Calculating Monthly Radiative Fluxes and Heating Rates fromMonthly Cloud Observations

Abstract

The radiative transfer model from NCAR?s general circulation model CCM3 is modified to calculate monthly radiative fluxes and heating rates from monthly observations of cloud properties from the International Satellite Cloud Climatology Project and temperature and humidity from ECMWF analysis. The calculation resolves the three-dimensional structure of monthly to interannual 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 of shortwave (SW) fluxes in a cloudy atmosphere. The first replaces an existing nonphysical parameterization of partially cloudy skies with a physically motivated one that increases substantially the accuracy of calculated SW fluxes while increasing the computational time of the calculation by only 10%. The second modification allows the specification of generalized cloud overlap properties. With these modifications, radiative fluxes are calculated from observed atmospheric properties without any tuning to observed fluxes. Based on a comparison with top-of-the-atmosphere (TOA) fluxes observed in the Earth Radiation Budget Experiment, calculated SW and longwave (LW) fluxes at TOA have errors of less than 10 W m?2 at 2.5° horizontal resolution, with smaller errors over ocean than over land. Errors in calculated surface fluxes are 10?20 W m?2 based on sensitivity tests and comparisons to surface fluxes from the GEWEX Surface Radiation Budget. In contrast, TOA and surface fluxes from the NCEP/NCAR reanalysis data, which rely on cloud properties from a general circulation model, have errors larger than 30 W m?2. Errors in the calculated fluxes result primarily from uncertainties in the observed cloud properties and specified surface albedo, with somewhat smaller errors resulting from unobserved aspects of the vertical distribution of clouds. Errors introduced into the calculation by using monthly observations 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 impact 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 tropical convection result in deep cloud radiative heating, whereas shallow cloud distributions in the subtropics result in low-level cloud radiative cooling there. Under all conditions, SW cloud radiative forcing is systematically of opposite sign to LW cloud radiative forcing, which reduces the impact of LW cloud radiative forcing.