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Biao Chen
,
Shawn R. Smith
, and
David H. Bromwich

Abstract

A case study investigation into the meridional and horizontal circulation over the South Pacific Ocean is presented for the 1986–89 El Niño–Southern Oscillation (ENSO) cycle. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses, annual average fields are created for the years before and after the 1987 minimum (warm phase) and 1989 maximum (cold phase) in the Southern Oscillation index. The analyses reveal a shift in the split jet stream over the south Pacific sector(180°–120°W)from a strong subtropical jet (STJ) and weak polar front jet (PFJ) during the warm phase to a weak STJ and strong PFJ during the cold phase.

Analysis of the momentum budget reveals how the split jet in the upper troposphere over South Pacific Ocean evolved during the 1986–89 ENSO cycle. During the warm phase, the strong STJ is associated with advection of the mean flow momentum flux from the Australian sector, which is approximately balanced by a large negative ageostrophic term; the PFJ is primarily associated with eddy momentum convergence, which is partially counterbalanced by the ageostrophic term. During the cold phase, the weakened STJ is related to an increasingly negative ageostrophic term and a less positive mean flow momentum convergence. The strengthened PFJ is associated with an increase in the convergence of eddy momentum flux that is mainly composed of 2.5–6-day poleward momentum transport from midlatitudes and 7–30-day equatorward momentum transport from high latitudes. In general, the impacts of eddy stress on the STJ and the mean momentum divergence on the PFJ in this sector are small.

The variations in the split jet may reflect the poleward propagation of the ENSO signal via the South Pacific convergence zone. The implications for the high southern latitudes are discussed as interannual variations are found in the low-level easterlies near Antarctica and the Amundsen Sea low.

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David H. Bromwich
,
Yang Du
, and
Keith M. Hines

Abstract

Clear-sky, wintertime surface winds over the Greenland Ice Sheet are simulated with a three-dimensional mesoscale numerical model. It is shown that the simulated winds blow from the broad gently sloped interior to the steep coastal margins. This general wind pattern is similar to that found over Antarctica due to the same governing dynamics. The longwave radiational cooling of the sloping ice terrain is the key driving force of this cold airflow. In some coastal areas the downslope winds converge into large fjords, such as Kangerlussuaq and Sermilik. This is consistent with the frequent presence in these areas of warm signatures on cloud-free thermal infrared satellite images that are generated by katabatic winds. The shape of the Greenland Ice Sheet plays an important role in directing the flow of the surface winds. The study demonstrates that the surface wind pattern is only moderately affected by climatological flow around and over the ice sheet. The mass redistribution associated with the katabatic wind circulation plays an important role in generating prominent features of the time-averaged sea level pressure and upper-level circulation fields near Greenland.

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Qiu-shi Chen
,
Le-sheng Bai
, and
David H. Bromwich

Abstract

In comparison to the Tatsumi’s spectral method, the harmonic-Fourier spectral method has two major advantages. 1) The semi-implicit scheme is quite efficient because the solutions of the Poisson and Helmholtz equations are readily derived. 2) The lateral boundary value problem of a limited-area model is easily solved. These advantages are the same as those of the spherical harmonics used in global models if the singularity at the pole points for a globe is considered to be the counterpart of the lateral boundary condition for a limited region.

If a limited-area model is nested in a global model, the prediction of the limited-area model at each time step is the sum of the inner part and the harmonic part predictions. The inner part prediction is solved by the double sine series from the inner part equations for the limited-area model. The harmonic part prediction is derived from the prediction of the global model. An external wind lateral boundary method is proposed based on the basic property of the wind separation in a limited region. The boundary values of a limited-area model in this method are not given at the closed boundary line, but always given by harmonic functions defined throughout the limited domain. The harmonic functions added to the inner parts at each time step represent the effects of the lateral boundary values on the prediction of the limited-area model, and they do not cause any discontinuity near the boundary.

Tests show that predicted motion systems move smoothly in and out through the boundary, where the predicted variables are very smooth without any other boundary treatment. In addition, the boundary method can also be used in the most complicated mountainous region where the boundary intersects high mountains. The tests also show that the adiabatic dynamical part of the limited-area model very accurately predicts the rapid development of a cyclone caused by dry baroclinic instability along the east coast of North America and a lee cyclogenesis case in East Asia. The predicted changes of intensity and location of both cyclones are close to those given by the observations.

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David H. Bromwich
,
Jorge F. Carrasco
, and
Charles R. Stearns

Abstract

Five winter months (April–August 1988) of thermal infrared satellite images were examined to investigate the occurrence of dark (warm) signatures across the Ross Ice Shelf in the Antarctic continent. These features are inferred to be generated by katabatic winds that descend from southern Marie Byrd Land and then blow horizontally across the ice shelf. Significant mass is added to this airstream by katabatic winds blowing from the major glaciers that flow through the Transantarctic Mountains from East Antarctica. These negatively buoyant katabatic winds can reach the northwestern edge of the shelf, a horizontal propagation distance of up to 1000 km, 14% of the time. Where the airstream crosses from the ice shelf to the ice-covered Ross Sea, a prominent coastal polynya is formed. Because the downslope buoyancy force is near zero over the Ross Ice Shelf, the northwestward propagation of this katabatic air mass requires pressure gradient support. The study shows that the extended horizontal propagation of this atmospheric density current occurred in conjunction with the passage of synoptic cyclones over the southern Amundsen Sea. These cyclones can strengthen the pressure gradient in the interior of West Antarctica and make the pressure field favorable for northwestward movement of the katabatic winds from West Antarctica across the ice shelf in a geostrophic direction. The glacier winds from East Antarctica are further accelerated by the synoptic pressure gradient, usually undergo abrupt adjustment beyond the exit to the glacier valley, and merge into the mountain-parallel katabatic air mass.

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William P. O'Connor
,
David H. Bromwich
, and
Jorge F. Carrasco

Abstract

The effect of the Transantarctic Mountains on cyclonically forced boundary-layer winds in the vicinity of Ross Island, Antarctica (77.5°S, 167°E), is discussed. When cyclones are present over the western Ross Ice Shelf and Ross Sea, the low-level easterly airflow is toward the mountains. A barrier wind regime is set up as the flow is turned northward and becomes parallel to the mountain range. IT is found that cyclonically forced barrier winds occurred around 5% and 8% of the time during 1984 and 1985, respectively.

The case histories of two well-defined barrier wind events lasting for 24 h are discussed in detail, with regional analyses based on satellite photographs and automatic weather station data. One case is for a katabatic wind-forced mesoscale cyclone forming to the north of Ross Island, and the other is for a synoptic-scale cyclone moving through the western Ross Ice Shelf-Ross Sea region.

A numerical model for the vertically integrated boundary-layer flow that calculates two horizontal velocities and the boundary-layer depth is used to investigate the mountain barrier effect on low-level airflow. The domain is the region of the western Ross Ice Shelf-Ross Sea from 82° to 76°S, between Byrd Glacier and Terra Nova Bay, and bounded to the west by the Transantarctic Mountains. The boundary-layer airflow is constrained to remain below the height of the mountains, so that the surface airflow is around the topographic features of Minna Blulff and Ross Island. The two cases of cyclonic forcing are modeled, with the isobars intersecting the mountains obliquely. The model depicts the pressure increases and stagnation zones south of Minna Bluff and Ross Island, and the surface airflow eastward past these features, which agree with observations.

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David H. Bromwich
,
Yang Du
, and
Thomas R. Parish

Abstract

Twenty-four-hour numerical simulations of wintertime surface winds under clear sky conditions over the West Antarctic ice sheet and its vicinity are performed using a hydrostatic, three-dimensional primitive equation model. Two initial states are examined: a state of rest, and a prescribed pressure field associated with katabatic winds from West Antarctica propagating across the Ross Ice Shelf. The Antarctic katabatic winds are mainly due to the strong radiative cooling of the ice slopes. The West Antarctic terrain is different from that of East Antarctica in two respects: its mean elevation is much lower, and the slope in the interior is steeper than near the margin at Siple Coast.

The simulated surface wind regime reveals confluence zones just inland from the coast and diffluence zones around the crest of the terrain. The model results suggest that the continuation of katabatic winds beyond coastal confluence zones, which are sustained by cold-air drainage in the interior, has an important impact on airflow over the flat Ross Ice Shelf adjacent to the Transantarctic Mountains. The prescribed pressure disturbance has little impact on the surface winds in the interior but markedly impacts those over and beyond the gently sloping coastal areas. Discussion of the impact of the surface wind on the polynya northwest of the Ross Ice Shelf is also provided. It is shown that the simulated surface-wind regime is consistent with the available, mostly surface observational data.

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Daniel F. Steinhoff
,
Saptarshi Chaudhuri
, and
David H. Bromwich

Abstract

A case study illustrating cloud processes and other features associated with the Ross Ice Shelf airstream (RAS), in Antarctica, is presented. The RAS is a semipermanent low-level wind regime primarily over the western Ross Ice Shelf, linked to the midlatitude circulation and formed from terrain-induced and large-scale forcing effects. An integrated approach utilizes Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery, automatic weather station (AWS) data, and Antarctic Mesoscale Prediction System (AMPS) forecast output to study the synoptic-scale and mesoscale phenomena involved in cloud formation over the Ross Ice Shelf during a RAS event. A synoptic-scale cyclone offshore of Marie Byrd Land draws moisture across West Antarctica to the southern base of the Ross Ice Shelf. Vertical lifting associated with flow around the Queen Maud Mountains leads to cloud formation that extends across the Ross Ice Shelf to the north. The low-level cloud has a warm signature in thermal infrared imagery, resembling a surface feature of turbulent katabatic flow typically ascribed to the RAS. Strategically placed AWS sites allow assessment of model performance within and outside of the RAS signature. AMPS provides realistic simulation of conditions aloft but experiences problems at low levels due to issues with the model PBL physics. Key meteorological features of this case study, within the context of previous studies on longer time scales, are inferred to be common occurrences. The assumption that warm thermal infrared signatures are surface features is found to be too restrictive.

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Jorge F. Carrasco
,
David H. Bromwich
, and
Andrew J. Monaghan

Abstract

The mesoscale cyclone activity observed in the portion of Antarctica that faces the South Pacific Ocean and Weddell Sea area is summarized from a study of 1991. In general, area-normalized results reveal much greater mesoscale cyclonic activity over the Ross Sea/Ross Ice Shelf and southern Marie Byrd Land than on both sides of the Antarctic Peninsula. More than 50% of the observed mesoscale vortices are of the comma cloud type. The average diameter of mesoscale vortices is approximately 200 km near Terra Nova Bay, 270 km near Byrd Glacier, and 280 km near Siple Coast. Near the Antarctic Peninsula, the average diameter is about 370 km over the Bellingshausen Sea and 380 km on the Weddell Sea side. The largest percentage of deep vortices occurs over the Bellingshausen Sea sector (38% of all cases), where convective instability frequently occurs. Over the Ross Sea/Ross Ice Shelf and Weddell Sea sectors the majority of the mesoscale vortices are low cloud features that probably do not exceed the 700-hPa level due to the prevailing lower-atmospheric stability. The areas identified as sources of mesoscale vortices concur with the locations of enhanced katabatic winds.

A synthesis of the available literature leads to some general characteristics of mesoscale cyclone formation and development. Mesoscale cyclogenesis is associated with areas of warm and/or cold air advection, low-level baroclinicity, and cyclonic vorticity resulting from the stretching mechanism. Subsequent intensification depends on the presence of upper-level support. Spatial and temporal variability in mesoscale cyclone formation is often related to the behavior of synoptic-scale cyclone tracks. Mesoscale cyclones can generate precipitation and severe weather conditions and thus present a critical forecasting challenge.

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Zhichang Guo
,
David H. Bromwich
, and
John J. Cassano

Abstract

Evaluation of a complete annual cycle of nonhydrostatic mesoscale model simulations of the Antarctic atmospheric circulation is presented. The year-long time series are compiled from a series of overlapping short-duration (72 h) simulations of the atmospheric state with the first 24 h being discarded for spinup reasons, and the 24–72-h periods used for model evaluation. The simulations are generated with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), which is modified for polar applications, and is referred to as the Polar MM5. With a horizontal resolution of 60 km, the Polar MM5 has been run for the period of January 1993–December 1993, creating short-term simulations from initial and boundary conditions provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) Tropical Ocean Global Atmosphere (TOGA) operational analyses. The model output is compared with observations from automatic weather stations, upper-air data, and global atmospheric analyses as well as climatological maps over timescales from diurnal to annual. In comparison with the observations, the evaluation shows that simulations with the Polar MM5 capture both the large- and regional-scale circulation features with generally small bias in the modeled variables. For example, the differences between the observations and simulations at the 500-hPa level are usually less than 2°C for temperature and dewpoint temperature, and 20 m for geopotential height. On the annual timescale the largest errors in the model simulations are the deficient total cloud cover and precipitation, and the colder near-surface temperature over the interior of the Antarctic plateau. The deficiencies in the cloud prediction and precipitation simulation follow from low-level dry biases found in the Polar MM5 simulations, and the cold bias is related to the low predicted downward longwave radiation under clear skies in the radiation parameterization scheme. The deficient predicted precipitation also reflects the limited ability of Polar MM5 to represent clear sky precipitation. On the seasonal timescale a persistent positive pressure bias is found in the model simulations, caused by the interaction between the gravity waves and the model upper boundary condition. The observed synoptic variability of the pressure, temperature, wind speed, wind direction, and water vapor mixing ratio, as well as the diurnal cycles of temperature, wind speed, and mixing ratio, are reproduced by the Polar MM5 with reasonable accuracy.

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David H. Bromwich
,
Lesheng Bai
, and
Gudmundur G. Bjarnason

Abstract

High-resolution regional climate simulations of Iceland for 1991–2000 have been performed using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesocale Model (MM5) modified for use in polar regions (Polar MM5) with three nested domains and short-duration integrations. The simulated results are compared with monthly mean surface observations from Iceland for 1991–2000 to demonstrate the high level of model performance; correlation coefficients exceed 0.9 for most variables considered.

The simulation results are used to analyze the near-surface climate over Iceland. The simulated near-surface winds in winter are primarily katabatic. The land–sea-breeze circulation is clearly evident in summer. The land is colder than the ocean during winter, with a strong (weak) temperature gradient along the southern (northern) coast. This temperature pattern over the sloping terrain forces the katabatic wind. The diurnal cycle of near-surface air temperature is marked in summer over the land areas, which drives the land–sea breeze. The near-surface climate variations for extremes of the North Atlantic Oscillation (NAO) index during winter and summer result from the large-scale atmospheric advection conditions.

The time-averaged mesoscale precipitation distribution over Iceland is reasonably well simulated by Polar MM5. Winter precipitation rates are double those during the summer, reflecting the much greater winter cyclonic activity. The simulated interannual precipitation variations during winter for 1991–2000 agree with those observed from snow accumulation measurements on the Vatnajökull ice cap. The winter precipitation decrease for 1991–2000 dominates the annual signal for all of Iceland except the northeastern and eastern parts where the precipitation increases. The large precipitation trends (decadal decrease of up to 50%) are caused by the eastward shift and weakening of the Icelandic low during the 1990s, as a result of changes in the NAO modulation of regional climate.

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