A series of numerical simulations of tropical cyclones in idealized large-scale environments is performed to examine the effects of vertical wind shear on the structure and intensity of hurricanes. The simulations are performed using the nonhydrostatic Pennsylvania State University-National Center for Atmospheric Research fifth-generation Mesoscale Model using a 5-km fine mesh and fully explicit representation of moist processes. When large-scale vertical shears are applied to mature tropical cyclones, the storms quickly develop wave-number one asymmetries with upward motion and rainfall concentrated on the left side of the shear vector looking downshear, in agreement with earlier studies. The asymmetries develop due to the storm's response to imbalances caused by the shear. The storms in shear weaken with time and eventually reach an approximate steady-state intensity that is well below their theoretical maximum potential intensity. As expected, the magnitude of the weakening increases with increasing shear. All of the storms experience time lags between the imposition of the large-scale shear and the resulting rise in the minimum central pressure. While the lag is at most a few hours when the storm is placed in very strong (15 m s 1) shear, storms in weaker shears experience much longer lag times, with the 5 m s 1 shear case showing no signs of weakening until more than 36 h after the shear is applied. These lags suggest that the storm intensity is to some degree predictable from observations of large-scale shear changes. In all cases both the development of the asymmetries in core structure and the subsequent weakening of the storm occur before any resolvable tilt of the storm's vertical axis occurs. It is hypothesized that the weakening of the storm occurs via the following sequence of events: First, the shear causes the structure of the eyewall region to become highly asymmetric throughout the depth of the storm. Second, the asymmetries in the upper troposphere, where the storm circulation is weaker, become sufficiently strong that air with high values of potential vorticity and equivalent potential temperature are mixed outward rather than into the eye. This allows the shear to ventilate the eye resulting in a loss of the warm core at upper levels, which causes the central pressure to rise, weakening the entire storm. The maximum potential vorticity becomes concentrated in saturated portions of the eyewall cloud aloft rather than in the eye. Third, the asymmetric features at upper levels are advected by the shear, causing the upper portions of the vortex to tilt approximately downshear. The storm weakens from the top down, reaching an approximate steady-state intensity when the ventilated layer can descend no farther due to the increasing strength and stability of the vortex at lower levels.

Five characteristic, low-level, large-scale dynamical patterns associated with tropical cyclogenesis in the western North Pacific basin are examined along with their capacity to generate the type of mesoscale convective systems that precede genesis. An 8-yr analysis set for the region is used to identify, and create composites for, the five characteristic patterns of monsoon shear line, monsoon confluence region, monsoon gyre, easterly waves, and Rossby energy dispersion. This brings out the common processes that contribute to tropical cyclogenesis within that pattern, which are described in detail. A 3-yr set of satellite data is then used to analyze the mesoscale convective system activity for all cases of genesis in that period and to stratify based on the above large-scale patterns. It is found that mesoscale convective systems develop in all cases of genesis except one. Seventy percent of cases developed mesoscale convective systems at more than one time during the genesis period and 44% of cases developed multiple mesoscale convective systems at a single time. Stratification by pattern type indicates some differentiation in mesoscale convective activity and it is inferred that this is due to the large-scale processes. Two of the five patterns, the monsoon shear line and the monsoon confluence region, had more than the average amount of mesoscale convective activity during the genesis period. These patterns also account for 70% of the total genesis events in the 8-yr period. The analysis for the other three patterns exhibit less mesoscale convective system activity during genesis. This may indicate either that genesis processes for these patterns are not as dominated by mesoscale convective system activity, or that genesis occurs more rapidly in these cases.

Numerical simulations of tropical-cyclone-like vortices are performed to analyze the effects of unidirectional vertical wind shear and translational flow upon the organization of convection within a hurricane's core region and upon the intensity of the storm. A series of dry and moist simulations is performed using the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model version 5 (MM5) with idealized initial conditions. The dry simulations are designed to determine the patterns of forced ascent that occur as the vortex responds to imposed vertical wind shear and translational flow, and the mechanisms that modulate the vertical velocity field are explored. The moist simulations are initialized with the same initial conditions as the dry runs but with a cumulus parameterization and explicit moisture scheme activated. The moist simulations are compared to the dry runs in order to test the hypothesis that the forced vertical circulation modes modulate the convection and hence latent heat release in the hurricane core, as well as to evaluate the net effect of the imposed environmental flow on the storm intensity and structure.The results indicate that the pattern of convection in the storm's core is strongly influenced by vertical wind shear, and to comparable degree by boundary layer friction. In the early stages of moist simulations, typical of the tropical depression stage, the regions of forced ascent and the mechanisms that cause them are similar to those in the dry runs. However, once the moist storm runs deepen enough to develop saturation in part of the eyewall, the patterns of vertical motion and associated rainfall differ between the paired dry and moist runs with identical initial conditions. The dry runs tend to produce a strong, deep region of ascent in the sector of the storm that lies downshear right of the center. The moist runs begin similarly, but as the storms intensify they strongly favor upward motion and rainfall downshear left of the center.It appears that the vertical motion patterns in the dry and moist simulations are dominated by similar adiabatic lifting mechanisms prior to the development of partial eyewall saturation. Once the moist runs reach saturation, this adiabatic lifting mechanism no longer occurs due to the latent heat release within the ascending air. Hence, the patterns of forced ascent in the dry runs should be relevant for understanding patterns of convection in loosely organized systems such as tropical depressions, but not in mature hurricanes. The rainfall patterns produced by the moist simulations are in good agreement with recent observational analyses of the relationships between rainfall distribution and vertical wind shear in Atlantic hurricanes.

With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors postulate that a major obstacle has been the complexity of scale interactions, particularly those on the mesoscale, which have only recently been observable. While there are well-known climatological requirements, these are by no means sufficient.A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development. Observations exist for only a few forming cases. In these, the moist convection in the preformation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevels. Interactions between these systems may lead to merger, growth to the surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon trough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the associated convective system.The authors develop this hypothesis by use of a detailed case study of the formation of Tropical Cyclone Oliver observed during TOGA COARE (1993). In this case, two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western North Pacific.