An assessment of rain “contamination” in ARM two-channel microwave radiometer measurements

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Description

Passive MicroWave Radiometers (MWRs) are the most commonly used and accurate instruments ARM has to retrieve cloud Liquid Water Path (LWP), and MWR data (brightness temperatures) are frequently combined with measurements from radar and other instruments for cloud microphysical retrievals. Unfortunately, MWR data are not easily used in precipitating conditions. The reasons for this are two-fold. First, the measurements are “contaminated” by water on the MWR radome, and second, precipitating particles are large enough to scatter (not simply emit) microwave radiation. Traditional MWR retrievals do not include scattering effects, which depend crucially on the particle size. Both wet radomes and scattering are thought to make a substantial contribution to the measured microwave brightness temperatures and typically result in retrievals of cloud liquid water and column water vapor that are biased high if these effects are ignored. While it is sometimes obvious when the contamination is large, much of the time it is difficult to know with confidence that the contribution from either a wet radome or scattering is small. At present there is no attempt to estimate or correct for these effects in ARM cloud LWP retrievals, and identification of poor LWP data is generally left to users. Typically users are advised to simply throw out all data when the MWR “wet window” resistance-based sensor indicates water is present, but this sensor is adjusted by hand and is known to be temperamental. To address the current limitations in MWR retrievals during precipitating conditions, a pair of ARM two-channel microwave radiometers was deployed to the University of Washington (UW) in Seattle between spring and fall of 2016. The radiometers were operated in a “scanning” or tip-cal mode, with one radiometer placed under a cover that prevented this radiometer’s radome from gathering water while still permitting MWR measurements away from zenith, while the other radiometer was operated normally (uncovered and with the radome exposed to the sky). We refer to these as the “covered” and “open” radiometers, respectively. The idea behind this experiment is that differences in the measurements could be used to identify and study the impact of water on the open radome, and isolate the effects of scattering from those due to having a wet radome. Further, while measurements along the off-zenith angles of the covered radiometer will still be affected by scattering, these data