Radiative forcing bias of simulated surface albedo modifications linked to forest cover changes at northern latitudes

[摘要] In the presence of snow, the bias in the prediction of surface albedo bymany climate models remains difficult to correct due to the difficulties ofseparating the albedo parameterizations from those describing snow andvegetation cover and structure. This can be overcome by extracting thealbedo parameterizations in isolation, by executing them with observedmeteorology and information on vegetation structure, and by comparing theresulting predictions to observations. Here, we employ an empirical data setof forest structure and daily meteorology for three snow cover seasons andfor three case regions in boreal Norway to compute and evaluate predictedalbedo to those based on daily MODIS retrievals. Forest and adjacent openarea albedos are subsequently used to estimate bias in top-of-the-atmosphere(TOA) radiative forcings (RF) from albedo changes (Δα, Open–Forest)connected to land use and land cover changes (LULCC).

As expected, given the diversity of approaches by which snow masking bytall-statured vegetation is parameterized, the magnitude and sign of thealbedo biases varied considerably for forests. Large biases at the opensites were also detected, which was unexpected given that these sites weresnow-covered throughout most of the analytical time period, thereforeeliminating potential biases linked to snow-masking parameterizations.Biases at the open sites were mostly positive, exacerbating the strength ofvegetation masking effects and hence the simulated LULCC ΔαRF. Despite the large biases in both forest and open area albedos by someschemes in some months and years, the mean Δα RF bias overthe 3-year period (November–May) was considerably small across models(−2.1 ± 1.04 Wm⁻²; 21±11%); four of six models hadnormalized mean absolute errors less than 20%. Identifying systematicsources of the albedo prediction biases proved challenging, although forsome schemes clear sources were identified.

[学科分类] 地球化学与岩石