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Abstract
A long-lived mesoscale convectively-generated vortex (MCV) associated with a mesoscale convective complex (MCC) is documented. The MCV, with a Rossby number of approximately 0.5, is investigated as a feature intrinsic to the organization of the MCC. On 6–7 July 1982 a particularly large and intense MCC developed in a region of high convective available potential energy (CAPE) but weak vertical wind shear (bulk Richardson number ∼150), and weak advection of temperature and vorticity. Convection initially was organized in a narrow line with elements propagating relative to the mean environmental flow. These elements subsequently developed a large semicircular area of stratiform precipitation and a surface mesolow to the rear. Heavy rain fell over a broad area; amounts as great as 10.9 cm accompanied by flooding were reported in central Oklahoma. As the large semicircular rain area dissipated, a three layered structure became evident: a large upper tropospheric anticyclone, a rain cooled mesoscale high pressure system at the surface with a trailing mesoscale low, and a middle level cyclonic vortex. The midlevel vortex is clearly identifiable in visible satellite imagery and in radar observations. The upper level anticyclone was initially unstable and dissipated rapidly. The midlevel cyclone, however, became inertially stable and persisted with little change for over two days in a relatively benign synoptic regime of low wind speed, weak shear and low CAPE. During this period, the MCV exhibited a deep column of convergence, positive relative vorticity and mesoscale saturated ascent with a layer of weak divergence aloft. The MCV was also accompanied by local showers and depressed afternoon surface temperatures.
The combination of high CAPE and weak vertical shear in the pre-MCC environment was conducive to the rapid formation of the large trailing stratiform cloud region in the middle and upper troposphere. Transformation of the environment from dry midlevels with high CAPE to moist midlevels and low CAPE resulted in a virtual warming that hydrostatically corresponded to about a 5–15 m height fall at midlevels. In the weak-gradient barotropic environment, perturbations of this magnitude were comparable to the height depression of the cyclonic disturbance. Following the establishment of inertial stability, the vortex was advected eastward with little change in relative vorticity for over 36 h. The presence of a surface-based cyclonic circulation during the period of inertial stability suggests that the dynamical forcing from differential vorticity advection may have been enhanced, particularly in the daytime, by frictional convergence. The combination of vorticity advection, frictional convergence and boundary layer warming was sufficient to initiate a new cycle of deep convection near the center of the vortex.
Abstract
A long-lived mesoscale convectively-generated vortex (MCV) associated with a mesoscale convective complex (MCC) is documented. The MCV, with a Rossby number of approximately 0.5, is investigated as a feature intrinsic to the organization of the MCC. On 6–7 July 1982 a particularly large and intense MCC developed in a region of high convective available potential energy (CAPE) but weak vertical wind shear (bulk Richardson number ∼150), and weak advection of temperature and vorticity. Convection initially was organized in a narrow line with elements propagating relative to the mean environmental flow. These elements subsequently developed a large semicircular area of stratiform precipitation and a surface mesolow to the rear. Heavy rain fell over a broad area; amounts as great as 10.9 cm accompanied by flooding were reported in central Oklahoma. As the large semicircular rain area dissipated, a three layered structure became evident: a large upper tropospheric anticyclone, a rain cooled mesoscale high pressure system at the surface with a trailing mesoscale low, and a middle level cyclonic vortex. The midlevel vortex is clearly identifiable in visible satellite imagery and in radar observations. The upper level anticyclone was initially unstable and dissipated rapidly. The midlevel cyclone, however, became inertially stable and persisted with little change for over two days in a relatively benign synoptic regime of low wind speed, weak shear and low CAPE. During this period, the MCV exhibited a deep column of convergence, positive relative vorticity and mesoscale saturated ascent with a layer of weak divergence aloft. The MCV was also accompanied by local showers and depressed afternoon surface temperatures.
The combination of high CAPE and weak vertical shear in the pre-MCC environment was conducive to the rapid formation of the large trailing stratiform cloud region in the middle and upper troposphere. Transformation of the environment from dry midlevels with high CAPE to moist midlevels and low CAPE resulted in a virtual warming that hydrostatically corresponded to about a 5–15 m height fall at midlevels. In the weak-gradient barotropic environment, perturbations of this magnitude were comparable to the height depression of the cyclonic disturbance. Following the establishment of inertial stability, the vortex was advected eastward with little change in relative vorticity for over 36 h. The presence of a surface-based cyclonic circulation during the period of inertial stability suggests that the dynamical forcing from differential vorticity advection may have been enhanced, particularly in the daytime, by frictional convergence. The combination of vorticity advection, frictional convergence and boundary layer warming was sufficient to initiate a new cycle of deep convection near the center of the vortex.
Abstract
Examination of NMC upper tropospheric analyses (300, 200 and 150 mb charts) indicates that significant perturbations are present in the wind fields in the vicinity of intense meso-α scale (250–2500 km) thunder-storm complexes (identified utilizing enhanced IR satellite imagery). This effect is investigated for each of 10 mesoscale convective complexes. Since the LFM convective adjustment procedure cannot infuse large amounts of mass, momentum and moisture into the upper troposphere and lower stratosphere, the 12 h LFM predicted winds are used as an indication of the unperturbed environmental flow. An estimate of the convective perturbation is obtained by subtracting the LFM predicted 200 mb winds from the observed winds. A large anticyclonic flow perturbation is present in each of the 10 events. Wind speed perturbations at individual sounding locations are commonly 10–20 m s−1 with maximum values as great as 38 m s−1. Detailed case analyses for two events are presented to illustrate these effects.
The predicted and observed fields are objectively analyzed over a common grid to develop a composite field for the 10 cases. The composite difference field is not only similar to that of the individual cases but it is also found that significant perturbations occur only in the vicinity of the convective complexes. Macroscale and mesoscale characteristics of these composite flow fields are also examined utilizing an objective technique for scale separation. Other characteristics of the perturbed fields are presented and implications of the convectively forced perturbations are discussed.
Abstract
Examination of NMC upper tropospheric analyses (300, 200 and 150 mb charts) indicates that significant perturbations are present in the wind fields in the vicinity of intense meso-α scale (250–2500 km) thunder-storm complexes (identified utilizing enhanced IR satellite imagery). This effect is investigated for each of 10 mesoscale convective complexes. Since the LFM convective adjustment procedure cannot infuse large amounts of mass, momentum and moisture into the upper troposphere and lower stratosphere, the 12 h LFM predicted winds are used as an indication of the unperturbed environmental flow. An estimate of the convective perturbation is obtained by subtracting the LFM predicted 200 mb winds from the observed winds. A large anticyclonic flow perturbation is present in each of the 10 events. Wind speed perturbations at individual sounding locations are commonly 10–20 m s−1 with maximum values as great as 38 m s−1. Detailed case analyses for two events are presented to illustrate these effects.
The predicted and observed fields are objectively analyzed over a common grid to develop a composite field for the 10 cases. The composite difference field is not only similar to that of the individual cases but it is also found that significant perturbations occur only in the vicinity of the convective complexes. Macroscale and mesoscale characteristics of these composite flow fields are also examined utilizing an objective technique for scale separation. Other characteristics of the perturbed fields are presented and implications of the convectively forced perturbations are discussed.
Abstract
The skill of an automated statistical forecasting system that uses only hourly (top of the hour) surface observations is compared with a system that utilizes hourly and high-frequency (interhour) surface observations to forecast low-ceiling and low-visibility events in the New York City, New York, area. Forecasts feature lead times of 1 h and less. Equations to forecast ceiling and visibility conditions 1 h in advance are developed for initialization times at the top of the hour, as well as at 15, 30, and 45 min past the hour. Two forecasting systems are created: a baseline system that utilizes only hourly surface observations and an alternative system that utilizes both hourly surface observations and high-frequency observations. Introduction of the high-frequency observations into the forecasting system produces an additional 1.5%–4.5% reduction in the mean-square error (MSE), as compared with the baseline system for 1-h forecasts made at the top of the hour. By 45 min past the hour, the reduction in MSE over the baseline system increases to 14%–17%. The high-frequency observations are also utilized to develop forecast equations with lead times of 5–55 min. Reduction in MSE for this rapid-update forecasting system in comparison with simple persistence increased from an average of 3% for a 5-min lead time to an average of nearly 22% for a 55-min lead time. Moreover, improvements over persistence climatology increased from an average of 1.5% for a 5-min lead time to an average of about 14% for a 55-min lead time. These findings indicate that current observations-based forecasting techniques can be improved by utilizing high-frequency surface weather observations. Therefore, the uncertainty in decisions affected by the arrival and duration of a low ceiling and low visibility can be reduced, thereby providing enhanced guidance for operational airport traffic delay programs.
Abstract
The skill of an automated statistical forecasting system that uses only hourly (top of the hour) surface observations is compared with a system that utilizes hourly and high-frequency (interhour) surface observations to forecast low-ceiling and low-visibility events in the New York City, New York, area. Forecasts feature lead times of 1 h and less. Equations to forecast ceiling and visibility conditions 1 h in advance are developed for initialization times at the top of the hour, as well as at 15, 30, and 45 min past the hour. Two forecasting systems are created: a baseline system that utilizes only hourly surface observations and an alternative system that utilizes both hourly surface observations and high-frequency observations. Introduction of the high-frequency observations into the forecasting system produces an additional 1.5%–4.5% reduction in the mean-square error (MSE), as compared with the baseline system for 1-h forecasts made at the top of the hour. By 45 min past the hour, the reduction in MSE over the baseline system increases to 14%–17%. The high-frequency observations are also utilized to develop forecast equations with lead times of 5–55 min. Reduction in MSE for this rapid-update forecasting system in comparison with simple persistence increased from an average of 3% for a 5-min lead time to an average of nearly 22% for a 55-min lead time. Moreover, improvements over persistence climatology increased from an average of 1.5% for a 5-min lead time to an average of about 14% for a 55-min lead time. These findings indicate that current observations-based forecasting techniques can be improved by utilizing high-frequency surface weather observations. Therefore, the uncertainty in decisions affected by the arrival and duration of a low ceiling and low visibility can be reduced, thereby providing enhanced guidance for operational airport traffic delay programs.
Abstract
A fine-mesh, 20-level, primitive equation model is used to study the generation of convectively driven weather systems in the vicinity of the tropopause. In a test simulation, a high-level (∼200 mb) mesoscale high pressure system forms in conjunction with the development of a convective complex. In response to this high-level mesohigh, winds aloft rapidly decelerate as they approach the convective complex. On the other hand, downstream of the convective system the mesoscale pressure gradient accelerates the wind to generate a jet maximum which is stronger than any wind speed prior to the development of the convection.
The formation of the high-level mesohigh appears to be linked to the convectively forced production of a layer of cold air above the tropopause. The cold layer of air is generated by cloud-scale cooling from overshooting tops and from adiabatic cooling by strong (∼0.5 m s−1) mesoscale lifting in response to the convective cloud warming below the tropopause.
The model-generated high-level convective system is compared to observed systems and briefly discussed in light of the interaction of these systems with their larger scale environment.
Abstract
A fine-mesh, 20-level, primitive equation model is used to study the generation of convectively driven weather systems in the vicinity of the tropopause. In a test simulation, a high-level (∼200 mb) mesoscale high pressure system forms in conjunction with the development of a convective complex. In response to this high-level mesohigh, winds aloft rapidly decelerate as they approach the convective complex. On the other hand, downstream of the convective system the mesoscale pressure gradient accelerates the wind to generate a jet maximum which is stronger than any wind speed prior to the development of the convection.
The formation of the high-level mesohigh appears to be linked to the convectively forced production of a layer of cold air above the tropopause. The cold layer of air is generated by cloud-scale cooling from overshooting tops and from adiabatic cooling by strong (∼0.5 m s−1) mesoscale lifting in response to the convective cloud warming below the tropopause.
The model-generated high-level convective system is compared to observed systems and briefly discussed in light of the interaction of these systems with their larger scale environment.
Abstract
An automated statistical system that utilizes regional high-density surface observations to forecast low ceiling and visibility events in the upper Midwest is presented. The system is based solely upon surface observations as predictors, featuring forecast lead times of 1, 3, and 6 h.
A test of the forecast system on a 5-yr independent sample of events shows that for a 1-h lead time, an additional 2%–4% reduction in the mean squared error (MSE) is obtained by the high-density forecasting system compared to that for a system utilizing only the standard synoptic observations. Meanwhile, tests on a 3-h lead time reveal an additional 0%–1.5% reduction in MSE by the high-density system over the synoptic system. Little improvement is gained by the high-density system at a 6-h lead time.
The results indicate that current observations-based forecasting techniques can be improved simply by utilizing a higher density of surface weather observations. With this enhanced guidance, it is likely that decisions impacted by the arrival and duration of low ceiling and visibility can be improved.
Abstract
An automated statistical system that utilizes regional high-density surface observations to forecast low ceiling and visibility events in the upper Midwest is presented. The system is based solely upon surface observations as predictors, featuring forecast lead times of 1, 3, and 6 h.
A test of the forecast system on a 5-yr independent sample of events shows that for a 1-h lead time, an additional 2%–4% reduction in the mean squared error (MSE) is obtained by the high-density forecasting system compared to that for a system utilizing only the standard synoptic observations. Meanwhile, tests on a 3-h lead time reveal an additional 0%–1.5% reduction in MSE by the high-density system over the synoptic system. Little improvement is gained by the high-density system at a 6-h lead time.
The results indicate that current observations-based forecasting techniques can be improved simply by utilizing a higher density of surface weather observations. With this enhanced guidance, it is likely that decisions impacted by the arrival and duration of low ceiling and visibility can be improved.
Abstract
A parameterization formulation for incorporating the effects of midlatitude deep convection into mesoscale-numerical models is presented. The formulation is based on the hypothesis that the buoyant energy available to a parcel, in combination with a prescribed period of time for the convection to remove that energy, can be used to regulate the amount of convection in a mesoscale numerical model grid element.
Individual clouds are represented as entraining moist updraft and downdraft plumes. The fraction of updraft condensate evaporated in moist downdrafts is determined from an empirical relationship between the vertical shear of the horizontal wind and precipitation efficiency. Vertical transports of horizontal momentum and warming by compensating subsidence are included in the parameterization. Since updraft and downdraft areas are sometimes a substantial fraction of mesoscale model grid-element areas, grid-point temperatures (adjusted for convection) are an area-weighted mean of updraft, downdraft and environmental temperatures.
Abstract
A parameterization formulation for incorporating the effects of midlatitude deep convection into mesoscale-numerical models is presented. The formulation is based on the hypothesis that the buoyant energy available to a parcel, in combination with a prescribed period of time for the convection to remove that energy, can be used to regulate the amount of convection in a mesoscale numerical model grid element.
Individual clouds are represented as entraining moist updraft and downdraft plumes. The fraction of updraft condensate evaporated in moist downdrafts is determined from an empirical relationship between the vertical shear of the horizontal wind and precipitation efficiency. Vertical transports of horizontal momentum and warming by compensating subsidence are included in the parameterization. Since updraft and downdraft areas are sometimes a substantial fraction of mesoscale model grid-element areas, grid-point temperatures (adjusted for convection) are an area-weighted mean of updraft, downdraft and environmental temperatures.
Abstract
A convectively generated mesoscale vortex that was instrumental in initiating and organizing five successive mesoscale convective systems over a period of three days is documented. Two of these convective systems were especially intense and resulted in widespread heavy rain with localized flooding. Based upon radar and satellite data, the detectable size of the vortex became much larger following the strong convective developments, nearly tripling its initial diameter over its three-day life cycle. During nighttime, when convection typically intensified within the vortex, movement of the system tended to slow. Following dissipation of the convection in the morning, the daytime movement accelerated.
Cross sections of potential vorticity taken through the vortex center clearly show a maximum at midlevels and a well-defined minimum directly above. The vortex and the potential vorticity maximum were essentially colocated and the system was nearly axisymmetric in the vertical. Over the three-day life cycle of the system, the strength of the vortex, as measured by the magnitude of the midlevel potential vorticity maximum, steadily increased.
At low levels, isentropic surfaces sloped upward from the rear of the potential vorticity anomaly into the vortex center so that relatively fast-moving low-level southwesterly flow, which was overtaking the slow-moving vortex from the rear, ascended as it approached the vortex center. Computations of the magnitude and duration of the ascent indicate that the lifting was sufficient to initiate new convection only if parcels realized the maximum possible ascent by flowing into the innermost region of the vortex circulation. In support of this interpretation, satellite observations show that new convection repeatedly developed near the vortex center instead of along well-defined surface outflow boundaries that encircled the convective system. A conceptual model describing the redevelopment mechanism is presented.
Analyses of the large-scale environment of the vortex show that it formed and persisted in a deep and broad zone of southwesterly flow just upstream of a synoptic-scale ridge. At tropopause levels, a large anticyclone covered the region. Potential buoyant energy in the vortex environment typically ranged from about 1000 J kg−1 at 1200 UTC to 1900 J kg−1 at 0000 UTC. Extreme values were as large as 3500 J kg−1. Except for a low-level jet, wind speed and vertical wind shear were relatively small throughout the troposphere, especially in the vortex-bearing layer (700–300 mb) where shear values were only about 0.8 × 10−3 s−1. The deep midlevel layer of weak shear provided a favorable environment for the formation and persistence of the nearly axisymmetric vertical disturbance.
Since the vortex formed and grew over land, this study demonstrates that warm-core mesovortex genesis and amplification do not require heat and moisture fluxes from a tropical marine surface. Evidently, ambient CAPE is sufficient for vortex formation and limited growth. However, since the vortex growth primarily occurred in the middle troposphere, and since anticyclonic outflow was usually present at the surface, marine surface fluxes may be necessary for transformation of such convectively generated vortices into surface-based tropical disturbances.
Abstract
A convectively generated mesoscale vortex that was instrumental in initiating and organizing five successive mesoscale convective systems over a period of three days is documented. Two of these convective systems were especially intense and resulted in widespread heavy rain with localized flooding. Based upon radar and satellite data, the detectable size of the vortex became much larger following the strong convective developments, nearly tripling its initial diameter over its three-day life cycle. During nighttime, when convection typically intensified within the vortex, movement of the system tended to slow. Following dissipation of the convection in the morning, the daytime movement accelerated.
Cross sections of potential vorticity taken through the vortex center clearly show a maximum at midlevels and a well-defined minimum directly above. The vortex and the potential vorticity maximum were essentially colocated and the system was nearly axisymmetric in the vertical. Over the three-day life cycle of the system, the strength of the vortex, as measured by the magnitude of the midlevel potential vorticity maximum, steadily increased.
At low levels, isentropic surfaces sloped upward from the rear of the potential vorticity anomaly into the vortex center so that relatively fast-moving low-level southwesterly flow, which was overtaking the slow-moving vortex from the rear, ascended as it approached the vortex center. Computations of the magnitude and duration of the ascent indicate that the lifting was sufficient to initiate new convection only if parcels realized the maximum possible ascent by flowing into the innermost region of the vortex circulation. In support of this interpretation, satellite observations show that new convection repeatedly developed near the vortex center instead of along well-defined surface outflow boundaries that encircled the convective system. A conceptual model describing the redevelopment mechanism is presented.
Analyses of the large-scale environment of the vortex show that it formed and persisted in a deep and broad zone of southwesterly flow just upstream of a synoptic-scale ridge. At tropopause levels, a large anticyclone covered the region. Potential buoyant energy in the vortex environment typically ranged from about 1000 J kg−1 at 1200 UTC to 1900 J kg−1 at 0000 UTC. Extreme values were as large as 3500 J kg−1. Except for a low-level jet, wind speed and vertical wind shear were relatively small throughout the troposphere, especially in the vortex-bearing layer (700–300 mb) where shear values were only about 0.8 × 10−3 s−1. The deep midlevel layer of weak shear provided a favorable environment for the formation and persistence of the nearly axisymmetric vertical disturbance.
Since the vortex formed and grew over land, this study demonstrates that warm-core mesovortex genesis and amplification do not require heat and moisture fluxes from a tropical marine surface. Evidently, ambient CAPE is sufficient for vortex formation and limited growth. However, since the vortex growth primarily occurred in the middle troposphere, and since anticyclonic outflow was usually present at the surface, marine surface fluxes may be necessary for transformation of such convectively generated vortices into surface-based tropical disturbances.
Abstract
Consensus forecasts from the control runs of several operational numerical models are compared to 1) the control-run forecasts of the individual models that compose the consensus and to 2) other consensus forecasts generated by varying the initial conditions of the various individual models. It is found that the multimodel consensus is superior to the individual control runs and to the consensus forecasts constructed from ensembles of runs generated by varying model initial conditions. The source of the forecast improvement by model consensus is not the result of a simple cancellation of errors as a result of an overall positive bias in one model and an overall negative bias in another. Rather the main improvement stems from overlapping differences in the sign of the errors associated with forecasts of individual traveling disturbances. The results suggest that variations in model physics and numerics play a substantial role in generating the full spectrum of possible solutions that can arise in a given numerical forecast.
Abstract
Consensus forecasts from the control runs of several operational numerical models are compared to 1) the control-run forecasts of the individual models that compose the consensus and to 2) other consensus forecasts generated by varying the initial conditions of the various individual models. It is found that the multimodel consensus is superior to the individual control runs and to the consensus forecasts constructed from ensembles of runs generated by varying model initial conditions. The source of the forecast improvement by model consensus is not the result of a simple cancellation of errors as a result of an overall positive bias in one model and an overall negative bias in another. Rather the main improvement stems from overlapping differences in the sign of the errors associated with forecasts of individual traveling disturbances. The results suggest that variations in model physics and numerics play a substantial role in generating the full spectrum of possible solutions that can arise in a given numerical forecast.
Abstract
A procedure for operationally predicting the movement of the mesobeta-scale convective elements responsible for the heavy rain in mesocale convective complexes is presented. The procedure is based on the well-known concepts that the motion of convective systems can be considered the sum of an advective component, given by the mean motion of the cells composing the system, and a propagation component, defined by the rate and location of new cell formation relative to existing cells. These concepts and the forecast procedure are examined using 103 mesoscale convective systems, 99 of which are mesoscale convective complexes.
It is found that the advective component of the convective systems is well correlated to the mean flow in the cloud layer. Similarly, the propagation component is shown to be directly proportional (but opposite in sign) and well correlated to the speed and direction of the low-level jet. Correlation coefficients between forecast and observed values for the speed and direction of the mesobeta-scale convective elements are 0.80 and 0.78, respectively. Mean absolute errors of the speed and direction are 2.0 m s−1 and 17°. These errors are sufficiently small so that the forecast path of the centroid of the mesobeta-scale elements would be well within the heavy rain swath of the typical mesoscale convective complex.
Abstract
A procedure for operationally predicting the movement of the mesobeta-scale convective elements responsible for the heavy rain in mesocale convective complexes is presented. The procedure is based on the well-known concepts that the motion of convective systems can be considered the sum of an advective component, given by the mean motion of the cells composing the system, and a propagation component, defined by the rate and location of new cell formation relative to existing cells. These concepts and the forecast procedure are examined using 103 mesoscale convective systems, 99 of which are mesoscale convective complexes.
It is found that the advective component of the convective systems is well correlated to the mean flow in the cloud layer. Similarly, the propagation component is shown to be directly proportional (but opposite in sign) and well correlated to the speed and direction of the low-level jet. Correlation coefficients between forecast and observed values for the speed and direction of the mesobeta-scale convective elements are 0.80 and 0.78, respectively. Mean absolute errors of the speed and direction are 2.0 m s−1 and 17°. These errors are sufficiently small so that the forecast path of the centroid of the mesobeta-scale elements would be well within the heavy rain swath of the typical mesoscale convective complex.
Abstract
An intense mesoscale convective complex developed over the central Mississippi Valley during the night and early morning hours of 24 and 25 April 1975. Analyses of upper tropospheric features during this period indicate strong changes in temperature, wind and pressure-surface heights occurred over the convective system in a period of only 6 h. It is hypothesized that the convective system is responsible for these changes. The question of whether the diagnosed changes reflect a natural evolution of large-scale meteorological fields or are a result of widespread deep convection is considered utilizing two separate numerical forecasts produced by the Drexel-NCAR mesoscale primitive equation model. A “dry” forecast, in which no convective clouds are permitted, is considered representative of the evolution of the large-scale environment. This forecast is contrasted with a “moist” forecast which, through the use of a one-dimensional, sequential plume cumulus model, includes the effects of deep convection. Differences between the forecasts are substantial and the perturbations produced by the convection are quite similar to diagnosed features. The numerical results support the contention that mososcale, convectively driven circulations associated with large thunderstorm complexes can significantly alter upper tropospheric environmental conditions.
Abstract
An intense mesoscale convective complex developed over the central Mississippi Valley during the night and early morning hours of 24 and 25 April 1975. Analyses of upper tropospheric features during this period indicate strong changes in temperature, wind and pressure-surface heights occurred over the convective system in a period of only 6 h. It is hypothesized that the convective system is responsible for these changes. The question of whether the diagnosed changes reflect a natural evolution of large-scale meteorological fields or are a result of widespread deep convection is considered utilizing two separate numerical forecasts produced by the Drexel-NCAR mesoscale primitive equation model. A “dry” forecast, in which no convective clouds are permitted, is considered representative of the evolution of the large-scale environment. This forecast is contrasted with a “moist” forecast which, through the use of a one-dimensional, sequential plume cumulus model, includes the effects of deep convection. Differences between the forecasts are substantial and the perturbations produced by the convection are quite similar to diagnosed features. The numerical results support the contention that mososcale, convectively driven circulations associated with large thunderstorm complexes can significantly alter upper tropospheric environmental conditions.
