on the movement and low level structure of cold fronts abstract this paper presents a review of theoretical and observational studies relating to the low level structure of cold fronts and explores the factors that are pertinent to frontal motion observational studies have shown that in some cases surface cold fronts move at speeds faster than the normal component of the wind at all levels in the lower troposphere and therefore propagate other case studies have shown that the low level flow immediately behind the front and normal to it is faster than the front and that the front has the local structure of a gravity current its speed of movement being well determined by the gravity current speed equation them different types of behavior are related to results of recent theoretical studies and the mechanism by which fronts can propagate is elucidated it is shown that a necessary requirement for propagation is the existence of an alongfront temperature gradient we question the relevance of the gravity current speed equation in general despite its appar

surface features of winter monsoon surges over south china abstract the surface features associated with two kinds of winter monsoon surges over south china are studied the easterly surge es and the northerly surge ns surface meteorological parameters over the region 15 50 n 90 130 e for the surges that occurred in the three winters october march from 1988 to 1991 are analyzed for the northerly surge the surface features found are 1 an abrupt temperature drop and wind direction turning from easterly to northerly which can be related to the passage of a cold front 2 an increase in the dewpoint depression and 3 a large north south pressure gradient on the other hand the easterly surge is found to be associated with strong easterly winds up to approximately 40 km h 1 little temperature or pressure change and a southeastward motion of a high pressure center from dahinggangling to the yellow sea together with a sharp pressure ridge along the east china coast furthermore an es and an ns are associated with different perturbations anomalies in

the structure and evolution of cold surges east of the rocky mountains abstract northerly surges of cold air often move southward along the eastern side of the rockies from southern canada into mexico the strongest surges which generally develop in midwinter are associated with temperature decreases and pressure rises of 20 30 c and 15 30 mb respectively within 24 h surges are usually accompanied by a meridionally elongated pressure ridge and strong low level ageostrophic winds that parallel the terrain the width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters mexico this paper provides a detailed description of a northerly surge to the east of the rocky mountains that occurred on 12 14 november 1986 using both observational and model data the structural evolution of the surge is analyzed in addition the dynamics of the event is explored by diagnosing the momentum thermodynamic energy and vorticity equations to determine the typical synoptic scale evolution of these

offshore wind forcing in the gulf of tehuantepec mexico the asymmetric circulation since the early surveys carried out by the eastern tropical pacific eastropic and scripps tuna oceanographic research stor projects in the tropical pacific off mexico the northerly winds which blow over the gulf of tehuantepec were described as an important factor controlling the dynamics of this coastal ocean in january february 1989 an international team carried out the experiment tehuano designed to study the response of the gulf to these wind pulses the complete evolution of the coastal ocean after an event of moderate intensity was observed the forcing is characterized by a mostly symmetric fan shaped offshore wind jet which in turn produces a remarkably asymmetric upper ocean response while analytical results based on ekman theory forced by a symmetric offshore wind predict the formation of a symmetric dipolar circulation the observed flow consists mainly of a large 200 km in diameter anticyclonic warm core eddy in the western gulf with only a weak cyclonic counterpart in the eastern gulf intense surface cooling under the wind jet is caused by entrainment of subsurface water into the upper layer the thermocline in the west deepens with the development of the eddy which is formed initially by the advection of warm surface waters from west of the gulf east of the axis of the wind the mixed layer deepens due to wind induced entrainment while at the same time shoaling and compression of the deeper isotherms by curl induced upwelling ekman pumping strengthen the thermocline

central australian cold fronts abstract this paper presents an observational study of the structure and behavior of cold fronts over central australia during the late dry season a time of year when the prefrontal convectively well mixed layer is particularly deep the study is based on the results of the central australian fronts experiments cafe held in 1991 three fronts were documented in unprecedented detail for the australian region using a greatly enhanced surface observing network and a boundary layer wind profiler as well as serial upper air soundings data on the surface energy balance were obtained also a common feature of the fronts observed during cafe was that they were dry shallow 1 km deep and moved into a deep 4 km convectively well mixed boundary layer one of them initiated major dust storms across central australia a prominent feature of the fronts was the marked diurnal variation of their surface signature as they moved through the network noteworthy was the tendency during the night for the initiatio

the tectonic development of the western margin of the gulf of elat aqaba rift abstract along the western coast of the gulf of elat a 30 km wide shear belt of subparallel faults trending n s to ne sw is developed this shear belt is observed mainly within the precambrian basement terrain sinistral movements on these faults have been recognized based on offsets of magmatic bodies and lithological contacts in rocks of precambrian age the cumulative displacement measured independently at several localities across the belt attains a total of 24 km early neogene volcanism in the form of long dikes trending nw se is known from the eastern desert of egypt of the gulf of suez and along the red sea coast of saudi arabia volcanic bodies of a similar pattern have been identified in eastern sinai these bodies mainly dolerites and basalts intruding mesozoic sediments are parallel to each other and to the gulf of suez they are tens to hundreds of kilometers long and spaced several tens of kilometers apart dating k ar of bodies in sinai saudi arabia and in the eastern desert of egypt suggest an age of 20 22 m y for this extensive volcanism all the dikes are sinistrally displaced by the individual faults of the eastern sinai shear belt the amount of movement recorded using the precambrian markers is identical to movement recorded by offsets of the early neogene dikes this proves that the total 24 km shear of this fault system postdates the 20 22 m y volcanic phase it is assumed that the recorded movements are part of the 105 110 km shear persumed to exist along the gulf of elat which is also younger than 20 22 m y preliminary results suggest that a similar amount of movement is observable along a shear zone developed on the eastern coast of the gulf of elat thus only some 60 km movement are taken up by faults in the gulf itself the evolving model suggests that 1 a the intrusion of very long basaltic dikes in a nw se direction on both sides of the red sea accompanied the initial stage of development of the red sea gulf of suez rift valley 2 b a basic change in the geotectonic regime occurs after the intrusion of this volcanic suite large scale horizontal movements are initiated being recorded as a 105 110 km sinistral movement along the arava jordan rift this movement implies a synchronous opening of the red sea

isthmus of tehuantepec wind climatology and enso signal abstract the statistical characteristics of the winds at the isthmus of tehuantepec and their seasonal and interannual variability are studied through the analysis of several datasets and a reconstruction of the winds for a 31 yr period observations show that the long term monthly mean wind speeds and frequency of occurrence of northerly winds have a strong seasonal signal with maximum values during december january minimum during may june and a relative maximum in july the frequency distribution of wind speed is bimodal a feature that is closely related to the wind direction with northerly winds being stronger based on these results and the close relationship between the across isthmus pressure differences and the local winds a statistical model is developed to get a reconstruction of 12 hourly winds through the isthmus of tehuantepec for 1964 95 the model reproduces fairly well the main characteristics of the observed winds the bimodal distribution of the wind speed and the seasonal signal in

cold surges in tropical and extratropical south america the strong event in june 1994 the authors investigate one case of surges of polar air that occasionally propagate into southeastern brazil during wintertime and are harmful to coffee production because of the freezing conditions associated with them the cooling is also observed in southern and with less intensity in western amazonia the event of 26 june 1994 is studied the frost event caused a sharp drop in coffee production and similarly dramatic increases in coffee prices the event was characterized by the presence of an anticyclonic perturbation off the coast of chile that enters south america and crosses the andes mountains south of 458s hours later the anticyclone center tracks equatorward until it reaches 208 308s and then shifts eastward toward the atlantic based on a diagnostic analysis using the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis a two stage process can be distinguished in the starting period midtropospheric troughing is established east of the andes over central argentina due to stretching and or vorticity advection this troughing is responsible for cold advection along the eastern flank of the andes which produces substantial temperature drops and deepens the upper trough locally the increase in the magnitude of the zonal temperature gradients east of the cold advection zone at levels near 700 hpa is another result of the cooling process this feature would ensure that the cold advection be extended eastward producing an upper level trough local deepening there this intensification would produce an increase in the cyclonic vorticity advection which would tend to produce sea level pressure drops underneath this near surface low pressure area would eventually contribute to the southeastern brazil sb cooling due to the associated southerly winds another feature is the near surface local anticyclonic generation due to midtropospheric cold advection associated with descending motions low level divergence and anticyclonic growth eventually this high pressure would also contribute to produce southwesterlies to transport cold air to sb the paper is divided in two sections the first part is a study of synoptic and climatic aspects of the cold surge episodes by using daily surface climatic observations the purpose of the second part is to analyze the dynamic aspects of this cold episode and to study the cooling mechanisms by using the four times daily surface and upper air ncep ncar reanalysis as well as to look for possible predictors

magnetostratigraphy of the ludlow member of the fort union formation lower paleocene in the williston basin north dakota to determine the geomagnetic polarity stratigraphy and the duration and age of the ludlow member of the fort union formation lower paleocene we constructed a 325 m composite lithostratigraphic section of the upper cretaceous hell creek formation and the lower paleocene ludlow and tongue river members of the fort union formation in the little missouri river valley of north dakota usa we analyzed paleomagnetic samples from nine of the logged sections the principal magnetic carrier in the ludlow member sediments is likely titanomaghemite as indicated by predominantly irreversible thermomagnetic curves measured from sandstone siltstone and carbonaceous shale samples the analyzed paleomagnetic samples document a series of polarity zones that can be correlated from c29n to c27r on the geomagnetic polarity time scale gpts we infer that the magnetization of the samples is primary because the characteristic directions are consistent with those of the paleocene of north america and the reversal stratigraphy from this section corresponds to the gpts with reasonable sediment accumulation rates by extrapolating the measured sediment accumulation rate from the cretaceous tertiary k t boundary to the top of c28n and then to the top of the ludlow member we estimate the duration of the member to range from 2 31 to 2 61 m y this is the fi rst estimate for the duration and age of the ludlow member and it can be used as an important tool for interpreting rates of biotic recovery after the k t extinction

variability in the mineral magnetic properties of soils and sediments within a single field in the cape fold mountains south africa implications for sediment source tracing abstract when tracing the sources of sediment reference to potential source groups is a necessity however the range of uncertainty present in source fingerprinting outputs has been shown to be increased by a high within source group variability the environmental factors controlling the magnetic properties of samples within a single field were examined on a particle size specific basis to continue building on the recent studies of the spatial variability associated with different tracer types the study area was in the eastern cape of south africa over a quartzite geology and was covered by rough grassland topsoils and gully walls were intensively sampled within the study field further samples were collected from weathering bedrock exposures burnt soil and wetland soil in order to explore potential controlling factors on source magnetism each sample was fractionated to 125 63 m 63 32 m 32 25 m 25 10 m and topographic position was a major controlling factor on soil magnetism with saturated lowland gully walls and wetland topsoils being less magnetic than dry topsoils especially within the little difference was found between the low frequency magnetic susceptibility of quartzite topsoils and the shale and sandstone topsoils found elsewhere in the eastern cape however igneous sources such as dolerite are far more magnetic than the quartzite topsoils discrimination between sediment sources can be highly variable within the

winter monsoon convection in the vicinity of north borneo part i structure and time variation of the clouds and precipitation abstract radar and satellite observations in the vicinity of northern borneo obtained during the international winter monsoon experiment wmonex showed that the convection in that region underwent an extremely regular diurnal cycle over the sea to the north of borneo the general level of convective activity was increased during monsoon surges and during the passages of westward propagating near equatorial disturbances convective activity was decreased during monsoon lulls the diurnal cycle was well defined regardless of whether the general level of convective activity was enhanced or suppressed by synoptic scale events the cycle of convection over the sea was especially well documented it was typically initiated at about midnight when an offshore low level wind began where this wind met the monsoonal northeasterly flow usually just off the coast convective cells formed after midnight the convection continued to develop and by 0800 lst it had evolved into an organized mesoscale system with a

airborne doppler radar and sounding analysis of an oceanic cold front abstract an analysis of a cold front over the eastern atlantic ocean based on airborne doppler wind syntheses and dropsonde data is presented the focus and unique aspect of this study is a segment of the front that was near the center of the cyclone the dual doppler wind synthesis of the frontal zone combined with an average dropsonde spacing of 30 km covers a total distance of 450 km in the cross frontal direction the finescale resolution and areal coverage of the dataset are believed to be unprecedented the cold front was characterized by a distinct wind shift and a strong horizontal temperature gradient the latter was most intense aloft and not at the surface in contrast to the classical paradigm of surface cold fronts the shear of the alongfront component of the wind was relatively uniform as a function of height within the frontal zone this observation is contrary to studies suggesting that frontal zones decrease in intensity above the surface the surface convergence within the frontal zon

magnetic polarity stratigraphy and biostratigraphy of paleocene and lower eocene continental deposits clark s fork basin wyoming paleomagnetic samples were collected at 273 sites within a 2012 m continental sedimentary sequence in the clark s fork basin near powell wyoming the lower 1158 m is in the polecat bench fm while the upper 854 m is within the willwood fm although significant secondary components of magnetization were present alternating field demagnetization to 300 oe peak field revealed the polarity of the primary component of the natural remanent magnetism a well defined polarity zonation is observed the polecat bench fm is dominantly of reversed polarity but does contain two normal polarity zones the willwood fm in this section is entirely of reversed polarity abundant tiffanian clarkforkian and wasatchian faunas indicate that the age range of the sediments is from late paleocene through early eocene these age constraints along with the characteristic polarity sequence allow the magnetic polarity sequence to be correlated with the magnetic polarity time scale the section correlates with the magnetic polari

high resolution paleomagnetic records from holocene sediments from the palmer deep western antartic peninsula thick holocene sedimentary sections 45 m cored in the palmer deep by the united states antarctic program usap and during ocean drilling program odp leg 178 provide the first opportunity to examine past geomagnetic field behavior at high southern latitudes after removal of a low coercivity drilling overprint the sediments display a stable single component remanent magnetization two short cores that recovered the uppermost 2 6 m of sediment have inclinations that fluctuate about the present day inclination 57 measured at faraday station and several features with wavelengths of 10 to 20 cm appear to be correlative however shipboard measurements of inclination fluctuations on split core samples from three holes drilled at odp site 1098 do not correlate well with each other even though the intensity and susceptibility data correlate very well and the overall mean inclination for cores from each hole is consistent with the expected geocentric axial dipole gad inclination the correlation is improved dramatically by using inclinations measured on u channels taken from the pristine center of a split core consequently the anomalous directions and the resulting poor between hole correlation of inclinations obtained from shipboard data can be attributed to coring induced deformation which is common on the outer edge of odp piston cores and or measurement artifacts in the split core data our preferred inclination record is thus derived from u channel results the upper 25 m represents continuous sedimentation over the past 9000 yr with an average sedimentation rate exceeding 250 cm kyr 0 25 cm yr given that remanence measurements on u channels average over an interval 7 cm long we obtained independent measurements of the paleo geomagnetic field that average over only 30 yr this high resolution record is characterized by an inclination that fluctuates within 15 of the current gad inclination

gap winds in a fjord part i observations and numerical simulation abstract gap winds in howe sound british columbia are described and placed in context by reviewing studies of similar phenomena in other locations an observational program consisting of a surface mesonetwork and vertical soundings shows that gap winds vary considerably along and across the channel as well as vertically wind strength generally increases down channel and strongest winds are found below 1000 m depth results from application of a 3d mesoscale numerical model to a gap wind case compare reasonably well with observations model output reveals more details of horizontal and especially vertical flow structure than is possible from observations model vertical cross sections and froude number output indicate similarity with hydraulic flow this is further substantiated by a force balance analysis of model output

the ohio valley wave merger cyclogenesis event of 25 26 january 1978 part ii diagnosis using quasigeostrophic potential vorticity inversion abstract the dynamical interactions between precursor disturbances during the wave merger cyclogenesis event of 25 26 january 1978 over eastern north america are diagnosed using quasigeostrophic potential vorticity qgpv inversion this case is characterized by two prominent preexisting upper level disturbances that induce rapid surface cyclogenesis as they come into close proximity static qgpv inversion is used to attribute a particular geopotential height field to the qgpv associated with each precursor disturbance the full flow is partitioned into the following components the northern upper precursor the southern upper precursor and the background flow prognostic qgpv inversion is used to quantify the instantaneous geopotential height tendencies attributable to each of these flow components the static inversion results for the upper precursors exhibit the structure of baroclinic vortices with maximum amplitude near the tropopause during the 48 h period spanning the period of study of this even

winds in the strait of gibraltar summary automated meteorological observations were made from october 1985 to september 1986 in the narrows of the strait of gibraltar additional surface and upper air measurements were made in the strait from may to july 1986 the main wind events in summer were mostly mesoscale levanters winds from the east through the strait and a lesser number of synoptic scale levanters and westerlies during mesoscale levanters the wind speed increased and the pressure decreased from gibraltar downstream through the narrows as far as tangier a mesoscale low and the highest winds were found over the axis of the strait north of tangier high velocity 20 m s winds occurred only under 1 km height the air temperature profile became progressively warmer moving from gibraltar towards tangier a weak air temperature inversion decreased in height to the west of the narrows venturi flow as an explanation for the downstream increase of the low level winds during a levanter is inconsistent with observations a new model is proposed to explain the wind field associated with levanter winds in the strait that includes some orographic blocking and low level convergence into the narrows some west side divergence due to drag over water and leeside heating that generates a leeside mesoscale low pressure the surface low accelerates the surface winds to their maximum strength well downstream of the narrows a similar flow regime has been observed recently in the sea level channel between hawaii and maui

a windstorm in the lee of a gap in a coastal mountain barrier abstract this paper describes a localized windstorm that struck some areas of northwest washington state on 28 december 1990 with winds exceeding 45 m s 1 resulting in extensive property damage treefalls and power outages arctic air originating within the interior of british columbia descended into a mesoscale gap in the coast cascade mountains and then accelerated ageostrophically to the west this gap acceleration is explained quantitatively by a three way balance among the pressure gradient force friction and inertia the flow maintained its integrity as a narrow current of high speed air as it exited the gap and subsequently accelerated over water troughing in the lee of the cascade mountains enhanced the horizontal pressure gradient over northwest washington this pressure gradient approximately balanced frictional drag resulting in only minimal acceleration farther south the flow decelerated as the current spread out horizontally

factors affecting cold air outbreaks east of the rocky mountains abstract a two layer isentropic model is used to examine the parameters controlling the severity of cold air outbreaks east of the rocky mountains and in particular the channeling effect of the mountain range the case that is scrutinized is the cold air outbreak of early february 1989 the parameters investigated include a the baro clinicity expressed as the ratio of upper to lower layer zonal scale velocities b the rossby number c the height of the topography d the initial longitudinal position of the cold air mass relative to the mountain ridge and e the presence of a wave in the upper level flow it is found that the rossby number determines the intensity of a cold air outbreak whereas the baroclinicity as well as the height of the orography and the initial position of the cold air mass control the extent to which the outbreak is channeled by the mountain range an upper level trough over the central united states favors the evolution of an outbreak

thermally activated viscous remanence in some magnetite and hematite bearing dolomites a brunhes age viscous remanent magnetization vrm carried by magnetite and hematite in the el paso and montoya dolomite formations has unblocking temperatures significantly higher than those predicted by single domain theory the samples appear to contain a substantial fraction of pseudo single domain magnetite suggesting transdomain processes may be responsible for the stable vrm a strong inverse correlation between the occurrence of high unblocking temperatures and high isothermal remanence fractions acquired above 0 3 tesla suggests that the hematite grain size may also be important in determining the range of observed temperatures larger grains are inferred to have lower coercivities but higher unblocking temperatures an older reverse polarity component is also observed and appears to be a record of late cretaceous tertiary tectonic activity

large scale antecedent conditions associated with the 12 14 march 1993 cyclone superstorm 93 over eastern north america abstract the results of a multiscale analysis of the 12 14 march 1993 superstonn ss93 over eastern north america are presented a time sequence of overlapping 10 day time mean 5oo hpa geopotential height and anomaly composites shows that the northern hemisphere nh flow pattern from 18 february to 15 march 1993 is characterized by 1 three persistent troughs situated over eastern asia and the northwestern pacific over eastern north america and over northwestern africa and southwestern europe eastward to central russia and 2 a massive blocking anticyclone located over the central and eastern atlantic beginning 8 9 march 1993 the planetary scale flow amplifies substantially the explosive ss93 cyclogenesis and the transport of cold air to very low latitudes occurs a few days later as the nh available potential energy content after peaking on 9 march 1993 decreases by about 6 7 a dynamical tropopause analysis is used to track coherent transient potential vorticity pv anomalies and show their q

paleomagnetism of the roskruge and gringo gulch volcanics southeast arizona paleomagnetic data were obtained from nine cooling units of the late cretaceous 72 ma roskruge volcanics and from 25 flows in the lower paleocene gringo gulch volcanics both from southeastern arizona alternating field demagnetization successfully erased the infrequent secondary components of magnetization the paleomagnetic pole position obtained from the roskruge volcanics is at 73 6 n 176 0 e with dp 6 2 and dm 8 8 the gringo gulch volcanics pole position is at 77 0 n 201 0 e with dp 1 2 and dm 1 7 in conjunction with other recently published paleocene and eocene paleomagnetic poles these data provide details of north american apparent polar wander during the laramide orogeny 80 to 40 ma an episode of rapid apparent polar wander from the cretaceous pole position in the bering strait to the eocene pole position near the present rotation axis occurred during that interval of time the initiation of this episode of apparent polar wander appears to be coincident with the major plate reorganizations which occurred at the onset of the laramide orogeny

importance of titanohematite in detrital remanent magnetizations of strata spanning the cretaceous paleogene boundary hell creek region montana intermediate composition titanohematite fe2 ytiyo3 with 0 5 y 0 7 is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature while largely overlooked in magnetostratigraphic studies intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of late mesozoic and early cenozoic laramide clastic deposits throughout the central united states intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite except its unique self reversing property due to these similarities and with detrital remanent magnetizations masking its self reversing nature intermediate titanohematite is often misidentified in sedimentary deposits past studies relied upon nonmagnetic techniques including x ray diffraction and electron microprobe analysis while these techniques can identify the presence of intermediate titanohematite they fail to test whether the mineral is the primary recorder to facilitate the identification of intermediate titanohematite in sedimentary deposits we characterize this mineral using low temperature magnetometry and high temperature susceptibility experiments and present a new identification technique based on titanohematite s self reversing property for sediments that span the cretaceous paleogene boundary hell creek region montana results from the self reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field but that intermediate self reversing titanohematite y 0 53 0 63 is an important ancillary carrier of remanence while earlier literature suggests that intermediate titanohematite is rare in nature reanalysis using specialized rock magnetic techniques may reveal that it is more abundant in the rock record particularly within depositional basins adjacent to calc alkaline volcanics than previously thought

inertial wind path and sea surface temperature patterns near the gulf of tehuantepec and gulf of papagayo during the northern hemisphere winter the surface atmospheric pressure on the gulf of mexico side of central america is often higher than that on the pacific side the resultant southward pressure gradient drives strong southward winds through a gap in the sierra madre mountain range at the isthmus of tehuantepec the atmospheric jet is narrow in comparison with the relevant radius of deformation so when it leaves the coast and flows out over the gulf of tehuantepec it is an inertial jet therefore the jet is expected to be deflected into a clockwise inertial circle by the coriolis force a similar process should also occur for central american atmospheric jets through mountain range gaps near the gulf of papagayo satellite measurements of sea surface temperature together with coastal wind data for the first 41 days of 1986 were examined to see if the expected wind path influenced sea surface temperature sst near the gulf of tehuantepec and the gulf of papagayo consecutive daily satellite images when the coastal southward tehuantepec wind increased suggest that the initial development of cold surface water in a clockwise loop is due to the wind mixing the shallow thermocline surface water along its inertial path a simple 1 layer analytical ocean model suggests in agreement with observations that the wind can drive surface quasi geostrophic currents of 1 m s 1 since the wind path is circular the wind actually generates a clockwise rotating ocean eddy with the approximate scale of the wind inertial circle the satellite sea surface temperature measurements also suggest that the wind driven surface ocean current is unstable in addition stronger gradients in sst to the right of the expected wind flow are consistent with advection of sea surface temperature by ocean surface ekman flow

a diagnostic analysis of the superstorm of march 1993 abstract in this study the synoptic evolution of the march 1993 superstorm is documented using conventional observations and the predictability and deepening mechanisms of the storm are investigated using a newly implemented mesoscale version of the canadian regional finite element rfe mode it is found that although the rfe model can predict well the large scale background flow associated with the storm it fails to predict various important mesoscale elements when it is initialized with fields that contain weak signals of the low level circulations however when the storm is located close to the data rich region the model has considerable skill in predicting those mesoscale elements such as a prefrontal squall line upper and low level jets as well as the quantitative aspects of their associated precipitation observational analysis reveals that the storm developed in a convectively unstable prestorm environment with ample moisture content it first experienced an antecedent surface vorticity g

a 2400 yr mesoamerican rainfall reconstruction links climate and cultural change droughts are a recurring feature of mexican climate but few high resolution data are available to test for climate change forcing of mesoamerican civilizations we present a quantitative 2400 yr rainfall reconstruction for the basin of mexico from a precisely dated and highly resolved speleothem that documents highly variable rainfall over the past 2400 yr dry conditions peaked during a 150 yr long late classic ca 600 900 ce common era mega drought that culminated at 770 ce which followed centuries of climatic drying that spanned the fall of the city of teotihuacan ca 550 ce the wettest conditions in the 1450s ce were associated with fl ooding in the basin of mexico our data suggest that rainfall variability was likely forced by the el nino southern oscillation and impacts on spring fed irrigation agriculture may have been a stressor on mesoamerican civilizations

the structure and evolution of a simulated midlatitude cyclone over land abstract using output from a mesoscale model simulation this paper describes the evolution of the three dimensional temperature and humidity structures of an intense cyclone that developed over the eastern half of the united states during 14 16 december 1987 some specific findings include the following the occlusion process at the surface appeared similar to the classical paradigm with the cold front catching up with the warm front aloft a warm type occlusion structure formed as an upper baroclinic zone merged with the surface based occluded front the frontogenesis associated with the cold and warm fronts was initially continuous but as the system evolved a break in the frontogenesis along the northern section of the cold front developed a single frontogenesis feature appeared to support both the warm and occluded fronts a quasi linear region of convective activity was observed in advance of the surface cold front this convective line was associated with an upper level humidity front wh

australian southerly busters part i analysis of a numerically simulated case study abstract a diagnostic study of the australian southerly buster is performed using a numerical weather prediction model it is demonstrated that the model run at 50 km resolution is capable of capturing the transition of a cold front into a southerly buster as it traverses the cut coast of the australian continent the main features of the modified front am identified through comparison of the simulation with an adiabatic simulation from which precipitation and surface processes including orography have been removed from this comparison the southerly buster effect is defined in terms of the deformation of the front from its adiabatic ostentation and in terms of enhancement of temperature gradients and southerly winds along the coastal strip sensitivity experiments are then analyzed with a view to objectively determining the relative roles of mountains precipitation and land sea contrasts of heating and friction in bringing about these perturbations the major conclusions are that i the primary e

appalachian cold air damming abstract appalachian cold air damming is investigated by means of 1 a 50 yr monthly climatology 2 a synoptic case study of the event of 21 23 march 1985 and 3 an investigation of the flow structure and force balance within the cold dome the climatology reveals cold air damming is a year round phenomenon in the southern appalachians with the most frequent prolonged and intense events occurring in winter particularly december and march when three five events per month can be expected cold air damming is least frequent and intense in july the synoptic case study reveals that cold air damming is critically dependent upon the configuration of the synoptic scale flow the cold dome can be identified by a u shaped ridge trough in the sea level isobar thermal patterns and the 930 mb height temperature fields representative of conditions at the base of the inversion overlying the cold dome differential horizontal and vertical thermal advection as well as adiabatic and evaporative cooling are r

doppler radar observation of a land breeze cold front abstract during the early morning hours of ii january 1981 a very narrow intense band of snow which had formed along the leading edge of a land breeze cold front was viewed by cp 4 radar near muskegon michigan the front formed well inland and moved through the area from the northeast shortly after dawn the driving force for the land breeze was the large temperature contrast that existed between radiationally cooled air over the northern highlands of lower michigan and the relatively warm lake modified air near the shore of lake michigan the possibility that the observed snowband was associated with a mesoscale feature other than the land breeze is investigated an analysis of minimum temperatures in the great lakes region for 11 january 1981 shows that a strong temperature gradient existed over western lower michigan this gradient reached a maximum value well inland rather than at the lake shore radar observations show the band to be over 80 km long and only 4 km wide oriented roughly parallel t

observations and modeling of a mesoscale cold surge during wispit a transient feature marked primarily by a sudden increase in wind speed and brief period of heavy snow was observed over northeastern colorado on 11 12 march 1993 little if any abrupt changes in surface pressure or temperature were noted such as might accompany a gravity current gradual temperature falls and pressure rises were observed behind the velocity surge however there was no cyclonic wind shift nor did the feature last for more than a few hours suggesting that it was not likely a classical front data indicate that the surge originated near the peak of the cheyenne ridge a 400 m east west elongated hill near the border between colorado and wyoming simulations using an idealized two dimensional isentropic model show that a transient resembling observed surge results from impulsively started flow over heated terrain heating creates convergence that is in phase with the velocity surge at the leading edge of the topographic wave the result is a coherent disturbance in velocity which moves downstream at nearly the mean flow speed simulations with the pennsylvania state university national center for atmospheric research nonhydrostatic model mm5 using a horizontal resolution of 6 7 km capture the evolution of the observed surge these simulations support the identification of the surge as a topographic wave modified by heating but they also indicate an important role played by latent heating in the updraft at the leading edge of the surge because the latent heating occurs mainly downstream from the cheyenne ridge the formation of the topographic wave is unaltered by it however the updraft at the leading edge of the surge it greatly intensified leading to a narrow propagating band of heavy snow

the strait surface outflow a three dimensional primitive equation model is used to simulate the flow through a narrow strait the flow within a strait has a simple two layered structure with the interface located approximately at the middepth however the flow experiences a sharp transition at the strait s exit the flow jumps from a subcritical velocity inside the strait to a supercritical velocity immediately outside the strait correspondingly the density interface also slopes sharply upward to the surface after exiting the strait the surface outflow tends to form an anticyclonic gyre to the right of the strait s exit most of the outflow water is trapped within the gyre although some manages to escape through a narrow coastal current these model results compare well with observations in the tiran strait and in the gibraltar strait and alboran sea the model study indicates that the flow transition at the strait s exit and the formation of an anticyclonic gyre are unique features of the nonlinear self advection of the density current

australian southerly busters part ii the dynamical structure of the orographically abstract a methodology is presented for comparing nonlinear 3d model output to the physical principles embodied in idealized and analytical models through the use of the transformation of wind components v vg va to accelerations k f fk v p dv dt this methodology is applied to numerical simulations of the interaction of a cold front with coastal topography leading to the formation of a southerly busier current along australia s east coast it is found that while the front is moving along the south coast mountain chain the dynamics of the perturbation are similar in character to an orographically trapped density current type perturbation on the background front along the east coast however the southerly busier current bears no dynamical relationship to either an orographically trapped density current or a kelvin wave the balance of forces here being dominated by inertial terms

sources and accumulation rates of organic carbon in an equatorial peat bog burundi east africa during the holocene carbon isotope constraints the relationship between organic carbon accumulation rates and 13c 12c ratios of total organic carbon toc was investigated in an highland peat bog core ru 3 from equatorial africa this core yielded a sequence spanning the last 14 kyr and was analysed with a 100 300 yr resolution for toc 13c values the holocene section shows contrasted toc accumulation regimes and toc 13c varying between 28 5 and 19 5 with a few very short isotopic excursions dated at ca 9 3 7 5 4 2 ka b p the organic carbon accumulation rates range from 2 to 20 mg c cm 2 yr 1 they increase when toc becomes more depleted in 13c notably between 12 and 9 8 ka b p 8 5 and 7 8 ka b p and after 1 6 ka b p periods of restricted carbon storage correspond to heavier toc accumulation at 9 3 and between 7 5 and 1 6 ka b p at the study site the variations can be related to variable c4 plant inputs and possibly to changes in the fractionation between co2 and the organic carbon in c3 vascular plants the ru 3 record indicates restricted carbon storage during the periods of increased contribution from c4 plants and or of decreased fractionation between co2 and organic carbon in c3 plants changes in toc 13c values in core ru 3 seem to match fluctuations of east equatorial african lakes high lake stands correspond to low 13c intervals and vice versa this points to indirect climatic forcing of 13c changes in intertropical peats

the end of the cretaceous sharp boundary or gradual transition evidence indicates that the cretaceous tertiary boundary is very sharp and within the limits of resolution it is apparently synchronous at the various boundary localities arguments to the contrary particularly those of officer and darke are shown to invalid

a multi proxy study of argentina loess marine oxygen isotope stage 4 and 5 environmental record from pedogenic hematite abstract a 12 m loess paleosol section la angostura from tafi del valle tucuman argentina was studied using rock magnetic measurements geochemistry mossbauer effect particle size and nd isotopes in order to elucidate changes in paleoenvironment at the site combining the results of these experiments allowed us to separate the effects of the two major controls parent material and pedogenesis on the measured parameters two behavior regimes were identified in the sediment 1 in the lower half of the section pedogenic processes though weak dominate the variation of the sediment s physical properties and 2 for the upper half of the section subtle changes in parent material control the majority of the measured parameters even those typically controlled by pedogenesis e g clay fraction applying rock magnetic mossbauer and bulk geochemistry measurements in concert we calculated an iron budget for the loess 80 of the iron in the sediment is in paramagnetic minerals 20 of the iron is in hematite 1 6 of the sediment s mass and 1 of the total iron is present as magnetite maghemite 0 1 of the sediment s total mass neither the pedogenic formation nor the depletion of ferrimagnetic material with increasing pedogenesis was consistently observed in this sediment throughout the entire section the concentration of hematite is argued to be sensitive to in situ alteration and it is further surmised to be sensitive to the available moisture at the site finally we present a curiosity within the error of our chronology the concentration of hematite seems to correlate with the coeval climate record from antarctica vostok it is possible that the sedimentary magnetic record from la angostura is a proxy for the strength of the south american summer monsoon

gap winds in the strait of juan de fuca abstract gap winds can be defined as a flow of air in a sea level channel which accelerates under the influence of a pressure gradient parallel to the axis of the channel in february 1980 two distinct cases of gap winds were observed in the strait of juan de fuca between western washington state and british columbia during a study that measured spatial variation of low level marine winds and other parameters from the noaa p 3 research aircraft and a dense network of surface stations which included eight meteorological buoys these two cases were a high pressure region over central british columbia and a low pressure system propagating northward seaward of the washington coast both cases produced strong easterly winds of 13 15 m s 1 at the western end of the strait of juan de fuea the high pressure region provided a drainage air mass from the interior of british columbia which flowed through the straits of georgia and juan de fuca and eventually into the pacific ocean this air mass remained nearly hom

magnetic ilmenite hematite detritus in mesozoic tertiary placer and sandstone hosted uranium deposits of the rocky mountains mesozoic and tertiary sandstones throughout the rocky mountains region of the united states contain a de trital fe ti oxide mineral that is generally regarded as unusual and is preserved wherever these sandstones have escaped destructive diagenesis this mineral a medial ilmenite hematite solid solution is highly magnetic and has sometimes been mistaken for magnetite we have encountered this mineral in paleomagnetic studies and in investigations of sandstone hosted uranium deposits and cretaceous shoreline placer deposits it is known to occur elsewhere in intermediate volcanic rocks consistent with quenching from hypersolvus high temperature conditions the detrital occurrences we describe are of economic importance in uranium deposits where they are among the reductants that precipitate uranium and in cretaceous placer deposits whose exploitation would require a separation procedure tailored to this mineral the distribution of detrital hematite ilmenite sandstones in cretaceous rocks suggests a contemporaneous volcanic source in the northern rocky mountains

the ohio valley wave merger cyclogenesis event of 25 26 january 1978 part i multiscale case study abstract the long standing observational view of cyclogenesis involves an interaction between tropopause and surface based finite amplitude disturbances there are however instances where more than one upper level disturbance contributes to low level development a subset of these events involves wave or trough merger which has been defined as the amalgamation of two or more distinct 500 hpa vorticity maxima an example of this phenomenon involving a case of very intense continental cyclogenesis 25 26 january 1978 over the eastern united states is selected to elucidate the origin and evolution of precursor disturbances from the planetary scale to the mesoscale the analysis of this event reveals that the two well defined tropopause based disturbances that contribute to cyclogenesis have distinctly different origins and are brought together by confluent planetary scale flow one of these disturbances originates over the western north pacific ocean 10 days prior to cyclogenesis and tracks eastward

the march 1993 superstorm cyclogenesis incipient phase synoptic and convective scale flow interaction and model performance abstract the incipient stages of the 12 14 march 1993 superstorm ss93 cyclogenesis over the gulf of mexico are examined noteworthy aspects of ss93 include 1 it is the deepest extratropical cyclone ever observed over the gulf of mexico during the 1957 96 period and 2 existing operational prediction models performed poorly in simulating the incipient cyclogenesis over the northwestern gulf of mexico a dynamic tropopause dt analysis shows that ss93 is triggered by a potent potential vorticity pv anomaly as it crosses extreme northern mexico and approaches the gulf of mexico the low level environment over the western gulf of mexico is warmed moistened and destabilized by a persistent southerly flow ahead of the approaching pv anomaly ascent and a lowering of the dt associated with a lowering of the potential temperature ahead of the pv anomaly contributes to further destabilization that is realized in the form of a massive convective outbreak an examination of the national centers for envi

a technique for diagnosing three dimensional ageostrophic circulations in baroclinic disturbances on limited area domains abstract the methodology developed by keyser et al for representing and diagnosing three dimensional vertical circulations in baroclinic disturbances using a two dimensional vector streamfunction referred to as the psi vector is restricted to f plane channel model geometry the vertical circulation described by the psi vector consists of the irrotational or divergent part of the ageostrophic wind and the vertical velocity a key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations allowing separation of a three dimensional vertical circulation into two two dimensional components and thus objective assessment of the extent to which a three dimensional vertical circulation is oriented in a preferred direction here the methodology for determining the psi vector is modified to be suitable for real data applications the modifications consists of reformulating the diagnostic equations to apply to conformal map pr

curie temperature analyses of upper jurassic and lower cretaceous pelagic limestones summary magnetic extracts were prepared from samples of upper jurassic and lower cretaceous pelagic limestones from france and spain thermomagnetic analysis of the magnetic extracts using a microbalance required careful monitoring of base weight changes during heating heating in argon gas atmosphere induced production of magnetite during heating while slight oxidation occurred during heating in air the dominant curie temperature detected by the thermomagnetic analyses was the 585 c curie temperature of magnetite the 680 c curie temperature of haematite was only detected when isothermal remanent magnetism irm data indicated large concentrations of haematite even when irm data indicated its presence the thermomagnetic analyses did not detect the neel temperature of goethite although thermomagnetic analyses of magnetic extracts provide more direct identification of the dominant strongly ferromagnetic minerals irm acquisition and subsequent thermal demagnetization is a superior technique in detecting high coercivity weakly ferromagnetic minerals such as goethite and haematite

a satellite time series of sea surface temperatures in the eastern equatorial pacific ocean 1982 1986 satellite measurements of sea surface temperature sst in the eastern equatorial pacific have been made since 1982 using the multichannel advanced very high resolution radiometers on noaa polar orbiting satellites a 4 year data set has been accumulated at weekly intervals and a spatial resolution of about 50 km on an interactive computer system the time lapse evaluation of the data reveals annual and interannual variations of the sst which are related to the 1982 1983 el nino and southern oscillation as well as to coastal and equatorial upwelling events the el nino was preceded by a weak equatorial cooling pattern from may to august 1982 the subsequent equatorial warming persisted until june 1983 from mid 1983 to early 1984 the equatorial sst pattern was dominated by pronounced upwelling and westward advection of cold water by the south equatorial current north of the equator the zonal equatorial sst front was distorted by a succession of westward moving equatorial long waves which extended nearly to the date line during both 1984 and 1985 the equatorial upwelling began to intensify during may and lasted until the following february the transition to seasonal equatorial warming occurred from february to march and was marked by abrupt increases of sst on time scales of less than 1 week the zonal equatorial sst fronts disappeared at this time but the upwelling off the coast of south america persisted from december to april of each year short term upwelling events appeared intermittently along the western coast of central america south of the gulf of tehuantepec and southwest of lake nicaragua and the gulf of panama these upwelling events were weak from 1982 to 1984 and were more intense during 1985 and 1986 during february and march of 1985 an extreme case of upwelling was observed south of the gulf of panama the sst was depressed by at least 9 c and cooler water appeared to be advected southwestward to the equator and the galapagos islands time series of the sea surface temperatures at fixed locations and color images of the sst distribution are used to illustrate these events

a case study of saharan cyclogenesis a case of saharan cyclogenesis associated with the equatorward intrusion of a trough at the end of the atlantic storm track is investigated it is shown that a potential vorticity anomaly resulting from a baroclinic wave life cycle associated with the polar jet interacts with the low level baroclinicity over subtropical africa beneath the subtropical jet it is suggested that low latitude synoptic scale cyclogenesis events of this type can be triggered only by upper level potential vorticity anomalies if they have a sufficient depth scale and that the interaction may be aided by the presence of low static stability associated with a well mixed boundary layer a weak cold front also forms during the cyclogenesis event associated with the convergence of the baroclinicity of the polar trough front with the baroclinicity of the subtropical heat low

magnetic mineralogy of continental deposits san juan basin new mexico and clark s fork basin wyoming magnetic concentrates were obtained from nine bulk samples from the late cretaceous through middle paleocene continental sedimentary section in the san juan basin new mexico and from two bulk samples from the late paleocene and early eocene section in the clark s fork basin wyoming strong field thermomagnetic js t curves of almost all the san juan basin concentrates show only a single curie temperature in the 175 195 c range indicating that the dominant ferrimagnetic mineral is detrital titanomagnetite fe3 xtixo4 of composition 0 51 x 0 54 the clark s fork basin concentrates exhibited curie temperatures of 200 c and 580 c indicating a mixture of x 0 5 titanomagnetite and magnetite respectively no evidence of hematite or other ferric oxides or oxyhydroxides was observed in the js t data isothermal remanent magnetization irm acquisition curves were determined for 56 samples from the clark s fork basin and 114 samples from the san juan basin although dominated by irm acquired in magnetizing fields of 300 mt additional irm was acquired in magnetizing fields from 300 to 700 mt the irm acquired above 300 mt is attributed to hematite or other ferric oxides or oxyhydroxides and is thought to be the result of minor oxidation probably during recent weathering these results indicate that irm acquisition behavior may be used to monitor the hematite content in continental sedimentary rocks and may indicate stratigraphic intervals within which the natural remanent magnetization nrm could contain significant chemical remanence overprints although minor hematite content is indicated the detailed sampling of irm acquisition behavior for the san juan basin and clark s fork basin sedimentary sequences did not reveal any significant correlation of irm behavior with polarity of the characteristic nrm these data thus support the previous conclusion based on paleomagnetic data that the characteristic nrm is a depositional remanence which provides a valid recording of the geomagnetic polarity sequence during deposition of these continental sedimentary sequences

meteorological tower measurements of a surface cold front abstract measurements from the boulder atmospheric observatory meteorological research tower are used to describe the structure and physical processes of a strong surface cold front analysis reveals that the horizontal gradients in temperature and wind velocity at the front are concentrated within a 200 m distance and measured vertical velocities exceed 5 m s 1 at the leading edge of the front calculations with the miller frontogenesis equation show the magnitude and relative importance of cross frontal confluence versus differential vertical motion tilting in forcing frontogenesis

interannual variability of tehuantepec eddies 1 topex poseidon satellite altimeter observations and the naval research laboratory layered ocean model simulations show interannual variability in the number and intensity of tehuantepec eddies off the mexican southwest coast analysis of the results illustrates that downwelling coastally trapped waves which are generated in the equatorial pacific play a crucial role in the modulation and generation of tehuantepec eddies and a dominant role in tehuantepec eddy interannual variability this introduces a new paradigm in which the generation and modulation of tehuantepec eddies is not exclusively explained in terms of the strong and intermittent tehuantepec wind events in fact the results show anticyclonic eddy formation during periods of calm tehuantepec winds that is specifically exemplified by the formation of two anticyclonic tehuantepec eddies during a 5 month period of weak gulf of tehuantepec winds during summer of 1997 furthermore the satellite observed and nlom simulated proliferation of tehuantepec eddies during el nino years is explained by the corresponding increase in downwelling coastally trapped waves and a lack of increase in the number and strength of tehuantepec wind events during el nino years

global climatologies of fronts airmass boundaries and airstream boundaries why the definition of front matters abstractclimatologies of fronts airmass boundaries and airstream boundaries can be calculated using automated approaches on gridded data such approaches may require choices to define a front in

magnetic polarity stratigraphy and 40 k 40 ar dating of late miocene and early pliocene continental deposits catamarca province nw argentina magnetostratigraphic and 40 k 40 ar data on a 2300 m thick sequence of continental sediments at puerta de corral quemado in catamarca province nw argentina permit calibration of land mammal faunas of late tertiary huayquerian and montehermosan age the sequence represents from oldest to youngest the chiquimil a araucanense and corral quemado formations paleomagnetic samples were collected from 99 stratigraphic levels strong field thermomagnetic and isothermal remanent magnetization experiments indicate that the dominant ferrimagnetic mineral is magnetite progressive alternating field af and themal demagnetization of the natural remanent magnetism nrm demonstrates that af demagnetization to 20 mt peak field is sufficient to isolate the primary nrm which is of depositional origin the resulting paleomagnetic data provide a well defined magnetic polarity zonation although sampling is less dense in the upper half of the section 40 k 40 ar data obtained from mineral separates of fo

mineralogy of magnetic minerals and revised magnetic polarity stratigraphy of continental sediments san juan basin new mexico detailed paleomagnetic and rock magnetic analyses have been performed on samples from multiple sections through the cretaceous tertiary k t boundary and throughout a 750 m thick sequence of late cretaceous through middle paleocene continental deposits in the san juan basin new mexico curie temperatures have been determined for magnetic separates from 52 levels and with only two exceptions they range from 180 to 300 c along with microprobe and x ray analyses these data indicate that the detrital ferrimagnetic mineral is titanohematite with composition 0 45 x 0 60 this magnetic mineralogy indicates derivation of the continental san juan basin sediments from a volcanic probably dacitic or andesitic source these mineralogical data along with other geological data and the pattern of magnetic polarity zonation in multiple sections across the basin argue strongly for deposition of the late cretaceous and paleocene continental deposits in the san juan basin as a clastic wedge derived from a sourc

a mechanism for african monsoon breaks mediterranean cold air surges 1 surges of cold air from the mediterranean into northern africa during the boreal summer are documented and their influence on monsoon breaks is analyzed using tropical rainfall measuring mission rainfall estimates and reanalysis products between 1998 and 2006 6 10 cold air surges occurred each summer with low level temperature anomalies ranging from less than 1 k to over 6 k composite analysis indicates that cold air surges over northern africa persist for 2 10 days and travel equatorward at approximately 5 5 m s 1 which is 0 5 1 5 m s 1 faster than the observed climatological low level meridional flow northern african cold surges have characteristics similar to surges observed elsewhere in the world including a hydrostatically induced ridge of surface pressure and an amplified upper tropospheric ridge trough pattern the african cold surge is preceded by the passage of a shortwave trough and an intensification of the upper tropospheric subtropical westerly jet streak over the mediterranean sea these events are associated with increased confluence in the jet entrance region over the central mediterranean an enhanced direct secondary circulation subsidence and low level ageostrophic northerly flow over northeastern africa composite analysis shows that the passage of a cold surge is associated with an enhancement in convective activity over southern algeria western niger northern mali and mauritania 2 to 5 days before the surge reaches the eastern sahel 17 5 n when northeasterly flow channeled between the atlas and ahaggar mountains strengthens and transports relatively moist air from the western mediterranean and eastern north atlantic over the region and increases moisture convergence over western africa north of 20 n over the eastern sahel of sudan and eastern chad the composite results reveal a break in convective activity and decrease in low level convergence when the surge arrives that persists for about 6 days these results offer great promise for improving the short range prediction of rainfall over northern africa

two severe freezes in brazil precursors and synoptic evolution abstract two freezes with repercussions on world coffee markets struck brazil in recent years one during fgge in 1979 the other in 1981 data from multiple sources including satellites and drifting buoys were analyzed for early warning signs and synoptic evolution of the freeze events two important precursors were found 1 a slowly moving long wave pattern in the central south pacific ocean amplified greatly one to two days before frontogenesis in south america and four to five days before the freezes in brazil 2 the long wave ridge when at maximum amplitude was located near the southern andes and the next long wave trough downstream was located in the south atlantic at the longitude of brazil this configuration channeled subantarctic air northward entirely over land into subtropical latitudes frontogenesis in low latitudes in the mid pacific was the first sign of wave amplification in the 1979 case subsequent frontogenesis in northern argentina was accelerated by the opportune approach of a s

magnetic polarity stratigraphy a series of previous review articles on paleomagnetic polarity stratigraphy has documented the growth of this field and its passage through various stages of development in which work was concentrated on young lavas young deep sea sediments from piston cores and red beds cox 1969 larson and helsley 1975 opdyke 1972 harrison 1974 irving and pulliah 1976 butler and opdyke 1979 the closely related development of the geomagnetic reversal time scale has been reviewed by ness et al 1980 at the time of the last quadrennial u s national report butler and opdyke 1979 noted that the recent development of digital spinner and cryogenic magnetometers was making it possible to measure large numbers of very weakly magnetized samples in the last four years cryogenic magnetometers have been acquired by many laboratories and studies of magnetic stratigraphy involving stepwise demagnetization of hundreds of samples have become fairly common in view of the time and effort that go into a careful study of paleomagnetic stratigraphy it seems unlikely that this technique will ever become as routine a correlation tool as biostratigraphy

evolution of the late cenozoic inselberg landscape of southwestern jordan abstract in southwestern jordan a spectacular inselberg landscape has resulted from dissection of paleozoic sandstones following stripping of cretaceous tertiary carbonates erosion of the sandstones was tied to development of the adjacent dead sea rift in two ways subsidence along the rift axis provided a low base level that facilitated erosion and faulting associated with the dead sea and red sea rifts led to differential erosion and isolation of inselbergs most of the erosion took place in and after miocene time the major drainage outlet wadi yutm probably existed throughout development of the landscape but was temporarily blocked during a normal faulting episode following graben faulting and stream erosion aggradation of sand and playa sediment created a relatively flat plain above which rise erosional outliers of the sandstones

effect of the andes cordillera on precipitation from a midlatitude cold front the effects of the andes cordillera the major mountain range in south america on precipitation patterns of baroclinic systems approaching from the southeast pacific remain largely unstudied this study focuses on a case in late may 2008 when an upper level trough and surface cold front produced widespread precipitation in central chile the primary goal was to analyze the physical mechanisms responsible for the structure and evolution of the precipitation weather research and forecasting wrf model simulations indicate that as an upper level trough approached central chile midtropospheric flow below 700 hpa was blocked by the high topography and deflected poleward in the form of a barrier jet this northerly jet had wind maxima in excess of 15 m s 21 was centered around 925 hpa and extended westward 200 km from the mountains it intersected the cold front which approached from the south near the coast thereby increasing convergence along the frontal surface slowing its equatorward progress and enhancing rainfall over central chile another separate region of heavy precipitation formed over the upwind slopes of the cordillera a trajectory analysis confirmed that the barrier jet moved low level parcels from their origin in the moist southeast pacific boundary layer to the coast when model topography was reduced to twenty percent of its original height the cold front advanced more rapidly to the northeast generated less precipitation in central chile between 338 and 368s and produced minimal orographic precipitation on the upwind andean slopes based on these findings the high topography appears responsible for not only orographic precipitation but also for substantially increasing precipitation totals over the central coast and valley

the 1993 superstorm cold surge frontal structure gap flow and tropical impact in the wake of the eastern united states cyclone of 12 14 march 1993 a cold surge originating over alaska and western canada brought northerlies exceeding 20 m s21 and temperature decreases up to 158c over 24 h into mexico and central america this paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale focusing on 1 the structure and evolution of the leading edge of the cold surge 2 the reasons for its extraordinary intensity and equatorward extent and 3 the impact of the surge on the tropics specifically on the strength of the trade winds and on the sea surface temperature in the eastern pacific the cold surge was initiated as a developing cyclone over the gulf of mexico and an upstream anticyclone east of the rockies caused an along barrier pressure gradient to form forcing topographically channeled northerlies along the rocky and sierra madre mountains to transport cold air equatorward on the mesoscale the leading edge of the cold surge possessed nonclassical frontal structure for example as the cold surge entered mexico the coldest air and the strongest wind arrived at about 900 hpa before affecting the surface suggestive of a tipped forward leading edge to the surge also satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air the cold surge had characteristics reminiscent of a kelvin wave a tipped forward cold front a pressure jump line a bore and a gravity current but none of these conceptual dynamical models was fully applicable associated with the cold surge gap winds up to 25 m s21 were observed in the gulfs of tehuantepec a tehuantepecer fonseca papagayo and panama owing to the strong cross mountain pressure gradient in the case of the tehuantepecer a rope cloud emanated from the isthmus of tehuantepec and turned anticyclonically consistent with an inertial oscillation on the synoptic and planetary scales the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold air damming by a low latitude upper tropospheric trough and by the lower branch of the secondary circulation associated with a confluent jet entrance region aloft the cold surge also impacted the tropical atmosphere and ocean by contributing to the strengthening of the northeast trade winds over the eastern pacific ocean and by inducing local cooling of the sea surface temperature in the gulfs of tehuantepec and papagayo by about 48 88c

the formation of internal bores in the atmosphere a laboratory model summary recently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low level temperature inversions it has been speculated that such disturbances are generated by some type of gravity current such as cold fronts sea breeze fronts and thunderstorm outflows interacting with an existing temperature inversion in this paper we describe a systematic laboratory study in a water channel of internal bores and their generation by the movement of gravity currents through a two layer model of the atmosphere we compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development

atmospheric structure and momentum balance during a gap wind event in shelikof strait alaska abstract gap winds occur in topographically restricted channels when a component of the pressure gradient is parallel to the channel axis aircraft flight level data are used to examine atmospheric structure and momentum balance during an early spring gap wind event in shelikof strait alaska alongshore sea level pressure ridging was observed vertical cross sections show that across strait gradients of boundary layer temperature and depth accounted for the pressure distribution geostrophic adjustment of the mass field to the along strait wind component contributed to development of the observed pressure pattern boundary layer structure and force balance during this event was similar to that often observed along isolated barriers however the rossby radius was lager than the strait width and atmospheric structure in the strait exit region indicates transition of the flow to open coastline conditions two across strait momentum budgets show that the coriolis force and across strait pressure gradient w