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Jenni L. Evans

Abstract

Increased occurrence of more intense tropical storms intruding further poleward has been foreshadowed as one of the potential consequences of global warming. This scenario is based almost entirely on the general circulation model predictions of warmer sea surface temperature (SST) with increasing levels of atmospheric C02 and some theories of tropical cyclone intensification that support the notion of more intense systems with warmer SST. Whether storms are able to achieve this theoretically determined more intense state depends on whether the temperature of the underlying water is the dominant factor in tropical cyclone intensification. An examination of the historical data record in a number of ocean basins is used to identify the relative importance of SST in the tropical cyclone intensification process. The results reveal that SST alone is an inadequate predictor of tropical cyclone intensity. Other factors known to affect tropical cyclone frequency and intensity are discussed.

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Jenni L. Evans
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Jenni L. Evans and Aviva Braun

Abstract

A 50-yr climatology (1957–2007) of subtropical cyclones (STs) in the South Atlantic is developed and analyzed. A subtropical cyclone is a hybrid structure (upper-level cold core and lower-level warm core) with associated surface gale-force winds. The tendency for warm season development of North Atlantic STs has resulted in these systems being confused as tropical cyclones (TCs). In fact, North Atlantic STs are a regular source of the incipient vortices leading to North Atlantic TC genesis. In 2004, Hurricane Catarina developed in the South Atlantic and made landfall in Brazil. A TC system had been previously unobserved in the South Atlantic, so the incidence of Catarina highlighted the lack of an ST climatology for the region to provide a context for the likelihood of future systems.

Sixty-three South Atlantic STs are documented over the 50-yr period analyzed in this climatology. In contrast to the North Atlantic, South Atlantic STs occur relatively uniformly throughout the year; however, their preferred location of genesis and mechanisms for this genesis do exhibit some seasonal variability. Rossby wave breaking was identified as the mechanism for the ST vortex initiation for North Atlantic STs. A subset of South Atlantic STs forms via this mechanism, however, an additional mechanism for ST genesis is identified here: lee cyclogenesis downstream of the Andes in the Brazil Current region—an area favorable for convection. This formation mechanism is similar to development of type-2 east coast lows in the Tasman Sea off eastern Australia.

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Giorgos Tsakraklides and Jenni L. Evans

Abstract

An automated objective classification procedure, the Convection Classification and Automated Tracking System (CCATS), is used to analyze the mean life cycles of organized convection in the global Tropics and midlatitudes (40°N–40°S). Five years (1989–93) of infrared satellite imagery are examined for the Pacific and Atlantic basins and one year (April 1988–March 1989) is studied for the Indian basin.

Two main classes of organized convection (lifetime of 6 h or more) are tracked: MCT and CCC. MCT represent a combined dataset of tropical cyclones and mesoscale convective complexes (MCC). Convective cloud clusters (CCC) meet the same cold cloud-top temperature, time, and size criteria used to distinguish MCC, but fail to sustain the same high degree of symmetry for at least 6 h. That is, CCC represent more elongated systems, such as squall lines. The frequency of CCC exceeds that of MCT by a factor of 30 over both land and sea.

MCT and CCC are each stratified to into 12 continental and oceanic regions and the diurnal variation of system characteristics in each geographic region are studied, leading to composite life cycle descriptions for each region. Oceanic CCC formed overnight and the shorter-lived, land-based CCC formed in the afternoon; apart from this time offset, oceanic and land-based CCC were found to have very similar life cycle evolution patterns.

Continental MCT exhibit a rapid size expansion early; this is not part of the oceanic system life cycle. Apart from this growth spurt, the evolution of land and ocean MCT follows the same pattern of CCC with early symmetry, then size expansion until just before termination. Land-based MCT are longer lived and more symmetric than oceanic MCT.

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Sytske K. Kimball and Jenni L. Evans

Abstract

A three-dimensional, nonhydrostatic, fine-resolution model, with explicitly resolved convective processes, is used to investigate the evolution of (a) a hurricane in two sheared flows, and (b) a hurricane interacting with four different upper-level lows. The negative impact of vertical shear on hurricane intensification is confirmed. The hurricanes display asymmetries that are most pronounced in higher shear flow. In both shear cases, the hurricane asymmetries seem to be related to a single upper-tropospheric outflow jet forcing convective activity below its right entrance region. Weak subsidence is confined to only part of the eye. Less eye subsidence leads to less inner-core warming, and hence a smaller fall in central surface pressure. A hurricane in zero flow (control) displays subsidence in the entire eye leading to a symmetric storm with a deep, strong warm core temperature anomaly and lower central surface pressure. In the weak shear and control cases, the radius of maximum wind (RMW) contracts as the storms intensifies via the mechanism of “symmetric intensification.” In the high-shear case the RMW and intensity remain almost steady.

When hurricanes interact with troughs, asymmetries are evident in the hurricanes and their RMWs expand as the storms slowly intensify. During the interaction, the troughs are deformed by the hurricane flow. Remnants of the deformed troughs prevent an outflow channel from developing on the eastern side of the hurricanes, hampering storm intensification in three of the four cases. In the fourth case, a strong and shallow trough merges with the hurricane causing a three-dimensional split of the trough, reduction of vertical shear over the vortex, followed by rapid intensification and RMW contraction. This vortex reaches the highest intensity of all four trough-interaction cases and comes close in intensity to the comparable no-trough case.

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Jenni L. Evans and Mark P. Guishard

Abstract

Subtropical cyclones (ST) have only recently gained attention as damaging weather systems. A set of criteria for identifying and classifying these systems is introduced here and employed to identify 18 ST cases forming in the 1999–2004 hurricane seasons. To be classified as an ST, these systems must have near-surface gale-force winds and show hybrid structure for more than one diurnal cycle. The 18 ST cases are partitioned into four classes based upon their genesis environments. Genesis over waters with SST in excess of 25°C is observed in almost 80% of warm-season cases, in contrast with only 55% in an ST climatology presented in a companion study. The low-shear magnitude constraint recognized for tropical cyclogenesis is less apparent for ST formation with over 50% forming in the two partitions characterized by shear in excess of 10 m s−1. This relatively high-shear environment corresponds to equatorward intrusion of upper troughs over the relatively warm SST present in the mid–late hurricane season. Anomaly composites confirm that ST genesis is associated with the intrusion of an upper trough in the westerlies into a region of relatively warm SST and weak static stability, with a corresponding reduction in the environmental shear near the time of ST genesis. These conditions correspond well with the conditions for tropical transition identified by Davis and Bosart. Indeed, these systems exhibit a propensity to continue development into a tropical cyclone; 80% eventually became named tropical systems. This result is consistent with a recent ST climatology but had not been widely recognized previously. This raises the possibility that tropical storms evolving from ST may have been overlooked or their tracks truncated in the National Hurricane Center Hurricane Database (HURDAT).

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Greg J. Holland and Jenni L. Evans

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The interactions between a barotropic vortex and an idealized subtropical ridge environment on a beta plane are examined and compared to the well-documented case of a single vortex with no environmental flow. First, the problems and advantages of several potential partitioning methods are discussed and then a three-part partition is chosen. Substantial variations are found from the single vortex case. In particular, the familiar gyres associated with the propagation of a single vortex are markedly distorted and relocated by the environment.

A vorticity budget is presented to help isolate the physical mechanisms. This analysis indicates that the major processes are associated with interactions with the gradients of absolute vorticity in the environment. Other nonlinear mechanisms can also be of significance in specific cases.

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Jenni L. Evans and Jeffrey J. Waters

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The impact of enhanced atmospheric CO2 concentrations on tropical convection and sea surface temperatures (SSTs) over the global tropics is assessed using five fully coupled atmospheric–oceanic general circulation models (AOGCMs). Relationships between SST and either outgoing longwave radiation or convective precipitation rates are evaluated for three climate states: present day, a doubled-CO2 scenario, and a quadrupled-CO2 scenario. All AOGCMs capture a relationship between present-day outgoing longwave radiation (OLR) and SST and between convective precipitation rate (PRC) and SST: deep tropical convection (DTC)—signified by rapidly decreasing OLR and rapidly increasing PRC rates—occurs above an SST threshold of around 25°C. Consistent across all AOGCMs, as concentrations increase to 2 × CO2 and 4 × CO2, the threshold SSTs for DTC to occur shift to 25.5°–28°C and 26.5°–30°C, respectively. Annual PRC rates in the 20°N–20°S region increase for two AOGCMs [Meteorological Research Institute Coupled General Circulation Model, version 2.3.2 (MRI CGCM2.3.2) and ECHAM5/Max Planck Institute Ocean Model (MPI-OM)] with increasing CO2, but PRC in the other three AOGCMs [Geophysical Fluid Dynamics Laboratory Climate Model versions 2.0 and 2.1 (GFDL CM2.0 and CM2.1) and National Center for Atmospheric Research (NCAR) Parallel Climate Model (PCM)] exhibits almost no change. Within this tropical zone, increased CO2 concentrations yield up to a 6.1% increase in the number of locations with monthly averaged PRC exceeding two established DTC thresholds (12 and 14 mm day−1). These results indicate that, although the SST threshold for DTC is projected to shift with increasing atmospheric CO2 concentrations, there will not be an expansion of regions experiencing DTC. One implication of these findings is that there will be little change in regions experiencing tropical cyclogenesis in future climate states.

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Robert E. Hart and Jenni L. Evans

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For over a century it has been known that each vortex in a multiple vortex configuration will move in response to the other vortices. However, despite advances since that time, the complexities of multiple vortex scenarios when sheared environments are present are still not completely understood. The interaction of binary vortices within horizontal environmental shear is explored here through shallow water simulations on a β plane. Due to nonlinear feedbacks, the combination of environmental vorticity (or vorticity gradient) and shear, as well as the multiple vortex situation, results in a more complicated track than for a storm experiencing any individual component. Despite the complexity of these vortex–environment interactions, the use of previous single-vortex studies greatly aids interpretation. Centroid-relative motion of the individual vortices is considered, as well as the propagation of the vortex pair centroid, to understand motion effects of the different vortex–environment combinations.

As the vortices interact, vortex Rossby waves are generated through distortion of the symmetric vorticity field by the opposing vortex. Initially, the high-frequency waves have an insignificant effect upon vortex intensity or propagation, and β-induced wavenumber one asymmetry dominates as expected. However, as the waves approach a critical radius (ζ = 0), wave potential vorticity filamentation and stretching by the circulation of the adjacent vortex leads to a coupling of the two vortices. This vortex coupling results in enhanced propagation speeds of the two vortices proportional to the effective size of the dual-vortex system.

The sign of vorticity of the environmental flow can act to enhance or negate β-drift such that single- or dual-vortex propagation is altered. Further, when environmental vorticity is present, the rate of mutual orbit from Fujiwhara rotation is altered. When the environmental flow is cyclonic, the cyclonic mutual rotation of the vortices is accelerated. Conversely, when the environmental flow is anticyclonic, the mutual rotation of the vortices is substantially decelerated, but remains cyclonic.

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Aaron S. Pratt and Jenni L. Evans

Abstract

Tropical cyclones have devastating impacts on countries across large parts of the globe, including the Atlantic basin. Thus, forecasting of the genesis of Atlantic tropical cyclones is important, but this problem remains a challenge for researchers and forecasters due to the variety of weather systems that can lead to tropical cyclogenesis (e.g., stalled frontal boundaries, African easterly waves, and extratropical cyclones), as well as the role of the surrounding environment in promoting or inhibiting the development into a tropical depression and beyond. In the North Atlantic, the effects of the Saharan air layer (SAL), a hot, dry dusty layer that moves into the eastern Atlantic basin, must be taken into account when forecasting whether genesis will occur. There are several characteristics of SAL that impact tropical cyclones (decreased midtropospheric moisture, increased midlevel shear, and enhanced stability). The purpose of this study is to examine the forecasting skill of the National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) model for the 2002 and 2003 Atlantic hurricane seasons, with particular regard paid to possible SAL effects on model genesis forecast accuracy. Cyclone phase space analyses of GFS 6-hourly forecasts were divided into three possible outcomes: S (successful forecasts that verified in cyclogenesis), F1 (cyclogenesis events that were not forecast to occur), and F2 (forecasted cyclogenesis that did not occur). The spatial variabilities of these outcomes for the early, middle, and late season were analyzed for both years, as well as the background environmental conditions. The large number of F2 forecasts that were seen in both years can be partly explained by the GFS model not capturing the detrimental effects of the SAL on cyclogenesis.

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