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J. M. Fritsch and J. M. Brown

Abstract

In an attempt to determine the relative contribution of the direct incorporation of cold air (detrainment from overshooting convective cloud tops) to the production of mesohighs in the vicinity of the tropopause, two numerical simulations were performed using a 20 km horizontal resolution, 20-level primitive equation model. One simulation included direct cooling and the other did not. The results showed that including the cooling increased the high-level pressure and wind perturbations by approximately 30 and 40%; respectively. The simulation results also showed that in spite of the omission of the direct cloud cooling, a high-level cold pool was still generated. The cooling was accomplished by adiabatic expansion in response to the lifting by the convectively driven mesoscale vertical circulation. Thus, it appears that the mesoscale adiabatic expansion is the dominant effect in elevated-mesohigh production and the detrainment of overshooting air is an important modifying factor.

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E. S. Takle and J. M. Brown

Abstract

A hybrid density function is given for describing wind-speed distributions having nonzero probability of “calm.” A Weibull probability graph paper designed specifically for plotting wind-speed distributions is used to determine distribution parameters to within a few percent of values obtained by the maximum likelihood technique. Data from the National Weather Service are used to demonstrate the use of the hybrid density function and the Weibull graph paper.

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M. Segal, G. Kallos, J. Brown, and M. Mandel

Abstract

The temporal variation of specific humidity during morning hours was evaluated by analytic and numerical model scaling as well as by observational means. The scaling quantified (i) the gradual increase in the shelter increase humidity as the surface temperatures inversion is eroded during the morning hours; (ii) the sharp decrease in the shelter specific humidity when the newly developed boundary layer merges with the previous day's elevated neutral layer. The relation of these patterns to the early-morning thermal stratification and the Bowen ratio was estimated. Observational data supported the general features suggested by the scaling evaluations. The applied significance of the presented specific-humidity patterns is outlined.

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B. M. Herman, R. S. Browning, and J. J. De Luisi

Abstract

A unique and promising method is outlined for determining the complex part of the index of refraction of atmospheric dust by utilizing measurements of diffuse and direct solar radiation. The solution requires comparison of measurements of the ratio of diffuse to direct solar radiation with theoretical calculations of the same values. A knowledge of the ground albedo is necessary for the solution. The ground albedo may, in fact, also be determined from the measured ratios, but the solution in this case tends to he more sensitive to measurement than when the ground albedo is known.

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L. J. Battan, S. R. Browning, and B. M. Herman

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Benjamin M. Herman, Samuel R. Browning, and Robert J. Curran

Abstract

Theoretical computations of the intensity and polarization of diffusively transmitted sunlight are presented for two wavelengths, λ = 4290 Å and λ = 5000 Å. The computations are for atmospheres containing various distributions of aerosols, as well as normal molecular constituents, and allow for all significant orders of scattering. The theoretical computations are compared with observations, and it is shown that inclusion of aerosols in the theoretical models results in considerably better agreement between observation and theory than can be achieved by assuming a pure molecular atmosphere for the theoretical computations.

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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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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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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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Robin T. Clark, Simon J. Brown, and James M. Murphy

Abstract

Changes in extreme daily temperature events are examined using a perturbed physics ensemble of global model simulations under present-day and doubled CO2 climates where ensemble members differ in their representation of various physical processes. Modeling uncertainties are quantified by varying poorly constrained model parameters that control atmospheric processes and feedbacks and analyzing the ensemble spread of simulated changes. In general, uncertainty is up to 50% of projected changes in extreme heat events of the type that occur only once per year.

Large changes are seen in distributions of daily maximum temperatures for June, July, and August with significant shifts to warmer conditions. Changes in extremely hot days are shown to be significantly larger than changes in mean values in some regions. The intensity, duration, and frequency of summer heat waves are expected to be substantially greater over all continents. The largest changes are found over Europe, North and South America, and East Asia. Reductions in soil moisture, number of wet days, and nocturnal cooling are identified as significant factors responsible for the changes.

Although uncertainty associated with the magnitude of expected changes is large in places, it does not bring into question the sign or nature of the projected changes. Even with the most conservative simulations, hot extreme events are still expected to substantially increase in intensity, duration, and frequency. This ensemble, however, does not represent the full range of uncertainty associated with future projections; for example, the effects of multiple parameter perturbations are neglected, as are the effects of structural changes to the basic nature of the parameterization schemes in the model.

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