Retrievals of ice microphysical properties using dual-wavelength polarimetric radar observations during stratiform precipitation events
Authors: Tetoni Eleni , Ewald Florian , Hagen Martin , Köcher Gregor , Zinner Tobias , Groß Silke 
Published: July 2022

Abstract. Ice growth processes within clouds affect the type and amount of precipitation. Hence, the importance of an accurate
representation of ice microphysics in numerical weather and numerical
climate models has been confirmed by several studies. To better constrain
ice processes in models, we need to study ice cloud regions before and
during monitored precipitation events. For this purpose, two radar
instruments facing each other were used to collect complementary
measurements. The C-band POLDIRAD weather radar from the German Aerospace
Center (DLR) in Oberpfaffenhofen and the Ka-band MIRA-35 cloud radar from the
Ludwig Maximilians University of Munich (LMU) were used to monitor
stratiform precipitation in the vertical cross-sectional area between the two
instruments. The logarithmic difference of radar reflectivities at two
different wavelengths (54.5 and 8.5 mm), known as the dual-wavelength ratio, was
exploited to provide information about the size of the detected ice
hydrometeors, taking advantage of the different scattering behavior in the
Rayleigh and Mie regime. Along with the dual-wavelength ratio, differential
radar reflectivity measurements from POLDIRAD provided information about the
apparent shape of the detected ice hydrometeors. Scattering simulations
using the T-matrix method were performed for oblate and horizontally aligned
prolate ice spheroids of varying shape and size using a realistic particle
size distribution and a well-established mass–size relationship. The
combination of dual-wavelength ratio, radar reflectivity, and differential
radar reflectivity measurements as well as scattering simulations was used
for the development of a novel retrieval for ice cloud microphysics. The
development of the retrieval scheme also comprised a method to estimate the
hydrometeor attenuation in both radar bands. To demonstrate this approach, a
feasibility study was conducted on three stratiform snow events which were
monitored over Munich in January 2019. The ice retrieval can provide ice
particle shape, size, and mass information which is in line with differential
radar reflectivity, dual-wavelength ratio, and radar reflectivity
observations, respectively, when the ice spheroids are assumed to be oblates
and to follow the mass–size relation of aggregates. When combining two
spatially separated radars to retrieve ice microphysics, the beam width
mismatch can locally lead to significant uncertainties. However, the
calibration uncertainty is found to cause the largest bias for the averaged
retrieved size and mass. Moreover, the shape assumption is found to be
equally important to the calibration uncertainty for the retrieved size,
while it is less important than the calibration uncertainty for the
retrieval of ice mass. A further finding is the importance of the
differential radar reflectivity for the particle size retrieval directly
above the MIRA-35 cloud radar. Especially for that observation geometry, the
simultaneous slantwise observation from the polarimetric weather radar
POLDIRAD can reduce ambiguities in retrieval of the ice particle size by
constraining the ice particle shape.