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Jan Paegle
and
Julia N. Paegle

Abstract

One year of geopotential data obtained from the National Meteorological Center and the National Center for Atmospheric Research are diagnosed for the occurrence of non-elliptic regions with respect to the balance equation. The highest frequencies of such occurrences appear at 200 mb over the subtropical oceans where there are few radiosonde observations. Substantial 200 mb frequencies are also found over the United States in the summer season above a reliable data net. A diagnosis of flow divergence implied for the non-elliptic data by a theoretical analysis of Paegle and Paegle (1974) produces values greatly in excess of. typical observations. This suggests that the gridding of the data by objective analysis may not have been adequate and/or that the aforementioned theory overestimates flow divergence in these regions. It is likely that non-elliptic data are important for initialization of primitive equation forecast models. It may be inferred that greater data accuracy, as well as better initialization techniques within non-elliptic regions, are required.

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Jan Paegle
and
Julia N. Paegle

Abstract

An efficient alternative to the customary balance equation solution procedures is described which gives very similar solutions for those cases when the balance equation is elliptic. This alternative invokes some assumptions that are not usually applied to the nonlinear balance equation, but which are justified by comparisons with the standard solutions to the balance equation in both rectangular and spherical geometries. The solution tends toward a flow with zero absolute vorticity as the pressure field tends toward configurations for which the balance equation is non-elliptic. Such non-elliptic pressure fields correspond to force fields with sufficient positive divergence with respect to space to generate flow divergence. In this case a non-divergent balanced solution may not exist, and is physically meaningless if it does exist, but a reasonable divergent balanced solution can be obtained by the proposed technique.

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Julia N. Paegle

Abstract

Regression techniques are applied on stratified and non-stratified data to obtain prediction equations for the probability of precipitation over the western continental United States. The stratification is based on 500-mb winter types. The equations are tested on an independent data sample and it is found that the stratification leads to considerable improvement of the forecasts.

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Julia N. Paegle

Abstract

A system of three Fourier components in a barotropic channel flow is considered to investigate the influence of topography on a neutral Rossby wave. Analytic solutions show how topography induces a nonlinear oscillation of the mean flow-wave system.

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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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Julia N. Paegle
and
Alexander E. MacDonald

Abstract

Two sets of initial conditions are used to integrate a three-level quasi-geostrophic model in spectral form. After a maximum perturbation kinetic energy is reached, a barotropic exchange in established between the zonal flow and the perturbation with an apparent periodicity from 2 to 4 days. The initial state includes a finite amplitude baroclinic mode which is highly unstable in the linear sense. This mode exhibits a negative growth rate for about the first two days of the integration due to barotropic exchanges with other modes. Spectra of kinetic and available potential energies, enstrophy, and omega2 are presented. The kinetic and available energy display a –3 slope for intermediate, scales for the initial integration period when perturbation kinetic energy is actively growing at the expense of the mean flow available energy. The effect of the variation of stratification with height, dissipation, heating, mountains, and truncation of the spectral system is discussed.

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Julia N. Paegle
and
Kingtse C. Mo

Abstract

A reconstructed rainfall dataset, and satellite estimates are used to analyze interannual to decadal variability of austral summer precipitation over South America. Rotated empirical orthogonal function (REOF) analysis is applied to isolate dominant patterns of rainfall. Links of these patterns to sea surface temperature anomalies (SSTAs) are examined.

The leading mode is related to El Niño–Southern Oscillation (ENSO), which explains 12% of the total variance. During warm ENSO events, the positive phase of this mode shows dry conditions over northern South America and wet conditions over the subtropical plains between 25° and 35°S. The situation reverses during cold events. The second REOF 2, which explains about 10.8% of the total variance, consists of positive loadings over northeast Brazil centered at 50°W near the equator and negative loadings over Colombia and the subtropical plains. For December–January–February (DJF), REOF 2 is influenced by tropical South Atlantic SSTAs through displacements of the intertropical convergence zone. Northeast Brazil receives most rainfall in March–April–May (MAM) and it is modulated by both the Atlantic SSTAs and ENSO. In the interannual frequency band, the North Atlantic Oscillation (NAO) has very limited influence on rainfall. On the decadal timescales, the NAO leads REOF 2 by three years.

Latitudinal variations of tropical convection are through the joint contribution of REOF 2 and REOF 4. REOF 4 is similar to REOF 2, but centers are displaced about 10° south. When these two EOFs are both positive, central South America is wet. The amplitudes of REOF 2 and REOF 4 are small during the mid-1950s to the mid-1960s and they are out of phase from 1968 to 1970, periods with persistent dry conditions over the upper La Plata River basin.

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Julia N. Paegle
and
Lawrence P. Kierulff

Abstract

An objective correlation method is used to obtain seven typical 500-mb winter flow Patterns Over the western United States and eastern Pacific Ocean. The frequency of occurrence of each type is presented. The atmospheric configurations of each type are described in terms of 850-, 500- and 300-mb heights, 850-mb dewpoint depressions, 500-nib absolute Yorticities and vertical velocities, relative frequency of precipitation, and other related fields. The distinctness of the types over different geographical locations is discussed. The relative frequency of precipitation at 42 western stations is well differentiated by the seven types. This suggests that the stratified relative frequencies could he used as a first refinement over simple climatology when other more accurate quantitative precipitation forecasts are not available.

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Julia N. Paegle
and
Reed B. Haslam

Abstract

Predictability times are estimated from 500 mb heights resolved for horizontal scales from 5 × 106 to 8 × 106 m. These scales are defined through eigenvector analysis of seven years of winter data over a portion of the Northern Hemisphere centered over the western United States.

An analysis of variance suggests characteristic times (T 0) of 6–12 days for effectively independent heights. A random process is chosen which fits well the frequency spectra of the time-dependent coefficients. The estimates of T 0 calculated from this random model are about half of those obtained from the variance analyses. These differences might be due to sampling fluctuations as to the existence of long-period oscillations poorly represented by the random process. Both methods show a decrease in T 0 with decreasing horizontal scale.

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Julia N. Paegle
and
Robert P. Wright

Abstract

A nonparametric statistical technique is developed which is capable of interfacing with dynamic atmospheric models to produce probability of precipitation forecasts. The technique is implicitly nonlinear, utilizing an efficient algorithm to represent joint probability densities as N-dimensional histograms. Operationally, the method can be updated on a daily basis.

The technique is tested in an atmospheric data sample. Initial predictor selection is based on a general nonlinear approach applying fields of the sample correlation ratio. An iterative algorithm optimization technique is described and utilized. The technique is tested and shown to have significant skill relative to climatology.

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