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Alistair B. Fraser
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Steven S. Fine and Alistair B. Fraser

Abstract

A statistical pattern recognition technique called IREW is described. IREW has several strengths, such as fast execution, small storage requirements, and increments learning, that may make the technique useful for many meteorological pattern recognition problems. A weakness of IREW is that it may not recognize complex patterns as well as more sophisticated pattern recognition techniques do.

IREW analyzes data by using a derivative of Bayes' rule to multiplicatively combine the influence of predictors. The weight assigned to each predictor is determined empirically from a training dataset containing the data and verifications for multiple cases. IREW uses several methods to select useful subsets of a large set of predictors.

The objective identification of surface fronts in Nested Grid Model forecasts is described to illustrate how IREW can be applied to a typical pattern recognition problem. The work consisted of identifying factors related to fronts and using some of those factors to make analyses. Given 27 000 predictors, IREW selected many that meteorologists associate with fronts. IREW's analyses were compared to subjective analyses for seven test cases. In this limited test, IREW performed similarly to meteorologists in terms of the number of grid points correctly classified as frontal or non-frontal.

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Alistair B. Fraser and Craig F. Bohren

Abstract

The visual phenomenon called virga, a sudden change in the brightness of a precipitation shaft below a cloud, is commonly attributed to evaporation of raindrops. It is said to be rain that does not reach the ground. The optical thickness of an evaporating rain shaft, however, decreases gradually from cloud base to ground. Thus, it is more likely that virga results from snowflakes melting in descent. Horizontal optical-thickness decreases of more than ten can occur in the short distance over which a snowflake is transformed into a raindrop. This decrease is caused by two factors: a smaller number density of hydrometeors because of the greater fall velocity of raindrops than of equivolume snowflakes, and a smaller scattering cross section: the first of these is dominant. An alternative explanation of virga is that it is precipitation that has not yet reached (rather than does not reach) the ground. This is a plausible explanation given the long time periods it may take hydrometeors, especially snowflakes, to descend from cloud to ground.

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Alistair B. Fraser and Craig F. Bohren

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No abstract available.

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Craig F. Bohren and Alistair B. Fraser

Green thunderstorms are observed occasionally, yet with one exception they have received no scientific attention, experimental or theoretical. Fraser suggested that thunderstorms themselves are not green but that a thick thunderstorm provides a dark backdrop for green airlight near sundown. Greenness is a consequence of reddened sunlight illuminating selective scatterers along the observer's line of sight. Bohren's alternative explanation is that green thunderstorms may be a consequence of the intrinsic blueness of clouds because of selective absorption by pure water, liquid or solid. Most clouds are so thin that the light transmitted by them is not markedly colored because of selective absorption. Only the most massive clouds—large both vertically and horizontally—are thick enough to shift the color of incident sunlight upon transmission. If that incident light is sunlight reddened at sundown, the transmitted light can be perceptually green. These two explanations do not exclude one another but allow for multiple causes, including those not yet identified.

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Peter V. Hobbs, Richard C. Easter, and Alistair B. Fraser

Abstract

Expressions are derived for the horizontal and vertical components of the wind, the temperature, and the mass of water vapor condensed when air flows over a long mountainous ridge. The growth of solid precipitation particles in the orographic clouds by deposition from the vapor phase, riming and aggregation are considered. The trajectories of these precipitation particles are then computed from their fallspeeds and the airflow model.

The model is used to investigate the effects of the microstructure of clouds on the growth and fallout of solid precipitation over the Cascade Mountains. It is shown that, under suitable conditions, increases in the concentration of ice particles in the clouds from about 1 to 100 liter−1 can cause the solid precipitation to be carried farther downwind and over the Cascade crest, so that snowfall is deposited on the eastern rather than the western slopes of the mountains.

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Alistair B. Fraser, Richard C. Easter, and Peter V. Hobbs

Abstract

A model for airflow over mountainous terrain is presented. The equations for steady, two-dimensional, laminar inviscid flow, including pseudo-adiabatic latent heat release, are derived. Approximate solutions to the linearized equations are obtained for stably stratified conditions, and a terrain consisting of broad ridges (width≳25 km), through an iterative transform technique which allows the nonlinear boundary conditions to be satisfied. The model indicates that the dynamical effects of latent heat are significant in some cases but are generally secondary to the barrier effect of the terrain.

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