Assessing the potential of frequencies around 205 GHz for improving snowfall estimation.

Snow, on the ground and falling to the surface, plays an important part in the global water cycle, yet the measurement of it remains problematic. The estimation of falling snow (or frozen precipitation) is difficult and relies upon the interpretation of observations from passive microwave sensors. However, the relationship(s) between the passive microwave observations and falling snow at/near the surface is very much dependent upon the type of snow (ice) crystals present which vary greatly between different precipitating weather systems. To date, much work has concentrated observations from current sensors using the 150/166 GHz and the 183.31 GHz water vapour channels to extract the scattering signature associated with frozen hydrometeors. However, the TROPICS pathfinder mission, launched in June 2021 into a near polar orbit, carries a new radiometer that provides an opportunity to assess the impact of including observations at 204.8 GHz. Such measurements are more sensitive to ice particles, resulting in a greater scattering signature, while being less sensitive to the water vapour around the 183.31 GHz region. Preliminary results are encouraging and will be presented here. A number of case studies, including lake-effect snowfall (November 2022) and widespread snow across North America (mid-December 2022), will be explored in detail. These initial results show observations at 204.8 GHz provide additional information that improves snow delineation and estimation: this is of great significance for the upcoming EUMETSAT EPS-SG missions.