The snow darkening effect has received a considerable amount of attention recently. The effect is mainly caused by deposition of solar radiation absorbing aerosolssuch as dust, black carbon (BC), and organic carbon (OC), and thus far ignored biological activities on glaciers, all greatly reducing visible snow albedos. It may accelerate snow melting due to enhanced solar absorption at the snow surface and generate feedbacks between the land and atmosphere. The NASA GEOS-5 is the latest version of awidely used global model that until recently completely ignored snow darkening effects. In the 3.5 years that I have been at Goddard, I have been working on rectifying this deficiency by developing schemes calculating snow albedo reductions due to the darkening effect and the associated mass concentration changes of snow impurities directly related to the snow albedo reductions.
I will show in this talk impacts of the snow darkening effect simulated by the latest version of the GEOS-5 model, using the latest version of the GOCART chemical transport model. So-called “replay” simulations are the best way to examine the current performance of the snow darkening parameterization because the atmospheric meteorological fields based on MERRA re-analysis data produced by GMAO are superior to those of the free-running version of the model. Climatological snow darkening effects will be discussed with outputs from 7-year replay run simulations (2005-2011).
I will also show some potential validation approaches of snow darkening including: 1) dust deposition episodes during precipitation over Japan for Oct 2008 and Jan. 2010 where GEOS-5 output at two horizontal resolutions (144x91 and 576x361) is compared to the available Japanese observations; 2) a winter migration of snow darkening in Sapporo, Japan, as seen both in direct output from GEOS-5 simulation and off-line GEOS-5 Catchment simulations forced by locally observed meteorological fields maintained by the Japan Meteorological Agency paired with aerosol deposition from the GEOS-5 simulation; 3) global comparisons of BC concentrations at the top snow layer between GEOS-5 and a limited number of available observations.