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J. A. Brown Jr.

Abstract

The hydrodynamic instability characteristics of planetary zonal flows are investigated through use of a quasi-geostrophic numerical model of high spatial resolution. An initial-value technique is employed to obtain solutions of the linear problem.

Certain zonal flows containing both vertical and lateral shears, which are representative of those observed in the earth's atmosphere, are found to be unstable with respect to the large-scale quasi-geostrophic disturbances. Westerly currents, each characterized by a latitudinally symmetric jet containing absolute vorticity extrema at various latitudes, amplify perturbations of some scales through a dominating baroclinic mechanism, and amplify perturbations of other scales through a dominating barotropic mechanism. For these flows, the unstable perturbations of relatively short zonal wavelength convert zonal available potential energy into perturbation energy and simultaneously strengthen the zonal kinetic energy of the basic flow. On the other hand, the unstable perturbations of relatively long zonal wavelength reduce both the zonal kinetic and available potential energies of the basic flow, with the former reduction dominating. For certain flows, these combinations produce two distinct wavelengths of maximum instability. Flows which are similar but contain no vanishing meridional gradient of absolute vorticity are found to produce baroclinically unstable perturbations with a tendency toward barotropic damping.

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J. A. Brown Jr.

Abstract

The numerical model of Part I is used in certain experiments designed to reveal special instability effects caused by the vertical walls which specify the lateral boundary conditions at the northern and southern boundaries of the atmosphere. In these examples the walls suppress instability of the barotropically dominated perturbations and have little influence on the westward progressions of the unstable waves.

Small-scale eddy momentum and heat diffusion processes are simulated in an example of a basic westerly wind field containing absolute vorticity extrema. The inclusion of these mechanisms is found to inhibit instabilities of all zonal wavelengths, with major effects noted for short shallow waves. The significant modifying influence is attributed to large effects of drag at the ground.

The behavior of an unstable wave interacting with the zonal current is obtained through nonlinear numerical calculations. The equilibrium state approached in the presence of a time-independent diabatic heating differential oscillates about a steady state. The several energy conversion rates vary in time in such a way as to minimize the time-rates-of-change of the different types of energies. The equilibrium energy levels appear to be governed by the required baroclinic process, and the resulting period of the oscillating regime is dictated by the barotropic mechanism.

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J. A. Brown Jr. and E. J. Pybus

Abstract

Data from eight water vapor soundings made with the dew-point hygrometer instrument at McMurdo Sound, Antarctica, are presented. These data are compared with various north temperate latitude data. The soundings are too sparse to present a valid time cross section, but structure (layering) is evident on individual soundings. The mean Antarctic summer mixing ratio profile shows decreasing moisture to 4×10−6 gm/gm at the tropopause, then increasing to 1×10−4 gm/gm at 30 km. The Antarctic stratosphere appears to be as moist as the mid-latitude stratosphere, but both sets of data are influenced by an unknown amount of sample contamination.

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John A. Brown Jr. and Emmett J. Pybus

Abstract

No abstract available.

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David Atlas, Keith A. Browning, Ralph J. Donaldson Jr., and Hugh J. Sweeney

Abstract

The advantages of a new radar digital processing device (called STRADAP) are demonstrated by a case study of the tornado storm which struck Charlton, Mass., on 12 October 1962. STRADAP furnishes patterns of echo height and maximum radar reflectivity in digital form, with a linear resolution of 5 sq n mi and with only a small time delay.

During the development of the Charlton storm, before. the appearance of the tornado, the STRADAP print-outs showed a substantial increase both in echo reflectivity and in the area of the storm echo exceeding 40,000 ft. Other severe weather phenomena, such as strong winds, heavy rain and lightning were also correlated fairly closely with the STRADAP patterns.

This study demonstrates the capability of STRADAP to depict, virtually in real time, the temporal and spatial development of storms, thereby indicating its potential as a valuable observing and short range forecasting tool. In addition, the digital patterns may he transmitted within 15 sec, permitting an integrated display of storm activity for the whole of the United States to be updated every 30 min.

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Leslie R. Lemon, Ralph J. Donaldson Jr., Donald W. Burgess, and Rodger A. Brown

Significant advances in single-Doppler radar application to severe storm study and identification have been made since 1965. Mesocyclones have been detected by Doppler radar and found to precede severe weather occurrence by several tens of minutes. A typical mesocyclone evolution leading to tornado development has also been documented. The tornado vortex itself has a revealing signature in Doppler radar data, the tornadic vortex signature (TVS). Statistics of both mesocyclone and TVS association with confirmed severe weather are presented in this paper. Doppler radar provides the potential for improving severe thunderstorm warnings. Experiments are underway to test the operational use of this new tool in storm warning and flight advisory services.

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Hung-Neng S. Chin, Martin J. Leach, Gayle A. Sugiyama, John M. Leone Jr., Hoyt Walker, J. S. Nasstrom, and Michael J. Brown

Abstract

A modified urban canopy parameterization (UCP) is developed and evaluated in a three-dimensional mesoscale model to assess the urban impact on surface and lower-atmospheric properties. This parameterization accounts for the effects of building drag, turbulent production, radiation balance, anthropogenic heating, and building rooftop heating/cooling. U.S. Geological Survey (USGS) land-use data are also utilized to derive urban infrastructure and urban surface properties needed for driving the UCP. An intensive observational period with clear sky, strong ambient wind, and drainage flow, and the absence of a land–lake breeze over the Salt Lake Valley, occurring on 25–26 October 2000, is selected for this study.

A series of sensitivity experiments are performed to gain understanding of the urban impact in the mesoscale model. Results indicate that within the selected urban environment, urban surface characteristics and anthropogenic heating play little role in the formation of the modeled nocturnal urban boundary layer. The rooftop effect appears to be the main contributor to this urban boundary layer. Sensitivity experiments also show that for this weak urban heat island case, the model horizontal grid resolution is important in simulating the elevated inversion layer.

The root-mean-square errors of the predicted wind and temperature with respect to surface station measurements exhibit substantially larger discrepancies at the urban locations than their rural counterparts. However, the close agreement of modeled tracer concentration with observations fairly justifies the modeled urban impact on the wind-direction shift and wind-drag effects.

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Abayomi A. Abatan, William J. Gutowski Jr., Caspar M. Ammann, Laurna Kaatz, Barbara G. Brown, Lawrence Buja, Randy Bullock, Tressa Fowler, Eric Gilleland, and John Halley Gotway

Abstract

This study analyzes spatial and temporal characteristics of multiyear droughts and pluvials over the southwestern United States with a focus on the upper Colorado River basin. The study uses two multiscalar moisture indices: standardized precipitation evapotranspiration index (SPEI) and standardized precipitation index (SPI) on a 36-month scale (SPEI36 and SPI36, respectively). The indices are calculated from monthly average precipitation and maximum and minimum temperatures from the Parameter-Elevation Regressions on Independent Slopes Model dataset for the period 1950–2012. The study examines the relationship between individual climate variables as well as large-scale atmospheric circulation features found in reanalysis output during drought and pluvial periods. The results indicate that SPEI36 and SPI36 show similar temporal and spatial patterns, but that the inclusion of temperatures in SPEI36 leads to more extreme magnitudes in SPEI36 than in SPI36. Analysis of large-scale atmospheric fields indicates an interplay between different fields that yields extremes over the study region. Widespread drought (pluvial) events are associated with enhanced positive (negative) 500-hPa geopotential height anomaly linked to subsidence (ascent) and negative (positive) moisture convergence and precipitable water anomalies. Considering the broader context of the conditions responsible for the occurrence of prolonged hydrologic anomalies provides water resource managers and other decision-makers with valuable understanding of these events. This perspective also offers evaluation opportunities for climate models.

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Bruce A. Boe, Jeffrey L. Stith, Paul L. Smith, John H. Hirsch, John H. Helsdon Jr., Andrew G. Detwiler, Harold D. Orville, Brooks E. Mariner, Roger F. Reinking, Rebecca J. Meitín, and Rodger A. Brown

The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification.

Of primary interest were tracer experiments that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide.

Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunderstorms has resulted.

The objectives of the various project experiments and the specific facilities employed are described. Project highlights and some preliminary results are also presented.

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L. Magnusson, J.-R. Bidlot, M. Bonavita, A. R. Brown, P. A. Browne, G. De Chiara, M. Dahoui, S. T. K. Lang, T. McNally, K. S. Mogensen, F. Pappenberger, F. Prates, F. Rabier, D. S. Richardson, F. Vitart, and S. Malardel

Abstract

Tropical cyclones are some of the most devastating natural hazards and the “three beasts”—Harvey, Irma, and Maria—during the Atlantic hurricane season 2017 are recent examples. The European Centre for Medium-Range Weather Forecasts (ECMWF) is working on fulfilling its 2016–25 strategy in which early warnings for extreme events will be made possible by a high-resolution Earth system ensemble forecasting system. Several verification reports acknowledge deterministic and probabilistic tropical cyclone tracks from ECMWF as world leading. However, producing reliable intensity forecasts is still a difficult task for the ECMWF global forecasting model, especially regarding maximum wind speed. This article will put the ECMWF strategy into a tropical cyclone perspective and highlight some key research activities, using Harvey, Irma, and Maria as examples. We describe the observation usage around tropical cyclones in data assimilation and give examples of their impact. From a model perspective, we show the impact of running at 5-km resolution and also the impact of applying ocean coupling. Finally, we discuss the future challenges to tackle the errors in intensity forecasts for tropical cyclones.

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