Physical Interpretation of the Spectral Radiative Signature in the Transition Zone Between Cloud-free and Cloudy Regions

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

Category: Radiation

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Observations on 18 May 2007 with a SZA of 45°: (a) total sky images (from TSI instrument), and (b) time-wavelength contour plot of SWS spectra measured from 21:35:24 to 21:40:24 UTC (300 sec). SWS-observed zenith radiances have been normalized by the extraterrestrial solar spectrum and by cos(SZA). In (b), arrows pointed at the time axis correspond to the times of the sky images shown in (a), while arrows pointed at the wavelength axis correspond to 870 and 1640 nm. We also see strong water vapor absorption bands at wavelengths of 930, 1120, 1400, and 1900 nm. (c) is time series of radiances at 870 and 1640 nm corresponding to two slices of (b). (d) is the plot of normalized radiance difference versus sum at wavelengths of 870 and 1640 nm. Letters S and E indicate the start and end of the time series, while two thick arrows indicate the flow of time evolution.

Studies on aerosol direct and indirect effects demand a precise separation of cloud-free and cloudy areas. However, a separation between cloud-free and cloudy areas from remotely sensed measurements is ambiguous at any scale. Therefore, it is important to understand the transition zone where strong aerosol-cloud interactions are taking place. This paper uses one-second-resolution zenith radiance measurements from the Atmospheric Radiation Measurement Program’s new shortwave spectrometer (SWS) to study how aerosol and cloud properties change around cloud edges. We will demonstrate that in the transition zone, there is a remarkable linear relationship between the sum and difference of radiances at 870 and 1640 nm wavelengths. The intercept of the relationship is determined primarily by aerosol properties, and the slope by cloud properties. We also will show that this linearity can be predicted from simple theoretical considerations and furthermore that it supports the hypothesis of inhomogeneous mixing, whereby optical depth increases as a cloud is approached, but the effective drop size remains unchanged.

This poster will be displayed at ARM Science Team Meeting.

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