| 21 Aug 2025
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 (σr) 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 σr ∝ 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 Dm > 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.
This ambitious article investigates the feasibility of retrieving snow ice water content and precipitation rates using vertically-pointed G-band radars. Often, centimeter-wavelength radars (S-band, C-band, X-band, Ku-band, etc.) are used for radar retrievals, especially for operational purposes. The authors however find that due to non-Rayleigh scattering effects, ice water content (IWC) and snowfall rate (S) retrievals are expected to vary considerably less at G-band than for these typical centimeter wavelength radars and even other millimeter radars (e.g., Ka- and W-) that are commonly used for ice retrievals – at least for large enough particles. The authors compare the computationally simpler Rayleigh Gans Approximation (RGA) for a number of particles from the ARTS database to the accurate and computationally rigorous Discrete Dipole Approximation (DDA) calculations for the same particles at G-band wavelengths. The authors use these simulations to justify the theoretical power-law scaling of non-Rayleigh scattering that they then use in the rest of the manuscript to derive retrieval equations; this power-law scaling of the parameter ‘f’ acts as a moment-based integration kernel when calculating IWC and S as well as other moment-based parameters. Overall, the authors show this scaling parameter ‘f’ leads to IWC and S being directly proportional to G-band Z and Z*MDV. For suitably sized aggregates, many of the parameters in the IWC and S equations can be treated as constants. The authors use simulations to determine and justify appropriate constant numbers for different types of particles. The authors include another set of numerical experiments where they vary the particle size distribution shapes for various mass-weighted diameters (Dm) and they find that there is only slight variability in results for typical Dm values of snow found near the surface. Finally, the authors utilize G-band data from real snow cases in the United Kingdom where they retrieve IWC and S and then statistically compare results to ground and in-situ measurements.
I found the manuscript to be exceptionally straightforward and easy to read. I thought the experiments were sensible and that the authors took good care of incorporating additional factors such as the impact of attenuation. The biggest limitation of this study really is whether the errors and limitations introduced when utilizing G-band radars are truly worth the benefits provided by the theoretically more accurate retrievals in a more practical sense. Overall, I’d like to see a more thorough discussion on the practical aspects and limitations of using vertically-pointed G-band compared to other radar wavelengths. I’m also wondering what the authors’ beliefs are regarding how G-band radars should or could be used; should these radars be used only in field campaigns or should they be deployed operationally? Also, I believe the authors should provide rough estimates of expected errors from G-band retrievals compared to similar errors from centimeter and maybe millimeter radars in order for readers to fully appreciate the benefits of utilizing G-band radars. Therefore, I recommend minor revisions.
Major Concerns:
Minor Concerns:
Suggestions/Typos/etc.:
Lines 6-7: The sentence should read: "This presents the opportunity for straightforward and accurate retrievals of ice microphysics."
“Straightforward” and “accurate” retrievals are both positive attributes.
Line 59: It is more appropriate to say that Z and IWC are proportional to PSD moments rather than they are the moments themselves.
Line 117: There should be a space between 2 and mm.
Line 177: Inconsistent usage of figure reference. This should probably say “Figs 1 and 2” similar to line 220 rather than spelling out "figure."