{'fraction_non_alphanumeric': 0.02394862418052892, 'fraction_numerical': 0.015163002274450341, 'mean_word_length': 4.976545842217484, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Passive tracer transport relevant to the TRACE A experiment

. This paper explores some of the mechanisms governing the accumulation of passive tracers over the tropical southern Atlantic Ocean during the northern hemisphere fall season. There has been a pioneering observation regarding ozone maxima over the South Atlantic during austral spring. The understanding of the formation of this maxima has been the prime motivation for this study. Using a global model as a frame of reference, we have carried out three kinds of experiments during the period of the Transport and Atmospheric Chemistry Near the Equator--Atlantic (TRACE A) project of 1992. The first of these is a simple advection of total ozone (a passive tracer) in time using the Florida State University global spectral model. Integration over the period of roughly 1 week showed that the model quite closely replicates the behavior of the observed total ozone from the total ozone mapping spectrometer (TOMS). This includes many of the changes in the features of total ozone over the tropical and subtropical region of the southern Atlantic Ocean. These studies suggest a correlation of 0.8 between the observed ozone over this region and ozone modeled from dynamics alone, i.e., without recourse to any photochemistry. The second series of experiments invoke sustained sources of a tracer over the biomass burn region of Africa and Brazil. Furthermore, sustained sources were also introduced in the active frontal descending air region of the southern hemisphere and over the Asian monsoon's east-west circulation. These experiments strongly suggest that air motions help to accumulate tracer elements over the z-(cid:127)uctHtl(cid:127), Uk, CctH. Z-(cid:127) fllay required to improve the deficiencies of the vertical stratification of ozone predicted by the model over the flight region of the tropical southern Atlantic during TRACE A. Here we use the global model to optimally derive plausible accumulation of burn elements over the fire count regions of Brazil and Africa to provide passive tracer advections to closely over the tropical southern Atlantic Ocean.


Introduction
Since the pioneering observational studies on tropospheric ozone distribution [Fishman, 1994;Fishman et al., 1990Fishman et al., , 1991, the issues on the source regions have focused on the biomass burning over South America and Africa [Fishman et al., 1990;Thompson et al., 1993]. Fishman's original studies examined two types of satellite observations: total ozone mapping spectrometer (TOMS) for the total atmospheric column ozone and stratospheric aerosol and gas experiment (SAGE) for the stratospheric component. A list of acronyms is provided in Table 1. These studies qualitatively provided a convincing picture of a tropospheric ozone maximum, which was centered near the Greenwich meridian and roughly 10 ø to the south of the equator. Plate 1 from Fishman et al. [1990] describes the residual tropospheric ozone obtained from the subtraction of the stratospheric ozone (as measured from SAGE) from the •Department of Meteorology, Florida State University, Tallahassee. 2National Meteorological Center, Washington, D.C. 3Earth System Science Division, NASA Ames Research Center, several sites provided insight into the possibility that accumulation of ozone can occur in the lower troposphere near the oceanic inversion and in the upper troposphere as well. Refinements of the early satellite measurements of total ozone [Fishman et al., 1990] were provided by Thompson et al. [1993], who emphasized the corrections for the oceanic stratocumulus. Satellite observations suggest that the tropospheric ozone has a relative maximum during the austral spring, i.e., September and October. The atmospheric circulations over a broad-scale region covering Africa, South America, and the Atlantic Ocean were summarized by Krishnamufti et al. [1993]. They described the circulation climatology (i.e., monthly means) as well as the transients of this region. It was recognized that atmospheric transport processes could include possible candidates such as (1) divergent planetary east-west circulations with ascending branches in the Asian monsoon region and descending air over the tropical southern Atlantic Ocean; (2) frontal descent of the southern hemispheric air, which was shown to subsequently circulate northward in the lower troposphere, i.e., counterclockwise around transient anticyclones of the southern tropics and subtropics; (3) biomass burn elements from South America, being eventually conveyed via the westerlies near 25øS and (4) biomass burn elements over Brazil being conveyed to the upper troposphere by deep cumulus convection and eventually conveyed by westerlies toward the tropical southern Atlantic; (5) the easterly flow near 700 mbar can convey low-level burn elements from the interior of the biomass burn regions of Africa; and (6) the upper tropospheric near-equatorial easterly flows over Africa appear to be possible vehicles for the transport of burn elements, which has been previously carried to the upper troposphere by deep cumulus convection. The determination of reliable flow trajectories that might provide insight on the relative importance of such candidates is not a simple matter [Pickering et al., 1994;Fuelberg et al., this issue]. Difficulties stem from uncertainties in data coverage, analysis assimilation differences from various centers, usage of modelbased data by users using alternate algorithms (different from those used by modelers), methodology, and space-time interpolations in the construction of trajectories. A nonlinear advection of passive tracers, using a fourth-order semi-Lagrangian advective scheme [Krishnamurti et al., 1990] used in this study eliminates most of these difficulties. There are important questions as to what dynamics only contributes, as compared to the contribution of the photochemistry of the biomass burn, toward the eventual pattern of the ozone accumulation over the tropical southern Atlantic Ocean. The role of the dynamics is something that we can perhaps address from our modeling capability. However, we feel that, given the half-life of the ozone, of roughly a week in the lower troposphere and roughly 3-4 weeks in the upper troposphere and stratosphere, the ad hoc nature of our ap-proach in neglecting the contributions of photochemistry may still be quite acceptable, since we are examining periods of 6 days only. The eventual strategy of long-term transport studies would, however, require four-dimensional global models at high resolution that include photochemical history variables.
The field experiment Transport and Atmospheric Chemistry Near the Equator--Atlantic (TRACE A) was conducted over this entire region of interest during the months of September and October 1992. A chronology of the components of the experiments appears in this special issue of the Journal of Geophysical Research [Bachmeier and Fuelburg, 1995]. One purpose of this field experiment was to provide a diverse set of observations for circulation and photochemical studies toward providing an understanding of the role of biomass burning on the accumulation of the tropical tropospheric ozone over the southern Atlantic Ocean.

Objectives of the Present Study
The main objective of the present study is to use results from the Florida State University (FSU) global spectral models at high resolution, to investigate the relative roles of passive tracer transports based on TOMS and those based on simply modeled biomass burn patterns on the accumulation of ozone over the tropical southern Atlantic Ocean.

Data Sources for the Present Study
For the initial state of atmosphere, we have used the National Meteorological Center (NMC) operational level III data sets, which are based on analysis of the conventional World Weather Watch observations data, the available global data from commercial aircraft, data from ships of opportunity, the polar orbital satellite based soundings, cloud-tracked winds, and some surface buoy data from the equatorial Pacific and the southern hemisphere.
Enhanced cloud-tracked winds over the tropical Atlantic Ocean were studied. During the months of September and October 1992 an enhanced extraction of cloud motion vectors was organized by Florida State University (carried out by Jack Beven) on the National Hurricane Center's Mcldas system. These data were passed on to the NMC and were assimilated in near real-time by their four-dimensional assimilation system [Kanamitsu, 1989]  TOMS, the total ozone global data, for this period were provided to us by NASA Goddard. These data sets did not include the cloud correction algorithm of Thompson et al. [1993].
A first guess vertical distribution of ozone for the TOMS was based on Datsch [1978] and Hilsenrath et al. [1977].
Tropopause height (pressure) data sets were obtained from the archives of the NMC.
In order to delineate the tropospheric ozone for our modelbased estimates, it was necessary to perform integrations from the Earth's surface to the tropopause. For this purpose, we have resorted to the NMC's operational analysis of the tropopause pressure. We examined the tropopause analysis for the period October 12-18. Typical values of tropopause location are around 120 mbar near the Greenwich meridian and 10øS.
The tropopause descends to roughly 250-300 mbar near 60øS, and the tropopause is somewhat above the 120-mbar surface over the northern summer tropics. In this study we have not addressed the specifics of tropospheric residue ozone, since such data were not available at the time of this research. Those aspects will be addressed in a future study.

The Global Model
The global model used in this study is identical in all respects to that used by Krishnamufti et al. [1989]. The characteristics of the global model are as follows: independent variables (x, y, o-, t); department variables of vorticity, divergence, surface pressure, vertical velocity, temperature, and humidity; horizontal resolution of 126 triangular waves; vertical resolution of 15 layers between roughly 30 and 1000 mbar; semi-implicit time differencing scheme; centered differences in the vertical for all variables except humidity, which is handled by an upstream differencing scheme (the humidity variables require a positive definite advective scheme); fourth-order horizontal diffusion [Kanamitsu et al., 1983 [Businger et al., 1971]; vertical distribution of fluxes utilizing diffusive formulation, where the exchange coefficients are functions of the Richardson number [Louis, 1979]; longwave and shortwave radiative fluxes based on band models [Lacis and Hansen, 1974;Harshvardan and Corsetti, 1984]; diurnal cycle; parameterization of low, middle, and high clouds based on threshold relative humidity for radiative transfer calculations; surface energy balance is coupled to the surface similarity theory [Krishnamufti et al., 1991]; nonlinear normal mode initialization of five vertical modes [Kitade, 1981].
Other aspects of global modeling entail the passive advections of ozone. For this purpose, we have designed an advection algorithm that makes use of the semi-Lagrangian conservation principle based on Kn'shnamurti et al. [1990].

Passive Tracer Experiments
In this section, we shall describe in sequence, the experiments based on dynamics alone, isolated source, and optimized burn.

Dynamics-Alone Experiment
Our interest lies with the issue of the contribution to the accumulation of tropical tropospheric ozone from dynamics, i.e., no photochemistry is invoked. The global model is first integrated for a period of 6 days and, using its threedimensional motion field (u, v, tr), the transports are next carried out by the predicted winds in order to study the time history of the passive tracer. The initial ozone is derived from the TOMS data set, which is the vertically integrated atmospheric ozone. Next we shall describe the results for an experiment that was initialized for October 12, 1992, at 0000 UTC.
The initial vertical distribution of ozone was determined from the TOMS data using the weighting function given by D These will be included in future studies. The heavy arrow in Figure 5 denotes a prominent divergent streamline illustrating the teleconnection between the ozone maximum and the Asian monsoon.
We make the assumption that, over a short period of the order of a week, the stratospheric changes in the ozone are small. Next we shall present the total ozone based on the TOMS data and those obtained from these global model forecasts. The period October 12-18, 1992, is considered here. The [1977] and as implied by the global model.
arising from interpolation to the T126 resolution, which has a transform grid separation of roughly 80 km. The predicted total ozone is in close agreement with the TOMS estimates throughout this 6-day period of October 12-18, 1992. If one looks at the 6-day forecast for October 18 (Figures 2k and 21), we can see that many of the small-scale details are reasonably predicted by the global model. An axis of a sharp gradient of total ozone extends from 20øW to 10øE across 40øS. This sharp feature is related to a frontal passage, and the circulation forecasts are able to convey these ozone transports accurately. A tongue of ozone maximum enters the Indian Ocean from the southern Atlantic Ocean south of South Africa; this is another of the many features that is reasonably predicted at day 6. The 300 Dobson unit (DU) isopleth over South America reflects a southward dip near 30øS; this is also reasonably portrayed by the forecast model. Over the tropical southern Atlantic Ocean the TOMS carries a maximum value of 314 DU, whereas the predicted maximum value over this region is around 308 DU. Overall, the 6-day forecast of the distribution of ozone appears quite realistic. The high skill in the prediction of ozone distribution is also reflected in the correlation diagram ( Figure 3). Here we show the correlation of the observed and the predicted ozone versus days of forecast. It may be seen from Figure 3 that the forecast skill falls to 0.6 at day 6. The errors arise from modeling approximations associated with forecast errors of the motion field and hence of the transports. The absence of photochemistry is, of course, another factor that must account for these errors, since  Table 2. It should be noted from Table 2 that the nature of these sources is akin to vertical plumes that provide a continuous supply of tracers. These are being advected by the motion field of the global model. These sources were introduced over a certain vertical depth (Table 2) of the atmosphere over the prescribed location (Table 2).  where D is a parameter that defines the lateral size of the source in degrees latitude-longitude, R is the transformed distance from the central point of the source (see Table 2), and S is the maximum sustained ozone concentration.
The rationale for the selection of these sustained sources is the following: the stratospheric sustained source was based on an examination of TOMS data for the week preceding October 14, where we noted a quasi-stationary ozone maximum over the Atlantic coast of South America near 30øS. The source of this maximum appeared to be stratospheric, since its earlier time history showed a passage to that region from much farther south of 25øS. On that basis it was decided to introduce a source near 25øS and 45øW with the specifications described in Table 2. The choice of the other two sustained source regions over South America and Africa were dictated by the mapping of the fire counts. These are illustrated in Plate 2 and Figure 4. The sustained source is introduced almost directly over the burn areas for both of these experiments. The vertical structure of the source (shown in Table 2) reflects plausible vertical distributions that might arise from effects of cumulus convection, which the biomass burn provides vertically. Given a prescribed vertical distribution (Table 2) over the burn regions, the global model is used to carry out a four-dimensional passive tracer advection for each case separately. A sustained biomass burn source is placed over Africa, between 300 and 900 mbar (Table 2) (Table 2). Westerly winds rapidly disperse the ozone toward West Africa and south of South Africa. A transient anticyclone in the middle of the southern Atlantic Ocean effectively transports ozone equatorward. Since the forecasts of the motion field were fairly accurate up to 6 days, these advective transports, unlike the single trajectories that followed, appear to present a reasonable perspective on the effects of transports. This computation suggests that a steady descent of air can provide a rich source of stratospheric air all the way across the Atlantic Ocean in a matter of 1 week. The choice of the maximum value of 4 DU per 100 mbar is certainly arbitrary. It can be scaled to any desired value, and the advection pattern can be handled accordingly. The descending motion over the source region is reflected in the time history of the evolution of ozone at 400 mbar. Initially, no sustained source was present at 400 mbar (Plate 6). Hence that panel is blank. With time, descent occurs all along the path of the 200-mbar flow toward West Africa. It is this descending flow that appears to be most interesting in contributing to ozone anomalies in the tropical southern Atlantic region at the 400-mbar level. The axis of the 400-mbar advection toward West Africa is not located exactly below that of the 200-mbar plume; the differential horizontal advection produces these features. The descent describes a southern stream at 400 mbar that moves eastward over South Africa. It is also interesting to note that a rather well marked sustained maximum value resides over the east coast of South America at 400 mbar. That was a region of sustained descent in the predicted circulation patterns between 200 and 300 mbar. Such sustained vertical motion features can advect stratospheric air into the tropical upper troposphere in a matter of a few days. However, this may not be crucial for our overall understanding of the ozone anomalies of the tropical southern Atlantic Ocean.
Next we shall illustrate the dispersal from a sustained source over the burn region. These were introduced between 300 and 900 mbar (Figure 4) Table 2 for further details on the location, extent, and intensity of this sustained source.) Plate 9 shows the time history of the evolution of a tracer at 200 mbar. A part of this evolution is westward near 10øN from the sustained source. As this tracer moves westward, it makes a counterclockwise, i.e., anticyclonic loop around the eastern tropical Atlantic Ocean. All this time, a gradual descent, carried out by vertical advection, brings this tracer to various upper tropospheric levels. Plate 10 shows its time history at the 300-mbar level. A marked vertical convergence of flux near 300 mbar was also noted in the computations, offering the suggestion that the ozone maximum we noted during TRACE A flights near 9 km above sea level could, in part, be ascribed to the monsoonal east-west teleconnections. In 6 days some of the tracers from the monsoon do appear to reach the lower troposphere over Africa, but the substantial effect is felt at 500 mbar and higher,     (Figure 7a), is plotted in vertical cross sections, and their comparison with the no burn ozone cross section (Figure 7b) suggests that in order to describe the ozone and its vertical structure over the tropical southern Atlantic, we need to invoke tracer elements from the biomass burn regions. If these are done carefully, we can make dramatic improvements in the simulation. The procedure we have adopted here is no more than an intermediate step toward the eventual photochemical modeling that is desirable.

Limitations of Present Work
There are several areas where further improvements in the present work are needed.

Concluding Remarks
The most striking result of this study is that we are able to demonstrate skills of the order of 0.8 (correlations of observed TOMS versus model-based estimates of ozone over the larger TRACE A domain) for the prediction of total ozone as a passive tracer for periods of the order of 1 week without recourse to photochemistry. The results presented here for the dynamics alone experiments may be dependent on the choice of the initial structure function. Further experimentation is needed to explore sensitivity in this area.
Dynamics alone models, which are based on transports of passive tracers, do not generate the fine structures of vertical distributions of ozone as are seen in the reconnaissance aircraft based UV DIAL measurements. There exists an obvious mismatch in the scales of UV DIAL observations and the global model's horizontal and vertical resolution. As a consequence, it is not possible to make a one-to-one comparison. We had resorted to a filtering of the smaller scales of the UV DIAL data. Insertion of biomass sources in the model integration appears to provide a vertical fine structure in the forecasts, something that was not possible from the so-called dynamics alone experiments. There is a need to refine the dynamics alone model further, i.e., to incorporate some measure of the photochemistry. The fine structure one sees in the ozonesonde data from Brazzaville, Ascension, and Natal and from the UV     A major observational contribution of TRACE A was the airborne UV DIAL measurements of ozone. These provided vertical cross sections of ozone with some definitive vertical structures with maximum ozone near the inversion layer and the upper troposphere of the tropical southern Atlantic Ocean. In order to model such fine features we had to invoke hypothetical tracer geometries over the fire count region of Africa and Brazil. That was done with amplitudes within the ranges of the extrema of ozone from the soundings of ozone at Brazzaville, Natal, and Ascension. It was possible to model the vertical cross sections of ozone along the aircraft track of TRACE A to some reasonable degree.
The dynamics alone experiments are quite revealing and bring out the use of numerical global models for such studies. In fact, the inclusion of explicit photochemistry in the model can provide answers to many unexplained features of atmospheric photochemistry.