Differences of Microphysical Processes in the Melting Layer Found for
Rimed and Unrimed Snowflakes Using Cloud Radar Statistics
Abstract
Understanding which microphysical processes are dominant while ice
particles pass through the melting layer is essential for precipitation
prediction by microphysics schemes and precipitation estimates by remote
sensing. Comparing the reflectivity flux at the top and bottom of the
melting layer reveals the overall effect of the microphysical processes
occurring within the melting layer on the particle population. If the
reflectivity flux increases more than expected due to the change in the
dielectric factor, growth processes dominate. In contrast, a weaker
increase in reflectivity flux indicates that shrinking processes
dominate. However, inference of growth or shrinking dominance from the
increase in reflectivity flux is only possible if other influences
(e.g., vertical wind speed) are negligible or corrected for. By
analyzing radar spectra and multi-frequency observations, we correct the
reflectivity fluxes for vertical wind speed and categorize the height
profiles by the riming degree at the melting layer top. Our statistical
analysis shows the slight dominance of growth processes for unrimed and
a clearer dominance of shrinking processes for rimed profiles. The
reflectivity flux profiles within the melting layer indicate that the
difference between unrimed and rimed profiles arises mainly in the upper
half of the melting layer, where the melting fraction increases the
strongest. We further narrow down which processes might be most
important to explain the observed signature by analyzing additional
radar variables. We suggest that whether the particle population is
overall growing or shrinking depends on the relative importance of
aggregation and collisional breakup of melting particles.