Snowfall retrieval at X, Ka and W bands: consistency of backscattering and microphysical properties using BAECC ground-based measurements
Abstract. Radar-based snowfall intensity retrieval is investigated at
centimeter and millimeter wavelengths using co-located
ground-based multi-frequency radar and video-disdrometer observations. Using data from four snowfall events, recorded
during the Biogenic Aerosols Effects on Clouds and Climate (BAECC) campaign in Finland, measurements of
liquid-water-equivalent snowfall rate  S are correlated to radar equivalent reflectivity factors  Ze ,
measured by the Atmospheric Radiation Measurement (ARM) cloud radars operating at X, Ka and W frequency bands. From these
combined observations, power-law Ze – S relationships are derived for all three frequencies considering
the influence of riming. Using microwave radiometer observations of liquid water path, the measured precipitation is
divided into lightly, moderately and
heavily rimed snow. Interestingly lightly rimed snow events show a spectrally distinct
signature of Ze – S with respect to moderately or heavily rimed snow cases. In order to understand the
connection between snowflake microphysical and multi-frequency backscattering properties, numerical simulations are
performed by using the particle size distribution provided by the in situ video disdrometer and retrieved ice particle
masses. The latter are carried out by using both the T-matrix method (TMM) applied to soft-spheroid particle models
with different aspect ratios and exploiting a pre-computed discrete dipole approximation (DDA) database for rimed
aggregates. Based on the presented results, it is concluded that the soft-spheroid approximation can be adopted to
explain the observed multi-frequency Ze – S relations if a proper spheroid aspect ratio is selected. The
latter may depend on the degree of riming in snowfall. A further analysis of the backscattering simulations reveals that
TMM cross sections are higher than the DDA ones for small ice particles, but lower for larger particles. The differences of computed cross sections for larger and smaller particles are compensating for each other. This may explain why the soft-spheroid approximation is satisfactory for radar reflectivity simulations under study.