Studies of snow growth processes using Polarimetric and Doppler radars in Finland

Sabine Goeke

Division of Atmospheric Sciences and Geophysics, Department of Physics,

University of Helsinki, Helsinki, Finland

Deducing bulk hydrometeor types and amounts from polarimetric radar data requires a thorough understanding of radar signatures of specific hydrometeor types and information about hydrometeor size distributions and characteristics. To achieve this goal, combined remote sensing and in situ measurements of dry and melting snow have been conducted since 2007 in the vicinity of Helsinki. During snowfall events, photographs of hydrometeors collected on black velvet were used to classify snow as single crystals, aggregates or rimed particles. For these three categories we obtained thresholds for polarimetric variables valid for C-band radars. Further we found that size distributions of aggregates are rather flat and wide while distributions of rimed particles are steeper. Estimated bulk densities of crystals are higher compared to densities of rimed particles or aggregates.

At large distances, weather radars sample hydrometeors well above the ground and certain assumptions are required to project the results of measurements aloft to the surface. We found that different growth processes like aggregation or riming aloft in the “ice only” layer influence the size distributions of rainfall below the melting layer. Whereas vertical profiles of polarimetric signatures in the “ice only” layer are influenced by advection and may give little information about the prevailing growth processes, vertical profiles through the melting layer on the contrary can be quite revealing of some microphysical processes that occurred above the bright band. For instance the relative intensity of the bright band (maximum reflectivity in the melting layer minus reflectivity of rain directly below the melting layer) varies noticeably with the degree of riming, and thus is an indicator whether riming or rather aggregation was the dominant growth mechanism in the “ice only” layer.