Arctic shortwave radiation under mixed-phase clouds

During the 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC), a shortwave spectroradiometer deployed at Barrow AK made continuous measurements in one-minute intervals of the spectral irradiance reaching the Arctic earth surface, covering the wavelength range 0.35 - 2.2 microns. The measurements reveal the surprising extent to which the surface radiation and energy budget is influenced by the presence of ice in the prevailing stratiform clouds. We compare these measurements with theoretical radiative transfer simulations for single-phase (liquid water) clouds, and find that within the 1.6 micron window, cloud ice water frequently increases the absorption of shortwave radiation by several watts per square meter. These measurements underscore the need for robust mixed-phase microphysical treatments in current polar climate model simulations. At the same time, these measurements are consistent with previous studies demonstrating the aerosol first indirect effect in Arctic stratiform clouds that are predominantly in the liquid phase.