars) deployed side by side at the Southern Great Plains (SGP) Central Facility. When both radars are observing the same raindrops, they measure different radial velocities. This velocity difference is due to Mie scattering from large raindrops influencing KAZR observations such that the KAZR-measured velocity is less than the UAZR-measured velocity. An outcome of this activity is a Rayleigh/Mie scattering retrieval methodology that estimates vertical air motion and three parameters of a gamma-shaped DSD in the vertical column. The second activity of this study applies the Rayleigh/Mie scattering retrieval method to over 100 hours of stratiform rain passing over the SGP Central Facility UAZR and KAZR radars from 2011 through 2016. By aggregating UAZR and KAZR observations to 1-minute intervals and assuming 10 m/s horizontal advection speed, retrievals and their uncertainties represent air motions and DSD parameters at approximately 600 m horizontal resolution and can be used to assess the representativeness of model parameterizations. To bridge the Precipitation Remote Sensing work with Precipitation Modeling, the third activity quantifies the vertical evolution of the retrieved falling raindrops in terms of liquid water content, total number concentration, and mass-weighted effective shape that incorporates both distribution size and breadth. Using vertical decomposition diagrams, changes in liquid water content with height quantify evaporation and accretion. When the raindrops are not evaporating, net raindrop breakup and coalescence are identified by changes in the total number of raindrops and changes in the DSD effective shape. Analysis of non-evaporating SGP stratiform rain events suggests that raindrops tend to coalescence as they fall, with smaller raindrops decreasing in number and larger raindrops increasing in number. These remote-sensing results pose an opportunity for the ASR community to determine if similar raindrop coalescence features occur in model simulations.