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J. M. Wallace

Abstract

Hourly data on the frequencies of all types of precipitation events, heavy precipitation events, trace precipitation events and thunderstorms for more than 100 stations in the United States were processed to generate statistics on the amplitude and phase of the diurnal and semidiurnal cycles at each station. Results are displayed on seasonal maps in a vectorial format that emphasizes the large scale geographical consistency of the diurnal variations.

During summer each of the four parameters listed above displays a distinctive geographical pattern of diurnal variations. Thunderstorm frequency tends to be the most strongly modulated by the diurnal cycle; trace precipitation the least strongly modulated. Over the central United States the maximum frequency of severe convective storms occurs during the early evening; thunderstorms exhibit their maximum frequency around midnight, while most precipitation falls later in the night. These amplitude and phase differences offer some insight into the relative importance of thermodynamical and dynamical processes in controlling the frequency and intensity of convective activity.

During winter heavy precipitation and thunderstorms are biased toward nighttime over much of the midwest and Atlantic seaboard. Trace precipitation exhibits a small but geographically consistent diurnal oscillation with a peak near or slightly after sunrise. It is suggested that this morning peak is associated with precipitation from low stratus decks.

The semidiurnal cycle is generally smaller than the diurnal. Effects of the S2 pressure wave are clearly evident over much of the tropics, but over middle latitudes they are often obscured by regional and local influences.

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J. Shukla and J. M. Wallace

Abstract

A number of recent observational and theoretical studies indicate that under certain conditions tropical sea surface temperature anomalies may be capable of producing climatic anomalies at extratropical latitudes. According to the hypothesis put forth in these studies, perturbations in the climatological mean distribution of precipitation in the tropics can influence the extratropical circulation through the action of forced, quasi-stationary, two-dimensional, Rossby wavetrains, which may tend to excite the fastest growing normal mode associated with barotropic instability.

Results of previous GCM experiments designed to simulate the atmospheric response to SST anomalies are re-examined in light of this hypothesis and are found to be generally consistent with it.

A modeling investigation consisting of three separate GCM experiments was carried out using the GLAS climate model with January initial conditions based on observed data and an equatorial Pacific sea surface temperature anomaly based on the recent analysis of Rasmusson and Carpenter (1982).

The observed eastward shift of the belt of heavy convective precipitation in the western Pacific during the episodes of positive sea surface temperature anomalies was correctly simulated in all three experiments. Associated with this, the ascending branch of the Walker circulation also shifted eastward and the north-south overturning intensified in the central Pacific. The 300 mb height difference field, averaged for the three pairs of experiments, showed evidence of two-dimensional Rossby wave propagation along a great circle path, poleward over the North Pacific and eastward across the North Pacific, in agreement with observations and with the results of experiments with simpler models. However, there was considerably variability from experiment to experiment and there were unexpected features over Eurasia in the average difference pattern. As in previous experiments, the simulations with sea surface temperature anomaly produced an increase of low-level westerlies along the equator, slightly to the west of the enhanced rainfall. It was the moisture convergence, associated with the anomalous low-level circulation rather than the local changes in evaporation, which accounted for most of the simulated changes in precipitation. The simulated circulation changes at extratropical latitudes exhibited an equivalent barotropic vertical structure, in agreement with observations. The sea level pressure associated with warm sea surface temperature anomalies was lower over the eastern Pacific and higher over the western Pacific and the Indian Ocean in agreement with the observed Southern Oscillation.

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R. Quadrelli and J. M. Wallace

Abstract

The low-frequency (>5 day period) variability observed within four different subsets of the climatology (H1, L1, H2, and L2) as defined by the high and low index polarities of the two leading principal components (PCs) of the sea level pressure field is compared, with emphasis on distinctive flow configurations and teleconnection patterns. The analysis is based on wintertime 500-hPa height, sea level pressure, and 1000–500-hPa thickness fields derived from the NCEP–NCAR reanalyses for the period of record, 1958–99.

“Spaghetti diagrams” display specified contours for ensembles of individual 10-day mean charts extracted from the four different subsets of the climatology. In L1, 10-day mean maps (weak zonal flow at latitudes ∼55°N) exhibit larger undulations in the barotropic component of the flow than those in H1, implying larger particle displacements and deeper penetration of Arctic air masses, particularly into Europe and the eastern United States. Maps in H2 and L2, separated in accordance with the Pacific–North American (PNA)-like second mode, exhibit quite different kinds of planetary wave patterns. The L2 subset (characterized by a retracted Pacific jet) exhibits greater variability over the Gulf of Alaska and over northern Europe.

Cold air outbreaks in Europe occur more frequently in L1 than H1, and over western North America, they occur more frequently in L2 than H2. The cold anomalies associated with low polarities of both PCs are observed more frequently than expected based on linear correlation; within the individual subsets of the climatology there are suggestions of multiple circulation regimes; teleconnection patterns for the subsets of the climatology are also discernibly different. These results constitute evidence of nonnormal or nonlinear behavior of 5- and 10-day mean fields and provide indications of how the intraseasonal variability depends on the mean state of the flow in which it is embedded.

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J. M. WALLACE and D. B. PATTON

Abstract

Vertical profiles of 12-hr temperature difference are examined for evidence of structure related to the topographically forced diurnal wind variations. Wavelike features are found with amplitude of the order of 1°C and wavelengths of the order of 5–15 km. These values agree with estimates inferred from the tidal wind field on the basis of continuity considerations.

The diurnal temperature range in the planetary boundary layer decreases exponentially with height, with the decay rate varying markedly from one geographical region to another.

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J. M. Wallace and C-P. Chang

Abstract

The study consists of two parts: a detailed investigation of wave disturbances in the tropical lower troposphere during a single 6 month period, and a brief survey of wave activity at a single station during four successive 6-month periods. Cross spectrum analysis of wind, temperature, relative humidity and surface pressure data reveals the existence of at least three types of disturbances:

  1. Easterly waves, with periods in the range of 4–5 days and horizontal wavelengths on the order of 3000 km. Contrary to the results of earlier studies, the axes of the waves show no inclination with height. There is some indication of a cold core structure, but the temperature fluctuations in the waves are very small. Some stations show a tendency for high relative humidities to occur in the troughs of the waves, in agreement with earlier studies.
  2. Low-frequency oscillations with periods >10 days and horizontal wavelengths on the order of 10,000 km. These are most strongly evident in the zonal wind component, for which they account for most of the variance during the period of the study.
  3. Planetary-scale pressure fluctuations with periods near 4 days and amplitudes on the order of 1 mb at the equator.

The relative importance of the easterly waves and the low-frequency oscillations as measured by their contributions to the total variance is found to vary considerably from one year to another and from one season to another.

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V. E. Kousky and J. M. Wallace

Abstract

The descent of westerly wind regimes of the quasi-biennial oscillation in the equatorial stratosphere is marked by two regions of strong zonal wind fluctuations in the 8–20 day period range: one in the easterly regime below the westerly shear zone and another in the upper level westerlies. There is a distinct minimum wave activity within the shear zone. The lower waves are identified as Kelvin waves, while the upper waves are shown to require the existence of a meridional wind component.

The vertical transport of zonal momentum by the waves is computed and compared to the balance requirements. It is shown that the distribution of momentum flux divergence due to Kelvin waves is in good agreement with the observed zonal accelerations during the descent of westerly regimes of the quasi-biennial oscillation. The Kelvin waves give up much of their momentum to the zonal flow in the shear zone which marks the leading edge of the descending westerly regime, even though they apparently do not encounter a “critical layer” within this zone. The occurrence of strong Kelvin waves appears to be confined to periods of descending westerly regimes.

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Justin J. Wettstein and John M. Wallace

Abstract

Month-to-month storm-track variability is investigated via EOF analyses performed on ERA-40 monthly-averaged high-pass filtered daily 850-hPa meridional heat flux and the variances of 300-hPa meridional wind and 500-hPa height. The analysis is performed both in hemispheric and sectoral domains of the Northern and Southern Hemispheres. Patterns characterized as “pulsing” and “latitudinal shifting” of the climatological-mean storm tracks emerge as the leading sectoral patterns of variability. Based on the analysis presented, storm-track variability on the spatial scale of the two Northern Hemisphere sectors appears to be largely, but perhaps not completely, independent.

Pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy-forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability. The North Atlantic Oscillation (NAO)–Northern Hemisphere annular mode (NAM) is associated with a pulsing of the Atlantic storm track and a meridional displacement of the upper-tropospheric jet exit region, whereas the eastern Atlantic (EA) pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper-tropospheric jet. Analogous patterns of storm-track and upper-tropospheric jet variability are associated with the western Pacific (WP) and Pacific–North America (PNA) patterns. Wave–mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere. The Southern Hemisphere annular mode (SAM) is associated with a latitudinally shifting storm track over the South Atlantic and Indian Oceans and a pulsing South Pacific storm track. The patterns of storm-track variability are shown to be related to simple distortions of the climatological-mean upper-tropospheric jet.

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Vincent J. Schaefer and Wallace M. Taylor

The operation, in a high flying airplane, of a simple 16 mm time lapse motion picture camera produces an excellent record of the cloud systems and other visual atmospheric patterns existing over large areas of continent and ocean.

Over the past several years such records were obtained from both jet and propeller driven planes through the cooperation of the Boeing Airplane Company and the Air Weather Service, U.S.A.F. with the Munitalp Foundation.

For best results it was found that the camera should be mounted at right angles or at 45 degrees with respect to the flight path and with a downward tilt of from five to twenty degrees depending on the cruising height of the plane. A two second interval was found to be suitable for good results with planes with an average speed in the range of 300–600 mph. Intervals of four to even ten seconds are permissible at these speeds for economy or convenience. When such a sequence is viewed as a movie, near clouds move by at a different rate from those in the middle or far distance thus giving an excellent illusion of three dimensions. Successive frames may be selected from a sequence and viewed in a stereoscope to get a three dimensional view of the area photographed. When projcted at sound speed in a movie projector, a film made with a two second interval sequence presents the illusion that the viewer is travelling through the sky at a speed of nearly 29,000 mph.

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Kevin J. Rennert and John M. Wallace

Abstract

Variability in daily wintertime [December–February (DJF)] 500-hPa heights on low [L: <(30 day)−1], intermediate [M: (6–30 day)−1], and high [H: >(6 day)−1] frequencies is examined using 40-yr ECMWF Re-Analysis (ERA-40) data. Leading EOFs of L correspond to planetary-scale teleconnection patterns; those of M to retrograding, eastward-dispersing long waves oriented along great circle routes; and those of H to baroclinic waves in the climatological-mean storm tracks. In the Atlantic sector, EOF 1 of M appears to be embedded in EOF 1 of L.

Cross-frequency coupling between L and M exhibits distinctive patterns. In the Atlantic sector the negative polarity of the North Atlantic Oscillation (NAO) with above-normal heights over Greenland is associated with enhanced M variability over Greenland. An analogous relationship is observed in the Pacific sector between an NAO-like pattern and the variance of M over Alaska. Cross-frequency coupling between L and H in both sectors is indicative of a reinforcement of the background flow by the baroclinic waves. Cross-frequency coupling between L and M is responsible for most of the skewness of the anomalies in the 500-hPa height field.

Linear wave dynamics evidently play an important role in M. Composites of high amplitude anomalies of contrasting signs over Baffin Bay exhibit similar spatial structures (apart from the sign reversal) and they exhibit a similar evolution, with westward phase propagation and downstream development characteristic of the behavior of Rossby waves. It is argued that teleconnection patterns exhibit memories much longer than the 7–10-day decorrelation time of daily indices formed by projecting unfiltered daily fields onto their spatial patterns.

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J. M. WALLACE and F. R. HARTRANFT

Abstract

Annual mean 12-hr wind differences are computed from monthly mean wind statistics at 27 levels between the surface and 10 mb for 105 radiosonde stations that have extended periods of record. The resulting wind difference vectors for 0000–1200 gmt and 0300–1500 gmt are plotted on constant pressure charts. At polar latitudes in both hemispheres, a simple pattern is observed, with flow directed across the Pole from the daytime hemisphere to the nighttime hemisphere at all levels. At low and middle latitudes, the wind difference patterns are strongly related to topography, even at stratospheric levels. Land-sea contrasts and terrain slope appear to be the controlling influences.

Analysis of hodographs at individual stations shows that the topographically induced tidal fluctuations propagate downward at all levels above 1 km. This is indicative of an upward flux of tidal energy from the planetary boundary layer.

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