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H. E. BROWN and R. A. BRINTZENHOFE

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A. P. Lock, A. R. Brown, M. R. Bush, G. M. Martin, and R. N. B. Smith
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G. M. Martin, M. R. Bush, A. R. Brown, A. P. Lock, and R. N. B. Smith

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A new turbulent mixing scheme, described in Part I of this paper, is tested in the climate and mesoscale configurations of the U.K. Met. Office’s Unified Model (UM). In climate configuration, the scheme is implemented along with increased vertical resolution below 700 hPa (the same as that in the mesoscale model), in order to allow the different boundary layer types and processes to be identified and treated properly. In both configurations, the new boundary layer (PBL-N) mixing scheme produces some improvement over the current boundary layer (PBL-C) scheme. The PBL-N scheme is able to diagnose different boundary layer types that appear to be consistent with the observed conditions, and the boundary layer structure is improved in comparison with observations. In the climate model, the boundary layer and cloud structure in the semipermanent stratocumulus regions of the eastern subtropical oceans are noticeably improved with the PBL-N scheme. The deepening and decoupling of the boundary layer toward the trade cumulus regime is also simulated more realistically. However, the cloud amounts in the stratocumulus regions, which were underestimated with the PBL-C scheme, are reduced further when the PBL-N scheme is included. Tests of the PBL-N scheme in the UM single-column model and in a development version of the UM, where the dynamics, time stepping, and vertical grid are different from the standard version, both show that realistic stratocumulus cloud amounts can be achieved. Thus, it is thought that the performance of the PBL-N scheme in the standard UM may be being limited by other aspects of that model. In the mesoscale model, improvements in the simulation of a convective case are achieved with the PBL-N scheme through reductions in layer cloud amount, while the simulation of a stratocumulus case is improved through better representation of the cloud and boundary layer structure. Other mesoscale model case studies show that there is a consistent improvement in fog probabilities and forecasts of cloud-base height. The root-mean-square errors in screen-level temperature are also reduced slightly. The weak daytime bias in wind strength is improved greatly through a systematic increase in the 10-m wind speed in unstable conditions. As a result of these trials, the scheme has been implemented operationally in the mesoscale model.

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A. P. Lock, A. R. Brown, M. R. Bush, G. M. Martin, and R. N. B. Smith

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A new boundary layer turbulent mixing scheme has been developed for use in the UKMO weather forecasting and climate prediction models. This includes a representation of nonlocal mixing (driven by both surface fluxes and cloud-top processes) in unstable layers, either coupled to or decoupled from the surface, and an explicit entrainment parameterization. The scheme is formulated in moist conserved variables so that it can treat both dry and cloudy layers. Details of the scheme and examples of its performance in single-column model tests are presented.

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CAPT. JOHN A. BROWN JR. and JAMES R. NEILON

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With the aid of an electronic computer, case studies of wind analyses at the 850-mb., 700-mb., 500-mb., 400-mb., 300-mb., and 200-mb. pressure levels have been made. The divergent and non-divergent wind components resulting from the u and v wind-component analyses are investigated. For the cases considered, the streamfunction fields are slightly superior to the Joint Numerical Weather Prediction operational fields, obtained initially through use of the “balance equation.” The magnitude of the horizontal wind divergence values are comparable to those obtained from the winds by previous investigators employing hand-analysis techniques. However, the divergence patterns are not sufficiently accurate for the strict requirements necessary for numerical weather forecasting.

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Leslie R. Lemon, Donald W. Burgess, and Rodger A. Brown

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Single-Doppler Velocity data reveal that a dominant feature in the Union City, Okla., tornadic thunderstorm is a core mesocyclonic circulation, 2–6 km in diameter, extending to at least 9 km above ground. There is an apparent flow through the precipitation echo at low levels and divergence at high levels. Considerable similarity appears between mid-level flow structure around the mesocyclone core and that observed around a solid rotating cyclinder embedded in classical potential flow. As tornado time approaches, core circulation tangential velocities increase while diameter decreases. Simultaneously, the collapse of storm top and extensive echo overhang suggest updraft weakening.

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Rodger A. Brown, Leslie R. Lemon, and Donald W. Burgess

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Doppler radar measurements in the Union City, Okla., tornadic storm of 24 May 1973 led to discovery of a unique tornadic vortex signature (TVS) in the field of mean Doppler velocity data. The distinct character of this signature and its association with the tornado are verified using a model that simulates Doppler velocity measurements through a tornado. Temporal and spatial variations of the TVS reveal previously unknown tornado characteristics. The TVS originates at storm mid-levels within a parent mesocyclone, descends to the ground with the tornado (extending vertically at least 10 km), and finally dissipates at all heights when the tornado dissipates. NSSL Doppler radar data from 1973 through 1976 reveal 10 signatures; eight were associated with tornadoes or funnel clouds, while no reports are available for the other two. Since the TVS first appears aloft tens of minutes before tornado touchdown, the signature has decided potential for real-time warning.

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K. A. Browning, C. G. Collier, P. R. Larke, P. Menmuir, G. A. Monk, and R. G. Owens

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This paper is concerned with the quantitative forecasting of hourly rainfall for the period 0–6 h ahead using linear extrapolation techniques. It deals with results obtained as part of the Meteorological Office Short Period Forecasting Pilot Project. The primary data used in this study are composite maps of rainfall echo distribution generated automatically and in real time using digital data received from a network of four weather radars covering parts of England and Wales. Forecasts have been derived during a total of 29 frontal rainfall events between November 1979 and June 1980. The forecasts wore derived both subjectively in real time and objectively using a computerized echo centroid tracking technique. The objective procedure, which was used to derive forecasts on a grid of 32 × 32, 20 km squares, is a practical way of quickly producing detailed forecasts for a large, number of target areas but its accuracy suffers from a number of factors. The subjective procedure, which was applied to a single target zone, was used to investigate some of the sources of error and their impact on forecasts. It is shown that radar rainfall measurement errors accounted for as much as half of the errors in the forecasts, and it is suggested that the biggest improvements in forecast accuracy are likely to accrue from improved analysis of the radar data prior to input into the forecast procedure. The radar measurement errors are due more to the variability of echo intensity with height than to straightforward radar calibration difficulties. Subtle procedures are required to identify these errors based on an analysis of the meteorological situation in which the radar data are viewed in the context of other kinds of meteorological information. Factors such as the development and decay of rainfall systems, which lead to the breakdown of the basic assumption underlying the linear extrapolation approach, accounted for about a quarter of the errors in the forecasts.

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K.A. Browning, J.C. Frankhauser, J.-P. Chalon, P.J. Eccles, R.G. Strauch, F.H. Merrem, D.J. Musil, E.L. May, and W.R. Sand

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A model of an evolving hailstorm is synthesized from data presented in four related papers in this issue. The storm model, which is applicable to a class of ordinary multicell hailstorms and similar to earlier models derived by workers in South Dakota and Alberta, is discussed in terms of the growth of hail and its implications for hail suppression. Hail is grown in time–evolving updrafts that begin as discrete new clouds on the flank of the storm. Low concentrations of embryos develop rapidly within each of these clouds. The embryos subsequently grow into small hailstones while suspended near or above, the −20°C level as each new cloud grows and becomes the main updraft. Recycling is not a feature of this model as it is in supercell models. To improve the chance of silver iodide seeding being effective in suppressing the growth of hall in multicell storms, it is proposed that the seeding should be carried out not in the main updraft as is often the practice, but, rather, in the regions of weaker updraft associated with the early stages of developing clouds an the flank of the storm.

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John L. Beven II, Lixion A. Avila, Eric S. Blake, Daniel P. Brown, James L. Franklin, Richard D. Knabb, Richard J. Pasch, Jamie R. Rhome, and Stacy R. Stewart

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The 2005 Atlantic hurricane season was the most active of record. Twenty-eight storms occurred, including 27 tropical storms and one subtropical storm. Fifteen of the storms became hurricanes, and seven of these became major hurricanes. Additionally, there were two tropical depressions and one subtropical depression. Numerous records for single-season activity were set, including most storms, most hurricanes, and highest accumulated cyclone energy index. Five hurricanes and two tropical storms made landfall in the United States, including four major hurricanes. Eight other cyclones made landfall elsewhere in the basin, and five systems that did not make landfall nonetheless impacted land areas. The 2005 storms directly caused nearly 1700 deaths. This includes approximately 1500 in the United States from Hurricane Katrina—the deadliest U.S. hurricane since 1928. The storms also caused well over $100 billion in damages in the United States alone, making 2005 the costliest hurricane season of record.

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