Snow particle orientation observed by ground‐based microwave radiometry

Abstract

This study analyzes the effect of snow particle orientation on polarization differences (PDs) observed with a ground‐based radiometer at 150 GHz on the basis of a 1 year time series of snowfall observations. The observations performed on Mount Zugspitze (German Alps) at 2650 meters above sea level reveal that brightness temperature (TB) differences between vertical and horizontal polarizations reach up to −10 K at an elevation angle of 34.8° during snowfall. The analysis of 458 h of snowfall data shows that PDs can be explained by the occurrence of oriented snow particles and supports the potential role of polarization measurements for improved retrievals of snowfall microphysical parameters. The dependence of measured PD and TB on integrated K band radar reflectivity (at 24.1 GHz) and independently derived cloud liquid water path (LWP) has been analyzed. The higher snow water path (SWP) indicated by high values of integrated reflectivity enhances both TB and PD because of the scattering of snow particles. TBs are also found to increase during snowfall when supercooled liquid water is present. The increase of LWP enhances the TB but damps the PD resulting from oriented snow particles. To evaluate the effects of SWP and LWP on PD and TB, radiative transfer simulations assuming horizontally aligned snow oblates using a radiative transfer model have been carried out. PD and TB observations can be captured well by the model given realistic assumptions on mass size relationship and aspect ratio of the oriented snow oblates.