The dependence of cirrus infrared radiative properties on ice crystal geometry and shape of the size-distribution function

The concept of an effective diameter is often used to describe the radiative properties of cirrus, and is commonly applied in the retrieval of cirrus microphysical and/or macrophysical properties. In this paper the applicability of an effective-diameter concept at thermal wavelengths (4.0-30 mu m) is further investigated. It is shown that at a wavelength of 8.2 mu m the concept begins to break down for small ice crystal effective diameters (<= 25 mu m) and has completely broken down beyond a wavelength of 20 mu m. At wavelengths in the far infrared (20-30 mu m) the potential impact of ice crystal geometry and assumed size-distribution shape on brightness-temperature measurements is quantified. It is found that the brightness-temperature difference at a wavelength of 25 mu m due to two different populations of ice crystal shapes, but with both populations having the same effective diameter and size-distribution shape, is similar to 5 K. However, if both populations have the same ice crystal shape and effective diameter but different size-distribution shapes, then the brightness-temperature difference is about similar to 9 K at the same wavelength. The impact of size-distribution shape on the brightness-temperature difference is almost twice as great as that of crystal shape. Given that there is sensitivity to the shape of the size-distribution function at far-infrared wavelengths, the potential for retrieving size-distribution shape using far-infrared brightness-temperature measurements is also investigated. The implications of these findings are that the concept of an effective diameter cannot be generally applied at infrared wavelengths unless a priori information is known about the shape of the size-distribution function.