Mesoscale Interactions in Tropical Cyclone Genesis
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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 . 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.Keywords:
Mesoscale convective system
Trough (economics)
Abstract : A WC-130 instrumented aircraft will be deployed in the Western North Pacific region near Guam during 21 July-18 August 1992 to obtain in situ measurements in Mesoscale Convective Systems embedded in tropical cyclones. Four hypotheses related to different tropical cyclone track modification or genesis mechanisms will be tested. The scientific basis for these hypotheses is described and observations and models of midlatitude mesoscale convective systems are reviewed to provide a basis for planning the WC-130 missions. Aircraft operations and the Experiment Operations Center are described, along with tentative flight tracks. Descriptions of the real-time observations and the data to be archived for post-experiment analyses are provided.
Mesoscale convective system
Middle latitudes
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Extratropical cyclone
Cyclogenesis
Typhoon
Intensity
Tropical cyclogenesis
Maximum sustained wind
Mesoscale convective system
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Abstract Compared to satellite-derived heating profiles, the Goddard Institute for Space Studies general circulation model (GCM) convective heating is too deep and its stratiform upper-level heating is too weak. This deficiency highlights the need for GCMs to parameterize the mesoscale organization of convection. Cloud-resolving model simulations of convection near Darwin, Australia, in weak wind shear environments of different humidities are used to characterize mesoscale organization processes and to provide parameterization guidance. Downdraft cold pools appear to stimulate further deep convection both through their effect on eddy size and vertical velocity. Anomalously humid air surrounds updrafts, reducing the efficacy of entrainment. Recovery of cold pool properties to ambient conditions over 5–6 h proceeds differently over land and ocean. Over ocean increased surface fluxes restore the cold pool to prestorm conditions. Over land surface fluxes are suppressed in the cold pool region; temperature decreases and humidity increases, and both then remain nearly constant, while the undisturbed environment cools diurnally. The upper-troposphere stratiform rain region area lags convection by 5–6 h under humid active monsoon conditions but by only 1–2 h during drier break periods, suggesting that mesoscale organization is more readily sustained in a humid environment. Stratiform region hydrometeor mixing ratio lags convection by 0–2 h, suggesting that it is strongly influenced by detrainment from convective updrafts. Small stratiform region temperature anomalies suggest that a mesoscale updraft parameterization initialized with properties of buoyant detrained air and evolving to a balance between diabatic heating and adiabatic cooling might be a plausible approach for GCMs.
Diabatic
Mesoscale convective system
Entrainment (biomusicology)
Radiative Cooling
Atmospheric convection
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Abstract Tropical cyclone formation close to the coastline of the Asian continent presents a significant threat to heavily populated coastal countries. A case study of Tropical Storm Mekkhala (2008) that developed off the coast of Vietnam is presented using the high-resolution analyses of the European Centre for Medium-Range Weather Forecasts/Year of Tropical Convection and multiple satellite observations. The authors have analyzed contributions to the formation from large-scale intraseasonal variability, synoptic perturbations, and mesoscale convective systems (MCSs). Within a large-scale westerly wind burst (WWB) associated with the Madden–Julian oscillation (MJO), synoptic perturbations generated by two preceding tropical cyclones initiated the pre-Mekkhala low-level vortex over the Philippine Sea. Typhoon Hagupit produced a synoptic-scale wave train that contributed to the development of Jangmi, but likely suppressed the Mekkhala formation. The low-level vortex of the pre-Mekkhala disturbance was then initiated in a confluent zone between northeasterlies in advance of Typhoon Jangmi and the WWB. A key contribution to the development of Mekkhala was from diurnally varying MCSs that were invigorated in the WWB. The oceanic MCSs, which typically develop off the west coast of the Philippines in the morning and dissipate in the afternoon, were prolonged beyond the regular diurnal cycle. A combination with the MCSs developing downstream of the Philippines led to the critical structure change of the oceanic convective cluster, which implies the critical role of mesoscale processes. Therefore, the diurnally varying mesoscale convective processes over both the ocean and land are shown to have an essential role in the formation of Mekkhala in conjunction with large-scale MJO and the synoptic-scale TC influences.
Typhoon
Madden–Julian oscillation
Mesoscale convective system
Tropical cyclogenesis
Tropical cyclone scales
Diurnal cycle
Rainband
African easterly jet
Synoptic scale meteorology
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Abstract The mesoscale features of 124 tropical cyclone formations in the western North Pacific Ocean during 1999–2004 are investigated through large-scale analyses, satellite infrared brightness temperature (TB), and Quick Scatterometer (QuikSCAT) oceanic wind data. Based on low-level wind flow and surge direction, the formation cases are classified into six synoptic patterns: easterly wave (EW), northeasterly flow (NE), coexistence of northeasterly and southwesterly flow (NE–SW), southwesterly flow (SW), monsoon confluence (MC), and monsoon shear (MS). Then the general convection characteristics and mesoscale convective system (MCS) activities associated with these formation cases are studied under this classification scheme. Convection processes in the EW cases are distinguished from the monsoon-related formations in that the convection is less deep and closer to the formation center. Five characteristic temporal evolutions of the deep convection are identified: (i) single convection event, (ii) two convection events, (iii) three convection events, (iv) gradual decrease in TB, and (v) fluctuating TB, or a slight increase in TB before formation. Although no dominant temporal evolution differentiates cases in the six synoptic patterns, evolutions ii and iii seem to be the common routes taken by the monsoon-related formations. The overall percentage of cases with MCS activity at multiple times is 63%, and in 35% of cases more than one MCS coexisted. Most of the MC and MS cases develop multiple MCSs that lead to several episodes of deep convection. These two patterns have the highest percentage of coexisting MCSs such that potential interaction between these systems may play a role in the formation process. The MCSs in the monsoon-related formations are distributed around the center, except in the NE–SW cases in which clustering of MCSs is found about 100–200 km east of the center during the 12 h before formation. On average only one MCS occurs during an EW formation, whereas the mean value is around two for the other monsoon-related patterns. Both the mean lifetime and time of first appearance of MCS in EW are much shorter than those developed in other synoptic patterns, which indicates that the overall formation evolution in the EW case is faster. Moreover, this MCS is most likely to be found within 100 km east of the center 12 h before formation. The implications of these results to internal mechanisms of tropical cyclone formation are discussed in light of other recent mesoscale studies.
Mesoscale convective system
Deep convection
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The purpose of this paper is to explore how a tropical cyclone forms from a pre-existing large-scale depression which has been observed and associated with cross-equatorial surges in the western North Pacific. Tropical cyclone Bilis（2000） was selected as the case to study.The research data used are from the results of the non-hydrostatic mesoscale model（MM5）,which has successfully simulated the transformation of a pre-existing weak large-scale tropical depression into a strong tropical storm.The scale separation technique is used to separate the synoptic-scale and sub-synoptic-scale fields from the model output fields. The scale-separated fields show that the pre-existing synoptic-scale tropical depression and the subsynoptic scale tropical cyclone formed later were different scale systems from beginning to end.It is also shown that the pre-existing synoptic-scale tropical depression did not contract to become the tropical cyclone. A series of weak,sub-synoptic-scale low and high pressure systems appeared and disappeared in the synopticscale depression,with one of the low systems near the center of the synoptic-scale depression having deepened to become the tropical cyclone. The roles of the synoptic-scale flow and the sub-synoptic scale disturbances in the formation of the tropical cyclone are investigated by diagnoses of the scale-separated vertical vorticity equation.The results show that the early development of the sub-synoptic scale vortex was fundamentally dependent on the strengthening synoptic-scale environmental depression.The depression was strengthened by cross-equatorial surges,which increased the convergence of the synoptic-scale depression at low levels and triggered the formation of the tropical cyclone.
Tropical cyclogenesis
Synoptic scale meteorology
African easterly jet
MM5
Cyclogenesis
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The large-scale environmental characteristics that are favorable for tropical cyclone formation are typically found to occur over most ocean basins that contain a monsoon system. Although the favorable conditions occur consistently in the monsoon basins, tropical cyclone activity is closely tied to variability in the monsoon that provides favorable or unfavorable modifications to the basic environmental characteristics. This variability may occur on interannual, intraseasonal, and synoptic time scales. Therefore, it is important to understand the various linkages between monsoon variability and tropical cyclone activity in each monsoon basin. Over recent years, several international field programs have been conducted to investigate various interactions among factors that link tropical cyclone activity and monsoon circulations. The primary programs have been conducted over the tropical western North Pacific. These include the special programs such as the THORPEX-Pacific Asian Regional Campaign (T-PARC) and the Tropical Cyclone Structure–2008 (TCS-08) that were conducted during August-September 2008, and the annual Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR). The anomalous monsoon environment that existed during T-PARC/TCS-08 is described. Additionally, the science objectives, observing platforms, and facilities associated with these programs are summarized with respect to observations of tropical cyclone characteristics in the monsoon environment.
Typhoon
Tropical cyclone scales
Tropical monsoon climate
African easterly jet
East Asian Monsoon
Tropical cyclogenesis
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Abstract This observational study is organized around three issues: the forcing of the monsoon flow by embedded deep convection, the thermodynamic conditions that support the convection, and the structures of the mesoscale convective systems (MCSs) that constitute much of the deep convection. The monsoon convective cloudiness occurred predominantly offshore, over the earth's warmest waters. Week‐long periods of widespread deep convection caused cycles of monsoon spin‐up, culminating in tropical‐cyclone formation. Momentum transports by the convection also created smaller‐scale vortex pairs in the upper troposphere. Sounding data suggest that the convection was modulated by low‐level processes, not by large‐scale deep forced ascent. Triggering by mesoscale boundary‐layer cold‐pool boundaries and coastlines determined specifically where convection occurred, while several positive feedbacks acted to keep regional thermodynamic conditions favourable. Hence monsoon convection, once initiated, persisted as self‐exciting ‘superclusters’, composed at any instant of many distinct MCSs. The observed MCSs all contained areas of deep convection and stratiform precipitation areas. Stratiform precipitation usually evolved in place from convective cells, although precipitation also occasionally fell from overhanging anvils created by upper‐level shear. the MCSs within similar synoptic wind environments tended to have similar mesoscale structures. This fact reflects the effects, illustrated herein, of environmental winds upon the relative motions of cold pools (which trigger new convective cells) and stratiform precipitation areas (which evolve from old convective cells).
Mesoscale convective system
Diurnal cycle
Free convective layer
Forcing (mathematics)
Squall line
Convective inhibition
Atmospheric convection
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Citations (126)