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3. Evaluating and improving ice sheet clouds, radiation, and precipitation in the Community Earth System Model

Author

Lenaerts, Jan, Camron, Michael D., Kay, Jennifer E., van Kampenhout, Leo, Gettelman, Andrew, L'Ecuyer, Tristan, Souverijns, Niels, Izeboud, Maaike, and Lhermitte, S.L.M.

Abstract

Clouds exert a pivotal control on the mass balance of the Greenland and Antarctic Ice Sheet and therefore their contribution to global sea level. Clouds transport moisture onto the marginal ice sheet, where steep topographic gradients force the air to rise and cool, inducing strong orographic precipitation and leaving the interior ice sheet dry (polar desert). Clouds further regulate the radiation balance at the surface and, consequently, surface melt. Depending on their frequency, phase, and structure, clouds not only mute incoming solar radiation but also enhance longwave radiation at the surface. With the advent of novel observations from space (CloudSat-CALIPSO) and in the field, we now have tools to start evaluating the representation of clouds, precipitation, and ice sheet surface radiation in climate models. Here we evaluate the Community Earth System Model version 1 (CESM1(CAM5)) to represent (1) precipitation frequency and phase, using a CALIPSO cloud simulator; (2) cloud radiative effect comparing to a CloudSat-CALIPSO based product; and (3) snowfall amounts and surface mass balance, comparing to CloudSat, in-situ observations, and regional climate model results. After discussing outstanding cloud biases in CESM1(CAM5), we present our efforts to reduce these in the recently released version 2 (CESM2). We show that clouds are considerably better represented in CESM2, leading to improvements in surface radiation, melt, and surface mass balance, although biases in precipitation phase persist. Our work demonstrates the need for high-quality, long-term observations of clouds and their effect on the ice sheet surface to enable continued climate model improvement.

Published
2018

4. Response of Greenland and Antarctic Ice Sheet to cloud radiative forcing.

Author

Izeboud, Maaike, Lhermitte, Stef, Lenaerts, Jan, Van Tricht, Kristof, Van Lipzig, Nicole, and Wever, Nander

Subjects
*ICE sheets, *GREENLAND ice, *RADIATIVE forcing, *ANTARCTIC ice, *ICE clouds, *ICE shelves, *SNOW
Abstract

The role of clouds on the surface mass balance of ice sheets remains a point of debate as there is no scientific consensus on the role of the cloud radiative effect (CRE) on the two major ice sheets. For the Greenland Ice Sheet, for example, conflicting studies argue either that clouds reduce surface melt through their radiative effect by blocking short-wave radiation (cloud cooling) or enhance surface melt by reducing meltwater refreezing (cloud warming). For the Antarctic Ice Sheet, on the other hand, the impact of the CRE effects on the ice sheet remain poorly quantified, although it can play a major role on the melt processes on ice shelves.To assess the impact of the CRE on surface snow and firn conditions on both ice sheets, simulations have been performed with SNOWPACK, a physically-based snow model forced with and without cloud radiative forcing. For this purpose, a state-of-the-art hybrid dataset has been created to represent the cloud forcing on the surface mass balance. This hybrid data set is a combination of CloudSat-CALIPSO satellite observations and data from the regional climate model, RACMO2.3. The combination of both data sets results in an observation-based forcing dataset with higher temporal resolution than the satellite observations can provide. Simulations are performed such that the response of the firn can be separated in a short-term and long-term component.Results show that the seasonal variation of the CRE is positive throughout the year, which implies a net cloud warming. Additionally, the simulations highlight that cloud cooling and cloud warming occur on different time scales: the first being a more direct, short-term effect and the latter having a long-term effect by initiating a feedback response that changes firn conditions. The long-term response of the firn to the CRE is shown to be dominant in summer, enhancing meltwater production and runoff by initializing a melt-albedo feedback.The long-term CRE impact on firn conditions during the melt season highlights the importance of the (initial) firn conditions when performing cloud radiation studies. It calls for the need of either detailed information of the firn surface layers or the inclusion of a snow-model that can accurately model firn response in such studies. [ABSTRACT FROM AUTHOR]

Published
2019

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