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- Author or Editor: J. M. Fritsch x
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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 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 20-level, three-dimensional, primitive equation model with 20 km horizontal resolution is used to predict the development of convectively driven mesoscale pressure systems. Systems produced by the model have life histories and structural characteristics similar to observed convectively driven meso-systems. Cooling by (parameterized) convective-scale moist downdrafts is largely responsible for meso-high formation, while warming by compensating subsidence strongly correlates with mesocyclogenesis.
An hypothesis for mesocyclogenesis associated with deep convective complexes is presented. The hypothesis recognizes that certain configurations of convective activity may produce focused areas of forced subsidence warming aloft. The warming in turn causes a thickness increase aloft which creates a hydrostatic circulation favorable for evacuating mass from the subsidence column. Consequently, pressure falls beneath the layer of high-level warming. Model results supporting this hypothesis are presented.
Abstract
A 20-level, three-dimensional, primitive equation model with 20 km horizontal resolution is used to predict the development of convectively driven mesoscale pressure systems. Systems produced by the model have life histories and structural characteristics similar to observed convectively driven meso-systems. Cooling by (parameterized) convective-scale moist downdrafts is largely responsible for meso-high formation, while warming by compensating subsidence strongly correlates with mesocyclogenesis.
An hypothesis for mesocyclogenesis associated with deep convective complexes is presented. The hypothesis recognizes that certain configurations of convective activity may produce focused areas of forced subsidence warming aloft. The warming in turn causes a thickness increase aloft which creates a hydrostatic circulation favorable for evacuating mass from the subsidence column. Consequently, pressure falls beneath the layer of high-level warming. Model results supporting this hypothesis are presented.
A generalized set of conventions for facilitating analysis of mesoscale boundaries is proposed, and an example of the utility of the new conventions is presented.
A generalized set of conventions for facilitating analysis of mesoscale boundaries is proposed, and an example of the utility of the new conventions is presented.
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
No Abstract available.
Abstract
No Abstract available.
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
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
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.
