mesoscale cyclogenesis dynamics over the southwestern ross...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. D7, PAGES 12,973-12,995, JULY 20, 1993

Mesoscale Cyclogenesis Dynamics Over the Southwestern Ross Sea, Antarctica

JORGE F. CARRASCO • AND DAWD H. BROMWICH

Byrd Polar Research •n tcr, A traosphcric Sciences Program, Otu'o State University, Columbus

Previous work has shown that frequent mesoscale cyclogenesis adjacent to Franklin Island is linked to the strong and persistent katabatic winds from East Antarctica which funnel into Terra Nova Bay and then blow out over the southwestern Ross Sea. Four mesoscale cyclones that formed near Terra Nova Bay between February 16 and 20, 1988 are examined to more clearly define the governing mechanisms. These events are investigated using all available observations, including automatic weather station data, high-resolution satellite images, satellite soundings, and hemispheric synoptic analyses. The first two cyclones formed on low-level baroclinic zones established by the synoptic scale advection of warm moist air toward the cold continental air blowing gently from East Antarctica. In the second case, baroclinic instability of this small-scale cold front was apparently triggered by the enhanced upward vertical motion associated with the approach of a midtropospheric trough. The third mesocyclone formed shortly after on a baroclinic zone over the polar plateau; the second vortex completely disrupted the usual katabatic drainage over the plateau and forced warm moist air over the coastal slopes. All three cyclones moved to the north in the prevailing cyclonic flow, but the plateau vortex lasted for only 6 hours. The fourth mesoscale low formed in conjunction with an abrupt and intense surge of katabatic air from Terra Nova Bay which resharpened the coastal baroclinic zone. At the same time a transiting midtropospheric trough probably associated with lower tropospheric upward vertical motion apparently accelerated the katabatic winds and triggered the vortex formation. A similar katabatic wind-forced mesocyclone formed near Byrd Glacier. The two vortices moved to the east-southeast and northeast, respectively, apparently being steered by the generating katabatic airstreams, and merged just to the north of the Ross Ice Shelf. The combined vortex reintensified as another trough passed overhead and moved eastward to West Antarctica where it dissipated two days later.

1. INTRODUCTION

The topography of the Antarctic continent defines several confluence zones where the surface winds from the interior of

the continent converge [Parish and Brorawich, 1987]. Such features provide an enhanced supply of radiatively cooled near-surface air to the coastal slopes and allow the resulting katabatic winds to be stronger and more persistent. One of these zones is located inland of Terra Nova Bay (Figure 1), where the intense katabatic winds that descend through Reeves and David glaciers blow out onto the southwestern Ross Sea, sometimes reaching the area around Franklin Island [Brom- v•ch, 1986, 1989a; Parish and Bromv•ch, 1989]. Bromv•ch [1991] used 2 years of automatic weather station (AWS) observations and satellite images to study mesoscale cyclogenesis along the Transantarctic Mountains and found that in the region adjacent to Terra Nova Bay, mesoscale cyclones form with great frequency. One or two mesoscale vortices formed each week during 1984 and 1985. Also, individual case studies[Bromvdch, 1987, 1989b, 1991] revealed that katabatic outflows play an important role in the formation of these mesoscale cyclones.

A boundary layer baroclinic zone can form when a cold jet of continental air invades the warmer maritime air located over

the southwestern comer of the Ross Sea. This feature, in

--•Permanently at Dirreccion Meteorologica de Chile, Santiago.

Copyright 1993 by the American Geophysical Union.

Paper number 92JD02821. 0148-0227/93/92JD-02821 $05.00

conjunction with a weak surface trough, constitutes sufficient conditions for mesoscale cyclogenesis [Broraadch, 1989 b]. The same conditions occur with lower frequency over the northwestern side of the Ross Ice Shelf. In this case, the formation

of a boundary layer baroclinic zone is linked to the intensified transport of cold katabatic air coming from East Antarctica through Byrd Glacier [Parish and Bromwich, 1987; Bromaq'ch, 1989c].

Studies have shown that mesoscale cyclones can develop in other coastal areas of the Antarctic continent. Turner and

Rove[1989] and Hcincmann [1990] demonstrated that mesoscale cyclones form over the Weddell Sea, particularly near Halley Station. Turner and Rove [1989] suggested that the most likely cause for most of these cyclones is the effect of the high topography of the Antarctic plateau inland of Halley. No dear role of the katabatic winds was observed. However, the easterly winds associated with most of the mesoscale cyclones indicate a cold air mass descending from the high plateau into the warmer maritime air located over the Weddell Sea [ Turner and Row, 1989]. The formations also took place adjacent to synoptic scale troughs, implying similar genesis conditions to those in the Ross Sea.

Carleton and Carpenter [1989, 1990] examined seven winters of Defense Meteorological Satellite Program (DMSP) images (5-kin resolution) to identify polar lows in the southern hemisphere. They found that most of the mesoscale cyclones were of comma cloud configuration and that they can be observed on satellite imagery at any longitude around the Antarctic continent, mainly over the open ocean. Because a distinctive cloud signature is required and the utilized satellite imagery was of relatively coarse spatial resolution, the satellite-observed formation location may not coincide with the

12,973

12,974 CARRASCO AND BROMW[CH: MESOSCALE CYCLOGENESIS

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actual genesis region. Parish [1992] used a numerical model (100-km spatial resolution) to study the interaction between Antarctic katabatic winds and tropospheric motions in high southern latitudes. His three-dimensional simulation of the

streamline pattern at about 100 m above the ice surface (second sigma level) suggests cyclonic circulations over the southern Ross and Weddell seas for the case of the continent surrounded

by sea ice. Besides these two regions the model simulation suggests, for the case of the continent surrounded by open ocean, other areas located over the Bellingshausen Sea, offshore from the Adelie Coast and others located in the

coastal areas facing the Indian Ocean. These locations are in agreement with the observations [Bromndch, 1991; Turner and Row, 1989; Heinemann 1990; Lyons, 1983; Carleton, 1992]

CARRASCO AND BROMWICH: MESOSCALE CYCLOGENESIS 12,975

suggesting that these places may be the actual source of the majority of the mesoscale cyclones noted by Carlaton and Carpenter[1989, 1990].

One or more mechanisms can influence the formation and

subsequent development of mesoscale cyclones, with the dominant mechanism varying during a cyclone's life cycle. Theoretical studies [ManstY½ld, 1974; Oldand, 1987; Moore and Peltier, 1989; Wiin-Niclsen, 1989; Emanuel and R otunno, 1989; Craig and Cho, 1989; Fantini and Buzz• 1992] have shown the importance of baroclinic instability as a main mechanism for formation of mesoscale cyclones, while other mechanisms, like CISK [Craig and Cho, 1989] or air-sea interaction [Emanuel and Rotunno, 1989], may be more important for their subsequent development. IVi/n-Niclscn [1989], using a three-level baroclinic model, found that baroclinic waves with a length scale of a few hundred kilometers can develop within the lower layers of the atmosphere due to baroclinic instability. This mechanism acts in the similar manner as for synoptic scale cyclogenesis [Mooca and P½]t•bc, 1989]. œmanucI and Rotunno [1989] concluded that during formation and/or development, polar low disturbances presumably require a nonaxisymmetric dynamic mechanism (like baroclinic instability) for the initial formation stage, while

for subsequent development, air-sea interaction mechanisms could be more important. When polar air masses move over relatively warm water, the fluxes of sensible and latent heat from the ocean surface can warm a deep layer of the atmosphere [EmanuclandRotunno, 1989]. If the heating rate is not too great, baroclinic instability is the primary mechanism for formation and development of mesoscale cyclones. On the other hand, diabatic heating helps to reduce the static stability of the near-surface atmosphere, and mesoscale cyclones can grow faster and with a shorter wavelength [Craig and Cho, 1989]. During the open water season, cold katabatic air blows over the relatively warm southwestern Ross Sea, and both baroclinic instability and diabatic heating can be present. Cyclogenesis studies [Brom•4ch, 1989b, 1991] during the winter season, when the southern Ross Sea is covered by sea ice, suggest that the primary mechanism responsible for mesoscale cyclogenesis near Terra Nova Bay is linked to baroclinic instability. This paper addresses the dynamic aspects related with mesoscale cyclogenesis during the open water season. Based upon the above literature review, the diabatic effects during the early part of the vortices' life cycle are presumed to be of secondary importance in comparison to baroclinic instability.

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Fig. 2. Partial reproduction of Australian surface synoptic analyses at 1200 UTC, (a) February 17, (b) February 18, (c) February 19, and (at) February 20, 1988. They were slightly modified to accord with AWS data and satellite images (90 = 990 hPa).

12,976 CAgRASCO AND BROMWICH: MESOSCALE CYCLOGENESIS

The formation of four mesoscale cyclones near Terra Nova Bay in February 1988 is described here. The original work carried out by Bromvffch and Parish [1988] is extended by incorporation of satellite data recorded at McMurdo Station [ Van Wocrt et al., 1992] and Australian southern hemisphere synoptic charts. The Australian 500-hPa analyses were adjusted by incorporating 500-hPa geopotential heights estimated from AWS observations over the polar plateau using the method developed by Phillpot [1991]. The mesoscale cyelogeneses and therefore the role of the katabatie airflow in the formation of these cyclones, are studied by using AWS observations from the Victoria Land and Ross Ice Shelf areas

along with high-resolution satellite images. The broadscale environment is analyzed using the synoptic charts produced by the Australian Bureau of Meteorology in conjunction with the satellite images and the TIROS operational vertical sounder (TOVS) [Sraith et al., 1979] data. The synoptic scale vertical motion near Franklin Island is diagnosed from the quasi- geostrophic omega equation.

2. SYNOPTIC SCALE ENVIRONMENT

To evaluate the broadscale environment over the South

Pacific Ocean, the Australian southern hemisphere sea level

pressure and 500-hPa charts were examined between February 14 and 22, 1988. Minor modifications were made to the surface analyses (Figures 2•-2d) to accord with the AWS data and with the interpretation of available satellite images. The 500-hPa analyses (Figures 3 •-3 d) were adjusted by incorporating 500-hPa heights estimated from the AWS observations at Dome C, D-80, and D-57 [Keller et aZ, 1989] using the tech- nique described by Pbillpot [1991]. The regression relation- ships needed to derive the 500-hPa heights for these AWS sites were taken from his work. Although some error is associated with this derivation, the reliability of the 500-hPa heights obtained is verified by the general agreement with the Austra- lian analyses and by comparison between the values calculated at South Pole Station and the 500-hPa heights given by the radioson& observations at the same site (both values are plotted in Figures 3•-3d).

During this period the surface synoptic analyses showed that the southwestern Ross Sea was affected by cyclonic flow associated with the passage of dissipating frontal systems. According to the Australian analyses a cold front (not shown)

passed over the northern Ross Sea between 0000 UT•C, February 14, and 0000 UTC, February 16, 1988. Figures 2•-2d are the Australian synoptic analyses at 1200 UTC, centered over the Ross Sea/Ross Ice Shelf, between February 17 and 20,

• 4.

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(c) (d)

Fig. 3. Same as Figure 2 but for 500 hPa. They were modified by incorporating the 500-hPa geopotential heights derived from AWS over the polar plateau. At South Pole Station the estimated 500-hPa geopotential height from AWS is plotted above the radiosonde-derived value. The arrows denote the observed 500-hPa wind directions at McMurdo and South Pole

(510 t 5100 gpm).


1988. The sequence reveals a low-pressure area over the southern Ross Sea. This circulation was associated with

another dissipating cold front (A in Figures 2a and 2b) which moved into the southern Ross Sea early on February 14 and remained quasi-stationary over this area until 1200 UTC, February 17, 1988. Analyses at 1200 UTC, February 18, and 1200 UTC, February 19, 1988 (Figures 2b and 2c) show that the synoptic eyelone subsequently moved slowly toward the east. Moreover, the NOAA 9 satellite image at 1250 UTC, February 19, revealed that a third dissipating cold front (B in Figures 2b and 2c) moved into the southern Ross Sea around 0000 UTC, February 19, maintaining the cyclonic circulation until February 21, 1988.

During the period under consideration the 500-hPa geopotential height analyses revealed the passage of four midtropospheric troughs over the Ross Sea sector. Figures 3a-3d summarize the midtropospheric environment every 24 hours between 1200 UTC, February 17, and 1200 UTC, February 20, 1988. They show the last three troughs that affected the Ross Sea area. According to the adjusted 500-hPa analyses the first trough crossed the Ross Sea between 1200 UTC, February 15, and 0000 UTC, February 16. The second passed over the northern Ross Sea at 1200 UTC, February 17 (II in Figure 3 a), and was followed by a shortwave trough, which crossed the

Terra Nova Bay around 0000 UTC, February 18 (III in Figures 3a and 3 b). The fourth trough passed over southwestern Ross Sea around 1200 UTC, February 20 (IV in Figures 3 b-3d). During the period under consideration the TOVS data (see section 3) placed a low thickness center over the Ross Ice Shelf that almost coincided with the position of the 500-hPa low center given by the Australian southern hemispheric charts (Figure 3a-3d ). This suggests that these midtropospheric troughs were of the cold core type. The role of these troughs is described later.

3. INCORPORATION OF TOVS DATA

3.1. Dcsc•ption

The international TOVS processing package (ITPP 3), developed by the University of Wisconsin, was used to convert raw TOVS data into temperature profries, which allow us to obtain the 1000- to 500-hPa thickness, and to derive the

500-hPa geopotential height field. The TOVS epplication package (TAP), incorporated in our analysis software (Tera- Scan), uses the "simultaneous physical" retrieval algorithm, which produces temperature and moisture profries from a 3x3 array of high-resolution infrared sounder (HIRS) and micro- wave sounding unit (MSU) radiance observations. This gives

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12,978 CAm•SCO AND BROMWICH: MESOSCALE CYCLOGENESIS

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a horizontal resolution of about 75 km [$m/tfi ½t M., 1983]. However, the distribution of the soundings is not uniform due to cloud that clusters the data over cloud-free areas, affecting the nominal resolution. Unfortunately, the MSU on NOAA 9 failed during 1987, so only TOVS data from the NOAA 10 satellite were available. Three passes of NOAA 10 were recorded at McMurdo Station [ Va• Wocrt ½t aI., 1992] during the period under investigation: one at 1723 UTC, February 17, another at 1703 UTC, February 18, and the third at 1438 UTC, February 20. Figures 4a and 4b, 5aand 5b, and 6aand 6bare the TOVS 1000- to 500-hPa thickness and the derived 500-hPa

geopotential height for these dates and times. The geopotential thickness analyses were obtained directly from the physical retrieval algorithm [Smith et M., 1985], using a regression first guess, incorporated in the TeraScan software package. No tuning for the polar environment was attempted. It was hoped that the simultaneous physical retrieval approach would provide better results over the high polar plateau and over snow-covered surfaces than the statistically based method [Lachlan-Cot•, 1992]. The 500-hPa fields were derived by adding the thickness fields to the 1000-hPa fields obtained from the concurrent AWS sea level pressure analyses (see Figures 9, 16, and 18, which are introduced later).

Due to the effect of the high topography and the fact that the distribution of the TOVS retrievals is not homogeneous,

the data must be used cautiously (a validation of the TOVS data is presented in section 3.2). According to the TOVS 1000- to 500-hPa analyses on February 17, 18, and 20 (Figures 4a, 5a and 6a), relatively low geopotenfial thickness (cold air) affected the area of Ross Ice Shelf and offshore from the Victoria Land

coast. On the other hand, relatively high thickness (warm air) was present over Victoria Land and the Transantarctic Moun- tains on February 17 and 18 (Figures 4a and 5a). At 1438 UTC, February 20 (Figure 6a), the TOVS satellite data reveal that the warm ridge was still present over Victoria Land. The 500-hPa analyses show a low center approximately over the northwestern quadrant of the Ross Ice Shelf for the three days (Figures 4 b, 5 b, and 6 b). On February 17 (Figure 4 b) a smaller center oflower geopotenfial height was located to the north of Franklin Island. Subsequent analyses, on February 18 and 20 (Figures 5 band 6 b), reveal only a trough over the northwestern comer of the Ross Sea and another located to the north of

AWS 00 over the Ross Sea. The TOVS 500-hPa analyses indicate a decrease of the geopotential height from February 17 to 20, over the northwestern side of the Ross Ice Shelf and an increase over the area to the north of Franklin Island. In

particular, a strengthening of the low over the Ross Ice Shelf between February 17 and 18, is clearly revealed by the TOVS 500-hPa analyses. Implications of these analyses for the development of the mesoscale cyclones are addressed later.

CARRASCO AND BROMWICH: MESOSCALE CYCLOGENESI$ 12,979

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3.2. Validation oœ TO VS Data

The TOVS data can be validated by comparing the TOVS 500-hPa analyses (Figures 4b, 5 b, and 6 b) with the adjusted 500-hPa charts constructed by the Australian Bureau of Meteorology (Figures 3a-3d). Although the Australian and TOVS analyses are not concurrent, a general comparison is possible. During the period 1200 UTC, February 17, to 1200 UTC, February 20, the southern hemisphere charts revealed a low center located over the Transantarctic Mountains, slightly to the west of the Ross Ice Shelf. It was possibly located over the ice shelf on February 18 and 20 (Figures 3band 3d), the position shown by the TOVS 500-hPa analyses. The Austra- lian analyses indicated that a shortwave trough (III in Figure 3) crossed the southwestern comer of the Ross Sea around 0000

UTC, February 18, and joined with the prior trough (II). Figure 3 b located this trough to the north of the Ross Ice Shelf. Although not in the same position, this trough is also suggested by the TOVS 500-h_Pa analyses, which at 1703 UTC, February 18, reveal a trough over the southern Ross Sea to the north of AWS 00 (Figure 5b: compare with Figure 3 b). The fourth trough shown by the Australian charts (Figures 3b-3d ) is suggested by the TOVS 500-h_Pa analysis at 1438 UTC, February 20, over the southern Ross Sea along longitude 175'W. The variation of the geopotential height revealed by

the TOVS 500-hPa data is also observed on the Australian 500-hPa charts. In both sets of analyses a slight decrease took place over tl,•.e ice shelf, while an increase occurred to the north of Franklin Island during the period February 17-20 (in Figures 3a-3d; see contour 510 dam).

A permanent subsynoptic scale upper level trough just offshore from the Victoria Land coast is suggested by the TOVS 500-hPa analyses. This is not confirmed by the Austra- lian charts but may be due to the lower horizontal resolution (500 kin) characterizing this type of analysis [Guyvnet, 1986]. The higher-resolution TOVS data (75 kin) may be capable of resolving this smaller-scale feature. Parish and Bromn4ch [1991] used a numerical model to simulate the 500-hPa and 250-hPa fields caused by the Antarctic katabatic wind regime. Their 1- and 2-day integration results (see their Figure 7) suggest a trough extending over Victoria Land, which may correspond to the subsynoptic midtropospheric trough. resolved by the TOVS analyses. The East Antarctic plateau is located to the west of the Ross Sea, and thus the prevailing westerly upper level flow [Taljaard et aZ, 1969] descends about 2000 m from the high plateau toward the ocean. In a homogeneous incompressible fluid with constant density the potential vorticity is conserved. Applying this concept to the Victoria Land/Ross Sea area, a column of air over the plateau expands vertically when it goes down onto the Ross Sea. This increase

12,980 CAm•sco AND BROMWICH: MESOSCALE CYCLOGENESIS

TABLE 1. Lower Tropospheric Vertical Motion Calculations Near Franklin Island Between February 16 and 20, 1988

Probable Differential Thickness Direction

February UTC Vorticity Advection Advection of Vertical Day Hour Term B Term C Motion

16 0000 AN CO down

16 1200 zero WA up 17 0000 CY WA up 17 1200 CY CO ?

18 0000 CY WA up 18 1200 CY CO L down 19 0000 AN CO down

19 1200 CY WA up 20 0000 CY* WA* up 20 1200 CY* WA* up

CY, cyclonic; WA, warm air advection; AN, anticyclonic; CO, cold air advection. Bemuse of the uncertainty in the magnitude of the terms, direction of the vertical motion is quoted only when the two terms act in the same manner. The letter L preceding the direction means "likely to be" and is quoted only when one of the terms is 1 order of magnitude larger than the other.

*Calculated for vicinity of automatic weather station (AWS) 00.

in depth implies a creation of negative (cyclonic) relative vorficity. Therefore the air column acquires cyclonic circulation over western Ross Sea, offshore from Victoria Land. In summary, simulation and theory could support the presence of a mesoscale midtropospheric trough just offshore from Victoria Land as resolved by the TOVS data. However, it should be noted that the difficulties associated with TOVS

retrievals over the Antarctic continent may lead to erroneous results.

In general, good qualitative agreement can be observed between both analyses over the Ross Sea and Ross Ice Shelf. However, a positive bias of around 200 gpm (geepotential meters) (a layer-average temperature bias of about + 5 ø C) was found by comparison between the TOVS-derived 500-hPa data over McMurdo Station and the geepotential heights obtained by the radiosonde at this location. This discrepancy agrees with the results obtained by Lutzct M. [1990]. They compared satellite-derived and radiosonde-measured 500-hPa heights for four stations located on the Antarctic continent. They found biases in ITPP 3 retrievals which ranged from 115 to 284 gpm with larger values over the plateau. Despite this an improvement in spatial resolution over conventional synoptic scale analysis and the general reliability of the spatial variations provided by TOVS analyses over high southern latitudes have been revealed by studies conducted by Warren and Turner [ 1989] and Heinemann [1990]. By studying mesoscale cyclones, they found that the TOVS analyses are able to resolve smaller- scale features that are missing from conventional synoptic analyses.

4. AWS OBSERVATIONS AND MESOSCALE ANALYSIS

Automatic weather stations started to be deployed in Antarctica during 1980 by the U.S. Antarctic Program [Stearns and Wendlet, 1988]. The AWS unit measures air temperature, wind speed, and wind direction at a nominal height of 3 m above the surface. Air pressure is measured at 1.5 m. The data

are broadcast by the stations and are collected by the ARGOS data collection system (DCS) on the NOAA series of polar orbiting meteorological satellites, eventually reaching the Department of Meteorology at the University of Wisconsin- Madison for processing and distribution [Keller ½t M., 1989]. Moreover, real-time collection at McMurdo Station of satellite data broadcast by the high-resolution picture transmission

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CARRASCO AND BROMWICH: MESOSCALE CYCLOGE:NESI$ 12,981

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(HRPT) system can provide concurrent AWS observations and advanced very high resolution radiometer (AVHRR) images, which can be processed and displayed for integrated analysis. Regional sea level pressure analyses were constructed every 3 hours and at image times by using the AWS observations [Keller at a/., 1989] collected by the platforms deployed in the Victoria Land and Ross Ice Shelf areas. Also, in order to

avoid the problem of reducing station pressures measured on the polar plateau to sea level, local analyses near Terra Nova Bay were constructed by using pressure anomalies, def'med as the difference between the AWS station pressure at the chart time and the average for February 1988.

5. VERTICAL MOTION CALCULATIONS

The vertical winds at 850 hPa in the European Centre for Medium Range Weather Forecast (ECMWF) numerical analyses were examined for the period under consideration. Unfortunately, over the southwestern Ross Sea the analyses became too noisy to permit precise interpretation of the vertical motion field. On the other hand, the Q-vector analysis [Hosla;ns et al., 1978] used in different studies in the northern hemisphere [Ledfew, 1988; Pedersen, 1989; Businger and Bailc, 1991] was not applied in this study because the coarse spatial resolution of the Australian numerical analyses [ Guymet, 1986]

12,982 CARPASCO AND BROMWICH: •'IESOSCALE CYCLOGENESIS

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Fig. 9. Mesoscale regional analysis at 1723 UTC, February 17, 1988. Sea level isobars are solid (90 = 990 hPa) and surface isotherms in degrees Celsius are dashed. Asterisk indicates position ofmesocyclone L1 at 1313 UTC, February 17, 1988.

does not permit accurate calculation of the Q-vector field. Furthermore, because the area under consideration is located immediately to the east of the high plateau of East Antarctica, required thickness values from that area cannot be estimated with any confidence. Therefore to evaluate the synoptic support for mesoscale cyclogenesis over the southwestern comer of the Ross Sea, the diagnostic omega equation was used to estimate the vertical motion in the lower troposphere (•700 hPa) over Franklin Island. The omega equation [Holton, 1992, pp. 166-169] can be written as

A B C

•2+•02]•a fø •[ V•'n•l •] •• •[-•p]] where term A is proportional to - to (vertical motion in pressure coordinates), term B is the differential vorticity advection, and term C is the thickness advection; o is the static

stability parameter, • the geopotential, V s the geostrophic wind, and fo a reference value of the Coriolis parameter. To estimate the vertical motion, term B was evaluated and expressed as

o •p[- <0 anticyclonic. (2)

Here, •s is the geostrophic relative vorticity. Term C was approximated by

with positive values corresponding to warm air advection, and negative values to cold air advection.

The relative vorticity was calculated from the Australian analyses for a grid of points at intervals of 2.5* of latitude and 10' of longitude centered over the Ross Sea; no calculations were made to the south of 77'S due to the uncertainty of the data. Because of the coarse spatial resolution the vertical motion was estimated for only one point located around Franklin Island. The observed wind at 500 hPa recorded by the McMurdo radiosonde was assumed to be geotrosphic and used, in term B, to evaluate the vorticity advection at 500 hPa. Scale analysis indicated that the vorticity advection at 500 hPa is 2 orders of magnitude larger than the vorticity advection at 1000 hPa; therefore the latter can be neglected and the differential vorticity advection determined from the 500-hPa height field. To approximate the geostrophic wind in the middle of the 1000- to 500-hPa layer for use in equation (3), the observed surface wind at Franklin Island and the 500-hPa radiosonde

wind recorded at McMurdo Station were averaged. The 1000- to 500-hPa thickness was obtained by subtracting the 1000-hPa heights obtained from the regional AWS analyses from the modified Australian 500-hPa heights. The thickness gradient was determined over the same grid as the relative vorticity. To estimate the vertical motion both terms were summed. The

results of the evaluation of terms B (equation (2)) and C (equation (3)), expressed in terms of their contribution to vertical motion, are given in Table 1.

6. CYCLOGENESIS EVENTS

6.1. First œ ven t

The regional mesoscale analyses revealed the formation of four mesoscale cyclones between February 16 and 20, 1988. Early on February 16 the first mesoscale low (L 1)could have formed to the north of Franklin Island, within the semipermanent surface trough located over the southwestern Ross Sea [Brom•4ch, 1991; Brom•4ch ½t M., 1993]. As revealed by the Australian analyses, a synoptic cyclone associated with a dissipating cold front remained almost stationary over the southern Ross Sea between February 14 and 17. This suggests warm air advection over Franklin Island by 1200 UTC, February 16. In addition, the 500-hPa charts revealed the approach of a midtropospheric trough (II) toward the northern Ross Sea. At 1200 UTC, February 16, this trough was located along longitude 150øE over the ocean and Wilkes Land and may have started to advect cyclonic vorticity over the area. Calculation of the omega equation (Table 1) indicates that upward synoptic scale vertical motion over the southwestern


I

160øE I

164øE 168øE

Priestley Neve

-4 -6

Tinker GI. •

-8 51

-8

Reeve8 Neve

G/.

L2 -lO

Terra Nova

Bay

6

ROSS SEA

-4 76's

Franklin le.

164OE 168øE

Fig. 10. Local analysis for the Terra Nova Bay area at 0600 UTC, February 17, 1988. Solid curves are contours of pressure anomaly in hectopascals. Thick dashed curve on the plateau represents the-5-hPa pressure anomaly isopleth. Dashed-dotted curves represent isopleths of potential temperature in degrees Celsius.

Ross Sea was present to support mesoscale cyclone formation within the semipermanent surface and midtropospheric troughs near Franklin Island. At 1200 UTC, February 16, the mesoscale vortex was already located next to Inexpressible Island, as shown by the sea level pressure at this site being lower than at Franklin Island (Figure 7a). Unfortunately, the lack of satellite images on this day impedes further study of the formation of this cyclone.

However, a NOAA 9 satelliteimage at 1313 UTC, February 17, showed a distinctive low-cloud spiral (L1) slightly to the northeast of the Borchgrevink Coast (marked by the asterisk in Figure 9). The next satellite image at 1723 UTC, February 17, showed that the vortex had moved to the northeast of its

earlier position. Figures 8 and 9 are the infrared satellite image and the concurrent mesoscale analysis at this time showing the subsequent position of the mesolow (L1) and revealing its

northeastward displacement. The cloud signature associated with the vortex revealed that it had also developed markedly over the 4-hour period, suggesting maximum strength about this time. The development of the vortex is associated with the passage of the second midtropospheric trough (II) which at 1200 UTC, February 17, was located just to the northeast of Victoria Land (Figure 3 a) according to the Australian 500-hPa numerical analyses. The "movie loop" of the satellite images at 1313 and 1723 UTC confumed the northward movement of the

mesoscale vortex and suggested that it originated in the southwestern Ross Sea. No dear evidence of the northeast-

ward displacement of the vortex is revealed by the AWS observations from Victoria Land. Weak southwesterly winds recorded at AWS 51 between 1500 and 2100 UTC, February 16, may signal the passage of the vortex along the Victoria Land coast. The TOVS 1000- to 500-hPa thickness at 1723

12,984 CARRASCO A•D BROMW•C•: MESOSCALE CYCLOGEI•IF_.SI8

160 ø E 180 ø

\

140 ø E + ./..

?

.

•oo (a)

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.. •-//'-)//,,,, ,,, .:?. tff,.; • I I , \\ ,,•' • ii1":",' '.'.:!:I,L,Y I t '-..•

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J:...' oL9.);.:'...•j: • '--- .... •--,,,,•

:'..,._ ' :::.::?':.-

ß ..

17/12 (b)

I / 16o ø E 180 ø

ß .

18/00 (c)

I / 160 ø E 180 ø

140 ø E -4- -/..

'.:::: ! ::'!.:: ::. , .• ..... ß .•:..•...-.•:. -/ --....,..,-•...:•./,,,,-,.._..•_._

...'(.: .....:.... ... :.'.'.-...

18/12 (d)

Fig. 11. Relative vorticity field at 500 hPa at 12-hr intervals from 0000 UTC, February 17, to 1200 UTC, February 20, 1988. Cont our interval is 3 x 10 '6 s 4, with negative values dashed. Observed 500-hPa winds at MeMurdo are plotted.

UTC, February 17 (Figure 4a), resolves a subsynoptic cold core approximately over the area where the northern low-cloud spiral (L1) is observed. No evidence of the vortex was observed on the following satellite image at 1302 UTC, February 18. To summarize, intensification of the semipermanent

surface and midtropospheric troughs and subsequent formation of the furst mesoscale eyelone could have been supported by the synoptic scale upward motion probably due mainly to synoptic scale warm air advection and seeondarily to advection of cyclonic vortieity at 500 hPa.

CAg_P. ASCO AND BROMWlCH: MESOSCALE CYCLOGENESIS 12,985

6.2. Second and Third Mesoscale Events

On February 17 the broadscale cyclonic regime continued affecting the southwestern Ross Sea. At 0300 UTC a second mesoscale cyclone (L2) developed over Drygalski Ice Tongue. Figure 10 [from Bromwich and Parish, 1988] is the pressure

anomaly analysis at 0600 UTC, February 17 (three hours later), showing the mesoscale vortex over Terra Nova Bay. The formation of this cyclone occurred under the presence of the strong and persistent easterly and southeasterly winds as recorded at Franklin Island between 0000 and 1800 UTC, February 17 (Figure 7a); these were associated with the

I / 160 ø E 180 ø

40 ø E 4-(.• (• ./.

ß ,)C t tll•/

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l I / 160 ø E 1800

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ß

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ß

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.:

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i / 160 ø E 180 ø

Fig. 11. (continued)

12,986 CARRASCO AND BROMWICH: MESOSCALE CYCLOGENESIS

160OE _

164øE 168øE

74øS

Reeves Neve

Priestley Neve

+4

COLD

Tinker GI.

74øS

\ .oss

x.•. SEA

./

ß

/

76øS

164øE

Fig. 12. Same as Figure 10 butat 0000 UTC, February 18, 1988.

Franklin IS.

1 168OE

synoptic cyclone tha' affected the southern Ross Sea (Figure 2a). This cyclonic circulation moved warmer maritime air toward Terra Nova Bay as revealed by the 5øC increase in air temperature at Inexpressible Island between 0000 and 0600 UTC, February 17. This warm air contrasted with the cold katabatic air coming down Reeves Glacier from the interior of the continent. This created a near-surface hor•ontal temperature gradient (boundary layer baroclinic zone), which in conjunction with the semipermanent surface trough over the southwestern Ross Sea established the necessary conditions for cyclogenesis; however, the primary mechanism appears to be baroclinic instability. Upper level support was provided by the approach of the second and third midtropospheric troughs. Figure 11 is the relative vorticity field from 0000 UTC, Febru- ary 17, to 1200 UTC, February 20. It was calculated every 12 hours from the 500-hPa geopotential height analyses (compare Figure 3), as described above. The increase of the cyclonic vorticity over the southwestern Ross Sea is revealed from the variation of the vorticity field from 0000 to 1200 UTC, Febru-

ary 17. This indicates cyclonic vorticity advection at 500 hPa and suggests upward synoptic scale vertical motion in the lower troposphere. This was confn'med by the evaluation of the differential vorticity advection term (B, equation (2)) which indicated cyclonic vorticity advection (Table 1) around Franklin Island. In addition, evaluation of term C (equation (3)) gave warm air advection over the area (Table 1). The summation of both terms suggests upward motion at around 0000 UTC, February 17, but at 1200 UTC, February 17, cold air advection toward Franklin Island could have decreased the

upward motion. The counterclockwise turning of the wind at Inexpressible

Island (AWS 05, Figures 7a and 7 b) between 0300 and 0600 UTC and the strong southwesterly wind (27 ms'l), recorded by AWS 51 at 1200 and 1500 UTC, indicate the northward displacement of vortex L2 along the Victoria Land coast. No evidence of this vortex was observed in the satellite image at 1313 UTC, February 17, due to the presence of high cloud around AWS 51. The subsequent satellite image at 1723 UTC,

CARPASCO AND BP, OMWtCH: MESOSCALE CYCLOGENESIS 12,987

Terra Nova

:Ross Is. '5:::

.........

.., :.:.

.,.

......

ß "'"'" "":-'"::?"::':'::'" ' '..::;" 5!i':: ::.,. :.:::..::::: .:::::::.:::.:::.

.:

:.:.:.:

.................

.................

Fig. 13. Thermal infrared satellite image at 1302 UTC, February 18, 1988.

February 17 (Figure 8), showed a weak low-cloud spiral (L2) to the east of AWS 51. The concurrent AWS analysis (Figure 9) places the vortex in a trough extending to the southwestern comer of the Ross Sea. The TOYS 1000- to 500-hPa thickness

analysis at this time (Figure 4a) shows a cold trough offshore from the Victoria Land coast, and the TOYS 500-hPa analysis (Figure 4b) resolves a mesoscale center of lower geopotential height to the north of Franklin Island, suggesting the presence of a deep mesoscale cyclonic circulation over this area. This can be associated with the surface mesolow (L2) that developed at 0300 UTC, February 17.

The AWS observations at Reeves Neve revealed that

another much smaller mesoscale cyclone (L3) formed over the plateau around 0600 UTC, February 17 (Figure 10). The typical wind pattern [see Bromndch andParish, 1988, Figure 1]

at AWS sites 23, 27, and 09 is completely modified at this time and reveals a cyclonic circulation over the plateau. The development of the third mesoscale cyclone could have been caused by the southeasterly wind that affected Inexpressible Island (AWS 05) from 0600 to 0900 UTC (Figure 7a and 7 b) Once again, a mesoscale baroclinic zone was formed, this time to the west of Terra Nova Bay, due to the maritime warm air being forced over the coastal slopes by the coastal vortex (L2). This interior baroclinic zone is shown by the 4øC potential temperature difference between AWS 23 and 21 (Figure 10). The short lifetime of this vortex is associated with the reestab-

lishment of the northwesterly katabatic wind at Inexpressib•!e Island. This was confn'rned by analysis at 1200 UTC, February 17 (not shown), which revealed that the vortex had already disappeared. During the time of cyclogenesis at Terra Nova

12,988 CARRASCO AND BROMWICH: MESOSCALE CYCLOGENESIS

180 ø

170øE '-F- 170øW 70ø8

98

g•2 75øS

L

Fig. 14. Same as Figure 9 but at 1302 UTC, February 18, 1988. Arrows indicate the trajectories of the kata. batic wind-forced mesoscale vortices that formed near Term Nova Bay and Byrd Glacier. A: 1250 UTC, February 19; B: 1438 UTC, February 20; C: 1057 UTC, February 21; D: 1732 UTC, February 21; E: 0532 UTC, February 22 (actual location at this time is just to the east of the figure edge).

Bay, another mesoscale cyclone developed over the west side of the Ross Ice Shelf sometime between 0300 and 1200 UTC,

February 17. The cyclonic vorticity and warm air advection observed over the southwestern Ross Sea at 0000 UTC,

February 17 (Table 1), could have been present over the northwestern side of the Ross Ice Shelf to give the synoptic support for mesoscale cyclogenesis adjacent to Byrd Glacier, as shown by Figure 9 (for further discussion of this event, see

CARRASCO AND BROMW[CH: MESOSCALE CYCLOGENESIS 12,989

............... .-

- .!:-:.:.:::::::::::::::::::::::::::::::: . ::: ..................

.

.

._ ............................

ß

..................................

..

......

...............

..

.

..

-..-. }::.. :.:..-::.......:.- ...... ..:.:..... :.. :.: :::::-::: ......

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................

:::::::::::::::::::::: ....' ...

ß ' . ....... ;::ii•!:::i•::•?ii::?::::!iii•ii!!ii:_'.'.' .............. !;i .. :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: .................. ..-..

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:::::::::::::::::::::::::::::::::::::::::: ========================= :.. :.

__ :•i•::ii::::i•::iii!!!! .............. :!::!;.' '. ......... i•::.?::.;:::iii::: .:i::½i:i::ii;?;:;iid: '" :• !' ..... ! .... :::i.' ' ' ' ::::i::!i!i:::.. '•;;:.':i:.½ii::::::::::!i! :•:: ' '

ß '.i:::!:... :!:i:!:i :i,.•.::.F' •?:,-'.:. ............ "::::i'• .... ::::i•.i::iZ!."•.•::

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' .'•: ';":ii:."::::.!!!:;':;:;.':;.:.. ::•i•!:•:;...:.:;;::•i:? •: ::.•i .!:'.::%:::::

!'":::".:Y :i:::::;.-:::"

Ross Is.

......

:.:.. ß ..:.::

.i: .;

Fig. 15. Thermal infrared satdlite image at 1703 UTC, February 18, 1988.

Catcasco and Bromwicb [1991]). Quasi-simultaneous mesoscale formation at these two locations was frequently noted by Bromrdch [ 1991] during his 2-year study period.

6.3. Fo uctb Even t

From 1500 UTC, February 17, the wind speed at Inexpress- S -1 ible Island started to increase reaching 30 m at 0000 UTC,

S -1 February 18 (an increase of about 28 m in 9 hours). This acceleration along with the clockwise turning of the wind at Franklin Island toward the northwest (Figure 7a) indicates the start of a katabatic surge across the southwestern Ross Sea. The 500-hPa analysis revealed that a shortwave trough (III) crossed this area around 0000 UTC, February 18 (Figures 3a and 3 b). The approach of this trough could have continued advecting negative relative vorficity over the southwestern

Ross Sea after 1200 UTC, February 17. The omega equation evaluation indicates upward motion over the southwestern Ross Sea at 0000 UTC, February 18. Figures 11 b and 1 lc suggest that the relative vorticity advection at 500 hPa was weak, with the thickness advection being the dominant synoptic mechanism determining the vertical motion. This synoptic environment could have modified the surface pressure gradient over the southwestern Ross Sea. Figure 7a reveals that the sea level pressure difference between Inexpressible Island and Franklin Island became negative, indicating lower pressure at Terra Nova Bay. Figure 7b shows the pressure anomaly difference between Shristi Station (AWS 09) and Inexpressible Island (AWS 05). It indicates that the difference significantly increased from 1800 UTC, February 17 to 0300 UTC, February 18, implying an increase in the pressure gradient along and beyond Reeves Glacier. As the pressure

12,990 CARR•SCO AND BROMAVICH: MESOSCALE CYCLOGENESIS

170øE 180 ø

170øW

70øS

98

w

{P13

COOL

! 0 w A

ß ..i

75øS

-/.

-./.

Fig. 16. Same as Figure 9 but for 1703 UTC, Februa• 18, 1988.

, gradient increase coincided with the abrupt acceleration of the katabatic airflow at Inexpressible Island, this may be an indication of the effect of the midtropospheric trough (III) on intensification of the katabatic airflow. The TOVS 500-hPa

•analyses at 1723 UTC, February 17, and 1703 UTC, February 18 (Figures 4b and 5 b) revealed a strengthening of the midtropospheric low over the Ross Ice Shelf. This may be a consequence of the acceleration of the katabatic winds at Terra

CARRASCO A/qD BR. OMWiCa: MESOSCALE CYCLOGEI'qESIS 12,991

Nova Bay [Parish and Brom•dch, 1991] that took place late on February 17.

Under the presence of strong katabatie winds at Terra Nova Bay and the passage of the third trough (III) a fourth mesoscale (L4) cyclone suddenly formed at 0000 UTC, February 18. Figure 12 [from Brom•dch and Parish, 1988] is the pressure- anomaly analysis at this time showing the mesolow centered over Drygalski Ice Tongue. At 0000 UTC, February 18, the potential temperature difference between Inexpressible and Franklin Island was about 8 ø C, revealing the presence of a new horizontal temperature gradient between these sites. On this occasion the near-surface baroclinic zone was caused by the katabatic airflow which brought cold air from the plateau into the maritime environment near Terra Nova Bay. The strong outflow of cold katabatic air, the temperature difference between the two sites, and the lower sea level pressure at Inexpressible Island with respect to Franklin Island (Figure 7a) are the same characteristics described by Brom•dch [1987], when a mesoscale cyclone formed near Franklin Island in February 1984. The strong katabatic airflow (-30 m s 'l) lasted for more than 6 hours, decreased to 23 m s '1 by 0900 UTC, and stayed at about the same intensity until 1800 UTC February 18. It steadily decreased thereafter. The NOAA 9 satellite image at 1302 UTC, February 18 (Figure 13), shows a very distinctive low cloud signature around Franklin Island revealing the presence of the mesoscale cyclone over the southwestern Ross Sea. Figure 14 is the mesoscale analysis using the concurrent AWS data taken directly by the NOAA 9 polar orbiting satellite. It shows the vortex centered slightly to the southeast of Franklin Island. On the same day, the next satellite image at 1703 UTC (Figure 15) showed the vortex slightly to the southeast of its earlier position revealing its displacement toward the ice shelf. Figure 16 is the concurrent mesoscale analysis at 1703 UTC, February 18, showing the sea level pressure field associated with the mesoscale cyclone. The gradual counterclockwise turning of the wind at Franklin Island (Figure 7a) from northwesterly to southwesterly, between 0900 and 1800 UTC, confirms the southeastward movement. Also, Figure 15 reveals that the cloud signature associated with the vortex was less organized, which indicates that the mesoscale cyclone had already begun to weaken. The diagnostic omega equation indicates downward motion over the southwestern Ross Sea at 0000 UTC, February 19, which is associated with the weakening of the mesoscale system.

The next available satellite image at 1250 UTC, February 19 (not reproduced), showed disorganized low clouds over the southern Ross Sea. However, the capability of enhancing the satellite image displayed on a computer screen resolved a low-cloud spiral feature to the east of Ross Island which is associated with the mesoscale vortex that developed near Terra Nova Bay. A similarly weak low-cloud feature was located to the southeast of Ross Island over the ice shelf (Figure 14). According to the mesoscale analyses (Figures 9, 14, and 16) the latter feature moved from near Byrd Glacier [Ca•wasco and Brom•dch, 1991]. A weak dissipating synoptic front over Ross Sea was also revealed by the satellite image and is analyzed in Figure 2c. Cyclonic vorticity and warm air advections were present over the southern Ross Sea at 0000 and 1200 UTC, February 20 (Figures 1 lg and 1 lb, Table 1). This is due to the transit of the fourth trough (IV) which crossed the area at

around 12000 UTC, February 20. The following image at 1438 UTC, February 20, revealed a well-defined low-cloud spiral, centered just to the southwest of AWS 00. Figures 17 and 18 are the infrared satellite image at this time, showing the subsynoptic low-cloud spiral, and the concurrent surface regional analysis. The apparent weakening of both mesoscale cyclones observed on the prior satellite image at 1250 UTC, February 19 (not shown), seems to have been reversed when both mesoscale systems merged into one subsynoptic cyclone sometime on February 20 (Figure 14). The combined system then moved toward Marie Byrd Land, where it dissipated sometime on February 22. The arrows in Figure 14 indicate the origin and track of the Terra Nova Bay vortex and the vortex that developed near Byrd Glacier. Unfortunately, the lack of an adequate sequence of satellite images prevents a more detailed analysis. Synoptic support for development of the combined subsynoptic system, formed by merging of the mesocyclones, is provided by the passage of the dissipating front over the Ross Sea (Figure 2c) and the passage of the fourth midtropospheric trough (IV) around 1200 UTC, February 20 (Figures 3c and 3d). Evaluation of the omega equation suggests synoptic scale support around AWS 00 for development of the combined system.

7. SUMMARY AND CONCLUSIONS

During a period of about 48 hours, four mesoscale cyclones formed around Terra Nova Bay. The surface synoptic analyses show a cyclonic circulation over the southern Ross Sea during this period, while the 500-hPa analyses reveal the passage of three troughs over the cyclogenesis area. The T¸VS analyses resolved a semipermanent shortwave trough offshore from the Victoria Land coast. This makes the southwestern Ross Sea a

vorticity-rich area [I-•s•s et al., 1985]. The semipermanent surface and midtropospheric troughs confn'rn that this area is very sensitive for mesoscale cyclogenesis. The formation of the mesoscale cyclones took place near the transit time of the midtropospheric troughs which moved around the cold-cored midtropospheric low. The omega equation analyses suggest that upward vertical motion in the lower troposphere accompanied the cyclogeneses. The last three mesoscale cyclones developed in association with the approach ofmidtropospheric troughs (II and III) that crossed the Ross Sea area at around 1200 UTC, February 17, and 0000 UTC, February 18. The synoptic cyclonic circulation around Franklin and Terra Nova Bay could have established a boundary layer baroclinic zone due to the advection of warm air which dammed up against the Scott Coast. On the other hand, the generally weak northwesterly katabatic winds between 0000 UTC, February 16, and 0300 UTC, February 17, seem to have brought sufficient cold air from the plateau for the establishment of baroclinic zones upon which the first two mesoscale cyclones formed. The third mesoscale cyclone that formed over the plateau of Victoria Land was caused by the southeasterly wind at Inexpressible Island which advected the warm maritime air over the plateau and established an interior baroclinic zone.

In contrast to the development of the three mesoscale cyclones described above, for which the surface synoptic scale cyclonic circulation over the southern Ross Sea seemed to play an important development role, the intense katabatic outflow

12,992 CARRASCO AND BROMWXCH: MESOSCALE CYCLOGENESIS

..

..... ß ?!.-.' ........ '.':..':::i:i:•::,!::-'-: '.'..•:'"•."-'":•'"'::.' ½ ........................ ,.'.:::.•,... '"'••' ..::,::¾:i,::...:,• ....... ":i?;'.'.. /

.. sUbsynopf ic..

Fig. 17. Thermal infrared satdlite image at 1438 UTC, February 20, 1988.

from Terra Nova Bay is the main factor that tr:ggered the formation of the fourth and most prominent mesoscale cyclone. The cold katabatic air from the East Antarctic ice sheet was monitored by the very strong winds at Inexpressible Island (AWS 05). On this occasion the baroclinic zone was established by this katabatic jet stream which brought cold continental air into the much warmer maritime environment in

the Ross Sea. Also, this development was accompanied by the passage of a midtropospheric trough (III in Figure 3). The initial formation of the fourth vortex is similar to that investi-

gated by Busiagera•d Baik[1991] where a low-level baroclinic zone was accompanied by upward synoptic scale vertical motion associated with a shortwave trough moving around a

cold-cored upper low. The fourth mesoscale cyclone merged with another, which developed near Byrd Glacier [Carcasco aad Brom•dcfi, 1991], sometime early on February 20 (Figure 14). The merging and the development of the combined system took place in association with the passage of another midtropospheric trough (IV in Figure 3). The by-now subsynoptic cyclone moved to West Antarctica where it dissipated sometime on February 22.

Figure 19 summarizes the key aspects related with mesoscale cyclogenesis near Terra Nova Bay. A mesoscale boundary layer baroclinic zone can form due to the enhanced cold katabatic airflow coming down from the high plateau into the warmer, maritime environment of the Ross Sea. Warm air

CARRASCO AI•D BROMWlCIt: MESOSCALE CYCLOGENESIS 12,993

(Y2 170øE 180 ø

170øW

70øS

WARM 75øS

...... :::::::::::::::::::::::::::: ..... ""ø15 '•24

...........

....... ß :.:.:.:.: ...........

o L

94

Fig. 18. Same as Figure 9 but for 1438 UTC, February 20, 1988.

advection usually associated with synoptic scale cyclonic circulation toward the vicinity of the Terra Nova Bay may also form the mesoscale baroclinic zone or enhance a preexisting one. Approach of midtropospheric troughs give synoptic support for triggering and/or subsequent development of the mesoscale cyclones. The same key aspects are present nea•

Byrd Glacier but with less frequent synoptic scale warm air advection affecting the area as well as less synoptic scale support from passing troughs.

Inadequate satellite coverage precludes further analysis of the formation of these mesoscale cyclones. However, from this and other studies the southern Ross Sea is demonstrably an

12,994 CARRASCO A_,•D BROMWmR: MESOSCALE CYCLOGENESI8

Ross Sea

Ross

2 Ice Shelf

L

cold katabatic airflow convergence area.

mesoscale cold front

mesoscale cyclogenesis zone 1) Terra Nova Bay 2) Byrd GI.

Passing synoptic-scale midtropospheric trough (upward motion over the Ross Sea and Ross Ice Shelf)

Synoptic near-surface warm air advection.

Fig. 19. Sketch showing key mesoscale and synoptic scale features involved in mesoscale cyclone formation near Terra Nova Bay and Byrd Glacier. Dashed arrow and dashed L indicate less frequent and/or weak warm air advection and less frequent mesoscale cyclogenesis, respectively.

active area for mesoscale cyclogenesis due to the presence of a katabatic wind confluence zone inland from Terra Nova Bay, and this occurs by baroclinic instability of the offshore katabatic jet. Although synoptic support may not be necessary for the formation of these mesoscale cyclones [Brorawich, 1991 ], it can facilitate and set up the appropriate environment. Throug- hout this study the favorable synoptic environment allowed the development of four mesoscale cyclones around Terra Nova Bay within a period of 48 hours. The mechanisms that govern the displacement of the mesoscale vortices that form over the southwestern corner of the Ross Sea are not clearly under- •tood. It seems that they are steered by the initial low-level circulation. Thus the first three vortices followed the circula-

tion associated with the synoptic cyclone, which dominated the southwestern Ross Sea on February 17. But the establishment of the strong katabatic wind over Terra Nova Bay could have steered the fourth mesoscale cyclone toward the east. Al- though this system did not develop beyond the subsynoptic scale, its fate suggests that such mesocyclones contribute significantly to the annual snowfall accumulation over West Antarctica.

In this study, the subsynoptic and synoptic environment associated with mesoscale cyclogenesis has been described. Surface and 500-kPa southern hemisphere charts provided by the Australian Bureau of Meteorology were integrated with mesoscale regional analyses and satellite information. Using the method developed by Phillpot [1991], the Australian 500-hPa analyses were adjusted by incorporating 500-hPa geopotential heights estimated from AWS deployed on the

East Antarctic plateau. The method improves the original charts by resolving and defining synoptic features over the plateau. Detailed description ofmesoscale cyclogenesis events were obtained by constructing mesoscale regional analyses and by examining high-resolution satellite images. The TOVS 500-hPa analyses revealed subsynoptic features that are not resolved by the Australian analyses. The most important is a semipermanent trough just offshore from the Victoria Land coast. This may be caused by the displacement of air from the plateau (elevation above 2000 m) to the Ross Sea. Conserva- tion of potential vorfieity predicts the formation of a trough in the general area. The sensitivity of the Terra Nova Bay area for mesoscale cyclone formation is supported by the presence of this subsynoptic midtropospheric trough offshore from Victoria Land. Although the TOVS analyses seem to be qualitatively in agreement with the Australian synoptic analyses, careful interpretation is required because ofthe errors introduced into the TOVS retrievals by the ice sheet and low clouds. Further studies are needed to determine whether or

not a semipermanent subsynoptic scale trough is actually located just offshore from Victoria Land.

Acknowledgments. This research was sponsored by the Division of Polar Programs of the National Science Foundation through grant DPP-8816792 to D. H. Bromwich. The satellite imagery was recorded by U.S. Navy personnel at McMurdo Station and obtained from Robert Whritner of the Antarctic Research Center at Scripps Institu- tion of Oceanography (NSF grant DPP-8815818). John Nagy drafted the figures. The satellite data were processed using the TeraScan software package developed by SeaSpace. This is contribution 816 of Byrd Polar Research Center.


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(Received January 24, 1992; revised October 26, 1992;

accepted November 27, 1992.)

mesoscale cyclogenesis dynamics over the southwestern ross...

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