Z to R relationships for snowfall-events in Austria, determined by modelling winterly precipitation particles realistically

Radar measurements of precipitation critically depend on the relation between radar reflectivity factor Z and precipitation rate R. The relationship has the general form Z = a R^b, where a and b are empirical constants. A great many of such relationships with varying parameters exist for different regions and precipitation types. In the present study the Z-R-relationships of several snowfall-events in the mountains of Austria were determined by realistically modelling a variety of mixed-phase precipitation particles and rigorously calculating their radar cross sections at 5.6 GHz frequency. The particles' distribution was measured by means of the 2D-Video-Distrometer (2DVD), a ground based imaging precipitation gauge. The 2DVD provides time, size, shape and falling velocity information of every single precipitation particle that falls through its 100 cm² sensing area. The whole spectrum of recorded particles was discretised in up to ten particle-size-classes with corresponding quantity. Particles of each class that tolerably represent the average radar cross section (RCS) of all particles within that class were realistically modelled in a CAD-tool and their RCSs were determined by means of the Finite Element Method. Together with the quantity information of each particle-class within a volume the reflectivity Z was determined. Because an imaging distrometer provides only sparse information on the inner structure of winterly mixed-phase precipitation particles and, as a consequence, on the precipitation rate, computational algorithms and a co-sited heated tipping bucked rain gauge were used to determine the equivalent liquid precipitation rate from data provided by the 2DVD. For all snowfall-events considered the particles' distributions were steadily re-calculated after several minutes, each of them providing a Z-R-pair as input to the a-b-regression analysis. Finally for all precipitation events the parameters a and b were identified, finding strong distinctions between different types of frozen precipitation. Also estimates for the reflectivity under the assumption of errors of the tipping bucked rain gauge (due to evaporation of melted snow flakes) have been determined.