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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.
Abstract
The hypothesis that clouds and precipitation enhance cold air damming is examined. A case example of cloud/precipitation-induced enhancement of damming is presented and a conceptual model is proposed.
It is found that the subcloud-layer diabatic effects associated with areas of precipitation produce mesoscale changes in pressure, wind, and static stability. These changes tend to maintain or strengthen damming in two fundamental ways: 1) Cloud cover maintains damming by preventing or reducing the radiative destabilization in the upslope layer. Without cloud cover, the lapse rate is more likely to increase so that upslope adiabatic cooling, and therefore the potential for damming, is decreased. 2) Subcloud-layer diabatic cooling from evaporation and reduced radiation produces a hydrostatic pressure rise in the precipitation zone. The low-level wind field adjusts to the pressure rise in such a manner that it enhances advection of low-level cold air southward under progressively warmer air just above the subcloud layer. As a result, the static stability, and therefore the damming potential, are increased as the cold air advances southward. The adjustment of the wind also increases both the upslope component of the wind field and the depth of the upslope layer, thereby enhancing the adiabatic cooling along the mountains and strengthening the wedge ridge. This combination of processes can create cold surges that propagate rapidly along the mountain chain.
Abstract
The hypothesis that clouds and precipitation enhance cold air damming is examined. A case example of cloud/precipitation-induced enhancement of damming is presented and a conceptual model is proposed.
It is found that the subcloud-layer diabatic effects associated with areas of precipitation produce mesoscale changes in pressure, wind, and static stability. These changes tend to maintain or strengthen damming in two fundamental ways: 1) Cloud cover maintains damming by preventing or reducing the radiative destabilization in the upslope layer. Without cloud cover, the lapse rate is more likely to increase so that upslope adiabatic cooling, and therefore the potential for damming, is decreased. 2) Subcloud-layer diabatic cooling from evaporation and reduced radiation produces a hydrostatic pressure rise in the precipitation zone. The low-level wind field adjusts to the pressure rise in such a manner that it enhances advection of low-level cold air southward under progressively warmer air just above the subcloud layer. As a result, the static stability, and therefore the damming potential, are increased as the cold air advances southward. The adjustment of the wind also increases both the upslope component of the wind field and the depth of the upslope layer, thereby enhancing the adiabatic cooling along the mountains and strengthening the wedge ridge. This combination of processes can create cold surges that propagate rapidly along the mountain chain.
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
The organizational mode of quasi-linear convective systems often falls within a spectrum of modes described by a line of discrete cells on one end (“cellular”) and an unbroken two-dimensional swath of ascent on the other (“slabular”). Convective events exhibiting distinctly cellular or slabular characteristics over the continental United States were compiled, and composite soundings of the respective inflow environments were constructed. The most notable difference between the environments of slabs and cells occurred in the wind profiles; lines organized as slabs existed in much stronger low-level line-relative inflow and stronger low-level shear.
A compressible model with high resolution (Δx = 500 m) was used to investigate the effects of varying environmental conditions on the nature of the convective overturning. The numerical results show that highly cellular convective lines are favored when the environmental conditions and initiation procedure allow the convectively generated cold pools to remain separate from one another. The transition to a continuous along-line cold pool and gust front leads to the generation of a more “solid” line of convection, as dynamic pressure forcing above the downshear edge of the cold outflow creates a swath of quasi-two-dimensional ascent. Using both full-physics simulations and a simplified cold-pool model, it is demonstrated that the magnitude of the two-dimensional ascent in slabular convective systems is closely related to the integrated cold-pool strength.
It is concluded that slabular organization tends to occur under conditions that favor the development of a strong, contiguous cold pool. The tendency to produce slabular convection is therefore enhanced by environmental conditions such as large CAPE, weak convective inhibition, strong along-line winds, and moderately strong cross-line wind shear.
Abstract
The organizational mode of quasi-linear convective systems often falls within a spectrum of modes described by a line of discrete cells on one end (“cellular”) and an unbroken two-dimensional swath of ascent on the other (“slabular”). Convective events exhibiting distinctly cellular or slabular characteristics over the continental United States were compiled, and composite soundings of the respective inflow environments were constructed. The most notable difference between the environments of slabs and cells occurred in the wind profiles; lines organized as slabs existed in much stronger low-level line-relative inflow and stronger low-level shear.
A compressible model with high resolution (Δx = 500 m) was used to investigate the effects of varying environmental conditions on the nature of the convective overturning. The numerical results show that highly cellular convective lines are favored when the environmental conditions and initiation procedure allow the convectively generated cold pools to remain separate from one another. The transition to a continuous along-line cold pool and gust front leads to the generation of a more “solid” line of convection, as dynamic pressure forcing above the downshear edge of the cold outflow creates a swath of quasi-two-dimensional ascent. Using both full-physics simulations and a simplified cold-pool model, it is demonstrated that the magnitude of the two-dimensional ascent in slabular convective systems is closely related to the integrated cold-pool strength.
It is concluded that slabular organization tends to occur under conditions that favor the development of a strong, contiguous cold pool. The tendency to produce slabular convection is therefore enhanced by environmental conditions such as large CAPE, weak convective inhibition, strong along-line winds, and moderately strong cross-line wind shear.
Abstract
Bow echo development within quasi-linear convective systems is investigated using a storm-scale numerical model. A strong sensitivity to the ambient water vapor mixing ratio is demonstrated. Relatively dry conditions at low and midlevels favor intense cold-air production and strong cold pool development, leading to upshear-tilted, “slab-like” convection for various magnitudes of convective available potential energy (CAPE) and low-level shear. High relative humidity in the environment tends to reduce the rate of production of cold air, leading to weak cold pools and downshear-tilted convective systems, with primarily cell-scale three-dimensionality in the convective region. At intermediate moisture contents, long-lived, coherent bowing segments are generated within the convective line. In general, the scale of the coherent three-dimensional structures increases with increasing cold pool strength.
The bowing lines are characterized in their developing and mature stages by segments of the convective line measuring 15–40 km in length over which the cold pool is much stronger than at other locations along the line. The growth of bow echo structures within a linear convective system appears to depend critically on the local strengthening of the cold pool to the extent that the convection becomes locally upshear tilted. A positive feedback process is thereby initiated, allowing the intensification of the bow echo. If the environment favors an excessively strong cold pool, however, the entire line becomes uniformly upshear tilted relatively quickly, and the along-line heterogeneity of the bowing line is lost.
Abstract
Bow echo development within quasi-linear convective systems is investigated using a storm-scale numerical model. A strong sensitivity to the ambient water vapor mixing ratio is demonstrated. Relatively dry conditions at low and midlevels favor intense cold-air production and strong cold pool development, leading to upshear-tilted, “slab-like” convection for various magnitudes of convective available potential energy (CAPE) and low-level shear. High relative humidity in the environment tends to reduce the rate of production of cold air, leading to weak cold pools and downshear-tilted convective systems, with primarily cell-scale three-dimensionality in the convective region. At intermediate moisture contents, long-lived, coherent bowing segments are generated within the convective line. In general, the scale of the coherent three-dimensional structures increases with increasing cold pool strength.
The bowing lines are characterized in their developing and mature stages by segments of the convective line measuring 15–40 km in length over which the cold pool is much stronger than at other locations along the line. The growth of bow echo structures within a linear convective system appears to depend critically on the local strengthening of the cold pool to the extent that the convection becomes locally upshear tilted. A positive feedback process is thereby initiated, allowing the intensification of the bow echo. If the environment favors an excessively strong cold pool, however, the entire line becomes uniformly upshear tilted relatively quickly, and the along-line heterogeneity of the bowing line is lost.
Abstract
A technique for generating analytical initial conditions for three-dimensional numerical models is presented. The technique combines trigonometric and other mathematical functions with meteorological constraints to construct an idealized atmosphere which exhibits commonly observed “real” atmosphere structural characteristics. For example, pressure and thermal waves which slope with height, tropopause, low-level moist tongue, phase differences in pressure and thermal waves, and a jet maximum at the tropopause level are all generated by the simple system of equations.
Examples of both mesoscale and synoptic-scale initial conditions are given, and results of integrating the mesoscale initial conditions in a three-dimensional model are shown. The initialization procedure is economical and flexible, and potential applications include testing weather modification sensitivity, finite-difference schemes, lateral boundary formulations, and various subgrid-scale parameterizations.
Abstract
A technique for generating analytical initial conditions for three-dimensional numerical models is presented. The technique combines trigonometric and other mathematical functions with meteorological constraints to construct an idealized atmosphere which exhibits commonly observed “real” atmosphere structural characteristics. For example, pressure and thermal waves which slope with height, tropopause, low-level moist tongue, phase differences in pressure and thermal waves, and a jet maximum at the tropopause level are all generated by the simple system of equations.
Examples of both mesoscale and synoptic-scale initial conditions are given, and results of integrating the mesoscale initial conditions in a three-dimensional model are shown. The initialization procedure is economical and flexible, and potential applications include testing weather modification sensitivity, finite-difference schemes, lateral boundary formulations, and various subgrid-scale parameterizations.
Abstract
Between 25 and 27 June 1995, excessive rainfall and associated flash flooding across portions of western Virginia resulted in three fatalities and millions of dollars in damage. Although many convective storms occurred over this region during this period, two particular mesoscale convective systems that occurred on 27 June were primarily responsible for the severe event. The first system (the Piedmont storm) developed over Madison County, Virginia (eastern slopes of the Blue Ridge Mountains), and propagated slowly southward producing 100–300 mm of rain over a narrow swath of the Virginia foothills and Piedmont. The second system (the Madison storm) developed over the same area but remained quasi-stationary along the eastern slopes of the Blue Ridge for nearly 8 h producing more than 600 mm of rain.
Analysis of this event indicates that the synoptic conditions responsible for initiating and maintaining the Madison storm were very similar to the Big Thompson and Fort Collins floods along the Front Range of the Rocky Mountains, as well as the Rapid City flood along the east slopes of the Black Hills of South Dakota. In all four events, an approaching shortwave aloft coupled with high-level difluence/divergence signaled the presence of local ascent and convective destabilization. A postfrontal ribbon of relatively fast-moving high-θ e air, oriented nearly perpendicular to the mountain range, provided a copious moisture supply and helped focus the convection over a relatively small area. Weak middle- and upper-tropospheric steering currents favored slow-moving storms that further contributed to locally excessive rainfall.
A conceptual model for the Madison–Piedmont convective systems and their synoptic environment is presented, and the similarities and differences between the Madison County flood and the Big Thompson, Fort Collins, and Rapid City floods are highlighted.
Abstract
Between 25 and 27 June 1995, excessive rainfall and associated flash flooding across portions of western Virginia resulted in three fatalities and millions of dollars in damage. Although many convective storms occurred over this region during this period, two particular mesoscale convective systems that occurred on 27 June were primarily responsible for the severe event. The first system (the Piedmont storm) developed over Madison County, Virginia (eastern slopes of the Blue Ridge Mountains), and propagated slowly southward producing 100–300 mm of rain over a narrow swath of the Virginia foothills and Piedmont. The second system (the Madison storm) developed over the same area but remained quasi-stationary along the eastern slopes of the Blue Ridge for nearly 8 h producing more than 600 mm of rain.
Analysis of this event indicates that the synoptic conditions responsible for initiating and maintaining the Madison storm were very similar to the Big Thompson and Fort Collins floods along the Front Range of the Rocky Mountains, as well as the Rapid City flood along the east slopes of the Black Hills of South Dakota. In all four events, an approaching shortwave aloft coupled with high-level difluence/divergence signaled the presence of local ascent and convective destabilization. A postfrontal ribbon of relatively fast-moving high-θ e air, oriented nearly perpendicular to the mountain range, provided a copious moisture supply and helped focus the convection over a relatively small area. Weak middle- and upper-tropospheric steering currents favored slow-moving storms that further contributed to locally excessive rainfall.
A conceptual model for the Madison–Piedmont convective systems and their synoptic environment is presented, and the similarities and differences between the Madison County flood and the Big Thompson, Fort Collins, and Rapid City floods are highlighted.
Abstract
An analysis of a flash flood caused by a lake-enhanced rainband is presented. The flood took place near Erie, Pennsylvania, on 17 September 1996. It was found that the flood resulted from a complex interplay of several scales of forcing that converged over the Erie region. In particular, the flood occurred during a period when 1) a lake-enhanced convective rainband pivoted over the city of Erie with the pivot point remaining quasi-stationary for about 5 h; 2) a deep, surface-based no-shear layer, favorable for the development of strong lake-induced precipitation bands, passed over the eastern portion of Lake Erie; 3) the direction of flow in the no-shear layer shifted from shore parallel to onshore at an angle that maximized frictional convergence; 4) an upper-level short-wave trough contributed to low-level convergence, lifting, and regional destabilization; and 5) a strong land–lake diurnal temperature difference produced a lake-scale disturbance that locally enhanced the low-level convergence.
Analysis of the Weather Surveillance Radar-1988 Doppler radar data from Buffalo, New York, and Cleveland, Ohio, revealed that most of the radar-derived precipitation estimates for the region were overdone except for the region affected by the quasi-stationary rainband, which was underestimated. Reconstruction of the conditions in the vicinity of the band indicate that cloud bases were considerably lower and equivalent potential temperatures higher than for the areas of precipitation farther east over northwestern Pennsylvania and southwestern New York State. It is postulated that, due to the long distance from the radar sites to the Erie area, the radar was unable to observe large amounts of cloud condensate produced by warm-rain processes below 4 km. Estimates of precipitation rates from a simple cloud model support this interpretation.
Abstract
An analysis of a flash flood caused by a lake-enhanced rainband is presented. The flood took place near Erie, Pennsylvania, on 17 September 1996. It was found that the flood resulted from a complex interplay of several scales of forcing that converged over the Erie region. In particular, the flood occurred during a period when 1) a lake-enhanced convective rainband pivoted over the city of Erie with the pivot point remaining quasi-stationary for about 5 h; 2) a deep, surface-based no-shear layer, favorable for the development of strong lake-induced precipitation bands, passed over the eastern portion of Lake Erie; 3) the direction of flow in the no-shear layer shifted from shore parallel to onshore at an angle that maximized frictional convergence; 4) an upper-level short-wave trough contributed to low-level convergence, lifting, and regional destabilization; and 5) a strong land–lake diurnal temperature difference produced a lake-scale disturbance that locally enhanced the low-level convergence.
Analysis of the Weather Surveillance Radar-1988 Doppler radar data from Buffalo, New York, and Cleveland, Ohio, revealed that most of the radar-derived precipitation estimates for the region were overdone except for the region affected by the quasi-stationary rainband, which was underestimated. Reconstruction of the conditions in the vicinity of the band indicate that cloud bases were considerably lower and equivalent potential temperatures higher than for the areas of precipitation farther east over northwestern Pennsylvania and southwestern New York State. It is postulated that, due to the long distance from the radar sites to the Erie area, the radar was unable to observe large amounts of cloud condensate produced by warm-rain processes below 4 km. Estimates of precipitation rates from a simple cloud model support this interpretation.
