Richard J. Reed

The Structure and Properties of African Wave Disturbances as Observed During Phase III of GATE

Abstract A compositing method is used to determine the average structure and properties of eight wave disturbances observed over west Africa and the eastern Atlantic during the period 23 August-19 September, 1974, a period marked by well-developed and regular wave activity. The disturbance centers propagated westward in the zone of cyclonic shear to the south of the 700 mb easterly jet, located at 16–17°N. The mean wave- length was about 25M km and the mean period 3.5 days. The mean zonal current satisfied the necessary condition for barotropic instability. The composite disturbance was most intense at 650 mb, being cold core below and warm core above. Two circulation centers were evident at the surface, one located below the upper center and the other displaced 10° to the north at about the latitude of the monsoon trough. When separate composites were constructed for land and ocean stations, the dual centers were found to be primarily a land phenomenon. Distinctive features of the high-level (200 inb) ci...

Structure and Properties of Synoptic-Scale Wave Disturbances in the Equatorial Western Pacific

Abstract A compositing technique is used to obtain the average structure of 18 disturbances which traversed an area in the equatorial western Pacific during the wet season (July–September) of 1967. Principal emphasis is placed on the wave properties in the triangular area described by Ponape, Kwajalein and Eniwetok within which it was possible to measure divergence and vertical motion and to compute moisture and heat budgets. Meridional wind maxima of nearly opposite phase occurred in the lower and upper troposphere. Negative temperature deviations were found in the vicinity of the wave trough at low and high levels; positive deviations were observed at intermediate levels. Highest relative humidities occurred in the trough region. This was also the region of strongest upward motion and greatest rainfall and cloud amount. The maximum upward velocity of 2.5 cm sec−1 was found at 300 mb. Convergence was strongest in the sub-cloud layer; divergence was concentrated near 175 mb. The maximum anticyclonic vorti...

Structure and Properties of Synoptic-Scale Wave Disturbances in the Intertropical Convergence Zone of the Eastern Atlantic

Abstract Data from a dense network of ship observations are used to study the structure and properties of westward-moving wave disturbances observed in the eastern Atlantic Intertropical Convergence Zone (ITCZ) during Phase III of the GAPP Atlantic Tropical Experiment (GATE). Comparisons are made with similar disturbances found in the ITCZ of the western Pacific. Wave fields are determined by fitting low-order polynomials to the ship data with use of the method of least squares. The wave structures in the two regions are found to be similar in many respects, the principal difference being in the divergence field and associated vertical motion. Unlike in the Pacific a multi-layer divergence pattern exists in the eastern Atlantic, leading us to hypothesize the existence of three main cloud populations with outflow levels near 800, 500 and 250 mb. The soundings for the Atlantic exhibit lesser parcel instability then the Pacific soundings in agreement with the reduced vigor of the convective cells and the gre...

A STUDY OF A CHARACTERISTIC TPYE OF UPPER-LEVEL FRONTOGENESIS

Abstract The potential vorticity on isentropic surfaces is used to study a characteristic type of upper-level frontogenesis — the development of a sloping stable layer marked by strong vertical wind shear and rapid upward decrease in humidity. In the case studied it was found that the intense portion of the frontal zone consisted of a thin wedge of stratospheric air which had descended to very low levels (700 to 800 millibars), the frontal boundaries being a folded portion of the original tropopause. The circulation within the frontal zone was indirect solenoidal, and surface cyclogenesis accompanied or slightly preceded the strengthening of the upper-level front. Although the frontal zone formed entirely within a polar air-mass, the strong adiabatic heating at and near the warm boundary could give the false impression that tropical air was present there at the end stage.

The Energetics of African Wave Disturbances as observed During Phase III of GATE

Abstract Fields of the meteorological variables in composite wave disturbance constructed for the region from IOOE to 31°W and 1°S to 26°N and for land and ocean subregions are used to diagnose energy transformations in African waves. The composites are based on data contained in the GATE Quick Look Data Set for the period 23 August to 19 September, 1974. The measurements indicate that for the region as a whole the kinetic energy of the waves is maintained almost equally by conversions from zonal kinetic energy and eddy available potential energy. Eddy available potential energy is supplied by the zonal available potential energy at a comparable rate. From the measured conversion rates it is estimated that in the absence of friction the kinetic energy of the waves would double in about 3 days. Measurements for the subregions show that the conversion from zonal to eddy kinetic energy is stronger over the limited oceanic region considered than over the land, while conversely, the conversion of eddy availabl...

A CONTRIBUTION TO THE PROBLEM OF STRATOSPHERIC DIFFUSION BY LARGE-SCALE MIXING1,,2

Abstract The assumption is commonly made in stratospheric diffusion studies that the large-scale eddy flux of various properties obeys the Fickian law which requires that the flux in a coordinate direction is proportional to and in the direction of the gradient. Measurements of eddy fluxes of ozone and sensible heat reveal, however, that the law is violated in the lower stratosphere where the meridional fluxes are countergradient at most latitudes. Use is made of the mixing length hypothesis to derive a more general expression for the large-scale eddy flux which is able to account for the phenomenon of countergradient flux. It is found that the critical parameter in determining whether the horizontal flux is in the gradient or countergradient direction is the ratio of the mean slope of the mixing path (or surface) to the mean slope of the isopleths of the property being considered. A procedure is outlined for evaluating the coefficients needed to solve the derived diffusion equation. Although exact soluti...

THE ROLE OF VERTICAL MOTIONS IN OZONE-WEATHER RELATIONSHIPS

Abstract The relative importance of horizontal advection and vertical motion in producing the day-to-day changes in total ozone amount is calculated, and the manner in which these two factors combine to produce the well-known ozone-weather relationships is explained. The calculations show that at most one third of the range of daily values is attributable to vertical motions, the remainder presumably being the result of horizontal advection.

Cyclogenesis in Polar Air Streams

Abstract Two case studies of cyclogenesis that occurred in polar air streams behind or poleward of major frontal bands are presented. Based on the results of the studies, and on other evidence, characteristics of the type of disturbance in question are described. The cyclones in polar air masses are generally of small dimension, being spaced at intervals of 1000–1500 km when they occur in multiple form. They form most often over the oceans in winter, originating in regions of low-level heating and enhanced convection and acquiring a comma-shaped cloud pattern as they mature. They are associated with well-developed baroclinity throughout the troposphere and are located on the poleward side of the jet stream in a region marked by strong cyclonic wind shear and by conditional instability through a substantial depth of the troposphere. Instability mechanisms for their formation are discussed. It is concluded that they are primarily a baroclinic phenomenon that owe their below average size to the effect that s...

Effects of Surface Energy Fluxes during the Early Development and Rapid Intensification Stages of Seven Explosive Cyclones in the Western Atlantic

Abstract A series of eight numerical experiments were conducted on seven cases of explosive marine cyclogenesis, using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. The main objective was to elucidate the role of surface energy fluxes both during and preceding rapid deepening. Results from the 24-h experiments initialized at the commencement of the period of most rapid deepening showed that the fluxes occurring during this stage had essentially no effect on the deepening rate. However, substantial effects of the fluxes were found in 48-h experiments initialized early in the development. When the fluxes were withheld from the 48-h simulations, the predicted cyclones were weakened on average by 7.1 mb at 24 hours and 13.5 mb at 48 hours. Evidently the fluxes occurring during the 24 hours preceding rapid deepening did affect the storm development throughout its lifetime. Additional 48-h experiments confirmed the relatively small impact of the concurren...

The Annual Temperature Variation in the Lower Tropical Stratosphere

Abstract The annual temperature variation in the lower tropical stratosphere has its maximum amplitude just above the equatorial tropopause and has nearly the same phase at a particular level throughout both hemispheres. Coldest temperatures occur during the Northern Hemisphere winter (January–February). It is hypothesized that the observed behavior is caused by a seasonal variation in the intensity of the Hadley cell of the Northern Hemisphere with strongest upwelling and cooling in the ascending branch of the meridional circulation occurring in January–February. Computations are made which suggest that a twofold increase in the upward motion near the equator from July to January is sufficient to account for the magnitude of the observed temperature decrease.