Cloud Fraction: Can it be Defined and Measured? And if we Knew it Would it be of any use to us? Schwartz S. E., American Geophysical Union Fall Meeting, December, 2013. A43H03.

Clouds greatly affect radiation transfer in the atmosphere and consequently climate. Globally clouds are thought to enhance reflected SW flux by about 48 W m-2 and reduce outgoing LW flux by about 26 W m-2 for a net cooling influence of about 22 W m-2 (Harrison, JGR, 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 projected area 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, Fig. 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. The SW and LW cloud radiative effects depend strongly on cloud optical depth and cloud top height, raising question over the utility of cloud fraction as a measure of cloud radiative effects. Here issues associated with definition and measurement of CF are examined and different measures of CF at the Department of Energy Atmospheric Radiation Measurement site in North Central Oklahoma are compared. Collocated measurements of cloud fraction by different methods differ often by several tens of percent and exhibit low correlation.

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