Assessing radar reflectivity retrieval methods with in-situ observations of cloud hydrometeor spectra

In order to infer cloud properties from ground and satellite radar measurements, the retrieval methods assume a relationship of radar reflectivity to particle size distributions and shape. Radar reflectivity is proportional to a power of the instrumentally determined particle size. Values of this power ranging between 3.8 and 4.8 are described in the literature for ice clouds. This implies that a few large particles can dominate the radar reflectivity. For example, it has been shown that more than 50% of the cumulative reflectivity calculated from project average spectra for ice clouds comes from particles with sizes between 6.4 and 12.8 mm. It is therefore very important to accurately characterize this part of the spectrum. In-situ aircraft measurements provide a means for validating and improving the retrieval methods. The goals of this study were to 1) examine recorded particle size spectra to determine how often and under what conditions radar reflectivity is contributed by large particles >12.8 mm, and then 2) estimate the contribution to reflectivity from large particles by fitting a gamma distribution to the spectra and extrapolating beyond the largest measured particle size. Data from 97 research flights collected during four field projects were examined. The projects are: CFDE1 (Maritime environment), CFDE3 (Continental environment), FIRE.ACE (Arctic environment) and AIRS1 (Continental environment). The data set represents 34,000 km of in-cloud data collected at temperatures <=0o C. Particle concentrations and dimensions were measured with three PMS 2D probes: a 2D-C 25-800 micron probe, a 2D-Grey 25-1600 micron probe and a 2D-P 200-6400 micron probe. Data were averaged over 30 second intervals and assessed as liquid, mixed or glaciated phase. For each interval the particle spectrum up to 12.8 mm was determined using the center-in method and the corresponding reflectivity spectrum was derived. Earlier work used two simple indicators (the slope of the reflectivity spectrum and the diameter below which 99% of the cumulative reflectivity occurs) to determine that 2% to 20% (depending on the field project) of 300 second records measured with the 2DP may be underestimating some reflectivity because of the presence of particles too large to be accurately recorded. A gamma distribution was fitted to the large particle tail of each 30sec observed particle size spectra. The missed reflectivity was calculated by extrapolating the gamma distribution beyond the largest observed particle size. Preliminary results suggest that although up to 15% of spectra are missing some reflectivity, significant missed reflectivity (> 5db) occurs rarely, in less than 1% of measured in-cloud observations. The dependence of these results on temperature range, cloud phase and averaging time interval will be discussed.