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M. Cheng and R. Brown

Abstract

Methods of optimizing the Lovejoy and Austin technique to delineate areas of precipitation using visible and infrared satellite data are investigated. The technique involves training the satellite data by correlation with real-time radar data. The choice of statistical measures to define the precipitation/no-precipitation boundary between satellite classes is investigated. Subjective evaluation of the satellite-diagnosed precipitation fields indicates that minimizing the difference between the observed and diagnosed number of precipitation pixels produces the most realistic results. Maximization of some standard skill scores tends to overestimate the areas extent of the precipitation. Examples of the variability of the accuracy of the technique and the variation in optimum boundary or threshold are given. Cases illustrating the improvement produced by using different correlation tables for different synoptic systems are presented. Use of time-averaged correlation tables is investigated and found to be nearly as accurate as use of tables formed at one time, when evaluated within the training area. Fixed predefined tables were rather less accurate when evaluated within the training area, especially with respect to the diagnosed areal extent. A method is presented to combine the use of instantaneous and time-averaged correlation tables, together with a predefined table. Ideally, the method should incorporate the use of different correlation tables for different synoptic systems.

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S. Sethuraman and R. M. Brown

Abstract

A three-dimensional hot-film probe, a Vector Vane, and an Aerovane were used to measure the mean wind speed and turbulence structure in the atmospheric surface layer at a location on the south shore of Long Island. A comparison was recently made of the characteristics of the three instruments to determine their capabilities in measuring the various meteorological parameters of interest. Results from the comparison indicated that the mean wind speed measured by the three instruments was the same.

The estimated spectral densities of the Vector Vane were approximately equal to those of the hot-film probe to a cyclic frequency of 1 Hz. The standard deviations of the velocity fluctuations were equal.

Comparison of the longitudinal velocity fluctuations measured by the Aerovane and Vector Vane were not significantly different to a frequency of 0.3 Hz. The Aerovane underestimated the lateral velocity fluctuations.

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M. Cheng, R. Brown, and C. C. Collier

Abstract

The relationship between precipitation and infrared and visible satellite data is investigated in the vicinity of the United Kingdom. The investigation uses histograms of Meteosat data, built up from many half-hourly fields, which represent the frequency distribution of the pixels as a function of temperature and brightness. Separate histograms are produced for pixels classified as “precipitation” and “no precipitation” by coincident radar observations. The study is conducted separately for four distinct synoptic types: cold fronts, warm fronts, cold-air convection, and mesoscale convective systems (MCSs). A method is presented that uses this information to delineate areas of precipitation.

It is found that the use of combined infrared and visible satellite data yields better results than using infrared alone for all four synoptic types and is better than visible date alone for the majority. Use of visible data alone is better than using infrared data by itself, except for warm-front cases. The results indicate that the ability of the satellite data to delineate precipitation decreases in the following order of synoptic regime: cold frontal, MCS, warm frontal. The most difficult regime to delineate is cold-air convection.

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John M. Brown and Kevin R. Knupp

Abstract

A severe thunderstorm which spawned at least four tornadoes, one of them anticyclonic, formed over central Iowa during the afternoon of 13 June 1976. This storm moved toward the east-northeast, approximately parallel to but slower than the mean tropospheric flow. The anticyclonic tornado (F3) and the most intense (F5) of the cyclonic tornadoes coexisted for 23 min and traveled on nearly parallel, cycloidal-like tracks, with the anticyclonic tornado 3–5 km southeast of the cyclonic. The major emphasis of this paper is on this pair of tornadoes and their relationship to the structure and evolution of the parent thunderstorm.

Radar recorded the development of a hook echo just prior to the genesis of the intense cyclonic tornado. A strengthening mesolow was centered somewhere south of this tornado soon after it formed. The mesolow is believed to have initiated a new updraft; the anticyclonic tornado formed in association with this updraft, south of the cyclonic tornado. It is hypothesized that the mesolow was responsible (through alteration of the storm-scale airflow) for the nearly simultaneous sharp right turns made by these tornadoes. Each of these tornadoes was observed to diminish in intensity soon after becoming associated with heavy rain.

It is argued that the parent thunderstom's distinctive airflow and thermodynamic structure at low levels provided a more favorable setting for the amplification of anticyclonic vorticity than is typical of most severe thunderstorms.

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Benjamin M. Herman and Samuel R. Browning

Abstract

A numerical method of solving the equation of radiative transfer for a plane parallel, horizontally homogeneous medium is presented. The method is applicable for problems with nonconservative scattering as well as for conservative scattering problems. Comparison of results for the reflected and transmitted radiation from this method with existing solutions for conservative Rayleigh scattering shows that, for optical depths up to 1-0, the present scheme is accurate to within ±0.007 unit total intensity and ±1.0 per cent polarization for an incident flux of π units per unit normal area. Results are presented for the reflected and transmitted intensity and per cent polarization for optical depths 2.0 and 4.0, for a particular problem of conservative Rayleigh scattering.

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Benjamin M. Herman and Samuel R. Browning

Abstract

In this paper, calculations are presented of the change in reflected flux by the earth-atmosphere system in response to increases in the atmospheric aerosol loading for a range of complex indices of refraction, solar elevation angle and ground albedo. Results show that, for small values of ground albedo, the reflected solar flux may either increase or decrease with increasing aerosol loadings, depending upon the complex part of the index of refraction of the aerosols. For high ground albedos (A > 0.4), an increase in aerosol levels always results in a decrease of reflected flux (i.e., a warming of the earth-atmosphere system).

The first part of the paper concerns itself with the computational techniques employed in this study. The method employs a numerical solution to the equation of radiation transfer and is essentially a modification of an older technique used by the authors. The modifications are detailed, and comparisons with the older technique are presented.

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

Abstract

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

Abstract

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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