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Paul R. Julian

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Paul R. Julian

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Anomalously high stratospheric temperatures were measured by rocket sounding techniques over McMurdo, Antarctica, in midwinter 1963. Because of the importance of these observations in determining whether the Southern Hemispheric circulation exhibits midwinter stratospheric warming events similar to those in the Northern Hemisphere, data sources independent of the rocket soundings are examined. The data from the 15-μ radiometer aboard TIROS VII, capable of determining the gross features of stratospheric temperature patterns, are used to check the rocket temperatures. Winds at McMurdo measured from rocketsonde techniques are used to determine the direction and magnitude of the 28-km thermal wind, which is compared with observed (rocket and radiosonde) temperatures. The TIROS VII radiometer data and the thermal wind and temperature gradient data are shown to be inconsistent with the high temperatures measured by the rocketsonde. Some suggestions are made as to the reasons for this apparent inconsistency.

Finally, a plea is made for adoption of a standard terminology for stratospheric warmings. The conclusion is made that no major stratospheric warming has yet been satisfactorily observed in the Southern Hemisphere.

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Paul R. Julian

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Paul R. Julian

A major component of the Tropical Observing System deployed during the Global Weather Experiment was the Aircraft Dropwindsonde System. The emphasis in this article is upon the processing of the data gathered by this system and upon factors that influence the quality of the data. The wind-finding algorithm is presented together with a description of the interactive raw data editing procedures. Some examples of drops illustrating various points are included.

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Paul R. Julian

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The tropical wind field presents some unique problems to an assimilation scheme designed for extratropical latitudes. Foremost among these is the relative increased importance of the divergent component of the upper and lower troposphere. Examination of tropical wind field analyses from conventional assimilation schemes points to some unsatisfactory results. Because of these problems an algorithm is proposed which is intended to handle the rotational and divergent components separately. The divergent component is estimated by transforming satellite-observed outgoing longwave radiation to a velocity potential field. The full analysis is then obtained by statistical optimum interpolation (OI) using a guess field which is an addition of a rotational forecast field with the divergent field estimated above. The scheme is tested against a conventional (single-level) OI scheme for a number of cases selected from the FGGE year. The proposed scheme is shown to perform comparably to the conventional OI scheme in regions of dense conventional observed data and, by induction, to be superior in areas of insufficient data.

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PAUL R. JULIAN

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The primary concern of this paper is an examination of the utility of spectral techniques in detecting wave modes of the large-scale tropospheric circulation. Sample time- and space-spectra of meridional winds in middle latitudes are presented. A maximum in the spatial spectra of the transient waves at hemispheric wave numbers 5–7 is shown, in agreement with similar, previously published spectra. The temporal spectra, unlike the spatial spectra, fail to show any significant features that can be attributed to the passage of baroclinic disturbances. (In this paper, the frequency range considered is limited to 0.03–0.50 per day.) The continuum of the coherence statistic between lower and upper tropospheric wind components is highly significant, but no significant “peaks” or maxima exist.

Because of recent interest in the dynamics of tropical regions, spectral techniques are employed on meridional wind data for five tropical stations with record lengths of from 3 to 7 yr. At 850 mb, the two stations in the west-central Pacific possess spectra with a peak in the 4–5-day period range. Similar spectra for two stations in the Atlantic Ocean and one in Indonesia do not exhibit this feature, and at 200 mb none of the five spectra has any indication of a significant peak at any frequency. The coherence between the two levels is not significant at any frequency, with the possible exception of the 0.20–0.25 per day band at the Pacific stations. As in the case of the midlatitude spectra, the peak in the spatial spectra of the transient 200-mb meridional winds does not possess a counterpart in the temporal spectra.

Finally, complex demodulation is used to investigate possible nonstationary aspects of the 4–5-day period wave mode for the Pacific stations. The results suggest nonstationary behavior with an annual modulation but of a sporadic nature. In addition, the demodulation procedure provides information on the questionable significance of the coherence between the lower and upper tropospheric tropical meridional winds.

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PAUL R. JULIAN

Abstract

Principal components or empirical orthogonal functions are virtually the sole statistical tool used to date for investigations of space-time variability of meteorological elements. Maximum statistical efficiency and possible physical interpretation of empirical orthogonal functions derives from the assumptions of stationarity and homoscedasticity of the scalar variables in space and time. In this study, a rank table technique is given in which temporal data from a number of stations is ranked time-wise, and rank sums for each time obtained by summing ranks over the total number of stations. The technique offers some advantages for investigations of joint space-time variability. First, it is nonparametric; second, analysis of variance schemes are simplified; and third, a test of homoscedasticity can easily be performed. Networks of streamflow and precipitation data over the conterminous 48 States are used to illustrate the use of the technique. As a result, streamflow and precipitation data are shown to be spatially heteroscedastic—dry periods are better correlated spatially than wet periods. A runs test on the temporally varying rank sums suggests that while precipitation is not temporally heteroscedastic (dry and wet periods are both essential randomly distributed), streamflow data might be. Apparently, years of deficient streamflow tend to be persistent while years of excessive streamflow are essentially randomly distributed in time.

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PAUL R. JULIAN

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In order to examine quantitatively the evidence for an index cycle, 21 years of daily 700-mb. zonal index data for the western longitudes of the Northern Hemisphere were subjected to cross-spectral analyses. The annual variation of the mean and standard deviation of the three zonal indices (polar, temperate, and subtropical) was removed by a standardizing procedure utilizing time-smoothed annual curves.

The variance (power) spectra of the individual indices indicated that their correlation structure can be closely approximated by a first-order Markov scheme. No evidence was found for an excess of variance in the range of period (3 to 8 weeks) assigned to the index cycle by subjective means. The cospectra all indicated negative covariability among the indices as would be expected from circulation patterns characteristic of index-cycle-like behavior, but none showed an excess of negative covariability within the period range of interest. Coherence values, indicating the degree of relationship among the various indices, also did not show maxima in the required period range.

The distributions of the standardized indices were found to be slightly skewed in the case of the subtropical and temperate indices and normal in the case of the polar index. In addition, a significant seasonal variation in the correlation and cross-correlation structure of the indices was found. Such nonstationarity complicates the interpretation of the cross spectra.

The conclusion of the statistical study of the zonal index data is that there is no evidence for sequential variations and covariations of the strength of the westerlies at various latitudes with any preferred range of period. Rather, significant relationships hold over a very broad range of period.

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Roland A. Madden
and
Paul R. Julian

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Observational aspects of the 40–50-day oscillation are reviewed. The oscillation is the result of large-scale circulation cells oriented in the equatorial plane that move eastward from at least the Indian Ocean to the central Pacific. Anomalies in zonal winds and the velocity potential in the upper troposphere often propagate the full circumference of the globe. Related, complex convective regions also show an eastward movement. There is a zonally symmetric component to the oscillation. It is manifest in changes in surface pressure and in the relative atmospheric angular momentum. The oscillation is an important factor in the timing of active and break phases of the Indian and Australian monsoons. It affects ocean waves, currents, and air-sea interaction. The oscillation was particularly active during the First GARP (Global Atmospheric Research Program) Global Experiment year, and some features that were evident during the Monsoon Experiment are described.

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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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