Cloud edge properties measured by the ARM shortwave spectrometers over ocean and land

Description

We use the spectrally invariant method to study the variability of cloud optical thickness τ and droplet effective radius reff in the transition between the cloudy and clear sky columns observed by Shortwave Array Spectroradiometer-Zenith (SASZe) at the Southern Great Plains (SGP C1) and during the MAGIC field campaign. The spectrally invariant method approximates the spectra in the transition zone as a linear combination of definitely clear and definitely cloudy spectra, where the coefficients (slope and intercept) characterize the cloud properties in the transition from cloudy to clear air. Similar to the results obtained for the oceanic areas, simulation results from the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model for the rural vegetation areas demonstrate that (i) the slope of the visible band is positively correlated with optical depth τ while the intercept of the near-IR band has a high negative correlation with reff ; (ii) The above properties are valid for all Solar Zenith Angles and for different cloud-contaminated skies. In observations of the 22 cases from SGP and MAGIC, we find that during cloudy-to-clear transitions (a) the slopes of the visible band decrease indicating the decrease of τ towards cloud edges, (b) the intercepts of the near-IR band show much more significant increases at SGP than from MAGIC. These findings suggest that while τ decreases during the cloudy-to-clear transition in all cases, the decreasing trend of droplet size is much more significant in SGP (over land) than at MAGIC (over ocean).