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Elmar R. Reiter and Maocang Tang

Abstract

The diurnal variation of 850 mb heights, the detailed distribution of which could be assessed by the inclusion of surface data, and of resultant winds over, and in the vicinity of, the Great Basin reveals clearly a plateau-wind circulation during summer. This circulation reverses between day and night and appears to include the low-level jet stream over Texas and Oklahoma, as well as the time of occurrence of thunderstorms. This plateau circulation system interacts with local mountain-valley breeze systems. The thickness of the daytime inflow and nighttime outflow layer over the plateau is approximately 2 km.

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Elmar R. Reiter and Abele Nania

Abstract

Cloud-photogrammetric studies conducted from the ground reveal the existence of wave perturbations near tropopause level, of a wavelength the same order of magnitude as is experienced in clear-air turbulence. A case study of the CAT occurrence on 13 April 1962 reveals the importance of the confluence mechanism of two jet streams, and the turning of wind with height in CAT generation.

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Elmar R. Reiter and Daniel Westhoff

Abstract

Ultralong and long planetary waves are analyzed at the 500 mb level in terms of their amplitudes, phases and stationarity characteristics, the latter described in terms of a stationarity index SIn. This index consists of the ratio between planetary-wave amplitudes computed from time-averaged 500 mb height patterns, and the wave amplitudes computed on a daily basis and averaged for the same time interval irrespective of their phase angles. The index assumes the value 1 for completely stationary waves and 0 for randomly variable waves.

Mean 500 mb height and planetary wave characteristics have been developed by calendar day. using NMC data between the years 1946 and 1979. Various significant singularities (i.e., departures from a low-order harmonic seasonal trend) in planetary-wave behavior are described. An index, SIn (d), computed by calendar day, was developed as a measure of probability that a certain planetary wave with number n would achieve its long-term mean phase angle on a certain date. This index, too, shows significant patterns with latitude and season, suggesting the presence of relatively short periods within the seasonal cycles of planetary-wave behavior, during which these waves may be extra sensitive to anomalies in their forcing parameters.

It is suggested that perturbation statistics derived from numerical general circulation models should be compared with the results presented in this study to permit an improved judgement on the veracity of such models.

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Maocang Tang and Elmar R. Reiter

Abstract

Detailed analyses are presented of the temperature and pressure fields of the planetary boundary layer (PBL) and their seasonal variability over the western United States and over the Plateau of Tibet (Qinghai-Xizang Plateau). Over the United States these analyses rely on 850 mb data, augmented by surface data. Over Tibet the 600 mb surface adequately describes the PBL.

The effects of a “plateau monsoon” appear prominently during winter and summer over both plateaus. Together with continental monsoon effects they help to shape prominent circulation features, such as the low-level jet stream (LLJ) over Texas and Oklahoma. The complex, seasonal characteristics of precipitation regimes over the North American continent can be explained, to a large extent, by considering these monsoonal changes in the PBL, especially over the mountains.

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Elmar R. Reiter and Daniel R. Westhoff

Abstract

Gridded NMC data for 500 mb geopotential height, and 300–500 mb and 500–700 mb thickness for the period 1951–78 wore subjected to linear trend analyses. These analyses were performed for each calendar month. Significant geographical and seasonal distributions of cooling and warming patterns emerged. An atmospheric cooling trend over the North Pacific during the winter months appears in a region where oceanic cooling has also been observed, but planetary-wave adjustments rather than ocean-atmosphere feedback mechanisms appear to dominate the atmospheric cooling on climatic time scales. Consistently large temperature trends also appear over the Asian continent. Comparisons between thickness trends in the layer 300–500 mb with those in the. layer 500–700 mb reveal well-pronounced patterns of stabilization and destabilization.

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Elmar R. Reiter and Harry P. Foltz

Abstract

Recent aircraft measurements of clear air turbulence over Australia have shown that the phenomenon of CAT in a thermally stable environment is associated with a breakdown of waves, presumably gravity waves or Helmholtz waves on a stable interface, into random turbulent eddies. The energy distribution in the wavelength range in which clear air turbulence is experienced seems to follow the “−5/3 law” postulated by Kolmogorov's similarity hypothesis. The “−5/3 law” seems to extend to much longer wavelengths than previously anticipated.

Combining these results of aircraft measurements with the theory on lee waves which has been derived by the use of perturbation equations, one finds that the energy involved in standing lee waves over mountains may “cascade” down from a wavelength range of approximately 10 km to a range near 100 m which then would be experienced as clear air turbulence, provided that the energy levels are high enough to cause any responses in an aircraft.

This physical model of turbulence being “fed” by mountain waves has been used in developing a forecasting scheme of CAT over mountains. Results of a preliminary, but very encouraging, study are reported in this paper.

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James F. Bresch and Elmar R. Reiter

Abstract

Observations of extremely high fine particulate sulfur concentrations during early April 1983 in the western United States are linked to a strong cyclone over the midwestern United States. The strong winds around this cyclone circulated polluted midwestern air as far west as the Pacific Coast. A retrograding upper wave pattern was conducive for this polluted air to move southwestward. Both a long-range trajectory analysis and a subjective evaluation of synoptic conditions confirm this hypothesis.

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James P. McGuirk and Elmar R. Reiter

Abstract

A strong, persistent and significant oscillation of about 24-day periodicity is observed in hemispheric-scale energy parameters during the winter season. The characteristics of this cycle are defined using 9.5 years of daily NMC gridded height and temperature fields. Analysis of northward eddy heat flux strongly suggests that baroclinic processes are involved in this oscillation. The vacillation acts primarily on the planetary-wave scale and some of its synoptic characteristics are investigated. A decomposition of eddy available potential energy by zone wavenumber shows that the vacillation tends to favor different wavenumbers in different years. The concept of a vacillation in orientation of trough and ridge line tilt is not substantiated.

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Longxun Chen, Elmar R. Reiter, and Zhiqiang Feng

Abstract

Estimates of the time and space variability of the atmospheric heat source over Tibet are presented for the summer of 1979. These estimates rely on new data from the People's Republic of China allowing a better assessment of the surface heat fluxes, and on new satellite data from Nimbus-7 giving the radiation balance at the top of the atmosphere. Our estimates of the atmospheric heat source turned out to be considerably smaller than those provided earlier in the literature, mainly because of different assumptions of the drag coefficient. The atmospheric heat source over Tibet is mainly modulated by the release of latent heat. Over the southeastern and southwestern plateau regions the heat source appears to be in phase with the precipitation yield of the Indian summer monsoon, whereas central Tibet reveals an out-of-phase behavior. Over western Tibet there appears to be hardly any net import of moisture from outside the region, whereas the maintenance of the hydrological cycle over eastern Tibet requires moisture flux convergence from outside the region of up to 40% of the mean rainfall, in agreement with what is known about the surface hydrology of Tibet.

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Elmar R. Reiter and Deng-Yi Gao

Abstract

The heating of the Plateau of Tibet (the Qinghai-Xizang Plateau) has been deemed to play an important role in the development and movement of the South Asian anticyclone which, to a great extent, affects the Indian monsoon development. In this paper the heating effects are estimated from synoptic data from that region for April 1979. During the transition season warming over the plateau interacts with advected synoptic systems and induces the South Asian high, normally located south of the Himalayas during that season, to develop strongly and shift northward.

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