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F. Caracena and J. M. Fritsch

Abstract

During the early morning of 2 August 1978, a stationary thunderstorm complex drenched the Balcones Escarpment of Texas and unleashed flash floods in the Hill Country which killed 27 people and produced extensive damage. After the storm, an unofficial 24 h total of at 1east 79 cm of rain was observed near the town of Bluff (about 90 km northwest of San Antonio). Five distinct mechanisms or meteorological features interacted to focus and anchor the storm complex: 1) a east-west oriented mesohigh outflow boundary was situated just south of the storm area; 2) a rapidly moving ribbon of extremely moist boundary-layer air was flowing toward the storm are from the southeast 3) an elevated warm, dry layer of air extended over the area east of the Hill Country and capped the southeasterly low level inflow; 4) a deep vertical motion field associated with a midtropospheric short-wave trough was advancing toward the Hill Country from Mexico; and 5) the diurnal heating cycle with its associated production of thermally forced convective clouds was ending. The juxtaposition of the first tour mechanisms over the storm are in combination with terrain lifting and the termination of convection initiated by boundary layer heating resulted in a forced, stationary storm complex that produced catastrophic flash floods in the Hill Country.

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David Miller and J. M. Fritsch

Abstract

A climatological study of mesoscale convective complexes (MCCs) during 1983–1985 over the western Pacific region (WPR), using full-disc, enhanced infrared satellite imagery from the Japanese Geostationary Meteorological Satellite is presented.

The results indicate that MCCs are common in the WPR and display many of the same characteristics as those found in the Americas. The systems are nocturnal and tend to form over or in the immediate vicinity of land. Cold-cloud shields in the Americas last for about 10 h while WPR shields last about 11 h. The cold-cloud-shield size distribution is similar to that of the Americas, with most systems exhibiting areas between 2 × 105 and 3 × 105 km2. Seasonal distributions of WPR systems are also similar to that in the Americas. Specifically, the frequency of midlatitude systems peaks in late spring and early summer while low-latitude MCCs are distributed uniformly throughout the warm season.

As with western systems, WPR MCCs occur in preferred zones. Climatologically, low-level jets of high-θe, air and upper-level diffluence are present in these zones. Tracks of WPR MCCs show that, like American systems, they typically move to the right (left in the Southern Hemisphere) of the climatological mean 700–500-mb flow. The deviation from the mean flow is in the direction of the source region of higher-θe air. A few MCCs that moved over water formed tropical storms. Likewise, a few tropical systems moved over land and formed MCCs.

It is concluded that the strong similarity of the properties and environment of WPR MCCs to that in the Americas indicates that they are essentially the same phenomenon. Their high frequency in the Americas and the WPR makes them potentially important contributors to the global hydrologic cycle.

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J. M. Fritsch and J. M. Brown

Abstract

In an attempt to determine the relative contribution of the direct incorporation of cold air (detrainment from overshooting convective cloud tops) to the production of mesohighs in the vicinity of the tropopause, two numerical simulations were performed using a 20 km horizontal resolution, 20-level primitive equation model. One simulation included direct cooling and the other did not. The results showed that including the cooling increased the high-level pressure and wind perturbations by approximately 30 and 40%; respectively. The simulation results also showed that in spite of the omission of the direct cloud cooling, a high-level cold pool was still generated. The cooling was accomplished by adiabatic expansion in response to the lifting by the convectively driven mesoscale vertical circulation. Thus, it appears that the mesoscale adiabatic expansion is the dominant effect in elevated-mesohigh production and the detrainment of overshooting air is an important modifying factor.

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Todd J. Miner and J. M. Fritsch

Abstract

Seven years of autumnal (September–November) precipitation data are examined to determine the characteristics of lake-effect precipitation downwind of Lake Erie. Atmospheric conditions for each lake-effect event are compiled and the mean atmospheric environment for rain events is constructed and compared to conditions for lake-effect snow events.

It is found that lake-effect precipitation occurs approximately one out of every five days with a diurnal peak in precipitation intensity during the afternoon and evening. The greatest number of lake-effect days occurs in October followed by November and then September. Comparison of these results to regional precipitation climatologies strongly suggests that the season of lake enhanced precipitation begins in late summer. Precipitation is predominantly rain throughout September and October and snow after the first week of November. A transition period of both rain and snow occurs in early November. Analysis of thunder events for the 7-yr period show a late September to mid-October peak with a decline in frequency by November. The decline in thunder events is due to a seasonal decrease in the depth of the conditionally unstable layer.

As might be expected, the mean atmospheric conditions during rain events are similar to those found during lake-effect snow events. This is particularly true with regard to the overall positions of transient synoptic features. Differences are most apparent in the thermodynamic profile of the lower troposphere. Extreme low-level instabilities typically observed in lake-effect snow events are absent from lake-effect rain events. However, in contrast to most snow events, a much deeper layer of conditionally unstable air is usually present during rain events.

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Robert F. Rogers and J. M. Fritsch

Abstract

A general framework for the trigger function used in convective parameterization routines in mesoscale models is proposed. The framework is based on the diagnosis of the accessibility of potential buoyant energy. Specifically, the trigger function 1) estimates the magnitude of the largest vertical velocity perturbation from a source layer and 2) calculates the total amount of inhibition between the source layer and the level of free convection. The calculation of perturbation magnitude accounts for such factors as subgrid-scale inhomogeneities, a convective boundary layer, and convergence within the source layer. Specific formulations to quantify these factors are proposed.

The trigger is tested in a simulation using the PSU–NCAR mesoscale model MM5. The event chosen for simulation is a summertime case exhibiting a variety of environments. The results of this simulation are compared with a simulation using the Fritsch–Chappell (FC) trigger function. It is found that decisions made by the new trigger function are more physically consistent with the local environment than decisions made by the FC trigger.

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J. M. Fritsch and R. A. Maddox

Abstract

High-resolution visible imagery for the eastern GOES satellite is used to document a convectively driven mesoscale weather system which propagates against the mean atmospheric flow and produces an apparent spiraling anvil outflow.

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J. M. Fritsch and R. A. Maddox

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.

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Stephen M. Leyton and J. Michael Fritsch

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.

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J. M. Fritsch and C. F. Chappell

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.

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J. M. Fritsch and R. A. Maddox

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.

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