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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
,
David P. Baumhefner
, and
Warren M. Washington

Abstract

The problem of obtaining initial values for vertical motion and the divergent component of horizontal velocity is examined for a global primitive equation model. Only diagnostic methods are considered, the emphasis being on uniform application over the globe rather than a high degree of accuracy. Results show that a very simple diagnostic equation similar in form to the omega equation provides for realistic values of vertical motion in high and middle latitudes and smooth variations across tropical latitudes. In terms of prediction accuracy, no improvement is noted by using the computed initial vertical motions instead of zero for the initial vertical motions in a six-layer, 5° mesh model. In both cases unrealistic oscillations occur during the first 12 hr.

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David D. Houghton
,
John E. Kutzbach
,
Michael McClintock
, and
David Suchman

Abstract

Sea temperature anomalies which departed from the December climatic mean by approximately 2C off the coast of Newfoundland were inserted into the NCAR six-layer, 5° mesh, general circulation model of the atmosphere in order to test the model's response to small perturbations in sea surface temperature. The response of the model to the anomalies was analyzed with respect to pressure patterns, heat flux, and cyclone frequency, path and intensity. This response was compared with a statistical analysis of the response of the atmosphere to similar sea temperature anomalies based on approximately 80 years of observations as described by Ratcliffe and Murray.

Analyses of the anomaly experiments are preceded by an analysis of the basic (control) statistics for both model and atmosphere. The most pronounced discrepancies between the two were noted in cyclone statistics. A calculation with double horizontal resolution greatly improved the model features. Detailed comparison was complicated by the fact that the model failed to achieve statistical stationarity.

The extensive verification data of Ratcliffe and Murray proved valuable in distinguishing meaningful anomaly responses from those that could be attributed to the many limitations in the model, including a pronounced natural variability. Both warm and cold anomaly cases were tested. Best agreement with observed data was obtained for the case of the warm anomaly; this agreement was most evident during the middle portions of the integrations and then only in the North Atlantic sector. The response in the case with a cold anomaly was not as satisfactory although there were clear distinctions between the warm and cold anomaly cases.

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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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Bette L. Otto-Bliesner
,
Grant W. Branstator
, and
David D. Houghton

Abstract

A global, spectral, primitive equation model is developed to study the seasonal climatology of the large-scale features of the atmosphere. The model resolution is five equally-spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. Included in the model are: orography; time-varying (but prescribed) sea-surface temperatures, snowcover, and solar declination angle; parameterizations for radiation, convection, condensation, diffusion, and surface transports; and a surface heat budget. The external seasonal forcing of the model atmosphere is composed of sinusoidal time variations in the incoming solar radiation and latitude of the snowline and more complicated variations in the albedo of the snow and the sea-surface temperatures. A five-year seasonal simulation has been analyzed. The model reasonably reproduces the general features of the observed atmospheric circulation, seasonal cycles, interannual variations and hemispheric differences. The success of this low-resolution model in simulating the large-scale features of the atmospheric seasonal cycle illustrates the usefulness of such models for climate studies in conjunction with high-resolution general circulation model simulations.

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Yun-Qi Ni
,
Bette L. Otto-Bliesner
, and
David D. Houghton

Abstract

An analysis is made of the effect of orography on the atmospheric energetics in a low-resolution general circulation model to determine the temporal and scale dependency of these effects. The numerical model is a global, spectral, primitive equation model of the atmosphere with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. A one-year seasonal simulation of the general circulation without mountains is compared to the results from a five-year seasonal simulation of the general circulation with mountains. The statistical significance of the topographic effects is evaluated by comparing them to magnitudes of model interannual variability determined from the five-year control simulation.

A small, but important, portion of the changes due to topography are significant. At northern extratropical latitudes, the increases of eddy activities and baroclinic instability in summer resulting from incorporation of the effect of the mountains give rise to significantly increased eddy components of atmospheric energetics and the conversion from eddy available potential energy to eddy kinetic energy. These increases are generally present and significant at each wavenumber and for the overall stationary component. In winter, topography significantly increases the zonal kinetic energy and dissipation. Examination of the individual zonal spectral components for winter reveals that topography increases long-wave energies and their transfer, but with proportional decreases at medium waves, resulting in little change in the total eddy components. A similar compensation occurs between the stationary and transient components of the heat transport. Less pronounced topographic features at tropical and southern extratropical latitudes result in fewer significant changes due to topography.

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John E. Kutzbach
,
Robert M. Chervin
, and
David D. Houghton

Abstract

Four numerical experiments are analyzed to determine the three-dimensional response of the NCAR general circulation model to large prescribed changes in mid-latitude North Pacific Ocean surface temperature. The ocean surface temperature (OST) boundary conditions are subjected to changes of opposite sign in the eastern and west-central portions of the North Pacific Ocean. The maximum amplitude of the temperature changes is either 12°C or 4°C. The model atmosphere response in the North Pacific sector includes changes in amplitude and vertical tilt of the long waves, an increased direct thermal circulation (i.e., warm air rises over the positive OST change and cold air sinks over the negative OST change), and locally enhanced westerlies to the north of the positive OST change. Cyclones form and/or intensify over the positive OST change and tend to be absent or weak over the negative OST change. The mid-tropospheric response extends downstream from the prescribed change region, and the response both over and downstream from the region depends strongly on the longitude of the prescribed changes. Many features of the response are statistically significant, although generally not over the continental United States. The amplitude and phase of the mid-tropospheric long waves (zonal wavenumbers 1–4) are also affected. The prescribed change response is largest and of greatest statistical significance when the prescribed change is very large (12°C maximum amplitude) but is also frequently detectable when the prescribed change is one-third as large (4°C maximum amplitude). A comparable experiment involving a prescribed North Atlantic OST change produces a similar mid-tropospheric response.

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