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- Author or Editor: J. M. Fritsch x
- Journal of Applied Meteorology and Climatology x
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Abstract
A fine-mesh 20-level, primitive equation model is used as a tool for preliminary study of the potential for modification of mesoscale convective systems. The governing system of the model is hydrostatic with the non-hydrostatic convective components parametrically introduced through a convective cloud model subroutine. Two modification possibilities are tested: 1) dynamic seeding, and 2) alteration of the timing and location of initial convection.
Results of artificially changing the time and location of initial convection indicate that the evolution, structure, dynamics and precipitation of mesoscale systems are sensitive to the location where the initial convection happens to develop. Changing the time and location of initial convection may also substantially alter the location and significance of subsequent severe weather as well as potentially beneficial rainfall.
For idealized dynamic seeding (i.e., freezing occurs at −10°C in all cloud updrafts), model results suggest that seeding enhances convective precipitation and strengthens the dynamics of the mesoscale system. Although mesoscale convergence is increased by the additional latent heat release, the most promising link to additional precipitation seems to be through the enhancement of new growth by strengthening or accelerating moist downdrafts and their associated mesohigh outflow.
Abstract
A fine-mesh 20-level, primitive equation model is used as a tool for preliminary study of the potential for modification of mesoscale convective systems. The governing system of the model is hydrostatic with the non-hydrostatic convective components parametrically introduced through a convective cloud model subroutine. Two modification possibilities are tested: 1) dynamic seeding, and 2) alteration of the timing and location of initial convection.
Results of artificially changing the time and location of initial convection indicate that the evolution, structure, dynamics and precipitation of mesoscale systems are sensitive to the location where the initial convection happens to develop. Changing the time and location of initial convection may also substantially alter the location and significance of subsequent severe weather as well as potentially beneficial rainfall.
For idealized dynamic seeding (i.e., freezing occurs at −10°C in all cloud updrafts), model results suggest that seeding enhances convective precipitation and strengthens the dynamics of the mesoscale system. Although mesoscale convergence is increased by the additional latent heat release, the most promising link to additional precipitation seems to be through the enhancement of new growth by strengthening or accelerating moist downdrafts and their associated mesohigh outflow.
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
The contribution of precipitation from mesoscale convective weather systems to the warm-season (April–September) rainfall in the United States is evaluated. Both Mesoscale Convective Complexes (MCC's) and other large, long-lived mesoscale convective systems that do not quite meet Maddox's criteria for being termed an MCC are included in the evaluation. The distribution and geographical limits of the precipitation from the convective weather systems are constructed for the warm seasons of 1982, a “normal” year, and 1983, a drought year. Precipitation characteristics of the systems are compared for the 2 years to determine how large-scale drought patterns affect their precipitation production.
The frequency, precipitation characteristics and hydrologic ramifications of multiple occurrences, or series, of convective weather systems are presented and discussed. The temporal and spatial characteristics of the accumulated precipitation from a series of convective complexes is investigated and compared to that of Hurricane Alicia.
It is found that mesoscale convective weather systems account for approximately 30% to 70% of the warm-season (April–September) precipitation over much of the region between the Rocky Mountains and the Mississippi River. During the June through August period, their contribution is even larger. Moreover, series of convective weather systems are very likely the most prolific precipitation producer in the United States, rivaling and even exceeding that of hurricanes.
Changes in the large-scale circulation patterns affected the seasonal precipitation from mesoscale convective weather systems by altering the precipitation characteristics of individual systems. In particular, for the drought period of 1983, the frequency of the convective systems remained nearly the same as in the “normal” year (1982); however, the average precipitation area and the average volumetric production significantly decreased. Nevertheless, the rainfall that was produced by mesoscale convective weather systems in the drought year accounted for most of the precipitation received during the critical crop growth period.
It is concluded that mesoscale convective weather systems may be a crucial precipitation-producing deterrent to drought and an important mechanism for enhancing midsummer crop growth throughout the midwestern United States. Furthermore, because mesoscale convective weather systems account for such a large fraction of the warm-season precipitation, significant improvements in prediction of such systems would likely translate into significant improvements in quantitative precipitation forecast skill and corresponding improvements in hydrologic forecasts of runoff.
Abstract
The contribution of precipitation from mesoscale convective weather systems to the warm-season (April–September) rainfall in the United States is evaluated. Both Mesoscale Convective Complexes (MCC's) and other large, long-lived mesoscale convective systems that do not quite meet Maddox's criteria for being termed an MCC are included in the evaluation. The distribution and geographical limits of the precipitation from the convective weather systems are constructed for the warm seasons of 1982, a “normal” year, and 1983, a drought year. Precipitation characteristics of the systems are compared for the 2 years to determine how large-scale drought patterns affect their precipitation production.
The frequency, precipitation characteristics and hydrologic ramifications of multiple occurrences, or series, of convective weather systems are presented and discussed. The temporal and spatial characteristics of the accumulated precipitation from a series of convective complexes is investigated and compared to that of Hurricane Alicia.
It is found that mesoscale convective weather systems account for approximately 30% to 70% of the warm-season (April–September) precipitation over much of the region between the Rocky Mountains and the Mississippi River. During the June through August period, their contribution is even larger. Moreover, series of convective weather systems are very likely the most prolific precipitation producer in the United States, rivaling and even exceeding that of hurricanes.
Changes in the large-scale circulation patterns affected the seasonal precipitation from mesoscale convective weather systems by altering the precipitation characteristics of individual systems. In particular, for the drought period of 1983, the frequency of the convective systems remained nearly the same as in the “normal” year (1982); however, the average precipitation area and the average volumetric production significantly decreased. Nevertheless, the rainfall that was produced by mesoscale convective weather systems in the drought year accounted for most of the precipitation received during the critical crop growth period.
It is concluded that mesoscale convective weather systems may be a crucial precipitation-producing deterrent to drought and an important mechanism for enhancing midsummer crop growth throughout the midwestern United States. Furthermore, because mesoscale convective weather systems account for such a large fraction of the warm-season precipitation, significant improvements in prediction of such systems would likely translate into significant improvements in quantitative precipitation forecast skill and corresponding improvements in hydrologic forecasts of runoff.
Abstract
Precipitation from 74 mesoscale convective complexes is examined to determine the total precipitation, areal extent, and characteristic precipitation pattern of an average convective complex. The relationship between the average precipitation pattern and the track of the centroid of the satellite-observed, cold-cloud shield is determined as an aid to forecasting. The amount and spatial distribution of precipitation during each stage (i.e., initiation, maturation and dissipation) of the average convective system's life cycle are presented, as well as the precipitation patterns for systems that form in particular synoptic environments. The precipitation characteristics of MCCs are compared to those from 32 other convective weather systems that are similar to MCCs but do not meet all the MCC-definition criteria.
Abstract
Precipitation from 74 mesoscale convective complexes is examined to determine the total precipitation, areal extent, and characteristic precipitation pattern of an average convective complex. The relationship between the average precipitation pattern and the track of the centroid of the satellite-observed, cold-cloud shield is determined as an aid to forecasting. The amount and spatial distribution of precipitation during each stage (i.e., initiation, maturation and dissipation) of the average convective system's life cycle are presented, as well as the precipitation patterns for systems that form in particular synoptic environments. The precipitation characteristics of MCCs are compared to those from 32 other convective weather systems that are similar to MCCs but do not meet all the MCC-definition criteria.
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.
