Search Results
You are looking at 1 - 9 of 9 items for
- Author or Editor: Robert L. Korty x
- Refine by Access: All Content x
Abstract
This work investigates the dynamic and thermal response of the winter stratosphere to the presence of a weak meridional surface temperature gradient. Previous work suggested that polar stratospheric clouds could have played a decisive role in maintaining high-latitude warmth, especially over continental interiors, during the polar nights of the late Paleocene and early Eocene epochs; both a chemical source of additional water vapor and a dynamical feedback between the surface climate and stratospheric temperatures have been proposed as mechanisms by which such clouds could form. A principal goal of this work is to investigate the latter problem using a general circulation model with stratospheric resolution that is forced with a very weak surface temperature gradient. It is found that temperatures in the lower stratosphere do not deviate significantly from the control run, which results from a robust flux of wave activity into the winter stratosphere. The strength of the stratosphere’s residual circulation increases slightly in the presence of the weak gradient, as wavenumber 3 begins to propagate to stratospheric altitudes. Changes in the zonal wind field that allow for the altered propagation are in balance with a weakened temperature gradient through the full depth of the troposphere. These simulations also suggest that the tropospheric thermal stratification could be maintained by moist convection at all latitudes in warm climate states with a weak temperature gradient.
Abstract
This work investigates the dynamic and thermal response of the winter stratosphere to the presence of a weak meridional surface temperature gradient. Previous work suggested that polar stratospheric clouds could have played a decisive role in maintaining high-latitude warmth, especially over continental interiors, during the polar nights of the late Paleocene and early Eocene epochs; both a chemical source of additional water vapor and a dynamical feedback between the surface climate and stratospheric temperatures have been proposed as mechanisms by which such clouds could form. A principal goal of this work is to investigate the latter problem using a general circulation model with stratospheric resolution that is forced with a very weak surface temperature gradient. It is found that temperatures in the lower stratosphere do not deviate significantly from the control run, which results from a robust flux of wave activity into the winter stratosphere. The strength of the stratosphere’s residual circulation increases slightly in the presence of the weak gradient, as wavenumber 3 begins to propagate to stratospheric altitudes. Changes in the zonal wind field that allow for the altered propagation are in balance with a weakened temperature gradient through the full depth of the troposphere. These simulations also suggest that the tropospheric thermal stratification could be maintained by moist convection at all latitudes in warm climate states with a weak temperature gradient.
Abstract
The condition of convective neutrality is assessed in the troposphere by calculating the saturation potential vorticity P* from reanalysis data. Regions of the atmosphere in which saturation entropy is constant along isosurfaces of absolute angular momentum, a state indicative of slantwise-convective neutrality, have values of P* equal to zero. In a global reanalysis dataset spanning the years 1970–2004, tropospheric regions are identified in which P* is near zero, implying that vertical convection or slantwise convection may be important in determining the local thermal stratification. Convectively neutral air masses are common not only in the Tropics but also in higher latitudes, for example, over midlatitude continents in summer and in storm tracks over oceans in winter. Large-scale eddies appear to stabilize parts of the lower troposphere, particularly in winter.
Abstract
The condition of convective neutrality is assessed in the troposphere by calculating the saturation potential vorticity P* from reanalysis data. Regions of the atmosphere in which saturation entropy is constant along isosurfaces of absolute angular momentum, a state indicative of slantwise-convective neutrality, have values of P* equal to zero. In a global reanalysis dataset spanning the years 1970–2004, tropospheric regions are identified in which P* is near zero, implying that vertical convection or slantwise convection may be important in determining the local thermal stratification. Convectively neutral air masses are common not only in the Tropics but also in higher latitudes, for example, over midlatitude continents in summer and in storm tracks over oceans in winter. Large-scale eddies appear to stabilize parts of the lower troposphere, particularly in winter.
Abstract
Analyses of two high-resolution reanalysis products show that high values of hurricane potential intensity (PI) are becoming more frequent and covering a larger area of the Atlantic, which is consistent with the lengthening of the tropical cyclone season previously reported. These changes are especially pronounced during the early months of the storm season (May–July) in subtropical latitudes. The western subtropical Atlantic features increases in mean PI as well as the areal coverage and frequency of high PI throughout the storm season; the length of the season with high PI has grown since 1980. The number of days with low vertical wind shear increases in the tropical North Atlantic during the early and middle months of the storm season, but trends are mixed and generally insignificant elsewhere. A thermodynamic parameter measuring the ratio of midlevel entropy deficits to the strength of surface fluxes that work to eliminate them is sensitive to the choice of the pressure level(s) used to calculate its value in the boundary layer, as well as to subtle differences in temperature and humidity values near the surface in different reanalysis datasets, leading to divergent results in metrics like the ventilation index that depend on its value. Projections from a high-resolution simulation of the remainder of the twenty-first century show that the number of days with high PI is likely to continue increasing in the North Atlantic basin, with trends especially strong in the western subtropical Atlantic during the early and late months of the season.
Abstract
Analyses of two high-resolution reanalysis products show that high values of hurricane potential intensity (PI) are becoming more frequent and covering a larger area of the Atlantic, which is consistent with the lengthening of the tropical cyclone season previously reported. These changes are especially pronounced during the early months of the storm season (May–July) in subtropical latitudes. The western subtropical Atlantic features increases in mean PI as well as the areal coverage and frequency of high PI throughout the storm season; the length of the season with high PI has grown since 1980. The number of days with low vertical wind shear increases in the tropical North Atlantic during the early and middle months of the storm season, but trends are mixed and generally insignificant elsewhere. A thermodynamic parameter measuring the ratio of midlevel entropy deficits to the strength of surface fluxes that work to eliminate them is sensitive to the choice of the pressure level(s) used to calculate its value in the boundary layer, as well as to subtle differences in temperature and humidity values near the surface in different reanalysis datasets, leading to divergent results in metrics like the ventilation index that depend on its value. Projections from a high-resolution simulation of the remainder of the twenty-first century show that the number of days with high PI is likely to continue increasing in the North Atlantic basin, with trends especially strong in the western subtropical Atlantic during the early and late months of the season.
Abstract
Large-scale environmental factors that favor tropical cyclogenesis are calculated and examined in simulations of the Last Glacial Maximum (LGM) from the Paleoclimate Modelling Intercomparison Project Phase 2 (PMIP2). Despite universally colder conditions at the LGM, values of tropical cyclone potential intensity, which both serves as an upper bound on thermodynamically achievable intensity and indicates regions supportive of the deep convection required, are broadly similar in magnitude to those in preindustrial era control simulation. Some regions, including large areas of the central and western North Pacific, feature higher potential intensities at the LGM than they do in the control runs, while other regions including much of the Atlantic and Indian Oceans are lower. Changes in potential intensity are strongly correlated with the degree of surface cooling during the LGM. Additionally, two thermodynamic parameters—one that measures midtropospheric entropy deficits relevant for tropical cyclogenesis and another related to the time required for genesis—are broadly more favorable in the LGM simulation than in the preindustrial era control. A genesis potential index yields higher values for the LGM in much of the western Pacific, a feature common to nearly all of the individual models examined.
Abstract
Large-scale environmental factors that favor tropical cyclogenesis are calculated and examined in simulations of the Last Glacial Maximum (LGM) from the Paleoclimate Modelling Intercomparison Project Phase 2 (PMIP2). Despite universally colder conditions at the LGM, values of tropical cyclone potential intensity, which both serves as an upper bound on thermodynamically achievable intensity and indicates regions supportive of the deep convection required, are broadly similar in magnitude to those in preindustrial era control simulation. Some regions, including large areas of the central and western North Pacific, feature higher potential intensities at the LGM than they do in the control runs, while other regions including much of the Atlantic and Indian Oceans are lower. Changes in potential intensity are strongly correlated with the degree of surface cooling during the LGM. Additionally, two thermodynamic parameters—one that measures midtropospheric entropy deficits relevant for tropical cyclogenesis and another related to the time required for genesis—are broadly more favorable in the LGM simulation than in the preindustrial era control. A genesis potential index yields higher values for the LGM in much of the western Pacific, a feature common to nearly all of the individual models examined.
Abstract
The thermodynamic factors related to tropical cyclone genesis are examined in several simulations of the middle part of the Holocene epoch when the precession of Earth’s orbit altered the seasonal distribution of solar radiation and in one transient simulation of the millennium preceding the industrial era. The thermodynamic properties most crucial for genesis display a broad stability across both periods, although both orbital variations during the mid-Holocene (MH) 6000 years ago (6ka) and volcanic eruptions in the transient simulation have detectable effects. It is shown that the distribution of top-of-the-atmosphere radiation 6ka altered the Northern Hemisphere seasonal cycle of the potential intensity of tropical cyclones in addition to slightly increasing the difference between middle tropospheric and boundary layer entropy, a parameter that has been related to the incubation period required for genesis. The Southern Hemisphere, which receives more solar radiation during its storm season today than it did 6ka, displays slightly more favorable thermodynamic properties during the MH than in the preindustrial era control. Surface temperatures over the ocean in both hemispheres respond to radiation anomalies more slowly than those in upper levels, altering the thermal stability.
Volcanism produces a sharp but transient temperature response in the last-millennium simulation that strongly reduces potential intensity during the seasons immediately following a major eruption. Here, too, the differential vertical temperature response is key: temperatures in the lower and middle troposphere cool, while those near the tropopause rise. Aside from these deviations, there is no substantial variation in thermodynamic properties over the 1000-yr simulation.
Abstract
The thermodynamic factors related to tropical cyclone genesis are examined in several simulations of the middle part of the Holocene epoch when the precession of Earth’s orbit altered the seasonal distribution of solar radiation and in one transient simulation of the millennium preceding the industrial era. The thermodynamic properties most crucial for genesis display a broad stability across both periods, although both orbital variations during the mid-Holocene (MH) 6000 years ago (6ka) and volcanic eruptions in the transient simulation have detectable effects. It is shown that the distribution of top-of-the-atmosphere radiation 6ka altered the Northern Hemisphere seasonal cycle of the potential intensity of tropical cyclones in addition to slightly increasing the difference between middle tropospheric and boundary layer entropy, a parameter that has been related to the incubation period required for genesis. The Southern Hemisphere, which receives more solar radiation during its storm season today than it did 6ka, displays slightly more favorable thermodynamic properties during the MH than in the preindustrial era control. Surface temperatures over the ocean in both hemispheres respond to radiation anomalies more slowly than those in upper levels, altering the thermal stability.
Volcanism produces a sharp but transient temperature response in the last-millennium simulation that strongly reduces potential intensity during the seasons immediately following a major eruption. Here, too, the differential vertical temperature response is key: temperatures in the lower and middle troposphere cool, while those near the tropopause rise. Aside from these deviations, there is no substantial variation in thermodynamic properties over the 1000-yr simulation.
Abstract
The tracks, intensities, and other properties of tropical cyclones downscaled from three models’ simulations of the Last Glacial Maximum (LGM) are analyzed and compared to those of storms downscaled from simulations of the present climate. Globally, the mean maximum intensity of storms generated from each model is lower at LGM, as is the fraction of all storms that reach intensities of category 4 or higher on the Saffir–Simpson hurricane wind scale. The median day of the storm season shifts earlier by an average of one week in all three models in both hemispheres. Two of the three models’ LGM simulations feature a reduction in storm count and global power dissipation index compared to the current climate, but a third shows no significant difference between the two climates. Although each model is forced by the same global changes, differences in the way sea surface temperatures and other large-scale environmental conditions respond in the North Atlantic impart significant differences in the climatology at LGM between models. Our results from the cold LGM provide a novel opportunity to assess how tropical cyclones respond to climate changes.
Abstract
The tracks, intensities, and other properties of tropical cyclones downscaled from three models’ simulations of the Last Glacial Maximum (LGM) are analyzed and compared to those of storms downscaled from simulations of the present climate. Globally, the mean maximum intensity of storms generated from each model is lower at LGM, as is the fraction of all storms that reach intensities of category 4 or higher on the Saffir–Simpson hurricane wind scale. The median day of the storm season shifts earlier by an average of one week in all three models in both hemispheres. Two of the three models’ LGM simulations feature a reduction in storm count and global power dissipation index compared to the current climate, but a third shows no significant difference between the two climates. Although each model is forced by the same global changes, differences in the way sea surface temperatures and other large-scale environmental conditions respond in the North Atlantic impart significant differences in the climatology at LGM between models. Our results from the cold LGM provide a novel opportunity to assess how tropical cyclones respond to climate changes.
Abstract
Tropical cyclones instigate an isolated blast of vigorous mixing in the upper tropical oceans, stirring warm surface water with cooler water in the thermocline. Previous work suggests that the frequency, intensity, and lifetime of these storms may be functions of the climate state, implying that transient tropical mixing could have been stronger during warmer equable climates with higher concentrations of carbon dioxide. Stronger mixing of the tropical oceans can force the oceans’ meridional heat flux to increase, cooling tropical latitudes while warming higher ones. This response differs significantly from previous modeling studies of equable climates that used static mixing; coupling mixing to climate changes the dynamic response. A parameterization of mixing from tropical cyclones is developed, and including it leads to a cooling of tropical oceans and a warming of subtropical waters compared with control cases with fixed mixing. The mixing penetration depth regulates the magnitude of the response.
Abstract
Tropical cyclones instigate an isolated blast of vigorous mixing in the upper tropical oceans, stirring warm surface water with cooler water in the thermocline. Previous work suggests that the frequency, intensity, and lifetime of these storms may be functions of the climate state, implying that transient tropical mixing could have been stronger during warmer equable climates with higher concentrations of carbon dioxide. Stronger mixing of the tropical oceans can force the oceans’ meridional heat flux to increase, cooling tropical latitudes while warming higher ones. This response differs significantly from previous modeling studies of equable climates that used static mixing; coupling mixing to climate changes the dynamic response. A parameterization of mixing from tropical cyclones is developed, and including it leads to a cooling of tropical oceans and a warming of subtropical waters compared with control cases with fixed mixing. The mixing penetration depth regulates the magnitude of the response.
Abstract
The spatial and temporal distribution of stable and convectively neutral air masses is examined in climate simulations with carbon dioxide levels spanning from modern-day values to very high levels that produce surface temperatures relevant to the hottest climate of the past 65 million years. To investigate how stability with respect to slantwise and upright moist convection changes across a wide range of climate states, the condition of moist convective neutrality in climate experiments is assessed using metrics based upon the saturation of potential vorticity, which is zero when temperature profiles are moist adiabatic profiles along vortex lines. The modern climate experiment reproduces previously reported properties from reanalysis data, in which convectively neutral air masses are common in the tropics and locally at higher latitudes, especially over midlatitude continents in summer and ocean storm tracks in winter. The frequency and coverage of air masses with higher stabilities declines in all seasons at higher latitudes with warming; the hottest case features convectively neutral air masses in the Arctic a majority of the time in January and nearly universally in July. The contribution from slantwise convective motions (as distinct from upright convection) is generally small outside of midlatitude storm tracks, and it declines in the warmer climate experiments, especially during summer. These findings support the conjecture that moist adiabatic lapse rates become more widespread in warmer climates, providing a physical basis for using this assumption in estimating paleoaltimetry during warm intervals such as the early Eocene.
Abstract
The spatial and temporal distribution of stable and convectively neutral air masses is examined in climate simulations with carbon dioxide levels spanning from modern-day values to very high levels that produce surface temperatures relevant to the hottest climate of the past 65 million years. To investigate how stability with respect to slantwise and upright moist convection changes across a wide range of climate states, the condition of moist convective neutrality in climate experiments is assessed using metrics based upon the saturation of potential vorticity, which is zero when temperature profiles are moist adiabatic profiles along vortex lines. The modern climate experiment reproduces previously reported properties from reanalysis data, in which convectively neutral air masses are common in the tropics and locally at higher latitudes, especially over midlatitude continents in summer and ocean storm tracks in winter. The frequency and coverage of air masses with higher stabilities declines in all seasons at higher latitudes with warming; the hottest case features convectively neutral air masses in the Arctic a majority of the time in January and nearly universally in July. The contribution from slantwise convective motions (as distinct from upright convection) is generally small outside of midlatitude storm tracks, and it declines in the warmer climate experiments, especially during summer. These findings support the conjecture that moist adiabatic lapse rates become more widespread in warmer climates, providing a physical basis for using this assumption in estimating paleoaltimetry during warm intervals such as the early Eocene.
Abstract
A method to simulate thousands of tropical cyclones using output from a global climate model is applied to simulations that span very high surface temperatures forced with high levels of carbon dioxide (CO2). The climatology of the storms downscaled from a simulation with modern-day conditions is compared to that of events downscaled from two other simulations featuring 8 and 32 times preindustrial-era levels of CO2. Storms shift poleward with warming: genesis locations and track densities increase in subtropical and higher latitudes, and power dissipation increases poleward of 20°S and 30°N. The average latitude at which storms reach their maximum intensity shifts poleward by more than 1.5° latitude in the 8 × CO2 experiment and by more than 7° latitude in the 32 × CO2 case. Storms live longer and are more numerous in both of the warmer climates. These increases come largely from an expansion of the area featuring favorable conditions into subtropics and midlatitudes, with some regions of the Arctic having the thermodynamic conditions necessary to sustain systems in the hottest case. Storms of category 5 intensity are 52% more frequent in the 8 × CO2 experiment but 40% less so in the 32 × CO2 case, largely owing to a substantial decline in low-latitude activity associated with increases in a normalized measure of wind shear called the ventilation index. Changes in genesis and track densities align well with differences in the ventilation index, and environmental conditions become substantially more favorable poleward of about 20° latitude in the warmer climates.
Abstract
A method to simulate thousands of tropical cyclones using output from a global climate model is applied to simulations that span very high surface temperatures forced with high levels of carbon dioxide (CO2). The climatology of the storms downscaled from a simulation with modern-day conditions is compared to that of events downscaled from two other simulations featuring 8 and 32 times preindustrial-era levels of CO2. Storms shift poleward with warming: genesis locations and track densities increase in subtropical and higher latitudes, and power dissipation increases poleward of 20°S and 30°N. The average latitude at which storms reach their maximum intensity shifts poleward by more than 1.5° latitude in the 8 × CO2 experiment and by more than 7° latitude in the 32 × CO2 case. Storms live longer and are more numerous in both of the warmer climates. These increases come largely from an expansion of the area featuring favorable conditions into subtropics and midlatitudes, with some regions of the Arctic having the thermodynamic conditions necessary to sustain systems in the hottest case. Storms of category 5 intensity are 52% more frequent in the 8 × CO2 experiment but 40% less so in the 32 × CO2 case, largely owing to a substantial decline in low-latitude activity associated with increases in a normalized measure of wind shear called the ventilation index. Changes in genesis and track densities align well with differences in the ventilation index, and environmental conditions become substantially more favorable poleward of about 20° latitude in the warmer climates.
