Stephen E. Schwartz — Brookhaven National Laboratory
Category
Cloud Properties
Description
Clouds greatly affect radiation transfer in the atmosphere and consequently climate. Globally, clouds enhance reflected shortwave (SW) flux by 47.5 ± 3 W m-2 and reduce outgoing longwave (LW) flux by 26.4 ± 4 W m-2 for a net cooling influence of 21.1 ± 5 W m-2 (Harrison, Journal of Geophysical Research 1990). The amount and properties of clouds are expected to change with increasing global temperature, but the amount and even the sign of resultant flux changes are not known, giving rise to much uncertainty in estimates of climate sensitivity and projections of climate change. Consequently it is essential that representation of clouds and their radiative influences in climate models be accurately assessed.
The conventional measure of the amount of clouds is "cloud fraction" (CF) the fraction of the atmosphere volume or column occupied by clouds. This raises the question of whether CF can be defined and how well it can be measured. If average CF is 0.5, then in round numbers, 1% error in CF corresponds to 1 W m-2 in SW and 0.5 W m-2 in LW globally. This sets the scene for how well CF must be known and provides context for differences in measurements by different techniques (Figure 1). Observationally, cloud fraction depends on resolution and threshold and on whether CF is evaluated as area average or as temporal average in a narrow-field-of-view vertical column.
This poster examines issues associated with definition and measurement of CF, compares different measures of CF at the ARM Southern Great Plains (SGP) site, and examines some of the literature. Given the importance of CF in models and the need for measurements, it might be useful to establish a Cloud Fraction Focus Group in ASR. The author invites discussion of this and suggestions for measurements or alternatives to CF, at the poster and/or by email.