Radiation observations are a key element in the evaluation of the 40-yr reanalysis at the European Centre for Medium-Range Weather Forecasts. This paper uses the High-Resolution Infrared Radiation Sounder/2 (HIRS/ 2) and Microwave Sounding Unit (MSU) observations on board the National Oceanic and Atmospheric Administration satellites, to assess the characteristics of the cloud fields produced by the forecasting system over midlatitude and tropical oceans. Infrared and microwave radiation have different sensitivities to clouds and are therefore complementary. Observed and model-generated radiances, as well as HIRS/2-derived cloud parameters, are compared. The model clouds are shown to be well distributed, with realistic seasonal cycles. However, deficiencies are identified and discussed: the cloud radiative impact may be too low in the midlatitudes, the frequency of occurrence of high clouds is overestimated in the intertropical convergence zone, and the stratocumulus off the west coast of the continents is underestimated. The methods described here provide a framework for assessing the impact of forthcoming improvements to the cloud scheme.

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An analytical solution to the two-flow equations developed by Bostater, et al.' is modified, and a sensitivity analysis is perfonned on the remote sensing model which contains both diffuse and specular light components. The resulting model simulates the solution to two different cases ofthe two-flow equations. The Case I model uses the two-flow irradiance equations where sub-surface collimated or specular irradiance is evaluated implicitly. All ofthe irradiance is assumed to become completely diffuse when it enters the water column. The Case II model uses equations that explicitly include collimated irradiance in the water column. Both models are simulated in three different ways in this paper based on the vertical distribution ofthe constituents in the water column. The concentrations ofthe water quality parameters can be assumed (a) constant with depth, (b) divided into three distinct layers with different concentrations in each layer, or (c) divided into n layers of differing concentrations. The solutions to the two-flow equations with (Case II) and without (Case I) the specular, collimated irradiance and assuming a uniform water column, are given by Bostater et. al.2. This paper focuses on the derivation ofthe layered Case I and II models, a sensitivity analysis performed on the Case II model coefficients, and comparisons ofthe output from the differing model assumptions are presented.