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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

Effective reasoning, analysis and communication regarding natural phenomena require the use of models to render tractable the complexities of nature. This paper attempts to put into perspective the proper roles of different types of models to maximize the effectiveness of their utilization. The advances in short term forecasting envisioned for the 1970's from full implementation of new knowledge, models and technology will materialize only if the managers and researchers join in an interagency effort to provide the operational meteorologists with the education, techniques, tools and, particularly, the challenging working environment needed to fully develop man's role in forecasting. A program to meet these requirements is outlined.

The types of models discussed include: descriptive or synoptic, dynamic or analytic, numerical or physical, statistical or optimized. The uses of models discussed include: education (basic concepts), research (experimental), operations (customized). Since the operational meteorologist is responsible for the intelligent use of these types of models, he must continually update his training and properly understand the potential contributions of the models.

It is anticipated that during the 1970's routine computer models will become more refined and specialized data such as trajectories and probabilities will become more common. Highly specialized products will be available from special purpose models on a special request basis as field forecasters gain access to remote terminals. Also, forecasters will have access to specialized consultants when unusual events or unusual forecast requirements arise. Background materials will be provided to the applied meteorologists so that he may gain physical understanding from educational and research models including systematic numerical experiments. Communication advances will provide for dynamic (motion picture) displays of radar, synchronous satellite, weather map and weather forecast data.

Only if the operational forecasters do receive the necessary management and scientific support, will their jobs be challenging and attractive to highly motivated and qualified students; only then will the customers of specialized short term forecasts receive the benefits made feasible by science and technology.

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Carl W. Kreitzberg

This paper outlines a mesoscale forecast system that could be implemented within a few years in spite of relatively sparse direct observations. Methods are discussed of using satellite information on large mesoscale features to initiate numerical models. The models develop further mesoscale structures through the influence of mesoscale geographic features and organized convective systems. The output of the numerical model serves as the physical foundation upon which the latest detailed satellite data can be interpreted.

Although many of the techniques described are not off-the-shelf items today, they are entirely feasible. It is important that the components of the forecast system be developed in parallel, rather than in series, if the system is to be completed within five years. The components include: polar-orbiting satellites for high latitudes; geosynchronous satellites for low latitudes; a mesoclimatological data base largely from satellite data; a mesoscale numerical prediction model with lateral boundary data supplied from a conventional large-scale model; and a variety of simple models and empirical schemes for treating special mesoscale phenomena.

A review of current activities in mesometeorology provides substantial evidence that the revolution in large-scale weather prediction in the past decade will be followed by a similar revolution on the mesoscale in the next five years.

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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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Robert A. Cohen and Carl W. Kreitzberg

The impact of atmospheric tides on surface pressure is studied by analyzing observations from the drifting-buoy network in the northwest Atlantic Ocean in winter 1988/89. These small, relatively inexpensive buoys, deployed for the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA), reported 10-min averages of air pressure, temperature, and sea surface temperature through satellites. Tidal oscillations are evident within the pressure variations and these tidal variations can be extracted and analyzed because they have a known constant period. The analyzed tides are compared with past observations and the similarities and differences are discussed. Many of the differences are attributed to the absence of local forcing in the homogeneous ocean environment, suggesting that the global-scale tide is being represented well. In this context, the observed variations agree well with what is expected, which demonstrates that the ability of the buoys to measure temporal changes is, on average, quite good. In addition, though the spatial gradients of tidal surface pressure variations at sea are negligible for most purposes, the magnitude of the temporal pressure variations are as large as 1 mb (3 h)−1, which is significant compared to the 3 mb (3 h)−1 indicative of rapid storm development. The different treatment of tide-producing mechanisms in different numerical prediction models may also complicate inter-comparison of pressure changes over a few hours in global models versus regional models.

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Ron Hadlock and Carl W. Kreitzberg

The Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) field study is designed to determine physical mechanisms and processes, and their critical spatial and temporal combinations, which can account for the wintertime phenomenon of explosively developing over-ocean atmospheric storms. Theoretical and numerical modeling research, during the five-year Office of Naval Research (ONR) Heavy Weather at Sea Accelerated Research Initiative ERICA program, comprises continuing effort, including the field study scheduled for 1 December 1988–28 February 1989. The ONR core field study is supplemented by the substantial participation of many other agencies and universities from the United States and Canada. Data will be obtained over the North Atlantic Ocean from Cape Hatteras to beyond Newfoundland, centered east of Cape Cod and south of Nova Scotia. The general timing and siting is chosen through consideration of historical storm occurrence data. Measurements in individual rapidly intensifying storms will be made from aircraft, buoys, and satellites, and by soundings and radars. Observations made during the pre-ERICA field test, January 1988, are discussed. This article describes the measurement objectives and the ways by which the field data will be collected.

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