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David D. Houghton

Abstract

A quasi-Lagrangian formulation for an inviscid barotropic fluid is presented and shown to afford a convenient basis for analysis of certain ageostrophic jet flows. Material fines serve as the references to delineate the north-south variations in the fluid, and Eulerian representation is used in the east west direction. Several desirable features are shown for the use of material lines in this manner. First, by orienting the lines approximately parallel to the jet axis, flows with finite horizontal curvature may be represented simply. This is illustrated by a development with east-west variations represented by only two harmonies and a mean. The solution of this semi-spectra model agrees closely with a non-spectral numerical solution for over three days. Second, the Lagrangian movement of the material lines gives an indication of non-linear adjustment motions and demonstrates a mixing in the fluid.

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David D. Houghton

Abstract

An incompressible, stratified, hydrostatic, inviscid fluid model is used to demonstrate non-linear effects in the interaction of finite amplitude gravity waves. A statically stable density stratification is approximated by the superposition of ten homogeneous fluid layers with a very deep layer on top. Computations are made using a two-step Lax-Wendroff finite difference system. The solutions reveal intrawave distortions comparable to those predicted by analytical studies of one- and two-layer fluid models. During the interaction of two waves, the solutions show overall changes in wave speed which are of the same magnitude as the variations in wave speed that cause the intrawave distortions. These changes are related differently to the horizontal fluid velocities in the waves.

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David D. Houghton

Abstract

Calculations are made with the NCAR six-layer general circulation model to determine the time evolution of errors initially confined to a region 4000 km in diameter superimposed upon real global data. Three experiments are made to distinguish between the effects of an error located initially on the northern or southern sides of the jet stream or in the tropical area. Results show that the largest error centers generally evolve in the jet stream; however, the propagation rate is much less than advection effects would suggest. Coverage of the Northern Hemisphere is accomplished as much by propagation across the north pole and via the tropical belt as it is via the jet stream. It is not complete even after seven days. As a whole the tropics are more sensitive than the middle latitudes to initial errors. Cross-equatorial effects are most pronounced at and just east of the initial longitude of the error.

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Dong Kyou Lee
and
David D. Houghton

Abstract

Quantitative characteristics and utility of mesoscale satellite winds are investigated for a low-level high spatial resolution data set obtained from a sequence of 6 min interval synchronous meteorological satellite images of the central region of the United States on 20 May 1977. Attention is focused on the quantitative errors introduced by height assignment in the presence of vertical wind shear and by the objective analysis of such irregularly spaced data to a regular grid point array.

It is shown that assignment of the wind vectors to a single level introduces a local variability and systematic horizontal shears due to the vertical wind-shear effect giving a variability comparable to that expected in natural mesoscale phenomena with 100 km length scale. The random component of the local variability can be reduced by appropriate averaging which is possible because of the data density.

The error introduced by the objective analysis procedure is estimated by examining the differences between various analysis methods. This sensitivity test is made both for grid spacing and for objective analysis method and includes the use of an analytical function field. Although there is a large variation in the results, it is estimated for the more reasonable cases that the variations in most areas are not greater than those expected from other error sources.

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Dong Kyou Lee
and
David D. Houghton

Abstract

Initialization of a comprehensive mesoscale numerical prediction model is investigated using actual low-level mesoscale satellite wind observations in a case study. Attention is focused on describing and understanding the impact of these data on the model simulation with basic dynamical variables as well as precipitation. Three 6-h forecast experiments are made with the Kreitzberg-Perkey mesoscale model with 35 km horizontal resolution. These provide information on the sensitivity of the results to the method of data insertion.

Results show that the mesoscale divergence and vorticity fields in the satellite wind information remain coherent and identifiable well into the forecast period. Examination of the fields for specific scale ranges obtained by an objective scale decomposition shows that both the small- and large-scale components for vorticity persist for the entire 6 h period, whereas the divergence patterns of the inserted data are dissipated by 0.5 and 3 h respectively for the small- and large-scale components. After 3 h the primary impact of the inserted data is to alter the amplitude of mesoscale features that develop in the control experiment particularly for the horizontal divergence field. The satellite data produce some noticeable changes in the precipitation forecast in the 1–3 h period. Variations in results due to using different insertion procedures are small compared to differences between all the insertion experiments and the control except for a rather large amplitude external gravity wave oscillation produced by a gradual insertion technique.

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Thomas A. Wilson
and
David D. Houghton

Abstract

An attempt is made to obtain fully three-dimensional mesoscale wind fields from satellite cloud displacement data for an area over the continental United States. A method to derive such fields and their likely accuracy is discussed prior to the presentation of a test case for 30 October 1974. The computed divergence and vertical motion fields are consistent with features of the observed mesoscale weather systems, particularly the locations of subsequent severe convective storms.

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David D. Houghton
and
Evan A. Rubin

Abstract

Twelve-hour quantitative precipitation forecasts from the National Weather Service National Meteorological Center's nested-grid model are evaluated for 12 stations in the upper Midwest for the period April–July 1987. Statistics for threat score, bias, post agreement, over and under forecasts, and “quantitative correctness” are determined from frequency distributions for precipitation forecasts and verifications categorized into four quantitative amount levels: none, low, medium, and high. The analysis is performed for all cases as a group and for subpopulations representing six different categories for the associated synoptic situation. The synoptic situation descriptors involve proximity to surface frontal or trough positions or lack thereof. It was found that the warm and occluded frontal situations had better forecast performance than the other synoptic situations reflecting the better handling of grid-scale in contrast to convective-scale precipitation by the model. Results provide an example of the aid that can be given to forecasters by suggesting relative levels of reliance to be assigned to specific model forecasts.

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Kenneth B. Mielke
and
David D. Houghton

Abstract

An extensive statistical study was made of the properties of radar echo systems in the upper Midwest region of the United States. Mesoscale groupings of echoes with distance scales 10–100 km and time scales 3–10 h instead of individual echoes were considered in the expectation that the former would be more relevant to deterministic short range prediction models.

A total of 203 case histories for 1974 was analyzed. Statistics for the area and duration were determined for the three categories of echo systems: those associated with warm fronts, with cold fronts and with neither. Correlations with the positions of the frontal zones and relationships with upper air winds were also made.

Consistent with earlier studies, it was found that the larger echo systems tend to last longer and that the direction of motion was usually somewhat to the right of upper level winds. Results provided many quantitative relationships that would be useful for prediction schemes and for developing models that carry mesoscale precipitation areas as explicit parameters.

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Walter L. Jones
and
David D. Houghton

Abstract

A numerical model of internal gravity waves allows momentum transport by the waves to interact with the mean flow. Momentum deposited at a critical level develops a “shelf” in the mean flow. Mean flow acceleration Doppler-shifts the wave frequency, allowing more penetration of wave energy than expected from linear theory.

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Walter L. Jones
and
David D. Houghton

Abstract

A simple numerical model is used to demonstrate that momentum exchange between wave and mean flow can substantially modify the process of “breaking” of internal gravity waves at great height. The momentum exchange results in appreciable transfer of energy from wave to mean flow.

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