Evaluation of cloud liquid absorption models at 90 and 150 GHz

The use of microwave observations in the frequency range of 90-150 GHz holds the promise to dramatically improve the retrieval of integrated liquid water in thin clouds due to the increased sensitivity of these frequencies to the presence of liquid water. One of the largest sources of retrieval uncertainty at these frequencies is the inaccuracy of liquid water absorption models at low temperatures. The purpose of this work is to assess the performance of four liquid water absorption models by comparing model simulations with measurements collected by a microwave high-frequency radiometer (MWRHF) located at the Atmospheric Radiation Measurement (ARM) Program Climate Research Facility (ACRF) in the Southern Great Plains, OK. Measurements at 90 and 150 GHz were collected during cloudy and cold conditions. In an attempt to isolate different sources of possible errors we give an assessment of the instrument calibration and compare clear-sky measurements with model simulations using an updated formulation for the water vapor continuum. With the help of a ceilometer and cloud radar we identified several cases of cold, thin liquid clouds and for those cases we independently retrieved liquid water path using infrared frequencies. The independently retrieved liquid water path was then used to compute brightness temperatures at 90 and 150 GHz with the four liquid absorption formulations. Simulation results were then analyzed and compared with observations.