Numerical simulation of planetary boundary-layer evolution and mesoscale flow over irregular terrain under daytime heating conditions Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0v838288w

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  • The influence of irregular terrain on the evolution of the daytime planetary boundary layer (PBL) and meso-β scale dry circulations is studied using two three-dimensional hydrostatic σ-coordinate models with different approaches for the PBL parameterizations; the 4-layer model uses the mixed-layer (bulk-layer) approach, while the 7-layer model adopts the eddy-diffusivity (multi-layer) approach. Numerical experiments are carried out under the conditions of a dry, sunny summer day with moderate prevailing westerly winds blowing over gently sloping idealized hills in a domain of 150 km on a side. The results from the two models are compared and their performances are evaluated. The behaviors of the mean PBL depth and inversion strength are analytically described using a simple one-point mixed-layer model. Counterclockwise rotation of the mean PBL winds with time observed in both model results can be explained only when the non-zero momentum flux at the PBL top is taken into account. However, stresses associated with entrainment at the PBL top are not sufficient to pull the cold air out of the valleys so as to result in breakup of the early morning stable layer, as is suggested in a previous study. The regions of weak winds that persist in the morning PBL are attributed largely to the baroclinic effect of horizontal variations of potential temperature θ in the PBL, while the effect of surface drag is quite small in these areas. Significant differences in the flow patterns near the surface in two results suggest the importance of the local pressure gradient force associated with terrain irregularities. The effect of horizontal θ advection is also significant in helping reduce the PBL θ anomalies and promote breakup of the stable layer. The well-mixed assumption generally applies quite well to the development of the θ profiles, while for momentum it seems valid only during the peak of convective mixing and the eddy-diffusivity approach is probably preferable for a better description of the low-level flows. The fields of the PBL top height obtained using different procedures in the two models are found to correspond fairly well to each other. Mass-flux convergence associated with terrain irregularities and resulting changes in the wind fields are shown to play a key role in the midday PBL height patterns. The development of the PBL structure as revealed by the θ cross sections obtained from either model corresponds favorably to that indicated by idealized cross sections previously constructed from observed data. The formation of a region of mass-flux convergence and accompanying updrafts near the surface on the leeward side of a mountain, processes which are likely to be important in terrain-induced cloud initiations, seem to be simulated.
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