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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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Richard D. Thomas Jr.
and
David D. Houghton

Abstract

This study examines the relationship between the radar echo parameters of area coverage and intensity, and the surface kinematic fields of divergence, divergence of moisture flux and relative vorticity. The study was limited to 11 cold frontal cases during the period March 1976–March 1977, and involved 99 echoes. Fine-resolution digitized radar data were used from two midwestern and one eastern United States sites. The data were analyzed on a computer-interactive video system. The area coverage did not correlate significantly with any of the surface parameters tested. However, the intensity parameters did show significant relationships with the surface parameters, the best being with relative vorticity. The correlations were higher when surface data from 1 h before the time of the echo were used, with coefficients as high as 0.5.

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David D. Houghton
and
William S. Irvine Jr.

Abstract

The performance of the National Weather Service, Air Force and Navy large-scale numerical prediction models was studied for the case of a relatively small-scale but important weather-producing frontal system in the Midwest over the period 1200 GMT 5 October 1974 to 0000 GMT 7 October 1974. Forecasts were analyzed both for the operationally important parameters of precipitation, surface pressure and 500 mb heights and for such key diagnostic parameters as vertical motion and thermal and vorticity advection. Results showed the importance of resolving small synoptic-scale features in the initial conditions as well as the role of model resolution, basic dynamics formulation, and planetary boundary layer representation in the forecasts. There was a wide range of performance among the four models. The National Weather Service (NWS) Limited Fine Mesh Model clearly gave the best 24 h forecasts, compared to all the other models including the NWS Primitive Equation Model.

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WILLIAM S. IRVINE JR
and
DAVID D. HOUGHTON

Abstract

A one-layer, mid-latitude, beta-plane channel model of an incompressible homogeneous fluid is constructed to study the propagation of systematic errors on a nearly stationary synoptic scale wave. A time- and space-centered difference scheme is used to evaluate the governing primitive equations. Data fields resulting from height field perturbations injected at various locations in the synoptic wave are compared to the unperturbed synoptic wave at 3-hr intervals for 5 model days. Results show that the low-frequency or quasi-geostrophic component of the error tends to move toward the core of maximum velocity in the basic state and that, after 5 days, these maximum height errors are in the core regardless of the location of the initial perturbation.

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David D. Houghton
,
William H. Campbell
, and
Nathaniel D. Reynolds

Abstract

Diagnostic methods me considered for isolating gravity-inertial motions in the output of a nonlinear atmospheric numerical model. The gravity-inertial component is defined by the nongeostrophic motions not directly incorporated with the synoptic-scale evolution according to quasi-scostrophic or balance model relationships. The analysis methods am applied to the solutions for a propagating jet stream maximum generated by a simple two-layer hydrostatic numerical model. Results identify a coherent pattern in the gravity-inertial motion component but details of the horizontal structure and propagation characteristics are only partially resolved. Results also elucidate relative merits of a number of physical variables and difference fields for defining the gravity-inertial component.

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Peter J. Pokrandt
,
Gregory J. Tripoli
, and
David D. Houghton

Abstract

On 15 December 1987, several long-lived, large-amplitude mesoscale wave disturbances accompanied a rapidly developing extratropical cyclone in the midwest United States. Previous observational and modeling studies have suggested that the disturbances had large amplitudes and long lifetimes as a result of a wave-CISK-type instability occurring within an imperfect wave duct and were initiated by convection. However. infrared (IR) satellite imagery and radar echoes shortly before the wave disturbances formed suggest that convection was not the primary feature in the wave genesis region at that time. Instead, a meso-β-scale comma-shaped cloud was present and appeared to evolve into the wave disturbances. The origins of the comma cloud can be traced back to a cloud streak and precipitation maximum in the left exit region of an approaching jet streak over northern Mexico 15 h earlier. In this study, satellite observations are examined in conjunction with numerical simulations of the case to explore a new hypothesis for the formation of the wave disturbances. Specifically, the transverse circulation about an approaching jet streak transports potential vorticity from a reservoir in the stable cold low-level air to produce a meso-β-scale potential vorticity anomaly at midlevels, which is subsequently rotated relative to the upper-level flow to force mesoscale waves.

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David D. Houghton
,
Ralph A. Petersen
, and
Richard L. Wobus

Abstract

Forecasts from different resolution versions of the National Meteorological Center Nested Grid Model (NGM) are compared for two case studies to assess an optimal ratio of model vertical and horizontal resolutions. Four combinations are considered: 1) 16 layers and 80-km horizontal grid over the United States (the operational version of the model), 2) 32 layers and 80-km horizontal grid, 3) 16 layers and 40-km horizontal grid, and 4) 32 layers and 40-km horizontal grid. Resolution impacts are evaluated for a number of weather system components such as extratropical cyclone evolution, baroclinic and frontal zone structure, jet-stream blow, moisture fields, and precipitation.

Resolution impacts for this limited sample are relatively small for synoptic-scale features such as the position of the extratropical cyclone and main jet-stream flows. Larger impacts are noted for smaller-scale horizontal structure and gradients, frontal zone associated circulations and hydrological cycle components. Vertical resolution enhancement effects on the NGM, which already has added resolution near the lower boundary, are less dramatic in the lower troposphere than those for horizontal resolution, but are important for defining upper-level frontal structures and circulations where the NGM's vertical structure is coarser. Conclusions concerning consistency of horizontal and vertical resolution impacts on baroclinic zone structure and spurious noise generation found in earlier studies with simpler models are confirmed and brought into perspective for comprehensive numerical models and operational weather prediction model applications for the two cases discussed. The effects of the improvements in small-scale forecast accuracy, however, are difficult either to generalize due to the limited number of case studies or to assess because of the lack of high-resolution verification information and evaluation techniques.

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Stephen D. Jascourt
,
Scott S. Lindstrom
,
Charles J. Seman
, and
David D. Houghton

Abstract

Satellite imagery dramatically portrays a mesoscale organization of deep convection over the south central United States on 5 June 1986. Free convection was expected over the region. The rapid development and organization of the convection simultaneously across a broad area suggests the presence of a mesoscale instability. Analysis of satellite and conventional data suggests that a layer of weak symmetric stability modified the atmosphere's response to free convective instability, contributing to the highly organized banded structure observed.

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