Before one can have confidence in the meaning of simulated reflectivity factor fields for interpretation of mesoscale models, it is important to understand how they are determined. The equivalent reflectivity factor is computed from the forecast mixing ratios of grid-resolved hydrometeor species, assuming Rayleigh scattering by spherical particles of known density and an exponential size distribution. During the DWFE, perceptible differences appeared in the general nature of the simulated reflectivity fields from the two WRF models, most notably a greater coverage of reflectivity below ~25 dBZ and higher maximum reflectivities in the case of the NMM compared to the ARW for winter storms. However, when attention focused on severe convective weather regions during the Spring Program, the NMM produced noticeably lower values of maximum reflectivity compared to the ARW versions, with the NMM values limited to less than 50 dBZ. These differences are mostly explained by the differences in physics packages, particularly the way various liquid water and ice species are treated in the model microphysics schemes. The WRF Single-Moment 5-class (WSM5) microphysics scheme used for the WRF-ARW model during DWFE treats the cloud condensate in the form of cloud water and cloud ice as a combined category, and precipitation in the form of rain and snow also as a combined category. The WRF-NMM used the Ferrier microphysics scheme, which accounts for four classes of hydrometeors. The most important difference between the two microphysical parameterizations concerns the assumed size distributions for snow: for the same snow mass content, differences in radar reflectivity will scale with differences in parameterized snow number concentrations between the two microphysical schemes.
It is also important to understand that it is not possible to make a strictly valid comparison between composite reflectivity computed from a model grid point and that measured by scanning radar. Owing to the fact that the radar resolution degrades with distance from the transmitter, that scanning radars cannot detect hydrometeors in the lower atmosphere due to the earth's curvature effect, and numerous other considerations (including ground clutter near the radar, anomalous propagation, etc.), any attempt to make direct comparisons between the model simulated reflectivity fields and radar measurements is replete with problems, though the NSSL product is experimenting with novel ways to overcome these problems.