QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Vol. 130 APRIL 2004 Part A No. 598 Q. J. R. Meteorol. Soc. (2004), 130, pp. 763–778 doi: 10.1256/qj.03.151 On the radiative properties of cirrus cloud at solar and thermal wavelengths: A test of model consistency using high-resolution airborne radiance measurements By A. J. BARAN ∗ and P. N. FRANCIS Met Office, Exeter, UK (Received 6 August 2003; revised 2 December 2003) SUMMARY Two models describing the single-scattering properties of cirrus cloud are tested for physical consistency at solar and thermal wavelengths using airborne high-resolution radiance data. The data were obtained from a case of semi-transparent cirrus cloud, which occurred north of Scotland during October 2000. The single-scattering models tested are randomly oriented hexagonal ice columns and randomly oriented ice aggregates. High-spectral resolution radiances were measured from above the cirrus at a number of wavelengths between 0.3 and 16.7 µm, thereby covering a large range of ice crystal size parameter space and complex refractive index. It is shown that consistency between retrieved optical thickness and ice crystal effective radius at both solar and infrared wavelengths could only be achieved if the ice aggregate model was assumed. Moreover, differences between the ice aggregate model and spectrally resolved brightness temperature measurements were generally well within ±1 K between the wavelengths of 3.3 and 16.0 µm in the clean atmospheric window regions. The paper shows that it is important to have simultaneous radiance measurements from both the solar and thermal spectral regions so that ice crystal scattering models and cirrus retrievals can be rigorously tested. KEYWORDS: Cirrus radiative properties High-resolution remote sensing Ice-crystal scattering 1. I NTRODUCTION Cirrus (ice crystal) clouds have an important influence on current climate, and any changes to their characteristics will have a substantial feedback on climate change (Mitchell et al. 1989). The single-scattering properties of cirrus cloud are important to quantify not only for climate models but also for remote sensing and assimilation of satellite cloudy radiances into Numerical Weather Prediction models (Mishchenko et al. 1996; Greenwald et al. 2002). The importance of cirrus cloud to the earth–atmosphere radiation balance is briefly summarized here. Cirrus clouds generally occur at altitudes greater than about 6 km and permanently cover 30% of the earth’s surface (Wylie et al. 1994), and they influence the earth’s climate in several ways. They are high-level thin cloud, which generally transmit solar radiation and absorb long-wave radiation. Since these clouds are cold, they emit little infrared radiation back to space, thereby tending to warm the earth-atmosphere system. However, ice cloud associated with deep convection can also reflect much incident solar radiation back to space, thereby tending to cool the ∗ Corresponding author: Met Office, Cordouan 2, FitzRoy Road, Exeter, Devon EX1 3PB, UK. e-mail: anthony.baran@metoffice.com c  Crown copyright, 2004. 763