Frozen hydrometeor fall velocity dependence on riming and the particle type
Matrosov, Sergey — University of Colorado
Area of research
Hydrometeor terminal fall velocity information is important for weather and climate studies as it influences cloud life times and the water cycle balance, and it also affects the vertical transport of moisture in the atmosphere. Additionally, the knowledge of these velocities is important for development of quantitative precipitation estimation methods for snowfall. Vertically pointing Doppler radars allow measurements of hydrometeor fall velocities while concurrent scanning polarimetric radars and/or in situ observations can be used for identifying particle type/shape and a degree of riming.
Hydrometeor fall velocity-radar reflectivity relations, which were derived in this study for different ice particle types, can be used in model parameterizations. These relations were derived for frozen hydrometeor species, which are typically considered in different weather and climate models. They include such frozen hydrometeor types as single pristine crystals, snow aggregates, and heavily rimed particles such as graupel.
Measurements from the vertically pointing and scanning polarimetric 35 GHz cloud radars at Oliktok Point, Alaska and the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) were used to derive power-law fall velocity (V)-reflectivity (Z) relations for frozen hydrometeor populations of different types observed in ice clouds and solid precipitation. The prefactor a and the exponent b in the observed V=aZ^b relations scaled to the sea level vary in the approximate ranges 0.5–1.4 and 0.03–0.13, respectively (reflectivities are in mm^6 m^-3 and velocities are in meters/second.
The coefficient a values are the smallest for single planar crystals (a ~ 0.5) and the largest (a ~ 1.2) for particles under severe riming conditions (e.g., graupel). Correspondences among coefficients in the V–Z relations for particle populations and those in the individual particle fall velocity-size relations were also analyzed. These correspondences and the observed V–Z relations can be used for evaluating different frozen hydrometeor fall velocity parameterizations in models. There is a general consistency of the results obtained from different arctic locations.