ALBERT

Description: The Weather Research and Forecasting (WRF) model is used to dynamically downscale the regional climate of the Arctic, an area undergoing rapid climate changes. Because the WRF model is increasingly being run over larger spatial and temporal scales, an assessment of its ability to reconstruct basic properties of regional climates, such as terms in the energy budget, is crucial. Estimates of the Arctic energy budget from WRF are compared with estimates from reanalyses and satellite observations. The WRF model was run on a large pan-Arctic domain continuously from 2000 to 2008. Apart from a few systematic shortcomings, WRF sufficiently captures the Arctic energy budget. The major deficiency, with differences from reanalyses and satellite observations as large as 40 W m−2 in summer months, is in the shortwave radiative fluxes at both the surface and top of atmosphere (TOA). WRF's positive bias in upwelling shortwave radiation is due to a specified constant sea ice albedo of 0.8, which is too high during the summer. When sea ice albedo in WRF is allowed to vary in a more realistic manner in a test simulation, both surface and TOA energy budget components improve, while showing little impact on the atmospheric energy convergence and storage. A second, similar WRF simulation was performed but with gridded nudging enabled. When the large-scale circulation is constrained to the forcing data, the two energy budget terms that are most dependent on weather patterns, the convergence of atmospheric energy transport and the tendency of column-integrated energy, closely resemble their reanalysis counterparts.

Description: The Weather Research and Forecasting (WRF) model is used to explore the sensitivity of the large-scale atmospheric energy and moisture budgets to prescribed changes in Arctic sea ice and sea surface temperatures (SSTs). Observed sea ice fractions and SSTs from 1996 and 2007, representing years of high and low sea ice extent, are used as lower boundary conditions. A pan-Arctic domain extending into the North Pacific and Atlantic Oceans is used. ERA-Interim reanalysis data from 1994 to 2008 are employed as initial and lateral forcing data for each high and low sea ice simulation. The addition of a third ensemble, with a mixed SST field between years 1996 and 2007 (using 2007 SSTs above 66°N and 1996 values below), results in a total of three 15-member ensembles. Results of the simulations show both local and remote responses to reduced sea ice. The local polar cap averaged response is largest in October and November, dominated by increased turbulent heat fluxes resulting in vertically deep heating and moistening of the Arctic atmosphere. This warmer and moister atmosphere is associated with an increase in cloud cover, affecting the surface and atmospheric energy budgets. There is an enhancement of the hydrologic cycle, with increased evaporation in areas of sea ice loss paired with increased precipitation. Most of the Arctic climate response results from within-Arctic changes, although some changes in the hydrologic cycle reflect circulation responses to midlatitude SST forcing, highlighting the general sensitivity of the Arctic climate.