Spectral Invariant Behavior of Zenith Radiance Around Cloud Edges Observed by ARM SWS
Marshak, A., NASA - Goddard Space Flight Center
Marshak A, Y Knyazikhin, JC Chiu, and WJ Wiscombe. 2009. "Spectral invariant behavior of zenith radiance around cloud edges observed by ARM SWS." Geophysical Research Letters, 36, L16802, doi:10.1029/2009GL039366.
Although clouds seem to have a distinct boundary, remote sensing measurements find it difficult to distinguish between cloudy and cloud-free air. The transition zone is neither precisely clear nor precisely cloudy. This problem has major climatic consequences, in particular on aerosol direct and indirect effect studies, which demand a precise separation of clear and cloudy zones.
ARM scientists studied the transition zone between cloudy and cloud-free areas using shortwave spectrometer (SWS) measurements. SWS looks straight up and every second measures downward solar radiation at 418 wavelengths between 350 and 2200 nm. Because of its high sampling resolution, the SWS provides a unique capability to study the transition zone and to address the important questions of what are the full-spectral radiative characteristics of the clear-cloud transition zone and what is the spectral signature of a weak evaporating cloud.
Analysis of high spectral and temporal resolution SWS measurements (see Figure 1 as an example) led to the surprising discovery of a wavelength-independent function that characterizes the transition zone between cloudy and cloud-free areas (Figure 2). The transition zone spectrum is determined fully by this function and radiances spectra of clear and cloudy sky areas. This finding is essential to both remote sensing and modeling communities. For example, this wavelength-independent function can serve as a good characteristic of pollution in a field with small cumulus clouds. In addition, if the transition zone between ice clouds and cloud-free area is longer and smoother than between water clouds and cloud-free area, the function may help determine the thermodynamic phase.