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- Author or Editor: RICHARD A. ANTHES x
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Abstract
An interesting ice storm of moderate severity occurred along the east slopes of the Appalachians on 13–14 January 1980. Though surface temperatures were initially below freezing in most of this region, objective guidance indicated that large-scale warm would render the atmosphere conducive to rain. Warm advection did occur above about 900 mb, but below this level warm advection was prevented by a cold ~ shaped ridge of high pressure which became entrenched along the east slopes. Temperature in the lowest 0.5–1 km remained below freezing and an ice storm resulted.
This case study documents the evolution of the wedge ridge and the temperature and wind fields associated with it. Comparisons are made between the evolution of these fields within the quasi-stationary wedge ridge (a weather regime known as cold-air damming and their evolution during the preceding period, when the pressure ridge was progressing eastward across the Midwest The processes controlling the charges of temperature in these regimes are analyzed; cold advection and upslope flow maintain the cold dome. Cross sections are used to present detailed analyses of the vertical structure and evolution of the temperatures and winds within the damming region. Interesting features include the development of an “extended coastal front”—the sloping inversion separating the trapped cold dome from the warm onshore flow above, a jet parallel to the mountain at low levels, and an enhanced flow over the mountain near its crest. Apparently due to the lack of vertical resolution sufficient to capture such features operational numerical models exhibited substantial errors in this case.
Abstract
An interesting ice storm of moderate severity occurred along the east slopes of the Appalachians on 13–14 January 1980. Though surface temperatures were initially below freezing in most of this region, objective guidance indicated that large-scale warm would render the atmosphere conducive to rain. Warm advection did occur above about 900 mb, but below this level warm advection was prevented by a cold ~ shaped ridge of high pressure which became entrenched along the east slopes. Temperature in the lowest 0.5–1 km remained below freezing and an ice storm resulted.
This case study documents the evolution of the wedge ridge and the temperature and wind fields associated with it. Comparisons are made between the evolution of these fields within the quasi-stationary wedge ridge (a weather regime known as cold-air damming and their evolution during the preceding period, when the pressure ridge was progressing eastward across the Midwest The processes controlling the charges of temperature in these regimes are analyzed; cold advection and upslope flow maintain the cold dome. Cross sections are used to present detailed analyses of the vertical structure and evolution of the temperatures and winds within the damming region. Interesting features include the development of an “extended coastal front”—the sloping inversion separating the trapped cold dome from the warm onshore flow above, a jet parallel to the mountain at low levels, and an enhanced flow over the mountain near its crest. Apparently due to the lack of vertical resolution sufficient to capture such features operational numerical models exhibited substantial errors in this case.
Abstract
During the period 11–15 July 1981, heavy rainfall occurred over the Sichuan basin in China, resulting in severe floods that took a large toll in human life and property damage. Synoptic analyses indicate that early in this period the southerly monsoon flow was particularly strong near the basin because of a favorable positioning of the Pacific subtropical high and the Indian monsoon depression. The passage of a deep midlatitude trough from the Lake Baikal region brought colder, drier air from Siberia into southwest China. The Siberian air stream met the monsoon current over the eastern plateau and the Sichuan basin, creating a region of large-scale, low-level convergence.
Mesoscale analyses show that the flood was directly related to the extreme development of a long-lived mesoscale vortex [called south (SW) vortex by Chinese meteorologists] over the basin as it merged with another mesoscale vortex generated over the Tibetan Plateau. Mesoscale heat and moisture budgets suggest that the latent heat release associated with cumulus convection contributed substantially to the development of the SW vortex. It is found that heating due to convective eddy flux convergence of sensible and Went heat is about half the amount of latent beat release due to condensation. Moisture flux analyses and isentropic trajectories indicate that the major moisture source for the good was from the Bay of Bengal. This moisture was transported over (and around) the southeastern corner of the Tibetan Plateau to the basin. The SW vortex finally dissipated, after the passage of a surface cold front associated with the Lake Baikal trough.
We hypothesize that the formation of the SW vortex was a consequence of the blocking of the southwesterly monsoon flow by the mesoscale mountain range (located at the southeastern corner of the Tibetan Plateau). This is suggested by the fact that the SW vortex was initiated completely within this southwesterly monsoon current, that it was strongest at the lower levels during its formation stage, and that the low-level cyclonic vorticity was present throughout the period that the southwesterly monsoon flowed around the plateau.
Abstract
During the period 11–15 July 1981, heavy rainfall occurred over the Sichuan basin in China, resulting in severe floods that took a large toll in human life and property damage. Synoptic analyses indicate that early in this period the southerly monsoon flow was particularly strong near the basin because of a favorable positioning of the Pacific subtropical high and the Indian monsoon depression. The passage of a deep midlatitude trough from the Lake Baikal region brought colder, drier air from Siberia into southwest China. The Siberian air stream met the monsoon current over the eastern plateau and the Sichuan basin, creating a region of large-scale, low-level convergence.
Mesoscale analyses show that the flood was directly related to the extreme development of a long-lived mesoscale vortex [called south (SW) vortex by Chinese meteorologists] over the basin as it merged with another mesoscale vortex generated over the Tibetan Plateau. Mesoscale heat and moisture budgets suggest that the latent heat release associated with cumulus convection contributed substantially to the development of the SW vortex. It is found that heating due to convective eddy flux convergence of sensible and Went heat is about half the amount of latent beat release due to condensation. Moisture flux analyses and isentropic trajectories indicate that the major moisture source for the good was from the Bay of Bengal. This moisture was transported over (and around) the southeastern corner of the Tibetan Plateau to the basin. The SW vortex finally dissipated, after the passage of a surface cold front associated with the Lake Baikal trough.
We hypothesize that the formation of the SW vortex was a consequence of the blocking of the southwesterly monsoon flow by the mesoscale mountain range (located at the southeastern corner of the Tibetan Plateau). This is suggested by the fact that the SW vortex was initiated completely within this southwesterly monsoon current, that it was strongest at the lower levels during its formation stage, and that the low-level cyclonic vorticity was present throughout the period that the southwesterly monsoon flowed around the plateau.
Abstract
The extratropical cyclone which damaged the liner Queen Elizabeth II in September 1978 is a well-documented example of explosive marine cyclogenesis in which the 24 h surface central pressure fall was 60 mb commencing 1200 GMT 9 September. Operational models of both the National Meteorological Center (NMC) and Fleet Numerical Weather Central (FNWC) predicted virtually none of the observed surface intensification. This study reports on results of simulations performed with a primitive equation model. Emphasis will be placed on discovering why such poor forecasts were made of this storm. The extensive data set compiled by Gyakum (1983a, b) is used both to initialize and verify the model in a series of 24 h simulations, in order to assess the impact of initializing the model with these supplementary data. Physical processes identified observationally by Gyakum as being important in the storm's evolution are also examined numerically for their relative importance. In a series of seven simulations in which initial condition, horizontal resolution, and physics are varied, the model storm intensity varies considerably. In the weakest, the minimum pressure and maximum boundary-layer wind speeds are 1001 mb and 15.0 m s−1; in the strongest, these parameters are 960 mb and 50.2 m s−1.
The model simulations without the supplementary data set show little improvement over the forecasts of NMC and FNWC. Those simulations with the supplementary data produce improvements in the S 1 score, the intensity of the storm and the track of the storm. The improvement in the model simulations with the introduction of the supplementary data appears due to their more realistic documentation of the shallow cyclonic circulation, the small low-level static stability, and enhanced lower-tropospheric water vapor content.
Physical processes also played a major role in the simulators. The effect of surface fluxes of sensible and latent heat were moderate on the 24 h pressure and wind forecasts. In addition, these fluxes produced large changes in the temperature and moisture structure of the planetary boundary layer over a large area of cold northerly flow to the rear of the cyclone.
Latent heating was important in determining the storm intensity and track. Including latent heating through a cumulus parameterization scheme with a horizontal resolution of 90 km produced an improvement in the simulated intensity and position, with a reduction in minimum pressure of 7 mb and an increase in boundary-layer wind speed of 5 m s−1. With 45 km horizontal resolution, use of explicit condensation heating rather than the cumulus parameterization produced a further reduction in minimum pressure of 12 mb. Although experiments with explicit rather than parameterized latent heating produced more intense storms, in agreement with observations, the model storm motion was slowed considerably during the last 6 h of the simulation, resulting in an increased position error.
The model storm showed a small increase in intensity when the horizontal grid length reduced from 90 km to 45 km, with the minimum pressure decreasing by 3 mb. A further reduction in horizontal resolution to 22.5 km produced only minor differences in storm intensity.
The most intense model storm was simulated when an explicit medium-resolution planetary boundary-layer formulation replaced the bulk formulation used in most of the experiments. With 45 km resolution, explicit latent heating, and the medium-resolution boundary-layer model, a storm with minimum pressure of 960 mb and a maximum wind speed of 50.2 m s−1 was obtained.
This study suggests that baroclinic instability in the weakly stratified lower troposphere is the major mechanism of growth for this cyclone, as discussed by Reed, although latent heat plays an important role in the later stages of development. The development of this strong, yet relatively shallow, storm has three major implications for improving operational forecasts Of similar storms. First, the vertical resolution of the model must be adequate; our estimate is that at least four model layers are required below 700 mb. Second, the lower-tropospheric winds, static stability, water vapor content, and sea-surface temperature must be resolved accurately in the initial analysis because of the sensitivity of the model storm to these fields. Third, continued improvement of modeling planetary boundary-layer and latent heating processes is likely to be important in cases of this type.
Abstract
The extratropical cyclone which damaged the liner Queen Elizabeth II in September 1978 is a well-documented example of explosive marine cyclogenesis in which the 24 h surface central pressure fall was 60 mb commencing 1200 GMT 9 September. Operational models of both the National Meteorological Center (NMC) and Fleet Numerical Weather Central (FNWC) predicted virtually none of the observed surface intensification. This study reports on results of simulations performed with a primitive equation model. Emphasis will be placed on discovering why such poor forecasts were made of this storm. The extensive data set compiled by Gyakum (1983a, b) is used both to initialize and verify the model in a series of 24 h simulations, in order to assess the impact of initializing the model with these supplementary data. Physical processes identified observationally by Gyakum as being important in the storm's evolution are also examined numerically for their relative importance. In a series of seven simulations in which initial condition, horizontal resolution, and physics are varied, the model storm intensity varies considerably. In the weakest, the minimum pressure and maximum boundary-layer wind speeds are 1001 mb and 15.0 m s−1; in the strongest, these parameters are 960 mb and 50.2 m s−1.
The model simulations without the supplementary data set show little improvement over the forecasts of NMC and FNWC. Those simulations with the supplementary data produce improvements in the S 1 score, the intensity of the storm and the track of the storm. The improvement in the model simulations with the introduction of the supplementary data appears due to their more realistic documentation of the shallow cyclonic circulation, the small low-level static stability, and enhanced lower-tropospheric water vapor content.
Physical processes also played a major role in the simulators. The effect of surface fluxes of sensible and latent heat were moderate on the 24 h pressure and wind forecasts. In addition, these fluxes produced large changes in the temperature and moisture structure of the planetary boundary layer over a large area of cold northerly flow to the rear of the cyclone.
Latent heating was important in determining the storm intensity and track. Including latent heating through a cumulus parameterization scheme with a horizontal resolution of 90 km produced an improvement in the simulated intensity and position, with a reduction in minimum pressure of 7 mb and an increase in boundary-layer wind speed of 5 m s−1. With 45 km horizontal resolution, use of explicit condensation heating rather than the cumulus parameterization produced a further reduction in minimum pressure of 12 mb. Although experiments with explicit rather than parameterized latent heating produced more intense storms, in agreement with observations, the model storm motion was slowed considerably during the last 6 h of the simulation, resulting in an increased position error.
The model storm showed a small increase in intensity when the horizontal grid length reduced from 90 km to 45 km, with the minimum pressure decreasing by 3 mb. A further reduction in horizontal resolution to 22.5 km produced only minor differences in storm intensity.
The most intense model storm was simulated when an explicit medium-resolution planetary boundary-layer formulation replaced the bulk formulation used in most of the experiments. With 45 km resolution, explicit latent heating, and the medium-resolution boundary-layer model, a storm with minimum pressure of 960 mb and a maximum wind speed of 50.2 m s−1 was obtained.
This study suggests that baroclinic instability in the weakly stratified lower troposphere is the major mechanism of growth for this cyclone, as discussed by Reed, although latent heat plays an important role in the later stages of development. The development of this strong, yet relatively shallow, storm has three major implications for improving operational forecasts Of similar storms. First, the vertical resolution of the model must be adequate; our estimate is that at least four model layers are required below 700 mb. Second, the lower-tropospheric winds, static stability, water vapor content, and sea-surface temperature must be resolved accurately in the initial analysis because of the sensitivity of the model storm to these fields. Third, continued improvement of modeling planetary boundary-layer and latent heating processes is likely to be important in cases of this type.
Abstract
Mixed-layer models have been used to simulate low-level flows under a variety of situations, including flow over complex terrain and in the vicinity of coastal zones. The advantage of mixed-layer models compared to multilevel models is their simplicity and minimal computational requirements. A disadvantage is that the atmosphere above the mixed layer is not modeled explicitly and approximations pertaining to this layer become necessary. This paper examines five approximations for treating this upper layer for a simple sea-breeze circulation. Approximating the flow immediately above the mixed-layer height h by the mixed-layer velocity and using this velocity to advect potential temperature above h gives a better simulation of the sea breeze than the approximation used by Anthes et al. (1980), which neglected horizontal advection at this level.
Abstract
Mixed-layer models have been used to simulate low-level flows under a variety of situations, including flow over complex terrain and in the vicinity of coastal zones. The advantage of mixed-layer models compared to multilevel models is their simplicity and minimal computational requirements. A disadvantage is that the atmosphere above the mixed layer is not modeled explicitly and approximations pertaining to this layer become necessary. This paper examines five approximations for treating this upper layer for a simple sea-breeze circulation. Approximating the flow immediately above the mixed-layer height h by the mixed-layer velocity and using this velocity to advect potential temperature above h gives a better simulation of the sea breeze than the approximation used by Anthes et al. (1980), which neglected horizontal advection at this level.
Abstract
A series of increasingly complicated meteorological circulations is modeled by a two-dimensional, multi-level primitive equation model (MLM) and a one-dimensional mixed-layer model (XLM) in order to determine the extent to which the simple mixed-layer model can provide accurate predictions of the mean structure of the planetary boundary layer (PBL). Under horizontally homogeneous conditions, the PBL structure in the XLM agreed closely with the average structure in the MLM. When horizontal inhomogeneities associated with differential heating over complex terrain and across a land-water boundary were introduced, the XLM solutions became less accurate when compared to the MLM solutions. For these conditions a multi-level model appears to be essential to the correct prediction of flow within the PBL, because mass-wind adjustments in the flow above the PBL produce important changes on the pressure gradient within the PBL.
Abstract
A series of increasingly complicated meteorological circulations is modeled by a two-dimensional, multi-level primitive equation model (MLM) and a one-dimensional mixed-layer model (XLM) in order to determine the extent to which the simple mixed-layer model can provide accurate predictions of the mean structure of the planetary boundary layer (PBL). Under horizontally homogeneous conditions, the PBL structure in the XLM agreed closely with the average structure in the MLM. When horizontal inhomogeneities associated with differential heating over complex terrain and across a land-water boundary were introduced, the XLM solutions became less accurate when compared to the MLM solutions. For these conditions a multi-level model appears to be essential to the correct prediction of flow within the PBL, because mass-wind adjustments in the flow above the PBL produce important changes on the pressure gradient within the PBL.
Abstract
A three-layer primitive equation model of an isolated stationary tropical cyclone is constructed. The major difference between this and previously published models is the elimination of the assumption of circular symmetry. The release of latent heat by organized cumulus convection is parameterized by use of techniques previously shown to give realistic results in symmetrical models. In particular, the total release of heat in a vertical column is given by the horizontal convergence of water vapor in the Ekman layer and the vertical distribution of the heating follows the proposals made by Kuo. In the preliminary calculation reported on here, water vapor content is not forecast but, rather, is treated implicitly as was the ease for the earlier circularly symmetric models.
The results show that the model reproduces many observed features of the three-dimensional tropical cyclone. Realistic portrayals of spiral rainbands and the strongly asymmetric structure of the outflow layer are obtained. The kinetic energy budget of the model compares favorably with empirical estimates and also shows the loss of kinetic energy due to truncation errors to be very small.
Large-scale horizontal asymmetries in the outflow are found to play a significant role in the radial transport of vorticity during the mature stage and are of the same magnitude as the transport by the mean circulation.
In agreement with empirical studies, the outflow layer of the model storm shows substantial areas of negative absolute vorticity and anomalous winds.
Abstract
A three-layer primitive equation model of an isolated stationary tropical cyclone is constructed. The major difference between this and previously published models is the elimination of the assumption of circular symmetry. The release of latent heat by organized cumulus convection is parameterized by use of techniques previously shown to give realistic results in symmetrical models. In particular, the total release of heat in a vertical column is given by the horizontal convergence of water vapor in the Ekman layer and the vertical distribution of the heating follows the proposals made by Kuo. In the preliminary calculation reported on here, water vapor content is not forecast but, rather, is treated implicitly as was the ease for the earlier circularly symmetric models.
The results show that the model reproduces many observed features of the three-dimensional tropical cyclone. Realistic portrayals of spiral rainbands and the strongly asymmetric structure of the outflow layer are obtained. The kinetic energy budget of the model compares favorably with empirical estimates and also shows the loss of kinetic energy due to truncation errors to be very small.
Large-scale horizontal asymmetries in the outflow are found to play a significant role in the radial transport of vorticity during the mature stage and are of the same magnitude as the transport by the mean circulation.
In agreement with empirical studies, the outflow layer of the model storm shows substantial areas of negative absolute vorticity and anomalous winds.
Abstract
Results from a three-layer asymmetric hurricane model previously described by the authors are compared with results from an axially symmetric analog to investigate the effect of the symmetry assumption on the internal dynamics of model cyclones. The symmetric model storm initially develops more rapidly than the asymmetric storm. The differences in intensity during the first 100 hr are related to differences in horizontal resolution produced by the staggered grid used with the symmetric model. The symmetric model, on the other hand, does not produce the second period of intensification that starts at 120 hr in the asymmetric model. This fact supports the conclusion reached in the earlier paper that the development of large-scale asymmetries at 100 hr is closely related to the subsequent intensification.
Although the life cycles of the two storms are different, the azimuthally averaged structure of the asymmetric storm at maximum intensity is similar to the corresponding structure of the symmetric model storm and supports the adequacy of symmetric models in investigating many aspects of tropical cyclone structure.
Abstract
Results from a three-layer asymmetric hurricane model previously described by the authors are compared with results from an axially symmetric analog to investigate the effect of the symmetry assumption on the internal dynamics of model cyclones. The symmetric model storm initially develops more rapidly than the asymmetric storm. The differences in intensity during the first 100 hr are related to differences in horizontal resolution produced by the staggered grid used with the symmetric model. The symmetric model, on the other hand, does not produce the second period of intensification that starts at 120 hr in the asymmetric model. This fact supports the conclusion reached in the earlier paper that the development of large-scale asymmetries at 100 hr is closely related to the subsequent intensification.
Although the life cycles of the two storms are different, the azimuthally averaged structure of the asymmetric storm at maximum intensity is similar to the corresponding structure of the symmetric model storm and supports the adequacy of symmetric models in investigating many aspects of tropical cyclone structure.
Abstract
We describe a procedure for the initialization of the divergent wind component in a mesoscale, numerical weather prediction model and evaluate it in terms of its ability to provide an improved short-range precipitation forecast. The divergent wind component was obtained from a vertical velocity field that was diagnosed using an omega equation. The diabatic term of the omega equation was dominant in regions of observed precipitation. Five precipitation forecasts were performed for the same 12 h period but each was initialized in a different manner. One procedure combined the diagnosed divergent component with the observed nondivergent wind. This total wind was used in a divergent balance equation to obtain the temperature field. The precipitation forecast based on these initial conditions was compared with those started from nondivergent, balanced initial conditions as well as from unbalanced data that contained the observed divergent component. The use of the diagnosed divergence significantly improved the precipitation forecast during the first three hours compared to the nondivergent forecasts. The forecast based on the unbalanced data with the observed divergence showed no improvement. A large fraction of the initial, diagnosed divergence was retained by the model because areas of observed precipitation were saturated in the initial moisture analysis causing the initial upward vertical motions to be sustained by latent heating. After 6 h of integration, the precipitation rates for all the forecasts were approximately the same.
Abstract
We describe a procedure for the initialization of the divergent wind component in a mesoscale, numerical weather prediction model and evaluate it in terms of its ability to provide an improved short-range precipitation forecast. The divergent wind component was obtained from a vertical velocity field that was diagnosed using an omega equation. The diabatic term of the omega equation was dominant in regions of observed precipitation. Five precipitation forecasts were performed for the same 12 h period but each was initialized in a different manner. One procedure combined the diagnosed divergent component with the observed nondivergent wind. This total wind was used in a divergent balance equation to obtain the temperature field. The precipitation forecast based on these initial conditions was compared with those started from nondivergent, balanced initial conditions as well as from unbalanced data that contained the observed divergent component. The use of the diagnosed divergence significantly improved the precipitation forecast during the first three hours compared to the nondivergent forecasts. The forecast based on the unbalanced data with the observed divergence showed no improvement. A large fraction of the initial, diagnosed divergence was retained by the model because areas of observed precipitation were saturated in the initial moisture analysis causing the initial upward vertical motions to be sustained by latent heating. After 6 h of integration, the precipitation rates for all the forecasts were approximately the same.
Abstract
Four numerical experiments are conducted using a mesoscale primitive equation model. The experiments illustrate the broad spectrum of applications made possible by the model's flexibility in treating subgrid-scale parameterizations, lateral boundary conditions and physical processes appropriate to the scale of each simulation. One experiment uses real synoptic-scale data to produce a 12 h forecast that is compared to the observed circulation and precipitation patterns. The other experiments are initialized with idealized flows over areas ranging in size from regional to small mesoscale. The idealized flow simulations produce qualitatively realistic features such as Ice side troughs and sea, lake and mountain-valley breezes resulting from differential thermal forcing.
Abstract
Four numerical experiments are conducted using a mesoscale primitive equation model. The experiments illustrate the broad spectrum of applications made possible by the model's flexibility in treating subgrid-scale parameterizations, lateral boundary conditions and physical processes appropriate to the scale of each simulation. One experiment uses real synoptic-scale data to produce a 12 h forecast that is compared to the observed circulation and precipitation patterns. The other experiments are initialized with idealized flows over areas ranging in size from regional to small mesoscale. The idealized flow simulations produce qualitatively realistic features such as Ice side troughs and sea, lake and mountain-valley breezes resulting from differential thermal forcing.
Abstract
Uccellini and Johnson present a case study of a severe weather event in Ohio on 10–11 May 1973 to show evidence for coupling between an upper-tropospheric jet streak and a low-level jet within an indirect transverse circulation found in the exit region of the upper-level jet. The differential advection of moisture and temperature created by the shear between the upper- and low-level jets reduced convective stability, thereby enhancing the potential for severe convection.
Two 12 h numerical simulations of the 10–11 May 1973 case are studied to determine 1) if a transverse indirect circulation with a low-level jet imbedded in its lower branch can be diagnosed in the exit region of the upper-level jet and studied using the model output at 3 h intervals and 2) if the initial magnitude and structure of the upper-level jet have a significant effect on the subsequent development of the low-level jet and the decrease in convectivc stability due to differential advection. In an adiabatic model simulation, an indirect transverse circulation having a low-level jet within its lower branch occurs in the exit region of the upper-level jet. The simulated vertical distribution of mass divergence and ageostrophic flow in the exit region agree with the diagnoses of Uccellini and Johnson. At upper levels, mass divergence (convergence) occurs on the cyclonic (anticyclonic) side of the exit region, while the opposite occurs at low levels. The, upper branch of the indirect circulation is dominated by the inertial–advective contribution to the ageostrophic wind which is related to the alongstream isotach gradient in the exit region. The lower branch is dominated by the wind tendency contribution to the ageostrophic wind. Ageostrophic shear associated with this circulation contributes to the development of differential moisture and temperature advection, which act to destabilize the preconvective environment.
A second simulation using a smoothed, nondivergent initialization with a weaker upper-level jet streak and weaker alongside isotach gradient in the exit region of the upper-level jet produces a weaker indirect transverse circulation even though diabatic heating effects are present. The indirect circulation for this simulation is marked by smaller vertical motions, a weaker low-level return branch, and weaker low-level thermal and moisture advection associated with the low-level flow. Comparison of the two simulations suggests that the indirect circulation in the exit region of the upper-level jet is strongly responsive to dynamical processes associated with the initial structure of the jet streak.
Abstract
Uccellini and Johnson present a case study of a severe weather event in Ohio on 10–11 May 1973 to show evidence for coupling between an upper-tropospheric jet streak and a low-level jet within an indirect transverse circulation found in the exit region of the upper-level jet. The differential advection of moisture and temperature created by the shear between the upper- and low-level jets reduced convective stability, thereby enhancing the potential for severe convection.
Two 12 h numerical simulations of the 10–11 May 1973 case are studied to determine 1) if a transverse indirect circulation with a low-level jet imbedded in its lower branch can be diagnosed in the exit region of the upper-level jet and studied using the model output at 3 h intervals and 2) if the initial magnitude and structure of the upper-level jet have a significant effect on the subsequent development of the low-level jet and the decrease in convectivc stability due to differential advection. In an adiabatic model simulation, an indirect transverse circulation having a low-level jet within its lower branch occurs in the exit region of the upper-level jet. The simulated vertical distribution of mass divergence and ageostrophic flow in the exit region agree with the diagnoses of Uccellini and Johnson. At upper levels, mass divergence (convergence) occurs on the cyclonic (anticyclonic) side of the exit region, while the opposite occurs at low levels. The, upper branch of the indirect circulation is dominated by the inertial–advective contribution to the ageostrophic wind which is related to the alongstream isotach gradient in the exit region. The lower branch is dominated by the wind tendency contribution to the ageostrophic wind. Ageostrophic shear associated with this circulation contributes to the development of differential moisture and temperature advection, which act to destabilize the preconvective environment.
A second simulation using a smoothed, nondivergent initialization with a weaker upper-level jet streak and weaker alongside isotach gradient in the exit region of the upper-level jet produces a weaker indirect transverse circulation even though diabatic heating effects are present. The indirect circulation for this simulation is marked by smaller vertical motions, a weaker low-level return branch, and weaker low-level thermal and moisture advection associated with the low-level flow. Comparison of the two simulations suggests that the indirect circulation in the exit region of the upper-level jet is strongly responsive to dynamical processes associated with the initial structure of the jet streak.
