Exponential size distributions for snow

Using airborne data from several recent field projects, the authors have taken another look at the properties of exponential ice particle size distributions (PSDs) when the PSDs are broad. Two primary questions are addressed: for what ranges of ice water content (IWC) and equivalent radar reflectivity (Ze) do exponentials produce accurate estimates of these higher moments of the PSD, and why is there a lower limit to the value to the slope of exponential fits to PSD, λ, as has been found from airborne measurements? Earlier studies at temperatures primarily above -10°C have found that λ measured in snow during spiral descents through deep ice cloud layers decreases to about 9 cm⁻¹ and then remains there. Several physical processes have been advanced to explain these observations. If reliable, the data could be used to improve retrieval of ice cloud properties through remote sensing and for cloud model representations of ice cloud microphysical properties. For data acquired from 2D probes, recent evidence indicates shattering of large ice particles ahead of, but attributable to, the probe's sensing area, generating small crystals that the probe then senses. Shattered artifacts have been objectively removed from the data. Comparisons of size distributions before and after removal of suspected shattered particles suggest that the reported minimum may have been due to shattering and/or other instrument errors. The revised PSDs indicate that for λ < 40 cm⁻¹, 0.1 g m⁻¹ < IWC, and 5 dB < Ze, moments two (IWC) through four (Ze) of the PSD are dominated by particles larger than a few hundred microns. Analytical representations with more variables than exponentials (e.g., gamma PSD) are not required to accurately derive these moments from the PSD. In these situations, the intercept parameter of the exponential PSD, N0 ≈ 1 cm⁻⁴, is 5 to 30 times larger than assumed earlier.