Katabatic Wind Regime Research Articles

Seasonal and interannual variability of the landfast ice mass balance between 2009 and 2018 in Prydz Bay, East Antarctica

Abstract. Landfast ice (LFI) plays a crucial role for both the climate and the ecosystem of the Antarctic coastal regions. We investigate the snow and LFI mass balance in Prydz Bay using observations from 11 sea ice mass balance buoys (IMBs). The buoys were deployed offshore from the Chinese Zhongshan Station (ZS) and Australian Davis Station (DS), with the measurements covering the ice seasons of 2009–2010, 2013–2016, and 2018. The observed annual maximum ice thickness and snow depth were 1.59 ± 0.17 and 0.11–0.76 m off ZS and 1.64 ± 0.08 and 0.11–0.38 m off DS, respectively. Early in the ice growth season (May–September), the LFI basal growth rate near DS (0.6 ± 0.2 cm d−1) was larger than that around ZS (0.5 ± 0.2 cm d−1). This is attributed to cooler air temperature (AT) and lower oceanic heat flux at that time in the DS region. Air temperature anomalies were more important in regulating the LFI growth rate at that time because of thinner sea ice having a weaker thermal inertia relative to thick ice in later seasons. Interannual and local spatial variabilities for the seasonality of LFI mass balance identified at ZS are larger than at DS due to local differences in topography and katabatic wind regime. Snow ice contributed up to 27 % of the LFI total ice thickness at the offshore site close to ground icebergs off ZS because of the substantial snow accumulation. Offshore from ZS, the supercooled water was observed at the sites close to the Dålk Glacier from July to October, which reduced the oceanic heat flux and promoted the LFI growth. During late austral spring and summer, the increased oceanic heat flux led to a reduction of LFI growth at all investigated sites. The variability of LFI properties across the study domain prevailed at interannual timescales, over any trend during the recent decades. Based on the results derived from this study, we argue that an increased understanding of snow (on LFI) processes, local atmospheric and oceanic conditions, as well as coastal morphology and bathymetry, are required to improve the Antarctic LFI modeling.

A Model-Based Climatology of Low-Level Jets in the Weddell Sea Region of the Antarctic

Low-level jets (LLJs) are climatological features in polar regions. It is well known that katabatic winds over the slopes of the Antarctic ice sheet are associated with strong LLJs. Barrier winds occurring, e.g., along the Antarctic Peninsula may also show LLJ structures. A few observational studies show that LLJs occur over sea ice regions. We present a model-based climatology of the wind field, of low-level inversions and of LLJs in the Weddell Sea region of the Antarctic for the period 2002–2016. The sensitivity of the LLJ detection on the selection of the wind speed maximum is investigated. The common criterion of an anomaly of at least 2 m/s is extended to a relative criterion of wind speed decrease above and below the LLJ. The frequencies of LLJs are sensitive to the choice of the relative criterion, i.e., if the value for the relative decrease exceeds 15%. The LLJs are evaluated with respect to the frequency distributions of height, speed, directional shear and stability for different regions. LLJs are most frequent in the katabatic wind regime over the ice sheet and in barrier wind regions. During winter, katabatic LLJs occur with frequencies of more than 70% in many areas. Katabatic LLJs show a narrow range of heights (mostly below 200 m) and speeds (typically 10–20 m/s), while LLJs over the sea ice cover a broad range of speeds and heights. LLJs are associated with surface inversions or low-level lifted inversions. LLJs in the katabatic wind and barrier wind regions can last several days during winter. The duration of LLJs is sensitive to the LLJ definition criteria. We propose to use only the absolute criterion for model studies.

Climatological aspects of cyclogenesis near Adélie Land Antarctica

The Ad´elie Land coastal region of Antarctica is one of the most prominent cyclogenesis regions in the Southern Hemisphere, and is adjacent to the continent’s most intense katabatic wind regime. However, the physical mechanisms responsible for cyclogenesis are not known. A manual analysis of cyclogenesis for the 2003–2005 period using output from the Antarctic Mesoscale Prediction System (AMPS) identifies two primary patterns of cyclogenesis near the Ad´elie Land coast. For “secondary development” cyclones, enhanced low-level cyclonic vorticity and baroclinicity result from the combination of an existing synoptic-scale cyclone to the west, coastal barrier winds and katabatic winds. “Lee cyclogenesis” occurs near 152◦E on the cyclonic-shear side of the Ad´elie Land katabatic jet, where a low-level warm potential temperature anomaly sets up a lee trough that becomes mobile with the arrival of upper-level synopticscale forcing. The representation of both “secondary development” and “lee cyclogenesis” cyclones in an automated cyclone-tracking scheme is explored, where it is found that the automated scheme overestimates cyclogenesis for this region. The location of the Antarctic coastal cyclogenesis maximum near Ad´elie Land is due to the unique juxtaposition of the extraordinary katabatic wind regime and dissipating synoptic-scale cyclones to the west.

The surface climatology of an ordinary katabatic wind regime in Coats Land, Antarctica

The surface climatology of Coats Land, Antarctica, is described through observations from automatic weather stations, from Halley station, from upper air soundings and from satellite remote sensing. Coats Land consists of the Brunt Ice Shelf and the adjoining continent to the south. The topography of this region is typical of much of the Antarctic coastal fringes: a modest slope (5% at most) and relative uniformity across the slope. A basic climatology broken into site and season is presented. In winter, and to an extent in the equinoctial seasons, the region clearly divides into two dynamical regimes. Over theice shelf winds are usually from the east or occasionally from the west, whereas over the continental slopes winds are from the east to south quadrant. Over the ice shelf the surface layer is about 10 K colder, in terms of potential temperature, than on the continent, and is also more stable than on the steeper parts of the slope. Motivated by case studies, three criteria are developed to select a subset of the data that are katabatic in the sense that the flow is believed to be primarily due to a downslope buoyancy forcing. On the continental slope, the criteria pick out a coherent subset of the data that aretightly clustered in wind speed and wind direction. Typical katabatic winds are from 10° to the east ofthe fall line and 7.5 m s^-1 at the steepest part of the slope (5.1 m s^-1 higher up). They are rarely morethan 15 m s^-1 in this region; hence their description as ordinary, in contrast with those extraordinary katabatic regimes that have been the focus of previous studies. The katabatic flow remains close to adiabatic as it moves down the slope, and is relatively dry near the slope foot. We estimate the flow tobe primarily katabatic at most 40—50% of the time, although it may appear to be katabatic, from windspeed and wind direction characteristics, some 60—70% of the time. There is no coherent katabatic-flow signature on the ice shelf.

The surface climatology of an ordinary katabatic wind regime in Coats Land, Antarctica

The surface climatology of Coats Land, Antarctica, is described through observations from automaticweather stations, from Halley station, from upper air soundings and from satellite remote sensing.Coats Land consists of the Brunt Ice Shelf and the adjoining continent to the south. The topography ofthis region is typical of much of the Antarctic coastal fringes: a modest slope (5% at most) and relativeuniformity across the slope. A basic climatology broken into site and season is presented. In winter, andto an extent in the equinoctial seasons, the region clearly divides into two dynamical regimes. Over theice shelf winds are usually from the east or occasionally from the west, whereas over the continentalslopes winds are from the east to south quadrant. Over the ice shelf the surface layer is about 10 Kcolder, in terms of potential temperature, than on the continent, and is also more stable than on thesteeper parts of the slope. Motivated by case studies, three criteria are developed to select a subset ofthe data that are katabatic in the sense that the flow is believed to be primarily due to a downslopebuoyancy forcing. On the continental slope, the criteria pick out a coherent subset of the data that aretightly clustered in wind speed and wind direction. Typical katabatic winds are from 10° to the east ofthe fall line and 7.5 m s-1 at the steepest part of the slope (5.1 m s-1 higher up). They are rarely morethan 15 m s-1 in this region; hence their description as ordinary, in contrast with those extraordinarykatabatic regimes that have been the focus of previous studies. The katabatic flow remains close toadiabatic as it moves down the slope, and is relatively dry near the slope foot. We estimate the flow tobe primarily katabatic at most 40—50% of the time, although it may appear to be katabatic, from windspeed and wind direction characteristics, some 60—70% of the time. There is no coherent katabatic-flowsignature on the ice shelf.

Studies of the Antarctic climate with a stretched‐grid general circulation model

A stretched‐grid general circulation model (GCM), derived from the Laboratoire de Météorologie Dynamique (LMD) GCM is used for a multiyear high‐resolution simulation of the Antarctic climate. The resolution in the Antarctic region reaches 100 km. In order to correctly represent the polar climate, it is necessary to implement several modifications in the model physics. These modifications mostly concern the parameterizations of the atmospheric boundary layer. The simulated Antarctic climate is significantly better in the stretched‐grid simulation than in the regular‐grid control run. The katabatic wind regime is well captured, although the winds may be somewhat too weak. The annual snow accumulation is generally close to the observed values, although local discrepancies between the simulated annual accumulation and observations remain. The simulated continental mean annual accumulation is 16.2 cm y−1. Features like the surface temperature and the temperature inversion over large parts of the continent are correctly represented. The model correctly simulates the atmospheric dynamics of the rest of the globe.

Forcing of the Austral Autumn Surface Pressure Change over the Antarctic Continent*

Pronounced seasonal variations in the surface pressure field are present over the Antarctic continent. Surface pressures over the ice sheet decrease during the austral autumn period January–April and increase during the austral springtime months September–December. The largest changes are found over the highest portions of the Antarctic ice sheets where seasonal surface pressure changes of up to 20 hPa are common. The outstanding feature of these surface pressure changes is that typically the isallobaric contours closely follow the Antarctic orography during both transition periods, suggesting a strong seasonal diabatic adjustment within the lower troposphere. During austral autumn, the pronounced cooling of the lower atmosphere adjacent to the ice sheets leads to an enhancement of the Antarctic katabatic wind regime and hence the lower branch of the mean meridional circulation over the high southern latitudes. The mass transport provided by these drainage flows is proposed as the mechanism behind the autumn pressure falls. Numerical simulations of the evolution of the Antarctic katabatic wind regime indicate that the radiative cooling of the sloping ice fields and attendant mass transport result in a modification of the temperature and pressure fields in the lower troposphere similar to what is seen during the early austral autumn period.

On the forcing of seasonal changes in surface pressure over Antarctica

A 10‐year record (1985–1994) of output statistics from the European Centre for Medium‐Range Weather Forecasts (ECMWF) model shows that profound seasonal changes in surface pressure take place over the Antarctic continent. The most pronounced changes occur during the periods straddling the brief Antarctic summer, from September to December and again from January to April. Surface pressures atop the high Antarctic plateau often display changes in excess of 20 hPa during these periods. Temperatures in the lower troposphere also exhibit marked changes during these transitional periods; surface temperature changes during these 3‐month periods reach a maximum near 40 K over the high interior of Antarctica. Hydrostatic considerations suggest that the thermal adjustment in the lowest levels of the atmosphere alters the vertical distribution of pressure with height and hence is consistent with the dramatic seasonal surface pressure changes over the elevated Antarctic ice sheets. A strong interplay exists between the thermal forcing and the katabatic wind circulation over the continent. The large seasonal changes in solar insolation reaching the Antarctic ice surface modulate the intensity of the katabatic wind regime and thus the resulting mean meridional circulation between the continent and the subpolar latitudes. It is proposed that the diabatic adjustment in the lower levels of the atmosphere over Antarctica disrupts the mean meridional circulation creating a seasonal mass imbalance and hence surface pressure changes. It is through the meridional transports that the mass and wind fields reach a quasi‐equilibrium adjusted state. The seasonal mass movement over Antarctica requires large‐scale mass compensation over much of the southern hemisphere and shows that the diabatic influences at the Antarctic surface have far‐field impacts.

Dynamics of a Holocene Cliff-Top Dune along Mountain River, Northwest Territories, Canada

The dynamics of a cliff-top dune system located at the foot of the Mackenzie Mountains was reconstructed by radiocarbon dating and tree ring analysis of forest paleosols in eolian sediments. The first evidence of eolian activity dates to ca. 3000 yr B.P. when the breaching of a bedrock sill initiated the erosion of glaciofluvial deposits that resulted in the exposure of a 75-m-high cliff of erodible sediment. Sandy sediment eroded from the cliff By katabatic winds from the Mackenzie Mountains buried a mature forest growing on alluvial sediments. Despite a minor eolian event between 750 and 860 cal yr A.D., dune development began ca. 1100 cal yr A.D. and has been marked by two major sedimentation periods between ca. 1100 cal yr A.D. and 1460 A.D. and from 1865 to present. Detailed analysis of tree morbidity and mortality indicates that the dune progressed at an average rate of 78 cm/yr for the last century. Dune dynamics are directly related to slope activity that controls the episodic nature of sediment availability and to the katabatic wind regime. Comparison of the dune dynamics and paleoclimatic data suggest a possible influence of warm climate on eolian activity.

On the Role of the Antarctic Continent in Forcing Large-Scale Circulations in the High Southern Latitudes

The Antarctic topography and attendant katabatic wind regime appear to play a key role in the climate of the high southern latitudes. During the nonsummer months, persistent and often times intense katabatic winds occur in the lowest few hundred meters of the Antarctic atmosphere. These slope flows transport significant amounts of cold air northward and thereby modify the horizontal pressure field over the high southern latitudes. Three-year seasonal cycle numerical simulations using the NCAR Community Climate Model Version 1 (CCM1) with and without representation of the Antarctic orography were performed to explore the role of the elevated terrain and drainage flows on the distribution and evolution of the horizontal pressure field. The katabatic wind regime is an important part of a clearly defined mean meridional circulation in the high southern latitudes. The position and intensity of the attendant sea level low pressure belt appears to be tied to the Antarctic orography. The seasonal movement of mass in the high southern latitudes is therefore constrained by the presence of the Antarctic ice sheet. The semiannual oscillation of pressure over Antarctica and the high southern latitutdes is well depicted in the CCM1 only when the Antarctic orography is included.

A numerical study of the diurnal variation of the Adelie Land katabatic wind regime

Katabatic winds occur with great frequency over the Adelie Land sector of Antarctica. Data collected by automatic weather stations (AWSs) along a string of stations from the coast of Adelie Land to the high interior plateau show a marked diurnal cycle of the wind and temperature regimes of the lower atmosphere during the short austral summer period. Numerical simulations of the katabatic wind regime have been conducted for a variety of solar forcing and synoptic conditions assuming a clear sky environment. Results suggest that the katabatic wind is a robust feature of the Antarctic boundary layer. Significant disruption of the low‐level drainage features occurs only during the midsummer months owing to the solar heating of the ice slopes and from synoptic activity along the Adelie Land coastline. During midsummer, modeled ground temperatures undergo a diurnal cycle with an amplitude of 15°C near the coast, in good agreement with the AWS observations. The katabatic wind decreases considerably during the middle of the day; wind speeds at coastal sites become reduced by more than one half of the early morning values. The numerical simulations depict a rapid summer to winter transition of katabatic wind conditions over the 1‐month period from mid‐February to mid‐March. The surface energy budget becomes negative for the entire 24‐hour period after mid‐February, corresponding to the rapid onset of the wintertime katabatic wind regime.

Intraseasonal variability of katabatic wind over east Antarctica and planetary flow regime in the southern hemisphere

The Intraseasonal variation (30‐ to 50‐day period) of katabatic wind has been found at Mizuho plateau, East Antarctica, as a predominant mode of the subseasonal time scale variability. The atmospheric circulation in the southern middle and polar latitudes associated with this fluctuation of katabatic wind has been deduced by using the NMC gridded 500‐mbar zonal wind and geopotential height field. The axisymmetric circulation pattern to Antarctica is noticeable in the anomalous zonal wind and height field. That is, the strong (weak) katabatic wind phase corresponds to the shallower and broadened (deepened and narrowed) tropospheric circumpolar vortex with weaker (stronger) westerlies over the surrounding oceans. These results strongly suggest that the intraseasonal variation of katabatic wind regime over East Antarctica is part of the modulation of planetary flow regime and meridional circulation in the southern middle and high latitudes, i.e., part of the index cycle of the southern hemisphere.

The influence of large-scale forcing on the katabatic wind regime at Ad�lie Land, Antarctica

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

On the Role of Antarctic Katabatic Winds in Forcing Large-Scale Tropospheric Motions

Abstract Katabatic winds are a dominant feature of the lower atmosphere over Antarctica. The radial diffluence displayed by the drainage flows implies that a continental-scale subsidence is present over Antarctica. From mass continuity considerations, a thermally direct meridional circulation must become established. The upper-level convergence above the Antarctic continent acting to feed the katabatic circulation generates cyclonic vorticity in the middle and upper troposphere. Model simulations show that a robust circumpolar circulation becomes established within a time scale of about a week. The adverse horizontal pressure gradients in the upper atmosphere result in a gradual decay of the low-level katabatic circulation. The katabatic wind regime appears to be an important forcing mechanism for the circumpolar vortex about the periphery of the Antarctic continent.

Continental-Scale Simulation of the Antarctic Katabatic Wind Regime

Abstract Katabatic winds are a common feature of the lower Antarctic atmosphere. Although these drainage flows are quite shallow, there is increasing evidence that the low-level circulations are an important component in establishing large-scale tropospheric motions in the high southern latitudes. Three-dimensional numerical simulations of the Antarctic katabatic wind regime and attendant tropospheric circulations have been conducted over the entire continent to depict the topographically forced drainage patterns in the near-surface layer of the atmosphere. Results of the simulation enable a mapping of katabatic wind potential and identification of coastal regions which may experience anomalously intense katabatic winds. A large upper-level cyclonic circulation forms rapidly in response to the evolving katabatic wind structure in the lower atmosphere, suggesting that the drainage circulations are an important component in prescribing the resulting resulting circumpolar vortex. These results imply that som...

The katabatic wind regime at Adelie Land, Antarctica

AbstractThe coastal sections of Adelie Land in East Antarctica experience the strongest and most persistent slope (katabatic) winds recorded about the continental periphery. The area was first explored during Mawson's 1911–14 Australasian Antarctic Expedition; the annual average surface wind speed at the base camp of Cape Denison was approximately 20 m s−1 during the course of the 2‐year stay in Antarctica. Field traverses conducted by the Mawson group and the subsequent establishment of additional bases and more recent deployment of automatic weather stations suggest that the zone of extreme katabatic winds is not confined to the Cape Denison site, but rather, extends several hundred kilometres inland from the coast and at least 60 km west along the coast. Numerical simulations of the Adelie Land katabatic wind regime have been conducted using a primitive equation three‐dimensional model. Results confirm the extreme wind conditions over the Adelie Land region and strongly suggest that the confluence of cold air drainage currents from the interior towards Adelie Land is responsible for the anomalous katabatic wind intensity.

The confluence zone of the intense katabatic winds at Terra Nova Bay, Antarctica, as derived from airborne sastrugi surveys and mesoscale numerical modeling

The surface wind field inland of the intense coastal katabatic wind regime at Terra Nova Bay, Antarctica, has been studied both observationally and numerically. Airborne surveys of wind‐induced features on the snow surface have been used to construct the time‐averaged winter surface airflow pattern. The surface motion field has also been simulated by a mesoscale primitive equation model using terrain slopes with a horizontal resolution of 32 km. Both methods of analysis demonstrate that the intense katabatic airstream at Terra Nova Bay is forced by converging air currents in the continental interior. The broadscale confluence zone becomes organized into two regions within about 180 km of the coast. The primary route for katabatic mass transport into the Terra Nova Bay area is Reeves Glacier valley, but an important secondary source is provided by airflow down David Glacier. The former is generated by the Coriolis‐induced concentration of mass transport on the left side (looking downwind) of the broadscale confluence zone as well as by the near‐coastal mountain deflection of airflow into the valley head. The confluence zone feeding into David Glacier valley stretches over 100 km into the interior and is forced by the broadscale terrain configuration of the ice sheet. Source areas for the two airstreams diner with Reeves Glacier valley being fed by cold air formed well into the interior, whereas the airflow down David Glacier is sustained by radiative cooling over lower parts of the ice sheet which are much closer to the Ross Sea. Evaluation of the observational and modeling results reveals both the strengths and weaknesses of each approach. Airborne surveys of sastrugi orientations are a highly successful method for establishing the detailed pattern of surface airflow. However, a systematic examination of sastrugi dimensions suggests that such work could also be carried out using SPOT‐type satellite observations, which is a more cost‐effective approach than aircraft surveying. The double‐jet structure of katabatic airflow into the Terra Nova Bay area is well represented by the primitive equation modeling, but much finer grids are needed to resolve the near‐coastal drainage details.

Instrumented Aircraft Observations of the Katabatic Wind Regime Near Terra Nova Bay

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 h...

An Extraordinary Katabatic Wind Regime at Terra Nova Bay, Antarctica

Abstract 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 ...