engleski

Systematic errors in regional climate models in the lower atmosphere

Near-surface and surface variables and the surface energy budget components from two simulations of the regional climate model RegCM4.2 over the European/north African domain during the period 1989-1998 are analysed. The simulations differ in selected planetary boundary layer (PBL) schemes: the Holtslag diagnostic non-local PBL scheme and UW prognostic local PBL scheme. Surface radiative and turbulent fluxes are compared against ERA- Interim while systematic errors in surface radiative fluxes are derived with respect to the satellite-based products. Substantial systematic errors and differences between the two simulations are present for some quantities. The most prominent error is an overestimation of the net surface shortwave radiation flux over eastern Europe during summer. This error strongly correlates with errors in the representation of total cloud cover, and less strongly with errors in surface albedo. During winter the amplitude of the surface energy budget components is more in line with reference datasets. Systematic errors may limit the usefulness of RegCM simulations in further applications. However, the use of the UW PBL scheme improves RegCM representation of the total cloudiness and net surface shortwave radiation and reduces near-surface temperature errors over eastern Europe and Russia. When compared with the default Holtslag scheme, the UW scheme, in the 10-year experiments over the European domain, shows a substantial cooling. It reduces winter warm bias over the north-eastern Europe by 2 °C and reduces summer warm bias over central Europe by 3 °C. A part of the detected cooling is ascribed to a general reduction in lower tropospheric eddy heat diffusivity with the UW scheme. While differences in temperature tendency due to the PBL schemes are mostly localized to the lower troposphere, the schemes show a much higher diversity in how vertical turbulent mixing of the water vapour mixing ratio is governed. Differences in the water vapour mixing ratio tendency due to the PBL scheme are present almost throughout the troposphere. However, they alone cannot explain the overall water vapour mixing ratio profiles, suggesting strong interaction between the PBL and other model parameterisations. An additional 18-member ensemble with the UW scheme is made, where two formulations of the master turbulent length scale in statically unstable conditions are tested and unconstrained parameters associated with (a) the evaporative enhancement of the cloud-top entrainment and (b) the formulation of the master turbulent length scale in statically stable conditions are systematically perturbed. These experiments suggest that the master turbulent length scale in the UW scheme could be further refined in the current implementation in the RegCM model. It was also found that the UW scheme is less sensitive to the variations of the other two selected unconstrained parameters. Near-surface and surface variables simulated by RegCM4.3 model using the two different PBL schemes under two scenarios of concentrations of the greenhouse gases (RCP4.5 and RCP8.5) and forced by the HadGEM2-ES Earth System Model are also analysed. Over the Mediterranean region, where substantial temperature increase and precipitation decrease are expected in the 21st century, near- surface air temperature T2m and total precipitation R projections are linked with the climate change of the components in the surface energy budget and total cloud cover, surface albedo and soil moisture. Although for the historical period the two RegCM simulations yield different climatology over the Mediterranean region, the climate change projections for the 21st century are not strongly sensitive to the choice of the PBL scheme.

regional climate model; systematic errors; planetary boundary layer; surface energy budget; climate change

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