Stephen E. Schwartz — Brookhaven National Laboratory
Dong Huang — NASA - Goddard Space Flight Center
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Category
General topics – Clouds
Description
Figure 1. Upper panel shows red-green-blue image and red and blue radiance fields of image at ARM SGP site; July 31, 2015, 16:35:01 UTC (10:05 local sun time) and, in false color, cloud optical depth field as determined from red and blue radiance. Lower panel shows blue and red radiance fields and red/(red + blue) radiance ratio, together with color-coded histograms and integrated histograms that correspond to cloud and clear fraction, as function of radiance or R/(R + B) threshold.
Clouds exhibit structures at a wide range of length scales. Passive radiometry from satellite shows structure on scales of tens to thousands of kilometers, but there is much structure at short spatial scales not resolved by satellite imagery. Here we report measurements with commercial cameras having high spatial resolution (~20, 110 µrad) and high dynamic range (16 bits in each of three color channels) in narrow field-of-view (20, 110 mrad), zenith-looking mode from the surface, to examine clouds at the scale of centimeters to a few hundred meters, focusing on non-precipitating single-layer clouds during daytime. Up-looking photography of clouds from the surface affords, in addition to resolution, the further advantage, relative to satellite imagery, of black background (space) with contributions to radiance only from blue sky (Rayleigh scattering), aerosols, and clouds, permitting determination of cloud optical depth, at the resolution of the camera, by inversion of the radiance image. Measurements at ARM SGP site in north central Oklahoma in July, 2015 show much short-range variability on scales down to 1 meter or less, Figure 1. Cloud height and vertical structure are determined by doppler lidar. Inversion of radiance field yields the 2D distribution of cloud optical depth; cloud optical depth is determined with precision of about 0.1 OD over range 0 to 3. Spatial inhomogeneity is attributed mainly to horizontal variation in vertical motion of the air and resultant condensation or evaporation associated with upward or downward motion, respectively. Cloud fraction depends strongly on threshold criterion, demonstrating the absence of any unique or unambiguous measure of cloud fraction.
