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- Author or Editor: RICHARD A. ANTHES x
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Abstract
The role of asymmetries (large-scale horizontal eddies) in satisfying the mean angular momentum budget for the steady-state hurricane is studied by computing transverse circulations for a prescribed tangential vortex on the scale of 1000 km. For realistic diabatic heating rates at large distances from the hurricane center, the correlation between radial velocity and absolute vorticity must be negative in the upper troposphere and positive in the lower troposphere.
The transverse circulations show small-scale oscillations that increase as internal mixing is increased. This paradox results from balancing “noise” in the prescribed tangential wind profiles by oscillations in the radial and vertical advection terms.
Abstract
The role of asymmetries (large-scale horizontal eddies) in satisfying the mean angular momentum budget for the steady-state hurricane is studied by computing transverse circulations for a prescribed tangential vortex on the scale of 1000 km. For realistic diabatic heating rates at large distances from the hurricane center, the correlation between radial velocity and absolute vorticity must be negative in the upper troposphere and positive in the lower troposphere.
The transverse circulations show small-scale oscillations that increase as internal mixing is increased. This paradox results from balancing “noise” in the prescribed tangential wind profiles by oscillations in the radial and vertical advection terms.
Abstract
A method for parameterizing the effects of deep cumulus clouds on the larger scale thermodynamic and moisture fields in numerical models is proposed. Rigorous derivations of the effect of cumulus clouds on their environment are derived for two definitions of the large-scale averaged variables. In the first, the classical Reynolds averaging method is used and the averaged variables vary continuously over the domain. In the second method, which has been popular in the derivation of cumulus parameterization schemes, the averages are defined by dividing an incremental area of the domain (usually the mesh aim) into a region occupied by convection and the remainder of the region which is free of convection. In this method, the large-scale averages assume discrete values over each incremental area. The differences between the large-scale equations that result from these two methods and some possible difficulties that may be encountered when the averaging interval approaches the aim of the convective clouds are discussed.
The process that determine the effect of deep cumulus convection on the larger scale variables are discussed. The vertical distribution of the net heating of the large scale by the cumulus clouds is determined primarily by the vertical distribution of beating on the cloud scale. A secondary effect is the vertical eddy flux of heat by warm updrafts, which shifts the large-scale heating maximum to slightly higher levels than the level of maximum cloud-scale beating. The major effect of convection on the large-scale moisture equation is to dry the lower troposphere and moisten the upper troposphere.
A method for determining the fractional area covered by deep cumulus updrafts is proposed. This method requires large-scale moisture convergence and estimates of the thermodynamic properties of the typical updraft.
The parameterization scheme conserves total energy in the large-scale equations. It requires representative values of temperature and moisture in the deep convection, as well as an estimate of the vertical distribution of cloud-scale heating. Any cloud model that provides these parameters may be used to complete the scheme; here a one-dimensional cloud model is utilized.
Vertical profiles of the net convective heating rate and the convective effects on the large-scale moisture field are computed for three clouds of different radii using a tropical and an extratropical sounding. The vertical partitioning of the net convective heating as determined by this method is compared to the partitioning given by Kuo's scheme.
Abstract
A method for parameterizing the effects of deep cumulus clouds on the larger scale thermodynamic and moisture fields in numerical models is proposed. Rigorous derivations of the effect of cumulus clouds on their environment are derived for two definitions of the large-scale averaged variables. In the first, the classical Reynolds averaging method is used and the averaged variables vary continuously over the domain. In the second method, which has been popular in the derivation of cumulus parameterization schemes, the averages are defined by dividing an incremental area of the domain (usually the mesh aim) into a region occupied by convection and the remainder of the region which is free of convection. In this method, the large-scale averages assume discrete values over each incremental area. The differences between the large-scale equations that result from these two methods and some possible difficulties that may be encountered when the averaging interval approaches the aim of the convective clouds are discussed.
The process that determine the effect of deep cumulus convection on the larger scale variables are discussed. The vertical distribution of the net heating of the large scale by the cumulus clouds is determined primarily by the vertical distribution of beating on the cloud scale. A secondary effect is the vertical eddy flux of heat by warm updrafts, which shifts the large-scale heating maximum to slightly higher levels than the level of maximum cloud-scale beating. The major effect of convection on the large-scale moisture equation is to dry the lower troposphere and moisten the upper troposphere.
A method for determining the fractional area covered by deep cumulus updrafts is proposed. This method requires large-scale moisture convergence and estimates of the thermodynamic properties of the typical updraft.
The parameterization scheme conserves total energy in the large-scale equations. It requires representative values of temperature and moisture in the deep convection, as well as an estimate of the vertical distribution of cloud-scale heating. Any cloud model that provides these parameters may be used to complete the scheme; here a one-dimensional cloud model is utilized.
Vertical profiles of the net convective heating rate and the convective effects on the large-scale moisture field are computed for three clouds of different radii using a tropical and an extratropical sounding. The vertical partitioning of the net convective heating as determined by this method is compared to the partitioning given by Kuo's scheme.
Abstract
A cumulus parameterization scheme that utilizes a one-dimensional cloud model is tested in a revised, axisymmetric hurricane model. The details of how the parameterization scheme may he incorporated into a larger wale model are presented. With a mean tropical sounding, the cumulus parameterization scheme yields a vertical distribution of heating that is appropriate for tropical cyclone development. The structure of the model hurricane and the properties of the convective clouds at various stages of the model storm development are described. The vertical distribution of the cloud-scale heating and the vertical eddy fluxes of heat, moisture and momentum are given as a function of radius from the storm center during the mature stage. The vertical fluxes of heat and moisture cool and dry the lower troposphere while they warm and moisten the upper troposphere. The vertical transport of radial momentum by the cumulus convection is relatively unimportant, however, the transport of tangential momentum is significant in reducing the vertical shear of the tangential wind.
Abstract
A cumulus parameterization scheme that utilizes a one-dimensional cloud model is tested in a revised, axisymmetric hurricane model. The details of how the parameterization scheme may he incorporated into a larger wale model are presented. With a mean tropical sounding, the cumulus parameterization scheme yields a vertical distribution of heating that is appropriate for tropical cyclone development. The structure of the model hurricane and the properties of the convective clouds at various stages of the model storm development are described. The vertical distribution of the cloud-scale heating and the vertical eddy fluxes of heat, moisture and momentum are given as a function of radius from the storm center during the mature stage. The vertical fluxes of heat and moisture cool and dry the lower troposphere while they warm and moisten the upper troposphere. The vertical transport of radial momentum by the cumulus convection is relatively unimportant, however, the transport of tangential momentum is significant in reducing the vertical shear of the tangential wind.
Abstract
Notable asymmetric features of an early experiment with a three-dimensional hurricane model were spiral bands of convection and large-scale asymmetries (eddies) in the outflow layer. Using an improved version of the model, we describe the formation and maintenance of these features in greater detail in this paper. The spiral bands in the model propagate cyclonically outward in agreement with bands in nature. The breakdown of symmetry into a chaotic pattern of eddies in the outflow region is shown to be the result of dynamic (inertial) instability, with the eddy kinetic energy derived from the kinetic energy of the azimuthal flow. This instability does not contribute to the overall intensification of the model storm, however.
We observe a curious anticyclonic looping of the vortex center in these experiments. This looping appears to be associated with asymmetries in the divergence pattern associated with the eddies in the outflow layer.
This paper also summarizes improvements made in the original version of the model. In contrast to the earlier model, the current version contains an explicit water vapor cycle. A staggered horizontal grid is used to provide a higher resolution in evaluating the pressure gradient forces. Some of the pragmatic assumptions made in the earlier model, notably those involving horizontal diffusion of heat and momentum, have been eliminated in the current version.
Abstract
Notable asymmetric features of an early experiment with a three-dimensional hurricane model were spiral bands of convection and large-scale asymmetries (eddies) in the outflow layer. Using an improved version of the model, we describe the formation and maintenance of these features in greater detail in this paper. The spiral bands in the model propagate cyclonically outward in agreement with bands in nature. The breakdown of symmetry into a chaotic pattern of eddies in the outflow region is shown to be the result of dynamic (inertial) instability, with the eddy kinetic energy derived from the kinetic energy of the azimuthal flow. This instability does not contribute to the overall intensification of the model storm, however.
We observe a curious anticyclonic looping of the vortex center in these experiments. This looping appears to be associated with asymmetries in the divergence pattern associated with the eddies in the outflow layer.
This paper also summarizes improvements made in the original version of the model. In contrast to the earlier model, the current version contains an explicit water vapor cycle. A staggered horizontal grid is used to provide a higher resolution in evaluating the pressure gradient forces. Some of the pragmatic assumptions made in the earlier model, notably those involving horizontal diffusion of heat and momentum, have been eliminated in the current version.
Abstract
Several physical processes and properties of the initial state that affect marine cyclogenesis are examined using a mesoscale numerical model. The sensitivity of an idealized cyclone to the effects of latent heat release, surface heat and moisture fluxes as well as the initial meridional temperature gradient and static stability is examined by comparing various numerical stimulations of cyclogenesis in a baroclinic channel-flow model. Idealized initial conditions are derived analytically and are characterized by strong low-level baroclinity and a very weak upper-level trough. These initial conditions are used to examine which factors in baroclinic cyclogenesis are most important for rapid development (1 m h−1 for 24 h or more) and how diabatic processes modify the development rate.
A strong low-level meridional gradient (40°C/2000 km) and low static stability (a mean lapse rate of 6.0°C km−1) resulted in rapid development of the model cyclone. The model cyclogenesis is more sensitive to small changes in the initial baroclinity than to physical processes during the development, which suggests that sustained rapid development requires substantial baroclinic instability. Inclusion of latent heat release during the development resulted in only a 10% increase in the average deepening rate. This effect of latent heating depended crucially upon the moisture distribution and is more representative of large-scale stable condensation than strong convection. Modification of the model cyclogenesis by various surface heat and moisture flux distributions indicated that the phase and magnitude of these fluxes relative to the low-level atmospheric baroclinity is important. A distribution of surface heating that enhanced the low-level baroclinity resulted in a 15% increase in growth rare, suggesting an important interaction during certain periods of development. Surface heating distributions that reduced the low-level baroclinity by counteracting thermal advection damped the development of the model cyclone as suggested by previous studies.
Abstract
Several physical processes and properties of the initial state that affect marine cyclogenesis are examined using a mesoscale numerical model. The sensitivity of an idealized cyclone to the effects of latent heat release, surface heat and moisture fluxes as well as the initial meridional temperature gradient and static stability is examined by comparing various numerical stimulations of cyclogenesis in a baroclinic channel-flow model. Idealized initial conditions are derived analytically and are characterized by strong low-level baroclinity and a very weak upper-level trough. These initial conditions are used to examine which factors in baroclinic cyclogenesis are most important for rapid development (1 m h−1 for 24 h or more) and how diabatic processes modify the development rate.
A strong low-level meridional gradient (40°C/2000 km) and low static stability (a mean lapse rate of 6.0°C km−1) resulted in rapid development of the model cyclone. The model cyclogenesis is more sensitive to small changes in the initial baroclinity than to physical processes during the development, which suggests that sustained rapid development requires substantial baroclinic instability. Inclusion of latent heat release during the development resulted in only a 10% increase in the average deepening rate. This effect of latent heating depended crucially upon the moisture distribution and is more representative of large-scale stable condensation than strong convection. Modification of the model cyclogenesis by various surface heat and moisture flux distributions indicated that the phase and magnitude of these fluxes relative to the low-level atmospheric baroclinity is important. A distribution of surface heating that enhanced the low-level baroclinity resulted in a 15% increase in growth rare, suggesting an important interaction during certain periods of development. Surface heating distributions that reduced the low-level baroclinity by counteracting thermal advection damped the development of the model cyclone as suggested by previous studies.
Abstract
As the U.S. polar-orbiting satellites NOAA-15, -18, and -19 and NASA’s Aqua satellite reach the ends of their lives, there may be a loss in redundancy between their microwave (MW) soundings, and the Advanced Technology Microwave Sounder (ATMS) on the Suomi–National Polar-Orbiting Partnership (NPP) satellite. With the expected delay in the launch of the next generation of U.S. polar-orbiting satellites, there may be a loss in at least some of the U.S. MW data. There may also be a significant decrease in the number of radio occultation (RO) observations. The mainstay of the global RO system, the COSMIC constellation of six satellites is already past the end of its nominal lifetime. A replacement of RO soundings in the tropics is planned with the launch of COSMIC-2 satellites in 2016. However, the polar constellation of COSMIC-2 will not be launched until 2018 or 2019, and complete funding for this constellation is not assured. Using the NCEP operational forecast system, forecasts for March–April 2013 are carried out in which various combinations of the U.S. MW and all RO soundings are removed. The main results are that the forecasts are only slightly degraded in the Northern Hemisphere, even with all of these observations removed. The decrease in accuracy is considerably greater in the Southern Hemisphere, where the greatest forecast degradation occurs when the RO observations are removed. Overall, these results indicate that the possible gap in RO observations is potentially more significant than the possible gap in the U.S. MW data.
Abstract
As the U.S. polar-orbiting satellites NOAA-15, -18, and -19 and NASA’s Aqua satellite reach the ends of their lives, there may be a loss in redundancy between their microwave (MW) soundings, and the Advanced Technology Microwave Sounder (ATMS) on the Suomi–National Polar-Orbiting Partnership (NPP) satellite. With the expected delay in the launch of the next generation of U.S. polar-orbiting satellites, there may be a loss in at least some of the U.S. MW data. There may also be a significant decrease in the number of radio occultation (RO) observations. The mainstay of the global RO system, the COSMIC constellation of six satellites is already past the end of its nominal lifetime. A replacement of RO soundings in the tropics is planned with the launch of COSMIC-2 satellites in 2016. However, the polar constellation of COSMIC-2 will not be launched until 2018 or 2019, and complete funding for this constellation is not assured. Using the NCEP operational forecast system, forecasts for March–April 2013 are carried out in which various combinations of the U.S. MW and all RO soundings are removed. The main results are that the forecasts are only slightly degraded in the Northern Hemisphere, even with all of these observations removed. The decrease in accuracy is considerably greater in the Southern Hemisphere, where the greatest forecast degradation occurs when the RO observations are removed. Overall, these results indicate that the possible gap in RO observations is potentially more significant than the possible gap in the U.S. MW data.
Abstract
A high-resolution, one-dimensional, moist planetary boundary layer (PBL) model is developed following Blackadar, and verified using the 10 April 1979 SESAME data set. The model consists of two modules to predict the time-dependent behavior of the PBL under various surface characteristics. Under stable conditions, turbulent fluxes are related to a local Richardson number. In contrast, under conditions of free convection, the exchange of heat, moisture and momentum occurs through mixing between convective elements originating at the surface and environmental air in the PBL.
Sensitivity tests showed that the daytime PBL structure is most sensitive to moisture availability, roughness length, albedo and thermal capacity, in that order. It is less sensitive in the nighttime to the above parameters. The wind profile is extremely sensitive to the specified geostrophic wind profile at all times. Simulations over both dry and moist terrain indicate that both the free convection (daytime) and the stable (nocturnal) modules are capable of accurately simulating the diurnal PBL evolution under nonsteady geostrophic conditions, provided accurate, time-dependent geostrophic wind profiles are available. With steady geostrophic forcing, the simulations are less realistic.
Abstract
A high-resolution, one-dimensional, moist planetary boundary layer (PBL) model is developed following Blackadar, and verified using the 10 April 1979 SESAME data set. The model consists of two modules to predict the time-dependent behavior of the PBL under various surface characteristics. Under stable conditions, turbulent fluxes are related to a local Richardson number. In contrast, under conditions of free convection, the exchange of heat, moisture and momentum occurs through mixing between convective elements originating at the surface and environmental air in the PBL.
Sensitivity tests showed that the daytime PBL structure is most sensitive to moisture availability, roughness length, albedo and thermal capacity, in that order. It is less sensitive in the nighttime to the above parameters. The wind profile is extremely sensitive to the specified geostrophic wind profile at all times. Simulations over both dry and moist terrain indicate that both the free convection (daytime) and the stable (nocturnal) modules are capable of accurately simulating the diurnal PBL evolution under nonsteady geostrophic conditions, provided accurate, time-dependent geostrophic wind profiles are available. With steady geostrophic forcing, the simulations are less realistic.
Abstract
ATS-III satellite data and conventional aerological data are used to construct detailed wind analyses of the outflow layer for four hurricanes and one tropical storm. Harmonic analysis of these data, and of the data for a mean Atlantic hurricane and a mean Pacific typhoon, shows that wave numbers 1 and 2 around the circumference of the storm account for most of the variance of momentum and kinetic energy. Subtraction of the symmetric part of the vortex circulation from the total flow to yield the “asymmetric wind” reveals two eddies located in preferred quadrants of the storm. An anticyclonic eddy is found to the right and a cyclonic eddy to the left of the storm motion. These eddies transport absolute vorticity inward, opposing the outward transport by the mean circulation. They also transport a significant amount of negative relative angular momentum outward.
The presence of inertial (or dynamic) instability is investigated. Although substantial areas of negative absolute vorticity and anomalous anticyclonic winds exist in all cases, these areas are correlated so well that the regions of dynamic instability are small.
Abstract
ATS-III satellite data and conventional aerological data are used to construct detailed wind analyses of the outflow layer for four hurricanes and one tropical storm. Harmonic analysis of these data, and of the data for a mean Atlantic hurricane and a mean Pacific typhoon, shows that wave numbers 1 and 2 around the circumference of the storm account for most of the variance of momentum and kinetic energy. Subtraction of the symmetric part of the vortex circulation from the total flow to yield the “asymmetric wind” reveals two eddies located in preferred quadrants of the storm. An anticyclonic eddy is found to the right and a cyclonic eddy to the left of the storm motion. These eddies transport absolute vorticity inward, opposing the outward transport by the mean circulation. They also transport a significant amount of negative relative angular momentum outward.
The presence of inertial (or dynamic) instability is investigated. Although substantial areas of negative absolute vorticity and anomalous anticyclonic winds exist in all cases, these areas are correlated so well that the regions of dynamic instability are small.
Abstract
Heat and moisture budgets are computed for the 40 km model simulations of moist frontogenesis described recently by Hsie and others. The apparent heat source and moisture sinks are dominated by the condensation term and have maxima in the middle troposphere. Both the large-scale (200 km) moisture convergence and the large-scale vertical motion are highly correlated with the mesoscale condensation rate.
Four alternative schemes for treating the effects of moist convection in primitive equation models are tested, and the results compared with those from the explicit scheme for calculating condensation and precipitation. A scheme in which only water vapor is predicted yields results similar to the explicit simulation, which included prediction equations for water vapor, cloud water and rainwater. The neglect of two opposing effects-water loading and evaporation-is apparently responsible for the similarity to the control. However, when both cloud water and water vapor are predicted, the presence of evaporation, but not water loading by rain, results in larger differences from the control. A third scheme, developed by Kessler, does not conserve total water, and the latent heat released is overestimated.
The cumulus parameterization proposed by Anthes is tested as the fourth scheme in a coarse-resolution (200 km) version of the model. The major deficiency with the coarse-mesh model is its failure to resolve a narrow, frictionally-driven updraft close to the surface cold front. The feedback of the moisture convergence and the cumulus heating parameterization produces large differences in the horizontal distribution of heating in the thermal and wind structure.
When the cumulus heating in the coarse-mesh simulation is specified from the average distribution given by the explicit scheme in the fine-mesh simulation, the coarse-mesh simulation is much closer to the large-scale average of the fine-mesh simulation.
Abstract
Heat and moisture budgets are computed for the 40 km model simulations of moist frontogenesis described recently by Hsie and others. The apparent heat source and moisture sinks are dominated by the condensation term and have maxima in the middle troposphere. Both the large-scale (200 km) moisture convergence and the large-scale vertical motion are highly correlated with the mesoscale condensation rate.
Four alternative schemes for treating the effects of moist convection in primitive equation models are tested, and the results compared with those from the explicit scheme for calculating condensation and precipitation. A scheme in which only water vapor is predicted yields results similar to the explicit simulation, which included prediction equations for water vapor, cloud water and rainwater. The neglect of two opposing effects-water loading and evaporation-is apparently responsible for the similarity to the control. However, when both cloud water and water vapor are predicted, the presence of evaporation, but not water loading by rain, results in larger differences from the control. A third scheme, developed by Kessler, does not conserve total water, and the latent heat released is overestimated.
The cumulus parameterization proposed by Anthes is tested as the fourth scheme in a coarse-resolution (200 km) version of the model. The major deficiency with the coarse-mesh model is its failure to resolve a narrow, frictionally-driven updraft close to the surface cold front. The feedback of the moisture convergence and the cumulus heating parameterization produces large differences in the horizontal distribution of heating in the thermal and wind structure.
When the cumulus heating in the coarse-mesh simulation is specified from the average distribution given by the explicit scheme in the fine-mesh simulation, the coarse-mesh simulation is much closer to the large-scale average of the fine-mesh simulation.
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