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Carl W. Kreitzberg

Abstract

Supersonic transport (SST) fuel consumption is very sensitive to ambient temperatures in the region of climb from 25,000 to 52,000 ft within 200 n mi and 20 min of takeoff. It has been suggested in the literature that extensive sounding networks may be required to provide adequate temperature forecasts for SST operations.

The thermal wind relation implies that the mean temperature in this deep layer will not change rapidly in space or time and that wind shears can be used as predictors of the changes. An empirical study using the Project Stormy Spring mesoscale rawinsonde network data confirms the thermal wind implication that these mesoscale temperature changes are small. Only 6-hr soundings at a single site near the airport are required for reasonably efficient SST operation. However, predictions based on wind shear explain only about 20% of the variance of the observed temperature changes.

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Carl W. Kreitzberg

Abstract

Data from a 10-site mesoscale rawinsonde network are used to document a number of features in the wind field in a mature cyclone of moderate intensity. The general nature of, and some particular phenomena in, the mesoscale wind field are discussed. Fields of divergence and vertical velocity are presented and the ageostrophic nature of the wind shear is demonstrated.

Specific features examined include mid-tropospheric oscillations in wind direction, a low-level jet stream, the coincidence of the tropopause with an ageostrophic level of maximum wind, and the jet-front shear zone.

In view of these results from AFCRL's project Stormy Spring, some of the capabilities and limitations of mesoscale rawinsonde data are discussed.

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Carl W. Kreitzberg and H. Albert Brown

Abstract

Precipitation systems within a mature extratropical cyclone are related to the mesoscale thermal and circulation fields aloft using data from Project Stormy Spring conducted by the Air Force Cambridge Research Laboratories. Precipitation systems were analyzed using radars and recording raingages, including a special mesoscale array; upper-air structures were deduced from a 10-site mesoscale rawinsonde network including serial soundings at 90-min intervals.

Results show that most of the widespread precipitation, in conjunction with the cyclonic-scale vertical motions in frontal baroclinic zones, occurs in bands and groups of showers. A sub-synoptic core of cold dry air in the middle troposphere ahead of the surface occlusion was found to be subsiding and surpressing wide-spread cloudiness, while it was furnishing a large amount of potential instability. The cyclone-scale ascending motions then released the potential instability around this cold core and also above the warm frontal stable layer. The convection became aligned in bands roughly parallel to the wind shear in the convective layer. These bands included clusters of cells of more intense precipitation.

The cyclonic-scale baroclinic zone associated with the synoptic fronts is made up of multiple mesoscale hyper-baroclinic zones which are shown to be related to the existence and production of potential instability and to precipitation bands and groups of cells.

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Carl W. Kreitzberg and Donald J. Perkey

Abstract

The release of potential instability by large-scale lifting and the subsequent interaction of cumulus convection and the hydrostatic mesoscale flow in a most complex scale-interaction process. This process is an essential part of tropical weather but it is also important in extratropical cyclones through the formation of mesoscale rainbands that contribute much of the precipitation. The purpose of this paper is to qualitatively and quantitatively clarify the potential instability release process within a framework that will permit calculation of convective/mesoscale interactions.

The approach is to use an extension of the Lagrangian form of the one-dimensional cumulus model to provide values of convective-scale changes to a hydrostatic primitive equation model. This cumulus sub-routine locates the base of the convection, computes the cumulus plume that will build, accounts for the environmental subsidence, and mixes the subsided environment with the cumulus plume after rainout. These plumes build sequentially when the subroutine is called every 20 min at each column in the hydrostatic model.

The convection model is explained in some detail along with its behavior within the hydrostatic model. The use of this scheme for convective adjustment is contrasted with other schemes; it is emphasized that this scheme is more generally applicable and includes the temporal evolution of mesoscale convective disturbances through consumption of pre-existing potential instability as well as the resupply of warm moist air (fuel).

Examples of convective/mesoscale interaction will he presented in Part II along with examples of the sensitivity of the results to variations in initial conditions and numerical coefficients.

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Carl W. Kreitzberg and Donald J. Perkey

Abstract

In Part I the convective processes important during the release of potential instability were described qualitatively and evaluated quantitatively in a parameterized cumulus model within a primitive equation model. Part II includes a more detailed examination of convective/mesoscale interactions through a basic simulation experiment and tests under different physical conditions and with different computational grids. The cumulus model was documented in Part I and the primitive equation model is documented herein. The example, for which detailed dynamical fields are shown, began with 6 h of convective activity that developed a saturated neutrally buoyant mesoscale updraft which produced the bulk of the precipitation by 12 h into the integration.

The potential instability process is readily understandable and verifiable in general terms by numerical simulation. Increasing moisture bandwidth or large-scale ascent results in a wider precipitation band. Permitting evaporation of convective precipitation above cloud base had surprising little effect on these rain-bands that are about 100 km wide. Decreasing cumulus updraft radius, thereby increasing entrainment effects, delays initial development of the mesoscale circulation and produces a much narrower and more intense circulation later on. Reducing the horizontal grid size from 20 to 10 km results in much narrower rainbands but does little to the area total precipitation. The rate of propagation inward of lateral boundary condition influences shows that rather large areas must be dealt with in mesoscale field projects and numerical weather prediction for phenomena with time scales of several hours.

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Robert A. Cohen and Carl W. Kreitzberg

Abstract

Distinct airstreams, separated by sharp boundaries, are present in numerical weather simulations and can be used to identify characteristic structures in baroclinic storms. To allow objective comparisons between different analyses, a rigorous treatment of airstream boundaries is performed based upon a numerical procedure in which the uncertainty of individual trajectory paths is related to the strength of an airstream boundary, as defined and quantified herein. The properties of the procedure are then investigated via application to a numerical simulation of the ERICA IOP 4 storm. The work not only provides the necessary framework within which future analyses of airstreams can be interpreted but also provides insights into quantitative properties of atmospheric flows.

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Donald J. Perkey and Carl W. Kreitzberg

Abstract

Before high-resolution numerical models can be of use operationally, they must be restricted to a limited domain, thus necessitating lateral boundary conditions which allow the changes outside the limited domain to influence the results while not contaminating the forecast with spurious boundary-reflected energy. Such a set of time-dependent lateral boundary conditions are presented in this paper. This boundary condition set is investigated using the linear analytic and finite-difference advection equations, the non-linear finite-difference shallow-water equations, and the hydrostatic primitive equations.

The results illustrate how the boundary condition transforms long- and medium-length interior advective and gravity waves into short waves which can then be removed by a low pass filter, thereby giving the appearance that the exiting wave simply passed through the boundary. The results also indicate that large-scale advective and gravity waves enter the forecast domain with little degradation. Thus, from the tests performed, the described boundary condition scheme yields a practical solution for prescribing time-dependent lateral boundaries for a limited-area model.

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Paul J. Neiman, M. A. Shapiro, Evelyn G. Donall, and Carl W. Kreitzberg

Abstract

On 25–27 January 1988, the National Oceanic and Atmospheric Administration's Wave Propagation Laboratory, Drexel University, and the Office of Naval Research carried out a combined pre-ERICA research aircraft investigation of a major marine cyclone moving northeastward over the Canadian Maritime Provinces. Flight-level and dropwindsonde observations documented the diabatic modification of the cyclone's warm sector marine boundary layer (MBL) as it moved out over cold underlying water. These observations and results from the Blackadar one-dimensional boundary layer model both show that heat fluxes were directed downward from the warm sector MBL into the cold ocean. Vertical gradients of these downward heat fluxes diabatically cooled the lower portion of the warm sector MBL and generated large static stability within the entire layer. The increase in stable stratification allowed large vertical wind shear to exist within this layer and strong wind speeds to exist at its top. The increase in static stability within the warm sector MBL acted to concentrate isentropic potential vorticity in this layer, but these changes also weakened the horizontal gradients of temperature, moisture, and wind velocity within the adjacent warm- and cold-frontal zones at the surface.

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