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Abstract
Turbulence in the planetary boundary layer (PBL) transports heat, momentum, and moisture in eddies that are not resolvable by current NWP systems. Numerical models typically parameterize this process using vertical diffusion operators whose coefficients depend on the intensity of the expected turbulence. The PBL scheme employed in this study uses a one-and-a-half-order closure based on a predictive equation for the turbulent kinetic energy (TKE). For a stably stratified fluid, the growth and decay of TKE is largely controlled by the dynamic stability of the flow as represented by the Richardson number. Although the existence of a critical Richardson number that uniquely separates turbulent and laminar regimes is predicted by linear theory and perturbation analysis, observational evidence and total energy arguments suggest that its value is highly uncertain. This can be explained in part by the apparent presence of turbulence regime-dependent critical values, a property known as Richardson number hysteresis. In this study, a parameterization of Richardson number hysteresis is proposed. The impact of including this effect is evaluated in systems of increasing complexity: a single-column model, a forecast case study, and a full assimilation cycle. It is shown that accounting for a hysteretic loop in the TKE equation improves guidance for a canonical freezing rain event by reducing the diffusive elimination of the warm nose aloft, thus improving the model’s representation of PBL profiles. Systematic enhancements in predictive skill further suggest that representing Richardson number hysteresis in PBL schemes using higher-order closures has the potential to yield important and physically relevant improvements in guidance quality.
Abstract
Turbulence in the planetary boundary layer (PBL) transports heat, momentum, and moisture in eddies that are not resolvable by current NWP systems. Numerical models typically parameterize this process using vertical diffusion operators whose coefficients depend on the intensity of the expected turbulence. The PBL scheme employed in this study uses a one-and-a-half-order closure based on a predictive equation for the turbulent kinetic energy (TKE). For a stably stratified fluid, the growth and decay of TKE is largely controlled by the dynamic stability of the flow as represented by the Richardson number. Although the existence of a critical Richardson number that uniquely separates turbulent and laminar regimes is predicted by linear theory and perturbation analysis, observational evidence and total energy arguments suggest that its value is highly uncertain. This can be explained in part by the apparent presence of turbulence regime-dependent critical values, a property known as Richardson number hysteresis. In this study, a parameterization of Richardson number hysteresis is proposed. The impact of including this effect is evaluated in systems of increasing complexity: a single-column model, a forecast case study, and a full assimilation cycle. It is shown that accounting for a hysteretic loop in the TKE equation improves guidance for a canonical freezing rain event by reducing the diffusive elimination of the warm nose aloft, thus improving the model’s representation of PBL profiles. Systematic enhancements in predictive skill further suggest that representing Richardson number hysteresis in PBL schemes using higher-order closures has the potential to yield important and physically relevant improvements in guidance quality.
Abstract
Several NWP centers currently employ a variational data assimilation approach for initializing deterministic forecasts and a separate ensemble Kalman filter (EnKF) system both for initializing ensemble forecasts and for providing ensemble background error covariances for the deterministic system. This study describes a new approach for performing the data assimilation step within a perturbed-observation EnKF. In this approach, called VarEnKF, the analysis increment is computed with a variational data assimilation approach both for the ensemble mean and for all of the ensemble perturbations. To obtain a computationally efficient algorithm, a much simpler configuration is used for the ensemble perturbations, whereas the configuration used for the ensemble mean is similar to that used for the deterministic system. Numerous practical benefits may be realized by using a variational approach for both deterministic and ensemble prediction, including improved efficiency for the development and maintenance of the computer code. Also, the use of essentially the same data assimilation algorithm would likely reduce the amount of numerical experimentation required when making system changes, since their impacts in the two systems would be very similar. The variational approach enables the use of hybrid background error covariances and may also allow the assimilation of a larger volume of observations. Preliminary tests with the Canadian global 256-member system produced significantly improved ensemble forecasts with VarEnKF as compared with the current EnKF and at a comparable computational cost. These improvements resulted entirely from changes to the ensemble mean analysis increment calculation. Moreover, because each ensemble perturbation is updated independently, VarEnKF scales perfectly up to a very large number of processors.
Abstract
Several NWP centers currently employ a variational data assimilation approach for initializing deterministic forecasts and a separate ensemble Kalman filter (EnKF) system both for initializing ensemble forecasts and for providing ensemble background error covariances for the deterministic system. This study describes a new approach for performing the data assimilation step within a perturbed-observation EnKF. In this approach, called VarEnKF, the analysis increment is computed with a variational data assimilation approach both for the ensemble mean and for all of the ensemble perturbations. To obtain a computationally efficient algorithm, a much simpler configuration is used for the ensemble perturbations, whereas the configuration used for the ensemble mean is similar to that used for the deterministic system. Numerous practical benefits may be realized by using a variational approach for both deterministic and ensemble prediction, including improved efficiency for the development and maintenance of the computer code. Also, the use of essentially the same data assimilation algorithm would likely reduce the amount of numerical experimentation required when making system changes, since their impacts in the two systems would be very similar. The variational approach enables the use of hybrid background error covariances and may also allow the assimilation of a larger volume of observations. Preliminary tests with the Canadian global 256-member system produced significantly improved ensemble forecasts with VarEnKF as compared with the current EnKF and at a comparable computational cost. These improvements resulted entirely from changes to the ensemble mean analysis increment calculation. Moreover, because each ensemble perturbation is updated independently, VarEnKF scales perfectly up to a very large number of processors.
Abstract
The importance of stratospheric influences for medium-range numerical weather prediction (NWP) of the troposphere has led to increases in the heights of global model domains at operational centers around the world. Grids now routinely extend to 0.1 hPa (approximately 65 km) in these systems, thereby covering the full depth of the stratosphere and the lower portion of the mesosphere. Increasing the vertical extent of higher-resolution limited-area models (LAMs) nested within the global forecasts is problematic because of the computational cost of additional levels and the possibility of inaccuracy or instability in the high-speed stratospheric jets. An upper-boundary nesting (UBN) technique is developed that allows information from high-topped driving grids to influence the evolution of a lower-topped (~10 hPa) LAM integration in a manner analogous to the treatment of lateral boundary conditions.
A stratospheric vortex displacement event in the winter of 2007 is used to study the effectiveness of the UBN technique. Tropospheric blocking over Europe leads to the development of an amplifying planetary-scale wave in the lower stratosphere that culminates in an anticyclonic wave break over Asia and a marked increase of wave-1 asymmetry. The rapid evolution of stratospheric potential vorticity (PV) is poorly represented in low-topped models, resulting in PV-induced forecast height errors throughout the depth of the troposphere on time scales as short as 2–5 days. Application of the UBN technique is shown to be an effective way for low-topped configurations to benefit from stratospheric predictability without the problems associated with the inclusion of the stratospheric flow in the higher-resolution model domain. The robustness and relative ease of implementation of the UBN technique may make this computationally inexpensive strategy attractive for a wide range of NWP applications.
Abstract
The importance of stratospheric influences for medium-range numerical weather prediction (NWP) of the troposphere has led to increases in the heights of global model domains at operational centers around the world. Grids now routinely extend to 0.1 hPa (approximately 65 km) in these systems, thereby covering the full depth of the stratosphere and the lower portion of the mesosphere. Increasing the vertical extent of higher-resolution limited-area models (LAMs) nested within the global forecasts is problematic because of the computational cost of additional levels and the possibility of inaccuracy or instability in the high-speed stratospheric jets. An upper-boundary nesting (UBN) technique is developed that allows information from high-topped driving grids to influence the evolution of a lower-topped (~10 hPa) LAM integration in a manner analogous to the treatment of lateral boundary conditions.
A stratospheric vortex displacement event in the winter of 2007 is used to study the effectiveness of the UBN technique. Tropospheric blocking over Europe leads to the development of an amplifying planetary-scale wave in the lower stratosphere that culminates in an anticyclonic wave break over Asia and a marked increase of wave-1 asymmetry. The rapid evolution of stratospheric potential vorticity (PV) is poorly represented in low-topped models, resulting in PV-induced forecast height errors throughout the depth of the troposphere on time scales as short as 2–5 days. Application of the UBN technique is shown to be an effective way for low-topped configurations to benefit from stratospheric predictability without the problems associated with the inclusion of the stratospheric flow in the higher-resolution model domain. The robustness and relative ease of implementation of the UBN technique may make this computationally inexpensive strategy attractive for a wide range of NWP applications.
Abstract
The role of moist processes in regulating mesoscale snowband life cycle within the comma head portion of three northeast U.S. cyclones is investigated using piecewise potential vorticity (PV) inversion, modeling experiments, and potential temperature tendency budgets. Snowband formation in each case occurred along a mesoscale trough that extended poleward of a 700-hPa low. This 700-hPa trough was associated with intense frontogenetical forcing for ascent. A variety of PV evolutions among the cases contributed to midlevel trough formation and associated frontogenesis. However, in each case the induced flow from diabatic PV anomalies accounted for a majority of the midlevel frontogenesis during the band’s life cycle, highlighting the important role that latent heat release plays in band evolution. Simulations with varying degrees of latent heating show that diabatic processes associated with the band itself were critical to the development and maintenance of the band. However, changes in the meso-α-scale flow associated with the development of diabatic PV anomalies east of the band contributed to frontolysis and band dissipation. Conditional stability was reduced near 500 hPa in each case several hours prior to band formation. This stability remained small until band formation, when the stratification generally increased in association with the release of conditional instability. Previous studies have suggested that the dry slot is important for the initial stability reduction at midlevels, but this was not evident for the three banding cases examined. Rather, differential horizontal temperature advection in moist southwest flow ahead of the upper trough was the dominant process that reduced the midlevel conditional stability.
Abstract
The role of moist processes in regulating mesoscale snowband life cycle within the comma head portion of three northeast U.S. cyclones is investigated using piecewise potential vorticity (PV) inversion, modeling experiments, and potential temperature tendency budgets. Snowband formation in each case occurred along a mesoscale trough that extended poleward of a 700-hPa low. This 700-hPa trough was associated with intense frontogenetical forcing for ascent. A variety of PV evolutions among the cases contributed to midlevel trough formation and associated frontogenesis. However, in each case the induced flow from diabatic PV anomalies accounted for a majority of the midlevel frontogenesis during the band’s life cycle, highlighting the important role that latent heat release plays in band evolution. Simulations with varying degrees of latent heating show that diabatic processes associated with the band itself were critical to the development and maintenance of the band. However, changes in the meso-α-scale flow associated with the development of diabatic PV anomalies east of the band contributed to frontolysis and band dissipation. Conditional stability was reduced near 500 hPa in each case several hours prior to band formation. This stability remained small until band formation, when the stratification generally increased in association with the release of conditional instability. Previous studies have suggested that the dry slot is important for the initial stability reduction at midlevels, but this was not evident for the three banding cases examined. Rather, differential horizontal temperature advection in moist southwest flow ahead of the upper trough was the dominant process that reduced the midlevel conditional stability.
Abstract
As subsaturated air ascends sloping isentropic surfaces, adiabatic expansion results in cooling and relative moistening. This process is an effective way to precondition the atmosphere for efficient moist processes while bringing parcels to saturation, and thereafter acts to maintain saturation during condensation. The goal of this study is to develop a diagnostic quantity that highlights circulations and regions in which the process of parcel moistening by isentropic ascent is active. Among the many features that rely on this process for the generation of an important fraction of their energy are oceanic cyclones, transitioning tropical cyclones, warm conveyor belts, diabatic Rossby vortices, and predecessor rain events. The baroclinic moisture flux (BMF) is defined as moisture transport by the component of vertical motion associated with isentropic upgliding. In warm conveyor belt and diabatic Rossby vortex case studies, the BMF appears to be successful in identifying the portion of the circulation in which this process is actively bringing parcels to saturation to promote the formation of clouds and precipitation. On a broader scale, the climatological maxima of the BMF highlight regions in which parcel moistening by isentropic ascent is anticipated to have a nonnegligible impact on the atmospheric state either through the action of the mean flow or via the repeated occurrence of isolated large-BMF events. The process-centric foundation of the BMF makes it useful as a filtering or exploratory variable, with the potential for extension into predictive applications.
Abstract
As subsaturated air ascends sloping isentropic surfaces, adiabatic expansion results in cooling and relative moistening. This process is an effective way to precondition the atmosphere for efficient moist processes while bringing parcels to saturation, and thereafter acts to maintain saturation during condensation. The goal of this study is to develop a diagnostic quantity that highlights circulations and regions in which the process of parcel moistening by isentropic ascent is active. Among the many features that rely on this process for the generation of an important fraction of their energy are oceanic cyclones, transitioning tropical cyclones, warm conveyor belts, diabatic Rossby vortices, and predecessor rain events. The baroclinic moisture flux (BMF) is defined as moisture transport by the component of vertical motion associated with isentropic upgliding. In warm conveyor belt and diabatic Rossby vortex case studies, the BMF appears to be successful in identifying the portion of the circulation in which this process is actively bringing parcels to saturation to promote the formation of clouds and precipitation. On a broader scale, the climatological maxima of the BMF highlight regions in which parcel moistening by isentropic ascent is anticipated to have a nonnegligible impact on the atmospheric state either through the action of the mean flow or via the repeated occurrence of isolated large-BMF events. The process-centric foundation of the BMF makes it useful as a filtering or exploratory variable, with the potential for extension into predictive applications.
Abstract
The threat posed to North America by Atlantic Ocean tropical cyclones (TCs) was highlighted by a series of intense landfalling storms that occurred during the record-setting 2005 hurricane season. However, the ability to understand—and therefore the ability to predict—tropical cyclogenesis remains limited, despite recent field studies and numerical experiments that have led to the development of conceptual models describing pathways for tropical vortex initiation. This study addresses the issue of TC spinup by developing a dynamically based classification scheme built on a diagnosis of North Atlantic hurricanes between 1948 and 2004. A pair of metrics is presented that describes TC development from the perspective of external forcings in the local environment. These discriminants are indicative of quasigeostrophic forcing for ascent and lower-level baroclinicity and are computed for the 36 h leading up to TC initiation. A latent trajectory model is used to classify the evolution of the metrics for 496 storms, and a physical synthesis of the results yields six identifiable categories of tropical cyclogenesis events. The nonbaroclinic category accounts for 40% of Atlantic TCs, while events displaying perturbations from this archetype make up the remaining 60% of storms. A geographical clustering of the groups suggests that the classification scheme is identifying fundamentally different categories of tropical cyclogenesis. Moreover, significant differences between the postinitiation attributes of the classes indicate that the evolution of TCs may be sensitive to the pathway taken during development.
Abstract
The threat posed to North America by Atlantic Ocean tropical cyclones (TCs) was highlighted by a series of intense landfalling storms that occurred during the record-setting 2005 hurricane season. However, the ability to understand—and therefore the ability to predict—tropical cyclogenesis remains limited, despite recent field studies and numerical experiments that have led to the development of conceptual models describing pathways for tropical vortex initiation. This study addresses the issue of TC spinup by developing a dynamically based classification scheme built on a diagnosis of North Atlantic hurricanes between 1948 and 2004. A pair of metrics is presented that describes TC development from the perspective of external forcings in the local environment. These discriminants are indicative of quasigeostrophic forcing for ascent and lower-level baroclinicity and are computed for the 36 h leading up to TC initiation. A latent trajectory model is used to classify the evolution of the metrics for 496 storms, and a physical synthesis of the results yields six identifiable categories of tropical cyclogenesis events. The nonbaroclinic category accounts for 40% of Atlantic TCs, while events displaying perturbations from this archetype make up the remaining 60% of storms. A geographical clustering of the groups suggests that the classification scheme is identifying fundamentally different categories of tropical cyclogenesis. Moreover, significant differences between the postinitiation attributes of the classes indicate that the evolution of TCs may be sensitive to the pathway taken during development.
Abstract
The period 5–15 June 2003, during the field phase of the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX), was noteworthy for the wide variety of mesoscale convective systems (MCSs) that occurred. Of particular interest was a long-lived MCV that formed in the trailing stratiform region of an MCS over west Texas at 0600 UTC 10 June. This MCV was noteworthy for its (i) longevity as it can be tracked from 0600 UTC 10 June to 1200 UTC 14 June, (ii) development of a surface cyclonic circulation and attendant −2- to −4-hPa sea level pressure perturbation, (iii) ability to retrigger convection and produce widespread rains over several diurnal heating cycles, and (iv) transition into a baroclinic surface cyclone with distinct frontal features. Baroclinic transition, defined here as the acquisition of surface fronts, occurred as the MCV interacted with a remnant cold front, left behind by a predecessor extratropical cyclone, over the Great Lakes region. Although the MCV developed well-defined frontal structure, which helped to focus heavy precipitation, weakening occurred throughout the baroclinic transition process. Energetics calculations indicated that weakening occurred as the diabatic and baroclinic energy conversion terms approached zero just prior and during baroclinic transition. This weakening can be attributed to (i) an increase in environmental wind shear, (ii) the development of a downshear tilt and associated anticyclonic vorticity advection over the surface low center, and (iii) the eastward relative movement of organized convection away from the MCV center.
Abstract
The period 5–15 June 2003, during the field phase of the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX), was noteworthy for the wide variety of mesoscale convective systems (MCSs) that occurred. Of particular interest was a long-lived MCV that formed in the trailing stratiform region of an MCS over west Texas at 0600 UTC 10 June. This MCV was noteworthy for its (i) longevity as it can be tracked from 0600 UTC 10 June to 1200 UTC 14 June, (ii) development of a surface cyclonic circulation and attendant −2- to −4-hPa sea level pressure perturbation, (iii) ability to retrigger convection and produce widespread rains over several diurnal heating cycles, and (iv) transition into a baroclinic surface cyclone with distinct frontal features. Baroclinic transition, defined here as the acquisition of surface fronts, occurred as the MCV interacted with a remnant cold front, left behind by a predecessor extratropical cyclone, over the Great Lakes region. Although the MCV developed well-defined frontal structure, which helped to focus heavy precipitation, weakening occurred throughout the baroclinic transition process. Energetics calculations indicated that weakening occurred as the diabatic and baroclinic energy conversion terms approached zero just prior and during baroclinic transition. This weakening can be attributed to (i) an increase in environmental wind shear, (ii) the development of a downshear tilt and associated anticyclonic vorticity advection over the surface low center, and (iii) the eastward relative movement of organized convection away from the MCV center.
Abstract
The development and tropical transition (TT) of a subsynoptic-scale cyclone in the Gulf of Genoa during the Mesoscale Alpine Project (MAP) demonstration of probabilistic hydrological and atmospheric simulation of flood events in the alpine region (D-PHASE) project is investigated using analyses and model simulations. Cyclogenesis occurs in association with the passage of a synoptic-scale trough and attendant surface cold front across the Alps on 15 November 2007. An embedded coherent tropopause disturbance (CTD) plays an important role in promoting the initial development of the lower-level vortex by simultaneously providing quasigeostrophic forcing for ascent and reducing the bulk column stability over warm Mediterranean waters. Persistent convection thereafter erodes the CTD as the storm transitions into a hurricane-like vortex.
In addition to this upper-level forcing, a pair of diabatically generated lower-level cyclonic potential vorticity (PV) features associated with distinct flow regimes is potentially important to the cyclogenetic process in this case. The first, a warm surface potential temperature anomaly, is generated during cross-barrier flow by prefrontal upslope precipitation on the Alpine northside, followed by parcel descent in the lee. The second PV feature is a mountain-scale PV banner that extends southward from the southwestern tip of the Alps as the flow is deflected around the mountain chain.
Numerical guidance for this case is evaluated on its ability to accurately depict the development and evolution of the cyclone. Comparison of a triply nested integration (grid spacings of 33, 10, and 2.5 km) with observations and analyses demonstrates that the model is capable of simulating the salient features of the event. Combining reliable guidance from high-resolution modeling systems with the paradigms of lee cyclone development and the emerging concepts of TT promotes an improved understanding of these potentially high-impact events.
Abstract
The development and tropical transition (TT) of a subsynoptic-scale cyclone in the Gulf of Genoa during the Mesoscale Alpine Project (MAP) demonstration of probabilistic hydrological and atmospheric simulation of flood events in the alpine region (D-PHASE) project is investigated using analyses and model simulations. Cyclogenesis occurs in association with the passage of a synoptic-scale trough and attendant surface cold front across the Alps on 15 November 2007. An embedded coherent tropopause disturbance (CTD) plays an important role in promoting the initial development of the lower-level vortex by simultaneously providing quasigeostrophic forcing for ascent and reducing the bulk column stability over warm Mediterranean waters. Persistent convection thereafter erodes the CTD as the storm transitions into a hurricane-like vortex.
In addition to this upper-level forcing, a pair of diabatically generated lower-level cyclonic potential vorticity (PV) features associated with distinct flow regimes is potentially important to the cyclogenetic process in this case. The first, a warm surface potential temperature anomaly, is generated during cross-barrier flow by prefrontal upslope precipitation on the Alpine northside, followed by parcel descent in the lee. The second PV feature is a mountain-scale PV banner that extends southward from the southwestern tip of the Alps as the flow is deflected around the mountain chain.
Numerical guidance for this case is evaluated on its ability to accurately depict the development and evolution of the cyclone. Comparison of a triply nested integration (grid spacings of 33, 10, and 2.5 km) with observations and analyses demonstrates that the model is capable of simulating the salient features of the event. Combining reliable guidance from high-resolution modeling systems with the paradigms of lee cyclone development and the emerging concepts of TT promotes an improved understanding of these potentially high-impact events.
Abstract
The development and subsequent tropical transition of a subsynoptic-scale cyclone over the Gulf of Genoa (GoG) on 15 November 2007 led to the rapid onset of tropical storm-force winds near the islands of Corsica and Sardinia. This study evaluates the influence of two key ingredients on the cyclogenesis event: a near-surface warm potential temperature perturbation in the lee of the Alps and a mountain-scale potential vorticity (PV) banner.
A high-resolution modeling system is used to perform a set of attribution tests in which modifications to the Alpine orography control the presence of the cyclogenetic ingredients. When either feature exists in the initial state, a GoG cyclone develops even when the Alpine barrier is removed; however, when neither the warm perturbation nor the PV banner is present, there is insufficient lower-level PV to couple with the upper-level trough to promote cyclogenesis. A conceptual model involving the complimentary interaction of the two PV features is presented that accurately describes the development location of the cyclone beneath a midlevel vorticity maximum.
Despite development in most of the attribution tests, the energy sources for the cyclones vary widely and represent a spectrum of cyclogenetic pathways from baroclinically to convectively dominant. Removal of the Alpine barrier allows for a stronger thermal wave and a baroclinic mode of development, rather than the diabatically generated hurricane-like vortex seen in the control and available observations. Similarly, insufficient flow interaction with the low-resolution representation of the Alps in the global-driving model is shown to favor a baroclinic mode of cyclogenesis in that integration. Adequate resolution of both the Alpine terrain and the incipient cyclone itself are shown to be important to correctly predict the evolution of the system from both structural and energetic perspectives.
Abstract
The development and subsequent tropical transition of a subsynoptic-scale cyclone over the Gulf of Genoa (GoG) on 15 November 2007 led to the rapid onset of tropical storm-force winds near the islands of Corsica and Sardinia. This study evaluates the influence of two key ingredients on the cyclogenesis event: a near-surface warm potential temperature perturbation in the lee of the Alps and a mountain-scale potential vorticity (PV) banner.
A high-resolution modeling system is used to perform a set of attribution tests in which modifications to the Alpine orography control the presence of the cyclogenetic ingredients. When either feature exists in the initial state, a GoG cyclone develops even when the Alpine barrier is removed; however, when neither the warm perturbation nor the PV banner is present, there is insufficient lower-level PV to couple with the upper-level trough to promote cyclogenesis. A conceptual model involving the complimentary interaction of the two PV features is presented that accurately describes the development location of the cyclone beneath a midlevel vorticity maximum.
Despite development in most of the attribution tests, the energy sources for the cyclones vary widely and represent a spectrum of cyclogenetic pathways from baroclinically to convectively dominant. Removal of the Alpine barrier allows for a stronger thermal wave and a baroclinic mode of development, rather than the diabatically generated hurricane-like vortex seen in the control and available observations. Similarly, insufficient flow interaction with the low-resolution representation of the Alps in the global-driving model is shown to favor a baroclinic mode of cyclogenesis in that integration. Adequate resolution of both the Alpine terrain and the incipient cyclone itself are shown to be important to correctly predict the evolution of the system from both structural and energetic perspectives.
Abstract
The devastating effects of Hurricane Katrina (2005) on the Gulf Coast of the United States are without compare for natural disasters in recent times in North America. With over 1800 dead and insured losses near $40 billion (U.S. dollars), Katrina ranks as the costliest and one of the deadliest Atlantic hurricanes in history. This study documents the complex life cycle of Katrina, a storm that was initiated by a tropical transition event in the Bahamas. Katrina intensified to a category-1 hurricane shortly before striking Miami, Florida; however, little weakening was observed as the system crossed the Florida peninsula. An analog climatology is used to show that this behavior is consistent with the historical record for storms crossing the southern extremity of the peninsula. Over the warm Gulf of Mexico waters, Katrina underwent two periods of rapid intensification associated with a warm core ring shed by the Loop Current. Between these spinup stages, the storm doubled in size, leading to a monotonic increase in power dissipation until Katrina reached a superintense state on 28 September. A pair of extremely destructive landfalls in Louisiana followed the weakening of the system over shelf waters. Despite its strength as a hurricane, Katrina did not reintensify following extratropical transition. The evolution of the storm’s outflow anticyclone, however, led to a perturbation of the midlatitude flow that is shown in a companion study to influence the Northern Hemisphere over a period of 2 weeks. An understanding of the varied components of Katrina’s complex evolution is necessary for further developing analysis and forecasting techniques as they apply to storms that form near the North American continent and rapidly intensify over the Gulf of Mexico. Given the observed overall increase in Atlantic hurricane activity since the mid-1990s, an enhanced appreciation for the forcings involved in such events could help to mitigate the impact of similar severe hurricanes in the future.
Abstract
The devastating effects of Hurricane Katrina (2005) on the Gulf Coast of the United States are without compare for natural disasters in recent times in North America. With over 1800 dead and insured losses near $40 billion (U.S. dollars), Katrina ranks as the costliest and one of the deadliest Atlantic hurricanes in history. This study documents the complex life cycle of Katrina, a storm that was initiated by a tropical transition event in the Bahamas. Katrina intensified to a category-1 hurricane shortly before striking Miami, Florida; however, little weakening was observed as the system crossed the Florida peninsula. An analog climatology is used to show that this behavior is consistent with the historical record for storms crossing the southern extremity of the peninsula. Over the warm Gulf of Mexico waters, Katrina underwent two periods of rapid intensification associated with a warm core ring shed by the Loop Current. Between these spinup stages, the storm doubled in size, leading to a monotonic increase in power dissipation until Katrina reached a superintense state on 28 September. A pair of extremely destructive landfalls in Louisiana followed the weakening of the system over shelf waters. Despite its strength as a hurricane, Katrina did not reintensify following extratropical transition. The evolution of the storm’s outflow anticyclone, however, led to a perturbation of the midlatitude flow that is shown in a companion study to influence the Northern Hemisphere over a period of 2 weeks. An understanding of the varied components of Katrina’s complex evolution is necessary for further developing analysis and forecasting techniques as they apply to storms that form near the North American continent and rapidly intensify over the Gulf of Mexico. Given the observed overall increase in Atlantic hurricane activity since the mid-1990s, an enhanced appreciation for the forcings involved in such events could help to mitigate the impact of similar severe hurricanes in the future.
