Calculating monthly radiative fluxes and heating rates from monthly cloud observations

The radiative transfer model from NCAR's general circulation model CCM3 is modified to calculate monthly radiative fluxes and heating rates from month ly observations of cloud properties from the International Satellite Cloud Climatology Project and temperature and humidity from ECMWF analysis. The c alculation resolves the three-dimensional structure of monthly to interannu al variations of radiative heating and is efficient enough to allow a wide range of sensitivity tests. Two modifications to the radiative transfer model improve the calculation o f shortwave (SW) fluxes in a cloudy atmosphere. The first replaces an exist ing nonphysical parameterization of partially cloudy skies with a physicall y motivated one that increases substantially the accuracy of calculated SW fluxes while increasing the computational time of the calculation by only 1 0%. The second modification allows the specification of generalized cloud o verlap properties. With these modifications, radiative fluxes are calculate d from observed atmospheric properties without any tuning to observed fluxe s. Based on a comparison with top-of-the-atmosphere (TOA) fluxes observed in t he Earth Radiation Budget Experiment, calculated SW and longwave (LW) fluxe s at TOA have errors of less than 10 W m(-2) at 2.5 degrees horizontal reso lution, with smaller errors over ocean than over land. Errors in calculated surface fluxes are 10-20 W m(-2) based on sensitivity tests and comparison s to surface fluxes from the GEWEX Surface Radiation Budget. In contrast, T OA and surface fluxes from the NCEP/NCAR reanalysis data, which rely on clo ud properties from a general circulation model, have errors larger than 30 W m(-2). Errors in the calculated fluxes result primarily from uncertaintie s in the observed cloud properties and specified surface albedo, with somew hat smaller errors resulting from unobserved aspects of the vertical distri bution of clouds. Errors introduced into the calculation by using monthly o bservations and neglecting high-frequency variations are small relative to other sources of error. Substantial uncertainty is found in many details of the vertical structure of cloud radiative forcing, which underscores the importance of performing a wide variety of sensitivity calculations in order to understand the impac t of clouds on radiative heating. However, certain general features of the calculated vertical structure of cloud radiative forcing in the atmosphere are robust. Deep vertical cloud distributions at locations of active tropic al convection result in deep cloud radiative heating, whereas shallow cloud distributions in the subtropics result in low-level cloud radiative coolin g there. Under all conditions, SW cloud radiative forcing is systematically of opposite sign to LW cloud radiative forcing, which reduces the impact o f LW cloud radiative forcing.