3548 MONTHLY WEATHER REVIEW—SPECIAL SECTION VOLUME 133 Identifying the Characteristics of Strong Southerly Wind Events at Casey Station in East Antarctica Using a Numerical Weather Prediction System NEIL ADAMS Australian Bureau of Meteorology, and Antarctic Climate and Ecosystems, Cooperative Research Centre, Hobart, Tasmania, Australia (Manuscript received 22 December 2004, in final form 23 June 2005) ABSTRACT Casey Station in East Antarctica is not often subject to strong southerly flow off the Antarctic continent but when such events occur, operations at the station are often adversely impacted. Not only are the dynamics of such events poorly understood, but the forecasting of such occurrences is difficult. The fol- lowing study uses model output from a 12-month experiment using the Antarctic Limited-Area Prediction System (ALAPS) to advance the understanding of the dynamics of such events and postulates that what are often described as katabatic wind events are more likely to be synoptic in scale, with mid- and upper-level tropospheric dynamics forcing the surface layer flow. Strong surface layer flows that have a katabatic signature commonly develop on the steep Antarctic escarpment but rarely extend out over the coast in the Casey area, most probably as a result of cold air damming. However, the development of a strong south- southwesterly jet over Casey provides a mechanism whereby the katabatic can move out off the coast. 1. Introduction nantly from the northeast, off Law Dome, East Ant- arctica (Fig. 1). A meteorological event that is poorly forecast in the The katabatic flow off the Vanderford Glacier is of- Casey Station area in East Antarctica is the onset of ten visible from Casey, with a gray “smudge” on the strong to gale-force southerly flow, often accompanied southern horizon, indicative of blowing snow advecting by clear-sky conditions. The events have the appear- out to sea in the strong south-southeasterly outflow. ance of a strong katabatic flow moving up the coast Given the common occurrence of the outflow off the from the Vanderford Glacier situated to the south of Vanderford Glacier, but the rare strong southerly flow Casey (Fig. 1). However, it is not a common occurrence at Casey, it has been difficult to identify the ambient to see the katabatic wind pushing as far north as Casey, conditions that lead to the katabatic wind reaching as or of such strength, despite the Vanderford Glacier be- far north as Casey. Simply having a strong flow off the ing only 30 km to the south, and an area that regularly glacier is not enough to predicate the strong flow reach- experiences strong katabatic winds (south-southeast- ing Casey, casting some doubt as to whether the strong erly) flow. For example, Fig. 2 shows a comparison of southerly flow at Casey is in fact a true katabatic wind, time series wind data from Casey and Haupt Nunatak, given that ambient conditions obviously need to be East Antarctica, some 34 km south-southeast of Casey. right for the strong southerly flow to reach as far north The nunatak is right on the northeast edge of the as Casey. In this context a “true” katabatic wind is Vanderford Glacier and experiences a very consistent defined as a surface wind resulting from gravitational Ϫ1 southeasterly wind (140°), often around 20 m s . forcing of cold air masses on inclined terrain (Schwerdt- Whereas, coincident with these strong wind events, Ca- feger 1984), but with the more strict interpretation im- Ϫ1 sey often experiences wind as little as 5 m s . Casey posed by Phillpot (1997) whereby a speed decrease oc- Ϫ generally experiences only light outflow, and predomi- curs from fairly high near-surface values to 5 m s 1 or less by about 1 km, or 850 hPa. There are many landmark studies of the Antarctic Corresponding author address: Neil Adams, Bureau of Meteo- katabatic wind regime, from the early work of Parish rology, GPO Box 727, Hobart, Tasmania 7001, Australia. and Bromwich (1987), through to more recent work by E-mail: [email protected] Parish and Cassano (2003) and van den Broeke and van © 2005 American Meteorological Society Unauthenticated | Downloaded 09/25/21 04:33 AM UTC DECEMBER 2005 ADAMS 3549 FIG. 1. Map of the Casey local area detailing the orography and the location of Casey Station and the surrounding significant locations. Lipzig (2003). Parish and Cassano (2003) used the fifth- dynamics occurring in the Casey area under a southerly generation Pennsylvania State University–National wind regime the Australian Bureau of Meteorology’s Center for Atmospheric Research (PSU–NCAR) Me- Antarctic Limited-Area Prediction System (ALAPS) soscale Model (MM5), to model the different forces was used to investigate such occurrences over the 12- acting on the surface flow, concluding that the persis- month period from July 2001 to June 2002. tency in wind direction is not necessarily indicative of a radiatively forced katabatic wind regime, but rather a 2. Model description and data analysis result of topographic adjustment of all pressure gradi- ent forces. Van den Broeke and van Lipzig (2003), used The ALAPS model is a modified version of the Aus- a medium-resolution regional atmospheric model to in- tralian Bureau of Meteorology’s Limited-Area Predic- vestigate the momentum budget of the Antarctic sur- tion System (LAPS). LAPS is a globally relocatable face layer and concluded that the near-surface wind limited-area gridpoint model employing full data as- field could be explained in terms of the katabatic pres- similation. The system uses a latitude–longitude hori- sure gradient force, the large-scale pressure gradient zontal grid and sigma coordinates in the vertical. A full force, and the thermal wind effect. The thermal wind description of the model can be found in Puri et al. effects were found to be significant in areas where weak (1998), but in essence the governing equations are the large-scale forces allowed cold air to build up over sea multilevel primitive equations for momentum, mass, ice or ice shelves and often opposed the katabatic pres- temperature, and moisture, written in advective form, sure gradient force. It is possible that this effect plays a except for the mass equation, which is in flux form. The role in modulating the strong southerly outflow in the model runs on an Arakawa A grid, and in the current Casey area. Murphy and Simmonds (1993), analyzed study, employed fully explicit Miller–Pearce time dif- strong wind events simulated in a GCM, in the Casey ferencing. High-order spatial differencing was used area, and looked at the relative roles of the katabatic wherever possible to ensure accuracy to at least that of flow and synoptic situation, and concluded that very second-order C-grid models. The physical parameter- strong katabatic flow appeared to be related to the pro- izations used in the model were the same as those em- duction of cold air inland of Casey by stronger-than- ployed in the Australian Global Assimilation and Pre- average surface temperature inversions a few days be- diction System (GASP) and described in Puri et al. fore the strong wind event. To further investigate the (1998). The analysis system used in the assimilation Unauthenticated | Downloaded 09/25/21 04:33 AM UTC 3550 MONTHLY WEATHER REVIEW— SPECIAL SECTION VOLUME 133 Ϫ1 FIG. 2. Time series data detailing the wind direction and speed (m s ) from (top two panels) Haupt Nunatak and (bottom two panels) Casey Station for the period 1200 UTC 9 Nov 2003–1200 UTC 13 Nov 2003. cycle was a limited-area adaptation of the global mul- the model should have captured any such southerly tivariate statistical interpolation (MVSI) used in the events during the 12-month trial period (July 2001– GASP system as described by Seaman et al. (1995). Modi- June 2002), and so provide a chronology of the devel- fications to the LAPS system for running over Antarctica oping dynamics associated with the events, and give were minor, including subtle changes in the sea ice zone valuable clues as to what precursors to the development to better represent surface fluxes, and fixes to defining may be observed in the Casey observations. During the surface temperatures over the Antarctic continent. 12-month verification period of the ALAPS system, The ALAPS domain, in this study, had a resolution seven strong to gale-force south-southeast flows were of 0.25° of latitude ϫ 0.50° of longitude, giving an ap- observed at Casey, with six of the seven events having proximate horizontal resolution of 27.5 km, with model occurred during periods of ambient light northeast– boundaries from 0°–180° to 80°–35°S. Twenty-nine ver- southeast flow at the station, and with one event di- tical sigma levels were used, ranging from 0.9988 near rectly preceding a gale-force easterly storm. the surface (approximately 8 m), to 0.05 (approximately In this study a strong wind is defined as one in which Ϫ 50 hPa), at the model upper boundary, with a concen- the wind speed exceeds 13.0 m s 1 and a gale where the Ϫ tration of levels in the planetary boundary layer. A full wind speed exceeds 17.0 m s 1. Table 1 details how description of the ALAPS system may be found in successful ALAPS was at forecasting the gale events at Adams (2004). The model was initialized twice daily, varying time steps throughout each model run, from the at 1100 and 2300 UTC, and run out to ϩ96 h. If the analysis to the ϩ48 h forecast. The number of false ALAPS dynamics and resolution were sufficient then alarms is also listed, where a false alarm is defined as an Unauthenticated | Downloaded 09/25/21 04:33 AM UTC DECEMBER 2005 ADAMS 3551 TABLE 1.