An extreme wind event at Casey Station, Antarctica

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D7, PAGES , APRIL 16, 2001 An extreme wind event at Casey Station, Antarctica John Turner, Tom A. Lachlan-Cope, and Gareth J. Marshall British Antarctic Survey, Natural Environment Research Council, Cambridge, England Stephen Pendlebury Bureau of Meteorology, Hobart, Tasmania, Australia Neil Adams Antarctic Cooperative Research Centre and Bureau of Meteorology, Hobart, Tasmania, Australia Abstract. Model output, satellite data, and in situ observations are used to investigate the conditions that gave rise to an extreme wind event at the Australian Casey Station (66.27 ø S, ø E) on the coast of East Antarctica. The event took place over the period March 20-22, 1992, and resulted in Casey Station's highest ever wind gust for March (66.9 ms-', 130 knots) and 10 m mean winds of near 50 m s TM. The event occurred when a deep low was located just north of the coast and there was high surface pressure inland. The rapid deepening of the low took place within a strong baroclinic zone lying north-south between a cold trough and a ridge bringing very warm air southward. A conceptual model is proposed for the very strong winds experienced at Casey Station. Key elements of the model are (1) a synoptic-scale high-low pressure couplet, providing a strengthening pressure gradient; (2) entrainment of radiatively cooled air by the supercritical synoptic gradient, leading to downslope flow; (3) the acceleration of the wind down the lee slope of Law Dome, occurring primarily in response to a topographically induced, long-period, vertically propagatingravity wave; and (4) sources of negative buoyancy, including prestorm radiatively cooled air and, later in the storm, maritime air cooled by heat flux to the ice surface. The topographically induced gravity wave increases the horizontal temperature difference, thus increasing the negative buoyancy of the surface airflow. 1. Introduction The coastal area of the Antarctic experiences some of the strongest winds on Earth as a result of the katabatic winds, the very deep depressions that occur over the Southern Ocean and the interactions of these two features. The persistence and strength of the katabatic drainage flow was noted by the early explorers and has been studied intensively over the subsequent years [Ball, 1956, 1960; Parish, 1981, 1982, 1988; Parish and Bromwich, 1987; Parish and Waight, 1987; Bromwich, 1989; Breckenridge et al., 1993; Gallde et al., 1996]. Investigation of the deep depressions north of the continent had to await the arrival of satellite data and the production of reliable numerical analyses, but in recent years great advances have been made in our knowledge of the formation, track, and dissipation of these lows [Jones and Simmonds, 1993; Turner et al., 1998; Simmonds and Keay, 2000]. The interactions between the synoptic-scale weather systems and the katabatic flow as the lows pass just north of the coast in the latitude of the circumpolar trough are complex, with either strengthening or weakening of the katabatic wind depending on the location of the lows in relation to the katabatic flow [Simmonds and Murphy, 1992]. The effects of such interactions are not just confined to the Antarctic and the immediate coastal region but can result in mass redistributions Copyright 2001 by the American Geophysical Union. Paper number 2000JD /01/2000JD that extend to the subtropical latitudes of the Southern Hemisphere [Parish and Bromwich, 1998]. In this paper we investigate a strong wind event (SWE) that occurred at the Australian Casey Station (66.27øS, øE, 40m elevation) as a result of the interaction of the synoptic pressure gradient, associated with a very deep low off the Antarctic coast and an Antarctic continental high pressure system, and the flow over the isolated topographic feature of Law Dome. The low in this case had a minimum central pressure of close to 950 hpa and there were 10 min mean winds of up to 49.4 m s" (96 knots) and the highest ever wind gust for March (66.9 ms-', 130 knots) at the Australian Casey Station in 39 years of data (see Figure 1 for places referred to in the text). In a comprehensive survey of the climatology of Casey, Russell-Head and Simmonds [1993] showed that over the period on only nine occasions did the wind speeds, collected every 3 hours as part of the synoptic observations, exceed 50 m s". The low formed well outside the Antarctic, in lower midlatitudes, but deepened rapidly and moved quickly into the Antarctic coastal area. The explosive development occurred within a strong baroclinic zone between a major cold trough to the west of the low and a warm ridge to the east. The low moved toward the Antarctic coast in a strong north to northwesterly steering flow and became slow moving as it came up against the high topography of the continent. The wind flow around Law Dome to the southeast of Casey has been examined by a number of workers. Wilson [1992] found that the surface wind field was dictated by the topography of the 7291

2 7292 TUR R ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA Windmill Islands Po nsett. _. ' i;.._ -I,,'"'-'... '""x 66øS-. :::.:.:::::: :?:m: :: :: :: ::?: : :;:: x. Law Dome ndedord, (1395m) Glacier AWS / 000m / / _ 5oom x..,,,.,./ 67S... Figure 1. The region of interest and places referred to in the text. The map of the Antarctic also shows the locations of Vostok (V), Dome C (C), and the AWSs (13), (3), and (5). upstream area and the immediate vicinity, the stability of the affecting East Antarctica was discussed at least as early as upstream air, the synoptic gradient, and the katabatic flow off the The storm of March 1992, albeit an extreme event, provides a East Antarctic ice sheet. Adams [1996] reported a diagnostic more recent opportunity to build on the concepts which, in study of a moderately severe wind storm and demonstrated the particular, Wilson, Murphy/Simmonds, and Adams, have likely existence of a signif]cant wave-like disturbance attributable developed or extended. to an internal atmospheric gravity wave generated in the flow In section 2 we consider the data available for this study from over Law Dome. the research stations and the meteorological satellites. This is Murphy and Simmonds [1993] (see also Murphy [1990]) followed in section 3 by a description of the broad-scale synoptic examined SWEs near Casey using a general circulation model environment in which the storm developed as determined from (GCM) run for a "perpetual" July case and proposed that such operational surface and upper air analyses. The form of the cloud events were associated with strong katabatic and gradient flow associated with the low is also considered using imagery from the operating together. Murphy and Simmonds' conceptual model for Japanese Geostationary Meteorological Satellite (GMS) and the these SWEs links a threefold increase in synoptic gradients due NOAA satellites. In section 4 the detailed effects of the storm on to the approach of a strong cyclonic system toward the coast the environment of the region around Casey Station are together with higher than average air pressure over the continent. considered with particular attention being paid to the role of Law The anomalously high inland air pressure in turn leads to the Dome on modifying the local wind field. In doing so, we also development of a threefold increase in katabatic flow [Murphy draw together the various proposed mechanisms for SWEs at and Simmonds, 1993, p. 533]. It may be inferred from Streten Casey into a more unified conceptual model. In the final section [1968, p. 52], when he referred to the work of Astapenko [1964], we examine this case in the light of other very deep lows around that the role of strengthening pressure gradients between oceanic the Antarctic and consider the implications for forecasting the lows and continental anticyclones in coastal wind storms Antarctic.

3 2. Data and Model Runs 22,.rRNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA 7293 Over the part of the Southern Ocean considered here, there are no synoptic observations from island stations and few observations from merchant vessels or drifting buoys. Conventional synoptic observations could therefore only be used over the Antarctic coastal region. We were, however, fortunate in that 3-hourly and autographic surface observations were available from Casey Station, along with 12-hourly radiosonde ascents. In addition, data from automatic weather stations (AWSs) were available for several sites around the station and in the immediate continental interior inland of the station. To supplementhe limited amount of in situ data, extensive use has been made of satellite remote sensing data. Three-hourly imagery from the GMS satellite was used to determine the broadscale distribution of cloud. Advanced Very High Resolution Radiometer (AVHRR) imagery with a horizontal resolution of 1 km collected at Casey provided higher-resolution data, although the coverage was less frequenthan that from GMS. The model fields were obtained from the U.K. Meteorological Office (UKMO) operational numerical weather prediction (NWP) system, which is a 19-level model with a horizontal resolution of about 90 km. The UKMO data assimilation scheme [Loteric et al., 1991] made use of the Casey surface and radiosonde data every 12 hours but was heavily reliant on satellite sounder data over the ocean areas. Initially, the output of the 12-hourly analyses and forecast runs were used, but additional runs from the operational analyses were carried out so that extra fields could be obtained and nonstandardiagnostics generated. The most important run carried out was a 24 hour integration starting at 1200 UTC on March 20 which covered the period of the rapid deepening of the low. This forecast was successful in predicting the deepening of the low (as was the European Centre for Medium-range Weather Forecasts (ECMWF) model [Pendlebury and Reader, 1993]) and gave a track that was close to that observed in the satellite imagery. This does not necessarily establish that the details match reality, but does lend confidence to using the model output An Overview of the Development of the Storm Daily mean sea level pressure (MSLP) and 500 hpa height charts for the period March 20-22, 1992 are shown in Figures 2 and 3. The surface low that later affected Casey (identified as low A on the accompanying charts) developed north of 40øS and moved southeastward toward the Antarctic. The markedeepening of low A and the start of its period of rapid movement toward the Antarctic took place on March 20. This can be seen in Figure 4, which shows the depression track and central pressure values of the low. The UKMO MSLP chart for 0000 UTC on March 20, 1992 (Figure 2a) indicates that low A was located approximately 800km to the north of an equivalent barotropic low in the coastal regionear Casey Station with the two systems being linked by a frontal band. The 500 hpa height chart for this time (Figure 3a) indicates that low A was located to the east of an upper trough and that a closed center of less than 504 dm was analyzed near 60øS, 95øE. The period prior to the rapid deepening of the low was characterized by marked meridional advection of warm and cold air so that a strong east - west baroclinic zone was established down the 110øE meridian. This can be seen in Figure 5, which shows the hpa thickness fields for 1200 UTC on March Figure 2. The U.K. Meteorological Office (UKMO) hand- 20, At this time the 528 dm hpa thickness drawn mean sea level pressure (MSLP) charts for (a) 0000 contour was over the Antarcti continent and the 546 dm contour UTC March 20, 1992; (b) 0000 UTC, March 21, 1992; and was near 62 ø S. (c) 0000 UTC, March 22, 1992.

4 7294 TURNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA Figure 3. The UKMO 500 hpa height charts for (a) 0000 UTC, March 20, 1992; (b) 0000 UTC, March 21, 1992; and (c) 0000 UTC, March 22, By 0000 UTC on March 21 (Figure 2b) low A was analyzed as having a central pressure of 957 hpa and a location close to 59øS, 113øE, having merged with the slow moving barotropic low near the coast. With the MSLP over the continent being relatively high and a deep low off the coast, the surface pressure gradient had increased in the coastal region, and Casey Station was reporting an easterly wind of 44 m s ' at this time. During March 21 the central pressure of low A dropped more slowly, although at 1200 UTC Casey was reporting a surface wind of 43 m s ' from the east as the pressure gradient was maintained between the deep low and the high pressure over the interior of the continent. The geostrophic wind over the station was analyzed to be only 30 m s ' (geostrophic winds were determined from the MSLP charts), suggesting local enhancement of the flow, as discussed in section 4. By 0000 UTC on March 21 the upper level trough had moved to the east, although there was little change in the low center north of Casey (Figure 3b). Low A was analyzed with its lowest central pressure of 950 hpa at 0000 UTC on March 22 (Figure 2c). The wind at Casey was still above 40 m s ' from the east. At this stage there was little change in the upper flow pattern from the previous day: however, the storm was about to weaken so that by 0600 UTC on March 22 the surface wind at Casey had fallen to below 20 m s '. The GMS infrared satellite imagery illustrating the development of low A is shown in Figure 6. Low A was first observed as a minor vortex center and associated frontal structure with the most marked feature being an active triple point low (a low developing at the point where the warm, cold, and occluded fronts meet) (indicated on the imagery as "T"). During the early part of March 20 there was rapid rotation of the cloud band associated with low A so that by 0830 UTC on March 20 (Figure 6c) there was a tight spiral of cloud centered just north of 50øS. Three concentric bands of cloud were apparent on the eastern side of the low, and the cloud top temperatures had decreased on the western side. The original north-south band of cloud lying down 120øE was still present and at its southern end the cloud curved anticyclonically around the pronounced ridge close to 125øE. As the low moved rapidly south, the banded structure in the cloud field became less clear as all the elements expanded horizontally. This can be seen in the image for 1730 UTC on March 20 shown in Figure 6d. At that time the cloud associated with the low was beginning to merge with the north - south cloud band close to 125øE and there was evidence of a clear slot developing along the 120øE meridian. By 2330 UTC on March 20 (Figure 6e) the merging of the cloud at the center of the low and the north-south band had advanced to the point where these features appeared to be part of a single system. The cloud over the vortex center had continued to expand, and it was not possible to observe the different bands apparent in earlier images. Two of the most marked changesince the 1730 UTC image were the sharpening of the western edge of the cloud band, which now had the form of an active cold front, and its rapid movement eastward to almost 130øE. The cloud associated with the cold front was to the east of the strongest gradient in the hpa thickness and followed its northeast - southwest orientation. An image of the system when the cold front was lying close to, and parallel with, the coast of the Antarctic at 1130 UTC on March 21 is shown in Figure 6f. The highest cloud is on the cold front, and in the center of the vortex there is only a featureless area of lower cloud. There is still a sharp rear edge to the cold front and an extensive area of low- and medium-level cold air cumulus clouds in the cold air to the north and northeast of the low center.

5 TURNER ET AL.' EXTREME WINDS AT CASEY, ANTARCTICA 7295 loo 2/19 x 991 x 0O/ O"8 x 120"E 12/22 x Figure 4. The track and central surface pressure values (hpa) of the low based on the UKMO analyses. 4. Airflow in the Law Dome to Casey Area 4.1. Standardization of Terminology and of Wind Reports It is relevant for us to define first our terminology with respect to the surface wind. This is important in view of the various definitions of katabatic flow which appear in the literature (see, for example, Murphy and Simmonds [1993, p. 528], Parish [1988], and Phillpot [1997, p. 63]) and because of surface wind data being reported from anemometers at different heights above the ice surface. For our purposes we note that the Meteorological Glossary [Meteorological Office, 1991, p. 166] clearly links the term "katabatic wind" with the downslope gravitational 11ow of air cooled by contact with ground which has lost heat through emission of long wave radiation, and so this is the context in which we will use the term "katabatic". The term "downslope wind" will be used to refer to any wind (or component) which blows down a terrain slope. In respect of the standardization of wind observations, the various AWS data quoted in this paper were adjusted to the World Meteorological Organismion (WMO) standard of 10 m height above the (snow-ice) surface using the well known logarithmic wind profile for neutral surface layer flow [Businger, 1973, p. 70]. The assumption of neutral stability is well justified in a turbulently mixed boundary layer (see, for example, Ohata et al. [1985, p ]); however, as will be seen from later discussion (section 4.3), this assumption would be invalid over the entire (several hundred meters) depth of the boundary layer in this SWE as marked thermal stratification was evident. For the several meters involved in the adjustment process we have no data of sufficient resolution which negates the neutral stability assumption used. A roughness length (Zo) of cm was considered appropriate for all the AWS sites (I. Allison, personal communication, 1999). Figure 5. The UKMO hpa thickness chart for 1200 UTC, March 20, The position of the surface low is indicated by a cross Outline of a Conceptual Model of the Airflow From Law Dome to Casey in Certain SWEs Following the lead given by Wilson [1992], we believe that the March 1992 Casey SWE provides prima facie evidence that aspects of the Murphy/Simmonds model (Murphy and Simmonds, 1993) can be combined with the work of others, in particular, Adams [ 1996], to form a conceptual model for the flow in the lee of Law Dome in situations in which synoptic forcing plays an important role. In brief, the model, which will be justified in detail below, is as œollows:

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8 7298 TURNER ET AL.: EXTREMIE WINDS AT CASEY, ANTARCTICA Direction : : MAR 1992 Speed (m/s) 20 MAR 21 MAR R 23 R Mean Sea Level Pressure {hpa) 24 MAR Figure 7. Casey wind direction, wind speed, and MSLP data for the period March 19-24, The synoptic-scale high-low pressure system couplet of pressure of the continental high can be obtained from station- Murphy and Simmonds [1993] provides two important level pressure recordings (not shown) at Dome C (74.50øS, components. First, the inland high-pressure system allows øE, 3280 m elevation), which is approximately 1200 km radiatively cooled air to exist in the general area. Second, the south-southeast of Casey, and from Vostok (78.45øS øE, synoptic low-pressure system develops and moves southward, 3488 m elevation) which is approximately 1450 km southward of and the combined high-low couplet causes the synoptic gradient Casey. At Dome C the station-level pressure was above the longflow over Law Dome to strengthen to some critical value term March monthly mean of hpa between 0000 UTC on (possibly around 10 m s"), and to then keep strengthening. March 15 and 0900 UTC on March 23, Between Following Grace and Holton [1988], entrainment of UTC on March 16 and 0900 UTC on March 22, 1992 the Dome radiatively cooled air by the supercritical synoptic gradient C surface pressure was at least 10 hpa above the March monthly contributes to the downslope surface flow. As the storm mean, and peaked at hpa on about 0000 UTC on March 21, progresses, the radiatively cooled air is displaced by maritime A similar pattern was evident at Vostok, where, for airflow, from which heat is lost to the ice surface by turbulent example, the station level air pressure on March 16, 1992 was as heat flux (by analogy with Parish and Waight [1987]), thus much as 16 hpa above the March long-term mean, and the contributing to the negative buoyancy of the boundary layer flow. station-level pressure did not return to the long-ter mean value However, a significant source of thermal contrast at this stage is until 0000 UTC on March 23, also provided by the lee wave system of part 3 of the model. In terms of the Murphy and Simmonds [1993] model, it is 3. In accord with Adams [ 1996], the acceleration of the wind clear that both parts of the anomalously high/low couplet were down the lee slopes of Law Dome occurs in response to a present. On the other hand, there is little or no observational topographically induced, long-period, vertically propagating evidence of the presence of abnormally (3 times greater) strong wave with higher oscillation waves further downstream. katabatic flow [Murphy and Simmonds, 1993, p. 533]. Given 4. The Casey wind speed is stronger than at the nearby Airstrip sparseness of data for the area, this does not necessarily mean AWS (89810) site (66.3øS, 110.8øE, 390 m elevation), at least in that such abnormality did not exist. However, Figure 8 shows the part because of a sudden terrain fall below the airstrip area, so variation of wind velocity, air temperature, and pressure at AWS that the negatively buoyant airflow increases acceleration down (71.6øS, 111.3øE, 2761 m elevation) for March This the steeper ice slope. AWS is close to, although not exactly within, the flow path from the source region (approximately near Dome C) that one might expect would supply the Casey-Law Dome area with cold air 4.3. Observational and Numerical Modeling Studies in drainage. Figure 9,,for example, shows idealized winter Support of the Conceptual Model streamlines for the area with AWS located on this figure Part 1 of the conceptual model is developed from the as "A", Dome C as "C", and Vostok as "V". following considerations. The effect of low A was to cause The time serie shown in Figure 8 is for the entire month of abnormally low pressure in the Casey area. The lowest pressure March 1992 to illustrate the character of the March 1992 for the storm at Casey was about 959 hpa recorded at about 0900 conditions at this AWS relative to the long-ter means, and in UTC on March 21, and again at around 0000 UTC on March 22 doing so, indicates thathere is little in this time series to suggest (Figure 7): This was 24 hpa below Casey's long-term average that any abnormally high "katabatic" flow (or, in the case of an pressure for March. An indication of the above-average surface inland site an "inversion wind" (see, for example, Schwerdtfeger

9 TURNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA o o 0 Speed (m/s) -2O -4O -6O +..+,k+ ß :... I '++.,.,+..%%¾+ 4'.. +,.+..+.'. '.. * ,. ' +.. :... *.., '... ;+++. +' :' '. :1 :... 6 MAR 11 I AR 16 IdAR 21 MAR 26 MR 1992 Temperature {C) Figure 8. Wind direction, wind speed, and temperature data for AWS for March [1970, p. 290]) was occumng at this site. I. Allison (personal reflecting a greater influence of cold air drainage over the AWS communication, 1999) advised that on the basis of 10 years of from the ice ridge just west of Vostok (Figure 9). However, record for this AWS, the mean March daily wind direction is during the period of above average air pressure over the area 148 ø (standard deviation (s.d.) 23ø); the directional constancy for (March 15 to 22) the wind direction at AWS commenced March is 0.97 (s.d. 0.02); the mean daily wind speed is 6.5 m s" near its long-term mean direction and then drifted into a more (s.d. 1.9 m s"); and the mean daily temperature is -45øC (s.d. easterly direction (no doubt due to the increasing synoptic 6.6øC). Early in March 1992 (particularly March 6 to 12) when influence), but the wind speed did not increase significantly temperatures were between -45 ø and-55øc, that is, colder than above the long-term mean until the occluded front reached the average, the wind direction showed a high degree of constancy area on March 22 (Figure 2c). Similar data (not shown) for AWS slightly southward of the mean March direction, perhaps 803 (68.49øS øE, 2125 m elevation) and AWS ø 65 ø Figure 9. Idealized katabatic streamlines for average winter conditions for the Antarctic continent near Casey Station [after Parish and Bromwich, 1987]. Reprinted with permission from Nature. Copyright 1987 Macmillan Magazines Limited.

10 7300 TURNER ET AL.: EXTREMt! WINDS AT CASEY, ANTARCTICA (74.13øS øE, 3092 m elevation) also show no evidence of greater than normal katabatic influence. It is also worth noting that Streten [1968, p. 52] supported the view that it is the tightening pressure gradient between slow moving cyclonic activity off the Antarctic coast and continental high pressure that leads to prolonged strong wind events. He argued though, that for Mawson Station (67.60øS, 62.88øE, 16 m elevation), which is prone to strong katabatic flow, the katabatic regime does not appear to affect the intensity of the strong wind events which he examined [Streten, 1968, p. 52]. The reference to a threshold value of synoptic flow above which the downslope flow becomes fully established has its basis in four independent sources. First, Wilson [1992] and Adams [1996] both indicated that the rapid change from light and variable winds to easterly gale force (or stronger) winds is typical of SWEs at Casey. In the current storm this occurred at about 0000 on UTC March 20 (Figure 7). Second, Wilson [1992, p. 124] reported that when the (synoptic) gradient increases sufficiently to produce speeds of about 15 m s -t in the area of the Casey Airstrip then surface winds at Casey itself will increase suddenly to 15 m s - from the east. Third, B. van Meurs [personal communication, 1999] reported on a numerical modeling study of flow around Law Dome and presented results which imply that a 3 m s ' northeasterly synoptic (geostrophic) wind allowed strong katabatic winds to flow down the Vanderford Glacier about 30 km to the southeast of Casey, but Casey and the airstrip itself were sheltered "being in the region of a weak eddy in the lee of Law Dome". However, when van Meurs increased the synoptic (geostrophic) flow to a 20 m s - northeasterly the flow over the Casey area became easterly at 35 m s ". Fourth, Grace and Holton [1988] summarized two conditions which relate to speed threshold values of fluids flowing over obstacles, and it may be seen that these conditions give values which closely match data relevant to the onset of the SWE at Casey. In applying these conditions the approach taken by Grace and Holton [1988, pp ] is summarized as follows. These authors cited Baines [1987], for example, when they assert that "observations and simple energetics arguments indicate that fluids will flow up and over obstacles if the following condition is met": NH/U <=2, (1) that, strictly speaking, N should be determined from conditions upstream of Law Dome, and the lapse rate should be specified from near the surface to an elevation above, but upstream off Law Dome. In the absence of observations in the required location, values of the Brunt-V'ais'alii frequency were calculated from the prestorm Casey radiosonde trace for approximately 1140 UTC on March 19, 1992 (Figure 10), and an average value from just above the low-level (-200 m) radiation inversion to about 1500 m was determined to be 22.3 x 10 ' s '2, giving a value of N = s '. Thus with H equal to 1395 m (top of Law Dome), and the right-hand side of criterion (1) being 2, U is determined to be no less than about 10 m s -. On the other hand, assuming the right-hand side of criterion (1) to be 0.4, the lowest of the options mentioned above, gives a value of U of no less than about 33 m s '. Thus for this SWE, equation (1) suggests that the synoptic flow impinging on Law Dome will go over this topographic feature if the synoptic flow speed is more than 10 m s TM and perhaps as high as approximately 33 m s '. Grace and Holton [ 1988, p. 22] provided another approach to ascertaining the value of the critical (if any) synoptic speed required to force boundary layer air over an obstacle. These authors adapted an argument from Turner [1973] and presented the momentum equation in terrain following coordinates as D du/dt =-D (g A0/0) sinct- Cd l.f - E(Ua- U), (2) where D is the depth of the boundary layer, U is the flow speed in the boundary layer and is parallel to the terrain (and du/dt is the acceleration of the flow), U, is the flow speed in the layer above U, ct is the terrain slope (< 0 for up slope); Cd is the coefficient of surface drag for the layer, and E is the coefficient of entrainment drag for the layer (E > 0 if U > U,). From Manins and Sawford [ 1979, pp ] the value of E is determined by E = A/(& Ri + k), (3) where A, St, and k are constants of approximate values 2, 0.5, and 0.02, respectively, and Ri is the Richardsonumber, given by N2Dq(UA-U). Grace and Holton [1988] went on to show that by substituting equation (3) into equation (2) then where H is the height of the obstacle, U is the speed of the upstream ambient wind speed, and N is the Brunt-V'ais'alii D du/dt =-D (g A0/0) sinct - Cd U = - A(UA- U)4/S, iv 2 D. (4) frequency (the angular frequency about which internal density waves oscillate) where N 2 = (g/o)x(do/dz), where 0 is potential These authors left it to the reader to show that uphill flow is thus temperature, g is the gravitational constant, and z is geopotential expected when height. It may be seen from Manins and Sawford [1982, p. 427], for Ua > N D ((S,/A) sinco ø.='. (5) example, that the expression on the left hand side of equation (1) This task is straightforward since for the uphill flow to just is the inverse of an internal Froude number and, as these authors start it is sufficient to have U as zero and du/dt negative, in reported, Baines [ 1979] has shown that blocking of the fluid flow which case, bearing mind that ct is negative for uphill flow, the over two-dimensional laboratory models of ridges occurs if right-hand side of equation (4) is negative when equation (1) is not met. (The square of the Froude number expresses the relative magnitudes of the kinetic energy of the ambient wind to the potential energy change required to lift a A(Ua- U)4/S, N 2 D: > D (g A0/0) sinct. (6) parcel from near the surface to clear an obstacle [see, for example, Manins and Sawford [1982, p. 427]). Manins and Sawford [1982, p. 427] also reported on their own field observations which, in effect, suggesthat the value of the righthand side of equation (1) is about 0.625, while other work they cited has this value varying from 0.8 to 0.4. Thus in the immediate discussion below we depart from Grace and Holton [1988] to suggest a range of values of U which would allow the synoptic airflow to move over Law Dome. In doing this we note Furthermore, substituting for g A0/0 in terms of the Brunt- V'ais'alli frequency (N) and solving for Ua leads to equation (5). Taking a typical windward (assuming an east or northeast synoptic flow) slope for Law Dome of 1 in 80 and a boundary layer depth of 300 m [Ohata et al., 1985, p. 10,655], then for N = s", criterion (5) requires a synoptic flow (above the boundary layer) of about 6 m s" to entrain the boundary layer flow up Law Dome.

11 TURNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA 7301 Figure 10. The aerological diagram for Casey for 1139 UTC, March 19, (The right-hand (left-hand) curve is air (dew point) temperature. The scale for temperature (øc) is indicated along the bottom and left-hand sides. Height (m) and pressure (hpa) are shown on the left-hand and right-hand side, respectively. It is of interesthat for this storm the wind speed at the Law Table 1, there was probably a general warming of the entire area Dome AWS site ( øS, 112.7øE, 1366 m elevation) as the maritime air associated with the Southern Ocean low (low exceeded 6 m s -1 just prior to 0000 UTC on March 20 and A) moved inland. Later in the storm (after about 1200 UTC on exceeded 10 ms-' in this storm at around 0100 UTC on March March 21) it is suggested that any radiationally cooled air had 20. This was abouthe time that the SWE event started at Casey. been largely replaced by the vigorous synoptically driven It is not known when the airstrip speed first reached the 15 ms-' maritime flow, a frontal band having crossed the area. For criterion noted by Wilson [1992], but the time would have been example, the Law Dome surface air temperature gradually rose to around 0000 UTC on March 20. The Airstrip value for this time -11øC and bearing in mind its elevation, this warming is in Table 1 (surface observations for Law Dome AWS, Casey appreciable. However, heat loss to the underlying ice surface Airstrip AWS, and Casey) is actually for 2100 UTC on March would still have been occurring to some significant degree, and 19, the closest observation available. with Casey near the end of the over-ice trajectory any Part 2 of the model is essentially the identification of sources contribution to the boundary layer inversion from this mechanism of negatively buoyant air. In relation to the above discussion on a would be maximized. In arriving at this possibility we note that critical value for downslope flow, it is noted that Grace and Parish and Waight [1987] reported that the application of a Holton [1988] argued that under suitable conditions boundary primitive equation model to the Antarctic katabatic wind problem layer air may be induced to ascend a windward mountain slope leads, in part, to the notion that heat transfer from the surface air by the drag of the viscou sheafing stress due to the geostrophic layer to the ice surface through turbulent heat flux increased as wind. The ascending boundary layer air cools adiabatically, and if the wind speed increased and was more significanthan radiation the environment is stable, then the boundary air reaches the crest effects [Parish and Waight, 1987, p and 2224]. several degrees cooler than the environmental air at the same Direct observational evidence for our hypothesis concerning altitude. The boundary layer air consequently accelerates down the loss of heat to the ice surface by turbulent heat flux may be the lee slopes as a gravity flow. These authors argued that it is the found from an examination, for example, of the Casey Station existence of the density gradient which is important, not the vertical temperature sounding for 1135 UTC on March 21, 1992 cause by which the density gradient is created. (Figure 11), along with a consideration of the satellite imagery. In the context of the current Casey SWE it is probable that From Figure 11 it may be seen that there is a marked temperature radiationally cooled air played an important role in providing inversion below about 550 m and a dry adiabatic rate of negatively buoyant air early in the event; the presence of high temperature decrease from about 1120 m to around 2000 m. It is pressure and clear skies is noted. However, as may be seen from also evident that the layer below about 1100 m is well mixed,

12 7302 TURNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA i i i i i i i i i i i i i i i i i up0 u o o o oo 8 oooooooooooo

13 TURNER ET AL.' EXTREME WINDS AT CASEY, ANTARCTICA 7303 AEROLOGICAL DIAGRAM FOR VALID 11:55Z 21/3/92 rn/s Figure 11. The aerological diagram for Casey for 1135 UTC, March 21, (Interpretation and scales as for Figure 10). since the mixing ratio is more or less constant below that level and is remarkably constant below about 500 m. Bearing in mind vertical velocity profiles of the radiosondes. With the data from 1135 UTC on March 21 (Figure 12a), there was a very strong that cloud began covering the general area from as early as about (+4.3 ms-') oscillation evident in the lowest 2500 m with a 1730 UTC on March 19 (Figure 6), it is our contention that the low-level inversion identified in Figure 11 is due to a cooling of the maritime flow by the underlying ice surface due to mechanical mixing. As will be discussed below, another very importanthermal source driving the low level flow is a vertically propagating wave. The adiabatic layer seen in Figure 11 between about 2000 and 1120 m is clear evidence for the adiabatic warming effect of this wave. If it were not for the conduction of heat to the underlying ice surface, the adiabatic warming would have extended to the surface. Moreover, if the warmth of the air +2.1 m s -I oscillation above, capped at around 7000 m. Wave-like signals were evident in all of the radiosonde profiles during the March 1992 storm with the wave gradually propagating vertically, as evident in the profile from 0050 UTC on March 22 (Figure 12b), where significant oscillations (+1.2 m s 'l) were evident throughouthe troposphere and lower stratosphere. Horizontal propagation of the wave may also be inferred from satellite imagery taken during the event. Figure 13 shows the band 4 (11 lam) NOAA image taken at 2322 UTC on March 20, when wave clouds can be seen clearly immediately down stream, mass below about 2000 m was entirely due to its maritime to the west of Casey. The cloud signature had a wavelength of character, then the temperature would have more closely around 8 km with cloud top temperatures at-21øc, placing the followed the dew point curve to the surface. cloud tops near 5000 m, with the measured wavelength very close Evidence for part 3 of the model comes directly from satellite to that observed in the January 1993 storm. Wave cloud was also and radiosonde observations during the SWE and indirectly from inferences from modeling studies of others. In the discussion of the January 1993 storm at Casey, Adams [ 1996] suggested that an internal atmospheric gravity wave was generated in the flow over Law Dome, heralding a switch from separated flow over the dome to a downslope wind. The strongradient generated by the passage of a cyclone close to Casey provided the forcing for the evident near 65.5øS, 98øE at 2322 UTC on March 20 (Figure 13) but had increased in areal extent by 0900 UTC on March 22 (Figure 14). The radiosonde flight from 0050 UTC on March 22 confirmed the existence of an atmospheric wave with the radiosonde motion (Figure 15) displaying distinct sinusoidal motion in the horizontal with a wavelength between 8 km and 10 km. The existence of a propagating wave train is consistent with wave over the dome, with the downslope wind, and induced the hypothesis of a stationary gravity wave generated near Casey gravity wave, producing the accelerated flow in the Casey area. in the flow over Law Dome [Adams, 1996] and beginning to The transient nature of the wave was probably due to changing propagate both horizontally and vertically once the storm force atmosphericonditions and forcing. The storm of March 1992 winds had abated and the forcing mechanism for the stationary displayed similar characteristics to the January 1993 storm with a wave eased. wave-like signal apparent during the event, as evident in the Other studies of SWEs near mountain ranges [e.g., Klemp and

14 7304 TURNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA '1' I Maximum- 10. Minimum- Mean- 5.8gm/s "I' I... I o (m) (m/,) Figure 12. The vertical velocity profiles from the Casey radiosonde ascents at (a) 1135 UTC, March 21, 1992 and (b) 0050 UTC, March 22, Lilly, 1975] have also drawn attention to the importance of propagating lee waves may be expected, with shorter-period standingravity waves in producing very strong winds in the lee waves further downstream. For convenience, their Figure 10(b) is of mountain barriers. As with the case investigated by Klemp and reproduced here as Figure 16, with Law Dome shown as "L" in Lilly [1975](see their Figure 4) there is evidence that with the the figure and Casey as "C". An appreciation of the relevance of March 1992 Casey SWE a long-period standing wave the asymmetrical nature of Miller and Durran's model mountain (characterized by subsidence and a lack of oscillations apparent to the Law Dome - Casey situation in northeasto southeasterly in the cloud sheet) was present in the lee of Law Dome and airflows may be gained by inspection of our Figure 1. The shorter-wavelength gravity waves were present further acceleration in the lee of these authors' model mountain for a downstream, as observed via the cloud on the satellite imagery. frictionless case is reproduced here as Figure 17. Moreover, the modeling work of Smith [1985], Smith and Sun Empirical evidence for part 4 of the model will be explored [ 1987], and Miller and Durran [ 1991 ] are relevant. Smith [ 1985], below, but we note here that over the relatively long Law Dome for example, through numerical modeling demonstrated how the to airstrip transecthe average ice slope is about 1:100 (over shape of a mountain might affect the downstream wind speed of approximately 110 km). On the other hand, the fetch between the lee flow. Moreover, Miller and Durran's [1991] Figures 9 and airstrip and Casey itself is only 12 km, and the average slope is 10(b) illustrate that for length scales and mountain asymmetry about 1:30, while for the last few kilometers the slope is as steep similar to the Law Dome-Casey situation, long-period, vertically as 1:20. Ball [ 1960, p. 11 ] provided an illumination on the effect

15 TURNER ET AL.' EXTREIV[E WINDS AT CASEY, ANTARCTICA ! I Maximum- 7.50m/.e Minimum- 4.20m/.s Mean- 5.81m,/e,,, I I I I I b 0 (m) 3.0 I, I I,. I I , I I, I Figure 12. (continued) of slope on the resultant wind. Moreover, following the lead of Blakers [1983], the model of Ball [1960, p. 16] may explain the difference between the constancy of the SWE wind direction at Casey (90 ø) and at the airstrip (130ø), which may be noted from Table 1. In brief, the difference is due to the difference in orientation of the local ice slopes at the two sites. By rotating Ball's [1960] Figure 7 anticlockwise by 70" to align the model topography with the local Casey topography, the mean flow direction at Casey is reproduced [see our Figure 18). While not shown, Ball's Figure 7 need only be rotated by 30" to reproduce the local ice terrain slope and the SWE observed mean wind direction near the airstrip Energetic Considerations in Support of the Conceptual Model Central to arguments for parts 2, 3, and 4 of the model is Grace's [1995, p. 7] statement that buoyancy forces are the ultimate causes of downslope winds. In applying this assertion to the current SWE, the following argument is from Grace and Holton [1988, p. 17]. The change in kinetic energy that a parcel experiences after descending from the cresto some point down a slope is given by equation (7), where the right-hand side is the thermodynamic energy represented by the density difference: = g I (ao/o) (7) where K is the chinage kinetic energy between the crest (at a height z = H) and some lower point (z = h) and the integral on the right-hand side is from h to H, and R represents the energy losses due to surface drag and due to interfacial entrainment at the top of the boundary layer. Per unit mass K is also given by K = 0.5 (U,, 2- U,,2), (8) where U is the wind speed at altitude z. Substituting K from equation (8) into equation (7) leads to equation (9):

16 TURNER ET AL.' EXTREME WINDS AT CASEY, ANTARCTICA Figure 13. An infrared image of the Casey areat 2322 UTC, March 20, 1992., ,.... :.::½:-:.:..: '!-?;::½.:' ::::.:. ; :;. ::.... '... %,, ::-...?;. : %. :.'... : :: ½".:.::.-.. :..:. ::'.: :-!:':" : ::: % ::...'..:-:::.::...:..- :? '...: ::m-' ':;... :.'... '- '.. ß.....:.>':. t >,..::.-::::...:.....,::::: ' '? ; ', :' :?:..-...::"?::.?... :...?::.:.-..'... ' '.'.. ::'? -.: : :: ':-'... ':?: b. ' -.. : :' ' : :' :. :?"' >": ".. '" ' -.-'...::::".....:: ': "'...:. :.:::?... >.:: - :...,, :... :..::... :..-.::..., ::.::;:.....,...:...:'... :..::::?::...:,.-:: :::..:..'-.?...: ¾...' -. ::..": :::?' ""?? :... :::'.-- ::& :. -..:...,. :.:::. -:. ::. :.:..: -'.-::--,.... "':'-:--"-.... ::.':: %;:?. ::;: :;.::':-" '.::?' :.::t -'.....' :'?:; ;:..' - :::'%:-%, ' ':.' ".. "...L :.,-.-. ::::':.:...:: :?. : :' ' " ::'.:: :::" ' ' : :..-,.:: :-:.?: ;:'::L:: :: '.,... ::?. '"":: ::".:':'::":" ':' -:-:.::::..Z: ":' "..?..::'::.:::':::'-.:-.. ::::.: :...:.'"' :...,... :..:::?z-" ::..¾ :::'"",:. -::.. -:'......:.: %: ::;::.:.::':.t:..:::??...,. -'...:.::::?:::-----':;.'.;..½.::?::..: :: ; ::- :. :?.½......'... :... ::';:. :':.-:.::. 4:.;... '...' :.? :'".(:.?.'.:;-; :: :.... _,.... ::..::: ;.,...,.:,..:.:.:.... :.::..,,:... ::<:: ; :½ ; :::'a:...:.: :...,.,,,:.:.: ;? '. -.. :... "......:....:. :.' :;:...::,: :.:.'.;::; ;%;' : ;-; :--,,.,:::.. :::,::½;;;,:.' ';:::.?,**.:.::...:bs; ;:: * :::, :,:..:::' ß, ' **' :,, ; ::::.;,::..:::;..,...:.....:..:½,, :.,.. :...,:,..;..... I ]... ß *:,:: -...** ;.:. '-*...---,-,-.,t-:-.:-..:-;r. : -..,:.,:'-,......,... ':: - *': *, '".,...,... :..' ; ½,.. i?:*½::? ':'";;'?::---:;:"": :*' '"" :':"::::': :":... :: '... ;$., *.-:'-'... ;;;, i '., : -*'. * "'... :";; S; :;-... : --;-:.: :*;... :::...;.... ::..,:... :; - ß.½.--: ;..... '"':=::...: :,.....,.:,, = ::½:? :'... -,:.; :..-.::, -..:-:::½,,- -,.:... Figure 14. An infrared image of the Casey area at 0900 UTC, March 22, 1992.

17 TURNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA õ5. B , , _578,4-66, B ,_ '0, ,1 31 Figure 15. The track followed by the radiosonde launched from Casey at 0050 UTC, March 22, (To assist with visualization in the vertical, the turn to a southwestward direction occurred at an altitude of 13,833 m, and the steady southeasterly track began 43 min into the flight at an altitude of 14,964 m; the balloon burst at an altitude of 27,936 m.) 12 (U,,) 2 - (U,) g AO (H-h)/O - 2R, (9) x (kin) Figure 16. Steady state isentropes (øk) for a two-layer model flow over an idealized mountain (x is the horizontal plan distance from the mountain crest) (adapted from Miller and Durran [1991, p. 1466]). The height of the idealized mountain, its shape in some airflow situations, and the distance of the peak to where the low-level lee waves are forming all closely match the Law Dome (L) to Casey (C) situation, although Law Dome is some 500 m higher. where A0 and 0 are averaged over the height interval z = H to z = h. Table 2 shows estimates of the potential temperatures at Law Dome summit and in the free air over Casey at the same altitude (approximately 1400 m) as Law Dome, and similar estimates for the Casey Airstrip and the free air over Casey at the same level (about 400 m). The relevant differences in potential temperature are also shown. These estimates were made by manual inspection of the radiosonde traces for Casey for the times shown. As mentioned, Figure 10 shows the prestorm sounding for around 1130 UTC on March 19, 1992, while Figure 11 is the sounding for around 1130 UTC on March 21 and is presented as an example of conditions during the storm. (No trace was available for 0000 UTC on March 20). Considering now the application of equation (9) (with R set to zero) to all available data from 0000 UTC on March 19 through to 0000 UTC on March 23 (Tables 1 and 2) leads to the results shown in Table 3. It would appear that application of equation (9) has the following features for this storm: 1. Equation (9) overestimates the surface wind speed at Casey when the airstrip to Casey potential temperature difference is used early in the event, that is, prior to about 0000 UTC on March 20, but gives reasonably accurate estimates of the wind speed at Casey thereafter. 2. Equation 9 overestimates the surface wind speed at the airstrip and at Casey itself when Law Dome to Casey potential temperature differences are used.

18 ._ 7308 TURNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA x Figure 17. As in Figure 16, excepthe plotted field is horizontal wind speed (m s ' ). (Here br is the lee slope half width [see Miller and Durran, 1991, p. 1460]. (Adapted from Miller and Durran [1991, p.1467].) 5. Discussion and Conclusions The case presented here is an example of a synoptically induced strong wind event that had a major impact on one sector of the Antarctic coastal region, causing great disruption to logistical operations (in particular those associated with M/V Icebird which was stationed very near Casey Station for resupply operations) and creating extremely dangerous conditions at Casey itself. In itself the deep low that moved into the coastal area was not particularly exceptional since an examination of the 3-hourly Casey meteorological record indicates that between February 1969 and November 1996 there were 278 reports of MSLP being less than 950 hpa. This is consistent with the results of Russell- Head and Simmonds [1993], who reported 368 of the 3-hourly observations of MSLP for the period being below 950 hpa. These low MSLP observations arise because of the many deep lows found in the circumpolar trough just north of the Antarctic with some of the centers moving close to the coast. What made this particular event so notable was the relatively high pressure that occurred over the interior of the Antarctic simultaneously with the arrival of the low, so that the surface pressure gradient over Casey was especially strong. In addition, the location of the station in the lee of Law Dome also provided the conditions for an enhancement of the flow over a rather limited area. Over the ocean areas the numerical weather prediction models now produce forecasts that have considerable skill for several days ahead, and the low considered here was Referring to point (1) above, we recall that the synoptic threshold values noted in part 1 of the conceptual model were only exceeded at around 0000 UTC on March 20 from which time onward the airstrip to Casey wind speed increase was well modeled. (Within about 30 min of the 0000 UTC on March 21, wind speed observation of 7.2 m s - at Casey, the wind had increased to between 15 and 20 m s -. In other words, the model data (Table 3) for 0000 UTC on March 20 are close to the winds that developed in the Casey area just after that time.) Referring to point (2), above, we point out that frictional losses (R) were assumed to be zero. Ohata et al. [1985, p. 10,657] and Miller and Durran [ 1992, pp ] discussed the effect of friction on katabatic and downslope winds. Note, for example, that a comparison between Miller and Durran's Figure 14 (b) and their Figure 10 (b) suggests that for a long shallow slope grass or shrub surface, the surface flow will be reduced by between 5 and 15 m s -. The effect over ice would have to be determined due to smaller coefficients of friction, but in the relatively long Law Dome to airstrip transect the frictional effects are likely to be significant and account for much of the discrepancy between the observed and modeled winds. Finally, in the above discussion on component 2 of our conceptual model it was implied that once the vertically propagating wave became established this wave generated a major source of thermodynamic energy (buoyancy differences) available so drive the downslope flow. This is the essence of the results in Table 3. As further evidence of the relevance of Equation (9), its application to Miller and Durran's frictionless case in their Figure 11 (see our Figure 17) using a crest wind speed of 20 ms-', a crest height of 900 m, a height down the lee slope of 180 m (at x/br = 2 where x is the horizontal distance from the crest and br is the half mountain width, taken to be 40 km in Miller and Durran's example), and A0 and 0 averaged as 25 ø and 295øK respectively, gives a surface wind speed of 39.9 m s ' which agrees well with their figure. Low / /, Figure 18. Idealized flow lines in the neighborhood of a depression near Casey: The geostrophic pressure gradient is superimposed on a "katabatic" pressure gradient resulting from an idealized topographic gradient. (Ball's[1960] Figure 7 is rotated 70 ø anticlockwise to simulate the coastal terrain in the immediate Casey area. The flow lines (thick solid lines) in the vicinity of Casey indicate an easterly wind [after Ball, 1960, p. 16; Blakers, 1983].)

19 TURNER ET AL.: EXTREME WINDS AT CASEY, ANTARCTICA o0 o o o o (D o o o o o o o o o o o o o o o o o o o t ' o t ' o t ' o t ' o t ' o,-, o, o,-.- o, o o o o o o o o o t ' o t ' o o t ' o t ' o, o,-, o,-, o, o o o o o o o o o o o o o o o o o o o o o ( 1 o ( 1 o ( 1 o ( 1 o o, o, o, o, o o o o o o o o o ( 1 o ( 1 o o,, o, o,.- o o

20 7310 TURNER ET AL.: EXTREM]E WINDS AT CASEY, ANTARCTICA forecast well by the ECMWF model. However, over the interior of the continenthe lack of data limits the predictability, and on many occasions forecasters have to adopt a nowcasting approach here. However, the availability of surface pressure observations from the AWSs allows broad-scale pressure rises, such as took place here, to be monitored, and these regional or local changes can be used in conjunction with the model fields when producing forecasts. Although some AWS data are assimilated into the computer models, it is felt that there could be improvements in the prediction of events such as the one considered here, where the conditions over the interior play a role, if more AWS data were available to the NWP systems. The development of the major oceanic low in a situation of amplified long-wave activity is consistent with other cases examined [Bromwich et al., 1996] where such conditions steer the vortices toward the coast of the Antarctic and can even result in systems penetrating well into the interior of the continent itselfi With such an exaggerated long-wave pattern there is very marked meridional transport of air, and the high hpa thickness values experienced over Casey were exceptional for March. In such a long-wave configuration the marked east-west thermal gradient created a baroclinic zone in which the low could develop. In terms of the Casey area surface wind we conclude that the work of Wilson, Murphy/Simmonds, and Adams all has relevance in this storm, and we have attempted to extend their contributions into a more unified conceptual model. The proposed model essentially identifies sources of density differences which will lead to negative buoyancy and, as such, should prove a useful tool for operational weather forecasters. Reasonably successful application of Grace and Holton's equation (9) to observed data in this SWE has given confidence that this part of the model might provide useful short-term objective forecast guidance, at least in a diagnostic sense. Through the use of combined model and observational data it was possible to gain valuable understanding of the reasons for the extremely strong winds experienced at Casey Station. It is hoped that the insight gained into this event will be of value to forecasters in the Antarctic and that it will encourage others to conduct similar investigations now that extensive data sets of meteorological data can be compiled using combinations of satellite observations, in situ measurements, and model fields. 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