Optically Thin Liquid Clouds: Detection and Assessment of Contribution to Greenland Melt Events Using Satellite Data

Clouds play a fundamental role in the mass budget of the world's major ice sheets both as a source, via precipitation, and as a sink, via surface radiative forcing. To understand present and future effects of changes to the world's ice sheets requires a robust understanding of the macro and microphysical properties of polar cloud systems, including their radiative effects on the surface. For this study, the Moderate Resolution Imaging Spectroradiometer (MODIS) is used to diagnose and quantify the contribution of surface radiative forcing from optically thin liquid clouds to melting of the Greenland ice sheet. Results from the Integrated Characterization of Energy, Clouds, Atmospheric State and Precipitation at Summit (ICECAPS) campaign noted a historically rare period of extended surface melting observed across the entire Greenland ice sheet in July 2012. A study by Bennartz et al. (2013), using ICECAPS surface instrument data and simple radiative transfer modeling, determined that low-level liquid clouds played a key role in that melt event by increasing surface temperatures.

Sensitivity studies performed using a robust radiative transfer model demonstrate that cloud liquid water path values observed by Bennartz, 10-40 g/m², corresponding to a cloud optical thickness between 1.5 and 6.5, produce a net warming effect at the surface over Greenland for the range of solar zenith angles that occurred during the melt event. The model also shows that varying the base height of such clouds has a negligible effect on the magnitude of surface radiative forcing, thus removing the “low-level” constraint in our evaluation. Both ICECAPS data and MODIS satellite data show an anomalously high presence of optically thin, liquid clouds over the Greenland ice sheet during the record melting event in July 2012. Further analysis of MODIS satellite data indicates that the frequency of occurrence of optically thin, liquid clouds in the Arctic increases dramatically during the summer months. Here we investigate the frequency and spatial extent of such melt events over the Greenland ice sheet and the Arctic (poleward of 55°N) as a whole over the past 30 years. Our results may help to improve climate model simulations of Arctic cloud properties and surface energy budget, which are vital to fully account for temperature feedbacks in the warming Arctic climate.