Evaluating Measurement of Supercooled Liquid Water Clouds
The retrieval of cloud liquid water is an important climatic measurement because the earth radiative balance is strongly affected by cloud cover. The microwave radiometer, which is deemed to be the primary instrument for making these measurements, uses radiative transfer models to determine this property. Because of the propensity of clouds with small liquid water path (LWP) amounts and the importance of these clouds, the research community has recently started using higher frequencies, in the 90-200 GHz region. These frequencies have higher sensitivity to small amounts of cloud liquid water and, thereby, can improve the retrieval of liquid water by reducing the random uncertainties in the current LWP retrievals. Thus the accuracy of the radiative transfer code in the spectral region above 90 GHz was a major research focus. Scientists used measurements from microwave radiometers at the Southern Great Plains and North Slope of Alaska sites to assess four radiative transfer models. An ancillary dataset of measurements from cloud radars, ceilometers, radiosondes, and atmospheric emitted radiance interferometers was used to derive additional information (such as cloud boundaries, cloud temperature, and cloud LWP) that could be used as input for the analysis of the code. The study compared measurements of microwave absorption with model computations in supercooled liquid clouds that have temperatures between 0°C and -30°C. Findings from this study will be implemented by the Atmospheric Radiation Measurement Climate Research Facility to improve the accuracy of these measurements.
Cadeddu, M. P. and D. D. Turner. 2011. “Evaluation of Water Permittivity Models from Ground-Based Observations of Cold Clouds at Frequencies Between 23 and 170 GHz,” IEEE Transactions on Geoscience and Remote Sensing 49(8), 2999-3008. DOI: 10.1109/TGRS.2011.2121074.