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

Abstract

We use the spectrally invariant method to study the variability of cloud optical thickness (τ) and droplet effective radius (r eff ) 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 r eff 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 r eff . 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 r eff is much more significant for cloud over land at the SGP site compared to the ocean counterpart during the MAGIC campaign.

The Shortwave Spectral Radiometer for Atmospheric Science: Capabilities and Applications from the ARM User Facility

“…When applying this method to the measurements of a ground-based shortwave spectroradiometer in continental and maritime conditions (Yang et al, 2016(Yang et al, , 2019, we found that cloud optical depth consistently decreases in both cases, while droplet size decreases much more substantially for the continental regime, suggesting different mixing processes for the two types of clouds. This is consistent with our theoretical understanding of the effect of relative humidity on the mixing types (Pinsky and Khain, 2018).…”

“…McBride et al 2011) or the shape (LeBlanc et al 2015) of the SW spectra mitigate challenges with absolute calibration accuracy allowing more effective use of the spectral measurements. For example, the studies of the mixing zone between clear and cloudy skies described in the following section use retrievals of cloud optical depth and effective radius from multiple spectral zenith radiance instruments with absolute accuracy differences of 15-20%, but these accuracy differences had little impact on the retrievals because ratios of cloudy and clear spectra are used (Yang et al, 2016(Yang et al, , 2019; Section S5).…”