Information Content of AVHRR Channels 4 and 5 with Respect to the Effective Radius of Cirrus Cloud Particles

Abstract

This paper investigates the important difference in the relationship between brightness temperatures between the 11-μm and the 12-μn AVHRR data and the microphysical properties of the semitransparent cirrus clouds. In the nonscattering approximation, the emittance for channels 4 and 5 are related through the absorption coefficient ratio that is the key parameter giving access to the size of cloud particles. The observed mean value of this parameter corresponds to effective radius of 18 μm for polydisperse spheres and 12 μm for polydisperse infinitely long ice cylinders. Taking the multiple scattering into account, the brightness temperature difference enhances much more for cylinders than for spheres owing to the fact that the forward peak of scattering is less large for cylinders. To obtain the size of cloud particles, the method developed in the nonscattering case is still applicable if one makes use of the effective emittance that implicitly includes the effects of mattering. Thus, an effective absorption coefficient ratio is defined and we derive a direct relationship between this ratio and the optical properties of the cloud particles. The mean value of the effective absorption coefficient ratio corresponds to ice spheres of effective radius of 26 μm or a bit less in the case of water spheres (supercooled droplets), but no agreement can be obtained for fully randomly oriented cylinders.

Parameterizing Size Distribution in Ice Clouds

“…Parol et al (1991) showed similar β eff results for a small region of cirrus. Giraud et al (1997) developed an automated split-window analysis using these AVHRR channels that estimates a maximum value for β eff , using 21 AVHRR images of cirrus during fall over the northern Atlantic and Europe.…”

Impacts of cloud heterogeneities on cirrus optical properties retrieved from space-based thermal infrared radiometry

“…Two related algorithms are used to retrieve cloud products: the split-window technique (Inoue, 1985;Parol et al, 1991;Dubuisson et al, 2008) to retrieve the effective diameter and an algorithm similar to the IIR operational algorithm to retrieve the effective emissivity and the effective optical thickness.…”

“…In the thermal infrared atmospheric window (8-13 µm), cloud optical properties (optical thickness and ice crystal effective size) are retrieved using the split-window technique (SWT) (Inoue, 1985;Parol et al, 1991;Dubuisson et al, 2008). This method is generally limited to thin cirrus clouds (optical thickness less than approximately 3 at 12 µm) and small crystals (effective diameters smaller than approximately 40 µm).…”

Scale dependence of cirrus horizontal heterogeneity effects on TOA measurements – Part I: MODIS brightness temperatures in the thermal infrared

“…However, several studies (Cooper et al, 2007;Cooper and Garrett, 2010;Wang et al, 2011) have shown that cirrus optical properties may be retrieved with a better accuracy using a combination of TIR channels instead of visible and near-infrared (VNIR) channels (such as the Nakajima and King method, Nakajima and King, 1990), as long as the cirrus is optically thin enough (with a visible optical thickness between roughly 0.5 and 3) and the CED is smaller than 40 µm. For example the split-window technique (Inoue, 1985) applied to the Advanced Very-High-Resolution Radiometer (AVHRR; Parol et al, 1991) and the Imaging Infrared Radiometer (IIR) onboard CALIPSO (Garnier et al, 2012(Garnier et al, , 2013) is used to retrieve CED and COT from the brightness temperature difference of two different channels in the infrared atmospheric windows where gaseous absorption is small. Based on the same spectral information, an optimal estimation method (OEM; Rodgers, 2000) is used for the Atmospheric Infrared Sounder V6 (AIRS, Kahn et al, 2014Kahn et al, , 2015) and in the research-level code of Wang et al (2016b, a) for MODIS.…”