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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

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
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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Joseph J. Tribbia
and
David P. Baumhefner

Abstract

This paper presents the results of an ensemble of 20 predictability experiments derived from the NCAR Community Climate Model (CCM). Particular emphasis is placed on the question of the predictability of dynamically driven low-frequency components of the model atmosphere. The conclusion drawn, using time averaging alone as a means of isolating low-frequency variability, is that in the ensemble mean there is little skill in a 30-day mean forecast. Examination of the variability of skill among the ensemble members indicates that approximately 40 percent of the perturbed monthly mean forecasts would be useful. Examples of skillful and poor monthly mean predictions am shown and conclusions are drawn as to the implications of the results with regard to the likelihood of success of extended range deterministic forecasts.

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Stephen J. Colucci
and
David P. Baumhefner

Abstract

A set of thirty 30-day mean 500-mb height anomaly forecasts run from National Meteorological Center initial analyses by the NCAR Community Climate Model is examined in order to learn if the forecast accuracy can be estimated with the initial conditions. Defining initial weather regimes by a 500-mb geostrophic zonal index anomaly difference between 50°W and 10°E discriminates between the best and worst 30-day mean forecasts in the sample. Initial regimes characterized by anomalously high zonal index (500-mb geostrophic westerlies) at 50°W and low index at 10°E yield on average lower 30-day mean forecast-observed anomaly correlation than initial regimes with opposite conditions (anomalously low zonal index at 50°N and high index at 10°E). It is suggested that initial regimes with abnormally fast geostrophic 500-mb westerlies at 50°W are followed in time by intense and poorly forecast synoptic-scale cyclones over the Atlantic Ocean. It is shown in a case study that the local synoptic- to planetary-scale interaction, as measured by its contribution to quasigeo-strophic 500-mb height tendencies, is misforecast early in the forecast cycle following these initial conditions. Early rapid synoptic-scale error growth, in this case, is followed by rapid planetary-scale error growth (incorrectly forecast demise of a blocking pattern), deterioration of forecast accuracy, and an unskillful 30-day mean forecast. It is further suggested that initial regimes with abnormally weak 500-mb geostrophic westerlies at 50°W are followed in time by less intense, but better forecast, cyclone waves over the Atlantic Ocean. In a representative case study, the local synoptic- to planetary-scale interaction is well forecast early in the forecast cycle. The local synoptic- and planetary-scale error growths are more restrained (a blocking pattern is correctly maintained), deterioration of forecast accuracy is postponed, and a skillful 30-day mean forecast results. It is hypothesized on the basis of this work that the accuracy of numerical 30-day mean forecasts may depend upon the accuracy with which the cyclones and their interactions with the planetary scale are predicted early in the forecast cycle, and that this accuracy in turn may depend upon the initial conditions.

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