The orographic effects of landfall location, approach angle, and their combination on track deflection during the passage of a cyclone vortex over a mesoscale mountain range are investigated using idealized model simulations. For an elongated mesoscale mountain range, the local vorticity generation, driving the cyclone vortex track deflection, is more dominated by vorticity advection upstream of the mountain range, by vorticity stretching over the lee side and its immediate downstream area, and by vorticity advection again far downstream of the mountain as it steers the vortex back to its original direction of movement. The vorticity advection upstream of the mountain range is caused by the flow splitting associated with orographic blocking. It is found that the ideally simulated cyclone vortex tracks compare reasonably well with observed tracks of typhoons over Taiwan’s Central Mountain Range (CMR). In analyzing the relative vorticity budget, the authors found that jumps in the vortex path are largely governed by stretching on the lee side of the mountain. Based on the vorticity equation, this stretching occurs where fluid columns descend the lee slope so that the rate of stretching is governed mostly by the flow speed and the terrain slope. In other words, the maximum stretching and associated track jump are located on the faster side of the vortex. In the type E and N landfalling tracks, the faster winds are well north of the mountain crest, and the vortex track has very little change across the mountain. For the S case, however, the stronger winds are near the center of the ridge, and the track jump is much larger. For the NE case, the jump in the vortex track occurs once the vortex center shifts south of the ridge. For the SE case, there is considerable stretching, but it is aligned with the original track, so there is no jump in track.

A series of idealized numerical experiments and vorticity budget analyses is performed to examine several mechanisms proposed in previous studies to help understand the orographic influence on track deflection over a mesoscale mountain range. When an idealized tropical cyclone (TC) is embedded in a uniform, easterly flow and passes over a mountain with a moderate Froude number, it is deflected to the south upstream, moves over the mountain anticyclonically, and then resumes its westward movement. The vorticity budget analysis indicates that the TC movement can be predicted by the maximum vorticity tendency (VT). The orographic effects on the above TC track deflection are explained by the following: 1) Upstream of the mountain, the easterly basic flow is decelerated as a result of orographic blocking that causes the flow to become subgeostrophic, which advects the TC to the southwest, analogous to the advection of a point vortex embedded in a flow. The VT is primarily dominated by the horizontal vorticity advection. 2) The TC passes over the mountain anticyclonically, mainly steered by the orographically generated high pressure. This makes the TC move southwestward (northwestward) over the upslope (lee slope). The VT is mainly contributed by the horizontal vorticity advection with additional contributions from vorticity stretching and the residual term (which includes friction and subgrid turbulence mixing). 3) Over the lee slope and downstream of the mountain, the northwestward movement is enhanced by asymmetric diabatic heating, making the turning more abrupt. 4) Far downstream of the mountain, the VT is mainly contributed by the horizontal vorticity advection.

North African climate is analysed between 1979 and 2010 with an emphasis on August using the European Center for Medium‐Range Weather Forecast (ECMWF) global dataset to investigate the effects of the subtropical anticyclones over North Africa and the Arabian Peninsula on the African easterly jet (AEJ). It was found that the AEJ encloses a core with a local wind maximum (LWM) in both West and East Africa, in which the west LWM core has a higher zonal wind speed. The strength of both cores is distinctly different by way of thermal wind balance. As found in previous studies, the AEJ is formed through baroclinicity with influence from Saharan lower level heating along and to the north of the Intertropical Front (ITF). As there are two separate anticyclonic centres, the AEJ is maintained by the anticyclonic systems in West Africa, East Africa as well as the Arabian Peninsula. The presence of these two anticyclonic centres provides the AEJ with two separate core maxima with the western LWM located at 15°W, 17°N and the eastern LWM located at 35°E, 15°N for August.

Numerical simulations of a gravity current in an environment characterized by complex stratification and vertical wind shear have been performed using a nonhydrostatic, two-dimensional, dry, primitive-equation model. Data from one of the most complete documentations to date of gravity waves associated with a gravity current, presented in an earlier study, are used both to prescribe the gravity current's environment and for validation of the simulated gravity current and its associated gravity waves. These comparisons indicate that the gravity current observed by a Doppler wind profiler and sodars was well simulated in terms of depth, density contrast, and propagation speed and that the model produced a variety of gravity waves similar in many ways to these observed. Because uncertainties remained concerning the gravity wave generation mechanisms derived from the observations (e.g., wavelengths were not observed), the validated simulations are used to test these tentative hypotheses. The simulations confirm that trapped lee-type gravity waves formed in response to flow over the head of the gravity current and that Kelvin-Helmholtz (KH) waves were created because of shear atop the cold air within the gravity current. The 2.8-km wavelength of the simulated KH waves agrees with the 2- to 3-km wavelength inferred from the observations. However, the 6.4-km wavelength of the simulated lee-type waves is significantly shorter than the 12.5-km wavelength inferred from the observational data, even though wave periods (20-23 minutes) are nearly identical. Sensitivity tests indicate that the curvature in the wind profile associated with the low-level opposing inflow and an elevated isothermal layer worked together to support the development of the trapped lee-type waves. The model produces a deep vertically propagating wave above the gravity current head that was not present in the observations. As deduced in the earlier study, sensitivity tests indicate that the prefrontal, near-surface stable layer was too shallow to support the generation of a bore; that is, conditions were supercritical. Synthesis of detailed observations and numerical simulations of these mesoscale phenomena thus offers the broadest examination possible of the complex physical processes.

The North African climate is analysed for August during a 32-year period using the European Centre for Medium Range Weather Forecast (ECMWF) global data set to investigate the intensity variability at 600 mb of the subtropical highs, Africa easterly jet (AEJ) with two embedded local wind maxima, and African easterly waves over North Africa and the Arabian Peninsula. The variability of these synoptic weather systems is higher in East Africa. The most noticeable variability of intensity occurred with easterly waves. Maintenance of easterly waves from the Arabian Peninsula into East Africa is dependent on strong zonal gradients from the AEJ through shear vorticity. These zonal gradients were induced by the strengthening of the subtropical highs and the presence of a westerly jet in Central Africa and south of the Arabian Peninsula. During positive ENSO periods, these systems are generally weaker while in negative periods are stronger. The focus of this research is to investigate the role of the Arabian High and eastern local wind maximum (LWM_E) on complementing the Saharan High and western local wind maximum (LWM_W). It is found that an intense local wind maximum in East Africa helps maintain the easterly waves and their westward propagation from the Arabian Peninsula.