Estimating Ice Water Content and Snowfall Rate from radar measurements in the G-band

Abstract. We present theory and simulations to show that at frequencies of order 200 GHz (G-band) the radar cross section (σ_r) of ice particles larger than ∼ a quarter wavelength (0.375 mm) is nearly directly proportional to their mass (m), and hence measurements of radar reflectivity (Z) at this frequency are directly proportional to the ice water content (IWC), with no other assumptions about the shape or breadth of the particle size distribution required. For the same reason, vertically pointing Doppler velocities at this frequency provide the mass-weighted mean vertical velocity of the particles, and the product of Z with the mean Doppler velocity (MDV) is proportional to the snowfall rate (S). This presents the opportunity for straightforward but accurate retrievals of ice microphysics.

We explore the sensitivity of such retrievals to the scattering model for ice particles. We find that all seven models examined, four with random orientation and three with horizontal orientation, have σ_r ∝ m in this regime, but that the coefficient of proportionality varies between models. The dominant factor controlling this coefficient is the mass-size relationship for the ice particles, and specifically the mass of a wavelength-sized ice particle. If this information is known, or can be assumed, then the ice population parameters above can be retrieved with high accuracy. For mass-weighted mean diameters D_m > 0.5 mm the variation in the IWC-Z relationship is within ≈ 30 %, and the variation in the S − (Z × MDV) relationship is within ≈ 15 %.

The method is applied to retrieve IWC and S during two case studies, with measurements from the GRaCE 200 GHz Doppler radar at Chilbolton Observatory in the UK. In the first of these case studies, retrieved snowfall rates from particles falling aloft in a precipitating ice cloud were compared to gauge data at the surface. In the second case study, retrieved ice water contents from a deep non-precipitating stratiform ice cloud were compared to measurements made using an evaporative water content probe on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 instrumented research aircraft. In both cases a statistical comparison was necessary because of imperfect colocation of the radar measurements and in-situ/gauge sampling. The retrievals fall within the distributions from the retrieved water content and snowfall fields, and follow consistent trends with time (Case 1) and height (Case 2), providing evidence that this method produces realistic retrievals.

Application of the same technique at even higher radar frequencies would allow clouds with smaller particles (e.g. in high altitude cirrus clouds) to be characterised. Because of the increased gaseous attenuation at such frequencies, the latter may be more practical from airborne or spaceborne platforms.