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- Author or Editor: Greg J. Holland x
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Abstract
A series of barotropic modeling experiments are used to examine the motion of a vortex in the vicinity of an idealized subtropical ridge. The vortex propagation is highly correlated with the absolute vorticity gradient in the initial imposed environment. The signal is degraded when radial band averages are used to calculate the environmental advection and absolute vorticity gradient components. It is shown here that this is due partially to the difficulties in separating the cyclone from its environment. Application of the modeling results to climatological mean fields indicates that considerable variations in vortex propagation can occur.
Abstract
A series of barotropic modeling experiments are used to examine the motion of a vortex in the vicinity of an idealized subtropical ridge. The vortex propagation is highly correlated with the absolute vorticity gradient in the initial imposed environment. The signal is degraded when radial band averages are used to calculate the environmental advection and absolute vorticity gradient components. It is shown here that this is due partially to the difficulties in separating the cyclone from its environment. Application of the modeling results to climatological mean fields indicates that considerable variations in vortex propagation can occur.
Abstract
Large-scale environmental variables known to be linked to the formation of tropical cyclones have previously been used to develop empirical indices as proxies for assessing cyclone frequency from large-scale analyses or model simulations. Here the authors examine the ability of two recent indices, the genesis potential (GP) and the genesis potential index, to reproduce observed North Atlantic cyclone annual frequency variations and trends. These skillfully estimate the mean seasonal variation of observed cyclones, but they struggle with reproducing interannual frequency variability and change. Examination of the independent contributions by the four terms that make up the indices finds that potential intensity and shear have significant skill, while moisture and vorticity either do not contribute to or degrade the indices’ capacity to reproduce observed interannual variability. It is also found that for assessing basinwide cyclone frequency, averaging indices over the whole basin is less skillful than its application to the general area off the coast of Africa broadly covering the main development region (MDR).
These results point to a revised index, the cyclone genesis index (CGI), which comprises only potential intensity and vertical shear. Application of the CGI averaged over the MDR demonstrates high and significant skill at reproducing interannual variations and trends in all-basin cyclones across both reanalyses. The CGI also provides a more accurate reproduction of seasonal variations than the original GP. Future work applying the CGI to other tropical cyclone basins and to the downscaling of relatively course climate simulations is briefly addressed.
Abstract
Large-scale environmental variables known to be linked to the formation of tropical cyclones have previously been used to develop empirical indices as proxies for assessing cyclone frequency from large-scale analyses or model simulations. Here the authors examine the ability of two recent indices, the genesis potential (GP) and the genesis potential index, to reproduce observed North Atlantic cyclone annual frequency variations and trends. These skillfully estimate the mean seasonal variation of observed cyclones, but they struggle with reproducing interannual frequency variability and change. Examination of the independent contributions by the four terms that make up the indices finds that potential intensity and shear have significant skill, while moisture and vorticity either do not contribute to or degrade the indices’ capacity to reproduce observed interannual variability. It is also found that for assessing basinwide cyclone frequency, averaging indices over the whole basin is less skillful than its application to the general area off the coast of Africa broadly covering the main development region (MDR).
These results point to a revised index, the cyclone genesis index (CGI), which comprises only potential intensity and vertical shear. Application of the CGI averaged over the MDR demonstrates high and significant skill at reproducing interannual variations and trends in all-basin cyclones across both reanalyses. The CGI also provides a more accurate reproduction of seasonal variations than the original GP. Future work applying the CGI to other tropical cyclone basins and to the downscaling of relatively course climate simulations is briefly addressed.
A brief description is given of the Cyclone Game, a direct simulation of many of the operational aspects of the Perth Tropical Cyclone Warning Centre. The game's main purpose is to provide tropical-cyclone forecasters with additional operational experience and skills. It may also be used to familiarize others with some of the complexities of tropical-cyclone forecasting. The game is controlled by a microcomputer and may be played by one or a team of “forecasters.” It has a modular structure and is designed for easy modification to suit local requirements.
We suggest that microcomputer-based simulations of severe weather events, such as described in this paper, have considerable potential as an educational and training tool.
A brief description is given of the Cyclone Game, a direct simulation of many of the operational aspects of the Perth Tropical Cyclone Warning Centre. The game's main purpose is to provide tropical-cyclone forecasters with additional operational experience and skills. It may also be used to familiarize others with some of the complexities of tropical-cyclone forecasting. The game is controlled by a microcomputer and may be played by one or a team of “forecasters.” It has a modular structure and is designed for easy modification to suit local requirements.
We suggest that microcomputer-based simulations of severe weather events, such as described in this paper, have considerable potential as an educational and training tool.
Abstract
Strong winds in a tropical cyclone over the ocean can produce high seas with substantial amounts of spray in the lower part of the atmospheric boundary layer. The effects that the evaporation of this sea spray may have on the transfer of energy between the ocean and the atmosphere, and consequent effects on the boundary layer structure, cumulus convection, and the evolution of the tropical cyclone, are largely unknown. In this study, a high-resolution tropical cyclone model with explicit cloud microphysics, developed by Y. Wang, has been used to study these potential effects. The sea spray evaporation is incorporated into the model by two bulk parameterization schemes with quite different properties.
The numerical results show that inclusion of the Fairall et al. sea spray parameterization increases the direct sensible heat flux from the ocean by about 70%, but has little effect on the direct latent heat flux. Sea spray itself causes a sensible heat flux of only about 6% of the direct sensible heat flux, while it contributes a latent heat flux by evaporation of sea spray droplets by 60%–70% of the direct latent heat flux. As a result, the total enthalpy flux with sea spray evaporation increases by about 20%, while the net contribution by sea spray is only about 1.5% of the total enthalpy flux. Consistent with this, the intensity of the model tropical cyclone is moderately increased by 8% in the maximum wind speed by the introduction of sea spray. The lower atmosphere becomes cooler and moister due to the evaporation of sea spray, which is supported by the available observations. The cooling in the surface layer further modifies the boundary layer structure and the activity of convection, especially in the near-core region where the highest concentration of sea spray exists.
On the other hand, with the Andreas and DeCosmo parameterization scheme, the intensity of the model tropical cyclone is increased by 25% in maximum wind speed. This dramatic increase in the model tropical cyclone intensity is due to both the large net sensible heat flux and the latent heat flux associated with the effect of sea spray by this parameterization scheme. The net upward sensible heat flux warms the air near the surface and results in a near-isothermal surface layer in the near-core environment under the tropical cyclone. Such a structure, however, is not supported by the available observations, which the authors argue is not physically realistic. The radically different results with this scheme are due to the unusual way that the feedbacks between direct and spray-mediated fluxes are handled within the parameterization.
Abstract
Strong winds in a tropical cyclone over the ocean can produce high seas with substantial amounts of spray in the lower part of the atmospheric boundary layer. The effects that the evaporation of this sea spray may have on the transfer of energy between the ocean and the atmosphere, and consequent effects on the boundary layer structure, cumulus convection, and the evolution of the tropical cyclone, are largely unknown. In this study, a high-resolution tropical cyclone model with explicit cloud microphysics, developed by Y. Wang, has been used to study these potential effects. The sea spray evaporation is incorporated into the model by two bulk parameterization schemes with quite different properties.
The numerical results show that inclusion of the Fairall et al. sea spray parameterization increases the direct sensible heat flux from the ocean by about 70%, but has little effect on the direct latent heat flux. Sea spray itself causes a sensible heat flux of only about 6% of the direct sensible heat flux, while it contributes a latent heat flux by evaporation of sea spray droplets by 60%–70% of the direct latent heat flux. As a result, the total enthalpy flux with sea spray evaporation increases by about 20%, while the net contribution by sea spray is only about 1.5% of the total enthalpy flux. Consistent with this, the intensity of the model tropical cyclone is moderately increased by 8% in the maximum wind speed by the introduction of sea spray. The lower atmosphere becomes cooler and moister due to the evaporation of sea spray, which is supported by the available observations. The cooling in the surface layer further modifies the boundary layer structure and the activity of convection, especially in the near-core region where the highest concentration of sea spray exists.
On the other hand, with the Andreas and DeCosmo parameterization scheme, the intensity of the model tropical cyclone is increased by 25% in maximum wind speed. This dramatic increase in the model tropical cyclone intensity is due to both the large net sensible heat flux and the latent heat flux associated with the effect of sea spray by this parameterization scheme. The net upward sensible heat flux warms the air near the surface and results in a near-isothermal surface layer in the near-core environment under the tropical cyclone. Such a structure, however, is not supported by the available observations, which the authors argue is not physically realistic. The radically different results with this scheme are due to the unusual way that the feedbacks between direct and spray-mediated fluxes are handled within the parameterization.
Abstract
A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. Analysis of the vorticity interactions that occur within the simulated rainbands demonstrates that couplets of cyclonic–anticyclonic mesovortices can be produced in outer bands. The primary source of this vorticity is the upward tilting of system-generated horizontal vorticity by diabatic heating gradients. The vertical heating gradient in the stratiform cloud also creates a potential vorticity (PV) dipole that accelerates the tangential flow and develops a midlevel jet. The strength of the jet is enhanced by the vortex pair that is oriented radially across the rainband. The Fourier decomposition of the absolute vorticity field shows two counterpropagating vortex Rossby waves associated with the rainband. The wave located on the inner side of the band transports energy toward the vortex center. The outer wave is made up of high wavenumbers and uses the vorticity gradients generated by the rainband. The results support the hypothesis that the heating profile in the stratiform regions of rainbands generates cyclonic PV across the freezing level, which develops a midlevel jet. This mechanism creates a vorticity gradient that enables the propagation of vortex Rossby waves that could allow the rainbands to interact with the mean flow and potentially influence the evolution of the storm by contributing to the symmetric component of vorticity and the development of secondary eyewalls.
Abstract
A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. Analysis of the vorticity interactions that occur within the simulated rainbands demonstrates that couplets of cyclonic–anticyclonic mesovortices can be produced in outer bands. The primary source of this vorticity is the upward tilting of system-generated horizontal vorticity by diabatic heating gradients. The vertical heating gradient in the stratiform cloud also creates a potential vorticity (PV) dipole that accelerates the tangential flow and develops a midlevel jet. The strength of the jet is enhanced by the vortex pair that is oriented radially across the rainband. The Fourier decomposition of the absolute vorticity field shows two counterpropagating vortex Rossby waves associated with the rainband. The wave located on the inner side of the band transports energy toward the vortex center. The outer wave is made up of high wavenumbers and uses the vorticity gradients generated by the rainband. The results support the hypothesis that the heating profile in the stratiform regions of rainbands generates cyclonic PV across the freezing level, which develops a midlevel jet. This mechanism creates a vorticity gradient that enables the propagation of vortex Rossby waves that could allow the rainbands to interact with the mean flow and potentially influence the evolution of the storm by contributing to the symmetric component of vorticity and the development of secondary eyewalls.
Abstract
Wind profiler and serial sounding observations extending to the upper troposphere are used to analyze Tropical Storm Flo (1990) as it passed within 115 km of the experimental site on Saipan. These data resolve details of the circulation and precipitation structure of the storm and its rainbands. Analysis of principal and secondary rainbands in outer radii indicate that there are considerable similarities with previous studies. Although the bands contained distinct precipitation maxima, there is no evidence of active convection and the mean structure is similar to that observed in the stratiform regions of squall lines. The vertical circulations in the rainbands are weak and complex, but distinct azimuthal wind maxima are observed that have maxima of relative vorticity and inertial stability on the inner edge. The divergence fields for the entire analysis period are strongly coherent and are indicative of vertically propagating gravity waves generated in the near inertially neutral outflow layer. The analysis thus demonstrates the usefulness of wind profilers for tropical cyclone observations.
Abstract
Wind profiler and serial sounding observations extending to the upper troposphere are used to analyze Tropical Storm Flo (1990) as it passed within 115 km of the experimental site on Saipan. These data resolve details of the circulation and precipitation structure of the storm and its rainbands. Analysis of principal and secondary rainbands in outer radii indicate that there are considerable similarities with previous studies. Although the bands contained distinct precipitation maxima, there is no evidence of active convection and the mean structure is similar to that observed in the stratiform regions of squall lines. The vertical circulations in the rainbands are weak and complex, but distinct azimuthal wind maxima are observed that have maxima of relative vorticity and inertial stability on the inner edge. The divergence fields for the entire analysis period are strongly coherent and are indicative of vertically propagating gravity waves generated in the near inertially neutral outflow layer. The analysis thus demonstrates the usefulness of wind profilers for tropical cyclone observations.
Abstract
A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. As a first step, the model rainbands have been qualitatively compared with observed rainband characteristics. The model-generated rainbands show many of the mesoscale and convective-scale features of observed tropical cyclone rainbands. Sensitivity studies of numerically simulated tropical cyclone convection to ice-phase microphysical parameters showed that the model was most sensitive to changes in the graupel fall speed parameters. Increasing the fall speeds saw graupel being confined to the convective regions and producing higher rain rates in the inner core of the storm. A greater region of stratiform precipitation was produced when the efficiency for the collection of snow and cloud ice by graupel was reduced and when the mean size of graupel was reduced. Both of these simulations resulted in a higher concentration of snow being transported into the stratiform region. Although the precipitation structure changed across the simulations, the surface rainfall rate and the fundamental dynamical variables showed little sensitivity to the parameter variations.
Abstract
A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. As a first step, the model rainbands have been qualitatively compared with observed rainband characteristics. The model-generated rainbands show many of the mesoscale and convective-scale features of observed tropical cyclone rainbands. Sensitivity studies of numerically simulated tropical cyclone convection to ice-phase microphysical parameters showed that the model was most sensitive to changes in the graupel fall speed parameters. Increasing the fall speeds saw graupel being confined to the convective regions and producing higher rain rates in the inner core of the storm. A greater region of stratiform precipitation was produced when the efficiency for the collection of snow and cloud ice by graupel was reduced and when the mean size of graupel was reduced. Both of these simulations resulted in a higher concentration of snow being transported into the stratiform region. Although the precipitation structure changed across the simulations, the surface rainfall rate and the fundamental dynamical variables showed little sensitivity to the parameter variations.
Abstract
The depth-averaged, numerical storm-surge model developed by Hubbert et al. (1990) has been configured to provide a stand-alone system to forecast tropical cyclone storm surges. The atmospheric surface pressure and surface winds are derived from the analytical-empirical model of Holland (1980) and require only cyclone positions, central pressures, and radii of maximum winds. The model has been adapted to run on personal computers in a few minutes so that multiple forecast scenarios can be tested in a forecast office in real time.
The storm surge model was tested in hindcast mode on four Australian tropical cyclones. For these case studies the model predicted the sea surface elevations and arrival times of surge peaks accurately, with typical elevation errors of 0.1 to 0.2 m and arrival time errors of no more than 1 h. Second order effects, such as coastally-trapped waves, were also well simulated. The model is now being used by the Australian Tropical Cyclone Warning Centres (TCWC's) for operational forecasting. It will also be released as part of a tropical cyclone workstation that has recently been recommended for use by WMO member nations.
Abstract
The depth-averaged, numerical storm-surge model developed by Hubbert et al. (1990) has been configured to provide a stand-alone system to forecast tropical cyclone storm surges. The atmospheric surface pressure and surface winds are derived from the analytical-empirical model of Holland (1980) and require only cyclone positions, central pressures, and radii of maximum winds. The model has been adapted to run on personal computers in a few minutes so that multiple forecast scenarios can be tested in a forecast office in real time.
The storm surge model was tested in hindcast mode on four Australian tropical cyclones. For these case studies the model predicted the sea surface elevations and arrival times of surge peaks accurately, with typical elevation errors of 0.1 to 0.2 m and arrival time errors of no more than 1 h. Second order effects, such as coastally-trapped waves, were also well simulated. The model is now being used by the Australian Tropical Cyclone Warning Centres (TCWC's) for operational forecasting. It will also be released as part of a tropical cyclone workstation that has recently been recommended for use by WMO member nations.
The Island Thunderstorm Experiment (ITEX) is a field and modeling study of the tropical thunderstorms that form regularly over Bathurst and Melville Islands north of Darwin, Northern Territory, Australia, during the transition season and breaks in the summer monsoon season. Such thunderstorms are of widespread occurrence in the tropics and they play an important role in tropical dynamics. ITEX is a joint project of the Bureau of Meteorology Research Centre and Monash University's Centre for Dynamical Meteorology. Preliminary studies have been used to plan an intensive period of observations that was carried out from 20 November to 10 December 1988. The resulting data will provide the basis for a series of analytical and numerical studies of tropical island thunderstorms.
The Island Thunderstorm Experiment (ITEX) is a field and modeling study of the tropical thunderstorms that form regularly over Bathurst and Melville Islands north of Darwin, Northern Territory, Australia, during the transition season and breaks in the summer monsoon season. Such thunderstorms are of widespread occurrence in the tropics and they play an important role in tropical dynamics. ITEX is a joint project of the Bureau of Meteorology Research Centre and Monash University's Centre for Dynamical Meteorology. Preliminary studies have been used to plan an intensive period of observations that was carried out from 20 November to 10 December 1988. The resulting data will provide the basis for a series of analytical and numerical studies of tropical island thunderstorms.
Abstract
This study uses a nonhierarchical cluster analysis to identify the major environmental circulation patterns associated with tropical cloud cluster (TCC) formation in the western North Pacific. All TCCs that formed in July–October 1981–2009 are examined based on their 850-hPa wind field around TCC centers. Eight types of environmental circulation patterns are identified. Of these, four are related to monsoon systems (trough, confluence, north of trough, and south of trough), three are related to easterly systems (low-latitude zone, west of subtropical high, and southwest of subtropical high), and one is associated with low-latitude cross-equatorial flow. The genesis potential index (GPI) is analyzed to compare how favorable the environmental conditions are for tropical cyclone (TC) formation when TCCs form. Excluding three cluster types with the GPI lower than the climatology of all samples, TCCs formed in monsoon environments have larger sizes, lower brightness temperatures, longer lifetimes, and higher GPIs than those of TCCs formed in easterly environments. However, for TCCs formed in easterly environments, the average GPI for those TCCs that later develop into TCs (developing TCCs) is higher than that for other TCCs (nondeveloping TCCs). This difference is nonsignificant for TCCs formed in monsoon environments. Conversely, the average magnitudes of GPI are similar for developing TCCs, regardless of whether TCCs form in easterly or monsoon environments. In summary, the probability of a TCC to develop into a TC is more sensitive to the environmental conditions for TCCs formed in easterly environments than those formed in monsoon environments.
Abstract
This study uses a nonhierarchical cluster analysis to identify the major environmental circulation patterns associated with tropical cloud cluster (TCC) formation in the western North Pacific. All TCCs that formed in July–October 1981–2009 are examined based on their 850-hPa wind field around TCC centers. Eight types of environmental circulation patterns are identified. Of these, four are related to monsoon systems (trough, confluence, north of trough, and south of trough), three are related to easterly systems (low-latitude zone, west of subtropical high, and southwest of subtropical high), and one is associated with low-latitude cross-equatorial flow. The genesis potential index (GPI) is analyzed to compare how favorable the environmental conditions are for tropical cyclone (TC) formation when TCCs form. Excluding three cluster types with the GPI lower than the climatology of all samples, TCCs formed in monsoon environments have larger sizes, lower brightness temperatures, longer lifetimes, and higher GPIs than those of TCCs formed in easterly environments. However, for TCCs formed in easterly environments, the average GPI for those TCCs that later develop into TCs (developing TCCs) is higher than that for other TCCs (nondeveloping TCCs). This difference is nonsignificant for TCCs formed in monsoon environments. Conversely, the average magnitudes of GPI are similar for developing TCCs, regardless of whether TCCs form in easterly or monsoon environments. In summary, the probability of a TCC to develop into a TC is more sensitive to the environmental conditions for TCCs formed in easterly environments than those formed in monsoon environments.
