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Dayton G. Vincent
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Dayton G. Vincent

Abstract

The circulation features associated with the South Pacific convergence zone (SPCZ) and its accompanying cloud band are reviewed and discussed. The paper focuses on the following topics: location, structure, and characteristics of the SPCZ; theories and observations concerning its existence; the significance and scope of the SPCZ in global-scale circulation patterns; quasi-periodic changes in its location and strength; and synoptic-scale features within its regional influence (e.g., cyclones, subtropical jets). It concludes with some challenging problems for the future.

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Dayton G. Vincent

Abstract

A diagnostic study of the time-averaged large-scale circulation features over the equatorial and South Pacific Ocean during 10–18 January 1979, FGGE SOP-1, is presented. Results are based on ECMWF Level III-b analyses for an area bounded by 10°N, 105°W, 42.5°S and 170°E. During the nine day period, the South Pacific Convergence Zone (SPCZ) and its accompanying cloud band were quasi-stationary features in the area of study. Analyses of surface pressure, geopotential height, horizontal winds, temperature, relative humidity, vertical p-velocity (ω), horizontal divergence and relative vorticity are presented and discussed. Particular attention is devoted to examining the structure of the SPCZ through the use of vertical cross sections along and across both the west-east (zonal) and northwest-southeast (diagonal) portions of the zone.

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Dayton G. Vincent and Herbert Borenstein

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The variability of subjective (hand) analyses is explored by examining the results of two synoptic laboratory experiments. Two groups of analysts participated in the first experiment, one consisting of 13 senior meteorology students (1977), the other consisting of six professional synopticians. Each participant analyzed maps of geopotential height and zonal and meridional wind components over the eastern two-thirds of the United States for 1200 GMT 25 June 1968 at 500 mb. Grid-point data were extracted from these analyses and are compared to those obtained from a prior subjective analysis and an objective analysis, both used as standards. Higher order quantities, consisting of geostrophic and ageostrophic wind components, horizontal divergence, relative vorticity, kinetic energy content and generation of kinetic energy, were computed from the grid-point data and also are compared. Methods of comparison include pattern analyses, difference maps and statistical tests.

In the second experiment, only one group of analysts, 11 senior meteorology students (1979), participated. The same data were used. Each participant analyzed zonal and meridional wind components as for the first experiment. In addition, individuals analyzed maps for isogons and isotachs from which wind components subsequently were derived. Comparisons. similar to those made in Experiment 1, are made between analyzed and derived components, as well as between kinetic energy, divergence and vorticity values computed from each set of components.

The significant findings are as follows: 1) in each experiment, analyses of wind components, regardless of the analysis scheme used (subjective, objective or components derived from wind direction and speed), are in very good agreement; 2) in each experiment, analyses are combined to form composite mean maps which are found to give the best representation of the flow features compared to any of the individual analyses; 3) in Experiment 1, the height and both wind components of the subjective and objective standards differ more from each other than they do from the corresponding composites of subjectively analyzed maps; 4) in Experiment 1. there is reasonably good agreement among analysis techniques for derived quantities, except those that depend on cross-contour flow; 5) average cross-contour flow angles for student composite, professional composite, overall composite, subjective standard and objective standard are 32, 24, 27, 31 and 15°, indicating the tendency of the objective scheme to minimize this variable; and 6) in Experiment 2, as in Experiment 1, kinetic energy and vorticity show good agreement, regardless of analysis scheme, but considerably less agreement is seen for divergence.

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Dayton G. Vincent and Robert G. Waterman

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A diagnostic analysis is presented of the large-scale atmospheric conditions in the Caribbean area during the 4-day period of Hurricane Carmen's (1974) development and intensification. Data taken during the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE) form the basis for the study and consist primarily of rawinsonde reports at standard pressure levels. Analyses of the following observed and computed quantities are performed at 12 h intervals: wind direction and speed, u and v wind components, geopotential height, air temperature, equivalent potential temperature, sea surface temperature, relative humidity, vertical motion, horizontal divergence, relative and absolute vorticity, and vorticity budget terms.

It is seen that several features of the large-scale circulation persist in Camen's vicinity throughout the 4-day period. These include warm sea surface temperatures, low-level conditional instability, a deep moisture-rich layer, low-level confluence and upper level diffluence, upward vertical motion, low and mid-tropospheric. cyclonic vorticity with upper tropospheric anticyclonic vorticity, and deep, easterly flow with weak vertical shear.

Results are also examined separately for Carmen's preburricane and hurricane stages. It is indicated that, as Carmen intensifies, its circulation has an impact on the large-scale flow. The upper level westerlies ahead of Carmen become easterlies as the hurricane approaches the western Caribbean and it appears that deep cumulus convection associated with Carmen plays an important role in producing large-scale vorticity.

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Perry G. Ramsey and Dayton G. Vincent

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An important quantity whose magnitude has not been thoroughly examined is the vertical distribution of heating in the Tropics. The details of the vertical distribution of heating have a significant impact on a number of phenomena, including the 30–60 day oscillation, sometimes known as the intraseasonal oscillation. Prior attempts to establish the structure of the heating relied on limited field data or assimilated data, coupled with climatological radiative heating parameters. The availability of high quality global-scale datasets has made it possible to make more accurate calculations than were possible a few years ago.

An important component of the apparent heat budget is the longwave radiative cooling, which in this paper is found by using the ECMWF/WCRP/TOGA Archive II and ISCCP C1 datasets, together with a well-established parameterization scheme. A method is developed that can be used to estimate the vertical structure of cloud amounts based on top-of-atmosphere cloud observations, and the results are used with a wide-band long-wave parameterization to produce longwave cooling rates over the tropical Pacific Ocean.

Outgoing longwave radiation is calculated and compared to ERBE results. The calculated values are generally higher than those from ERBE, though the spatial distributions are similar. Some significant problems exist with the ECMWF upper-tropospheric water vapor amounts, which could imply uncertainties of 0.5°C day−1 in the calculated cooling rates. This is comparable to the differences associated with the minimum or random overlap assumptions used to generate cloud profiles.

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Dayton G. Vincent and Johan van Heerden
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Dayton G. Vincent and Thomas Q. Carney

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Upper air data collected during AVE-SESAME 1, 1200 GMT 10 April 1200 GMT 11 April 1979, are used to detect differences between calculations of kinetic energy budget terms derived from two sets of analyses. Barnes's objective analysis scheme is used to grid both sets of analyses, one based on all data taken in the regional AVE-SESAME network (SES) and the other based on data taken only at the National Weather Service stations (NWS) within that network. The SES analyses were derived from data taken at 23 NWS stations plus 16 supplementary stations.

Four areas are examined: 1) the total analysis area, 2) a smaller fixed area that contains most of the active convection throughout the period, 3) a still smaller, but varying area that contains the strongest convective activity as determined from Manually-Digitized Radar (MDR) data and enhanced IR satellite imagery and 4) another varying small area downstream from the strong convective activity that contains no detectable convection. The most significant difference between SES- and NWS-derived budget terms occurs in the kinetic energy generation term. In all areas except the no convection area, generation values are considerably more positive when the SES data set is used. Most of the difference occurs in the upper troposphere. Furthermore, both data sets show that generation occurs in convective areas, whereas destruction predominates elsewhere.

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Ken-Chung Ko and Dayton G. Vincent

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A composite of 10 cases of zonal wind maxima at 200 hPa over the subtropical region stretching from Australia to the central Pacific is examined for the six-month period, November 1984-April 1985. This region is unique in that distinct westerly jets frequently form and propagate eastward at latitudes between 20° and 35°S in the summer season. Some statistical tests were applied and suggest that the flow patterns are quasi periodic, consisting of a tendency for new jet streaks to develop over the eastern Australian region approximately every one to two weeks. These jets then take about 10 days to propagate across the western Pacific before dissipating or, perhaps, moving toward higher latitudes. Their average propagation speed is approximately 4 m s−1. An examination of the case-to-case variability of the jets provides additional evidence that they are significant features. A diagnosis of the trough/ridge systems at 200 and 850 hPa, together with calculations of the vertically integrated mean and shear kinetic energies suggests that baroclinic processes dominate in the entrance and center regions of the jet, whereas barotropic processes dominate in the exit and downstream regions.

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Catherine B. Pedigo and Dayton G. Vincent

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This study presents global estimates of precipitation rates from 30°N to 30°S, derived from the “apparent” heat source (Q 1) and “apparent” moisture sink (Q 2) budgets using the NASA Goddard Laboratory for Atmospheres Level III-b analyses collected during the first special observing period (SOP-1) of the Global Weather Experiment. In each technique precipitation is solved as the residual. Because of the assumptions made, the techniques yield the most reliable results in the summer hemisphere. Therefore, area averages of precipitation rates are presented to examine the variability of rainfall among selected regions in the Southern Hemisphere tropics. These regions include Africa, the Indian Ocean, the Australian monsoon, the South Pacific convergence zone (SPCZ), and the South American/South Atlantic convergence zone (SACZ). Time averages of precipitation rates are also calculated for two periods (10–24 January and 28 January–11 February 1979) that were selected because of significant changes observed in the convective activity. In the first period intense convection was indicated in the SPCZ, with a subsequent lack of activity there in the latter period. During the second period, a buildup of convective activity was noted in the Indian Ocean. Vertical profiles of heating are also presented for each region and comparisons are made between the profile for the SPCZ and convectively active regions investigated elsewhere in previous studies. Finally, precipitable water (W) is computed and compared to results derived from satellite microwave measurements, as well as to the budget-produced precipitation patterns.

Results indicate that the heat and moisture budget estimates of precipitation compare favorably. The vertical advection term in both techniques is found to be the dominant contributor to Q 1 and Q 2. Vertical profiles reveal that maximum convective heating occurs in the middle troposphere and the profile of the SPCZ region compares best with those over the western North Pacific. In general, the largest values of W are observed in the area of lowest outgoing longwave radiation and strongest rainfall rates. Patterns of W from GLA compared well with those from the satellite measurements.

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