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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

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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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David D. Houghton
and
Robert M. Chervin

Abstract

Vertically-averaged meridional transports of westerly momentum are analyzed in sampled ensembles of January simulations of an NCAR GCM and an equivalent ensemble of five years of observational January data according to a simple time-domain decomposition. Ensemble averages and standard deviations are compared in terms of both zonally-averaged and grid-point presentations for the steady and transient flux components highlighting the relative characteristics of the fundamental time-domain elements. Results from 5 and 2.5° horizontal resolution versions of the model demonstrate the impact of truncation error on model simulations of these flux statistics.

Comparing grid point measures constitutes a more stringent model performance evaluation since regional differences between observed and simulated transports often are found to he considerably larger than zonally-averaged differences. Such regional considerations also reveal substantial differences between model and observations in the location and orientation of transport maxima and minima. Typically the transient flux component is smaller in the model simulations than in the observations although there are some regional exceptions. The steady flux component, however, is generally larger in the model simulations (particularly the 2.5° version) than in the observations and is affected more than the transient component by resolution changes. Analysis of the estimated standard deviations of the flux components shows that the model's inherent variability is typically at least a factor of two lower than the observed interannual variability with substantial regional differences.

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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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Robert G. Gallimore
and
David D. Houghton

Abstract

The approximation of ocean heat storage by the net surface energy flux and the implications for zonal mean SST simulation using mixed layer ocean formulation are examined. The analysis considers both constant and variable depth mixed layers. Simulated zonal-mean net surface energy fluxes, taken from a low-resolution atmospheric GCM with prescribed SST, are compared to observed flux and ocean heat storage data. The impact of limitations of the mixed layer ocean formulation on SST simulation are assessed to impacts of simulation errors of the atmospheric model.

The results indicate the important in determining errors in the atmospheric model simulation for the net surface energy flux when assessing anticipated improvement in SST simulation as neglected physical processes (e.g., ocean heat transport) are incorporated in the ocean component of an interactive model. Noting the current limited availability of observations, the approximation of ocean heat storage by the simulated net surface energy flux is cautiously assessed for middle latitudes of the Northern Hemisphere. Due to the uncertainty in observational estimates for both seasonal net surface energy flux and seasonal ocean heat transport, the quality of the flux simulation and the question as to whether or not the model heat storage approximation would improve with the addition of seasonal ocean heat transport are assessed with less certainty.

The inferred annual cycle of zonal mean SST is calculated by applying both model and observed net surface energy fluxes (approximated heat storage) and observed heat storage data to the mixed-layer ocean formulations. The results show that the change from a constant 50 m depth to a variable depth mixed layer ocean formulation (after Meehl), yields significant improvement in zonal mean SST simulation with the sensitivity to the approximation for heat storage being a lesser factor. The large uncertainty in seasonal heat transport data, however, warrants sensitivity examination of their impact on climate in a coupled ocean–atmosphere model. The results demonstrate that examination of biases in atmospheric model simulation and calculation of inferred SST using the atmospheric model results can be useful in diagnosing SST simulation in coupled ocean–atmosphere models.

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Robert G. Gallimore
and
David D. Houghton

Abstract

The simulation of zonal mean ocean surface temperature and heat storage (time rate of change of heat content) obtained from a low resolution, atmospheric general circulation model (GCM) coupled to a variable depth upper-ocean formulation (A/VO model) is compared with observations and with a parallel simulation using a constant (50 m) depth upper-ocean component (A/CO model). Variable depths are used to specify the heat content in the seasonal mixed layer and thermocline; the depths are prescribed to vary latitudinally and seasonally but not longitudinally. The Northern Hemisphere depths are taken from Meehl; for the Southern Hemisphere, Meehl's depths are modified to account for differences in ocean thermal structure between the hemispheres.

The annual variation of upper ocean depth produces important effects on seasonal simulation of ocean surface temperature in the extratropics. In this study, the extremes of ocean temperature in both hemispheres occur earlier by about 30 days in the A/VO model, compared to the A/CO model. This leads to a warmer ocean in spring/summer and a colder ocean in fall/winter for the A/VO model. In contrast to the A/CO model, an important asymmetry in the structure of the monthly departures of ocean temperature from the annual mean is produced by the A/VO model. In the southern extratropics, the A/VO model simulates a reduced annual cycle of ocean temperature (by about 25–30%) from that produced by the A/CO) model.

Both models underestimate the observed seasonal amplitude of zonal mean heat storage; the underestimation is greatest for the A/VO model. The differences in simulations of heat storage are linked to differences in ocean surface temperature computation between the two models.

Errors in zonal mean ocean surface temperature for the A/VO model are less than for the A/CO model in the extratropics. particularly the Southern Hemisphere. However, the errors in the coupled A/VO model simulation are larger in the northern extratropics than in the previously published uncoupled calculations using prescribed heat storage estimates. It is argued that significant improvement in ocean temperature simulation by the A/VO model can be achieved with better approximation of heat storage in conjunction with a small adjustment to the prescribed variable depths.

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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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