Sensitivity of the regional climate model RegCM4.2 to planetary boundary layer parameterisation
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This study investigates the performance of two planetary boundary layer (PBL) parameterisations in the regional climate model RegCM4.2 with specific focus on the recently implemented prognostic turbulent kinetic energy parameterisation scheme: the University of Washington (UW) scheme. 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 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 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 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, supporting the choice of these parameters in the default formulation of the UW scheme.
KeywordsEddy heat diffusivity Structural uncertainty RegCM Systematic errors
ECMWF ERA-Interim data used in this study have been obtained from the ECMWF data server. University of East Anglia CRU data used in this study have been obtained from http://badc.nerc.ac.uk. Surface flux measurements from the EUMETNET organized C-SRNWP Project have been obtained from the COSMO consortium database (http://www.como-model.org/srnwp/content) and provided by the FMI, KNMI, DWD and Meteo-France. Computations and visualizations in this study have been performed using cdo (https://code.zmaw.de/projects/cdo), GrADS (http://www.iges.org/grads) and R (http://www.R-project.org/) software. Branko Grisogono is supported by the Croatian Ministry of Science, Education and Sports (MZOS) and Croatian Science Foundation through projects BORA-MZOS 119-1193086-1311 and CATURBO-HRZZ 09/151. Ivan Güttler and Čedo Branković are supported by the MZOS project 004-1193086-3035. The contribution by T.A. O’Brien was supported by the Director, Office of Science, Office of Biological and Environmental Research of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 as part of the Regional and Global Climate Modeling Program (RGCM). We thank to two anonymous reviewers for their constructive criticism, comments and suggestions that greatly improved the original manuscript.
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