Review of “Simulations of Spectral Polarimeric Variables measured in rain at W-band” by I. Tsikoudi, A. Battaglia, C. Unal, and E. Marinou.

Summary: This manuscript outlines simulations of spectral polarimetric radar observations for rain. The single-scattering properties are calculated using the T-matrix method for spheroids with aspect ratios determined from previous empirical relations. The spectra are then simulated using two methods for randomly generating radar signals, and the spectral ZDR and backscatter differential phase are computed from these signals and compared to observations. The simulated power spectra are fit using gamma PSD parameters and a parameter for the wind speed variability due to turbulence. The general shapes of the simulated polarimetric spectra are similar to the measurements; the magnitudes show substantial differences.

General comments: This manuscript provides some interesting insight into using the spectral polarimetric radar measurements to better understand rain microphysics. However, there are some issues with this study that need to be addressed before it is acceptable for publication.

The first issue I have with this study is that the fitting of the PSD parameters and the air motion variability parameter (σ_t) are only done with respect to the spectral power. Therefore, the PSD parameters controlling the width of the particle spectrum may be compensated by the σ_t to best fit the spectrum. As such, the assumed σ_t may deviate substantially from the σ_t associated with the measurements, introducing errors into the comparisons with the spectral polarimetric variables. To address this issue, the authors may want to show the sensitivity of the spectral polarimetric variables to σ_t and the PSD parameters. Additionally, showing the range of PSD parameters and values of σ_t that have similar RMSE during the fitting process would clarify how well constrained these parameters are.

Similarly, the authors mention that the spheroid shape may not adequately represent the scattering of natural raindrops due to processes such as drop oscillations. However, a variety of spheroids with the same fall speed but different aspect ratios could better represent this process and provide evidence as to whether the assumption of fixed aspect ratios for a given particle size is responsible for the poor comparisons between the simulated and measured spectral ZDR. Randomly sampling aspect ratios for particles of the same fall speed would help demonstrate whether the broadening of the spectral polarimetric variables due to aspect ratio variability produces simulated spectra that are more consistent with the measurements.

Finally, there should be more discussion of simulating spectral polarimetric variables for radars with different transmission and reception strategies. For instance, fully polarimetric radars that transmit horizontal, receive horizontal and vertical, transmit vertical, and receive horizontal and vertical are processed differently than simultaneous transmit/receive radars. These differences could also explain some of the discrepancy between the observed and simulated spectral polarimetric variables. It is unclear what the transmission and reception strategy is for the radar observations presented in the manuscript. This information needs to be included to better understand how faithfully the method for simulating the spectral radar variables emulates the processing algorithm of the radar.

Specific comments:

• Lines 24-25: Vertically pointing radar are also able to do this. Please add that radars in slant polarization mode can take advantage of polarimetric measurements.
• Line 39: Do you mean vertically profiling here? Please clarify.
• Lines 50-52: “Describing the methodology to compute spectral polarimetric variables” doesn’t really address a science question. Based on the previous line and my impression of the study, a stronger goal might be to explore how different assumptions impact the simulated spectral polarimetric variables.
• Lines 57-58: The T-matrix method can simulate the scattering properties of arbitrary shaped particles (as long as the numerical integration converges; Wriedt 2002). However, these codes are not widely available and are much less efficient. Please change this sentence accordingly.
• Lines 74-75: Does equation (1) come from one of these studies specifically? Please clarify.
• Line 144: Please add some reference(s) for these equations.
• Line 157: Where does this equation come from? Please address.
• Line 158: Shouldn't this denominator be in terms of d v_los instead of d v_t? What if the horizontal wind is large compared to the fall velocities or the radar has an elevation angle near 0? If this equation is an approximation, please indicate under what conditions it is valid.
• Line 178: It is a bit unclear why these two methods for calculating the spectra are being discussed. Are they the only two such methods? Please add some brief discussion on this point at the end of the previous section.
• Lines 240-241: What is the transmission and reception strategy of this radar? Can it measure retrieved co-polar signals independently?
• Line 257: I think the “e” is missing from the subscript on the elevation angle symbol.
• Line 264: Please be more specific about the degree of turbulence during this case and the following case. Are these cases on the extreme ends of the turbulence that might be observed during these such events?
• Lines 278-279: How were they adjusted? According to the measured values of the smallest particles? Please clarify.
• 7: What does the gray shading on the plot represent? Please add this information to the figure caption.

References:

Wriedt, T. (2002), Using the T-Matrix Method for Light Scattering Computations by Non-axisymmetric Particles: Superellipsoids and Realistically Shaped Particles. Part. Part. Syst. Charact., 19: 256-268. https://doi.org/10.1002/1521-4117(200208)19:4<256::AID-PPSC256>3.0.CO;2-8

Please find my comments in the attached pdf