Using the ACRF Shortwave Spectrometer to Study the Transition Between Clear and Cloudy Regions

Marshak, A., NASA - Goddard Space Flight Center



Radiation Processes

Radiative Processes

Chiu C, A Marshak, Y Knyazikhin, P Pilewskie, and W Wiscombe. 2009. "Physical interpretation of the spectral radiative signature in the transition zone between cloud-free and cloudy regions." Atmospheric Chemistry and Physics, 9(4), 10.5194/acp-9-1419-2009.


(a) Total sky images on 18 May 2007, and (b) plot of SWS normalized zenith radiances. In (b), arrows pointed at the time axis correspond to the times of the images shown in (a), while arrows pointed at the wavelength axis correspond to 870 and 1640 nm. (c) is the plot of radiance difference vs sum at 870 and 1640 nm. Letters S and E indicate the start and end of the time series. Two arrows show the flow of time evolution. A remarkable linear relationship is found in the transition zone.


(a) Total sky images on 18 May 2007, and (b) plot of SWS normalized zenith radiances. In (b), arrows pointed at the time axis correspond to the times of the images shown in (a), while arrows pointed at the wavelength axis correspond to 870 and 1640 nm. (c) is the plot of radiance difference vs sum at 870 and 1640 nm. Letters S and E indicate the start and end of the time series. Two arrows show the flow of time evolution. A remarkable linear relationship is found in the transition zone.

To the naked eye, clouds appear to have sharp boundaries; however, this is merely an illusion. Cloud boundaries are actually somewhat fuzzy, with the transition from cloud to clear stretching over as little as 50 m to as much as several hundred meters. Fuzzy cloud boundaries create major headaches for studies of aerosol indirect effect and aerosol radiative forcing – especially when, as with most satellite instruments, spatial resolution is too poor to resolve the transition zone. This argues strongly for the use of ground-based instruments with spatial resolution on the order of meters, and temporal resolution better than a few seconds, to study the transition zone.

One-second-resolution zenith radiance measurements from the new Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) shortwave spectrometer (SWS) provide a unique opportunity to analyze the transition zone. We have used two wavelengths, 870 and 1640 nm, from the SWS spectra to study the transition zone on the sides of clouds. These two wavelengths provide information about optical depth and particle size and are nearly free of the confounding effect of Rayleigh scattering.

In the transition zone, we find a remarkable linear relationship between the sum and difference of radiances at 870 and 1640 nm wavelengths. The linear behavior allows us to neatly separate effects of aerosols and clouds. The intercept of the line is determined mostly by aerosol optical depth and size, while the slope of the line is determined mostly by cloud droplet size. This linearity also can be predicted from simple theoretical considerations and furthermore supports the hypothesis of inhomogeneous mixing, whereby optical depth increases as a cloud is approached but the effective drop size remains unchanged.