Spectral Invariant Behavior of Clear Sky Near Clouds Found on ARM SWS Measurements

Alexander Marshak NASA - Goddard Space Flight Center
Yuri Knyazikhin Boston University
J.-Y. Christine Chiu University of Reading
Warren Wiscombe Brookhaven National Laboratory
Peter Pilewskie University of Colorado

Category: Radiation

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Example of scale invariance. Left panel: time-wavelength contour plot of SWS spectra measured from 21:35:24 to 21:40:24 UTC (300 sec). Middle panel: total sky image at 21:38:30. Left panel: ratios of zenith radiance I_t(lambda)/I_T(lambda) versus cloudy-to-clear ratio I_t0(lambda)/I_T(lambda), where t0 = 150 sec; T = 250 sec, t = 160 sec, 170 sec, 180 sec and 190 sec; labda from 350 nm to 900 nm.

The ARM SWS looks straight up and measures zenith radiance at 418 wavelengths ranging between 350 and 2200 nm. With a 1-sec sampling resolution, it provides a unique opportunity to study the transition zone between cloudy and clear sky areas. Analyzing hyperspectral properties of zenith radiance in the vicinity of clouds we found a remarkable spectral invariant behavior between the ratios of spectral measurements taken at a different time from cloudy to cloud-free. From here, we were able to deduce a spectral signature of the transition zone between cloudy and cloud-free areas as a linear mixture between the two extreme (definitely cloudy and definitely clear) observations. This new finding helps us to better understand and quantify such physical phenomena as humidification of aerosols in the relatively moist cloud environment and evaporation and activation of cloud droplets.

This poster will be displayed at ARM Science Team Meeting.

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