Effects Of Topography And Boundary Layer Process On Baroclinic Wave

The effects of topography and boundary layer process on the strcture, intensity and propagation speed of cyclones and low-level fronts within baroclinic wave in dry and moist air were examined using a three-dimensional, compressible, primitive equation model WRFv3.0. The main work and findings are as follows.The result shows that the most important effect of mountains is to alter the distribution of surface potential temperature, particularly around mountains. Barclinic wave accelerates where the distribution of surface potential temperature extruded by mountains, vice versa. Cyclones are weakened with their pressure rising, and low-level fronts are also weakened due to the decrease of the gradient of surface potential temperature as baroclinic wave crosses mountains. Much of the development of baroclinic disturbances is the result of superposition of mean flow features with wave features, so that cyclones are strengthened after going up mountains, whereas their intensity is less than the classic ones yet. Cold fronts are weakened yet with less influence than warm fronts, so that cold fronts move faster than warm ones. The flow pattern is changed little with different factors such as the maximum height and form factor of mountains. Substantially different evolutions were obtained in the idealized model depending on whether mountains lay north or south of the integration region. For mountains to the north, cyclones are weakened as they meridionally elongate; nevertheless to the south, cyclones and warm fronts are little influenced, which leaves a dominant cyclonic circulation in the lee. The gradient of the surface potential temperature of cold-warm fronts is strongly decreased so that fronts are decelerated and weakened in the case with boundary layer process and without mountains. The low-level momentum is continuously dissipated beceause of the surface drag, in the meantime, the middle-up-level momentum cannot exchange with the low-level one in time owing to the blocking of descending high PV air. So energy near the surface layer is less and less, and then cyclones and fronts are weakened bit by bit. In the case of the dual influence of mountains and the boundary layer process, the developing process is actually the result of superposition. Yet, the latter plays a bigger role.Diabatic heating cannot change the basic structue of cylones and fronts within baroclinic wave, but it can intensify the growth rate of baroclinic wave. Warm fronts can attain much more frontogenesis energy especially in the zone on the border with cold fronts. There are one frontogenesis and another frontolysis zone paralleled in cold fronts. As time goes on, cold fronts are under the control of frontolysis zone. Cyclones attain more engergy result in increasing "flow over", more frontogenesis energy giving to warm fronts, meanwhile, more frontolysis energy to cold fronts in the case with boundary layer and without mountains. High PV air descends more faster and the larger amount of precipitation shows up earlier due to higher mountains. In comparison with the dry case, the drag is strongly reduced, so that warm fronts achieve less weakening. The situation of cold fronts is more complicated. The intensity of the frontogenesis zone in cold fronts is reduced, at the same time, the one of the frontolysis zone is also reduced, so that cold fronts are under the control of a small frontogenesis zone at the end of the process. So we can conclude that the effect of boundary layer on cold fronts has its two sides. In the case with mountains and boundary layer process, the intensity of cyclones and cold-warm fronts is stronger after going up mountains, comparison of the case without mountains. Mountains can make rainbands a tendency of eastward expanding, so the rainbands are west-eastward. Moreover, boundary layer can make rainbands a tendency of southward spreading, so the rainbands are north-southward. Baroclinic waves are so intensitive to the initial humidity. The larger humidity employs, the larger perturbation of potential temperature and pressure produces due to the stronger reducing of the drag. So waves move faster, high PV air descends faster, and then rainbands show up earlier with narrow "T" shaped north-southward zone.