Sensitivity of Arctic climate simulations to different boundary layer parmeterizations in a regional climate model

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Arctic climate simulations with the high resolution regional climate model HIRHAM show some deviations fromstation data in the planetary boundary layer (PBL) during winter, which indicates the necessity of improvementsin the atmospheric PBL parameterization for a better description of the vertical stratification and atmosphere-surface energy exchange. A one-dimensional single column model scheme has been used to investigate theinfluence of two different PBL parameterizations in monthly integrations for January 1991 and July 1990. The firstscheme uses the boundary layer parameterization of the atmospheric circulation model ECHAM3, including theMonin-Obukhov similarity theory in the surface layer and a mixing length approach above. The second schemeapplies the Rossby-number similarity theory for the whole PBL, connecting external parameters with turbulent fluxes and with universal functions determined on the basis of Arctic data. For both schemes the heat and humidityadvection has been determined as residual term of the PBL balance equations. Diabatic sources have beencomputed from the current model solution and local temperature and humidity changes are estimated fromradiosonde data. The simulated vertical structure and the atmosphere-surface energy exchange during Januarystrongly depends on the used PBL parameterization scheme.These different PBL parameterization schemes were then applied for simulations of the Arctic climate in the three-dimensional regional atmospheric climate model HIRHAM, using ECHAM3 with Monin-Obukhov similarity theory,ECHAM3 with Rossby-number similarity theory and ECHAM4 parameterizations with a turbulent kinetic energyclosure. The near surface temperature, the large-scale fields of geopotential and horizontal wind are simulatedsatisfactorily by all three schemes, but strong regional differences occur. The results show a sensitivity to thetype of turbulence exchange scheme used. The comparison with ECMWF analyses and with radiosonde datareveals that during January ECHAM3 with Rossby number similarity theory more succesfully simulates the coldand stable PBL over land surfaces, whereas over the open ocean ECHAM3 with Monin Obukhov similarity worksbetter. ECHAM3 with Rossby-number similarity theory delivers a better adapted vertical heat exchange understable Arctic conditions and reduces the cold bias at the surface. The monthly mean surface turbulent heat fluxdistribution strongly depends on the use of different PBL parameterizations and leads to different Arctic climatestructures throughout the atmosphere with the strongest changes at the ice edge for January.

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Dethloff, K. , Abegg, C. , Rinke, A. , Hebestadt, I. and Romanov, V. (2001): Sensitivity of Arctic climate simulations to different boundary layer parmeterizations in a regional climate model , TellusA, 53 , pp. 1-26 .

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