The impact of the PBL scheme and the vertical distribution of model layers on simulations of Alpine foehn

SummaryThis paper investigates the influence of the planetary boundary-layer (PBL) parameterization and the vertical distribution of model layers on simulations of an Alpine foehn case that was observed during the Mesoscale Alpine Programme (MAP) in autumn 1999. The study is based on the PSU/NCAR MM5 modelling system and combines five different PBL schemes with three model layer settings, which mainly differ in the height above ground of the lowest model level (z1). Specifically, z1 takes values of about 7 m, 22 m and 36 m, and the experiments with z1 = 7 m are set up such that the second model level is located at z = 36 m. To assess if the different model setups have a systematic impact on the model performance, the simulation results are compared against wind lidar, radiosonde and surface measurements gathered along the Austrian Wipp Valley. Moreover, the dependence of the simulated wind and temperature fields at a given height (36 m above ground) on z1 is examined for several different regions.Our validation results show that at least over the Wipp Valley, the dependence of the model skill on z1 tends to be larger and more systematic than the impact of the PBL scheme. The agreement of the simulated wind field with observations tends to benefit from moving the lowest model layer closer to the ground, which appears to be related to the dependence of lee-side flow separation on z1. However, the simulated 2 m-temperatures are closest to observations for the intermediate z1 of 22 m. This is mainly related to the fact that the simulated low-level temperatures decrease systematically with decreasing z1 for all PBL schemes, turning a positive bias at z1 = 36 m into a negative bias at z1 = 7 m. The systematic z1-dependence is also observed for the temperatures at a fixed height of 36 m, indicating a deficiency in the self-consistency of the model results that is not related to a specific PBL formulation. Possible reasons for this deficiency are discussed in the paper. On the other hand, a systematic z1-dependence of the 36-m wind speed is encountered only for one out of the five PBL schemes. This turns out to be related to an unrealistic profile of the vertical mixing coefficient.

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