Optical cavity resonances in water micro-droplets: Implications for shortwave cloud forcing

[1] The influence of narrow optical resonances, which result from trapping of light rays via total internal reflection in water droplets, on the absorption of shortwave (SW) solar radiation has been estimated through high resolution Mie scattering calculations. Our results indicate that these resonances engender an increase in absorption of solar radiation by cloud droplets that is several W/m above the linear direct absorption process. Mie scattering calculations performed at the Dx = 0.1 (x = 2pr/l) resolution typically implemented in cloudy sky radiative transfer models are shown to be insufficient for accurate determination of the attenuation of SW radiation when considered over relatively narrow wavelength ranges, consistent with the recent finding of Nussenzveig [2003]. However, for broadband calculations we find positive and negative errors in Mie calculations at Dx = 0.1 nearly cancel resulting in reasonable estimates of SW attenuation.