the influence of large-scale forcing on the katabatic wind regime at adélie land, antarctica

Meteorol. Atmos. Phys. 51, 165-176 (1993) Meteorology, and Atmospheric

Physics �9 Springer-Verlag 1993 Printed in Austria

i Department of Atmospheric Science, University of Wyoming, Laramie, U.S.A. z METEO-FRANCE, Centre National de Recherches M6t6orologiques, Toulouse, France 3 Geophysical Institute, University of Alaska-Fairbanks, Fairbanks, Alaska

551.555 (99)

The Influence of Large-Scale Forcing on the Katabatic Wind Regime at Ad61ie Land, Antarctica

T. R. Parish I, P. Pettr6 2, and G. Wendler 3

With 7 Figures

Received October 27, 1992 Revised December 28, 1992

Summary

The Ad61ie Land coastal section of East Antarctica is known for strong katabatic winds. Although the primary forcing of these persistent drainage flows has been attributed to the radiative cooling of the sloping ice topography, effects of ambient horizontal pressure gradients can play a central role in shaping the Antarctic surface ~wind regime as well. Observations of the katabatic wind at the near-coastal Ad61ie Land station D-10 have been sorted into strong and weak wind classes. Concurrent radiosonde ascents at nearby Dumont D'Urville have been used to depict the time- averaged large scale conditions accompanying the katabatic wind classes. Results suggest that strong katabatic wind cases are associated with low pressure over the coastal margin and easterly upper level motions. Numerical simulations have been conducted to examine the effect of prescribed large scale forcing on the evolution of the katabatic wind. The model runs indicate that the ambient environment plays a key role in the development and intensity of the katabatic wind regime.

I. Introduction

Katabatic winds are prevalent features of the lower boundary layer over Antarctica. Without question, topographic forcing is the dominant mechanism in the establishment of the Antarctic surface wind regime. The strong radiational cooling of the sloping Antarctic ice fields sets up a horizontal pressure gradient at the surface directed in a down-

slope direction. The magnitude of the forcing is dependent on both the terrain slope and cooling rate; it is not surprising that the strongest katabatic winds are found above the steep coastal ice slopes of East Antarctica during the non-summer months. Many of the early katabatic wind studies examined the relationship between the topographic forcing and the intensity of the katabatic wind (see, for example, Ball, 1960). Directional wind constancy, a ratio of the vector resultant wind to the scalar mean wind speed, is character- istically near 0.90 for Antarctic stations (Parish, 1982), indicating the unidirectional nature of the surface wind. Surface wind directions are in general related to the fall line of the terrain.

While this topo-dynamic forcing dominates the drainage flows over most of Antarctica, the effect of the ambient horizontal pressure field on the near-surface windfield cannot be neglected. Syn- optic charts clearly show the coastline about Antarctica to be an active storm track. Schwerdt- feger (1984) notes that the sea level trough of low pressure about the continental periphery is the site of maximum cyclonic activity. Horizontal pressure gradients associated with the passage of cyclones appear to modulate the intensity of the katabatic wind although observations suggest the directional wind change is usually small.

166 T.R. Parish et al.

However, studies of the interaction between the katabatic wind and the ambient environment have produced inconclusive and often contra- dictory results. Ball (1960) has suggested that the horizontal pressure field associated with the western side of a trough should enhance the katabatic outflow. A similar conclusion was reached by Tauber (1960) who noted that strong katabatic winds seem to occur when the geostrophic wind in the free atmosphere was directed offshore. However, Loewe (1974) notes that the strongest katabatic winds at Port Martin are associated with pressure falls in advance of predominantly west-to-east moving cyclonic disturbances; the upper level geostrophic wind presumably acts in a direction opposite to that of the katabatic wind. Bromwich (1989) has indicated that the strongest katabatic winds at Terra Nova Bay appear during periods in which the horizontal pressure gradient over the Ross Sea is quite weak. For episodes of strong southerly geostrophic winds, which at Terra Nova Bay are associated with an offshore horizontal pressure gradient force, weaker than usual katabatic winds prevail. Thus, differing conclusions have been reached regarding the interaction between the katabatic wind and the large-scale environment. Such discrepancies can in a broad sense be attributed to the sparse data coverage over Antarctica and the surrounding Southern Ocean and attendant uncertainties in the horizontal pressure field about the continent.

In a larger context, it has become apparent that the katabatic wind regime cannot be viewed as isolated from its surrounding environment. Egger (1985), James (1989) and Parish (1992) have shown that the continental-scale katabatic wind drainage of the Antarctic influences the large- scale circulation throughout much of the troposphere and possibly even extending into the stratosphere. The mass transport provided by the katabatic wind removes cold, dense air from low levels over Antarctica. Continuity requirements imply warmer air from aloft must replenish the katabatic wind regime. Thus, a thermally-direct secondary circulation becomes established in the high southern latitudes comprised of the northward-directed katabatic wind regime at low levels, rising motion just offshore from Antarctica, a return branch in the middle and upper troposphere and a general subsidence over the continent.

Upper level convergence above Antarctica acts to generate cyclonic vorticity; a circumpolar vortex must become established in time. Egger (1985, 1992) has noted that angular momentum must be transported northward if the katabatic wind regime is to persist. Baroclinic eddies are the most likely transport agent; such a constraint offers more evidence as to the coupling between the katabatic wind and cyclonic activity. Although the dominance of the topography in establishing the katabatic wind cannot be denied, the interaction between the drainage flow and the environment is undoubtedly significant.

Here we will investigate the interaction of the katabatic wind with the ambient horizontal pressure field along the Ad61ie Land sector of East Antarctica, a section known for intense katabatic winds. An evaluation will be made of observational evidence incorporating available automatic weather station (AWS) data in the Ad61ie Land sector and daily soundings taken along the coastal stretch of Ad61ie Land. In addition, results from a series of numerical modeling experiments will be synthesized in order to depict the coupling between the Adelie Land drainage flows and the large-scale environment.

2. Dumont D'Urville Soundings

Extremely strong and persistent katabatic winds prevail along the stretch of Antarctic coastline in Ad61ie Land from approximately 140-145 ~ The 1911-14 Australasian Antarctic Expedition under the direction of Mawson established the base camp of Cape Denison. The group maintained a meteorological log during the two-year stay and found the average annual wind speed to be in excess of 19 ms- 1, by far the highest wind speeds recorded for a sea level location. Similar intense katabatic winds were also seen some 60km to the west of Cape Denison at the French station Port Martin during 1950 and 1951. During the past decade a joint U.S.-France study has taken place to better understand the katabatic wind in this region. The Interactions-Atmosphere-Glace- Ocean (IAGO) experiment has established a string of five AWSs from a central point atop the nearly flat Dome C some 1000km inland and extending down to the coast of Ad61ie Land near the station Dumont D'Urville (see Fig. 1). In addition, detailed summer field campaigns were

The Influence of Large-Scale Forcing on the Katabatic Wind Regime at Ad61ie Land, Antarctica t67

135~ 150 ~

70 ~ - 75 ~ 120 ~ 80 ~ 135 ~ ~50 ~

Fig. 1. Adelie Land, Antarctica

conducted during two seasons (see Wendler et al., 1988). Routine upper air soundings have been taken at least once per day at the French station Dumont D'Urville situated on a rocky outcrop some 2 km from the Ad61ie Land coast. Here we will report on the sounding data from the years 1979-1984 and 1987-1988. Soundings were taken twice daily prior to 1980; this was reduced to once per day in the following years. We will consider only the 00Z (1000 LT) sounding information for the eight-year period.

To give some perspective on the upper level conditions near Ad61ie Land, the Dumont D'Urville sounding data were grouped into multi-annual monthly means. Figure 2 illustrates the monthly mean soundings of temperature, wind speed and wind direction from 500 m above the surface to around 22kin for the eight-year period. Owing to the height of the first level considered in this analysis, katabatic wind events are masked for the most part. The monthly mean thermal structure of the troposphere (Fig. 2a) shows only minor seasonal variations; a near- constant lapse rate of nearly 6 ~ is present above approximately 2 km to the tropopause near 9 km or so. Note that during the summer months a well-mixed lower boundary layer is evident;

during the non-summer months the temperature sounding suggests some degree of stratification which may reflect the katabatic wind. The stratosphere above Dumont D'Urville provides ample evidence of dramatic seasonal change. The mean monthly soundings for the eight-year period suggest an annual temperature oscillation of near 40~ near 20km. Note that the temperature change near 20 km during the austral springtime period from August to October amounts to 30 ~ This rapid stratospheric temperature increase is in response to the absorption of solar energy by ozone with the return of the sun to the Antarctic.

Wind speed profiles (Fig. 2b) for the multi- annual monthly mean soundings display the characteristic katabatic signature in the lowest 2 kin. Since the lowest sounding level used in the analysis is 500 m, much of the katabatic wind cannot be explicitly resolved. The maximum katabatic wind speed near Ad61ie Land usually is found in the lowest 200 m or so (see Kodama et al., 1989). Nevertheless, some seasonal variation in the intensity of the low level windfield is present. A hint of the equinoctial maxima in tropospheric wind speed (Schwerdtfeger, 1984) can be seen for this eight-year period. Maximum wind speeds between 5 and 10km occur between May and


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TEMPERATURE (C) a

~ , ~ , ~ \ ~ , ~ , , , / , , , / , , , , 2 / , , ~ - , , - , ~ ,

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WIND SPEED (m/s) b

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4

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WIND DIRECTION c

Fig. 2. Mean monthly vertical profiles of(a) temperature, (b) wind speed, and (c) wind direction for 00Z soundings at Dumont D'Urville for 1979-84, 1987-88 period. A refers to January, B February, etc

June and again near October. As seen previously, the outstanding seasonal variation is seen in the stratosphere. Monthly mean wind speeds above 20 km undergo an annual oscillation in excess of 30 ms-~ with the maximum winds seen during the late winter and early spring periods when the stratospheric circumpolar vortex about the continent is most pronounced. Note that from October to December the mean wind speed at 22 km is reduced from 28 ms-1 to just 2ms-1 . Limited sounding data above the 22 km level (not shown)

suggests an even larger wind speed change during the austral springtime.

Wind directions show little seasonal change (Fig. 2c). The katabatic drainage and attendant sea-level easterlies can be seen in the lowest 2 km. The orientation of the fall line on the ice slopes inland from Dumont D'Urville is around 180~ the resultant wind direction at this site is 135 ~ . Note that above 2km the wind abruptly shifts from east to west indicating that the entire katabatic regime is contained within the lowest 1500 m

The Influence of Large-Scale Forcing on the Katabatic Wind Regime at Ad61ie Land, Antarctica 169

9 9 l ' ~ ' ~ ' ~ ' ~ ' ~ ' ~ ' - ' ' ~ ' ~ '

9 9 0

9 8 9

988 / ,, 1 / / " / /

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Fig. 3. Mean monthly sea-level pressures at Dumont D'Urville for 1979-84, 1987-88 period

or so. The sharp wind shift is consistent with the strong horizontal temperature contrast owing to the effect ofdiabatic cooling over the steep coastal terrain slopes in the lower atmosphere. This feature is in all likelihood seen at other katabatic wind-prone sites about the Antarctic periphery. Note that during the austral summertime period the wind direction shifts from the predominant westerly flow to an easterly circulation above approximately 20 km in response to the absorption of shortwave energy by ozone.

Of relevance to the katabatic wind regime is the mean monthly variation in sea level pressure (Fig. 3). Note that a well-defined semi-annual oscillation is present with maxima in sea level pressure seen during the solstice periods and minima during the equinoxes. Schwerdtfeger (1967) has noted that such a pressure trace is indicative of large-scale movements of atmospheric mass in response to meridional differences in solar radiation and attendant latitudinal heat budgets. The corresponding pressure trace implies a net mass transport northward from Antarctica at the end of summer and at the end of winter with a southward net mass flux from middle to high southern latitudes during late spring and autumn. It is not clear how closely coupled the katabatic wind is to this bulk movement of atmospheric mass although there seems no doubt that the katabatic wind is an important agent in the meridionat exchange. It is ~vorth noting that

the most intense monthly period of katabatic winds at the surface of the Antarctic continent took place at Port Martin during March of 1951 (mean wind speed 24.9ms-1). Other katabatic wind records at coastal sites such as Cape Denison and Terra Nova Bay suggest no detectable equinoctial wind speed maxima. Schwerdtfeger (1984) notes that cyclonic activity about the continent is at a maxima during the equinoctial periods; again, the association between the katabatic wind regime and the broad-scale environment is ambiguous in a general sense.

3. Episodes of Strong Katabatic Winds

To examine the possible relationship between the katabatic wind regime near Ad61ie Land and forcing in the free atmosphere, it was first necessary to sort the surface wind data at Ad61ie Land on the basis of wind speed. Here we have used the AWS data from the near-coastal station D-10 (elevation 240m, installed in early 1980) as the reference site for the strong versus weak katabatic wind cases; strong winds were defined as those in excess of 15 ms - 1 while weak winds were 3 ms- 1 or less. The selection of D-10 is probably of little impact to the results of the study. The AWS at D-47, situated on the ice slope at an elevation of 1560m approximately l l 0 k m from the coast, displayed similar trends to those of D-10. To be consistent with the sounding data collected at nearby Dumont D'Urville, only the 00Z observations from the D-10 AWS were considered. During the seven-year period 1980-1984, 1987- 1988, a total of 244 strong wind cases at D-10 were selected. Several factors contributed to limit the number of cases. First, the selection of 00Z (corresponding to around 1000 LT) as the reference time is not ideal to examine the katabatic wind. A marked diurnal cycle in the Ad61ie Land wind is present during the short summer period (October- February) with a minimum wind speed occurring shortly after the 00Z observing period (see P6riard and Pettr6, 1991). Second, the AWS record is in- complete due to instrument malfunction in the harsh Antarctic climate. The actual number of cases used in the study was further reduced due to difficulties in conducting soundings under intense katabatic wind conditions. Concurrent successful rawinsonde ascents occurred on only 138 of the strong wind case study days. A total


of 184 weak wind case study days were identified from the seven-year record; concurrent rawinsonde ascents were made on 156 of the study days.

For all case study days the wind speed and direction, temperature and pressure at constant levels of 500, 1000, 1500, 2000, 3000, 4000, 5000, 6000, 7000 and 8000 m above sea level were analyzed from the Dumont D'Urville soundings. Composite strong and weak wind soundings were then constructed. Of interest is the zonal component of the wind, which above approximately 1000 m we assume is close to geostrophic. Because of the sparse data coverage about the Southern Ocean, little direct observational evidence per- taining to the horizontal pressure gradient is available. Thus, the zonal component of wind will be used as an indication as to the strength of the pressure gradient force. Since the Ad~lie Land coastline runs essentially along the 67 ~ parallel, a horizontal pressure gradient force directed northward acts to support the katabatic drainage off the continental ice sheet. This corresponds to an easterly geostrophic wind. From the work of Ball (1960), it is to be expected that some difference in the x-component of the wind in the lowest levels should differentiate the strong and weak wind composite soundings.

Tables 1 and 2 depict the results of the composite soundings for the strong and weak wind cases, respectively. The zonal wind components and attendant ambient large-scale horizontal pressure gradients in the near-coastal environment are significantly different for the two sets of conditions. The strong wind composite sounding

Table 1. Mean Pressures, Wind Components and Tempera- tures in the Lowest 8000 m from OOZ Soundings at Dumont D'Urville for 1980-1984, 1987-1988 for Strong Wind Cases (D-10 wind speed 15 ms- 1 or greater)

Ht (m) P (hPa) u (ms- 1) v (ms- i) T (C)

500 920.8 -9.0 5.8 - 10.0 1000 862.5 -9.2 3.3 - 13.5 1500 807.4 - 8.1 1.7 - 15.2 2000 755.6 - 5.9 1.5 - 16.1 3000 661.0 - 3.6 0.9 - 20.1 4000 576.6 - 1.5 0.9 - 25.8 5000 501.4 - 0.5 0.8 - 32.0 6000 434.3 0.1 0.5 -38.8 7000 374.5 0.9 0.0 -45.8 8000 323.3 1.8 -0.2 -52.1

Table 2. As in Table 1, Except for Weak Wind Cases (D-10 wind speed 3 ms- 1 or less)

Ht (m) P (hPa) u (ms- 1) v (ms- 1) T (C)

500 927.7 -2.5 1.3 - 13.0 1000 868.5 - 1.9 1.5 - 15.1 1500 812.8 -0.3 1.2 - 15.6 2000 760.6 1.3 0.9 - 16.6 3000 665.0 3.2 0.5 -21.3 4000 579.8 4.6 0.2 - 27.0 5000 503.8 5.8 - 0.2 - 33.0 6000 436.2 6.7 - 0.4 - 39.6 7000 376.0 7.5 -0.7 -46.4 8000 322.7 8.6 - 0.9 - 52.4

suggests easterly geostrophic winds persist throughout much of the troposphere. Both the strength and the depth of the easterly flow appear to be correlated with katabatic wind intensity. For the strong katabatic wind episodes, the strongest tropospheric zonal current is found at lowest levels; it appears as though a supporting meridional horizontal pressure gradient equivalent to approximately a 10 ms - 1 easterly geostrophic wind is present during katabatic wind episodes at D-10 in excess of 15ms -x. The decrease in the easterly current with height (or, similarly, the increase of the westerly wind regime with height) seen in Table 1 is consistent with the terrain- induced forcing of the katabatic wind and also the meridional temperature contrast arising from solar geometry following the thermal wind relationship.

By contrast, the 156 cases of weak katabatic winds at D-10 are characterized by relatively weak large-scale ambient forcing in the lowest 1 km equivalent to an easterly geostrophic wind component of around 2 ms - 1. Upper levels of the troposphere are characterized by westerly flow which reaches approximately 10 ms - 1 by 300 mb. This westerly circulation is embedded within the very broad circumpolar vortex which stretches to mid-latitudes of the Southern Hemisphere. Note that even during poorly developed katabatic wind conditions, the time-averaged ambient meridional horizontal pressure gradient force at low levels is directed northward. The same statement holds when one considers the yearly-mean soundings at Dumont D'Urville; the mean zonal winds in the lowest two kilometers are also from the east (see Table 3). This may be a reflection of the

T h e In f luence o f L a rge -S ca l e F o r c i n g on the K a t a b a t i c W i n d R e g i m e at Ad61ie L a n d , A n t a r c t i c a 171

Tab l e 3. As in Table I, Except for All Cases

Ht (m) P (hPa) u ( m s - ~) v ( m s - 1) T (C)

500 926.0 - 5.2 2.7 - ! 1.8

lO00 867.1 - 4 . 3 1.6 - 14.6

1500 811.6 - 2 . 6 1.1 - 1 5 . 5

2000 759.5 - 0 . 9 0.7 - 16.4

3000 664.2 1.2 0.2 - 20.8

4000 579.2 2.7 - O. 1 - 26.4

5000 503.5 3.8 - 0.4 - 32.6

6000 436.0 4.9 - 0.6 - 39.2

7000 375.9 5.9 - 0 . 9 - 4 5 . 9

8000 322.8 6.9 - 1.2 - 52.0

circumpolar low pressure trough which is found over the ocean north of the East Antarctic coastline near Ad61ie Land. This may also be a mani- festation of the persistent katabatic outflow. As the cold, dense air becomes transported northward across the Antarctic coastline, psuedo-inertial turning of the airstream may occur with a few hundred kilometers of the continent. The mass adjustment in the lowest levels of the atmosphere offers support to the climatologically-observed sea level easterlies.

It can also be seen from Tables 1 and 2 that the strong katabatic wind events are associated with warmer temperatures and lower pressures than found during weak katabatic wind episodes. Surface pressures (not shown) during the strong wind cases are nearly 14 hPa lower than seen for corresponding weak wind cases and nearly 12 hPa lower than the average for all cases. The higher temperatures may be due to the pronounced mixing and/or adiabatic compression of airflow off the continent attendant with strong katabatic winds. The lower pressures may be indicative of the large-scale synoptic environment with a deeper low pressure trough over the coastal margin of Ad61ie Land during the strong wind events.

Analyses of pressure change accompanying the strong and weak wind cases yield results similar to those reported by Loewe (1974). The strong wind cases are associated with pressure falls at the surface. For the 138 strong wind cases considered, the average pressure fall was 0.4 hPa in the previous six hours. Weak wind cases were accompained by a small rise in pressure with time; the mean six-hour pressure change amounted to a 0.09hPa rise. It is likely that the isallobaric component of the wind at times can be an impor-

rant factor in the strength of the katabatic wind. The coastal periphery about Antarctica is known for frequent and often intense cyclonic disturbances. Examination of the D-10 AWS record indicates abrupt pressure changes are quite common; pressure falls in excess of 20 hPa in a 24-hour period are not unusual. The entire Ad61ie Land coastline is subject to marked changes in pressure; Port Martin has experienced a range of surface pressures in excess of 100 hPa (Schwerdt- feger, 1984). The intensity of the cyclones and their often rapid movement can give rise to large isallobaric wind components which, for an approach- ing cyclone, are directed offshore. Kidson (1946) has noted that strong katabatic winds always increased at Cape Denison with the approach of a cyclonic depression. The lack of spatial and temporal coverage in high southern latitudes restricts assessment of the isallobaric component of the ageostrophic wind, yet it is clear that this is an important consideration at times. However, it can be stated with confidence that the strong katabatic wind events along the Ad61ie Land coast usually occur with lower than normal pressure in the offshore environment and under falling pressures.

While there are significant differences in the zonal wind component during strong and weak katabatic wind episodes and hence differences in the horizontal pressure gradient, we stop short of implying that the katabatic wind regime is solely determined by the large-scale conditions. After all, a plethora of surface observations indicate the overwhelming importance of the terrain forcing in producing the unidirectional katabatic wind regime. In addition, the radiative cooling of the sloping terrain acts to create a horizontal pressure gradient supporting easterly geostrophic winds in the near-coastal environment. It is possible that the terrain influences alone may be responsible for a 15 ms-1 katabatic wind near the coast of Ad61ie Land. Thus, the existing ambient low level pressure field in the strong katabatic wind cases may be due in large part to the katabatic forcing itself and the attendant adjustment of the low level horizontal pressure field. Likewise, it is not clear that the weak wind cases are due solely to the dominance of the large-scale ambient pressure field. Thick cloud shields may be present and disrupt the radiative cooling of the sloping terrain necessary for the development of the katabatic wind


regime. In addition, the katabatic wind may be responsible in part for creating an adverse environment for its continuance. As indicated earlier, a strong interaction between the katabatic wind regime and the ambient atmosphere takes place. Egger (1985) has pointed out that the Antarctica katabatic wind regime cannot be viewed as a steady-state system. The mass flux imparted by the drainage flows tend to increase the surface pressure near the coast and decrease the surface pressure over the high plateau. This implies the existence of a secondary circulation throughout much of the troposphere which will produce a component of the horizontal pressure gradient adverse to the katabatic wind regime.

4. Model Simulations of the Interaction Between the Katabatic Wind and the Large-Scale Environment

To quantify the relationship between the katabatic wind and large-scale forcing, a series of numerical experiments have been performed. The model used is a two-dimensional version of that described by Anthes and Warner (1978), adapted for katabatic wind studies. The model is written in terrain- following sigma coordinates to allow for irregular terrain. Both an explicit longwave radiation parameterization scheme and a high resolution boundary

layer are present to capture the essential physics of the katabatic wind evolution. Complete details of the model equations, parameterization of the model physics, initial and boundary conditions can be found in Parish and Waight (1987). The numerical experiments are designed to examine the development of the katabatic wind over the Ad61ie Land terrain profile under a variety of prescribed initial horizontal pressure fields. Terrain data were obtained from the Drewry (1983) map of Antarctica and digitized to a 40-km grid scale. A total of 40 grid points comprise the horizontal domain, covering a distance of approximately 1200 km inland from D-10 and extending 400 km out over the ocean. In all simulations, it is assumed that the ocean is covered with a thick ice shelf. The vertical grid used in the simulations consists of 15 levels (a = 0.998, 0.99, 0.98, 0.97, 0.96, 0.94, 0.92, 0.90, 0.85, 0.775, 0.70, 0.60, 0.50, 0.30, 0.10); the lowest level corresponds to approximately 10 m above the ground.

Five 24-hr numerical simulations incorporating different background values of the horizontal pressure field will be described. The initial horizontal pressure fields correspond to geostrophic winds of 20 and 10 ms- 1 from the west (opposing the katabatic wind), 0, and 10 and 20 ms-1 from the east (supporting the katabatic wind) throughout the model atmosphere. In all experiments, the

GRID POINT 50 (D-IO) KATABATIC WIND SPEED

le L , i i i i i i [ i i i i i i i J i J i i J i i

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7

Z o

( J I t / r r

320

300

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260

240

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2OO

180

160

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GRID POINT 50 (D-IO) DIRECTION OF KATABAT, IC WIND

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,,,,, ,-.,,,,,,,,,,1,,. 0 4 8 12 16 20 24 4 8 12 !6 20 24

TIME (hrs) TIME (hrs)

Fig. 4. Model evolution of the wind speed (left) and wind direction (right) at the first sigma level for the D-10 grid point for 24-h integration


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12

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GRID POINT 21 (D-80) KATABATIC WIND SPEED

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4 8 12 16 20 24

TIME (hrs)

Fig. 5. As in Fig. 4, except for the D-80 grid point

5

GRID POINT 21 (D-80) DIRECTION OF KATABATIC WIND

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540 t ~ " ~ " ~ A "- '--- '- '----

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TIME (hrs)

model has been initialized about a geostrophic state. The numerical simulations assume ideal clear sky, nocturnal conditions; individual case studies of the D-10 katabatic wind presented in the previous section may not conform to such ideal conditions.

Figure 4 illustrates the evolution of the wind speed and direction at the lowest sigma level (approximately 10 m above the ice surface) at the grid point corresponding to the D-10 location for each of the five model simulations. In this and other figures to follow, curves A and B refer to model results from simulations with initial westerly geostrophic winds (opposing the katabatic wind) of 20 and 10ms- l , respectively. Curve C illustrates results of the model run with no ambient pressure gradients at the start of the integration period; curves D and E refer to the simulation with initial easterly geostrophic winds (supporting the katabatic wind) of 10 and 20 ms- 1, respectively. The simulated D-10 katabatic winds appear to reach a quasi-steady state within the first six hours for all except the 20 ms - 1 westerly wind initializa- tion (curve A). It can be seen that the horizontal pressure field has a noticeable influence on the intensity of the katabatic wind regime. In absence of any initial horizontal pressure gradients, the simulated D-10 katabatic wind speed (curve C) reaches approximately 12ms-1. Introduction of

initial ambient forcing modifies the intensity of the katabatic wind. An initial 10ms-1 westerly wind (curve B) acts to retard the development of the katabatic wind regime such that the near- steady wind speed is approximately 10ms -1. Preexisting horizontal pressure gradients supporting the katabatic wind tend to enhance the intensity of the katabatic wind by a similar incre- ment; initial 10 and 20 ms-1 geostrophic winds lead to quasi-steady kat/tbatic wind speeds of approximately 14 and 16ms-1

Wind directions for the near-surface wind regime at the grid point corresponding to D-10 show the overwhelming influence of topography. By 24 hours all five simulations' show essentially the same wind direction, clearly reflecting the topography of the region. This suggests that although the synoptic variability may be reflected in the katabatic wind intensity at D-10, the directional change accompanying changes in the large-scale forcing are minimal. This again under- scores the fundamental importance of the terrain in the development of the surface wind over the steeply-sloping coastal environment of Antarctica.

The above analysis is i n accord with observations from the coast of Ad61ie Land. The AWS records show that winds at the D-10 site are almost always off the ice sheet. Northerly winds occur less than 17o of the time and are invariably


very weak. Figure 5 illustrates the evolution of the wind speed and wind direction at the grid point corresponding to D-80 situated at an elevation of approximately 2500 m. The terrain-induced forcing at this site is considerably less than that found at D-10 owing to the gentle terrain slope of approximately 0.002 which is a factor of ten less than that of the coastal region near D-10. The resulting surface wind at this point is much more sensitive to the large-scale forcing than the D-10 grid. The intensity of the katabatic wind in absence of horizontal pressure gradients at the start of the model run (curve C) reaches approximately 6 ms - 1. Intro- duction of favorable pressure gradients equivalent to 10 and 20 ms-1 geostrophic winds (curves D and E) lead to quasi-steady katabatic wind speeds of 10 and I3 ms- ~, respectively. Note thatadverse horizontal pressure gradients corresponding to westerly geostrophic winds of 10 and 20ms-1 (curves B and A) lead to near-surface wind speeds at D-80 of 2 and 6 ms - 1. The evolution of the wind directions suggests that for adverse forcing at the D-80 site, the large-scale forcing is able to over- come the terrain-induced forcing for even moderate disturbances.

Figure 6 illustrates the vertical profiles of wind speed in the lowest 1400 m of the atmosphere after the 24-hr model integration for the grid points associated both D-10 and D-80 locations. Note that

the characteristic katabatic wind signal is present in all but the most extreme adverse case at the D-10 grid. At the D-80 grid, no trace of the katabatic signature can be seen for both initial 10 and 20 ms- 1

westerly geostrophic wind cases. This again empha- sizes the importance of the slope of the ice sheet in the establishment of the surface windfield. The intensity of the maximum wind speed and depth of the katabatic layer are sensitive to the initial ambient forcing. Little modification takes place in the free atmosphere above 500 m during these short integration periods; Parish (1992) has shown that this is not the case for extended model simulations of the Antarctic katabatic wind. The wind directions (not shown) can be inferred from the wind speed profiles; consistent wind directions of 140-150 ~ are seen within the katabatic layer for all five numerical experiments with a sharp return to the ambient conditions between 200-400 m.

To depict the adjustment in the horizontal pressure field attendant with the katabatic wind evolution, the vertical profiles of the zonal component of the geostrophic wind after the 24-hr simulations for both D-10 and D-80 grids are shown in Fig. 7. Radiative cooling of the sloping ice terrain dramat- ically alters the horizontal pressure field in the lowest levels of the atmosphere and leads to a downslope-directed horizontal pressure field in each case. Note that the effect of the initial forcing

GRID POINT 30 (D-IO) WIND SPEED (m/s) 1400 I . . . . . . . / . . . .

1200 t

IO001-C D S E ,4

800

600

122 0 0 2 4 6 8 I0 12 14 16 18 20 22 24 26 28 50

WIND SPEED (m/s)

E I - - -i '- (.9

LtJ I

1400 I

1200

C I000

800

600

400

200

0 0

GRID POINT 21 (D-80) WIND SPEED (m/s)

"1'1 . . . . I" O O , d E

4 8 12 16 20 24 28 WIND SPEED (m/s)

32

Fig. 6. Vertical profiles of wind speed at D-10 and D-80 grid points from 24-hr model integration for ambient initial forcing conditions. Curves A and B denote simulation incorporating initial 20, 10ms-1 westerly winds, respectively; curve C denotes no ambient initial horizontal pressure gradients; curves D, E denote initial 10, 20 ms-~ easterly winds


T

( .9

U J "1-

,1400

1200

IO00

800

600

GEOSTROP ' i I i

_ B

GRID POINT 30 (D-10) 41C WIND SPEED (m/s)

i I l

400 -

200 f r

-20 - I0 0 WIND SPEED

I l l l l l

0

E

I0 20 30 40 (m/s)

I - - - r - (..9

I L l "1-

iiii ! I000

800

600

400

20O

0 I -2( -15 - I0

GRID POINT 21 GEOSTROPHIC I I I

B

(D - 80) WIND SPEED (m/s)

C D E

-5 0 5 I0 15 WIND SPEED (m/s)

/ Fig. 7. As in Fig. 6, except for geostrophic wind conditions

I

20 25

is evident. Horizontal pressure fields at the end of the integration period range from 41 ms- ~ for the initial 20ms -~ easterly geostrophic wind run (curve E) to 5 ms- 1 for the initial 20 ms- 1 westerly wind case (curve A) for the D-10 grid. Note that over the more gentle slopes associated with the D-80 grid point, the adjustment in the horizontal pressure gradient force in the lower atmosphere is much less pronounced. While relatively large adjustments take place in the lowest few hundred meters (up to 25 ms-1 for the D-10 grid point), little change is seen above 400 m.

Results of the numerical simulations suggest that, given ideal radiative conditions, the terrain- induced forcing is the dominant mechanism in shaping the surface wind over the steeply-sloping coastal regions and often even over the gently- sloping interior regions. Large ambient horizontal pressure gradient forces are required to disrupt the coastal drainage flows although katabatic winds over the interior slopes seem considerably more sensitive to external forcing. Presumably the infrequent penetration of cyclones onto the East Antarctic continent serves to buffer the interior surface wind field from strong, adverse horizontal pressure gradients. Observations (see Parish, 1982) reveal that surface winds over the interior of Antarctica also display high directional constaricy values.

It is concluded that the ambient conditions

appear to regulate the intensity of the drainage flows.

5. Summary

Composite wind profiles for the station Dumont D'Urville reveal significant differences between strong and weak wind events at the nearby AWS D-10. The ambient atmosphere appears to modulate the intensity of the katabatic wind, although the directional change appears small. Strong katabatic winds at D-10 are associated with lower than normal pressure in the coastal vicinity of Ad61ie Land and predominantly easterly flow above the katabatic layer implying a south-to- north horizontal pressure gradient force. This concurs with results from a two-dimensional numerical model which suggests that the strength of the katabatic wind is sensitive to the initial large-scale forcing. The terrain-induced forcing significantly adjusts the horizontal pressure field in the lowest few hundred meters of the atmosphere above Antarctica such that a downslope-directed horizontal pressure gradient force develops in the lowest levels in all but the most extreme cases at sites along the sleep Antarctic coastal perimeter. This is in agreement with an extensive body of observations which suggests that the katabatic wind along the coast of East Antarctica is essentially unidirectional. It is also proposed that

176 T.R. Parish et al.: The Influence of Large-Scale Forcing on the Katabatic Wind Regime

isallobaric components of the ageostrophic wind may be significant during periods of intensification of cyclonic disturbances or the approach of deep cyclones along the coastal periphery of the continent. Verification of the importance of such ageostrophy awaits more detailed data assimila- tion about the continent.

Acknowledgements

This work was supported in part by the National Science Foundation grants DPP-8916998 and DPP-9117202 (TRP) and DPP-9017969 (GW). We wish to thank the many people from Expeditions Polaires Francaises and U.S. Antarctic Program for their help in the data collection. Special thanks go to Professor Charles Stearns and colleagues at the University of Wisconsin-Madison for the dissemination of the automatic weather station data.

References

Anthes, R. A., Warner, T. T., 1978: The development of hydrodynamical models suitable for air pollution and other mesometeorological studies. Mort. Wea. Rev., 106, 1045-1078.

Ball, F. K., 1960: Winds on the ice slopes of Antarctica. Antarctic Meteorology, Proceedings of the Symposium in Melbourne, 1959. New York: Pergamon, pp. 9-16.

Bromwich, D. H., 1989: An extraordinary katabatic wind regime at Terra Nova Bay, Antarctica. Mon. Wea. Rev., 117, 688-695.

Drewry, D. J., 1983: The surface of the Antarctic ice sheet. In: Drewry, D. J. (ed.) Antarctica: Glaciological and Geo- physical Folio, sheet 2. Cambridge: Scott Polar Research Institute.

Egger, J., 1985: Slope winds and the axisymmetric circulation over Antarctica. J. Atmos. Sci., 42, 1859-1867.

Egger, J., 1992: Topographic wave modification and the

angular momentum balance of the Antarctic troposphere. J. Atmos. Sci., 49, 327-334.

James, I. N., 1989: The Antarctic drainage flow: Implications for hemispheric flow on the Southern Hemisphere. Antarct. Sci., 1, 279-290.

Kidson, E., 1946: Discussion of observations at Adrlie Land, Queen Mary Land, and Macquarie Island. Austr. Ant. Exp. 1911-1914. Scient. Rep. Ser. B, vol. 6.

Kodama, Y., Wendler, G., Ishikawa, N., 1989: On the diurnal variation of the boundary layer in summer in Adrlie Land, Eastern Antarctica. J. Appl. Meteor., 28, 16-24.

Loewe, F., 1974: Considerations concerning the winds of Adrlie Land. Z. Gletscherkd. Glazialgeol., 10, 189-197.

Parish, T. R., 1982: Surface airflow over East Antarctica. Mon. Wea. Rev., 110, 84-90.

Parish, T. R., 1992: On the role of Antarctic katabatic winds in forcing large-scale tropospheric motions. J. Atmos. Sci., 49, 1374-1385.

Parish, T. R., Waight, K. T., 1987: The forcing of Antarctic katabatic winds. Mon. Wea. Rev., 115, 2214-2226.

Prriard, C., Pettrr, P., 1991: Climatology of Dumont D'Urville, Adrlie Land, Antarctica. Centre National de Recherches Mbtrorologiques, Toulouse, 19 pp.

Schwerdtfeger, W., 1984: Weather and Climate of the Antarctic. New York: Elsevier Science Publishers, 261 pp.

Schwerdtfeger, W., 1967: Annual and semi-annual changes of atmospheric mass over Antarctica. J. Geophys. Res., 72, 3543-3547.

Tauber, G. M., 1960: Characteristics of Antarctic katabatic winds. Antarctic Meteorology, Proceedings of the Symposium in Melbourne, 1959. New York: Pergamon, pp. 52-64.

Wendler, G., Ishikawa, N., Kodama, Y., 1988. On the heat balance of the icy slope of Adrlie Land, Eastern Antarctica. J. Appt. Meteor., 27, 52-65.

Authors' addresses: T. R. Parish, Department of Atmos- pheric Science, University of Wyoming, Laramie, WY 82071, U.S.A.; P. Pettrr, METEO-FRANCE, Centre National de Recherches Mrt6orologiques, Toulouse, France; G. Wendler, Geophysical Institute, University of Alaska-Fairbanks, Fair- banks, Alaska 99701.

the influence of large-scale forcing on the katabatic wind regime at adélie land, antarctica

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