Cloud Edge Properties Measured by the ARM Shortwave Spectrometer Over Ocean and Land

Abstract

AbstractWe use the spectrally invariant method to study the variability of cloud optical thickness (τ) and droplet effective radius (reff) in transition zones between the cloudy and clear‐sky columns observed by Shortwave Array Spectroradiometer‐Zenith at the Southern Great Plains Central Facility site (SGP C1) and during the Marine ARM GPCI Investigation of Clouds (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. The slope and intercept of the linear relations characterize τ and reff in the transition region, respectively. The radiative transfer model simulations show that that (i) the slope of the visible band is positively correlated with τ, while (ii) the intercept of the near‐infrared band has a high negative correlation with reff. We have analyzed 22 cloud edge cases from the SGP and MAGIC and found that from cloud to clear in the transitions, (a) the slopes of the visible band decrease, indicating the decrease of τ toward cloud edges, and (b) the intercepts of the near‐infrared band show a much more significant increase at the SGP than from the MAGIC. The results from observed cases suggest that while τ decreases for all cases, the decrease in reff is much more significant for cloud over land at the SGP site compared to the ocean counterpart during the MAGIC campaign.