Abstract

The mechanisms by which mesoscale midlevel vortices that form in the stratiform anvil regions of mesoscale convective systems develop downward in the atmosphere are explored in the context of tropical cyclone genesis. Using simple two- and three-dimensional models, a theory for the processes by which midlevel vortices may interact both with each other, and with their large-scale environment in order to develop a storm-scale vortex, is developed. It is found that absorption of the circulation of one vortex by another results in a vortex of greater horizontal and vertical extent. Embedding the vortices in an enhanced vorticity environment such as might be found in the monsoon trough results in more efficient merger and greater downward development of the circulation associated with the merged vortex.

This theory is used to interpret a real case of the development of Tropical Cyclone (TC) Oliver in the Australian region during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) experiment in 1993. High-resolution flight-level and dropwindsonde data were collected during the interaction and merger phase of two large mesoscale convective systems that were embedded in the monsoon trough. Multiple mesoscale vortices were observed to interact and merge during the development phase of TC Oliver with consequences for the downward development of the vortex, and subsequent eye development.

Abstract

Tropical Cyclone Oswald (2013) is considered to be one of the highest-impact storms to make landfall in northern Australia even though it only reached a maximum category 1 intensity on the Australian category scale. After making landfall on the west coast of Cape York Peninsula, Oswald turned southward, and persisted for more than 7 days moving parallel to the coastline as far south as 30°S. As one of the wettest tropical cyclones (TCs) in Australian history, the favorable configurations of a lower-latitude active monsoon trough and two consecutive midlatitude trough–jet systems generally contributed to the maintenance of the Oswald circulation over land and prolonged rainfall. As a result, Oswald produced widespread heavy rainfall along the east coast with three maximum centers near Weipa, Townsville, and Rockhampton, respectively. Using high-resolution WRF simulations, the mechanisms associated with TC Oswald’s rainfall are analyzed. The results show that the rainfall involved different rainfall mechanisms at each stage. The land–sea surface friction contrast, the vertical wind shear, and monsoon trough were mostly responsible for the intensity and location for the first heavy rainfall center on the Cape York Peninsula. The second torrential rainfall near Townsville was primarily a result of the local topography and land–sea frictional convergence in a conditionally unstable monsoonal environment with frictional convergence due to TC motion modulating some offshore rainfall. The third rainfall area was largely dominated by persistent high vertical wind shear forcing, favorable large-scale quasigeostrophic dynamic lifting from two midlatitude trough–jet systems, and mesoscale frontogenesis lifting.

Abstract

Tropical Cyclone Debbie (2017) made landfall near Airlie Beach on 28 March 2017 causing 14 fatalities and an estimated $2.67 billion (U.S. dollars) economic loss and was ranked as the most dangerous cyclone to hit Australia since TC Tracy in 1974. In addition to the extreme flooding as TC Debbie moved onshore and down the east coast of Australia, it intensified rapidly just offshore from category 2 to category 4 on the Australian TC intensity scale in under 18 h prior to making landfall. A high-resolution WRF simulation is used to analyze the inner-core structure and evolution during the offshore intensification period. Two stages are identified: a slow intensification (SI) stage characterized by an asymmetric eyewall contraction and a rapid intensification (RI) stage characterized by three eyewall breakdown and redevelopment events. Each round of breakdown and reestablishment brings high potential vorticity and equivalent potential temperature air back into the eyewall, reinvigorating eyewall convection activity and driving intensification.

Abstract

A dataset of 88 recurving western North Pacific tropical cyclones from 2004 to 2015 is investigated for rainfall characteristics during their period of recurvature. The TCs are categorized into two groups based on different large-scale patterns from empirical orthogonal function analysis. Group 1 is characterized by an intense midlatitude baroclinic zone and close distance between the zone and TC, while Group 2 is characterized by a weaker midlatitude baroclinic zone and more remote distance between the zone and TC at the time of recurvature. The results show the large-scale environment has substantial impact on TC rainfall patterns. In Group 1, as the TC approaches and is embedded into the baroclinic zone, a relatively strong interaction between the TC and midlatitudes occurs, which is reflected by a rapid increase of environmental vertical wind shear and TC translation speed, the alignment of the shear vector and motion vector, and a sharp contrast of temperature and moisture. Higher rainfall and wider coverage of rainfall tends to be produced along the track after recurvature, and the rainfall pattern turns from a right-of-track (ROT) to a left-of-track (LOT) preference. Conversely, in Group 2, a relatively weak interaction between the TC and midlatitude circulation occurs, which is reflected by weaker vertical wind shear and slower TC motion, a separation of the shear vector and motion vector, and a weak gradient of temperature and moisture. The corresponding rainfall swath for Group 2 exhibits a narrower rainfall swath after recurvature. The rain pattern changes from a LOT to ROT preference.

Abstract

Lightning flashes in convective tropical clusters of the eastern North Pacific Ocean are detected by the Long-Range Lightning Detection Network and are analyzed for temporal patterns in electrical activity. The rates of lightning flash discharge in the 2006 season are analyzed for both tropical cyclones and nondeveloping cloud clusters to 1) determine if there is a difference in the convective activity of these two populations and 2) find a level of electrical activity that constitutes development in a particular system. Convective activity is associated with tropical cyclogenesis and thus we use the rate of electrical discharge as a proxy for convection associated with the likelihood of organization of individual cloud clusters into a tropical depression strength system. On the basis of the rates of lightning flashes in the cloud clusters, four levels of development are defined, ranging from non- and partially developing to fully developing cloud clusters. The levels of development are further supported by the analysis of other remotely sensed observations, such as surface scatterometer winds, that allow for the description of the mesoscale and large-scale circulation patterns in which the cloud clusters are embedded. It is found that lightning flash rates distinguish those cloud clusters that do not fully develop into tropical depressions from those that do. Receiver operating characteristic curves for these groupings are calculated, and a level of flash rate can be chosen that gives a probability of detection of 67% for a false-alarm rate of 24%.

Abstract

The physical mechanisms associated with the transformation stage of the extratropical transition of a tropical cyclone are simulated with a mesoscale model using initial environmental conditions that approximate the mean circulations defined by Klein et al. The tropical cyclone structural changes simulated by the U.S. Navy Coupled Ocean–Atmosphere Model Prediction System mesoscale model during the three steps of transformation compare well with available observations. During step 1 of transformation when the tropical cyclone is just beginning to interact with the midlatitude baroclinic zone, the main environmental factor that affects the tropical cyclone structure appears to be the decreased sea surface temperature. The movement of the tropical cyclone over the lower sea surface temperatures results in reduced surface heat and moisture fluxes, which weakens the core convection and the intensity decreases. During step 2 of transformation, the low-level temperature gradient and vertical wind shear associated with the baroclinic zone begin to affect the tropical cyclone. Main structural changes include the development of cloud-free regions on the west side of the tropical cyclone, and an enhanced “delta” rain region to the northwest of the tropical cyclone center. Gradual erosion of the clouds and deep convection in the west through south sectors of the tropical cyclone appear to be from mechanically forced subsidence due to the convergence between the midlatitude flow and the tropical cyclone circulation. Whereas the warm core aloft is advected downstream, the mid- to low-level warm core is enhanced by subsidence into the tropical cyclone center, which implies that the low-level cyclonic circulation may continue to be maintained.

Step 3 of transformation is the logical conclusion of structural changes that were occurring during steps 1 and 2. Even though the tropical cyclone circulation aloft has dissipated, a broad cyclonic circulation is maintained below 500 mb. Although the low-level warm core is reduced from step 2, it is still significantly stronger than at step 1, and a second warm anomaly is simulated in a region of strong subsidence upshear of the tropical cyclone remnants. Whereas some precipitation is associated with the remnants of the northern eyewall and some cloudiness to the north-northeast, the southern semicircle is almost completely clear of clouds and precipitation.

Abstract

The development of Typhoon Irving is investigated using a variety of data, including special research aircraft data from the Tropical Cyclone Motion (TCM-92) experiment, objective analyses, satellite data, and traditional surface and sounding data. The development process is treated as a dry-adiabatic vortex dynamics problem, and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation. Synoptic-scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development. Mesoscale interactions with the low-level, large-scale circulations and with other midlevel, mesoscale features result in development of vorticity in the midlevels and enhancement of the low-level vorticity associated with the developing surface cyclone.

Multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low-level confluence and a tendency toward recurrent development of associated mesoscale convective vortices. This is observed in both aircraft data and satellite imagery where subsequent interactions, including mergers with the low-level, tropical depression vortex, are observed. A contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices. Results from a simple baroclinic experiment show that the midlevel vortices affect the large-scale, low-level circulation in two ways: 1) initially, interactions between midlevel vortices produce a combined vortex of greater depth; 2) interaction between midlevel vortices and the low-level circulation produces a development downward of the midlevel vorticity. This strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere.

Abstract

A 42-yr study of eastern North Pacific tropical cyclones (TCs) undergoing extratropical transition (ET) is presented using the Japanese 55-yr Reanalysis dataset. By using cyclone phase space (CPS) to differentiate those TCs that undergo ET from those that do not, it is found that only 9% of eastern North Pacific TCs that developed from 1971 to 2012 complete ET, compared with 40% in the North Atlantic.

Using a combination of CPS, empirical orthogonal function (EOF) analysis, and composite analysis, it is found that the evolution of ET in this basin differs from that observed in the North Atlantic and western North Pacific, possibly as a result of the rapidly decreasing sea surface temperatures north of the main genesis region. The presence of a strong, deep subtropical ridge extending westward from North America into the eastern North Pacific is a major factor inhibiting ET in this basin. Similar to other basins, eastern North Pacific ET generally occurs in conjunction with an approaching midlatitude trough, which helps to weaken the ridge and allow northward passage of the TC. The frequency of ET appears to increase during developing El Niño events but is not significantly affected by the Pacific decadal oscillation.

Abstract

Numerical simulations of tropical cyclones are performed to examine the effects of a variable Coriolis parameter on the structure and intensity of hurricanes. The simulations are performed using the nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model using a 5-km fine mesh and fully explicit representation of moist processes. When a variable Conolis parameter ( f ) environment is applied to a mature tropical cyclone, a persistent north-northwesterly shear develops over the storm center as a result of an interaction between the primary circulation of the storm and the gradient in absolute vorticity. As a result, the variable-f storm quickly develops a persistent wavenumber-1 asymmetry in its inner-core structure with upward motion and rainfall concentrated on the left side of the shear looking downshear, in agreement with earlier studies. In comparison, the constant-f storm develops weak transient asymmetries in structure that are only partially related to a weak vertical wind shear. As a result, it is found that the tropical cyclone with variable f intensifies slightly more slowly than that with constant f, and reaches a final intensity that is about 5 mb weaker. It is argued that this “beta shear” is not adequately represented in large-scale analyses and so does not figure into calculations of environmental shear. Although the effect of the beta shear on the tropical cyclone intensity seems small by itself, when combined with the environmental shear it can produce a large net shear or it can reduce an environmental shear below the apparent threshold to impact storm intensity. If this result proves to be generally true, then the presence of an additional overlooked beta shear may well explain differences in the response of tropical cyclone intensification to westerly versus easterly shear regimes.

Abstract

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 wavenumber 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.