Microwave absorption of supercooled clouds and implications for the dielectric properties of water

The absorption of liquid clouds was determined in the frequency range from 21 to 31 GHz and over the temperature range from 246 to 279 K. The information was derived from continuous surface-based microwave and infrared radiometer observations at Bern, Switzerland, from 2006 to 2010 and using satellite data for cloud-top temperature. The results indicate that common dielectric models of liquid water are inaccurate at temperatures below 265 K, possibly owing to the poor representation of the main relaxation frequency of water. The best agreement with our observations is found for a model published in report form in 1995. Part of the scatter in the temperature dependence of the analyzed data can be explained by correlated fluctuations in water vapor and liquid water. The results are relevant for remote sensing of supercooled clouds using passive and active microwave techniques and, more generally, for the physics of water.