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TOMISLAVA VUKICEVIC and JAN PAEGLE

Abstract

The influence of one-way interacting lateral bounday conditions upon the predictability of flows in boundeddomains is studied using the barotropic nondivergent model in global and local domains. Past studies haveattempted to reconcile the apparent contradiction between pessimistic forecast of predictability theory and thehigh predictability actually found in regional models. Those investigations have emphasized the rather differentspectra and forcing mechanisms that are not considered in the theoretical estimates. We demonstrate that thepredictability remains high in an unforced, inertially driven local flow characterized by a typical synoptic scalespectrum, and constrained only by lateral boundary specification. We also offer a possible reconciliation ofthese results with the classical theory. It is shown that one-way interacting boundary conditions enhance thepredictability of flow in a local region which, without the boundary constraints, has limited predictabifity. Thedegree of this boundary constraint is dependent on the size of the domain, on the nature of flow in the domainand on the scale structure of the error field. The boundary constraint is particularly strong when a substantialportion of the larger scale flow in the domain is imposed through the boundary condition. In that case, smallscale initial uncertainties have limited interaction with the basic flow field because of scale separation andbecause the largest scales in the domain do not react to internal dynamics.

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Jan Paegle, Julia N. Paegle, and Hong Yan

Abstract

Among the various energy transfer mechanisms that might be relevant for teleconnections between tropical and higher latitudes, Rossby wave propagation is certainly one of the most important. In view of this, it is of interest to understand how the propagation of Rossby waves might be affected by ambient flows which locally may be unable to sustain oscillations due to vorticity gradients. This concept is re-examined based on the observation that there are large areas over the Pacific where the upper tropospheric absolute vorticity and its horizontal gradient are small. These key areas for teleconnections might be suspected to be unfavorable to the local propagation of Rossby waves. Results of integrations of the barotropic vorticity equation on the sphere are presented to show the role that regions with small absolute vorticity gradient play in this problem. Implications regarding the maintenance of blocks and the influence of divergent effects are also addressed.

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Jan Paegle and David W. Mclawhorn

Abstract

We document the development and Sensitivity testing of a numerical model designed to predict diurnal cycles of boundary-layer flows of synoptic horizontal scale above sloping terrain. This application requires detailed vertical resolution of low-level jets, mixed layers and sharp inversions above terrain with large pointwise variations. The truncation errors of the pressure gradient term are of special concern. This term is computed using a thermodynamic state carried as a deviation from a standard state. Turbulent mixing is based upon a simplified diagnostic treatment. The final model produces rather stable solutions because of the rather careful pressure gradient calculation and strong physical dissipation.

Sensitivity testing indicates that the model prediction of diurnal convergence cycles depends upon soil parameters. Results are also sensitive to absolute rotation and mixing parameterizations, but not equally sensitive to longwave radiative flux divergence. Applications over complex North America terrain and coasts produce boundary-layer ascents that are generally in phase with observed summer diurnal thunderstorm distributions, where data on the latter are available. Equatorward of ∼30° latitude, the phase is ∼3–6 h retarded with respect to the response poleward of 30°.

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Jan Paegle and Wayman E. Baker

Abstract

Time averages of the latitudinal distribution of kinetic energy and terms of the kinetic energy equation are presented as depicted by the Goddard Laboratory for Atmospheric Science (GLAS) analyses of the First GARP Global Experiment (FGGE) during the First Special Observing Period (SOP-1). Monthly averages display peaks in the stationary wave energy at 30°N and in the tropics. Global decompositions of the streamfunction and velocity potential in spherical harmonics are truncated at the fourth degree. The kinetic energy distribution of the associated wind field displays peaks in the tropics and northern mid-latitudes in January, but only a tropical peak in February.

The Eliassen-Palm relationship for latitudinal momentum and geopotential wave transport appears to have some support in these analyses, particularly in the mid-latitudes. In the deep tropics, latitudinal convergence of the fully resolved stationary wave momentum transport accompanies latitudinal convergence of stationary wave geopotential flux in westerly flow, in contradiction to this relationship.

Such results make it difficult to interpret time averages in terms of idealized wave propagation and linear theories. However, there is an interesting correspondence between observed weekly averages of the global-scale kinetic energy and the zonally averaged wind.

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JAN PAEGLE and DAVID W. McLAWHORN

Abstract

An analysis is made of 11 Great Plains nocturnal thunderstorm occurrences that have no obvious synoptic scale support. The pressure patterns and local terrain configurations are input to a numerical boundary-layer model that computes the vertically integrated boundary-layer convergence. The time and space phasing of the vertical velocities thus obtained are in good agreement with the time and space phasing of all 11 thunderstorm occurrences for the beginning of the activity, but they are acceptable in only 6 cases for the termination of the activity. There is a tendency for the model to forecast boundary-layer convergence where no thunderstorms occur; but in many such cases, stability and humidity data are unfavorable for thunderstorm activity.

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Gonzalo Miguez-Macho and Jan Paegle

Abstract

In this study the impact of initial uncertainty in localized regions on midrange forecast sensitivity over North America is studied. The local regions considered are the North American domain and two areas upstream, one covering the northeast Pacific and another extending farther west to include most of the North Pacific. The University of Utah Global Model and an estimate of initial uncertainty given by the differences between ECMWF and NCEP reanalyses are used. Control runs are performed with NCEP initial data globally. The effect of initial uncertainty on the control forecasts is simulated by a change of initial data from NCEP to ECMWF reanalysis first globally, and then only inside or only outside the selected domains. The impact of local initial uncertainty is quantified in comparison to the impact of initial uncertainty over the whole globe. Results from 17 cases show that regional state differences are less important than global state differences, unless the considered region covers most of the North Pacific.

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Jan Paegle and J. E. Geisler

Abstract

The low-level circulation in summer over the western Indian Ocean is characterized by southeast trades that are channeled near the African coast into a concentrated southerly flow across the equator and thence north-eastward into the southwest monsoon over the Arabian Sea. It is widely accepted that deflection by the East African highlands is responsible for this flow configuration. Existing theoretical models to a greater or lesser extent, build in this deflection by imposing a western boundary extending all the way to the top of the fluid or by prescribing longitudinally dependent sources and sinks for driving the flow.

The purpose of this study is to determine what flow configuration occurs when these constraints are removed. For this we use zonally symmetric forcing to drive a planetary boundary layer model formulated in a terrain-following coordinate system that permits fluid to flow over as well as along, a topographic barrier. The results support the conclusion that East African topography alone can channel incident flow into a pattern with most of the observed features. An analysis of the diurnal oscillation in the model suggests a mechanism for the diurnal variation of low-level wind observed in northeastern Somalia.

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Jan Paegle and Wayman E. Baker

Abstract

The influence of tropical latent heating on the short-to-medium-range numerical prediction of ultralong waves is examined. Two integrations of the GLAS general circulation model are made from the came initial state. One forecast utilizes the full model physics, the other contains no latent heating in the tropical belt from 20°S to 20°N.

The tropical and subtropical divergence fields react to differences in the latent heating within half a day. Differences in the rotational wind field within these latitudes are noted in about three days, while at mid-latitudes (30°–60°) the influence is not felt until five days. At higher latitudes the five-day predictions of a strong North Atlantic block with and without latent heating are quite similar.

Although the greatest changes in the heating field occur in the Southern Hemisphere, the largest temperature, height and wind change take place in the Northern Hemisphere. It appears that the upper troposheric tropical westerlies are sustained in the model by tropical heat sources, and reverse to easterlies when these sources are removed.

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Gonzalo Miguez-Macho and Jan Paegle

Abstract

A global research model is initialized with reanalysis datasets obtained from NCEP and ECMWF. The globally averaged accuracy of the resulting 120-h predictions varies little between the different initializations, but a perceptible difference arises in the mid- to high latitudes of the Southern Hemisphere, where ECMWF initialized forecasts have somewhat greater skill. Most of this benefit is explained by differences of the longer-wave components (wavenumbers 0–15) of the initial data. This motivates further diagnoses of globally computed sensitivity measures to initial data changes. Approximately 67% of the 120-h forecast difference produced by changing initial data from ECMWF to NCEP reanalyses is due to initial changes only in wavenumbers 0–15, and more than 85% of this difference is produced by initial changes in wavenumbers 0–20. The result implies downscale uncertainty growth and contradicts several recent predictability investigations based upon singular vector analyses, which emphasize upscale uncertainty growth. The results do not imply that singular vector analyses are in error. They only suggest that large-scale errors of the initial state may play a more prominent role than suggested in some singular vector analyses. Downscale uncertainty evolution may be due to greater analysis uncertainty at large spatial scales than considered in prior recent studies emphasizing upscale predictability loss.

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Jan Paegle and Wayman E. Baker

Abstract

The vertical and temporal Structure of the global scale flow in the Goddard Laboratory for Atmospheric Sciences analyses of data from the First GARP Global Experiment are presented. The fields are represented in terms of spherical harmonic expansions of the streamfunction, velocity potential and geopotential. The global scale patterns are obtained from triangular truncations of such series, neglecting terms past the fourth degree.

Some modes display prominent high frequency oscillations in the velocity potential and geopotential height that may be related to diurnal cycles of cumulus convection. Such oscillations are not apparent in the streamfunction. Low-order harmonics with nodes only along latitude circles are nearly equivalent barotropic. However, those harmonics that have no nodes between the poles reverse phase with height.

The following conclusions are drawn regarding the global scale patterns: 1) Since the divergent component of the meridional flow is not much smaller than the rotational part, the global scale pattern resembles forced modes of linear tidal theory more closely than it resembles free modes. 2) Because the vertical structure reverses, longitudinal heating gradients are probably important to the forcing. 3) The high frequency oscillations of the velocity potential and height field imply high frequency components in the forcing due to heating. 4) Monthly and weekly averages display suggestive teleconnection patterns.

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