Cloud Fraction: Can it be defined, can it be measured, and if we knew it would it be of any use to us anyway? And what can we learn from high resolution digital photography of clouds? Schwartz S. E., BECS Seminar, BNL, December 5, 2014.

Clouds greatly affect radiation transfer in the atmosphere and consequently climate. Globally clouds are thought to enhance reflected SW flux by about 48 -2 and reduce outgoing LW flux by about 31 W m-2 for a net cooling influence of about 17 W m-2. A common measure of the amount of clouds is areal cloud fraction, CF, the fraction of the 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 0.7, then in round numbers, 1% error in CF corresponds to 0.7 W m-2 in SW and 0.4 W m-2 in LW globally. However 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.

Commercially available digital cameras provide an unprecedented opportunity for detailed study of cloud structure. Key attributes of such cameras include large number of pixels, (e.g., 3456 x 4608) yielding rich detail of spatial structure, high spatial resolution (e.g., 20 μrad, corresponding to 20 mm for a cloud at 1 km height), and high dynamic range (16 bit in each of three color channels). Photography of clouds from the surface looking upwards affords the further advantage, relative to satellite imagery looking downward, that the background is black (space) with contributions to path radiance only from blue sky (Rayleigh scattering), aerosols, and clouds, without complication of surface-leaving radiance.

Here preliminary results from measurements at Long Island, NY, in summer 2014 are presented. The camera was pointed vertically, typically with field of view 22 × 29 mrad (cf. solar diameter 9.3 mrad), corresponding to 22 × 29 m at 1 km. Even at this scale CF is frequently highly dependent on threshold and resolution (several tens of percent). An alternative color-based measure of cloud influence on downwelling radiance is proposed.

This page was last updated 2014-12-14.

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