Microphysical properties of stratiform clouds by cloud radar measuraments

In this contribution we consider the measurements of stratiform clouds taken with a 35GHz vertically pointing Doppler radar. We retrieve their microphysical structure and their dynamics after decomposition of the Doppler spectra.

The Doppler spectra are determined by the velocity and the scattering cross section of all the particles present in the radar sampling volume. The structure of the spectra has a complexity that arises from the variegate collection of particles forming clouds: ice crystals of different habits and size, or ice crystals mixed with cloud droplets (mixed-phase clouds). Moreover, vertical air motions influence the particles fall velocity.

To decompose the spectra we assume that every class of cloud particles coexisting in the sampling volume produces a Gaussian spectrum. Thus we develop an algorithm that fits the measured spectrum with a linear superposition of Gaussian curves.

The moments of the Doppler spectrum, used to retrieve cloud properties, are then evaluated for every class of cloud particles, by associating them with the parameters of the relative Gaussian curve.

Above the melting layer the Doppler spectra profile not infrequently split up into two classes, which indicates the existence of mixed phases or separate habits of ice crystals.

The observation of these coherent multiple peak structures in the spectra profiles may be used for mixed-phase cloud studies.

Between cloud top and melting layer/cloud base, the fall velocities have amplitudes increasing with decreasing height. We evaluate the gradient of the fall velocity as a function of altitude and the evolution of this gradient with time. We find that within a cloud the fall velocity increases with an average of about 10 cm/s per km fall path.

In stratiform clouds, for which it is possible to minimize the effect of the vertical air motion, the gradient variability around the mean value shows a substantial reduction.

In order to prove if this behaviour is a consequence of ice particle growth, and to deduce which microphysical interaction processes are involved, we will compare these results with a non-operational configuration of the COSMO model making different assumption on ice particle geometry.