PBL Parameterization And Its Application In Mesoscale Numerical Model

Planetary boundary layer (PBL) parameterization is an essential part of numerical weather prediction model. There have been many PBL schemes applied in all kinds of models, such as MM5, WRF and so on. There are several layers in boundary layer for all PBL schemes except for the Bulk boundary layer scheme in MM5. Therefore, the PBL scheme itself is part of model's function. Each PBL scheme is different in its formulation of vertical mixing within the PBL and the surface fluxes, which in turn leads to the difference in upper free convection atmosphere. The main work and findings of this study are shown as follows:In this paper, the nonhydrostatic version of mesoscale numerical model MM5 is used to simulate heavy rain with the same conditions except for the PBL parameterization schemes. Tests are conducted to determine the sensitivity of the simulation to the available PBL parameterization schemes, including the HIR scheme, BT scheme, ETA scheme, MRF scheme and GS scheme. Significant sensitivity is seen that the structure of meteorological fields and the precipitation vary greatly with different PBL schemes. The MRF boundary layer scheme produced the strongest storm while NO boundary layer scheme produced the weakest storm. Results suggest that accurate forecasts of heavy rain can be just as sensitive to the formulation of the PBL process as they are to the cloud microphysical parameterization.During the period of 4 -5 July 2003, a heavy rain occurred in Nanjing, resulting in severe flood catastrophe. Using the MM5 model to conduct investigation of numerical simulation for this case, it was basically possessed of a capability to reproduce the genesis, development and evolution of the large-scale and mesoscale weather systems. Special analysis is made to see the impact of different boundary layer parameterization schemes on some factors which affect the heavy rainfall such as vertical velocity, divergence, vorticity, divergence of water vapor flux, se field and so on. The simulated results using a two-way interactive nesting procedure revealed that the precipitation simulated by the model is sensitive to the boundary layer parameterization scheme. Different parameterization schemes have different influences on the maximum intensity of rainfall and the distribution of rain area. Suitable boundary layer parameterization scheme has obvious improvement on simulated results.Then we select 4 "Ya-An-Tian-Lou" cases to see the role of PBL under the condition of precipitous topography. Ya'an is located on the west edge of the Sichuanbasin, the east periphery of the Qinghai-Xizang (Tibetan) Plateau. Whether in the aspect of total rain days or total rainfall amount in a year, Ya'an is always an extreme range, which has been called "Ya-An-Tian-Lou" in Chinese. The predicted average physical structures in the raining process are as follows:(l) positive vortex and convergent center in the lower and middle troposphere and negative vortex and divergent center in upper troposphere;(2) the superposition of the low layer zonal-vertical circulation and the up layer zonal-vertical circulation forms a strong upward current.The possible mechanism of "Ya-An-Tian-Lou" is as follows: the daytime sensible heat flux and latent heat flux transmission of PBL at the east steep slope of the Tibetan Plateau heat the lower atmosphere, which weakens the downward current associated with the west wind over the Tibetan Plateau, leads the low layer quasi-stationary air in the Ya-an trumpet terrain area and the Sichuan basin flowing along the slope, forms a weak east wind in the east lower layer, changes the downward current into the upward current associated with the east wind climbing the slope, forms a lower layer vertical circulation in the east periphery of the Tibetan Plateau. The lower layer east wind strengthens gradually along with the vertical circulation. The momentum transmission from the lower layer and upper troposphere weakens the west wind in the middle troposphere above the Sichuan basin, which produces more convergence ov...