Discrimination between riming and aggregation using quasi-vertical profiles of polarimetric radar variables during MC3E

 

Author

Alexander Ryzhkov — NOAA - National Severe Storms Laboratory

Category

CAPI Deep Convective Clouds

Description

Discrimination between the microphysical processes of riming and aggregation in clouds is of great importance. Riming is associated with supercooled liquid water lofted by upward air motions, which has a strong impact on precipitation formation in ice parts of the clouds and may potentially pose an icing hazard for aviation. Two storms occurring on 27 April and 20 May 2011 during the MC3E campaign have been penetrated by the University of North Dakota Citation instrumented aircraft and closely monitored by a number of remote sensing instruments including the polarimetric C-band Scanning ARM Precipitation Radar (CSAPR) radar. A new methodology for processing and representing polarimetric radar data has been introduced which allows one to monitor microphysical processes such as aggregation, riming, and melting with unprecedented vertical resolution in a height vs time format conducive for comparisons with single-column models and observations from vertically pointing instruments. It involves azimuthal averaging of polarimetric radar variables at high antenna elevation, which produces “quasi-vertical profiles” (QVPs) of radar reflectivity Z, differential reflectivity ZDR, differential phase ΦDP, and cross-correlation coefficient ρhv with remarkable statistical accuracy. The use of the QVP methodology reveals subtle but noticeable differences in the radar signatures of aggregation and riming. It is found that riming is marked by higher Z, lower ZDR, higher ρhv just above the freezing level and by “sagging” of the melting layer. In situ aircraft probes confirm that riming occurs in the areas identified as “rimed snow” by the CSAPR radar. This is manifested by the abundance of needle crystals likely produced by the Hallett-Mossop ice multiplication process during the 27 April event and a large number of small, round particles which are classified as “rimed snow” during the 20 May event. A simple one-dimensional spectral bin microphysical model of melting snowflakes with varying degrees of riming successfully reproduces the “sagging” melting layer radar signatures observed in experiments.