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Raymond K. W. Wong
and
Keith D. Hage

Abstract

By examining chain rule transformation and tensor transformation results, it is shown that the small slope assumption mentioned in Pielke and Martin (1981) is not required for the validity of the hydrostatic equation in terrain-following coordinates.

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Herbert D. Entrekin
,
James W. Wilson
, and
Keith D. Hage

Abstract

Two years of mesoscale weather data collected from the Federal Aviation Administration's Atlantic City mesometeorological network (mesonet) were used for the development and testing of techniques for 15-, 30- and 60-min predictions of runway visual range, cloud-base height and wind. The primary purpose of this work was to evaluate the utility of the mesonet for improved prediction of aviation terminal weather.

The first year of mesonet data was used for development work. Studies included preliminary development and testing of a prediction model based on advection; and, using detailed data analyses and a numerical-physical boundary-layer model, studies of methods for predicting changes due to non-advective processes.

The second year of mesonet data was used as independent data for testing six forecast techniques. Of the six techniques tested, three did not utilize mesonet data and were used for control purposes. The control forecasts were prepared from persistence, from trend, and by experienced U. S. Weather Bureau terminal weather forecasters. The forecasts based on mesonet data were prepared by a simple advection model which used low-level winds, by The Travelers Research Corporation (TRC) meteorologists and by U. S. Weather Bureau terminal weather forecasters.

The mesonet evaluation was made by comparing results from techniques that used mesonet data with those that did not use mesonet data. The results showed, in general, that none of the techniques that used mesonet data provided forecasts significantly better than those provided by persistence. It is therefore concluded, using the forecast methods evaluated, that the mesoscale data provided from the Atlantic City mesonet are not adequate for providing significantly improved short-range terminal weather forecasts.

There were three weather situations in which forecasts significantly better than persistence were obtained by TRC meteorologists. These forecasts were made by using a combination of mesonet and weather radar data. These special situations were 1) visual range restricted by snow, 2) dense fog accreted by rain, and 3) wind shift lines identified by radar or surface wind reports.

The general failure of the techniques that used mesonet data to provide better forecasts is attributed to the following: low-level winds alone are frequently not adequate for estimating the movement of the fields of cloud-base height and visual range; the number and arrangement of mesonet stations is not adequate for describing these fields or for determining their movement by analysis; and the techniques are not capable, in general, of predicting changes due to formation or dissipation.

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Raymond K. W. Wong
,
Keith D. Hage
, and
Leslie D. Phillips

Abstract

A two-dimensional numerical model is used to simulate nocturnal drainage flow in a small urban valley with light prevailing winds and conditions of supercritical Richardson numbers (Ri). The model uses a hydrostatic and Boussinesq system of equations written in terrain-following coordinates. Radiative transfer is represented by Brunt's method of radiative diffusivity. Eddy diffusivities are specified in the subgrid parameterization for conditions where Ri is supercritical. Tests show the dependence of drainage wind on slope angle, cooling rate, surface drag and prevailing wind speed, and also the insensitivity of wind and temperature to the eddy diffusivities under supercritical Ri conditions. The drainage wind cells are asymmetric, with a shallow surface layer of drainage flow and a thicker upper region of slower return flow. The predicted wind profiles show low-level maxima and the predicted temperature profiles are exponential in shape, in good agreement with observations obtained in Edmonton, Alberta in the summer of 1978. The model is also able to predict the quasi-stationary slope flow observed in the field.

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Yong-Seung Chung
,
Keith D. Hage
, and
Erhard R. Reinelt

Abstract

A comparative study for the year 1958 of 259 cyclones in the lee of major mountain barriers in Fast Asia, and of 146 cyclones in the lee of the Canadian Rocky Mountains, shows complex multiple-frequency maxima of initial cyclogenesis in juxtaposition to these major mountain systems. The initial formation of most lee cyclones occurred under a different, cross-barrier flow in the mid-troposphere. The difluent characteristics of the upper-level flow appeared to be intensified by the mountains. On the other hand, appreciable development or intensification of the cyclones begins with the approach of an upper-level cold trough or low. In general, this intensification occurs in regions away from the principal cyclogenetic zones. However, formation and intensification do take place in the same region in some instances.

About 41% of the lee cyclones sampled in East Asia were weak, local cyclones of orographic origin. In western Canada, only 25% of the cyclones belong into this category. Many cyclones that formed in the lee of the Tibetan Plateau did not acquire appreciable intensity as they tracked across China. Moreover, lee cyclones of moderate to high intensity were observed to be much less common in central and southern China than in North America.

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