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- Author or Editor: Jenni L. Evans x
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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.
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.
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.
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.
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.
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.
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).
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).
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.
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.
Abstract
A modified formula for calculating tropical cyclone (TC) potential intensity (PI) from a balance between energy production and frictional dissipation in the TC surface layer is developed. This modified formula accounts for energy production and frictional dissipation at multiple radii (and therefore at multiple wind speeds) along the TC inflow trajectory. The PI maximum wind speed values V MAX are calculated using this expanded formula for four canonical radial profiles of wind speed. These results are compared to PI V MAX values calculated using the standard assumption that all energy production and frictional dissipation relevant to maximum intensity occurs at the radius of maximum winds (RMW).
The new PI formulation developed here results in PI V MAX values substantially higher than the standard PI V MAX for all four of the radial wind speed profiles examined; the difference is explained by the increase in the outer radial limit of energy production. This increase holds true even if outflow temperature increases with increasing radius, although the V MAX increases with increasing outer radius are somewhat more modest in this case. The extended PI formula yields V MAX values 3–17 m s−1 higher than the standard PI V MAX value when calculated with outer energy production–dissipation limits of 2.0–2.5 RMW, although it yields potentially unrealistic values when calculated with larger outer limits (e.g., 6 RMW). These results are presented as a potential explanation for why individual TCs can exceed their standard PI V MAX values in terms of the storm thermodynamics.
Abstract
A modified formula for calculating tropical cyclone (TC) potential intensity (PI) from a balance between energy production and frictional dissipation in the TC surface layer is developed. This modified formula accounts for energy production and frictional dissipation at multiple radii (and therefore at multiple wind speeds) along the TC inflow trajectory. The PI maximum wind speed values V MAX are calculated using this expanded formula for four canonical radial profiles of wind speed. These results are compared to PI V MAX values calculated using the standard assumption that all energy production and frictional dissipation relevant to maximum intensity occurs at the radius of maximum winds (RMW).
The new PI formulation developed here results in PI V MAX values substantially higher than the standard PI V MAX for all four of the radial wind speed profiles examined; the difference is explained by the increase in the outer radial limit of energy production. This increase holds true even if outflow temperature increases with increasing radius, although the V MAX increases with increasing outer radius are somewhat more modest in this case. The extended PI formula yields V MAX values 3–17 m s−1 higher than the standard PI V MAX value when calculated with outer energy production–dissipation limits of 2.0–2.5 RMW, although it yields potentially unrealistic values when calculated with larger outer limits (e.g., 6 RMW). These results are presented as a potential explanation for why individual TCs can exceed their standard PI V MAX values in terms of the storm thermodynamics.
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.
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.
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.
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.
Abstract
Hurricane Irene (1999) is examined as a case study of extratropical transition. Irene began its life in the Gulf of Mexico and made quick landfalls over both Cuba and Florida before entering the Atlantic Ocean at about 0900 UTC 16 October; the storm then paralleled the East Coast, tracking along the Gulf Stream for 2 days. Extratropical transition took place from 1800 UTC 17 October to 0000 UTC 19 October; during this time, the interaction of Irene with an upper-level jet streak contributed to intensification during and after transition. The details of the interaction are examined here using simulations of storm interactions with surface and upper-level features as well as quasigeostrophic omega and potential vorticity diagnostics. These analyses reveal that the extratropical transition of Irene was facilitated by the presence of an upper-level trough and jet streak, which contributed to the cyclogenesis ahead of the transitioning storm as well as to the posttransition intensification of the storm.
Abstract
Hurricane Irene (1999) is examined as a case study of extratropical transition. Irene began its life in the Gulf of Mexico and made quick landfalls over both Cuba and Florida before entering the Atlantic Ocean at about 0900 UTC 16 October; the storm then paralleled the East Coast, tracking along the Gulf Stream for 2 days. Extratropical transition took place from 1800 UTC 17 October to 0000 UTC 19 October; during this time, the interaction of Irene with an upper-level jet streak contributed to intensification during and after transition. The details of the interaction are examined here using simulations of storm interactions with surface and upper-level features as well as quasigeostrophic omega and potential vorticity diagnostics. These analyses reveal that the extratropical transition of Irene was facilitated by the presence of an upper-level trough and jet streak, which contributed to the cyclogenesis ahead of the transitioning storm as well as to the posttransition intensification of the storm.
