Ice water content assessment in the single-, dual-, and triple-frequency radar scenarios

With the advent of the Global Precipitation Measurement (GPM) mission and the associated Ground Validation campaigns, there has been a strong development of studies related to dual-frequency and more recently to triple-frequency radar. In this context, one requirement is that at least one of the radar frequencies operates in the Rayleigh regime while the others have to ensure a measurable difference in reflectivities. A common radar coupling for triple frequency systems is the Ku-, Ka-, and W-band. Multi-frequency radars, in addition to the classic single-frequency reflectivity (SFR) measurement for each frequency, allow a further parameter, the dual-frequency ratio (DFR) defined as the ratio between two reflectivities at two frequencies. Referring to the same measurement volume, and for a fixed microphysical ice particle model, SFR and DFR allow to better constraint parameters of the particle size distribution, such as the mass-weighted mean diameter (Dm) and the normalized intercept parameter (Nw) when a normalized gamma distribution is assumed. This paper deals with various topics with the preliminary purpose of assessing the accuracy of the ice water content (IWC) estimate obtained using SFR and DFR methods to evaluate the improvements brought by the use of DFR. To pursue this goal, a simple microphysical model was used to choose the form of the SFR and DFR estimation algorithms and to evaluate their performances in a simulated framework. The most important aspect revealed by the study is that the cloud water content (CWC) plays a very important role both in the mass vs. diameter relationship as well as in the IWC estimation. The combined use of specific radar algorithms according to the different CWC values has shown notable improvements for the IWC estimation. Since CWC is not an operational measure, a substitute parameter was sought in the (DFRaou, DFRwoa) domain defined by the Ka- and Ku-band and by the W- and Ka-band measurements. This new parameter provides improvements similar to those obtained with the use of CWC. Data from the OLYMPEX field campaign that include an airborne triple-frequency radar at Ku-, Ka-, and W-band, as well as airborne measurements of in-situ bulk microphysics and meteorological parameters were used to validate the robustness of the methodology.