Last update
5 Apr 2001

This page is maintained by
Mark Fielding

All content Copyright © The University of Reading unless otherwise stated.

Abstract

It has long been recognised that cirrus clouds play a fundamental role in the earth's radiation balance. In numerical models the IR emissivity is generally computed from the optical depth. The relationship between optical depth and emissivity is non-linear so if cirrus clouds are inhomogeneous then emissivities calculated from a grid-box mean value of optical depth will be biased high. An anomalous effect in stratocumulus involving the non-linearity of albedo and liquid water path is well known and to compensate for sub-grid fluctuations the grid-box average liquid water is multiplied by 0.7 to calculate albedo. We analyse a three month series of profiles of radar reflectivity in ice clouds to derive the spatial variability if optical depth and quantity any biases introduced into emissivities calculated from grid-box mean values of optical depth. For a grid-box of 10km or 20km the effect is small but becomes appreciable for larger sized boxes and the average optical depth should be multiplied by correction factors as low as 0.698 for calculating emissivities for a 100km box in mid-latitude winter cirrus.

Key results

Variation of mean emissivity bias with grid box size: crosses; error bars indicate one standard deviation plus and minus the mean. Bias from calculating emissivity from

• (a) optical depth running mean;
• (b) 0.7 times optical depth running mean;
• (c) linear fit to emissivity running mean data;
• (d) optical depth running mean reduced by a constant factor,

Grid-length	10km	20km	30km	40km	50km	100km
	0.9403	0.8971	0.8613	0.8295	0.8007	0.6976