Cloudy sky shortwave flux closure experiments in the tropical western Pacific

An important criteria for evaluating remote sensing retrievals of cloud microphysical properties is that the retrieved cloud properties should be consistent with the observed radiation field. A flux closure experiment is performed to analyze whether cloud microphysical properties retrieved from millimeter wave cloud radar (MMCR) and microwave radiometer (MWR) measurements at the Atmospheric Radiation Measurement (ARM) Program's site on the island of Nauru can be used to accurately predict the observed shortwave downwelling surface fluxes. Testing whether retrieved cloud properties can be used to predict the effect of clouds on surface fluxes improves understanding of the uncertainties associated with the cloud retrievals, radiative transfer models, and measurements.; A new algorithm, based on Bayes' theorem of conditional probability, is developed for the retrieval of microphysical properties of liquid clouds from MMCR and MWR observations. Tests using tropical cumulus cloud fields from a large-eddy simulation model show that the Bayesian algorithm has smaller errors in effective radius and optical depth than current retrieval methods.; In the closure experiment, nine months of cloud retrievals and aerosol optical depth measurements are input to a one-dimensional radiative transfer model, shortwave fluxes at the surface are calculated, and modeled and observed fluxes are compared. Model flux errors are analyzed as a function of cloud type, cloud fraction, and time of day. Clear sky is modeled well, but the model underestimates the observed surface fluxes for cloudy skies. A comparison of cloud amount from the MMCR dataset and a hemispheric sky imager indicates that the radar dataset predicts too much cloud, due to the Nauru island effect and the interpolation of data from the individual radar modes in the creation of the merged radar product. The variability of boundary layer clouds at Nauru cannot be fully resolved with the current configuration of the MMCR modes. The MWR is found to be insensitive to clouds with optical depths <10, although it adds useful information to retrievals for clouds with high liquid water paths. The addition of a third channel near 90 GHz to the MWR would be useful in remotely sensing shallow cumulus clouds at Nauru.