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

Transcription

1 920 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 first 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 fields to be constructed for the region south of 50 S. 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 influences 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 identification of circulation features in satellite cloud imagery when a favorable broadscale environment has been established and the interpretation of anomaly fields using Antarctic AWSs offer possibilities for the better prediction of the tracks of these small but significant systems. 1. Introduction The difficulties associated with the identification of atmospheric pressure systems over the Antarctic continent are well known to analysts and have been discussed by, inter alia, Schwerdtfeger (1984). Additionally, weather systems moving inland from the Antarctic coast have proved very difficult to track. Apart from the obvious limitations of the observational network, cloud signatures of systems are difficult to identify over the underlying ice surface. Furthermore, the elevation of the Antarctic plateau, which rises from approximately 2 km close to the coast to over 4 km at its highest point, requires that synoptic and mesoscale systems moving inland have well-defined vertical structures in order to survive. Inferences about pressure systems over Antarctica Corresponding author address: Michael J. Pook, Antarctic CRC, University of Tasmania, GPO Box , Hobart 7001, Australia. M.J.Pook@utas.edu.au have been influenced, in some cases, by charts of equivalent mean sea level (MSL) pressure. However, the method of constructing MSL charts requires that station barometric pressures are reduced to sea level by assuming that a column of air of known mean virtual temperature exists between the station and the datum. Errors introduced by this process make the practice of producing MSL pressure (MSLP) analyses over the Antarctic interior and other elevated continents highly dubious. Hence the regular appearance of very high pressure over Antarctica on MSL synoptic charts cannot be given a physical significance, a point emphasized by Schwerdtfeger (1984). Figure 1 shows output from the Australian Global Assimilation and Prediction model (GASP), a wave 53 spectral model on 19 sigma levels (Bourke et al. 1995), which estimated an MSLP in excess of 1040 hpa over East Antarctica at 0000 UTC on 27 July Analysts are faced with the difficulty of selecting a pressure level that is not compromised by the topography of Antarctica but that is sufficiently close to the 1999 American Meteorological Society

2 DECEMBER 1999 NOTES AND CORRESPONDENCE 921 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 field. 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 upperair program of observations throughout the year. This contrasts with the situation during the International Geophysical Year (IGY) of when there were nine staffed scientific 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 system for estimating 500-hPa geopotential heights from station-level observations of pressure and temperature at AWSs with elevations exceeding 2500 m. His analyses of the 500-hPa geopotential field over East Antarctica were incorporated in a set of analyses covering the region south of 50 S for the month of July These analyses form part of a project known as the First Regional Observing Study of the Troposphere (FROST), which provided an opportunity to investigate pressure systems over the interior of Antarctica. The FROST project (Turner et al. 1996) was designed to study the effects of all sources of late data on meteorological analyses over the Southern Ocean and Antarctic region and the probable impacts of these data on FIG. 2. (a) Map of Antarctica. (b) An enlarged section of East Antarctica showing the locations of AWSs and meteorological stations referred to in the text. the performance of numerical weather prediction models. It was constructed around three special observing periods (SOP): R SOP-1 in July 1994, R SOP-2 from 16 October to 15 November 1994, and R SOP-3 in January The preliminary analyses for SOP-1 were completed at the Bureau of Meteorology in Hobart by August 1995 and the reanalysis of a special week in SOP-1 (viz July 1994) was accomplished in September 1996.

3 922 WEATHER AND FORECASTING VOLUME 14 FIG. 3. SW-1 analyses of the 500-hPa geopotential surface (m) south of 50 S at 0000 UTC for (a) 22 Jul 1994, (b) 23 Jul 1994, (c) 24 Jul 1994, (d) 25 Jul 1994, TABLE 1. Geographical coordinates and elevations of selected climatological stations over Antarctica and the Southern Ocean. Station Lat ( S) Long ( E) Elevation (m) D-80 Dome C GC 41 GF 08 AGO 4 Macquarie Island Vostok The reanalysis for this special week was conducted with the addition of late data that included verified observations from AWSs, some drifting buoys, and, significantly, Advanced Very High Resolution Radiometer satellite imagery from National Oceanic and Atmospheric Administration satellites and Operational Linescan (OLS) imagery and Special Sensor Microwave/ Imager data from the Defense Meteorological Satellite Program (DMSP). The previously unavailable visible and infrared data from DMSP have been analyzed for

4 DECEMBER 1999 NOTES AND CORRESPONDENCE 923 FIG. 3.(Continued) (e) 26 Jul 1994, (f) 27 Jul 1994, and (g) 28 Jul the data-sparse Indian Ocean and Australasian sectors of the Southern Ocean using the semiobjective technique reported by Guymer (1978). The technique has been employed to locate cyclonic vortices over the ocean and, in some cases, as an aid in making estimates of the intensities of these systems. As well, the technique has been used to determine the structure, orientation, and intensity of key features in the hPa thickness field. A detailed description of the FROST analysis program is given in Hutchinson et al. (1999, this issue). Of particular significance to the reanalysis has been the ability to track weather systems inland over the Antarctic continent using the DMSP OLS thermal infrared channel as well as the visible channel with its ability to detect reflected moonlight during the polar night. For the most part, cyclonic systems appeared to remain north of the Antarctic coast but cloud bands from these systems were observed to move inland at regular intervals. In this paper we present a summary of the synoptic systems analyzed over the Antarctic continent during the special week analysis period of SOP-1, from 22 to 28 July 1994, inclusive. As well, we make comparisons with numerical analysis schemes and present a study of a period toward the end of the special week during

5 924 WEATHER AND FORECASTING VOLUME 14 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 final week of SOP-1 (b). which several vortices identified 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 The apparent anticyclone over the Antarctic continent on MSLP charts has little significance and arises from attempts to estimate equivalent mean sea level pressure 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 useful level at which to investigate the presence of significant anticyclonic systems in the free atmosphere above Antarctica. In SOP-1, manual analyses of the 500- hpa surface over East Antarctica were carried out by Phillpot (1997) and those over West Antarctica by one of the authors (Cowled) and subsequently blended into the 500-hPa analysis for the region south of 50 S for FIG. 5. Mean 500-hPa analysis at 0000 UTC for the period Jul 1994 from the SW-1 analysis set (a), and the difference between the mean of the SW hPa analyses and the mean Australian GASP analysis at 0000 UTC for the same period (b). the special week of reanalysis from 22 to 28 July These analyses will be referred to as SW-1 analyses while the first analysis set in SOP-1 for the complete month will be referred to as FROST-1. The SW-1 analyses at 0000 UTC are shown in Fig. 3. At the beginning of the reanalysis period (0000 UTC 22 July 1994), the SW-1 analysis of geopotential height at the 500-hPa level (Fig. 3a) identified a significant

6 DECEMBER 1999 NOTES AND CORRESPONDENCE 925 FIG. 6. Time series for Jul 1994 (SOP-1) of 500-hPa geopotential height (m) at Macquarie Island (54 S, 159 E) in the Southern Ocean, and station-level pressure at the automatic weather station located at dome C (74 S, 123 E) on the Antarctic plateau. high pressure ridge (R1) over East Antarctica between the Greenwich meridian and 120 E. This ridge remained the dominant anticyclonic system over the continent for the first half of the SW-1 analysis set (see Figs. 3b and 3c). The estimated central value of the high decreased gradually during the period from its initial peak in excess of 5050 geopotential m (gpm) to approximately 4900 gpm at 0000 UTC on 25 July 1994 (Fig. 3d). The high was absorbed subsequently by an intense ridge (R2) that extended southwestward from a blocking anticyclone in the southwest Pacific Ocean toward Vostok (see Table 1) on the Antarctic plateau (Figs. 3e, 3f, and 3g). As discussed in Turner et al. (1996) significant variations of surface pressure were found to occur over East Antarctica during SOP-1. In Fig. 3 the increase in geopotential over Victoria Land, the site of the R2 ridge, during the SW-1 analysis period was in excess of 250 m. Whereas the ridge at the start of the special week was confined to the continent or, possibly, weakly linked to a high in the Indian Ocean (Fig. 3a), the Pacific ridge appeared to propagate from the east across the plateau toward the end of the period. Time series of pressure anomalies at AWSs at elevated locations (Fig. 4a) shows pressure increasing at the easternmost stations first. Daily mean pressure values for dome C and the Automatic Geophysical Observatory (AGO 4) display similar behavior (Fig. 4b). Surface pressure variations for a selection of Australian AWSs on the Antarctic continent throughout SOP-1 have previously been shown by Turner et al. (1996), their Fig. 10. The mean 500-hPa analysis at 0000 UTC for the SW-1 analysis period is shown in Fig. 5a. It can be contrasted with the output from the Australian GASP model during SOP-1. The difference field between the mean SW-1 analyses at 0000 UTC for the 500-hPa level and the mean Australian GASP analysis (SW-1 GASP) 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 45 E 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 significant difference in the intensity of the ridge extending inland along 120 W. 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. The monthly mean of the 500-hPa geopotential height over East Antarctica from FROST-1 (see Turner et al. 1996, their Fig. 11) identified three maxima. Centers near 50 and 90 E closely mirror the maxima in SW-1. The center exceeding 5000 gpm that is apparent near 120 E in the monthly mean is, however, not evident in the SW-1 mean. This appears to be the result of a slight eastward displacement during the special week of the trough normally located near 100 E. b. Cyclones over Antarctica at 500 hpa Broad regions of relatively low geopotential are detectable over the Antarctic continent in the mean. Dalrymple (1966) identified four main features of the mean circulation in the middle troposphere over Antarctica. Drawing on data from the IGY, regions of relatively high geopotential were found to occur over central East Antarctica and over eastern Marie Byrd Land in West Antarctica with minima over the Ross and Weddell Seas. Schwerdtfeger (1970) demonstrated that these features are preserved in the seasonal cycle for the most part but there is a decrease of total mass over Antarctica in winter and the region of lowest geopotential tends to be more pole centered in summer. The mean SW hPa geopotential has been calculated from the analyses at 0000 UTC during the special week and the resulting pattern is shown in Fig. 5a. The wavenumber 4 pattern reveals minima over West Antarctica, the Weddell Sea, Enderby Land, and Wilkes Land. The GASP mean 500-hPa field for the same period (0000 UTC analyses) differs considerably in the Ross Sea region where the minimum value is over 100 m lower than our SW-1 minimum. The mean GASP analysis for the month of July 1994 (not shown) also produced a significant low over the Ross Ice Shelf. As shown in the previous section in relation to the ridge over West Antarctica, it appears that the GASP model had some difficulties in the southern Pacific region during SOP-1.

7 926 WEATHER AND FORECASTING VOLUME 14 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 circulations were in evidence over the continent in the daily analyses and underwent significant evolution with time. In the Indian Ocean sector the trough was oriented parallel to the coast throughout the SW-1 period with individual centers over the ocean and, on some occasions, just inland. However, the low over Victoria Land on 22 July 1994 was part of an extensive trough (T1) located well inland for the first 4 days of the period and subsequently appeared to be absorbed into an intense depression near the Amundsen Sea on 28 July. It was noted in section 2a that Victoria Land was a region of very large pressure increase during SW-1. Equally, these analyses demonstrate that the region between the Ross Sea and the Amundsen Sea experienced corresponding decreases of geopotential over the same period. 3. Cyclones migrating inland from the Southern Ocean Climatologies of cyclones over the Southern Hemisphere reveal an almost complete absence of lows over the Antarctic continent, particularly over East Antarctica south of 70 S (Sinclair 1994; Murray and Simmonds 1991). Similarly, studies of polar air vortices, or polar lows, have shown an almost total absence of these systems over the continental interior (Carleton and Carpenter 1990). It is generally accepted that cyclones rarely move over East Antarctica because of the barrier effect of the elevated ice sheet (Bromwich 1988). However, for several reasons, it is often very difficult to track cyclonic vortices over the Antarctic continent. Problems are encountered in discriminating cloud from the ice surface on satellite imagery because of the similarities in brightness temperatures observed in the thermal infrared channels and the similar albedos of the two surfaces in the visible channels (Turner and Row 1995). Other difficulties arise because of the limited nature of the conventional observational network. Recent improvements in the network of AWSs over East Antarctica combined with high quality satellite imagery provide opportunities to detect the movement inland of synoptic-scale cyclones. During the special week of SOP-1 several vortices

8 DECEMBER 1999 NOTES AND CORRESPONDENCE 927 FIG. 7.(Continued) (b) 0140 UTC 27 Jul 1994, and were observed to move across the coast of Antarctica and migrate inland over Wilkes Land. These vortices were apparent in the DMSP satellite cloud imagery in the latter part of the week. On 27 July 1994 a meridional cloud band had become quasi-stationary to the south of Australia with its western edge aligned approximately along 135 E and a sharply defined eastern boundary along 145 E. A cutoff low was apparent to the north of Macquarie Island and there was evidence of extensive ridging over the Tasman Sea and southward toward Antarctica. In Fig. 6, the 500-hPa geopotential at Macquarie Island, a sub-antarctic island to the southeast of Australia, is compared with the station level pressure at dome C on the Antarctic plateau. The time series for Macquarie Island shows a progression of midlatitude systems during the month of July In the middle of the month a cutoff low passed over the island, resulting in a temporary drop in geopotential height. This coincided with the peak station-level pressure for the month at dome C. However, toward the end of July the 500-hPa geopotential height rose sharply at Macquarie Island as a blocking anticyclone developed in the southwest Pacific. This pressure peak appeared to propagate across East Antarctica from the Pacific sector of the Southern Ocean as discussed in section 2a. The sky was clear of cloud over much of the Ross Sea and over East Antarctica south of 75 S. Along with surface and upper-air data from Macquarie Island over preceding days these indications of strong subsidence provided evidence of atmospheric blocking in the region. Waves forming on the cloud band were observed to move rapidly southward, and by approximately 2300 UTC on 26 July 1994 two apparent cyclonic vortices in the cloud field had tracked across the coast of Antarctica (Fig. 7a). Estimated cloud-top temperatures on the western side of the vortex near 69 S, 127 E were approximately 230 K, which corresponded to the 450- hpa pressure level on the radiosonde trace from Casey (66.3 S, E). The easternmost of these vortices (A) continued moving southward and by 0110 UTC 27 July was located near 71.5 S, 124 E (Fig. 7b). During this period the western vortex (B) moved slightly westward and its associated cloud bands became less distinct. In Fig. 7c, one orbit later, vortex A was still clearly evident in the IR cloud field and had moved well south of 70 S. The

9 928 WEATHER AND FORECASTING VOLUME 14 FIG. 7.(Continued) (c) 0220 UTC 27 Jul DMSP visible light image (not shown), which was obtained from reflected moonlight, revealed that the system had preserved its vertical structure. The vortex appeared to be tracking toward dome C. Evidence that the eastern vortex (A) was detectable at the surface was provided by the AWS at dome C (74.5 S, 123 E; 3280 m). This station had indicated a steadily rising barometric pressure over several days FIG. 8. Wind speed (m s 1 ) and direction ( true 10) time series for the dome C AWS during the period Jul (see Fig. 4a) but experienced a fall in pressure of approximately 3 hpa between 0600 and 1500 UTC on 27 July. Figure 8 demonstrates that the anemometer recorded a rapid increase in wind speed from approximately 2 m s 1 at 0000 UTC on 27 July to 7.1 m s 1 at 1200 UTC and back to 0 m s 1 by 0000 UTC on 28 July. The wind changed direction from easterly to northerly and back to easterly within 24 h. The mean wind speed at dome C for the month of July 1994 was 2.6 ms 1 with a standard deviation of 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 The vortices were not identified 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 2.5 latitude (275 km) and remained in the mesocyclone scale length throughout its lifetime. 4. Discussion and conclusions The FROST exercise has provided an opportunity to study in closer detail the characteristics of synoptic-

10 DECEMBER 1999 NOTES AND CORRESPONDENCE 929 scale and mesoscale systems over the Antarctic continent. Analyses of the 500-hPa surface during SW-1 revealed detailed structure at this level and evolution of individual systems throughout the period. Variations in station-level pressure experienced at AWS on the high plateau during SOP-1 were similar to ranges experienced at lower latitudes of the Southern Hemisphere and confirm that the atmospheric circulation is complex, even at these high latitudes and elevations. This study demonstrates that vortices originating over 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 the Tasman Sea sector appears to have been a critical factor in this case. Occurrences of this type have the potential to influence precipitation events over the Antarctic interior in a significant way and further study is required to attempt to quantify the effects of these cyclones. Clearly, the numerical models in operation during FROST did not have the necessary resolution to identify the relatively small cyclonic systems, with their comparatively short lifetimes, that penetrated inland during this study. Nor was it possible to locate them using traditional methods of manual analysis as the density of observations in Antarctica is too low. The new generation of mesoscale models nesting within global models may provide possibilities of modeling these systems over the Antarctic in the future. However, now that good climatologies have been developed for many AWSs in Antarctica the technique employed in this paper to detect cyclonic systems over the Antarctic Plateau using AWS anomaly fields and high-resolution satellite imagery appears capable of adaptation to operational use. The possibility exists for analysis of AWS pressure anomalies in the observational stream, allowing much finer temporal resolution of features because of the near-hourly reporting frequency of AWSs. A further possibility would be to extend the use of anomaly fields to temperature, dewpoint, or wind. Additionally, the presentation of the same anomaly fields, but derived from numerical analyses and prognoses, especially in the next generation of high-resolution models, could provide a useful predictive capability. Acknowledgments. This research has been partially funded by an Australian Research Council large grant. Daily means of meteorological data for the Automatic Geophysical Observatory, AGO 4, were obtained from the World Wide Web site of the University of Maryland, and data from the automatic weather stations, GC 41 and GF 08, were supplied by the Australian Antarctic Division. We would like to thank Professor Bill Budd and Dr. Tim Gibson of the Antarctic Co-operative Research Centre for helpful suggestions on the original manuscript and four anonymous reviewers for constructive criticisms. REFERENCES Bourke, W., T. Hart, P. Steinle, R. Seaman, G. Embery, M. Naughton, and L. Rikus, 1995: Evolution of the Bureau of Meteorology s Global Assimilation and Prediction system. Part 2: resolution enhancements and case studies. Aust. Meteor. Mag., 44, Bromwich, D. H., 1988: Snowfall in high southern latitudes. Rev. Geophys., 26, Carleton, A. M., and D. A. Carpenter, 1990: Satellite climatology of polar lows and broadscale climatic associations for the Southern Hemisphere. Int. J. 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Mag., 39, , 1997: Some observationally-identified meteorological features of East Antarctica. The Impact of Project FROST, Meteorological Study No. 42, Australian Government Publishing Service, Schwerdtfeger, W., 1970: The climate of the Antarctic. World Survey of Climatology, S. Orvig, Ed., Vol. 14, Elsevier, , 1984: Weather and Climate of the Antarctic. Elsevier, 261 pp. Sinclair, M. R., 1994: An objective cyclone climatology for the Southern Hemisphere. Mon. Wea. Rev., 122, Turner, J., and M. Row, 1995: Polar phenomena. Images in Weather Forecasting, M. J. Bader et al., Eds., CUB, , and Coauthors, 1996: The Antarctic First Regional Observing Study of the Troposphere (FROST) project. Bull. Amer. Meteor. Soc., 77,

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