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 µm) is further investigated. It is shown that at a wavelength of 8.2 µm the concept begins to break down for small ice crystal effective diameters (≤25 µm) and has completely broken down beyond a wavelength of 20 µm. At wavelengths in the far infrared (20–30 µm) 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 µm due to two different populations of ice crystal shapes, but with both populations having the same effective diameter and size‐distribution shape, is ∼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 ∼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. Copyright © 2005 Royal Meteorological Society