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  • Author or Editor: J. M. Fritsch x
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J. M. Fritsch
and
C. F. Chappell

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.

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J. M. Fritsch
and
C. F. Chappell

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.

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J. M. Fritsch
,
J. D. Murphy
, and
J. S. Kain

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.

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R. A. Maddox
,
D. J. Perkey
, and
J. M. Fritsch

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.

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J. M. Fritsch
,
J. Kapolka
, and
P. A. Hirschberg

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.

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