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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
and
Zhong Liu

Abstract

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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Thomas R. Parish
and
David H. Bromwich

Abstract

Two aircraft missions to sample the boundary layer dynamics associated with the intense katabatic wind regime at Terra Nova Bay, Antarctica were flown on successive days in early November 1987. Light winds averaging 5 m s−1 were monitored at the 170 m flight level over the interior of the ice sheet. Dramatic acceleration of the airflow and abrupt 5°–7°C cooling were encountered on both days near the head of Reeves Glacier just upslope from where the terrain steepens considerably. These results suggest that much of the airflow convergence which sustains the coastal katabatic winds is forced by localized topographic channeling into Reeves Glacier, and that the descending airstream is negatively buoyant. The horizontally propagating katabatic winds were followed for 250 km directly offshore and for 200 km southward parallel to the Victoria Land coast the airstream momentum gradually decreased along both flight paths.

In conjunction with the descent of negatively buoyant air down Reeves Glacier and horizontal flow acres Nansen Ice Sheet, thermal infrared satellite images showed a warm katabatic signature along the trajectory. This paradox is explained by vigorous vertical mixing within the katabatic layer which makes the temperature of the emitting snow surface beneath the katabatic jet much warmer than that for adjacent light-wind areas. Thermal images often suggest that katabatic winds propagate for hundreds of kilometers beyond the slope break; this interpretation is strongly supported by the offshore aircraft data.

Primitive-equation model simulations for the aircraft flight level reproduced the light Dearly frictionless contour-parallel winds wen in the interior. The model also reproduced the abrupt airflow acceleration near the head of Reeves Glacier. Maximum speeds within the steeply-sloping glacier valley are underestimated, however, and it appears that a much finer rid spacing than 32 km is required to accurately simulate katabatic drainage through the complex coastal mountains.

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Thomas R. Parish
and
David H. Bromwich

Abstract

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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Jorge F. Carrasco
and
David H. Bromwich

Abstract

A one-year (1988) statistical study of mesoscale cyclogenesis near Terra Nova Bay and Byrd Glacier. Antarctica, was conducted using high-resolution digital satellite imagery and automatic weather station data. Results indicate that on average two (one) mesoscale cyclones form near Terra Nova Bay (Byrd Glacier) each week, confirming these two locations as mesoscale cyclogenesis areas. The maximum (minimum) weekly frequency of mesoscale cyclones occurred during the summer (winter). The satellite survey of mesoscale vortices was extended over the Ross Sea and Ross Ice Shelf. Results suggest southern Maric Byrd Land as another area of mesoscale cyclone formation. Also, frequent mesoscale cyclonic activity was noted over the Ross Sea and Ross Ice Shelf, where, on average, six and three mesoscale vortices were observed each week, respectively, with maximum (minimum) frequency during summer (winter) in both regions. The majority (70%–80%) of the vortices were of comma-cloud type and were shallow. Only around 10% of the vortices near Terra Nova Bay and Byrd Glacier were classified as deep vortices, while over the Ross Sea and Ross Ice Shelf around 20% were found to be deep.

The average large-scale pattern associated with cyclogenesis days near Terra Nova Bay suggests a slight decrease in the sea level pressure and 5OO-hPa geopotential height to the northwest of this area with respect to the annual average. This may be an indication of the average position of synoptic-scale cyclones entering the Ross Sea region. Comparison with a similar study but for 1984-85 shows that the overall mesoscale cyclogenesis activity was similar during the three years, but 1985 was found to be the year with greater occurrence of “Significant” mesoscale cyclones. The large-scale pattern indicates that this greater activity is related to a deeper circumpolar trough and 5OO-hPa polar vortex for 1985 in comparison to 1984 and 1988. This means that 1985 had more frequent and/or stronger warm air advection toward the Ross Sea area caused by synoptic-scale cyclones decaying near Maric Byrd Land and had more frequent and/or stronger cold katabatic air outbreaks from East Antarctica onto the southwestern corner of the Ross Sea. The convergence of these two air masses creates boundary layer baroclinic zones that can undergo mesoscale cyclogenesis.

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Thomas R. Parish
and
David H. Bromwich

Abstract

Previous work has shown that winds in the lower atmosphere over the Antarctic continent are among the most persistent on earth with directions coupled to the underlying ice topography. In 1987, Parish and Bromwich used a diagnostic model to depict details of the Antarctic near-surface airflow. A radially outward drainage pattern off the highest elevations of the ice sheets was displayed with wind speeds that generally increase from the high interior to the coast. These winds are often referred to as “katabatic,” with the implication that they are driven by radiational cooling of near-surface air over the sloping ice terrain. It has been shown that the Antarctic orography constrains the low-level wind regime through other forcing mechanisms as well. Dynamics of the lower atmosphere have been investigated increasingly by the use of numerical models since the observational network over the Antarctic remains quite sparse. Real-time numerical weather prediction for the U.S. Antarctic Program has been ongoing since the 2000–01 austral summer season via the Antarctic Mesoscale Prediction System (AMPS). AMPS output, which is based on a polar optimized version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, is used for a 1-yr period from June 2003 to May 2004 to investigate the mean annual and seasonal airflow patterns over the Antarctic continent to compare with previous streamline depictions. Divergent outflow from atop the continental interior implies that subsidence must exist over the continent and a direct thermal circulation over the high southern latitudes results. Estimates of the north–south mass fluxes are obtained from the mean airflow patterns to infer the influence of the elevated ice sheets on the mean meridional circulation over Antarctica.

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Qiu-shi Chen
and
David H. Bromwich

Abstract

In σ coordinates, a variable ϕ e (x, y, σ, t) whose horizontal gradient − ϕ e is equal to the irrotational part of the horizontal pressure gradient force is referred to as an equivalent isobaric geopotential height. Its inner part can be derived from the solution of a Poisson equation with zero Dirichlet boundary value. Because − ϕ(x, y, p, t) is also the irrotational part of the horizontal pressure gradient force in p coordinates, the equivalent geopotential ϕ e in σ coordinates can be used in the same way as the geopotential ϕ(x, y, p, t) used in p coordinates. In the sea level pressure (SLP) analysis over Greenland, small but strong high pressure systems often occur due to extrapolation. These artificial systems can be removed if the equivalent geopotential ϕ e is used in synoptic analysis on a constant σ surface, for example, at σ = 0.995 level. The geostrophic relation between the equivalent geopotential and streamfunction at σ = 0.995 is approximately satisfied.

Because weather systems over the Tibetan Plateau are very difficult to track using routine SLP, 850-hPa, and 700-hPa analyses, equivalent isobaric geopotential analysis in σ coordinates is especially useful over this area. An example of equivalent isobaric geopotential analysis at σ = 0.995 shows that a secondary high separated from a major anticyclone over the Tibetan Plateau when cold air affected the northeastern part of the plateau, but this secondary high is hardly resolved by the SLP analysis. The early stage of a low (or vortex), called a southwest vortex due to its origin in southwest China, over the eastern flank of the Tibetan Plateau is more clearly identified by equivalent isobaric geopotential analysis at σ = 0.825 and 0.735 than by routine isobaric analysis at the 850- and 700-hPa levels. Anomalous high and low systems in the SLP analysis over the Tibetan Plateau due to extrapolation are all removed by equivalent isobaric geopotential analysis at σ = 0.995.

Use of equivalent geopotential ϕ e in the vorticity and divergence equations is presented, and the equivalent geopotential equation is derived. These equations can be used in numerical models, initializations, and other dynamical studies. As an example, it is shown how these equations are used to derive a velocity potential form of the generalized ω equation in σ coordinates. As a check, retrieval of precipitation over Greenland using this ω equation shows that the computed precipitation distributions for 1987 and 1988 are in good agreement with the observed annual accumulation.

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Thomas R. Parish
and
David H. Bromwich

Abstract

Surface pressure decreases of up to 20 hPa occurred over much of the Antarctic continent during a 4-day midwinter period of 1988. The widespread change in the pressure field accompanied intense cyclonic activity to the north of the ice sheet. The equatorward mass transport across the Antarctic coastline resulted in a redistribution of atmospheric pressure that extended to the subtropics of the Southern Hemisphere. Most of the mass flux from Antarctica was the result of low-level processes and appears tied to the katabatic wind circulation. The observed surface pressure decrease over the continent reflects a perturbation of the mean meridional circulation between Antarctica and the subpolar latitudes by synoptic-scale processes. Zonally averaged circulations over Antarctica were examined using output from the European Centre for Medium-Range Weather Forecasts model. Results suggest that only a poorly defined return branch of the meridional circulation exists in the middle and upper troposphere. This southward-directed flow does not compensate for the northward mass transport provided by the katabatic wind outflow in the lower atmosphere. Isallobaric contours over the Antarctic ice sheet roughly match the area of the large-scale drainage catchment associated with katabatic wind transport through the Ross Sea sector. An intense extratropical cyclone was present in the circumpolar oceanic belt to the north of the continent. The horizontal pressure gradient associated with the cyclone prompted enhanced drainage off the high interior plateau. The resulting katabatic flow issued from the continent through a narrow corridor across the Ross Ice Shelf and out over the Southern Ocean.

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Keith M. Hines
and
David H. Bromwich

Abstract

A polar-optimized version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was developed to fill climate and synoptic needs of the polar science community and to achieve an improved regional performance. To continue the goal of enhanced polar mesoscale modeling, polar optimization should now be applied toward the state-of-the-art Weather Research and Forecasting (WRF) Model. Evaluations and optimizations are especially needed for the boundary layer parameterization, cloud physics, snow surface physics, and sea ice treatment. Testing and development work for Polar WRF begins with simulations for ice sheet surface conditions using a Greenland-area domain with 24-km resolution. The winter month December 2002 and the summer month June 2001 are simulated with WRF, version 2.1.1, in a series of 48-h integrations initialized daily at 0000 UTC. The results motivated several improvements to Polar WRF, especially to the Noah land surface model (LSM) and the snowpack treatment. Different physics packages for WRF are evaluated with December 2002 simulations that show variable forecast skill when verified with the automatic weather station observations. The WRF simulation with the combination of the modified Noah LSM, the Mellor–Yamada–Janjić boundary layer parameterization, and the WRF single-moment microphysics produced results that reach or exceed the success standards of a Polar MM5 simulation for December 2002. For summer simulations of June 2001, WRF simulates an improved surface energy balance, and shows forecast skill nearly equal to that of Polar MM5.

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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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