| << | Profiling of Water Vapour and Temperature | >> |
Abstract
The monitoring of low humidity levels at dry and cold locations like Antarctica is of interest for the modelling of earth’s climatology. Global warming leads to increased concentrations of atmospheric water vapour and the strongest gradients can be measured in polar areas where warming effects are more dramatic than elsewhere. Accurate cloud liquid measurements are essential for the understanding of cloud physics and the atmosphere’s water cycle. LWP measurements with reasonable accuracy under raining conditions are complicated due to the presence of a mixture of Rayleigh and Mie scattering hydro-meteors. Polarized microwave observations offer the possibility to distinguish between cloud and rain liquid, and therefore to determine the total amount of liquid water more precisely.
This contribution is a description of design concepts and built instruments for the described applications, as well as methods for improving the water vapour profile vertical resolution. The combination of a 183 GHz, 6 channel DSB filter bank system with a 50-60 GHz temperature profiler turns out to be an ideal tool to resolve the vertical humidity distribution in particular for the boundary layer with IWVs of less than 4 mm. The high time resolution of the radiometer system allows for monitoring of rapid WV changes. The applied boundary layer scanning method resolves the low level extreme temperature inversions present in Antarctica. Inversions of >20 K over 200 m vertical extension are common and their detection requires a high spatial resolution of the antenna beams to be able to scan at low elevation angles down to 5°.
Polarized instruments cannot use scanning mirrors for elevation scanning because the mirror changes the polarisation planes at different reflection angles. Consequently, the radiometers together with their antennas have to be rotated in elevation. In this configuration, external calibration targets for automatic calibration procedures are not practical, at least at frequencies < 30 GHz. Internal auto-calibration systems have been designed to achieve the required radiometric long term stability. Allan variance tests are presented to demonstrate the performance of this calibration approach.