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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
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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 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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Perry G. Ramsey and Dayton G. Vincent

Abstract

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 Robert G. Waterman

Abstract

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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James W. Hurrell and Dayton G. Vincent

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A modified set of FGGE Level III-b gridpoint analyses, originally produced by ECMWF, is used to diagnose the eddy energy budgets of four equal-area regions within the tropical Southern Hemisphere (0–30°S) during the SOP-1 period of 10–27 January 1979. Each region is approximately centered on a wave axis of maximum warm, rising air. Three of the four wave axes are tied to the continents of Africa, Australia, and South America, while the fourth coincides extremely well with the South Pacific Convergence Zone (SPCZ). Daily variations of the energy conversions are examined. In addition, time-averaged results of energy contents, conversions and boundary transports are compiled for a 15–day period, 10–24 January, when the, SPCZ was most active.

Results show that the eddy kinetic energy (KE) exceeds the eddy available potential energy (AE) in all four regions, with that in the SPCZ being the largest. Of the conversion and boundary flux terms, only the conversion of AE to KE is significant. Again, the region containing the largest value is the SPCZ. The main flow of energy in each region appears to consist of a generation of AE by diabatic heating, a conversion of AE to KE by thermally-direct eddy circulations, and a dissipation of KE.

The relationships among the four subareas are investigated, primarily through evaluations of the boundary fluxes of KE. Results indicate that the only significant transport between regions is a flow of KE from the SPCZ region into the South American region. Thus, it appears likely that some of the KE from the SPCZ is helping to maintain the KE of the South American region and, in particular, the South Atlantic Convergence Zone (SACZ). These results seem to be in good agreement with the modeling results produced by the NASA Goddard Laboratory for Atmospheres (GLA) General Circulation Model.

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Huo-Jin Huang and Dayton G. Vincent

Abstract

A modified set of Level III-b grid point analyses, originally produced by ECMWF, is used to diagnose the circulation features and energy conversions in the Southern Hemisphere during the FGGE SOP-1 period of 10–27 January 1979. One of the dominant features during the period was the South Pacific Convergence Zone (SPCZ), a large-scale, quasi-stationary, convectively-active cloud band over the South Pacific Ocean. The study focuses on the significance of the SPCZ on Southern Hemisphere energy conversions by partitioning the conversions into zonal and eddy (transient and standing) components. The mean state is examined for a 15-day period, 10–24 January, when the SPCZ was most active. After 24 January it dissipated. In addition, daily variations are examined for the entire period and a zonal wavenumber analysis fox. wavenumbers 1–15 is performed.

The major findings are that 1) the baroclinic conversion of eddy potential to eddy kinetic energy (CE) is the dominant conversion term in the tropics (0–30°S), and it is particularly important in the vicinity of the SPCZ; 2) all conversion terms in middle latitudes (30–60°S) are comparable and equally important; 3) standing (transient) eddies make the most significant contribution to CE (all eddy conversion terms) in the tropics and SPCZ area (midlatitudes); 4) wavenumber 4 dominates the CE conversion in the tropics, whereas wavenumbers 5–8 dominate all the eddy conversions in middle latitudes; 5) one of the four waves in the n=4CE conversion in the tropics coincides with the SPCZ, while the remaining three correspond to the continental areas of Africa, Australia and South America; and 6) during the last three days, when the SPCZ is decaying, the importance of the n=4 contribution to CE is negligible.

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Dayton G. Vincent and Thomas Q. Carney

Abstract

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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Matthew D. Eastin and Dayton G. Vincent

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The climatology of the kinetic energy associated with the subtropical jet over the Australian–South Pacific region is investigated for a 6-yr period, January 1985–December 1990, using monthly mean data. The total kinetic energy (TKE) is partitioned into vertically averaged mean kinetic energy (KM) and level-by-level departure from the mean, or so-called shear kinetic energy (KS). A comparison of the two components during the annual cycle reveals that KM within the region of the subtropical jet is usually greater than KS. An out-of- phase relationship between the annual cycle of TKE and the annual cycle of the percentage of TKE represented by KS is found. A higher percentage of KS occurs in the summer season, when the jet is weakest. During late summer, KS dominates in the entrance region of the jet over Australia and the western Pacific. This appears to coincide with the annual strengthening of the jet. During winter, when the jet reaches its maximum intensity, KM dominates. It also dominates throughout the year in the exit region of the jet.

In addition, a comparison of TKE during an El Niño–Southern Oscillation cycle is made. Results indicate an increase of kinetic energy during El Niño over the central Pacific coupled with a decrease over Australia, indicating eastward movement of the jet. Subsequently, during La Niña, an opposite pattern is observed as the jet moves westward. The results of this climatological study, which appear to be in good agreement with the previous seasonal studies of the subtropical jet, could be beneficial to seasonal or year-to-year forecasting.

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