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Weather Vol. 57 September 2002 The meteorological setting of the `TMI-2’ nuclear accident on 28 March 1979 Reinhold Steinacker1 and Ignaz Vergeiner2 1 2 Department of Meteorology and Geophysics, University of Vienna Department of Meteorology and Geophysics, University of Innsbruck On 28 March 1979 at 0400 EST (EST = GMT 75 h), the Three Mile Island Unit 2 pressurised water reactor (TMI-2) began automatic shutdown following a loss of feed water, initiating a cascade of equipment failures and operator errors which caused a loss of coolant water and the destruction of the reactor core (Special Inquiry Group 1980). Roughly half of the reactor core melted and significant amounts of the fuel were vaporised. This was the most severe accident, by far, within the `civilian’ nuclear industry up to that time. The first few days were the critical ones. It appears that an outright nuclear catastrophe on the scale of the 1986 Chernobyl disaster was avoided only barely. The quantity, composition and timing of the releases of noble gases and other b-emitting nuclides, iodines, activated particulates and fuel into the environment are still in dispute, as well as the consequences. There is an enormous discrepancy in the claimed radiation exposures to the public between millirems and hundreds of rems*: (i) (ii) Low-dose engineering estimates by official agencies rely on postulated low emissions, on remote sensing by &20 thermo-luminescent dosimeters, and on inferences drawn from Gaussian plume modelling, where the Appalachian Hills are taken to force strong diffusion. High-dose biological estimates are based on a spotty, but consistent, pattern of symptoms of acute radiation poisoning of people, pets, trees and plants, and on later medical tests and * A more recent unit of radiation dose is 1 Sievert (Sv) = 100 rems. cancer statistics (Aamodt and Aamodt 1984, 1985; Wing et al. 1997). All such biological direct evidence of locally high exposures, despite being abundant and available from many distant locations, as opposed to the engineering-type dose reconstructions, has consistently been officially dismissed as `psychological stress’ . There can be no doubt that the pertinent reports must have been feared to be potentially ruinous to the nuclear establishment. In an attempt to help resolve the extraordinary discrepancy through a re-evalution of the dispersion conditions, the synoptic setting was investigated. Meteorological analysis of the TMI case had been conspicuously non-existent for 15 years. Weather conditions turned out to be very complex. Accordingly, we offer a casestudy of two slowly moving warm fronts being distorted by the Appalachian Hills. Whereas resolution of all the issues is obviously far beyond a single discipline, the present analysis demonstrates extreme thermal stability with weak and variable winds during the first critical days of the accident, a combination which makes the impaction of almost undiluted, highly concentrated plumes or puffs of contaminated air on exposed hilltop sites up to 30 km away from TMI quite plausible. The topography The hilly relief surrounding the TMI reactor site is highly contorted. Overall, the terrain rises toward the west and south-west. The reactor site is at an elevation of 100 m above sea-level on an island in the broad Susquehanna River, which flows from the northern Appalachians southward through the eastern 341 Weather Vol. 57 Appalachian foothills, with a local orientation from north-west to south-east, eventually reaching Chesapeake Bay. There are various ranges topping 200 to 300 m and valleys with a general south-west to north-east orientation, leading up to the Susquehanna River valley. There are several gaps cut into these ranges. The city of Harrisburg, capital of Pennsylvania (population 200 000), located about 15 to 20 km north-west of TMI and 150 km north of Washington, DC, is situated in a wide valley running almost west± east, but parts of the city itself are hilly terrain. The Blue Mountains, reaching up to 450 m, frame this valley to the north. Meteorological data sources and notation We used National Meteorological Center (NMC 1979) surface and 850 mbar weather maps, and rawinsonde soundings from Pittsburgh, Pennsylvania, Washington, DC, and Albany, New York, from 0000 GMT on 28 March to 1200 GMT on 2 April 1979. Charts at 500 mbar from the National Weather Service (NWS) were also available for inspection, as well as published charts from the Deutscher Wetterdienst (1979). Fortunately, the NMC analyses contain the original observations, otherwise a reanalysis could not have been done. In an attempt to secure all the available relevant regional weather information in the vicinity of the TMI plant, records of hourly surface weather observations from more than two dozen airports in Pennsylvania, Maryland and Delaware between 27 March and 2 April 1979 were obtained from the NWS (US Department of Commerce 1979). The general weather situation The general weather pattern during the last days of March 1979 reflects the planetary circulation regime for the entire month of March, as described by Taubensee (1979), and to some extent continuing into April (Wagner 1979). The quasi-stationary upper ridge over the northern Rocky Mountains was displaced westward, forming a blocking structure 342 September 2002 together with an upper trough off California and Mexico (see Fig. 1). Between that and a trough over the western Atlantic linking up to a massive closed low over the Canadian Arctic a shorter wave pattern became established, consisting of a broad upper trough over the Great Plains and a flat ridge of high pressure across the Appalachians and the Atlantic seaboard. This regime of general westerlies with some shifts and rearrangements was largely responsible for the warmth of March experienced across much of the USA, and some heavy precipitation in Midwestern states. A surge of cold air had invaded the northern USA between 23 and 26 March. Thereafter, the broad frontal zone ahead of the Midwestern upper trough became stalled to the west of the Appalachian upper ridge to cause massive advection of warm and humid air from the south-west towards the north-eastern states. During the three days following 1900 EST on 27 March, the entire troposphere in the region of concern, up to heights above 10 km, warmed up continuously, the lower levels, up to 800 mbar, by approximately 20 degC! This general regime of warm-air advection was responsible for heavy rains and flooding across the south-east well into April. In and around Pennsylvania lower temperatures returned by 2 April, but the southerly regime was not conclusively terminated until 5/6 April, when a severe storm swept across the Great Lakes area. At 1900 EST on 27 March 1979, nine hours before the start of the accident, the surface high pressure core was still west of the Appalachians (not shown). By 0400 EST on 28 March the surface high had shifted right over the TMI region. Surface winds veered from west-northwesterly to northerly, north-easterly, easterly and south-easterly (see Fig. 2) by afternoon. At 1900 EST on 28 March 1979, the surface anticyclone had moved east of the Appalachians (Fig. 1), and southerly winds prevailed in the TMI area. At this point we show two consecutive soundings from Washington (Fig. 3). The first one, from the late afternoon of the accident day, has the cooler `old’ airmass still almost well mixed up to about 1200 m by insolation on this perfectly clear day. As it has become Weather Vol. 57 September 2002 Fig. 1 Hemispheric chart for 1900 EST on 28 March 1979 showing sea-level pressure (mbar, solid lines) and 500 mbar height (dam, dashed lines). Adapted from German Weather Service. warmer already on the western and north-western plateau around Pittsburgh, this pool of cool air, higher than the mountain crest height, tends to flow westward, seeping across mountain passes and regions of lower terrain. This configuration (Fig. 1) has a striking resemblance to the surface weather map accompanying the `shallow foÈhn’ across the European Alps: cyclones tracking along the Bavarian foreland (north-western plateau); cool air forming a wedge of high pressure across the Po valley (Pennsylvania and southern foreland east of the Appalachian Mountains), draining into the northern Alpine forelands across mountain passes. Such foÈhn effects on the western slopes of the Alleghennies are apparent on 28 March. Figure 3 shows how warm-air advection at and above the 850 mbar level has put a `lid’ on the atmosphere. The sounding 12 hours later has a very strong frontal inversion near 400 m. Accordingly, the lower atmosphere upstream of TMI had developed extreme stagnation beneath the strong inversion during the night of 28/29 March. The sequence of hourly weather events and winds at Harrisburg (Fig. 2) demonstrates how on 29 and 30 March surface winds slackened and became very weak and variable, with near calm conditions for many hours. Warm-air advection finally penetrated to the surface during 29 March, the grip of the inversion began to loosen across the middle Atlantic states, and by 30 March record high temperatures were reported from there (Washington and Baltimore). Winds, however, still remained generally weak until 5 April. Beyond the foregoing general description of events, the analysis becomes demanding. The structure of the frontal zone is quite complicated, as indicated by inconsistencies in the various published surface analyses. Advances of warm air from the south-west came in surges space-wise and time-wise. The frontal zone lies almost parallel to the upper-level flow. For a complete analysis, boundary-layer effects must be understood and taken into account, too. 343 Weather Vol. 57 September 2002 Fig. 2 Time-series of hourly values of weather and winds at Harrisburg for 28, 29 and 30 March 1979. Measured winds (m s-1 ) are shown from three locations for comparison: HAR = Harrisburg, Capital City Airport, west shore; MDT = Middletown, present HAR International Airport, east shore; TMI = TMI meteorological tower, 30 m above ground level. In this notation indicates 2 m s-1 and visibility is in kilometres. Surface 3-hourly and 850 mbar 12-hourly charts were reanalysed in detail for that purpose. Some comments on methods of synoptic analysis Regarding synoptic analysis, the concept and 344 positioning of fronts is one of the essential features. One basis for the idealised concept of a `polar front’ was a homogeneous oceanic lower boundary. Subsequently, it turned out that boundary-layer effects, such as stable stratification and topographic relief, can confuse frontal signals severely. Classic evidence of a boundary layer decoupled from frontal behaviour aloft Weather Vol. 57 September 2002 Fig. 3 Washington, DC, tephigrams for 1900 EST on 28 March 1979 and 0700 EST on 29 March 1979. Temperatures (8C) are heavy lines; dew points (8C) are dashed lines; wind direction and speed plotted to the left; height scale (mbar) with approximate height (m above sea-level) added on the left. The upper troposphere with a fairly standard decrease of temperature with height has been omitted. The soundings near the tropopause are shown again at the top of the figure. was given by Browning and Monk (1982) and cast into the concept of `split fronts’ . Similar experiences prompted Scherhag (1948) to concentrate on analysing airmass boundaries at 850 mbar, removed from the disturbing boundary-layer influences. For this purpose, potential temperature, y, or, preferably, equivalent potential temperature, ye, or wet-bulb potential temperature, yw, remain as the only useful frontal parameters. The analysis of thermal gradients at 850 mbar can, in principle, easily be accomplished by objective procedures, and much progress has been made in developing thermal frontal parameters (TFPs) since Renard and Clarke’s (1965) original work (Steinacker 1992; Hewson 1998). A TFP chart at 850 mbar, be it automated or drawn by a skilled analyst, produces a clearer, smoother picture than can be obtained from surface observations. With this picture it becomes easier to return to the surface observations with all their boundary-layer complexity and to look in a systematic way for corroborating additional information (e.g. precipitation), taking into account the possible degree of decoupling. The idea is to obtain a space- and time-consistent picture by applying an older technique in reverse (`Aufbaumethode’ ), where hydrostatic and other consistency relations are used to trans345 Weather Vol. 57 port data coverage from the dense surface observation network upward to higher levels (Steinacker 1981). The effect of complex topography on fronts does not appear to have received the attention it might deserve. There are a few classics on the deformation of a cold front by the Alps (Bergeron 1934; Cantu 1977; Steinacker 1982). To our knowledge, there is hardly any published analysis on orographically modified warm fronts save for some schematic illustrations by Bjerknes and Solberg (1921). This, we believe, makes our present TMI analysis worthwhile in its own right, irrespective of the unusual accident background. Discussion of the continuing conceptual arguments on fronts and frontal analysis is beyond the scope of this work. With all the progress in objective analysis schemes, subjective experience and conceptual models are still needed in the very complicated, extremely stable synoptic setting surrounding the TMI-2 accident. Detailed synoptic analysis for 28 and 29 March The evolution of temperatures, winds, weather and atmospheric stability can be understood in terms of two warm fronts progressing across September 2002 the region of interest from the south-west, and latterly becoming stalled. This double structure is apparent in the data; it is also very plausible as the overall temperature and humidity contrast across the frontal zone, expressed in ye, is an enormous 50 degC. The warm fronts stretched over more than 1000 miles (1600 km), with warm-air advection progressing in individual surges, partly because the cold air being replaced offered more or less resistance to the oncoming warm airmasses. Warm air had a difficult time gradually eroding the cold air from above, especially as that air was dammed up and thereby protected by a mountain range. In addition to this deformation of the warm fronts seen on the charts (Figs. 4 and 5), the frontal zones became disfigured within the lowest 1 km or so above the ground by the daily march of temperature. Special analyses supporting our interpretation cannot be shown here for lack of space, among them a ye chart at 850 mbar and three time± height sections for Albany, Pittsburgh and Washington on which the isentropes drawn mesh the radiosondes’ high resolution in the vertical with the high frequency (hourly) of surface observations. A highly idealised picture of these two frontal zones would show three airmasses, which Fig. 4 Isochrones of the first (dash-dotted) and second warm fronts at 850 mbar. Times in EST. See text. Shaded areas are above 500 and above 1000 m above sea-level, respectively. 346 Weather Vol. 57 September 2002 (a) (b) Fig. 5 Isochrones of (a) the first warm front and (b) the second warm front at the surface. Times in EST (see text). Shaded areas are above 500 and above 1000 m above sea-level, respectively. In (a) the cold front/occlusion to the northwest is only indicated. In (b) not all dates are entered, as the front lines may be either too close to each other, masked by nocturnal inversions or otherwise not identifiable. The cold front/occlusion to the north-west is entered fully, with an indication of where the first warm front joins it. we call cold, moderate and warm. Typical daytime temperatures at the lower elevations would be 5 to 10 8C, 15 8C, and 20 to 25 8C, respectively, with an additional general warm- ing of up to 5 degC by 29 and 30 March outside the Canadian cold core mass. Within each airmass, horizontal temperature gradients were relatively small; gradients became concen347 Weather Vol. 57 trated, however, within the frontal zones between airmasses. The same is true in the vertical direction, where frontal inversions separate the airmasses. The general sense of progression of the two warm fronts from the south-west towards the north-east should not be confused with wind speeds, although the airflow happened to be from south-easterly to south-westerly directions at low levels, as evidenced, for example, by the TMI plume detected at Albany (Wahlen et al. 1980). From the 850 mbar pressure level upwards, airflow was generally from the west. At any fixed location, the frontal inversion progressively descended from higher levels, until finally the surface front passed the station. This means that inversion conditions at low levels, with all their inhibiting effects on atmospheric transport and dispersion, worsened during descent, to be relieved only if and when the surface front actually passed through. A strong frontal inversion would not easily be broken by the sun’s heating alone. We first present the results of detailed analyses of 12-hourly 850 mbar charts from 0700 EST on the 28th to 0700 EST on the 30th and the respective 3-hourly surface charts in distilled form in Figs. 4 and 5. The first warm front was always `ahead’ of the second, and each of the fronts was further ahead at 850 mbar than at the surface. The warm fronts were part of the entire system of cold front, warm fronts and occlusion winding up in the cyclone centre north of the Great Lakes region (Fig. 1). A cold front was approaching our region of interest from the north-west as indicated by the short segments (for the sake of clarity) in Fig. 5(a), and more fully in Fig. 5(b), but, as it had no direct impact on the Pennsylvania region during the first three days, we have refrained from visualising it in detail. At 850 mbar (Fig. 4), the first warm front had already crossed Pennsylvania by 1900 EST on 28 March. The associated surface front (Fig. 5(a)) had, however, barely reached the south-west corner of Pennsylvania by the end of the day. This first front had a stronger temperature contrast across it than the second one, but hardly any conspicuous weather activity. A pertinent signal, however, may have been the 348 September 2002 sudden high cloud deck appearing between 1600 and 1900 EST (see Fig. 2), clearing up completely before rain set in at midnight. The second warm front at 850 mbar came in two distinct tongues of warm air, of which the northern one more closely resembled a warm conveyor belt carrying warm, moist air upwards. The southern tongue moved across the southernmost strip of Pennsylvania during the night of 28/29 March. This tongue, due to conditional instability released by forced ascent, gave rise to frequent rain showers, even some thunderstorms, shifting from Indiana and Ohio on the afternoon and evening of 28 March across eastern Pennsylvania and on to the Atlantic coast in the early morning hours of 29 March. The northern warm tongue was even more active in this respect. The shores of Lake Erie and Lake Ontario, as well as parts of Illinois, Indiana and Ohio, experienced rain showers from the morning of 29 March until 31 March when the cold front finally crossed Pennsylvania in the afternoon. The two tongues were seen to be drifting backwards toward the south at a later stage. This was in response to the general southward drift of the cold front/occlusion, pushing the overrunning air ahead of it. Very significantly, the two tongues missed a region between them, which included part of Pennsylvania. We will pursue this feature and its consequences on the surface maps. Investigating the displacement of the surface warm fronts (Figs. 5(a) and (b)), a `steering’ action by the warm-air tongues described above will be apparent, including even steering of the first surface warm front by the second warm frontal zone lying on top of it. Such steering is an expression of the necessary coupling between near-surface and upper-air thermal and flow features. Pursuing the first surface warm front (Fig. 5(a)), it appeared to leap forward at times, e.g. between 1000 and 1300 EST. This is, of course, mostly not kinematic propagation, but a reflection of the fact that a shallow nocturnal inversion, which had made the edge of the warmer air at the surface stay far towards the southwest, was `burnt away’ by insolation. In such Weather Vol. 57 daytime conditions, the second surface warm front almost caught up with the first, so that a large temperature contrast could occur between the cold and the very warm airmasses across a small distance. For example, at 1900 EST, Huntington, West Virginia, reported 21 8C, but Cleveland, Ohio, only 8 8C. Similarly, Evansville, Indiana, had 21 8C and St. Louis, Missouri, 23 8C, but Green Bay, Wisconsin, only 3 8C! The US surface analysis at least showed a warm front between these stations, but the global surface map of the German Weather Service for the same date (Fig. 1), which had a selection of these and similar stations plotted, showed them all in one warm sector without a single front in between, indicating the confusion of analyses. At 1000 EST on 29 March, Morgantown, West Virginia, near the south-western corner of Pennsylvania, reported 23 8C, but Penn State College, a mere 200 km to the north-east, reported 8 8C! The first warm front pushed ahead towards the south-west corner of Pennsylvania late in the afternoon of 28 March, staying retarded both to the west, over Illinois, Indiana and Ohio, and to the east. Here, the surface high September 2002 was centred right over the Harrisburg area at 0400 EST on the 28th (time zero), drifting off towards the east-south-east thereafter, but the relatively shallow cool air dome, which was deeper further north, was still pushing southwards along the eastern slopes of the Appalachian mountain ranges across Virginia and North Carolina until the early afternoon of 28 March, as shown by the rising pressure and by the northerly surface winds. Even after the first warm front had begun to make some headway across North Carolina on the afternoon of 28 March, isobars remained tightly packed across the Appalachians (Fig. 6), and the cool air was sucked into the lower pressure on the western side across gaps in the terrain, in the manner of the `shallow foÈhn’ discussed earlier. Later in the evening of 28 March, the warm front briefly retreated southward and westward, pushed back across the mountain passes by the cool air to the east. Both the steering by the upper warm conveyor belts and the blocking effect of the Appalachian ranges, protecting the cold airmass and deforming the warm front accordingly, will be repeated by the second surface warm front. Fig. 6 Detailed surface analysis for 1900 EST on 28 March 1979. Isobars drawn every 1 mbar; two surface warm fronts entered. See text. Shaded areas are above 500 and above 1000 m above sea-level, respectively. 349 Weather Vol. 57 Figure 6 brings out the tight packing of the isobars across the Appalachians, their distinct `bulge’ westward beyond north-western Pennsylvania, suggestive of both the above-mentioned `shallow foÈhn’ and a tendency for stagnation west of Harrisburg, and the highly characteristic deformation of both warm fronts. On 29 March, the first warm front progressed along northern Ohio and Lake Erie first, until the nocturnal surface-based inversion had been heated up east of the Appalachians. By 1000 EST, the front had achieved the double structure reminiscent of the second front at 850 mbar. Its northern surge brought noticeable warming for Buffalo, Rochester and even Toronto between 0700 and 1000 EST. Its southern advance took the warm front to within 50 km of Harrisburg by 1000 EST, and just beyond Harrisburg by 1300 EST on 29 March, thereby alleviating the inversion condition. The cool air, however, tended to push the surface warm front back slightly in the evening hours of 29 March, when the pressure minimum was already to the south, and Harrisburg remained a `borderline case’ until 30 March. The second warm front did not quite reach Harrisburg (Fig. 5(b)). Cities such as Williamsport, Wilkes-Barre and Albany did not experience the passage of either surface warm front, remaining in the shallow cool air throughout. In addition, any transport of air from the Atlantic would bring cool air at this time of the year. At upper levels, however, the warm air did come through. Both warm fronts came through at Pittsburgh and Washington. The second warm front was, in most ways, a repeat of the first. It brought a new feature, however ± a warm-frontal wave, which carried extremely warm subtropical air towards the south-west corner of Pennsylvania by 1000 EST on the 29th and even up to the shores of Lake Erie by 1300 EST, propagating further east thereafter. In its rear, Pittsburgh experienced the striking return of moderately cooler air from the west between 1300 and 1600 EST. At this time of day, there is no mistaking the cooler air and accompanying inversion for an action of local night-time cooling, but in other cases the distinction between a nocturnal inversion and a frontal inversion near the ground may be difficult. 350 September 2002 In summary, Harrisburg saw the moderately warm surface air only in a rather brief `window’ during the early afternoon hours of 29 March. The two warm fronts approached each other closely in the course of the afternoon and became quasi-stationary across south-eastern Pennsylvania, with pressure gradients vanishing almost completely. Although only 150 km further south, Washington was already in the very warm air from the late morning of 29 March, and the inversion was broken there. Thus, the Washington sounding is not representative of the more stable lowest atmospheric layers near Harrisburg, where the conditions for the dispersion of TMI releases remained unfavourable. Looking at Fig. 5(b), the cold front/occlusion approached Pennsylvania from the northwest across the Great Lakes late on 29 March, but it did not hit the state then because a wave had formed along it. The ensuing `see-saw’ of winds and temperatures, familiar by now, will not be pursued in detail. Periods of extreme stagnation across eastern Pennsylvania occurred (cf. Fig. 2, 30 March), as well as temperatures approaching 25 8C at Harrisburg and 30 8C further south. The long regime of warmair advection, with its pronounced inversion and stagnation conditions, was finally ended by the passage of a cold front late on 5 April and the main upper trough on 6 April. Conclusion The balmy wind and weather conditions during the first few days of the TMI-2 accident were highly unfavourable for plume dispersion. Two successive warm fronts were slowed and deformed by the Appalachian Mountains. Locally very high exposures to radioactive releases due to plume impingement on hill sites are plausible. In order to trace and understand the consequences of accidents like this one, careful meteorological analysis and reasoning is required. To outline the synoptic situation is an important first step in visualising possible dispersion scenarios. Standard Gaussian procedures with built-in heavy averaging and strictly abstract presentation, time and again applied in official nuclear environmental impact stu- Weather Vol. 57 dies, may miss the essential interplay of individual plumes with wind, weather and hilly terrain completely. Acknowledgements Deep gratitude is extended to Norman and Marjorie Aamodt for sharing their knowledge of the case and their superior critical judgement of scientific and other matters with us, and for their persistent encouragement in trying to keep track of all the available data and facts. These two people and Nancy Laufer are wistful reminders of the motto of our academic teacher, H. Hoinkes, that a synoptic analysis should ``mate truth with grace’’ . Thanks are due to D. Whiteman and M. Staudinger for competent advice, to N. Span for drafting most of the figures, and to G. Mittermaier, C. Eller and B. Rainer for typing various stages of the manuscript. References Aamodt, N. and Aamodt, M. 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(1957) Dynamic meteorology and weather forecasting, published jointly by the American Meteorological Society and Carnegie Institution of Washington to the memory of Vilhelm Bjerknes. See specifically Fig. 16.43.1, p. 612.) Bjerknes, J. and Solberg, H. (1921) Meteorological conditions for the formation of rain. Geofys. Publ., 2, No. 3 (Excerpts published later in: Godske, C. L., Bergeron, T., Bjerknes, J. and Bundgaard, R. C. (1957) Dynamic meteorology and weather forecasting, published jointly by the American Meteorological Society and Carnegie Institution of September 2002 Washington to the memory of Vilhelm Bjerknes. See specifically Fig. 16.44.1, p. 613.) Browning, K. A. and Monk, G. A. (1982) A simple model for the synoptic analysis of cold fronts. Q. J. R. Meteorol. Soc., 108, pp. 435± 452 CantuÂ, V. (1977) The climate of Italy. In: Landsberg, H. (Ed.) 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Vergeiner, Institut fuÈr Meteorologie und Geophysik, UniversitaÈt Innsbruck, Innrain 52, A-6020 Innsbruck, Austria. e-mail: [email protected] # Royal Meteorological Society, 2002. 351