Diagnosing Raindrop Evaporation, Breakup, and Coalescence in Vertical Radar Observations

Williams, C. C., University of Colorado Boulder

Cloud Processes

Cloud Life Cycle

Williams CR. 2016. "Reflectivity and Liquid Water Content Vertical Decomposition Diagrams to Diagnose Vertical Evolution of Raindrop Size Distributions." Journal of Atmospheric and Oceanic Technology, 33(3), 10.1175/jtech-d-15-0208.1.


Example of evaporation and coalescence processes diagnosed using a liquid water content (LWC) vertical decomposition diagram (LWC-VDD) during stratiform rain on 20 May, 2011. Retrieved parameters are expressed in logarithmic units and are (a) liquid water content (LWC), (b) total raindrop number concentration (Nt), (c) characteristic raindrop size (Dq), and (d) evolution of Nt and Dq with height as indicated with colors.


Example of evaporation and coalescence processes diagnosed using a liquid water content (LWC) vertical decomposition diagram (LWC-VDD) during stratiform rain on 20 May, 2011. Retrieved parameters are expressed in logarithmic units and are (a) liquid water content (LWC), (b) total raindrop number concentration (Nt), (c) characteristic raindrop size (Dq), and (d) evolution of Nt and Dq with height as indicated with colors.

Science

In our quest to better understand precipitating cloud systems, we must realize that radars do not directly observe microphysical processes. Rather, radars observe raindrops and the changes in the number and size of those observed raindrops over time and space provide us with clues to the microphysical processes acting upon those raindrops. Thus, evaporation and accretion will appear as the subtraction or addition of mass to the observed raindrop size distribution over time and space, while breakup and coalescence will appear as modifications of how liquid is distributed between different-sized raindrops.

Impact

To help identify and isolate mass-modifying processes from size-modifying processes, Williams (2016) introduced vertical decomposition diagrams (VDDs) that express rainfall parameters in logarithmic units so that liquid water content (LWC) is a linear combination of the total number of raindrops (Nt) and the characteristic raindrop size (Dq). This formulation allows evaporation and accretion processes to be directly estimated from changes in LWC with height, and breakup and coalescence processes to be estimated from changes in Nt and Dq.

Summary

During a MC3E stratiform rain event (20 May, 2011), LWC vertical decomposition diagrams (LWC-VDDs) indicated signatures of net evaporation and net raindrop coalescence as the raindrops fell a distance of 2 km below a well-defined radar bright band. In summary, the LWC-VDD is a tool to characterize rain microphysics with quantities related to mass-modifying, number-modifying, and size-modifying processes.