Relations between the reflectivity Z and the ice water content IWC or the reflectivity Z and the snow rate S are needed both for direct measurements of snow (if it snows at the ground) or measurements of rain at the ranges where the radar beam is above the melting layer. Of course, this is of great importance for NWP modeling of ice aloft and assimilation of radar data. To derive reliable Z-IWC relations, however, is a challenging taks, because they strongly dependent on the snow habit. Thus, classification and quantification of ice above the freezing level exploiting so-called quasi-vertical profiles (QVPs) is a new upcoming research topic within the research group. First, a climatology is needed for a classification and quantification of ice. Based on the classifications derived, different Z - IWC or Z - S relations can be applied. For the classification, we aim at 4 classes of ice aloft:
Towards this classification, we start with the investigation of different polarimetric signatures aloft:
In Figure 1 ongoing riming processes are nicely visible. ZDR decreases and Z increases below the layer of dendritic growth. Compared to aggregation processes, ZDR is in most cases 0.1- 0.3 dB lower above the freezing level in case of riming (Ryzhkov et al. 2016).Another indication of riming is a sagging of the melting layer. The sagging is caused by the higher fall velocity of rimed particles, thus rimed particles completely melt at lower altitudes compared to non-rimed snow.
Figure: QVPs of the polarimetric variables ZH, ZDR, Rhv, and PHIdp measured with BoXPol at elevation 28° on 6 May 2014. Overlaid are contours of Z (thin lines) and wet bulb temperature retrieved from COMSO-DE (thick lines).