Search Results

You are looking at 1 - 10 of 40 items for

  • Author or Editor: Jenni L. Evans x
  • All content x
Clear All Modify Search
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.

Full access
Jenni L. Evans
Full access
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.

Full access
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.

Full access
Jenni L. Evans and Jeffrey J. Waters

Abstract

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.

Full access
Alex M. Kowaleski and Jenni L. Evans

Abstract

Track and cyclone phase space (CPS) forecasts of Hurricane Sandy from four global ensemble prediction systems are clustered using regression mixture models. Bayesian information criterion, cluster assignment strength, and mean-squared forecast error are used to select optimal model specifications. Fourth-order (third order) polynomials for 168-h forecasts (60-h forecast segments) and 5 (6) clusters for track (CPS) forecasts are selected.

Mean cluster paths from eight initialization times show that track and CPS clustering meaningfully partition potential tracks and structural evolutions, distilling a large number of ensemble members into several representative and distinct solutions. Rand index and adjusted Rand index calculations demonstrate a relationship between track and CPS cluster membership for both 168-h forecasts and 60-h forecast segments, indicating that certain tracks are preferentially associated with certain structural evolutions. These relationships are explained in greater detail using forecasts initialized at 0000 UTC 25 October.

Storm-centered cluster composite maps of 500-hPa geopotential height and 850-hPa equivalent potential temperature for the 120-h forecast valid at 0000 UTC 30 October (initialized at 0000 UTC 25 October) indicate that both track and CPS clustering successfully capture variations in the Sandy–trough interaction and the strength of the lower-troposphere warm core of Sandy at the time of observed landfall. Together, these results illustrate the relationship between the track and structural evolution of Sandy and suggest the potential of multiensemble mixture-model path clustering for tropical cyclone forecasting.

Full access
Jenni L. Evans and Robert E. Shemo

Abstract

A fully automated, objective classification system has been developed to analyze infrared satellite imagery. This automated system facilitates tracking and categorization of convective weather systems into various classes. The classes chosen reflect the maximum degree of organization attained by each weather system. Four classes of convective weather system are defined; tropical cyclones (TS; including prestorm clusters through to decaying storms), mesoscale convective complexes (MCC), convective cloud clusters (CCC), and disorganized shortlived convection (DSL). Systems are identified, tracked, and then classified. If a system satisfies the criteria for any of the organized convection classes (TS, MCC, or CCC) for at least two time periods, the entire track is allocated to that class. In cases where a system satisfies the criteria for more than one type of organized convection (commonly MCC and CCC), it is assigned to the “most organized” class (in this case, MCC). Thus, the characteristics of each class incorporate the life cycles of systems that satisfy the imposed criteria for at least a 6-h period.

Two satellite infrared-based (IR) rain-rate algorithms are applied to the convective areas in order to obtain precipitation amounts for the various classes of convection. The domain of interest extends from the eastern Pacific margin to the African coast (15°W) and 40°N–40°S.

In addition to the IR data, rain rates derived from Special Sensor Microwave/Imager data are compared with the infrared retrieved rain rates at available times for a subset of each of the three organized convection classes. Rainfall amounts obtained from these infrared algorithms are also compared with ground-based station observations over Florida. Comparison of the inferred rainfall with station data reveals that the TS precipitation is in approximate agreement (in the mean), whereas the precipitation contributions from the other forms of convection are somewhat overestimated. DSL is overestimated the most and CCCs are overestimated the least.

According to the infrared-based rain-rate algorithms, DSLs (short-lived systems) contribute the most total (basinwide, annual) precipitation, CCCs contribute the second largest amount, MCCs are third in the contribution of precipitation, and TSs contribute the least to the total precipitation.

Full access
Alex M. Kowaleski and Jenni L. Evans

Abstract

An ensemble of 72 Weather Research and Forecasting (WRF) Model simulations is evaluated to examine the relationship between the track of Hurricane Sandy (2012) and its structural evolution. Initial and boundary conditions are obtained from ECMWF and GEFS ensemble forecasts initialized at 0000 UTC 25 October. The 5-day WRF simulations are initialized at 0000 UTC 27 October, 48 h into the global model forecasts. Tracks and cyclone phase space (CPS) paths from the 72 simulations are partitioned into 6 clusters using regression mixture models; results from the 4 most populous track clusters are examined. The four analyzed clusters vary in mean landfall location from southern New Jersey to Maine. Extratropical transition timing is the clearest difference among clusters; more eastward clusters show later Sandy–midlatitude trough interaction, warm seclusion formation, and extratropical transition completion. However, the intercluster variability is much smaller when examined relative to the landfall time of each simulation. In each cluster, a short-lived warm seclusion forms and contracts through landfall while lower-tropospheric potential vorticity concentrates at small radii. Despite the large-scale similarity among the clusters, relevant intercluster differences in landfall-relative extratropical transition are observed. In the easternmost cluster the Sandy–trough interaction is least intense and the warm seclusion decays the most by landfall. In the second most eastward cluster Sandy retains the most intact warm seclusion at landfall because of a slightly later (relative to landfall) and weaker trough interaction compared to the two most westward clusters. Nevertheless, the remarkably similar large-scale evolution of Sandy among the four clusters indicates the high predictability of Sandy’s warm seclusion extratropical transition before landfall.

Full access
Greg J. Holland and Jenni L. Evans

Abstract

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.

Full access
Alex M. Kowaleski and Jenni L. Evans

Abstract

Thermodynamic variables including temperature, humidity, and equivalent potential temperature are obtained and calculated from 88 buoy and C-MAN time series of 38 Atlantic hurricanes. Radial profiles of these variables are compared to the tropical cyclone (TC) boundary layer idealization in potential intensity (PI) theory. For the composite hurricane, temperature decreases by 2.4 K between the environmental far field and the radius of maximum winds (RMW), in contrast to the PI boundary layer profile, which is radially isothermal outside the RMW. Observationally derived moisture and equivalent potential temperature (moist entropy) begin to increase with decreasing radius beyond the RMW, especially for the subset of category 3–5 hurricanes. This suggests the relevance of ocean–air fluxes beyond the RMW to increasing the moist entropy of eyewall updrafts. Ocean–air enthalpy fluxes produced by 85 time series with sea surface temperature data are explored using the bulk aerodynamic flux formulation and two methods that explicitly account for sea spray. Formulations incorporating sea spray produce greater total enthalpy fluxes, especially near the RMW. Total enthalpy fluxes calculated using composite observed conditions differ substantially from fluxes calculated using the idealizations of classic PI theory, though the sign of the difference depends on the calculation method used. Observed conditions may yield higher maximum intensities if maximum intensity is governed by the energy production–frictional dissipation balance under the eyewall. However, if TC intensity is governed by the entropy gained by inflow air, no matter where entropy is acquired, observed conditions may yield lower intensities than the classic PI theory boundary layer.

Full access