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- Author or Editor: Kerry Emanuel x
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Abstract
Cumulative distribution functions (CDFs) of tropical cyclone wind speeds are calculated using best track data from the North Atlantic and western North Pacific basins. Wind speeds are normalized by theoretical potential wind speeds derived from reanalysis datasets, and the individual storms are classified according to whether their maximum intensities were limited by landfall, passage over cold water, or other factors. For each classification, CDFs were calculated and the evolution of the storm wind speed was composited relative to the time at which each storm achieved its lifetime maximum wind speed.
For storms whose maximum intensity is not limited by declining potential intensity, the evolution of storm intensity is remarkably similar in the Atlantic and western North Pacific basins, with average intensification and decay rates of around 12 m s−1 day−1 and 8 m s−1 day−1, respectively. The average hurricane-strength storm in both basins reaches a sharp peak in intensity followed by a decline at a rate roughly two-thirds that of its prior intensification, a behavior distinctly different from that of axisymmetric numerical models. Moreover, this class of storms achieves almost the same intensity in the Atlantic and western North Pacific regions, while storms whose maximum intensity is limited by declining potential intensity are significantly more intense in the Pacific region, showing that the main reason for the greater intensity of western North Pacific tropical cyclones is the greater length of the average storm track over warm water. Other results from this study include the finding that average rates of decline of tropical cyclone intensity over warm and cold water are very similar and are about half the average rate of decline of landfalling storm intensity.
Abstract
Cumulative distribution functions (CDFs) of tropical cyclone wind speeds are calculated using best track data from the North Atlantic and western North Pacific basins. Wind speeds are normalized by theoretical potential wind speeds derived from reanalysis datasets, and the individual storms are classified according to whether their maximum intensities were limited by landfall, passage over cold water, or other factors. For each classification, CDFs were calculated and the evolution of the storm wind speed was composited relative to the time at which each storm achieved its lifetime maximum wind speed.
For storms whose maximum intensity is not limited by declining potential intensity, the evolution of storm intensity is remarkably similar in the Atlantic and western North Pacific basins, with average intensification and decay rates of around 12 m s−1 day−1 and 8 m s−1 day−1, respectively. The average hurricane-strength storm in both basins reaches a sharp peak in intensity followed by a decline at a rate roughly two-thirds that of its prior intensification, a behavior distinctly different from that of axisymmetric numerical models. Moreover, this class of storms achieves almost the same intensity in the Atlantic and western North Pacific regions, while storms whose maximum intensity is limited by declining potential intensity are significantly more intense in the Pacific region, showing that the main reason for the greater intensity of western North Pacific tropical cyclones is the greater length of the average storm track over warm water. Other results from this study include the finding that average rates of decline of tropical cyclone intensity over warm and cold water are very similar and are about half the average rate of decline of landfalling storm intensity.
Abstract
The advent of the polar front theory of cyclones in Norway early in the last century held that the development of fronts and air masses is central to understanding midlatitude weather phenomena. While work on fronts continues to this day, the concept of air masses has been largely forgotten, superseded by the idea of a continuum. The Norwegians placed equal emphasis on the thermodynamics of airmass formation and on the dynamical processes that moved air masses around; today, almost all the emphasis is on dynamics, with little published literature on diabatic processes acting on a large scale. In this essay, the author argues that a lack of understanding of large-scale diabatic processes leads to an incomplete picture of the atmosphere and contributes to systematic errors in medium- and long-range weather forecasts. At the same time, modern concepts centered around potential vorticity conservation and inversion lead one to a redefinition of the term "air mass" that may have some utility in conceptualizing atmospheric physics and in weather forecasting.
Abstract
The advent of the polar front theory of cyclones in Norway early in the last century held that the development of fronts and air masses is central to understanding midlatitude weather phenomena. While work on fronts continues to this day, the concept of air masses has been largely forgotten, superseded by the idea of a continuum. The Norwegians placed equal emphasis on the thermodynamics of airmass formation and on the dynamical processes that moved air masses around; today, almost all the emphasis is on dynamics, with little published literature on diabatic processes acting on a large scale. In this essay, the author argues that a lack of understanding of large-scale diabatic processes leads to an incomplete picture of the atmosphere and contributes to systematic errors in medium- and long-range weather forecasts. At the same time, modern concepts centered around potential vorticity conservation and inversion lead one to a redefinition of the term "air mass" that may have some utility in conceptualizing atmospheric physics and in weather forecasting.
Abstract
A century ago, meteorologists regarded tropical cyclones as shallow vortices, extending upward only a few kilometers into the troposphere, and nothing was known about their physics save that convection was somehow involved. As recently as 1938, a major hurricane struck the densely populated northeastern United States with no warning whatsoever, killing hundreds. In the time since the American Meteorological Society was founded, however, tropical cyclone research blossomed into an endeavor of great breadth and depth, encompassing fields ranging from atmospheric and oceanic dynamics to biogeochemistry, and the precision and scope of forecasts and warnings have achieved a level of success that would have been regarded as impossible only a few decades ago. This chapter attempts to document the extraordinary progress in tropical cyclone research over the last century and to suggest some avenues for productive research over the next one.
Abstract
A century ago, meteorologists regarded tropical cyclones as shallow vortices, extending upward only a few kilometers into the troposphere, and nothing was known about their physics save that convection was somehow involved. As recently as 1938, a major hurricane struck the densely populated northeastern United States with no warning whatsoever, killing hundreds. In the time since the American Meteorological Society was founded, however, tropical cyclone research blossomed into an endeavor of great breadth and depth, encompassing fields ranging from atmospheric and oceanic dynamics to biogeochemistry, and the precision and scope of forecasts and warnings have achieved a level of success that would have been regarded as impossible only a few decades ago. This chapter attempts to document the extraordinary progress in tropical cyclone research over the last century and to suggest some avenues for productive research over the next one.
Abstract
Recent research has shown that a variety of wavelike oscillations in the tropics may be explained by instabilities driven by wind-induced surface heat exchange (WISHE). All such studies to date have implicitly assumed that moist convection is in quasi equilibrium with the flow in question. Here that assumption is relaxed by accounting for a small but nonzero lag between the large-scale forcing of convection and its response. Reaction times as short as 30 minutes damp the higher-frequency Kelvin-like equatorial modes, favoring zonal wavenumbers 1–4, and strongly bias the higher-order modes to westward-propagating disturbances of synoptic scale. An analysis of off-equatorial disturbances reveals a preference for poleward- and westward-propagating modes with wavelengths of the order of 1000 km.
Abstract
Recent research has shown that a variety of wavelike oscillations in the tropics may be explained by instabilities driven by wind-induced surface heat exchange (WISHE). All such studies to date have implicitly assumed that moist convection is in quasi equilibrium with the flow in question. Here that assumption is relaxed by accounting for a small but nonzero lag between the large-scale forcing of convection and its response. Reaction times as short as 30 minutes damp the higher-frequency Kelvin-like equatorial modes, favoring zonal wavenumbers 1–4, and strongly bias the higher-order modes to westward-propagating disturbances of synoptic scale. An analysis of off-equatorial disturbances reveals a preference for poleward- and westward-propagating modes with wavelengths of the order of 1000 km.
Abstract
Hurricane intensity is sensitive to fluxes of enthalpy and momentum between the ocean and atmosphere in the high wind core of the storm. It has come to be recognized that much of this exchange is likely mediated by sea spray. A number of representations of spray-mediated exchange have appeared in recent years, but when these are applied in numerical simulations of hurricanes, storm intensity proves sensitive to the details of these representations. Here it is proposed that in the limit of very high wind speed, the air–sea transition layer becomes self-similar, permitting deductions about air–sea exchange based on scaling laws. In particular, it is hypothesized that exchange coefficients based on the gradient wind speed should become independent of wind speed in the high wind limit. A mechanistic argument suggests that the enthalpy exchange coefficient should depend on temperature. These propositions are tested in a hurricane intensity prediction model and can, in principle, be tested in the field.
Abstract
Hurricane intensity is sensitive to fluxes of enthalpy and momentum between the ocean and atmosphere in the high wind core of the storm. It has come to be recognized that much of this exchange is likely mediated by sea spray. A number of representations of spray-mediated exchange have appeared in recent years, but when these are applied in numerical simulations of hurricanes, storm intensity proves sensitive to the details of these representations. Here it is proposed that in the limit of very high wind speed, the air–sea transition layer becomes self-similar, permitting deductions about air–sea exchange based on scaling laws. In particular, it is hypothesized that exchange coefficients based on the gradient wind speed should become independent of wind speed in the high wind limit. A mechanistic argument suggests that the enthalpy exchange coefficient should depend on temperature. These propositions are tested in a hurricane intensity prediction model and can, in principle, be tested in the field.
Abstract
Tropical cyclones intensify and are maintained by surface enthalpy fluxes that result from the thermodynamics disequilibrium that exists between the tropical oceans and atmosphere. While this general result has been known for at least a half century, the detailed nature of feedbacks between thermodynamic and dynamic processes in tropical cyclones remains poorly understood. In particular, the spatial relationship between surface fluxes and the radial entropy distribution apparently does not act to amplify the entropy gradient and therefore the surface winds. In previous work, this problem was addressed by accounting for the radial distribution of convective fluxes of entropy out of the boundary layer; this led to the conclusion that a radial gradient of such convective fluxes is necessary for intensification.
Part I showed that the assumption of constant outflow temperature is incorrect and argued that the thermal stratification of the outflow is set by small-scale turbulence that limits the Richardson number. The assumption of Richardson number criticality of the outflow allows one to derive an equation for the variation of outflow temperature with angular momentum; this in turn leads to predictions of vortex structure and intensity that agree well with tropical cyclones simulated using a full-physics axisymmetric model. Here it is shown that the variation of outflow temperature with angular momentum also permits the vortex to intensify with time even in the absence of radial gradients of entrainment into the boundary layer. An equation is derived for the rate of intensity change and compared to simple models and to simulations using a full-physics model.
Abstract
Tropical cyclones intensify and are maintained by surface enthalpy fluxes that result from the thermodynamics disequilibrium that exists between the tropical oceans and atmosphere. While this general result has been known for at least a half century, the detailed nature of feedbacks between thermodynamic and dynamic processes in tropical cyclones remains poorly understood. In particular, the spatial relationship between surface fluxes and the radial entropy distribution apparently does not act to amplify the entropy gradient and therefore the surface winds. In previous work, this problem was addressed by accounting for the radial distribution of convective fluxes of entropy out of the boundary layer; this led to the conclusion that a radial gradient of such convective fluxes is necessary for intensification.
Part I showed that the assumption of constant outflow temperature is incorrect and argued that the thermal stratification of the outflow is set by small-scale turbulence that limits the Richardson number. The assumption of Richardson number criticality of the outflow allows one to derive an equation for the variation of outflow temperature with angular momentum; this in turn leads to predictions of vortex structure and intensity that agree well with tropical cyclones simulated using a full-physics axisymmetric model. Here it is shown that the variation of outflow temperature with angular momentum also permits the vortex to intensify with time even in the absence of radial gradients of entrainment into the boundary layer. An equation is derived for the rate of intensity change and compared to simple models and to simulations using a full-physics model.
Abstract
Revised estimates of kinetic energy production by tropical cyclones in the Atlantic and western North Pacific are presented. These show considerable variability on interannual-to-multidecadal time scales. In the Atlantic, variability on time scales of a few years and more is strongly correlated with tropical Atlantic sea surface temperature, while in the western North Pacific, this correlation, while still present, is considerably weaker. Using a combination of basic theory and empirical statistical analysis, it is shown that much of the variability in both ocean basins can be explained by variations in potential intensity, low-level vorticity, and vertical wind shear. Potential intensity variations are in turn factored into components related to variations in net surface radiation, thermodynamic efficiency, and average surface wind speed.
In the Atlantic, potential intensity, low-level vorticity, and vertical wind shear strongly covary and are also highly correlated with sea surface temperature, at least during the period in which reanalysis products are considered reliable. In the Pacific, the three factors are not strongly correlated. The relative contributions of the three factors are quantified, and implications for future trends and variability of tropical cyclone activity are discussed.
Abstract
Revised estimates of kinetic energy production by tropical cyclones in the Atlantic and western North Pacific are presented. These show considerable variability on interannual-to-multidecadal time scales. In the Atlantic, variability on time scales of a few years and more is strongly correlated with tropical Atlantic sea surface temperature, while in the western North Pacific, this correlation, while still present, is considerably weaker. Using a combination of basic theory and empirical statistical analysis, it is shown that much of the variability in both ocean basins can be explained by variations in potential intensity, low-level vorticity, and vertical wind shear. Potential intensity variations are in turn factored into components related to variations in net surface radiation, thermodynamic efficiency, and average surface wind speed.
In the Atlantic, potential intensity, low-level vorticity, and vertical wind shear strongly covary and are also highly correlated with sea surface temperature, at least during the period in which reanalysis products are considered reliable. In the Pacific, the three factors are not strongly correlated. The relative contributions of the three factors are quantified, and implications for future trends and variability of tropical cyclone activity are discussed.
Abstract
It has been proposed that tropical cyclogenesis rates can be expressed as the product of the frequency of “seeds” and a transition probability that depends on the large-scale environment. Here it is demonstrated that the partitioning between seed frequency and transition probability depends on the seed definition and that the existence of such a partition does not resolve the long-standing issue of whether tropical cyclone frequency is controlled more by environmental conditions or by the statistics of background weather. It is here argued that tropical cyclone climatology is mostly controlled by regional environment and that the response of global tropical cyclone activity to globally uniform radiative forcing may be more controlled by the regionality of the response than by the mean response.
Abstract
It has been proposed that tropical cyclogenesis rates can be expressed as the product of the frequency of “seeds” and a transition probability that depends on the large-scale environment. Here it is demonstrated that the partitioning between seed frequency and transition probability depends on the seed definition and that the existence of such a partition does not resolve the long-standing issue of whether tropical cyclone frequency is controlled more by environmental conditions or by the statistics of background weather. It is here argued that tropical cyclone climatology is mostly controlled by regional environment and that the response of global tropical cyclone activity to globally uniform radiative forcing may be more controlled by the regionality of the response than by the mean response.
Abstract
Global models comprising the sixth-generation Coupled Climate Model Intercomparison Project (CMIP6) are downscaled using a very high-resolution but simplified coupled atmosphere–ocean tropical cyclone model, as a means of estimating the response of global tropical cyclone activity to increasing greenhouse gases. As with a previous downscaling of CMIP5 models, the results show an increase in both the frequency and severity of tropical cyclones, robust across the models downscaled, in response to increasing greenhouse gases. The increase is strongly weighted to the Northern Hemisphere, and especially noteworthy is a large increase in the higher latitudes of the North Atlantic. Changes are insignificant in the South Pacific across metrics. Although the largest increases in track density are far from land, substantial increases in global landfalling power dissipation are indicated. The incidence of rapid intensification increases rapidly with warming, as predicted by existing theory. Measures of robustness across downscaled climate models are presented, and comparisons to tropical cyclones explicitly simulated in climate models are discussed.
Abstract
Global models comprising the sixth-generation Coupled Climate Model Intercomparison Project (CMIP6) are downscaled using a very high-resolution but simplified coupled atmosphere–ocean tropical cyclone model, as a means of estimating the response of global tropical cyclone activity to increasing greenhouse gases. As with a previous downscaling of CMIP5 models, the results show an increase in both the frequency and severity of tropical cyclones, robust across the models downscaled, in response to increasing greenhouse gases. The increase is strongly weighted to the Northern Hemisphere, and especially noteworthy is a large increase in the higher latitudes of the North Atlantic. Changes are insignificant in the South Pacific across metrics. Although the largest increases in track density are far from land, substantial increases in global landfalling power dissipation are indicated. The incidence of rapid intensification increases rapidly with warming, as predicted by existing theory. Measures of robustness across downscaled climate models are presented, and comparisons to tropical cyclones explicitly simulated in climate models are discussed.
Abstract
Recent work has highlighted the possible importance of changing upper-ocean thermal and density stratification on observed and projected changes in tropical cyclone activity. Here seven CMIP phase 5 (CMIP5)-generation climate model simulations are downscaled under IPCC representative concentration pathway 8.5 using a coupled atmosphere–ocean tropical cyclone model, generating 100 events per year in the western North Pacific from 2006 to 2100. A control downscaling in which the upper-ocean thermal structure is fixed at its monthly values in the year 2006 is compared to one in which the upper ocean is allowed to evolve, as derived from the CMIP5 models. As found in earlier work, the thermal stratification generally increases as the climate warms, leading to increased ocean mixing–induced negative feedback on tropical cyclone intensity. While trends in the frequency of storms are unaffected, the increasing stratification of the upper ocean leads to a 13% reduction in the increase of tropical cyclone power dissipation over the twenty-first century, averaged across the seven climate models. Much of this reduction is associated with a moderation of the increase in the frequency of category-5 storms.
Abstract
Recent work has highlighted the possible importance of changing upper-ocean thermal and density stratification on observed and projected changes in tropical cyclone activity. Here seven CMIP phase 5 (CMIP5)-generation climate model simulations are downscaled under IPCC representative concentration pathway 8.5 using a coupled atmosphere–ocean tropical cyclone model, generating 100 events per year in the western North Pacific from 2006 to 2100. A control downscaling in which the upper-ocean thermal structure is fixed at its monthly values in the year 2006 is compared to one in which the upper ocean is allowed to evolve, as derived from the CMIP5 models. As found in earlier work, the thermal stratification generally increases as the climate warms, leading to increased ocean mixing–induced negative feedback on tropical cyclone intensity. While trends in the frequency of storms are unaffected, the increasing stratification of the upper ocean leads to a 13% reduction in the increase of tropical cyclone power dissipation over the twenty-first century, averaged across the seven climate models. Much of this reduction is associated with a moderation of the increase in the frequency of category-5 storms.
