Evaluation of uncertainties affecting the retrievals of cloud liquid water using microwave frequencies at 90 and 150 GHz

Description

The Atmospheric Radiation Measurement (ARM) Climate Research Facility is in the process of deploying new microwave radiometers that use frequencies at 90 and 150 GHz to improve the retrieval of cloud liquid water path. One of these radiometers has been operating at the Southern Great Plains site for one year, and the new datastream is about to be released to the archives. A considerable effort has been devoted to the development of reliable calibration procedures and to the improvement of absorption models at these frequencies. Accurate calibration of the instrument and modeling are essential elements to ensure the highest accuracy of LWP retrievals. We present the new datastream, evaluate the calibration of the instrument, and compare model computations with observations. With the help of infrared measurements we retrieve cloud liquid water path for several cases of thin clouds, and we use the IR-derived LWP as input to a radiative transfer code to compute microwave brightness temperatures with four different cloud absorption models. The focus of the study is to analyze the model behavior for cases of very cold clouds (T< -10° C) where the microwave absorption models are known to suffer from the lack of laboratory measurements and where the largest discrepancies between the models are expected. Modeled brightness temperatures are then compared to observations, and the results are used to infer the accuracy of the models and the level of uncertainty that can be expected in the retrievals. Examples of liquid water path retrievals from the new instruments are provided and compared to existing LWP retrievals from the ARM two-channel microwave radiometers (MWR).