The influence of mixed-phase clouds on surface shortwave irradiance during the Arctic spring

Abstract

[1] The influence of mixed-phase stratiform clouds on the surface shortwave irradiance is examined using unique spectral shortwave irradiance measurements made during the Indirect and Semi-Direct Aerosol Campaign (ISDAC), supported by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program. An Analytical Spectral Devices (ASD, Inc.) spectroradiometer measured downwelling spectral irradiance from 350 to 2200 nm in one-minute averages throughout April–May 2008 from the ARM Climate Research Facility's North Slope of Alaska (NSA) site at Barrow. This study examines spectral irradiance measurements made under single-layer, overcast cloud decks having geometric thickness < 3000 m. Cloud optical depth is retrieved from irradiance in the interval 1022–1033 nm. The contrasting surface radiative influences of mixed-phase clouds and liquid-water clouds are discerned using irradiances in the 1.6-μm window. Compared with liquid-water clouds, mixed-phase clouds during the Arctic spring cause a greater reduction of shortwave irradiance at the surface. At fixed conservative-scattering optical depth (constant optical depth for wavelengths λ < 1100 nm), the presence of ice water in cloud reduces the near-IR surface irradiance by an additional several watts-per-meter-squared. This additional reduction, or supplemental ice absorption, is typically ∼5 W m−2 near solar noon over Barrow, and decreases with increasing solar zenith angle. However, for some cloud decks this additional absorption can be as large as 8–10 W m−2.