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R. M. Brown, L. A. Cohen, and M. E. Smith

Abstract

Recent studies of particulate and gaseous materials in the atmosphere have raised important questions about diffusion at distances of 10–100 km. A photometric densitometer, initially developed for a quantitative study of oil-fog concentrations at ground level, has been adapted for use in an aircraft. Real-time measurements of ground-level and airborne particle concentrations are presented to distances of 120 km, and the implications of these data in terms of large-scale dispersion are discussed.

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

Abstract

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

Abstract

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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A. R. Brown, M. K. MacVean, and P. J. Mason

Abstract

Large eddy simulations sometimes use monotone advection schemes. Such schemes are dissipative, and the effective subgrid model then becomes the combined effect of the intended model and of the numerical dissipation. The impacts on simulation reliability are examined for the cases of dry convective and neutral planetary boundary layers. In general it is found that the results in the well-resolved flow interior are insensitive to the details of the advection scheme. However, unsatisfactory results may be obtained if numerical dissipation dominates where the flow becomes less well resolved as the surface is approached.

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Jean-François Gayet, Philip R. A. Brown, and Frank Albers

Abstract

During a preparatory experiment (PREICE) for the field campaign of the International Cirrus Experiment (ICE), six different Particle Measuring Systems (PMS) 2D-C probes belonging to five research organizations were intercompared. Three of these probes were original versions (2D-C), the three others being updated instruments (2D2-C version). The comparisons were performed using data obtained during flights in various types of warm and glaciated clouds.

The probe-by-probe comparisons show that relative particle-size response is in good agreement for all the probes and a variety of particle shapes. Similarly, measurements of the mean volume particle size agree to within about 10%. There are, however, noticeable discrepancies up to a factor of about 1.5 in values of the particle concentration. This can lead to similar large uncertainties in values of derived parameters, such as ice water content (IWC) and extinction coefficient. These differences are found to be related primarily to the probe version. the updated 2D2-C instruments appear to detect some 50% more images than the original version (2D-C).

Large differences may also be obtained when two different but common methods of calculation of the sample time are applied to data from a single probe. This appears to be related to timing errors within the probe data stream. There is a need for the standardization of processing schemes, where possible, in order to reduce the uncertainties in results obtained during multiaircraft cooperative experiments.

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N. E. Busch, R. M. Brown, and J. A. Frizzola

Abstract

The response characteristics obtained through wind tunnel tests of the Brookhaven National Laboratory bivane are presented and used in correcting the measured values of (ω′2/u*2), the variance of the vertical wind velocity fluctuations normalized by the square of the friction velocity. In neutral through unstable atmospheres the corrected values appear to be independent of thermal stability and height of observation, except for extreme instability at heights >46 m. The average value based on measurements at heights of 23, 46 and 92 m is 1.48 with a standard deviation of 0.38.

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Benjamin C. Trabing, Michael M. Bell, and Bonnie R. Brown

Abstract

Potential intensity theory predicts that the upper-tropospheric temperature acts as an important constraint on tropical cyclone (TC) intensity. The physical mechanisms through which the upper troposphere impacts TC intensity and structure have not been fully explored, however, due in part to limited observations and the complex interactions between clouds, radiation, and TC dynamics. In this study, idealized Weather Research and Forecasting Model ensembles initialized with a combination of three different tropopause temperatures and with no radiation, longwave radiation only, and full diurnal radiation are used to examine the physical mechanisms in the TC–upper-tropospheric temperature relationship on weather time scales. Simulated TC intensity and structure are strongly sensitive to colder tropopause temperatures using only longwave radiation, but are less sensitive using full radiation and no radiation. Colder tropopause temperatures result in deeper convection and increased ice mass aloft in all cases, but are more intense only when radiation was included. Deeper convection leads to increased local longwave cooling rates but reduced top-of-the-atmosphere outgoing longwave radiation, such that the total radiative heat sink is reduced from a Carnot engine perspective in stronger storms. We hypothesize that a balanced response in the secondary circulation described by the Eliassen equation arises from upper-troposphere radiative cooling anomalies that lead to stronger tangential winds. The results of this study further suggest that radiation and cloud–radiative feedbacks have important impacts on weather time scales.

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Benjamin M. Herman, Samuel R. Browning, and John A. Reagan

Abstract

It can be shown, theoretically, that the polarization properties of laser light scattered by a volume of air containing aerosols include considerable information as to the size distribution of the aerosols. A theoretical inversion model, utilizing the above information, is developed, which uses the Stokes parameters of the angularly scattered laser light as input data. These input data are generated theoretically from assumed size distribution functions of the aerosols. Both “perfect” measurements and measurements into which random errors are introduced are employed. These data are then used in the inversion model to generate predicted size distribution functions. Numerical experiments are performed with 0, 1 and 2% random error in the observations, in order to determine what accuracy is required in the lidar measurements. Comparisons between the actual and predicted functions are then made in order to assess the accuracy of the model.

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J. A. Dumas, G. M. Flato, and R. D. Brown

Abstract

Projections of future landfast ice thickness and duration were generated for nine sites in the Canadian Arctic and one site on the Labrador coast with a simple downscaling technique that used a one-dimensional sea ice model driven by observationally based forcing and superimposed projected future climate change from the Canadian Centre for Climate Modelling and Analysis global climate model (CGCM2). For the Canadian Arctic sites the downscaling approach indicated a decrease in maximum ice thickness of 30 and 50 cm and a reduction in ice cover duration of 1 and 2 months by 2041–60 and 2081–2100, respectively. In contrast, there is a slight increase in simulated landfast ice thickness and duration at Cartwright in the future due to its sensitivity to snow–ice formation with increased snowfall and to a projected slight cooling over this site (along the Labrador coast) by CGCM2. The magnitude of simulated changes in freeze-up and break-up date was largest for freeze up (e.g., 52 days later at Alert by 2081–2100), and freeze-up date changes exhibited much greater regional variability than break up, which was simulated to be 30 days earlier by 2081–2100 over the Canadian Arctic sites.

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J. A. Vermersch, R. C. Beardsley, and W. S. Brown

Abstract

The wintertime circulation in the western Gulf of Maine has been studied with a moored current, temperature and pressure array which was deployed from November 1974 to January 1975. These observations have been interpreted with three additional data sets: coastal sea level records, Portland Lightship meteorological data, and offshore hydrographic transect data which describe the evolution of the density field on weekly time scales. The observed mean currents are consistent with the idea of a cyclonic Gulf of Maine gyre. The subtidal current fluctuations were coherent in the vertical at each mooring but incoherent between the moorings which were separated by about 50 km in both the alongshore and offshore direction. Furthermore, the currents showed only weak coherence with the winds.

The pressure field was highly coherent over the whole Gulf of Maine. Therefore, estimates of the pressure gradient vector inside and outside the 100 m isobath were made using coastal subsurface and bottom pressure records. The alongshore pressure gradient for the deeper water was found to be quite coherent with the winds for periods between 35 and 200 h. The relation of the pressure gradients and the winds in the shallower water suggests the development of a transient coastal boundary layer.

The incoherence between the observed current and pressure gradient fields is due in part to the existence of geostrophic currents associated with a highly variable density field. The density field variability is caused by incomplete mixing of three water masses: advected Scotian shelf water, deeper more saline slope water, and local winter water which is formed in the region of the experiment.

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