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David P. Baumhefner

Abstract

A pilot study that evaluates the potential forecast skill of winter 10–30-day time-mean flow from a low-resolution (R15) climate simulation model is presented. The hypothesis tested is that low-resolution climate model forecasts might be as skillful as high-resolution numerical weather prediction model forecasts at extended-range timescales, if the low-frequency evolution is primarily a large-scale process and if the systematic error of the climate model is less detrimental than high-resolution forecast model error.

Eight forecast cases, each containing four ensemble members, are examined and compared to high-resolution forecasts discussed by Miyakoda et al. The systematic error of the climate model is examined and then used to reduce the forecast error in an a posteriors fashion. The operational utility of these climate model forecasts is also assessed.

The low-resolution climate model is quite successful in duplicating the skill of the high-resolution forecast model. If the forecast systematic component of error evaluated from the same eight cases is removed, the climate model forecasts improve in a comparable fashion to the high-resolution results. When information from the low-resolution climate simulation is used to estimate the forecast systematic error, the improvement in skill is less successful. These results show that a low-resolution climate model can be a viable tool for numerical extended-range forecasting and imply that large ensembles can be integrated for the same cost as higher-resolution model integrations.

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David P. Baumhefner

Abstract

No abstract available.

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DAVID P. BAUMHEFNER

Abstract

A diagnostic, nonlinear balanced model is applied in order to describe numerically the three dimensional structure of the tropical atmosphere. Several comparisons and experiments are made to gain insight into the physical processes and reliability of the model. These include different types of stream functions and temperature analyses, and the addition of surface friction and latent heat. A comparison between the kinematic vertical motion and the final numerical result is performed.

Obtained by using the complete form of the balance model, the derived vertical motion for August 12–14, 1961, in the Caribbean is presented in the form of cross sections. The vertical velocity fields, which are displayed in partitioned form, are compared with the analyzed moisture distribution. The validity of the computed vertical motion is discussed along with its possible influence on the tropical weather.

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David P. Baumhefner

Abstract

Experiments with a global, 2½° primitive equation model show that for periods of one week a wall placed at the equator does not appreciably affect the results of a forecast of large-scale mid-latitude atmospheric flows. The success of the hemispheric model depends on the lack of distortion of the mean and eddy quantities of the large-scale processes in the subtropical region.

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David P. Baumhefner

Abstract

An attempt is made to investigate the effect of the tropics and the Southern Hemisphere on Northern Hemisphere mid-latitude numerical forecasting. Using the NCAR general circulation model and real atmospheric data for January 1958, several experiments were conducted in which a wall was inserted at various latitudes. These forecasts were compared with real data and a global, real-data forecast without a wall. Verification statistics were evaluated for comparison purposes. Several different boundary conditions at the wall were also examined.

Walls inserted in the model at or below the equator did not influence the forecast in the mid-latitudes for nearly too weeks. However, walls north of the equator damaged the results after less than a week. Different boundary conditions have little effect on the forecast except near regions of high wind speed.

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DAVID P. BAUMHEFNER

Abstract

A number of global real-data numerical forecasts have been calculated using the two-layer NCAR (National Center for Atmospheric Research) general circulation model. The purpose of these experiments was threefold: 1) to evaluate the model's ability to predict the real atmosphere, 2) to develop a global forecasting model which will make use of the data obtained by the proposed GARP (Global Atmospheric Research Program), and 3) to help determine some of the internal, empirical constants of the model. In order to evaluate the accuracy of the predictions, several “skill scores” were calculated from the forecasted and observed variables. A by-product of this research was the testing of five different types of data-initialization schemes. Over 50, 4-day forecasts have been run, in which the initialization schemes and internal constants were varied.

The results from these experiments indicate that the present two-layer model is capable of forecasting the real atmosphere with reasonable skill out to 2 days at the surface and 4 days in the middle troposphere. The best initialization scheme for this particular model, thus far, appears to be the complete balance equation. However, several of the simplified initialization techniques are very close in terms of forecasting skill.

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Thomas W. Bettge
and
David P. Baumhefner

Abstract

The total and systematic errors in the 500 mb geopotential height forecasts from the NMC grid-point and spectral operational models are compared and contrasted for two recent winters. The spectral model is shown to be an improvement in the forecasts through a more skillful prediction of the planetary-scale (zonal wavenumbers 1-2) quasi-stationary wave amplitudes, and through the elimination of the grid-point model's large systematic error at low latitudes.

In agreement with estimates from related studies, the systematic error in the NMC spectral model accounts for 15-20% of the total error variance. Approximately one-half of the total systematic error resides in the planetary scales.

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Thomas W. Bettge
and
David P. Baumhefner

Abstract

The design of a digital filter is outlined and its application as a band-pass filter to separate various scales from an atmospheric field in a limited domain is discussed. The accuracy of the filter is demonstrated by decomposing both a function with specified wave components and a 500 mb geopotential field within a 90°longitudinal area of the globe. The boundary effects of the non-periodic domain are not negligible, but tests using various boundary conditions show that little contamination exists inside 7–10 grid points from the boundaries. The suitability of the technique to examine the spatial wavenumber characteristics of a geopotential field within a limited domain is demonstrated.

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David P. Baumhefner
and
Paul R. Julian

Abstract

A reference level is defined as a level of known altitude at which temperature, pressure, and perhaps wind are specified as functions of time. This study is concerned with the optimum location of a reference level without wind information. Experiments were performed with the NCAR six-layer general circulation model to compare the usefulness of a surface reference level with an upper tropospheric reference level. We first performed a control integration using real atmospheric initial data. We then ran several comparison runs with initial conditions differing from those of the control run. The initial pressure distribution at the reference level was kept the same as the control run. The distribution of temperature pseudo-error employed in calculating the initial pressure distributions at the other levels was chosen to simulate possible error patterns in temperatures radiometrically derived from satellites. The initial conditions in all cases were in hydrostatic and geostrophic balance. Three data sets were used and the experiments were integrated to five or seven days. In addition, two horizontal distributions of initial temperature pseudo-error and two horizontal mesh lengths of the model were used for one of the three data sets. The results were examined using an rms difference of the distribution of pressure and meridional wind normalized (in the vertical) by the difference statistics derived from randomly chosen states of the model.

It appears that pseudo-error growth rates are nearly independent of the location of a reference level, but details of the pseudo-error patterns depend on the initial synoptic conditions. Pseudo-error growth rates differed depending on the manner in which the horizontal pseudo-error was initially distributed (but did not differ with the location of the reference level). The most significant change in the pseudo-error growth rates was observed when the mesh length was changed; halving the mesh length produced much faster growth rates, particularly in the lower layers.

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David P. Baumhefner
and
Paul R. Julian

Abstract

The error produced by an observational system of remotely sensed temperature profiles is partially simulated with aid of an operational retrieval scheme. A given temperature distribution is converted to radiances and then back to temperatures. A comparison is made between the retrieved temperatures and the original values.

The sensitivity of the retrieval scheme to various input parameters, such as guess profiles, the statistical coefficients used in the retrieval scheme, and cloudiness, is examined. Experiments with the placement of a reference level from which to integrate the geopotential field are performed.

The relative growth rate of the simulated initial error is examined by forecasting with two initial states, one case with the observational error and one central case. Error growths are calculated for different reference levels, clear, and cloudy cases.

The results show that the generated error fields do not seriously contaminate forecasts of the large-scale baroclinic waves for periods up to one week, providing the retrievals are free of the effects of clouds and an accurate guess is used. In general, the results support the need for reference-level information, and if forecast error in the low troposphere is to be a minimum, placing of the reference level at sea level.

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