University of Cambridge > Talks.cam > Scott Polar Research Institute - Physical Sciences Seminar > High-resolution modelling of polar climates using the regional climate model RACMO2.3

High-resolution modelling of polar climates using the regional climate model RACMO2.3

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If you have a question about this talk, please contact Poul Christoffersen.

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Monitoring the current climate in polar regions such as the Greenland (GrIS) and Antarctic ice sheets (AIS), encompasses many limitations. Automatic weather stations, stake sites, firn and ice cores located all over the accumulation and ablation zones of the GrIS and AIS only measure point climate conditions, not necessarily representative of larger areas. Since the late 50s, airborne and satellite measurements provide spatially continuous coverage of the ice sheets at relatively low resolutions. These limitations can be solved using regional climate models (RCMs), covering the entire ice sheets at high spatial and temporal resolutions, typically 5 km to 30 km at a hourly-to-daily time scale. The polar version of the Regional Atmospheric Climate Model (RACMO2) is adapted to specifically model the surface mass balance (SMB) and energy budget (SEB) over Greenland, Antarctica and other glaciated regions. RACMO2 incorporates a multilayer snow module, interactively coupled to the atmosphere, that simulates meltwater percolation, refreezing and runoff; an albedo scheme based on prognostic snow grain size, and a drifting snow routine, accounting for interactions between drifting snow, the surface and the lower atmosphere. RACMO2 has been thoroughly evaluated in many glaciated regions including Greenland, Canadian Arctic, Svalbard, Patagonia and Antarctica.

Recently, RACMO2 has been updated from version 2.1 to 2.3. These updates include major modifications in the description of cloud microphysics, surface and boundary layer turbulence, and radiation transport. Most notably, the new cloud scheme enables ice supersaturation, which prolongs the vapour phase at low temperatures and delays cloud formation at higher elevations. Furthermore, the auto-conversion coefficient, controlling the conversion rate of water-vapour into precipitation in convective clouds, has been modified to favour solid at the expense of liquid precipitation. The implications of these physics updates on modelled SMB components are discussed for Greenland (1958-2014, 11 km) and Antarctica (1979-2014, 27 km). In addition, RACMO2 .3 performances improve on the previous version 2.1; the evaluation against observations shows a better agreement for both the GrIS and AIS .

Despite physics improvements, the current spatial resolution in RACMO2 .3 remains too coarse to represent the local variability in SMB components, especially over narrow glaciated features, i.e. the Antarctic Peninsula (AP), the GrIS and surrounding ice caps (GICs). The relatively low-resolution surface elevation prescribed in RACMO2 .3 contributes to underestimate topographically-forced precipitation over the mountainous regions of the AP. In Greenland, underestimated elevation and thus near-surface temperature result in too low surface melt and runoff at the GrIS rough margins. To address these issues, a high-resolution simulation at 5 km is carried out over the AP for the period 1979-2014, allowing for a better representation of local precipitation patterns. Another approach is applied in Greenland, where the data of RACMO2 .3 at 11 km (1958-2014) are statistically downscaled to the topography and ice mask of a down-sampled version of the GIMP DEM at 1 km, using a daily specific elevation dependent technique. This method allows to provide more realistic SMB patterns over Greenland narrow ablation zones, marginal outlet glaciers and peripheral ice caps, owing to enhanced runoff at the GrIS margins. These high-resolution SMB products are evaluated against ablation/accumulation measurements collected in the AP and Greenland. Finally, the current GICs contribution to the total GrIS mass loss is estimated based on the 1 km dataset of daily downscaled SMB components.

This talk is part of the Scott Polar Research Institute - Physical Sciences Seminar series.

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