Improving Supercooled Liquid Water Absorption Models in the Microwave Using Multi-Wavelength Ground-based Observations

Accurate liquid water path (LWP) measurements are critical for a large number of atmospheric research topics. The most commonly used method to derive LWP is to retrieve it from sky brightness temperature data observed by microwave radiometers (MWRs). However, this measurement approach requires an accurate radiative transfer model that has the strength of the liquid water absorption well characterized; biases in the strength of the absorption in the model will result in biases in the retrieved LWP. However, the accuracy of liquid water absorption models in the microwave for supercooled liquid (i.e., liquid water less than 0 deg C) is highly uncertain due to the paucity of accurate laboratory measurements at different microwave frequencies over a range of supercooled temperatures. Thus, commonly used absorption models can result in differences in LWP that range from -40% to +40% at -30 deg C if only frequencies below 35 GHz are used; if higher frequencies (such as 90 GHz which have enhanced sensitivity to low amounts of LWP) are used, then the differences can be as large as +70%. Three microwave radiometer datasets that span the frequency range from 23 to 225 GHz have been compiled that include a large number of cases from temperatures from 0 to -30 deg C. These datasets include the AMF deployment to the Black Forest in Germany, a dataset from the Zugspitze (the highest peak in Germany), and from Summit Station in central Greenland. These observations demonstrate that none of the current microwave absorption models properly predict the absorption at all microwave frequencies across the super cooled temperature range. These field experiment datasets, together with original laboratory data, have been used along with a variational approach to derive a new liquid water absorption model. The use of the variational approach allows the uncertainties in the model coefficients to be estimated, and thus provides an estimate of the uncertainty in the liquid water absorption coefficient for different frequencies and temperatures.