Path Attenuation Estimates from the GPM Dual-Frequency Precipitation Radar

An initial assessment of path-attenuation estimates from the DPR show encouraging results from the dual-frequency surface reference technique (DSRT). This tentative conclusion follows from the rain-free statistics of the normalized surface cross sections, sigma-0, and from the degree of consistency among path attenuation estimates derived from different types of reference data. However, despite generally favorable results, there are a number of questions related to the method including ways in which it might be improved and approaches by which the estimates can be directly validated.

There are two principle disadvantages to the DSRT. The first is that in heavy rain, the surface signal at Ka-band can be attenuated below the minimum detectable signal. The second is that the method provides an estimate of differential attenuation, delA=A(Ka)-A(Ku) and not the individual path attenuations. To obtain A(Ku) or A(Ka) an assumption must be made as to the attenuation ratio, p=A(Ka)/A(Ku). Results from measured raindrop size distributions show that while p=6 is a good approximation, some error will be introduced in the conversion of delA to A(Ku) or A(Ka) because of fluctuations in the DSD. Correlations between Z(Ku) and the differential specific attenuation (delk = k(Ka)-k(Ku)) suggest that the use of the Z(Ku) profile may provide a more accurate way of estimating p than by using a constant value.

Another disadvantage of the DSRT is that it is applicable only to the inner swath where both Ku- and Ka-band measurements are made. It may be possible, however, to use information from the inner swath to improve estimates in the outer swath where only Ku-band measurements are available.

One way to improve both the single- and dual-frequency surface techniques is to use a data base of previously measured rain-free surface cross sections. This takes the form of a look-up table ordered according to latitude, longitude and incidence angle. There are several ways of constructing this table; the objective is to find a choice of averaging domains that minimizes the average variance. Space-borne rain retrieval methods use a combination of the SRT and other techniques, such as the Hitschfeld-Bordan, to estimate attenuation. As a consequence, comparisons between space-borne and ground-based estimates of Z and R provide insight into the effective attenuation estimate but not the SRT alone. Using retrieval methods that use only a single method for attenuation correction should help assess the various attenuation correction methods that are available. Multiple samples of the surface, within each field of view, are obtained at off-nadir incidence angles. These data provide higher sampling of the cross-track path attenuations since multiple path attenuation estimates can be obtained at each field of view. This information has the potential to give insight into non-uniform beam-filling effect in the cross-track direction and to provide data to algorithms that attempt to mitigate the effect.