The spectral-invariant behavior near cloud edges from ground-based zenith radiances: observation and simulations

Fuzzy cloud edges, with the transition from cloudy to clear air spanning as little as 50 m to as much as several hundred meters, are hard to measure with satellites and research aircrafts. Yet the transition zone is where strong aerosol-cloud interactions take place, and thus has major consequences on determination of aerosol direct and indirect forcing. Recently, one-second-resolution, 418-wavelength zenith radiance measurements from the Atmospheric Radiation Measurement (ARM) program shortwave spectrometer (SWS) provide a unique opportunity to analyze the transition zone. We demonstrate a striking spectral-invariant relationship between ratios of zenith radiances found in the SWS spectra. Accordingly, the spectral invariance suggests that the shortwave spectrum near cloud edges can be determined by zenith radiance spectra of the cloudy and clear regions. The observed spectral-invariant behavior has been confirmed with SBDART radiative transfer calculations. We will also discuss sensitivity to changes in aerosol and cloud properties. Sensitivity results shed light on mixing hypothesis and retrieval capability near cloud edges.