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920 WEATHER AND FORECASTING VOLUME 14

On the Detection of Weather Systems over the Antarctic Interior in the FROST Analyses

MICHAEL POOK Institute of Antarctic and Southern Ocean Studies and Antarctic CRC, University of Tasmania, Hobart, Australia

LANCE COWLED Bureau of Meteorology, Hobart, Australia

28 February 1998 and 25 March 1999

ABSTRACT The ®rst Special Observing Period (SOP-1) of the Antarctic First Regional Observing Study of the Troposphere (FROST) was completed in July 1994 and provided a unique opportunity to assemble a comprehensive dataset for the Antarctic region. Data obtained from this intensive collection effort have been undergoing analysis at several centers around the world, including Hobart in Australia. The synoptic analysis program for SOP-1 has been completed in Hobart and, additionally, a reanalysis of a ``special week'' (22±28 July) has been undertaken, enabling 500-hPa contour ®elds to be constructed for the region south of 50ЊS. Results of these analyses for continental Antarctica are presented and comparisons made with operational analyses from numerical models. Satellite imagery from the Defense Meteorological Satellite Program (DMSP) was employed in the special week reanalysis and has provided evidence of several vortices that moved southward over East Antarctica during the latter part of July 1994 and appeared to decay over the high plateau. Observations from the network of automatic weather stations (AWSs) over East Antarctica were combined with satellite imagery to infer the movement inland of these cyclones. It is demonstrated that broadscale and synoptic-scale in¯uences contributed to the migration of cyclones over East Antarctica during SOP-1 and, in particular, an association is established between the incidence of atmospheric blocking activity in the Tasman Sea and the inland penetration of lows. The early identi®cation of circulation features in satellite cloud imagery when a favorable broadscale environment has been established and the interpretation of anomaly ®elds using Antarctic AWSs offer possibilities for the better prediction of the tracks of these small but signi®cant systems.

1. Introduction have been in¯uenced, in some cases, by charts of equiv- The dif®culties associated with the identi®cation of alent mean sea level (MSL) pressure. However, the atmospheric pressure systems over the Antarctic con- method of constructing MSL charts requires that station tinent are well known to analysts and have been dis- barometric pressures are reduced to sea level by assum- cussed by, inter alia, Schwerdtfeger (1984). Addition- ing that a column of air of known mean virtual tem- ally, weather systems moving inland from the Antarctic perature exists between the station and the datum. Errors coast have proved very dif®cult to track. Apart from the introduced by this process make the practice of pro- obvious limitations of the observational network, cloud ducing MSL pressure (MSLP) analyses over the Ant- signatures of systems are dif®cult to identify over the arctic interior and other elevated continents highly du- underlying ice surface. Furthermore, the elevation of bious. Hence the regular appearance of very high pres- the Antarctic plateau, which rises from approximately sure over Antarctica on MSL synoptic charts cannot be 2 km close to the coast to over 4 km at its highest point, given a physical signi®cance, a point emphasized by requires that synoptic and mesoscale systems moving Schwerdtfeger (1984). Figure 1 shows output from the inland have well-de®ned vertical structures in order to Australian Global Assimilation and Prediction model survive. (GASP), a wave 53 spectral model on 19 sigma levels Inferences about pressure systems over Antarctica (Bourke et al. 1995), which estimated an MSLP in excess of 1040 hPa over East Antarctica at 0000 UTC on 27 July 1994. Analysts are faced with the dif®culty of selecting a Corresponding author address: Michael J. Pook, Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart 7001, Australia. pressure level that is not compromised by the topog- E-mail: [email protected] raphy of Antarctica but that is suf®ciently close to the

᭧ 1999 American Meteorological Society

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FIG. 1. MSLP analysis from the Australian GASP model for 0000 UTC 27 Jul 1994 indicating a central pressure exceeding 1043 hPa over East Antarctica. surface to be linked dynamically to the surface wind ®eld. In this study, we have followed Phillpot (1991) and selected the 500-hPa surface as the most suitable level for analysis over Antarctica. Despite the gradual expansion in the network of surface observations that has been achieved in recent years by the installation of automatic weather stations (AWSs) there has been a reduction of upper-air observations in the interior of the continent. In 1994, South Pole station was the only inland station still conducting an upper- air program of observations throughout the year. This contrasts with the situation during the International Geo- physical Year (IGY) of 1957±58 when there were nine staffed scienti®c stations providing meteorological data from elevations of 1500 m or more (Dalrymple 1966). Clearly, the lack of rawinsonde stations makes conventional upper-air analysis impossible without the input of other data. To overcome this problem Phillpot (1991) devised a FIG. 2. (a) Map of Antarctica. (b) An enlarged section of East system for estimating 500-hPa geopotential heights Antarctica showing the locations of AWSs and meteorological sta- from station-level observations of pressure and tem- tions referred to in the text. perature at AWSs with elevations exceeding 2500 m. His analyses of the 500-hPa geopotential ®eld over East the performance of numerical weather prediction mod- Antarctica were incorporated in a set of analyses cov- els. It was constructed around three special observing ering the region south of 50ЊS for the month of July periods (SOP): 1994. These analyses form part of a project known as R the First Regional Observing Study of the Troposphere SOP-1 in July 1994, R (FROST), which provided an opportunity to investigate SOP-2 from 16 October to 15 November 1994, and R pressure systems over the interior of Antarctica. The SOP-3 in January 1995. FROST project (Turner et al. 1996) was designed to The preliminary analyses for SOP-1 were completed study the effects of all sources of ``late'' data on me- at the Bureau of Meteorology in Hobart by August 1995 teorological analyses over the Southern Ocean and Ant- and the reanalysis of a ``special week'' in SOP-1 (viz. arctic region and the probable impacts of these data on 22±28 July 1994) was accomplished in September 1996.

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FIG. 3. SW-1 analyses of the 500-hPa geopotential surface (m) south of 50ЊS at 0000 UTC for (a) 22 Jul 1994, (b) 23 Jul 1994, (c) 24 Jul 1994, (d) 25 Jul 1994,

The reanalysis for this special week was conducted TABLE 1. Geographical coordinates and elevations of selected climatological stations over Antarctica and the Southern Ocean. with the addition of late data that included veri®ed observations from AWSs, some drifting buoys, and, sig- Station Lat (ЊS) Long (ЊE) Elevation (m) ni®cantly, Advanced Very High Resolution Radiometer D-80 70.02 134.72 2500 satellite imagery from National Oceanic and Atmo- Dome C 74.50 123.00 3280 spheric Administration satellites and Operational Line- GC 41 71.60 111.25 2740 GF 08 68.50 102.18 2118 scan (OLS) imagery and Special Sensor Microwave/ AGO 4 82.01 96.76 3565 Imager data from the Defense Meteorological Satellite Macquarie Island 54.50 158.93 6 Program (DMSP). The previously unavailable visible Vostok 78.45 106.87 3488 and infrared data from DMSP have been analyzed for

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FIG.3.(Continued) (e) 26 Jul 1994, (f) 27 Jul 1994, and (g) 28 Jul 1994.

the data-sparse Indian Ocean and Australasian sectors arctic continent using the DMSP OLS thermal infrared of the Southern Ocean using the semiobjective tech- channel as well as the visible channel with its ability nique reported by Guymer (1978). The technique has to detect re¯ected moonlight during the polar night. For been employed to locate cyclonic vortices over the the most part, cyclonic systems appeared to remain north ocean and, in some cases, as an aid in making estimates of the Antarctic coast but cloud bands from these sys- of the intensities of these systems. As well, the technique tems were observed to move inland at regular intervals. has been used to determine the structure, orientation, In this paper we present a summary of the synoptic and intensity of key features in the 1000±500-hPa thick- systems analyzed over the Antarctic continent during ness ®eld. A detailed description of the FROST analysis the special week analysis period of SOP-1, from 22 to program is given in Hutchinson et al. (1999, this issue). 28 July 1994, inclusive. As well, we make comparisons Of particular signi®cance to the reanalysis has been with numerical analysis schemes and present a study of the ability to track weather systems inland over the Ant- a period toward the end of the special week during

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FIG. 4. Time series of station-level pressure anomaly (hPa) at selected AWSs on the Antarctic plateau during the special week of SOP-1 (a), and comparison of the daily mean station-level pressure (hPa) at dome C and AGO 4 during the ®nal week of SOP-1 (b). which several vortices identi®ed on cloud imagery were observed to move across the coast of East Antarctica and penetrate well inland. Meteorological stations discussed in the text are shown in the map in Fig. 2 and their geographical coordinates and elevations are given in Table 1. A complete listing of the stations employed in the FROST analysis program together with a map can be found in Turner et al. (1996).

2. Synoptic-scale systems over Antarctica during SOP-1 a. Anticyclones at 500 hPa FIG. 5. Mean 500-hPa analysis at 0000 UTC for the period 22±28 The apparent anticyclone over the Antarctic continent Jul 1994 from the SW-1 analysis set (a), and the difference between on MSLP charts has little signi®cance and arises from the mean of the SW-1 500-hPa analyses and the mean Australian attempts to estimate equivalent mean sea level pressure GASP analysis at 0000 UTC for the same period (b). from station-level pressures at elevations generally exceeding 2000 m. However, the 500-hPa surface does not intersect the Antarctic terrain and can be regarded as a the special week of reanalysis from 22 to 28 July 1994. useful level at which to investigate the presence of sig- These analyses will be referred to as SW-1 analyses ni®cant anticyclonic systems in the free atmosphere while the ®rst analysis set in SOP-1 for the complete above Antarctica. In SOP-1, manual analyses of the 500- month will be referred to as FROST-1. The SW-1 analy- hPa surface over East Antarctica were carried out by ses at 0000 UTC are shown in Fig. 3. Phillpot (1997) and those over West Antarctica by one At the beginning of the reanalysis period (0000 UTC of the authors (Cowled) and subsequently blended into 22 July 1994), the SW-1 analysis of geopotential height the 500-hPa analysis for the region south of 50ЊS for at the 500-hPa level (Fig. 3a) identi®ed a signi®cant

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and the mean Australian GASP analysis (SW-1ÐGASP) for the same period (22±28 July 1994) and also at 0000 UTC is shown in Fig. 5b. Over East Antarctica, the peak geopotential occurs in the SW-1 analysis near 45ЊE while the GASP analysis has a ridge closer to the pole and the strongest Antarctic ridge is found to the south of Australia. In other respects there is good agreement between SW-1 and GASP in the East Antarctic sector but over West Antarctica, there is a signi®cant difference in the intensity of the ridge extending inland along 120ЊW. The GASP estimate of the intensity of the ridge in this region is approximately 100 gpm higher than the SW-1 result. This is a region where Turner et al. (1996) found that the GASP model had not performed well during SOP-1. The other region where there was a marked difference between SW-1 and GASP was in the strength of the trough near the Ross Sea, and this will be discussed in the following section. FIG. 6. Time series for Jul 1994 (SOP-1) of 500-hPa geopotential The monthly mean of the 500-hPa geopotential height height (m) at Macquarie Island (54ЊS, 159ЊE) in the Southern Ocean, over East Antarctica from FROST-1 (see Turner et al. and station-level pressure at the automatic weather station located at 1996, their Fig. 11) identi®ed three maxima. Centers dome C (74ЊS, 123ЊE) on the Antarctic plateau. near 50Њ and 90ЊE closely mirror the maxima in SW-1. The center exceeding 5000 gpm that is apparent near high pressure ridge (R1) over East Antarctica between 120ЊE in the monthly mean is, however, not evident in the Greenwich meridian and 120ЊE. This ridge remained the SW-1 mean. This appears to be the result of a slight the dominant anticyclonic system over the continent for eastward displacement during the special week of the the ®rst half of the SW-1 analysis set (see Figs. 3b and trough normally located near 100ЊE. 3c). The estimated central value of the high decreased gradually during the period from its initial peak in ex- b. Cyclones over Antarctica at 500 hPa cess of 5050 geopotential m (gpm) to approximately 4900 gpm at 0000 UTC on 25 July 1994 (Fig. 3d). The Broad regions of relatively low geopotential are de- high was absorbed subsequently by an intense ridge tectable over the Antarctic continent in the mean. Dal- (R2) that extended southwestward from a blocking an- rymple (1966) identi®ed four main features of the mean ticyclone in the southwest Paci®c Ocean toward Vostok circulation in the middle troposphere over Antarctica. (see Table 1) on the Antarctic plateau (Figs. 3e, 3f, and Drawing on data from the IGY, regions of relatively 3g). As discussed in Turner et al. (1996) signi®cant high geopotential were found to occur over central East variations of surface pressure were found to occur over Antarctica and over eastern Marie Byrd Land in West East Antarctica during SOP-1. In Fig. 3 the increase in Antarctica with minima over the Ross and Weddell Seas. geopotential over Victoria Land, the site of the R2 ridge, Schwerdtfeger (1970) demonstrated that these features during the SW-1 analysis period was in excess of 250 m. are preserved in the seasonal cycle for the most part but Whereas the ridge at the start of the special week was there is a decrease of total mass over Antarctica in winter con®ned to the continent or, possibly, weakly linked to and the region of lowest geopotential tends to be more a high in the Indian Ocean (Fig. 3a), the Paci®c ridge pole centered in summer. appeared to propagate from the east across the plateau The mean SW-1 500-hPa geopotential has been cal- toward the end of the period. Time series of pressure culated from the analyses at 0000 UTC during the spe- anomalies at AWSs at elevated locations (Fig. 4a) shows cial week and the resulting pattern is shown in Fig. 5a. pressure increasing at the easternmost stations ®rst. Dai- The wavenumber 4 pattern reveals minima over West ly mean pressure values for dome C and the Automatic Antarctica, the Weddell Sea, Enderby Land, and Wilkes Geophysical Observatory (AGO 4) display similar be- Land. The GASP mean 500-hPa ®eld for the same pe- havior (Fig. 4b). Surface pressure variations for a se- riod (0000 UTC analyses) differs considerably in the lection of Australian AWSs on the Antarctic continent Ross Sea region where the minimum value is over 100 throughout SOP-1 have previously been shown by Turn- m lower than our SW-1 minimum. The mean GASP er et al. (1996), their Fig. 10. analysis for the month of July 1994 (not shown) also The mean 500-hPa analysis at 0000 UTC for the produced a signi®cant low over the Ross Ice Shelf. As SW-1 analysis period is shown in Fig. 5a. It can be shown in the previous section in relation to the ridge contrasted with the output from the Australian GASP over West Antarctica, it appears that the GASP model model during SOP-1. The difference ®eld between the had some dif®culties in the southern Paci®c region dur- mean SW-1 analyses at 0000 UTC for the 500-hPa level ing SOP-1.

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FIG. 7. DMSP images of cyclonic vortices over East Antarctica at approximately (a) 2300 UTC 26 Jul 1994,

As can be seen from Fig. 3, closed cyclonic circu- the Antarctic continent, particularly over East Antarctica lations were in evidence over the continent in the daily south of 70ЊS (Sinclair 1994; Murray and Simmonds analyses and underwent signi®cant evolution with time. 1991). Similarly, studies of polar air vortices, or polar In the Indian Ocean sector the trough was oriented par- lows, have shown an almost total absence of these sys- allel to the coast throughout the SW-1 period with in- tems over the continental interior (Carleton and Car- dividual centers over the ocean and, on some occasions, penter 1990). It is generally accepted that cyclones rare- just inland. However, the low over Victoria Land on 22 ly move over East Antarctica because of the barrier July 1994 was part of an extensive trough (T1) located effect of the elevated ice sheet (Bromwich 1988). How- well inland for the ®rst 4 days of the period and sub- ever, for several reasons, it is often very dif®cult to track sequently appeared to be absorbed into an intense de- cyclonic vortices over the Antarctic continent. Problems pression near the Amundsen Sea on 28 July. It was noted are encountered in discriminating cloud from the ice in section 2a that Victoria Land was a region of very surface on satellite imagery because of the similarities large pressure increase during SW-1. Equally, these an- in brightness temperatures observed in the thermal in- alyses demonstrate that the region between the Ross Sea frared channels and the similar albedos of the two sur- and the Amundsen Sea experienced corresponding de- faces in the visible channels (Turner and Row 1995). creases of geopotential over the same period. Other dif®culties arise because of the limited nature of the conventional observational network. Recent im- provements in the network of AWSs over East Antarc- 3. Cyclones migrating inland from the tica combined with high quality satellite imagery pro- Southern Ocean vide opportunities to detect the movement inland of syn- Climatologies of cyclones over the Southern Hemi- optic-scale cyclones. sphere reveal an almost complete absence of lows over During the special week of SOP-1 several vortices

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FIG.7.(Continued) (b) 0140 UTC 27 Jul 1994, and

were observed to move across the coast of Antarctica across East Antarctica from the Paci®c sector of the and migrate inland over Wilkes Land. These vortices Southern Ocean as discussed in section 2a. were apparent in the DMSP satellite cloud imagery in The sky was clear of cloud over much of the Ross the latter part of the week. On 27 July 1994 a meridional Sea and over East Antarctica south of 75ЊS. Along with cloud band had become quasi-stationary to the south of surface and upper-air data from Macquarie Island over Australia with its western edge aligned approximately preceding days these indications of strong subsidence along 135ЊE and a sharply de®ned eastern boundary provided evidence of atmospheric blocking in the re- along 145ЊE. A cutoff low was apparent to the north of gion. Waves forming on the cloud band were observed Macquarie Island and there was evidence of extensive to move rapidly southward, and by approximately 2300 ridging over the Tasman Sea and southward toward Ant- UTC on 26 July 1994 two apparent cyclonic vortices arctica. In Fig. 6, the 500-hPa geopotential at Macquarie in the cloud ®eld had tracked across the coast of Ant- Island, a sub-Antarctic island to the southeast of Aus- arctica (Fig. 7a). Estimated cloud-top temperatures on tralia, is compared with the station level pressure at the western side of the vortex near 69ЊS, 127ЊE were dome C on the Antarctic plateau. The time series for approximately 230 K, which corresponded to the 450- Macquarie Island shows a progression of midlatitude hPa pressure level on the radiosonde trace from Casey systems during the month of July 1994. In the middle (66.3ЊS, 110.5ЊE). of the month a cutoff low passed over the island, re- The easternmost of these vortices (A) continued mov- sulting in a temporary drop in geopotential height. This ing southward and by 0110 UTC 27 July was located coincided with the peak station-level pressure for the near 71.5ЊS, 124ЊE (Fig. 7b). During this period the month at dome C. However, toward the end of July the western vortex (B) moved slightly westward and its as- 500-hPa geopotential height rose sharply at Macquarie sociated cloud bands became less distinct. In Fig. 7c, Island as a blocking anticyclone developed in the south- one orbit later, vortex A was still clearly evident in the west Paci®c. This pressure peak appeared to propagate IR cloud ®eld and had moved well south of 70ЊS. The

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FIG.7.(Continued) (c) 0220 UTC 27 Jul 1994.

DMSP visible light image (not shown), which was ob- (see Fig. 4a) but experienced a fall in pressure of ap- tained from re¯ected moonlight, revealed that the sys- proximately 3 hPa between 0600 and 1500 UTC on 27 tem had preserved its vertical structure. The vortex ap- July. Figure 8 demonstrates that the anemometer re- peared to be tracking toward dome C. corded a rapid increase in wind speed from approxi- Evidence that the eastern vortex (A) was detectable mately 2 m sϪ1 at 0000 UTC on 27 July to 7.1 m sϪ1 at the surface was provided by the AWS at dome C at 1200 UTC and back to 0 m sϪ1 by 0000 UTC on 28 (74.5ЊS, 123ЊE; 3280 m). This station had indicated a July. The wind changed direction from easterly to north- steadily rising barometric pressure over several days erly and back to easterly within 24 h. The mean wind speed at dome C for the month of July 1994 was 2.6 msϪ1 with a standard deviation of 2.15. The peak value on 27 July was the highest wind speed recorded at dome C during the 10-day period from 22 to 31 July 1994. The vortices were not identi®ed on the SW-1 analyses at 500 hPa previously discussed in section 2. It is by no means certain why this is so but appears to be related to the relative size of the vortices and the rapid movement inland of the easternmost vortex, A. The diameter of this system was probably never more than 2.5Њ lat- itude (275 km) and remained in the mesocyclone scale length throughout its lifetime.

4. Discussion and conclusions

FIG. 8. Wind speed (m sϪ1) and direction (Њtrue ϫ 10) time series The FROST exercise has provided an opportunity to for the dome C AWS during the period 26±28 Jul 1994. study in closer detail the characteristics of synoptic-

Unauthenticated | Downloaded 10/02/21 07:18 AM UTC DECEMBER 1999 NOTES AND CORRESPONDENCE 929 scale and mesoscale systems over the Antarctic conti- Daily means of meteorological data for the Automatic nent. Analyses of the 500-hPa surface during SW-1 re- Geophysical Observatory, AGO 4, were obtained from vealed detailed structure at this level and evolution of the World Wide Web site of the University of Maryland, individual systems throughout the period. Variations in and data from the automatic weather stations, GC 41 station-level pressure experienced at AWS on the high and GF 08, were supplied by the Australian Antarctic plateau during SOP-1 were similar to ranges experi- Division. We would like to thank Professor Bill Budd enced at lower latitudes of the Southern Hemisphere and Dr. Tim Gibson of the Antarctic Co-operative Re- and con®rm that the atmospheric circulation is complex, search Centre for helpful suggestions on the original even at these high latitudes and elevations. manuscript and four anonymous reviewers for construc- This study demonstrates that vortices originating over tive criticisms. the Southern Ocean can penetrate the high plateau of East Antarctica and move well inland before decaying. The development of an intense blocking anticyclone in REFERENCES the Tasman Sea sector appears to have been a critical factor in this case. Occurrences of this type have the Bourke, W., T. Hart, P. Steinle, R. Seaman, G. Embery, M. Naughton, potential to in¯uence precipitation events over the Ant- and L. Rikus, 1995: Evolution of the Bureau of Meteorology's Global Assimilation and Prediction system. Part 2: resolution arctic interior in a signi®cant way and further study is enhancements and case studies. Aust. Meteor. Mag., 44, 19±40. required to attempt to quantify the effects of these cy- Bromwich, D. H., 1988: Snowfall in high southern latitudes. Rev. clones. Geophys., 26, 149±168. Clearly, the numerical models in operation during Carleton, A. M., and D. A. Carpenter, 1990: Satellite climatology of FROST did not have the necessary resolution to identify `polar lows' and broadscale climatic associations for the South- ern Hemisphere. Int. J. Climatol., 10, 219±246. the relatively small cyclonic systems, with their com- Dalrymple, P. C., 1966: A physical climatology of the Antarctic Pla- paratively short lifetimes, that penetrated inland during teau. Studies in Antarctic Meteorology, M. J. Rubin, Ed., Vol. this study. Nor was it possible to locate them using 9, Antarctic Research Series, Amer. Geophys. Union, 195±231. traditional methods of manual analysis as the density of Guymer, L. B., 1978: Operational application of satellite imagery to observations in Antarctica is too low. The new gener- synoptic analysis in the Southern Hemisphere. Bureau of Me- teorology Tech. Rep. 29, 87 pp. [Available from Bureau of Me- ation of mesoscale models nesting within global models teorology, GPO Box 1289 K, Melbourne, Victoria 3001, Aus- may provide possibilities of modeling these systems tralia.] over the Antarctic in the future. Hutchinson, H. A., and Coauthors, 1999: On the reanalysis of South- However, now that good climatologies have been de- ern Hemisphere charts for the FROST project. Wea. Forecasting, veloped for many AWSs in Antarctica the technique 14, 909±919. Murray, R. J., and I. Simmonds, 1991: A numerical scheme for track- employed in this paper to detect cyclonic systems over ing cyclone centers from digital data. Part III: Application to the Antarctic Plateau using AWS anomaly ®elds and January and July general circulation model simulations. Aust. high-resolution satellite imagery appears capable of ad- Meteor. Mag., 39, 167±180. aptation to operational use. The possibility exists for Phillpot, H. R., 1991: The derivation of 500-hPa height from auto- analysis of AWS pressure anomalies in the observational matic weather station surface observations in the Antarctic continental interior. Aust. Meteor. Mag., 39, 79±86. stream, allowing much ®ner temporal resolution of fea- , 1997: Some observationally-identi®ed meteorological features tures because of the near-hourly reporting frequency of of East Antarctica. The Impact of Project FROST, Meteorolog- AWSs. A further possibility would be to extend the use ical Study No. 42, Australian Government Publishing Service, of anomaly ®elds to temperature, dewpoint, or wind. 181±211. Additionally, the presentation of the same anomaly Schwerdtfeger, W., 1970: The climate of the Antarctic. World Survey ®elds, but derived from numerical analyses and prog- of Climatology, S. Orvig, Ed., Vol. 14, Elsevier, 253±331. , 1984: Weather and Climate of the Antarctic. Elsevier, 261 pp. noses, especially in the next generation of high-reso- Sinclair, M. R., 1994: An objective cyclone climatology for the South- lution models, could provide a useful predictive capa- ern Hemisphere. Mon. Wea. Rev., 122, 2239±2256. bility. Turner, J., and M. Row, 1995: Polar phenomena. Images in Weather Forecasting, M. J. Bader et al., Eds., CUB, 484±490. , and Coauthors, 1996: The Antarctic First Regional Observing Acknowledgments. This research has been partially Study of the Troposphere (FROST) project. Bull. Amer. Meteor. funded by an Australian Research Council large grant. Soc., 77, 2007±2032.

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