Arctic Mixed-Phase Cloud Radiative Closure Study from ISDAC

Description

Spectrally resolved shortwave irradiance measurements obtained from the North Slope of Alaska (NSA) site during the 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC) allow for a test of the fidelity of mixed-phase cloud microphysical models in terms of how their simulated cloud properties yield realistic surface energy budget components. Given that surface radiative fluxes are a standard output field with regional models as well as global climate model, radiative closure comparisons are a valuable exercise at any location. Here, we compare a microphysical simulation for the ISDAC “Golden Day” stable mixed-phase cloud deck of April 8, 2008 with surface irradiance measurements obtained continuously in one-minute averages with a spectral resolution of 3 nm between 350-1000 nm and 10 nm between 1000-2500 nm. The simulated cloud properties were used as input for a 179-band discrete-ordinates-based radiative transfer model. Comparison of the model results with the irradiance data suggests that the microphysical model has a slight but consistent tendency to underestimate total cloud optical depth and total near-infrared absorption by ice particles, although the spectral shape of ice absorption is realistically reproduced. This result applies to an aggregate of ice particle optical properties. A family of radiative transfer simulations with ice particle sizes and habits considered individually is able to bracket the measurements in the near-infrared. Suggestions will be discussed for performing useful radiative closure studies with cloud microphysical models when shortwave irradiance spectra are available.