Relationships between Doppler Spectral Moments within Large-Scale Cirro- and Altostratus Cloud Fields Observed by a Ground-Based 95-GHz Cloud Radar

Abstract

In this study, data obtained from measurements with a ground-based vertical-pointing 95-GHz polarimetric cloud radar are analyzed. The investigations concentrate on the relationships between the Doppler spectral moments observed in different regions of nonprecipitating cirrostratus and altostratus decks connected with warm fronts approaching the radar site. In some of these cases, a remarkably well-defined relationship between the radar reflectivity and the spectral width is found. It is demonstrated how this relationship can be used to obtain information on the size distributions and the fallspeeds of the particles in the investigated cloud sections. It is found that if single parameters of the size distributions, for example, are parameterized by a lognormal distribution, they cannot be determined with an acceptable accuracy. However, at least the changes of these parameters, such as mean particle diameter and particle concentration, with changing reflectivity as well as the behavior of the corresponding particle fallspeeds, can be described with the help of empirical relations between the Doppler moments. A main result is that significant changes in reflectivity within a cloud section (e.g., of 10 dBZe) must correspond with a change in the relation between particle size and fallspeed, most commonly described by empirical power laws, and, therefore, probably with changes in particle shapes. This kind of radar data analysis will help to come to a better understanding of the microphysical and dynamical properties of the investigated cloud types, especially if further information from simultaneous measurements with other remote sensors is available.