synoptic climatology and intrahemispheric associations of cold air mesocyclones in the australasian sector satellite ir images for seven months in 1992 are interpreted for cold air mesoscale cyclones mesocyclones occurring in the australasian sector 70 e 150 w of the southern ocean time averaged monthly seasonal distributions of mesocyclogenesis mesocyclolysis and tracks of movement along with statistical summaries of mesocyclone attributes e g cloud vortex size speeds of movement are presented and discussed in the context of the larger scale atmospheric circulation maximum frequencies of mesocyclones occurred in the transitional months of april and october 1992 with a secondary peak in july statistically significant differences in mesocyclone track length between months appear related dominantly to changes in speed of the background flow associated with the semiannual oscillation sao of tropospheric pressure height the associations of mesocyclone outbreaks with composite anomaly fields of pressure and height are identified for three subareas of the australasian sector suggested by the analysis of mesocyclone spatial patterns outbreaks occur in the strong southerly geostrophic airflow located between pressure and height anomalies that are negative positive to the eastward westward when outbreaks occurred in the new zealand subarea in 1992 a similarly strong couplet of pressure height anomalies developed in the southern south america antarctic peninsula sector but not when outbreaks occurred south of australia the mesocyclone remote association that is suggested is evaluated by using polar orbiter ir imagery for the southeastern pacific region frequencies of mesocyclones increase decrease west of chile but decrease increase through drake passage when mesocyclone outbreaks occur near new zealand south of australia these long distance associations of mesocyclone outbreaks are consistent with the connectivity of the baroclinic waves and might prove useful in the development of techniques to forecast mesocyclones over the southern ocean

mesoscale vortices in the weddell sea region antarctica abstract preliminary results of investigations of antarctic mesocyclones in the weddell sea region are presented for the antarctic summer periods 1983 88 based on noaa and meteor satellite images a total of 195 mesoscale vortices scale less than 1000 km were observed and classified using a simple classification scheme depending on lifetime and cortex diameter the highest frequency of vortices was found over the ice free part of the eastern weddell sea mean synoptic conditions for vortices with scales less than 400 km near halley station indicate an offshore flow being most favorable for their development a case study with satellite images surface measurements synoptic analyses derived from tovs data shows the evolution of several mesoscale vortices in a pool of cold air in the eastern weddell sea

a climatology of cyclogenesis for the southern hemisphere abstract this study surveys life cycle characteristics of cyclones in the southern hemisphere sh an objective technique for locating instances of cyclone formation intensification maturation and decay is applied to a 7 year dataset of sh cyclone positions obtained from an automated finding and tracking scheme based on twice daily ecmwf analyses during 1980 86 geostrophic vorticity g at 1000 hpa is used to locate cyclones and determine their intensity cyclones form and develop within middle latitudes and migrate eastward and poleward during their lives preferred genesis and cyclogenesis regions include eastern coasts of australia and south america in winter and leeward of the andes and near the baroclinic zones associated with the principal upper tropospheric jet streams year round over open water winter cyclones tend to form and intensify near zones of strongest sst gradient rapid cyclogenesis is most frequent east of south america southeast of africa south of australia and near new zeal

climatological aspects of mesoscale cyclogenesis over the ross sea and ross ice shelf regions of antarctica 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 a

hemispheric atmospheric variations and oceanographic impacts associated with katabatic surges across the ross ice shelf antarctica numerical simulations and surface based observations show that katabatic winds persistently converge toward and blow across the siple coast part of west antarctica onto the ross ice shelf about 14 of the time during winter april to august 1988 thermal infrared satellite images reveal the horizontal propagation of this negatively buoyant katabatic airstream for about 1000 km across the ice shelf to its northwestern edge a trajectory that nearly parallels the transantarctic mountains this takes place when the pressure field supports such airflow and is caused by synoptic scale cyclones that decay near and or over marie byrd land the northwestward propagation of the katabatic winds is accompanied by other changes in the hemispheric long wave pattern an upper level ridge develops over wilkes land resulting in an enhancement of the split jet in the pacific ocean then more frequent and or intensified synoptic scale cyclones are steered toward marie byrd land where they become nearly stationary to the northeast of the climatological location the resulting isobaric configuration accelerates the katabatic winds crossing siple coast and supports their horizontal propagation across the ross ice shelf an immediate impact of this katabatic airflow that crosses from the ice shelf to the ross sea is expansion of the persistent polynya that is present just to the east of ross island this polynya is a conspicuous feature on passive microwave images of antarctic sea ice and plays a central role in the salt budget of water masses over the ross sea continental shelf the impact of this katabatic airflow upon mesoscale cyclogenesis over the south pacific ocean is also discussed

an objective cyclone climatology for the southern hemisphere abstract an objective method is developed and used to derive a climatology of centers of cyclonic vorticity for the southern hemisphere based on twice daily european centre for medium range weather forecasts ecmwf 1000 hpa analyses during 1980 86 these centers were computed as local minima of geostrophic relative vorticity g extending previous studies based on pressure minima this use of g avoids a bias favoring slower and or deeper cyclones that occurs when pressure is used and includes a large number of additional mobile vorticity centers in the 45 55 s band that are missed where a local pressure minimum cannot be found an automated tracking algorithm similar to that of murray and simmonds is used to match predicted location pressure and g from previous track history with available centers large numbers of g centers were found south of 60 s and near the midlatitude continents as in previous studies these were a mixture of migratory centers and other fixed topographic features the maxi

distribution and characteristics of mesoscale cyclones in the antarctic ross sea eastward to the weddell sea 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

mesoscale cyclone activity over antarctica during 1991 2 near the antarctic peninsula a survey of mesoscale cyclogenesis near the antarctic peninsula has been carried out for 1991 based on digital satellite imagery collected at palmer station slightly higher cyclonic activity was found over the bellingshausen sea sector bss than in the weddell sea sector wss both sides were characterized by a pronounced formation maximum in summer and a minimum in winter comma clouds were the dominant vortex type and their average diameter was around 370 km trajectories show that many vortices in the bss move northeastward toward the drake passage with some of the most well developed moving north northeastward to the southern tip of south america bringing severe weather conditions to that area little systematic motion was noted over the wss the fraction of mesoscale cyclones with middle and or high cloud on the west side of the antarctic peninsula was much higher than on the east side these numerous and deep mesoscale cyclones are a consequence of the frequent outbreaks of cold air associated with synoptic scale cyclones that move cold air northward over the relatively warm bellingshausen sea with convective instability resulting this happens much less often in the wss where low level baroclinic instability and vortex stretching are the dominant formation mechanisms and air sea interaction is usually much less vigorous due to the stabilizing influence of the extended sea ice cover

cyclonically forced barrier winds along the transantarctic mountains near ross island 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

numerical simulation of winter katabatic winds from west antarctica crossing siple coast and the ross ice shelf 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 impa

simulations of topographically forced mesocyclones in the weddell sea and the ross sea region of antarctica abstract mesocyclones mcs are a frequently observed phenomenon in the coastal regions of antarctica numerical simulations of topographically forced mcs in the weddell sea and the ross sea region are presented using a three dimensional mesoscale weather forecast model simulations are performed for an idealized case without synoptic forcing three realistic cases of smaller mcs with diameters of 200 300 km and two larger systems with diameters of up to 1000 km the simulation results show that the orography of the coastal regions can play an important role in mesocyclogenesis one key factor is the katabatic wind system which is able to initiate low level mcs in areas of suitable orography structure the second key factor is the support of the synoptic environment leading to vorticity production by vertical stretching of the synoptically supported katabatic winds besides this stretching mechanism katabatic winds can have a second impact on the generation of mcs by transporting cold air into the coa

the tectonostatigraphic evolution of the huronian basement and the subsequent basin fill geological constraints on impact models of the sudbury event abstract whereas both meteorite impact and volcanic mechanisms have been proposed to explain the rocks and associated ore deposits within the sudbury structure 1 85 ga few models take into consideration the paleogeographic constraints implied by the history of the region before and following the sudbury event huronian depositional style south of the sudbury structure appears to have been strongly influenced by the early development of transtensional extensional basins these basins were initiated by sinistral strike slip movement along part of a paleoproterozoic continental margin which later evolved into a passive margin transition from a rifted strike slip margin to a passive margin was accompanied by major slope failure and soft sediment deformation south of the murray fault diabase dikes were intruded into unconsolidated sediments at considerable depth producing round stone breccias which are texturally similar to sudbury breccia minor compressional folding occurred prior to emplacement of the nipissing diabase suite at around 2 2 ga the abundance of diabase intrusions near the sudbury structure suggests that the area may have been a focal point for magma emplacement the post event fill of the paleoproterozoic sudbury basin reflects initial deposition in a deep water anoxic setting there is no evidence to suggest that sedimentation was influenced by the steep walls of an impact crater carbonate in the basal vermilion member is of sedimentary exhalative origin and was not derived from a shallow marine shelf turbidites in the chelmsford formation show no evidence of centripetal fill as might be expected from a restricted circular basin they appear to have been emplaced along the depositional axis of an elongate foreland basin developed in front of the rising penokean mountain chain penokean deformation continued after the sudbury event and influenced strata well into the archean craton deformation of huronian strata in paleovalleys north and west of the sudbury structure does not appear to be genetically related to the sudbury event if superfaults were generated around the sudbury structure in response to compression and crustal rebound following a meteorite impact these do not appear to have caused offset of pre existing steeply dipping contacts between the huronian and underlying archean basement a new model consistent with these observations needs to be developed

a climatology of southern hemisphere extratropical cyclones a climatology of extratropical cyclones determined by an objective automatic scheme applied to 15 years 1975 89 of once daily australian bureau of meteorology hemispheric analyses is presented contour maps of the positions of formation cyclogenesis dissipation cyclolysis together with other cyclone statistics are presented the distribution of cyclones through the hemisphere was found to be dominated by a permanent high latitude core coincident with the circumpolar trough during the winter and intermediate seasons two mid latitude branches are evident in the cyclone density originating in the tasman sea and south american sectors both spiraling poleward and merging with the circumpolar core in the southern oceans systems were observed to move in an castsouth east direction away from their location of formation exhibiting peak speeds of migration in the mid latitudes little seasonality was evident in the densitiy distribution of cyclones through the southern oceans but a considerable amount was found in their central pressure

simulation of the mesocyclonic activity in the ross sea antarctica the mesocyclonic activity in the southwestern ross sea is examined with emphasis on its forcing by katabatic winds the three dimensional version of the meso gamma scale primitive equation model modele atmospherique regional is used in which a representation of cloud microphysical processes has been introduced idealized boundary conditions are prescribed in particular the ocean is assumed to be completely ice free or partially ice covered the former case corresponds to a fall situation that coincides with the climatological maximum of estimated precipitation at mcmurdo station on ross island due to the propagation of katabatic winds over the ocean boundary layer fronts form clouds are generated in the fronts a surface pressure trough also forms and extends northeastward from terra nova bay mesocyclones are simulated in the trough when the ocean is ice free strong sensible heat fluxes toward the atmosphere amplify the strength of the trough up to typically observed values tn this case snow precipitation is associated with the boundary layer fronts and occurs in particular over mcmurdo sound it is also found that latent heat release due to precipitation formation in the fronts does not significantly affect the deepening of the trough

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

satellite observations of katabatic wind propagation for great distances across the ross ice shelf 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 propagat

mesoscale cyclogenesis dynamics over the southwestern ross sea antarctica 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

mesoscale cyclogenesis over the southwestern ross sea linked to strong katabatic winds 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

a mesoscale vortex over halley station antarctica abstract a detailed analysis of the evolution and structure of a mesoscale vortex and associated cloud comma that developed at the eastern edge of the weddell sea antarctica during the early part of january 1986 is presented the system remained quasi stationary for over three days close to the british research station halley 75 36 s 26 42 w and gave severe weather with gale force winds and prolonged snow the formation and development of the system were investigated using conventional surface and upper air meteorological observations taken at halley analyses from the u k meteorological office 15 level model and satellite imagery and sounder data from the tiros n noaaseries of polar orbiting satellites the thermal structure of the vortex was examined using atmospheric profiles derived from radiance measurements from the tiros operational vertical sounder details of the wind field were examined using cloud motion vectors derived from a sequence of advanced very high resolution radiometer images

the surface windfield over the antarctic ice sheets the intense radiative cooling of air over the ice slopes of antarctica generates a surface wind regime that is strongly controlled by topography and plays a key role in determining the behaviour of the atmosphere and ocean in high southern latitudes1 7 resultant surface winds are intimately linked to the orientation of the ice terrain fig 1 and display the highest degree of persistence found on earth the close coupling between wind and topography allows estimation of the former if the latter is known with some precision here we report on time averaged near surface airflow over the antarctic continent during winter diagnosed from a recent accurate synthesis of terrain slopes and from estimates of the lower atmospheric temperature structure the simulated drainage pat tern exhibits strong spatial variability with the airflow concentrated into several zones near the coastal margin these confluence regions are responsible for strong persistent katabatic winds over downstream coastal stretches and are indicative of zones of greatest katabatic potential