CLDMWVALIDATION

COPS - Cloud Microwave Validation Experiment in Support of CLOWD

22 June 2007 - 31 December 2007

Lead Scientist: Andrew Vogelmann

Observatory: AMF, FKB

The overarching objective is improving our understanding of existing uncertainties in the observations and spectroscopy in the microwave region. Cloud property retrievals using microwave radiometers (MWRs) offer a promising method for obtaining the needed observations of cloud liquid-water path (LWP). However, unresolved issues in the microwave radiometry for clouds cause uncertainties in retrieved cloud liquid-water paths that are presently too large when the clouds are thin (e.g., < 100 gm2), hereafter referred to as Clouds with Low Optical (Water) Depth (CLOWD). These clouds are important because, from the tropics to the Arctic, 50% or more of the liquid-water clouds have LWPs below this limit; because the Earth’s radiative energy balance is particularly sensitive to small perturbations in their LWP, small uncertainties in their optical properties can easily affect changes in the local radiative energy balance in excess of that for doubled carbon dioxide (Turner et al., Bull. Amer. Met. Soc., February, 2007). Thus, resolving such uncertainties would have a huge impact on CLOWD-related science issues (e.g., cloud radiative impacts, aerosol indirect effects). The current accuracy of MWR retrievals is limited by: 1) uncertainties in the gas spectroscopy and liquid water dielectric constants used by the microwave absorption models; and 2) the uncertainty associated with inverting the forward model (i.e., the retrieval method itself). Additionally, uncertainties in the radiometric observations (i.e., their calibration) might contribute to instrument-to-instrument differences. The key to addressing this problem is unraveling the different types of uncertainties (instrumental, state variable, microwave absorption models). To address the pressing issues, the ARM Climate Research Facility (ACRF) deployed one of the new 90/150 GHz microwave radiometers with the ARM Mobile Facility (AMF) during its 9-month observation period involved with the Convective and Orographically-induced Precipitation Study (COPS) in 2007. This deployment addressed some of the issues by taking advantage of the multiple microwave radiometer measurements that were present at this AMF deployment. The AMF had its standard MWR instruments that included its two-channel MWR, and the 12 channel MWR profiler. European instrumentation were co-sited with the AMF and included a second 90/150 GHz MWR (very similar to that purchased by ACRF), and a 14-channel profiling MWR. There was also excellent water vapor and temperature-profiling information from frequent sonde launches, and retrievals from a water vapor DIfferential Absorption Lidar (DIAL) and temperature profiles from a Raman lidar. Together, these observations offered an excellent database to address the unresolved microwave uncertainties and were further aided by additional measurements made by the AMF. The primary objectives of the proposal were to conduct calibration comparisons between the different microwave sensors, and investigate the synergies possible with all of the microwave information available. The expected benefits were: 1) Take advantage of the presence of multiple MWR instruments to intercompare the calibrations of our 90/150 GHz MWR and the 12 Channel MWR with those from like instruments, 2) Develop a single, long-term, comprehensive MWR radiometric database with complementary data on atmospheric state and cloud properties, leading to the following key objective, 3) Resolve the nature of the uncertainties that exist in the MWR absorption models by intercomparing the different microwave absorption models that are used within the field with these observations; and 4) Use the LWP obtained from the MWRs to evaluate with those obtained from the visible and infrared retrieval methods. This objective was of direct relevance to the ARM Climate Research Facility (ACRF) mission of studying and monitoring the Earth’s system, since clouds are an essential element of the Earth’s climate. Steering Committee: Andrew M. Vogelmann, Co-Chair (BNL), David D. Turner, Co-Chair (University of Wisconsin-Madison), Maria Cadeddu (ANL), Christine Chiu (JCET), Jennifer Comstock (PNNL), Jim Liljegren (ANL), Alexander Marshak (NASA Goddard)