Abstract
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.
Highlights
Radar-based quantitative precipitation estimation (QPE) is a challenging task
The T-matrix method (TMM) and discrete dipole approximation (DDA) results can provide some microphysical insights into the considered snowfall events
Since the ice particle mass growth rate due to riming is proportional to liquid water path (LWP) along the particle fall trajectory, the LWP can be seen as a proxy for riming
Summary
To derive a relation between radar observables and precipitation rate knowledge of the particle size distribution (PSD) is required. This problem is compounded by the uncertainty in ice particle microphysical and microwave scattering properties. The relation between equivalent reflectivity factor, Ze, and snowfall intensity, S, is usually assumed to follow a powerlaw form defined by two parameters, i.e. the prefactor a and exponent b. These parameters have been derived for weather radars operating in the centimeter wavelength range, either
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