Towards an improved estimation of cloud boundaries from combined 95 GHz radar, ceilometer, and microwave radiometer measurements

An accurate geometrical cloud information like the height of the cloud base and top as well as number of layers is an essential for assessing the influence of clouds on global climate and related feedback mechanisms.

Within the EU project CLIWA-NET (Cloud LIquid WAter NETwork) a network of ground-based observation sites are combined with satellite measurements to deduce highly accurate fields of the cloud liquid water path (LWP) for the entire BALTEX (Balitc sea EXperiment) area. The results will then be used for evaluation and improvement of the parametrisation of clouds, where special focus is on pure liquid water clouds, i.e. comparably low and geometrically thin clouds.

During the first CLIWA-NET observation period, which took place from 1 August 2000 to 30 September 2000, the GKSS Research Center in Geesthacht (Germany) hosted a ground-based site. The instrumentation consisted of a 95 GHz cloud radar, a Vaisala laser ceilometer, and a 22 channel microwave radiometer.

In principle, the two active instruments supply information about the vertical distribution of the hydrometeors. Scattering by cloud particles is subject to different mechanisms at radar and lidar wavelength, respectively. While the radar signal is dominated by size of particles, the lidar signal is governed by their number. Thus, a few large particles falling out from the cloud base (drizzle) can cause non-negligible radar signals, but contribute virtually nothing to the liquid water path. Because of those 'drizzle signals', the radar-estimated cloud base is often distinctively below the cloud base height as derived at optical wavelength, i.e. from ceilometer measurements. It should be noted that the difference between the instruments is dependent on the specified sensitivity.

The radiometer provides a quantitative estimation of path integrated liquid water content which can be used to estimate the attenuation of the radar signal within the cloud. Such information is required to indicate, whether the radar signal detects the cloud top correctly or is attenuated within the cloud before reaching the top.

For the clouds or cloud elements (in case of attenuation) not detected by the radar, an estimated maximum corresponding LWC is given.