Characteristic Radiative Heating Rate Profiles of Arctic Clouds Observed at Barrow, Alaska

Radiative transfer is an important component of the global energy balance, and radiative processes must be accurately represented in global climate models (GCMs). Clouds are a strong modulator of radiation, which is especially significant in the Arctic where the trapping of longwave radiation has the ability to alter ice growth and melting rate at the surface. Arctic clouds, however, are complex and the microphysics of mixed-phase clouds, which occur frequently in the Arctic, are poorly understood. To address these uncertainties, there have been increased efforts to investigate and accurately profile Arctic clouds and their characteristics. In 1997, the Atmospheric Radiation Measurement (ARM) program installed a number of active and passive ground-based remote sensors to collect a long-term dataset of atmospheric measurements at Barrow, Alaska. The measurements from these instruments allowed for the creation of macro and microphysical datasets that help improve the understanding of Arctic clouds. One such dataset, the Shupe-Turner Microbase (ST Microbase), uses a combination of remote sensing instruments to accurately determine cloud phase and retrieve profiles of ice and liquid water content within these Arctic clouds. Using the ST Microbase dataset, this study creates characteristic radiative heating rate profiles for different cloud classifications that can be used to verify, compare, and ultimately improve GCMs.

Using a 2-year cloud dataset derived from the ARM site and created by ST Microbase, the radiative flux divergence was calculated using a radiative transfer model. The clouds were then binned into distinct classifications (liquid-only versus mixed-phase, single cloud layer versus two cloud layers, etc.) and the characteristic radiative heating rate profiles were derived for each. While single layer mixed-phase clouds were observed to contain more liquid than single layer liquid-only clouds, liquid-only clouds were observed to have a higher longwave radiative cooling rate at the top of the cloud compared to mixed-phase clouds. Liquid water path variations were observed to be an important modulator of the radiative cooling rate profile. The presence of an ice-only layer at the top of mixed-phase clouds greatly reduced the radiative cooling rate at the top of mixed-phase clouds, which would cause the mixed-phase cloud to be less efficient at generating cloud top turbulence. Lastly, in two cloud layer cases that contain two liquid bearing clouds, the radiative impact was significant, with the radiative cooling at the top of the lower cloud layer being greatly reduced by the presence of the upper liquid bearing cloud.