Three‐dimensional radiative transfer in midlatitude cirrus clouds

Abstract

AbstractThree different types of cirrus cloud field, reconstructed in three dimensions directly from midlatitude observations by a cirrus stochastic model, are used to study the effects of three‐dimensional radiative transfer in both the long‐wave and short‐wave spectral regions. Calculations of three‐dimensional radiative transfer (3D), the independent column approximation (ICA) and the plane‐parallel approximation are compared to quantify the effects on heating rates, radiative fluxes and related properties. Locally the heating rate difference between 3D and ICA reaches more than 10 K day−1 in both the long‐wave and short‐wave, depending upon the distributions of ice water content, which indicates that horizontal radiation transport plays an important role in structures of heating rate. The domain‐averaged heating profiles of 3D agree within a few tenths of a K day−1 with ICA but show a systematic low bias. The domain‐averaged heating rates in cloud layers are increased in 3D by up to 7% in the long‐wave and more than 20% in the short‐wave. The root‐mean‐square differences at individual points are up to ten times larger than the corresponding domain‐averaged differences, representing the cancellation of opposing 3D effects. The ICA biases in long‐wave net flux and emissivity have their maximum values (∼2–3%) near cloud top for the thinnest cloud with lowest fractional coverage. In general, ICA tends to reduce the reflected upwelling short‐wave flux at the top of clouds; the layer‐averaged albedo at cloud top agrees with 3D within 1%, although the corresponding RMS difference may differ by up to 30% from 3D at high solar zenith angles. Similar results are found for whole‐sky (cloudy plus clear) short‐wave reflectances and transmittances for which ICA agrees with 3D within 5%. For domain‐averaged short‐wave absorptance, however, ICA errors can reach 20%. The corresponding RMS differences may differ by up to 50% in reflectance and transmittance but exceed 200% in absorptance. The effects of solar zenith angle are also discussed. Copyright © 2007 Royal Meteorological Society