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David H. Bromwich
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David H. Bromwich

Abstract

Two years of automatic weather station (AWS) observations and satellite images have been used to study mesoscale cyclogenesis along the Transantarctic Mountains. Twice-daily regional sea-level pressure analyses revealed the frequent formation of mesoscale cyclones adjacent to two regions where the discharge of cold boundary-layer air from east Antarctica is concentrated: near Terra Nova Bay/Franklin Island and Byrd Glacier. Between one and two new vortices on average formed each week in the former location with weak frequency maxima in December–March and August–September. There was a large difference between the cyclogenetic activity in the two years. The AWS array expanded in 1985 and resolved another cyclogenetic area near Byrd Glacier. This feature was half as active as the Franklin Island area and exhibited many of the same characteristics. About half of the Byrd Glacier cyclones developed simultaneously with vortices near Franklin Island.

These developments are the result of a dry baroclinic process with marked baroclinicity and weak cyclonic vorticity appearing to be boundary-layer prerequisites. There is little consistent upper-air support associated with the cyclogeneses, but such factors often play a key role in subsequent storm evolution. The evidence suggests that synoptic forcing plays a significant genetic role via troughs attached to, but ahead of, maritime cyclones centered to the northwest of the Ross Sea.

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David H. Bromwich

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Three years of automatic weather station observations for the months of February to April show that intense katabatic winds persistently blow across the western shore of Terra Nova Bay. The data demonstrate that the anomalously strong katabatic winds of Adelie Land are not unique, and thus strongly support the proposition that most of the cold boundary layer air from the ice sheet crosses the coastline in a small number of narrow zones. Furthermore the observations prove that katabatic winds can routinely blow for substantial distances across flat terrain in marked contrast to the abrupt dissipation previously monitored just offshore from East Antarctica. Winter wind conditions onset suddenly in mid-February and are characterized by negligible directional variations and by speeds mostly ranging between 10 and 30 m s−1.

Katabatic winds at Terra Nova Bay both affect and are affected by the regional atmospheric circulation. This katabatic airflow is a time-averaged source of cold boundary layer air for the western Ross Sea. Maximum thermal contrast with the regional temperature field occurs between January and June. Temperature observations suggest that the katabatic winds at Inexpressible Island am primarily of the boratype throughout the year. Strong southerly geostrophic winds over the western Ross Sea appear to suppress the katabatic outflow during winter while weak zonal pressure gradients coincide with intensified katabatic drainage. This relationship is suggested to arise because clouds modulate the radiative production of cold surface air over the interior of the ice sheet.

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David H. Bromwich

Prominent warm signatures of strong, negatively buoyant, katabatic airstreams are present at thermal infrared wavelengths as a result of intense vertical mixing and drift-snow transport within stable boundary layers. These tracers are used to illustrate several aspects of the behavior of katabatic winds in the Ross Sea sector of the Antarctic. The satellite features are compared with surface-based observations whenever possible. Converging surface-wind signatures upslope from Terra Nova Bay are shown to closely follow the observed time-averaged streamlines of drainage airflow. The satellite-observed core of the katabatic airstream descends to sea level via a direct route, but complex three-dimensional trajectories are manifested in marginal regions. Katabatic winds propagating horizontally for hundreds of kilometers over the southwestern Ross Sea do not exhibit the expected influence of the Coriolis force. Katabatic signatures are shown to be climatological features over the Ross Ice Shelf which closely follow surface wind measurements. An approximate proportionality appears to exist between average signature size over the shelf and the magnitude of katabatic mass transport from the plateau.

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David H. Bromwich
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Zhong Liu and David H. Bromwich

Abstract

The surface wind pattern over the ice sheets of Antarctica is irregular with marked areas of airflow confluence near the coastal margins. Where cold air from a large interior area of the ice sheet converges (a confluence zone), an anomalously large supply of air is available to feed the coastal katabatic winds, which, as a result, are intensified and more persistent. The confluence zone inland of Siple Coast, West Antarctica, differs from its East Antarctic counterparts in that the terrain slopes become gentler rather than steeper as the coast is approached. In addition, synoptic processes exert substantially more impact on the behavior of the surface winds.

A month-long field program to study the dynamics of the springtime katabatic wind confluence zone has been carried out near Siple Coast. Two sites, Upstream B (83.5°S, 136.1°W) and South Camp (84.5°S, 134.3°W), were established roughly perpendicular to the downslope direction. The field program involved the use of the ground-based remote sensing equipment (sodar and RASS) along with conventional surface and balloon observations. Previous analyses revealed the cross-sectional structure of the confluence zone as consisting of a more buoyant West Antarctic katabatic airflow overlying a less buoyant katabatic airflow originating from East Antarctica.

The force balances inside the confluence zone are here investigated for three situations: mean (all available wind profiles from balloon launches), and two extreme cases (light and strong winds). A linear regression method is used to estimate the mean vertical wind shears and horizontal temperature gradients. The vertical wind shears are used to examine whether or not the airflows are in geostrophic balance. The results are 1) the airflow above the surface at both sites is in geostrophic balance for the three situations; 2) inside the West Antarctic katabatic wind zone, there are three forces in the north–south direction—the restoring pressure gradient force associated with blocking of the katabatic and synoptic winds, the downslope buoyancy force, and the synoptic pressure gradient force associated with the time-averaged low in the South Pacific Ocean; 3) above the West Antarctic katabatic wind layer, the observed easterly wind is due to the synoptic pressure gradient force associated with the low; 4) inside the East Antarctic katabatic wind zone, in addition to the above three forces, there is the downslope buoyancy force associated with the inversion; and 5) large-scale transient synoptic systems strongly influence the downslope wind speed and the boundary layer depth, resulting in the light and strong wind cases.

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Zhong Liu and David H. Bromwich

Abstract

The blocking effect of Ross Island and Hut Point peninsula, Antarctica, has been investigated since the early part of this century. Due to lack of continuous measurements of boundary-layer winds, the investigations were limited to an overall description of the blocking effect with no information on the diurnal variation or the detailed vertical structure of the approaching airflow.

An acoustic sounder (sodar) was deployed during the 1990/91 austral summer season at Williams Field in the upwind area south of Ross Island, Antarctica. Such equipment can continuously measure three-dimensional winds from a few tens of meters above the surface up to an altitude of several hundred meters, thus providing a new opportunity to study the dynamics of the stably stratified planetary boundary layer.

In addition to confirming earlier work, the sodar winds show a significant diurnal variation of the blocking effect, which amplifies with height. Such variation is dominated by the changes in the upstream air mass in which katabatic airflow from Byrd, Mulock, and Skelton glaciers plays a central role.

Through case studies, the breakdown of the prevailing wind regime in the Ross Island area was associated with the influence of meso- and synoptic-scale pressure gradients on the katabatic airflow approaching from the south and with very localized geostrophic winds deflected around the topography of Ross Island.

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David H. Bromwich and Zhong Liu

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A month-long field program to study the springtime katabatic wind confluence zone (where katabatic winds converge) has been carried out near Siple Coast, West Antarctica. Based on previous observations and numerical studies, two surface camps, Upstream B (83.5°8, 136.1°W) and South Camp (84.5°S, 134.3°W), were established. Ground-based remote sensing equipment (sodar and RASS), along with conventional observations, were used. Combining the analyses of surface observations and wind and temperature profiles at the above camps, the following picture for the cross-sectional structure of the confluence zone emerges. A relatively cold katabatic airflow, which probably comes from Fast Antarctica, occupies the layer between the surface and roughly 500 m ACL. Low-level jets are present below 200 m AGL and are stronger near the Transantarctic Mountains. Diurnal variation is present in this cold drainage flow and decreases toward the Transantarctic Mountains. Weak-inversion-layer tops are found near 500 m AGL, which is roughly equal to the depth of the cold katabatic flow. The warmer West Antarctic katabatic airflow overlies the cold drainage flow from East Antarctica and has a depth of approximately 1000 m at Upstream B and more than 1500 m at South Camp; this is caused by blocking of the converging West Antarctic airflow by the Transantarctic Mountains. This warm flow originates near the surface far upslope in the vicinity of Byrd Station (80°S, 120°W). A baroclinic zone, formed where the two drainage flows are horizontally adjacent, appears to become unstable with sonar frequency to generate mesoscale cyclones.

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Ryan L. Fogt and David H. Bromwich

Abstract

Antarctic Mesoscale Prediction System (AMPS) forecasts of atmospheric moisture and cloud fraction (CF) are compared with observations at McMurdo and Amundsen–Scott South Pole station (hereafter, South Pole station) in Antarctica. Overall, it is found that the model produces excessive moisture at both sites in the mid- to upper troposphere because of a weaker vertical decrease of moisture in AMPS than observed. Correlations with observations suggest AMPS does a reasonable job of capturing the low-level moisture variability at McMurdo and the upper-level moisture variability at South Pole station. The model underpredicts the cloud cover at both locations, but changes to the AMPS empirical CF algorithm remove this negative bias by more than doubling the weight given to the cloud ice path.

A “pseudosatellite” product based on the microphysical quantities of cloud ice and cloud liquid water within AMPS is preliminarily evaluated against Defense Meteorological Satellite Program (DMSP) imagery during summer to examine the broader performance of cloud variability in AMPS. These comparisons reveal that the model predicts high-level cloud cover and movement with fidelity, which explains the good agreement between the modified CF algorithm and the observed CF. However, this product also demonstrates deficiencies in capturing low-level cloudiness over cold ice surfaces primarily related to insufficient supercooled liquid water produced by the microphysics scheme, which also reduces the CF correlation with observations.

The results suggest that AMPS predicts the overall CF amount and high cloud variability notably well, making it a reliable tool for longer-term climate studies of these fields in Antarctica.

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Thomas R. Parish and David H. Bromwich

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The surface windfield over the gently sloping interior ice fields of Antarctica is characterized by a high degree of persistence in terms of both direction and speed. The forcing of the surface wind is due primarily to the radiational cooling of the air adjacent to the sloping terrain. The representativeness of a simple diagnostic equation system in inferring the surface winds from a knowledge of terrain slope and temperature inversion structure is examined. Results suggest at least qualitatively accurate surface drainage patterns over the Antarctic continent are possible using this technique. A wintertime surface wind simulation for West Antarctica has been generated based on an accurate ice topography map. Close agreement is seen between the simulated surface windfield with field observations and sastrugi orientations. Implications of the simulation are discussed.

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