summertime cold fronts in southeast australia behavior and low level structure of main frontal types abstract the cold fronts research programme cfrp concentrated upon cold frontal systems synoptic scale fronts occurring in southeast australia during late spring and early summer november and december many of these tend to the complex with change lines associated with sea breezes prefrontal squall lines and related cold outflows garratt et al the present paper extends the earlier work to include additional observations of frontal events from field phase 3 of the cfrp held in late 1984 in addition we distinguish between this frontal system type 1 and a frontal system type 2 occurring throughout summer but predominantly late in the season which is often dry with a single major change line identified as a mesoscale coastal front 0bservations from the cfrp and other sources are described covering a wide range of frontal intensities with frontal speeds varying between 5 and 25 m s 1 the nature of the low level flow behind and in the vicinity of the active surface cold front scf is

research aircraft measurements of jet stream geostrophic and ageostrophic winds abstract radar altimeter and inertial navigation wind velocity measurements from research aircraft are shown to give the ageostrophic wind velocity distribution within a jet stream system near the tropopause results from one case study reveal cross height contour ageostrophic winds exceeding 20 m s 1 and centripetal acceleration induced ageostrophic winds approaching 100 m s 1 in the exit region of a jet streak which has entered into the trough region of a synoptic wave diagnostic calculations are made of the horizontal velocity divergence and the geostrophic and ageostrophic components of the observed wind velocity acceleration the results suggest caution in the application of near straight jet streak dynamics and the geostrophic momentum approximation to jet streak systems embedded within large amplitude synoptic wave regimes

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

observation of a cold front with rope cloud abstract observations are presented of a cold front which passed over the florida peninsula and produced a rope cloud visible on satellite imagery the low level structure of the leading edge of the front is revealed with data from the 150 m tower at cape canaveral air force station it is shown that the leading edge of the front is very similar to thunderstorm gust fronts and the speed is predicted well by the gust front speed equation this indicates that the rope cloud is directly analogous to the arc cloud sometimes observed along thunderstorm gust fronts further the rope cloud appears to be a less intense manifestation of the forced convection that produces the narrow cold frontal rainbands observed in some other cold front studies

climatology of strong intermountain cold fronts abstract motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern utah this paper presents a 25 yr climatology of strong cold frontal passages over the intermountain west and adjoining western united states using conventional surface observations and the north american regional reanalysis strong cold frontal passages are identified based on a temperature fall of 7 c or greater in a 2 3 h period a concurrent pressure rise of 3 hpa or greater and the presence of a large scale 700 hpa temperature gradient of at least 6 c 500 km 1 the number of strong cold frontal passages exhibits a strong continental signature with very few events 10 along the pacific coast and more than 200 events east of the continental divide the number of events increases dramatically from the cascade mountains and sierra nevada to northern utah indicating that the intermountain west is a frequent cold front breeding ground a composite of th

importance of diabatic processes on ageostrophic circulations observed during the fronts 87 experiment the impact of condensation and evaporation on real cross frontal circulations is studied using data from the fronts 87 field experiment the sawyer eliassen equation is solved numerically in its primitive equation form for one intensive observing period of fronts 87 three different schemes of parametrization of condensation heating are used comparison between diagnosed and observed ageostrophic circulations shows that latent heat release plays an important role in enhancing the ascending branch of the ageostrophic circulation ahead of the frontal zone in particular when a parametrization scheme for the low level moisture convergence is applied a cell of strong ascending motion is formed ahead of the frontal zone in good agreement with the observations further a simple parametrization of evaporative cooling below the frontal discontinuity leads to a reinforcement of downdraughts in the cold sector of the front as observed in regions where precipitation originating from the strong line convection evaporates in comparison with their adiabatic counterparts the current solutions show strong similarities with the observed circulation and indicate the predominance of diabatic effects in the frontal zone

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

a transition phase teleconnection of the pacific quasi decadal oscillation the atmospheric circulation patterns associated with the pacific quasi decadal oscillation qdo are investigated using available observational data from 1948 to 2007 previous studies indicate that the pacific qdo is characterized by a distinct lifecycle in the form of sea surface temperature sst patterns in the warm and cool phases of the pacific qdo the sst patterns resemble those associated with the el nino southern oscillation enso during the warm cool and cool warm transitions of the pacific qdo recurrent sst patterns are also clearly visible the rotated empirical orthogonal function analysis on the 10 15 year filtered data shows that the evolutions of sst and atmospheric circulation are well coupled while the warm cool phase pacific qdo generates an enso like circulation pattern the transition phases form a distinct short wave train emanating from southeast asia towards north america this short wave train is particularly robust in the streamfunction of water vapor flux diagnostic analyses of the heat budget the stationary wave flux and a barotropic model indicate that this short wave train is thermodynamically maintained and is likely forced by diabatic heating near southeast asia additional modulations of forcing sources in the western and eastern tropical pacific on this short wave teleconnection are also discussed

seismic attenuation and velocity dispersion in heterogeneous partially saturated porous rocks summary using a numerical approach we explore wave induced fluid flow effects in partially saturated porous rocks in which the gas water saturation patterns are governed by mesoscopic heterogeneities associated with the dry frame properties the link between the dry frame properties and the gas saturation is defined by the assumption of capillary pressure equilibrium which in the presence of heterogeneity implies that neighbouring regions can exhibit different levels of saturation to determine the equivalent attenuation and phase velocity of the synthetic rock samples considered in this study we apply a numerical upscaling procedure which permits to take into account mesoscopic heterogeneities associated with the dry frame properties as well as spatially continuous variations of the pore fluid properties the multiscale nature of the fluid saturation is taken into account by locally computing the physical properties of an effective fluid which are then used for the larger scale simulations we consider two sets of numerical experiments to analyse such effects in heterogeneous partially saturated porous media where the saturation field is determined by variations in porosity and clay content respectively in both cases we also evaluate the seismic responses of corresponding binary patchy type saturation patterns our results indicate that significant attenuation and modest velocity dispersion effects take place in this kind of media for both binary patchy type and spatially continuous gas saturation patterns and in particular in the presence of relatively small amounts of gas the numerical experiments also show that the nature of the gas distribution patterns is a critical parameter controlling the seismic responses of these environments since attenuation and velocity dispersion effects are much more significant and occur over a broader saturation range for binary patchy type gas water distributions this analysis therefore suggests that the physical mechanisms governing partial saturation should be accounted for when analysing seismic data in a poroelastic framework in this context heterogeneities associated with the dry frame properties which do not play important roles in wave induced fluid flow processes per se should be taken into account since they may determine the kind of gas distribution pattern taking place in the porous rock

the interaction of a pacific cold front with shallow air masses east of the rocky mountains abstract the study presented here describes the interactions that occurred between an advancing pacific cold front and shallow gulf of mexico and arctic air masses situated east of the rocky mountains during the verification of the origins of rotation in tornadoes experiment vortex field campaign on 17 18 april over oklahoma and adjacent states these interactions were driven largely by the complex topography of the region four air masses of distinctly different origin i e pacific polar high altitude continental gulf of mexico and arctic and the boundaries that separated them i e pacific cold front dryline and arctic front were observed within the experimental domain this event produced more than 1 million worth of damage in the experimental domain due to severe weather a dense network of ground based in situ and remote observing systems and two research aircraft equipped with in situ sensors and doppler radars gathered data that allowed the authors to document the passage of a vigoro

multi reanalysis climatology of intermountain cyclones abstract the topography in and around the intermountain west strongly affects the genesis migration and lysis of extratropical cyclones here intermountain i e nevada or great basin cyclone ic activity and evolution are examined using the ecmwf re analysis interim era interim the north american regional reanalysis narr and the ncep ncar reanalysis from 1989 to 2008 the period during which all three are available the ics are defined and tracked objectively as 850 hpa geopotential height depressions of 40 m that persist for 12 h the monthly distribution of ic center and genesis frequency in all three reanalyses is bimodal with spring absolute and fall secondary maxima although the results are sensitive to differences in resolution topographic representation and reanalysis methodology both the era interim and narr produce frequent ic centers and genesis in the great basin cyclone region which extends from the southern high sierra to northwest utah and the canyonlands cyclone reg

a north pacific short wave train during the extreme phases of enso abstract sea surface temperatures ssts exhibit an interannual seesaw between the eastern and western tropical pacific in concert with the el nino southern oscillation enso cycle evidence accumulated from previous studies suggests that a teleconnection link may exist between enhanced rainfall convection in the western pacific and precipitation severe weather over north america the aforementioned link could possibly be established by a teleconnection wave pattern induced by the anomalously warm cold sst anomalies in the western tropical pacific to demonstrate this possibility a fourier scale separation was introduced to divide the enso anomalous circulation into two wave regimes long wave waves 1 3 and short wave waves 4 15 the classic pacific north american teleconnection pattern is formed by the long wave regime in contrast emerging in the short wave regime is a well organized wave train that propagates from the western subtropical pacific along the north pacific rim into north america in

on the interaction between bathymetry and climate in the system dynamics and preferred levels of the great salt lake 1 the great salt lake is a terminal lake whose level is determined by the balance between inflows and outflows we examine the causes for multimodality in the distributions of lake level and hence volume and area that have previously been examined from a system dynamics perspective we focus on the role of bathymetry in the dynamics of this system and show that some of the modes that are observed and that represent preferred system states are attributable to features of the bathymetry described using the topographic area volume relationship being a terminal lake the only outflow is evaporation which depends directly on lake area which adjusts with stochastic fluctuations to balance inflows where the topographic side slopes are relatively flat and the lake goes through a large change in area for a given increase in level the outflux evaporation will go through a corresponding large change tending to stabilize the lake at that point conversely where topographic side slopes are relatively steep the stabilizing effect is diminished this effect was quantified using the derivative of the lake area lake volume function determined from u s geological survey topographic and bathymetric surveys we show how some of the observed modes that represent preferred lake volume states are attributable to peaks in this area volume derivative while a complete description of the observed distribution of lake volume requires combining the bathymetry represented by the area volume derivative with a multimodal lake area distribution that may be connected to multimodality in the aggregate hydrological forcing

morphology and paleoclimatic significance of pleistocene lake bonneville spits abstract pleistocene lake bonneville of western utah contains a variety of spits associated with shorelines and other features that formed between 21 000 and 12 000 14 c yr bp field studies in the low lying mountain ranges of the central portion of lake bonneville identified 17 spits of various types the spits are connected to small mountain ranges and islands vary in size from 0 02 to 0 5 km 2 and are composed of coarse grained well rounded poorly sorted sedimentary material sixteen of the 17 spits have a northeasterly to southwesterly orientation implying that winds were from the northwest to northeast approximately 180 out of phase with modern winds in the eastern great basin lake bonneville spit orientation is best explained as the result of persistent northerly winds caused by the high atmospheric pressure cell of the continental ice sheet and passage of low pressure extratropical storms south of the lake similar strong persistent winds are a common feature of modern continental ice sheets and passing low pressure systems if so the north american jet stream tracked south of lake bonneville as recently as 12 000 14 c yr bp well past the height of the last glacial maximum

planetary and synoptic scale signatures associated with central american cold surges abstract motivated by outstanding issues from a previous case study of a midlatitude cold surge that affected mexico and central america the climatology of central american cold surges is examined in this paper an independently derived listing of 177 cold surge events is employed for which the following properties are tabulated onset date duration time between cold surge events latitude of maximum equatorward penetration min and 48 h maximum surface temperature change at merida mexico t these data show that 75 of the cold surges have durations of 2 6 days the same timescale as mobile disturbances in the westerlies also there does not appear to be any relationship between t and the duration of the event although cold surges that penetrate to low latitudes min 7 10 n have a weak tendency to persist longer than those that do not penetrate to low latitudes min 15 20 n in addition the reding data indicate that the cold surges tend to reach their most equatorward extent where

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

the front range blizzard of 1990 part i synoptic and mesoscale structure abstract over 1 m of snow fell in the foothills of southeast wyoming and northeast colorado during the storm of 6 7 march 1990 the heavy snowfall combined with strong winds to product blizzard conditions resulting in major highways being closed for several days the heaviest snow fell in the vicinity of a narrow northerly barrier jet that developed in place along the front range of the colorado rockies strong warm air advection from the southeast was observed during the development of the barrier jet there was no cold air advection from the north rapid intensification of the barrier jet took place only after precipitation started and was concurrent with the development of heavy precipitation a mesoscale front marked the transition from southeasterly to northerly flow this front remained very close to the rain snow line and progressed toward the east at 1 2 m s 1 for about 15 h melting precipitation appears to be the dominant process contributing to the development of the barrier jet and mesoscale f

an observational and numerical study of an intense landfalling cold front along the northwest coast of the united states during coast iop 2 abstract this paper presents an observational and numerical study of an intense wintertime cold front that made landfall along the northwest coast of the united states during iop 2 3 4 dec 1993 of the coastal observation and simulation with topography experiment observations suggest that the offshore frontal zone was associated with two transitions the first characterized by a substantial temperature gradient a nearly vertically oriented wind shift from southwesterly ahead to westerly behind and a convective rainband while the second transition had a slantwise wind shift zone from westerly ahead to northwesterly behind the frontal zone was quite narrow 5 km wide and nearly vertical below 850 mb and its width increased by 1 2 orders of magnitude above 850 mb well before the frontal landfall low level flow to the west of the olympics was associated with geostrophic balance in the cross shore direction and downgradient acceleration in the alongshore direction which contributed to the formation

effects of pile cap thickness on the maximum scour depth at a complex pier an experimental campaign of 21 long duration tests on scouring at complex piers of different configurations was performed to investigate the effect of pile cap thickness on the temporal evolution of the maximum scour depth and the development of equilibrium conditions two test series were performed with two significantly different pile cap thicknesses over a wide range of pile cap elevations with respect to the expected scour hole and the approach flow depth in addition the accuracy of existing methods to calculate the maximum scour depth was evaluated and discussed the results show that in general the thicker the pile cap the deeper the corresponding scour hole the increase in scour depth that is attributable to the pile cap thickness depends on the distance between the pile cap and the original bed level the observed maximum scour depth was greater with the thicker pile cap with respect to the thinner pile cap when it was entirely immerged into the flow partially buried or totally buried at a small depth from the initial bed level depending on the distance between the pile cap and initial bed level the pile cap thickness may also influence the temporal evolution of the maximum scour depth by increasing or reducing the scouring rate doi 10 1061 asce hy 1943 7900 0000704 2013 american society of civil engineers

discrete frontal propagation over the sierra cascade mountains and intermountain west abstract on 25 march 2006 a complex frontal system moved across the sierra cascade mountains and intensified rapidly over the intermountain west where it produced one of the strongest cold frontal passages observed in salt lake city utah during the past 25 yr observational analyses and numerical simulations by the weather research and forecast wrf model illustrate that the frontal system propagated discretely across the sierra cascade mountains and western nevada this discrete propagation occurs in a synoptic environment that features a mobile upper level cyclonic potential vorticity pv anomaly that is coupled initially with a landfalling pacific cyclone and attendant occluded front the eastward migration of the upper level cyclonic pv anomaly ultimately encourages the development of a new surface based cold front ahead of the landfalling occlusion as troughing confluence and convergence downstream of the sierra nevada intensify preexisting baroclinity over nevada trajectories show that the n

comment on dependence of shear wave seismoelectrics on soil textures a numerical study in the vadose zone by f i zyserman l b monachesi and l jouniaux

terrain influences on synoptic storm structure and mesoscale precipitation distribution during ipex iop3 abstract the influence of topography on the evolution of a winter storm over the western united states and distribution of precipitation over northern utah are examined using data collected during the third intensive observing period iop3 of the intermountain precipitation experiment ipex the analysis is based on high density surface observations collected by the mesowest cooperative networks special radiosonde observations wind profiler observations next generation weather radar nexrad data and conventional data a complex storm evolution was observed beginning with frontal distortion and low level frontolysis as a surface occluded front approached the sierra nevada as the low level occluded front weakened the associated upper level trough moved over the sierra nevada and overtook a lee trough the upper level trough which was forward sloping and featured more dramatic moisture than temperature gradients then moved across nevada with a weak surface reflection as a pressure trough over no

experimental study on local scour at complex bridge pier under combined waves and current abstract this paper presents an experimental study on scour at complex bridge piers exposed to combined waves and current regular waves were produced with various strength and superimposed on co directional current with constant velocity the key parameters tested and discussed include keulegan carpenter k c number relative current strength u c w pile cap elevation and pier skewness the scour hole patterns and scour mechanism were found to depend on the flow type combination and compared with steady current the superposition of waves may restrain the development of scour hole due to migrating bed forms three types of scour temporal evolution i e ascend descend asymptotic and transferred type were identified and the scouring processes structure exposure cyclic sediment motion etc at the scour initiation and development stages were also discussed the dimensional time scale decreased for higher bed mobility with a power functional relationship while the non dimensional time scale peaked when the maximum near bed velocity components contributed by current and waves are similar the dependence of the equilibrium scour depths both original and normalized on the tested parameters was also analysed it was found that the normalized scour depth at complex piers increases with greater k c and u c w and tends to be much larger than that for single piers the increase of bed mobility did not lead to scour peaks at the clear water threshold as what is usually observed for current only scour the influence of pile cap elevation on the equilibrium scour depth was found to be much weaker than that under steady current furthermore new scour prediction method is proposed to incorporate extra terms taking the influence of pile cap elevation and pier skewness into account the prediction accuracy is improved significantly finally more suggestions regarding design considerations are also given

an observational study of fronts and frontal mergers over the continental united states abstract the stormscale operational and research meteorology fronts experiment systems tests storm fest field program was carried out over the central united states to investigate the structure and dynamics of fronts and their attendant precipitation systems adjacent to complex topography and to evaluate new observing networks and analysis techniques for mesoscale research and weather forecasting this study presents the analysis of data taken between 8 and 10 march 1992 which was the most active period of storm fest this three day observing period was characterized by the passage of an extratropical cyclone and related fronts the synoptic scale conditions related to the cyclone and frontal evolutions are described multiple frontal zones i e pacific and arctic cold fronts quasi stationary polar front dryline maritime warm front and associated regions of precipitation are documented and their origins and interactions are explored these features both well defined and subtle were observed wi

cold fronts with and without prefrontal wind shifts in the central united states time series of cold fronts from stations in the central united states possess incredible variety for example time series of some cold fronts exhibit a sharp temperature decrease coincident with a pressure trough and a distinct wind shift other time series exhibit a prefrontal trough and wind shift that precedes the temperature decrease associated with the front by several hours in early march 2003 two cold fronts passed through oklahoma city oklahoma okc representing each of the above scenarios the cold front on 4 march was characterized by a coincident sharp wind shift pressure trough and a strong temperature decrease of 108c in 2 min on the other hand the cold frontal passage on 8 march was characterized by a prefrontal wind shift occurring over a 7 h period before the temperature decrease of 108c in 2 h twelve hours before frontal passage at okc both fronts had the same magnitude of the horizontal potential temperature gradient and petterssen frontogenesis by the time of frontal passage at okc the magnitude of the horizontal potential temperature gradient for the 4 march front was double that of the 8 march front and the frontogenesis was nearly four times as great the simultaneity of the surface horizontal potential temperature gradient and deformation and convergence maxima coincident with the wind shift was primarily responsible for the greater strength of the cold front in okc on 4 march compared to that on 8 march whether a prefrontal wind shift occurred was determined by the timing and location of cyclogenesis in the central united states on 4 march a cyclone was adjacent to the slope of the rocky mountains and developed on the cold front as it moved through oklahoma permitting greater frontogenesis and resulting in a cold frontal passage at okc with a simultaneous temperature decrease and wind shift on 8 march the cyclone moved eastward through oklahoma before the arrival of the cold front resulting in a prefrontal wind shift associated with the northerlies behind the cyclone followed by the frontal passage a 2 yr climatology of cold frontal passages at okc supports the results from the two cases above indicating that the timing and location of cyclogenesis was responsible for these two different cold frontal structures these results imply that for situations resembling those of this study the prefrontal trough is not directly associated with the cold front but is caused by external processes related to the lee troughing

induced polarization of shaly sands equations have been developed that relate induced polarization ip in shaly sands to measurable petrophysical parameters the induced polarization process has been modeled in terms of two mechanisms clay counterion displacement and membrane blockage the resulting equations can be used to determine shaliness brine conductivity and oil saturation from in phase and out of phase conductivities laboratory measurements have confirmed the ip dependence on these variables as well as on temperature

airborne observations of a front near a col during fastex abstract an analysis of an oceanic front situated near a col defined by the surface pressure field is presented there have been few observational examples of this type of front presented in the literature the primary source of information for this study was data recorded by an aircraft equipped with a doppler radar the front was approximately two dimensional and the cross frontal scale at low levels was 30 40 km a prefrontal low level jet was identified in the high resolution analyses and was shown to be supergeostrophic surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance twi in the alongfront direction large negative values of twi the vertical shear is less than predicted for the given horizontal temperature gradient were located near a region of frontogenesis the strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in g

regional variability in dust on snow processes and impacts in the upper colorado river basin dust deposition onto mountain snow cover in the upper colorado river basin frequently occurs in the spring when wind speeds and dust emission peaks on the nearby colorado plateau dust loading has increased since the intensive settlement in the western usa in the mid 1880s the effects of dust on snow have been well studied at senator beck basin study area sbbsa in the san juan mountains co the first high altitude area of contact for predominantly southwesterly winds transporting dust from the southern colorado plateau to capture variability in dust transport from the broader colorado plateau and dust deposition across a larger area of the colorado river water sources an additional study plot was established in 2009 on grand mesa 150 km to the north of sbbsa in west central co here we compare the 4 year 2010 2013 dust source deposition and radiative forcing records at grand mesa study plot gmsp and swamp angel study plot sasp sbbsa s subalpine study plot the study plots have similar site elevations environments and differ mainly in the amount of dust deposited and ensuing impacts at sasp end of year dust concentrations ranged from 0 83 mg g 1 to 4 80 mg g 1 and daily mean spring dust radiative forcing ranged from 50 65 w m 2 advancing melt by 24 49 days at gmsp which received 1 0 mg g 1 less dust per season on average spring radiative forcings of 32 50 w m 2 advanced melt by 15 30 days remote sensing imagery showed that observed dust events were frequently associated with dust emission from the southern colorado plateau dust from these sources generally passed south of gmsp and back trajectory footprints modelled for observed dust events were commonly more westerly and northerly for gmsp relative to sasp these factors suggest that although the southern colorado plateau contains important dust sources dust contributions from other dust sources contribute to dust loading in this region and likely account for the majority of dust loading at gmsp copyright 2015 john wiley sons ltd

recent rainfall cycle in the intermountain region as a quadrature amplitude modulation from the pacific decadal oscillation 1 precipitation in the intermountain region has experienced a pronounced 10 20 yr cycle during the last four decades we present evidence of a quadrature phase coupling of these 10 20 yr cycles between the pacific decadal oscillation pdo and the intermountain rainfall the pdo has been linked to precipitation anomalies in the northwest and southwest u s while the intermountain region remains marginal during the transition phase of a 10 20 yr cycle in the pdo an anomalous circulation dipole over the gulf of alaska is formed this circulation dipole modulates the synoptic activity that produces rainfall in the intermountain region as a result the intermountain rainfall cycle has repeatedly lagged the pdo cycle by a quarter phase amounting to about 3 to 4 years future understanding of the 10 20 yr cycle embedded in the pdo and its quadrature phase modulation may help predict the intermountain climate

north south precipitation patterns in western north america on interannual to decadal timescales the overall amount of precipitation deposited along the west coast and western cordillera of north america from 258 to 558n varies from year to year and superimposed on this domain average variability are varying north south contrasts on timescales from at least interannual to interdecadal in order to better understand the north south precipitation contrasts their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large scale climatic patterns spatial empirical orthogonal functions eofs and spatial moments domain average central latitude and latitudinal spread of zonally averaged precipitation anomalies along the westernmost parts of north america are analyzed and each is correlated with global sea level pressure slp and sea surface temperature series on interannual defined here as 3 7 yr and decadal 7 yr timescales the interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much p

principles of electrography applied to self potential electrokinetic sources and hydrogeological applications 1 the electrical potential field passively recorded at the ground surface of the earth and termed self potential can be analyzed to determine the shape and the depth of the piezometric surface the coupling between hydraulic flow and electrical current density is electrokinetic in nature the electrokinetic coupling coefficient entering into the integral equation relating the depth of the water table to self potential signals is analyzed for various types of porous materials it is simply related to the electrical conductivity of the pore water in steady state conditions each element of the water table can be seen as an elementary dipole with an inclination locally perpendicular to the water table and strength proportional to the water table elevation then we propose three methods to obtain the shape and range of possible depths of the water table from the study of the self potential distribution recorded at the ground surface the nonuniqueness of the solution is removed if one knows either the electrokinetic coupling coefficient or the water table at one location and under the assumption of the spatial homogeneity of the electrokinetic coupling coefficient two field cases are discussed to show the success of the proposed methods for estimating the shape and depth of the water table at two different scales of investigations they concern the study of self potential signals associated with the shape of the water table in the vicinity of a pumping well and in the flank of the kilauea volcano

effects of relative column width and pile cap elevation on local scour depth around complex piers abstractan experimental campaign of 25 long duration 13 47 days laboratory tests was carried out with three complex pier models under steady clear water flow conditions each model characterized by a different relation between the column and the pile cap widths dc dpc was tested for a variety of pile cap positions relatively to the initial bed hc the experimental data were used to describe the temporal evolution of the scour depth as a function of hc h h approach flow depth the common criterion to stop experimental tests on complex piers was analyzed and a new criterion was introduced the equilibrium scour depth dse was calculated by extrapolation of data series the results are used to evaluate the effect of dc dpc and hc h on dse when the pile cap is above the bed situation 1 partially buried in the bed situation 2 and completely buried in the bed situation 3 the analysis includes the definition of hc at which the maximum dse occurs through an equation that takes into account th

the frontal hydraulic head a micro α scale 1 km triggering mechanism for mesoconvective weather systems abstract measurements from the noaa boulder atmospheric observatory bao 300 m tower the national center for atmospheric research ncar sabreliner aircraft and the noaa goes 5 satellite give evidence for the cross front scale collapse of nonprecipitating surface cold frontal zones to horizontal distances of 1 km or less the leading edges of these frosts possess the characteristic structure of density current flows an elevated hydraulic head followed by a turbulent wake vertical motions at the frontal heads exceed 5 m s 1 at 300 m agl the ascent at the frontal head may act as a 1 km scale triggering mechanism for the release of potential instability and the formation of intense squall line mesoconvection

multiscale analysis of a meso β frontal passage in the complex terrain of the colorado front range abstract data from a mesoscale observing network are used to describe the evolution of a complex boundary between a dry air mass near the foothills of the rocky mountains and a shallow moist air mass over the eastern plains synoptic scale analyses revealed that the origin of the moist air mass was associated with lee cyclogenesis mesoscale analyses provided a detailed picture of a localized anticyclonic circulation that developed within the larger scale flow mixing ratio data from the mesoscale observing network indicated the position of the boundary between the air masses it is shown that the cooler moister air on the plains advanced toward and retreated away from the foothills during the evening eventually downslope winds that were opposing the motion of the mesoscale boundary decreased and the anticyclonic circulation on the plains became more organized on an even smaller scale doppler lidar measurements revealed characteristics of the wind flow associated with the mesofront and the interacti

enso like interdecadal variability 1900 93 a number of recent studies have reported an enso like eof mode in the global sea surface temperature sst field whose time variability is marked by an abrupt change toward a warmer tropical eastern pacific and a colder extratropical central north pacific in 1976 77 the present study compares this pattern with the structure of the interannual variability associated with the enso cycle and documents its time history back to 1900 the analysis is primarily based on the leading eofs of the sst anomaly and anomaly deviation fields in various domains and the associated expansion coefficient or principal component time series which are used to construct global regression maps of sst sea level pressure slp and a number of related variables the use of anomaly deviations i e departures of local sst anomalies from the concurrent global mean sst anomaly reduces the influence of global mean sst trends upon the structure of the eofs and their expansion coefficient time series an important auxiliary time series used in this study is a southern oscillation index based on marine surface observations by means of several different analysis techniques the time variability of the leading eof of the global sst field is separated into two components one identified with the enso cycle related variability on the interannual timescale and the other a linearly independent residual comprising all the interdecadal variability in the record the two components exhibit rather similar spatial signatures in the global sst slp and wind stress fields the sst signature in the residual variability is less equatorially confined in the eastern pacific and it is relatively more prominent over the extratropical north pacific the corresponding slp signature is also stronger over the extratropical north pacific and its counterpart in the cold season 500 mb height field more closely resembles the pna pattern the amplitude time series of the enso like pattern in the residual variability reflects the above mentioned shift in 1976 77 as well as a number of other prominent features including a shift of opposite polarity during the 1940s

an observational and numerical study of a cold front interacting with the olympic mountains during coast iop5 abstract this paper documents the three dimensional flow and precipitation structures associated with a weak cold front interacting with the olympic mountains and the subsequent development of a puget sound convergence zone this study utilizes data collected during coast iop5 the fifth intensive observing period of the coastal observation and simulation with topography field experiment that took place on 11 12 december 1993 one of the most important data sources was a noaa p 3 aircraft which provided flight level data radar reflectivity and doppler winds as it circumnavigated the olympics initially frontal passage along the western foothills of the olympics was accompanied by a 2 3 c temperature drop a rapid wind shift to northwesterlies and an intense line of precipitation 35 45 dbz however the wind shift and associated precipitation structures attenuated when the front began to ascend the windward slopes of the olympics surface and p 3 observations document the deformation of the front

mesoscale modeling of a cold front and its interaction with a diurnally heated land mass abstract a mesoscale numerical model incorporating a detailed planetary boundary layer scheme including momentum heat and moisture exchange with the lower boundary is used to study the change in structure of a dry summertime front as it moves towards a coastline two dimensional experiments show that the diurnal pressure fall over land causes a trough to form in the coastal region ahead of the approaching front frontogenesis at this mesoscale trough leads to the formation of a new front near the coast giving the appearance of strong acceleration of the original frontal system in the offshore region this process can occur over a wide range of times from early afternoon to late evening but no such acceleration occurs for fronts crossing the coast in the early morning hours these results indicate a tendency for more fronts to cross the coastline in the afternoon early evening period than at other times a statistic which is observed in southeast australia and oregon usa they also imply fronts sho

coherence between the great salt lake level and the pacific quasi decadal oscillation the lake level elevation of the great salt lake gsl a large closed basin lake in the arid western united states is characterized by a pronounced quasi decadal oscillation qdo the variation of the gsl elevation is very coherent with the qdo of sea surface temperature anomalies in the tropical central pacific also known as the pacific qdo however such coherence denies any direct association between the precipitation in the gsl watershed and the pacific qdo because in a given frequency the precipitation variation always leads the gsl elevation variation therefore the precipitation variation is phase shifted from the pacific qdo this study investigates the physical mechanism forming the coherence between the gsl elevation and the pacific qdo pronounced and coherent quasi decadal signals in precipitation streamflow water vapor flux and drought conditions are found throughout the great basin recurrent atmospheric circulation patterns develop over the gulf of alaska during the warm to cool and cool to warm transition phases of the pacific qdo these circulation patterns modulate the water vapor flux associated with synoptic transient activities over the western united states and in turn lead to the qdo in the hydrological cycle of the great basin as the gsl integrates the hydrological responses in the great basin the hydrological qdo is then transferred to the gsl elevation because the gsl elevation consistently lags the precipitation by a quarter phase about 3 yr in the quasidecadal time scale these processes take an average of 6 yr for the gsl elevation to eventually respond to the pacific qdo this creates a half phase delay of the gsl elevation from the pacific qdo thereby forming the inverse yet coherent relationship between them tree ring reconstructed precipitation records confirm that the quasi decadal signal in precipitation is a prominent feature in this region

investigations of a winter mountain storm in utah part i synoptic analyses mesoscale kinematics and water release rates abstract a winter storm passing across the north south orientated tushar mountains in southwest utah is investigated in this multipart paper this part i describes the evolving synoptic pattern mesoscale kinematics and calculated water release rates condensation or deposition in clouds over the western upstope part of the mountains horizontal mesoscale kinematic variables come from direct application of volume velocity processing to single c band doppler radar data water release rates are computed from updrafts derived from the radar data and from the vertical gradient of saturation mixing ratio obtained from soundings in stage i of the storm altostratus was present on the leading side of a long wave trough weak updrafts occurred only at the higher altitudes within the clouds where there was convergence and large scale synoptically forced lift downdrafts as great as 0 6 m s 1 occurred in the lower parts of the cloud where there was divergence the downdrafts were induced in part by sublimation c

comment on streaming potential dependence on water content in fontainebleau sand by v allegre l jouniaux f lehmann and p sailhac summary allegre et al recently presented new experimental data regarding the dependence of the streaming potential coupling coefficient with the saturation of the water phase such experiments are important to model the self potential response associated with the flow of water in the vadose zone and the electroseismic seismoelectric conversions in unsaturated porous media however the approach used to interpret the data is questionable and the conclusions reached by allegre et al likely incorrect

multiphase streaming potential in sandstones saturated with gas brine and oil brine during drainage and imbibition 1 we have measured the streaming potential in water wet sandstones during both drainage and imbibition and demonstrate that it behaves differently in oil brine and gas brine displacements during drainage the streaming potential remains significantly greater than zero at the irreducible water saturation in oil brine displacements but falls to zero in gas brine displacements during imbibition the streaming potential at partial saturation in oil brine displacements exceeds that measured at saturation but not in gas brine experiments our results have application to streaming potential measurements in the vadose zone hydrocarbon reservoirs contaminated aquifers and during co2 sequestration

a proposed method of surface map analysis present surface frontal analyses suffer from the defect that frontal positions are typically not collocated with zones of intense temperature contrast further individuals typically do not agree as to the existence type and location of fronts the author argues that the lack of a surface temperature analysis is mainly responsible for these flaws and it is proposed that such analysis preferably of potential temperature in regions of variable terrain elevation become part of routine procedure such an analysis will reveal nonfrontal baroclinic zones of considerable intensity most cold fronts except the strongest ones are denoted as baroclinic troughs propagating eastward in the prevailing westerly flow it is argued that when a meridional cold front exists in the presence of even a small meridional temperature gradient the wind shift should propagate away from the intense surface temperature gradient which then weakens an explanation is provided based on quasigeostrophic theory it follows that fronts are short lived phenomena

seismoelectric effects due to mesoscopic heterogeneities 1 while the seismic effects of wave induced fluid flow due to mesoscopic heterogeneities have been studied for several decades the role played by these types of heterogeneities on seismoelectric phenomena is largely unexplored to address this issue we have developed a novel methodological framework which allows for the coupling of wave induced fluid flow as inferred through numerical oscillatory compressibility tests with the pertinent seismoelectric conversion mechanisms simulating the corresponding response of a water saturated sandstone sample containing mesoscopic fractures we demonstrate for the first time that these kinds of heterogeneities can produce measurable seismoelectric signals under typical laboratory conditions given that this phenomenon is sensitive to key hydraulic and mechanical properties we expect that the results of this pilot study will stimulate further exploration on this topic in several domains of the earth environmental and engineering sciences

airborne doppler observations of a landfalling cold front upstream of steep coastal orography abstract this study uses airborne doppler radar observations to describe the mesoscale structure and evolution of a cold frontal system as it made landfall on the mountainous coast of oregon and northern california on 1 december 1995 during the coastal observations and simulations with topography experiment this section of coastline constitutes a steep approximately two dimensional north south oriented orographic barrier the front exhibited a northeast southwest orientation and thus intersected the axis of high terrain at an acute angle the along barrier pressure gradient and low level winds increased with time along the coastal zone and reached a maximum as the front made landfall stably stratified prefrontal flow was strongly blocked by the orography resulting in a confluent transition from pervasive southwesterly winds offshore to a narrow zone of accelerated south southwesterly flow near the coast where wind speeds approached 30 m s 1 at a height of 750 m above mean sea level postfrontal flow

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

pakistan s two stage monsoon and links with the recent climate change 1 meteorological conditions related to the pakistan floods of 2010 were examined in the context of monsoon dynamics and large scale circulations case and climatological analyses suggest that summer precipitation in northern pakistan comprises two distinct phases 1 a premonsoon trough phase july whose rainfall is more episodic and intense occurring prior to arrival of the monsoon trough and 2 a monsoon trough phase august whose rainfall is persistent yet less episodic driven by northward migration of the monsoon trough analyses of conditional instability moisture flux and circulation features support a persistent increase in conditional instability during the july premonsoon trough phase accompanied by increased frequency of heavy rainfall events conversely evidence does not support intensification of the august monsoon trough phase the increased convective activity during the premonsoon trough phase agrees with the projected increase in the intensity of heavy rainfall events over northern pakistan large scale circulation analysis reveals an upper level cyclonic anomaly over and to the west of pakistan a feature empirically associated with weak monsoon the analysis also suggests that the anomalous circulation in 2010 is not sporadic but rather is part of a long term trend that defies the typical linkage of strong monsoons with an anomalous anticyclone in the upper troposphere

the eastern united states side door cold front of 22 april 1987 a case study of an intense atmospheric density current abstract a case study is presented of an unusual cold front that affected the east coast of the united states on 22 april 1987 noteworthy aspects of this front are its genesis behind a preexisting back door cold front its propagation to the southwest the shallow nature of the cold air associated with it 600 m and the absence of precipitation this front has been termed a side door cold front because of the significant differences between it and typical back door cold fronts such as its direction of propagation and vertical structure weather conditions in the mid atlantic states changed abruptly from partly cloudy skies light winds and 25 30 c surface temperatures ahead of the front to a windy gust to 20 m s 1 low overcast regime with temperatures 10 15 c cooler behind the front analysis of satellite imagery surface data and sounding data revealed that the front possessed density current structure during a portion of its lifetime observed front relative flow as well as agreement betwe

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones part v the substructure of narrow cold frontal rainbands abstract the organization and structure of a narrow cold frontal rainband ncfr on the small mesoscale and the microscale have been investigated through quantitative radar reflectivity doppler radar observations airborne observations and surface measurements the ncfr was composed of small mesoscale regions of heavy precipitation called precipitation cores pcs oriented at an angle to the synoptic scale cold front in horizontal cross section the pcs were roughly elliptical in shape areas of lighter precipitation called gap regions grs separated the pcs the pcs were so oriented that their loading edges were regions of strong low level convergence the weather associated with the passage of a pc resembled that of a squall line gust front with concurrent windshifts and pressure checks occurring 5 min before heavy precipitation and a fall in temperature the changes in surface weather that accompanied the passage of a gr were more variable but tended to be less marked than for pcs thus the se

climatology and variability of warm and cold fronts over north america from 1979 to 2018 abstractthis paper describes the development and analysis of an objective climatology of warm and cold fronts over north america from 1979 2018 fronts are detected by a convolutional neural networ

reply to comment on dependence of shear wave seismoelectrics on soil textures a numerical study in the vadose zone by f i zyserman l b monachesi and l jouniaux by revil a

observations of nonclassical frontal propagation and frontally forced gravity waves adjacent to steep topography through the integrated analysis of remote sensing and in situ data taken along the front range of colorado this study describes the interactions that occurred between a leeside arctic front and topographically modulated flows these interactions resulted in nonclassical frontal behavior and structure across northeastern colorado the shallow arctic front initially advanced southwestward toward the front range foothills before retreating eastward then a secondary surge of arctic air migrated westward into the foothills during its initial southwestward advance the front exhibited obstacle like density current characteristics its initial advance was interrupted by strong downslope northwesterly flow associated with a high amplitude mountain wave downstream of the continental divide and by a temporal decrease in the density contrast across the front due to diurnal heating in the cold air and weak cold advection in the warm air the direction and depth of flow within the arctic air also influenced the frontal propagation the shallow obstacle like front actively generated both vertically propagating and vertically trapped gravity waves as it advanced into the downslope northwesterly flow resulting in midtropospheric lenticular wave clouds aloft that tracked with the front because the front entered a region where strong downslope winds and mountain waves extended downstream over the high plains the wave field in northeastern colorado included both frontally forced and true mountain forced gravity waves a sequence of scorer parameter profiles calculated from hourly observations reveals a sharp contrast between the prefrontal and postfrontal wave environments consequently analytic resonant wave mode calculations based on the scorer parameter profiles reveal that the waves supported in the postfrontal regime differed markedly from those supported in the prefrontal environment this result is consistent with wind profiler observations that showed the amplitude of vertical motions decreasing substantially through 16 km above mean sea level msl after the shallow frontal passage

an analytical study of seismoelectric signals produced by 1 d mesoscopic heterogeneities the presence of mesoscopic heterogeneities in fluid saturated porous rocks can produce measurable seismoelectric signals due to wave induced fluid flow between regions of differing compressibility the dependence of these signals on the petrophysical and structural characteristics of the probed rock mass remains largely unexplored in this work we derive an analytical solution to describe the seismoelectric response of a rock sample containing a horizontal layer at its center that is subjected to an oscillatory compressibility test we then adapt this general solution to compute the seismoelectric signature of a particular case related to a sample that is permeated by a horizontal fracture located at its center analyses of the general and particular solutions are performed to study the impact of different petrophysical and structural parameters on the seismoelectric response we find that the amplitude of the seismoelectric signal is directly proportional to the applied stress to the skempton coefficient contrast between the host rock and the layer and to a weighted average of the effective excess charge of the two materials our results also demonstrate that the frequency at which the maximum electrical potential amplitude prevails does not depend on the applied stress or the skempton coefficient contrast in presence of strong permeability variations this frequency is rather controlled by the permeability and thickness of the less permeable material the results of this study thus indicate that seismoelectric measurements can potentially be used to estimate key mechanical and hydraulic rock properties of mesoscopic heterogeneities such as compressibility permeability and fracture compliance

airborne dual doppler observations of an intense frontal system approaching the pacific northwest coast airborne doppler radar data collected off the pacific northwest coast by a noaa wp 3d orion aircraft over an 8 h period on 8 december 1993 during the coastal observations and simulations with topography experiment reveal the mesoscale structure of an intense frontal system while it was well offshore and as it approached within 20 km of the oregon coastline during the offshore stage a portion of the narrow coldfrontal rainband was characterized by deep convective cores pseudo dual doppler analyses characterize the kinematic and precipitation structure of the deep convection pseudo dual doppler analyses describe the subsequent evolution of the narrow cold frontal rainband as it approached to within 20 km of the oregon coast deformation of the frontal zone appeared to cause the dissipation of one of three precipitation cores contained within the dual doppler area the precipitation cores and the strong convergence zone associated with the front conformed to some degree to the shape of the coastline near cape blanco oregon as the front neared the coast changes in the prefrontal flow that occurred as the front approached the coast were qualitatively consistent with theoretical and numerical studies of upstream orographic influence comparison of pseudo dual doppler derived velocity profiles with idealized numerical model calculations suggests that the nearshore evolution of the frontal rainband was significantly affected by an upstream influence of the coastal orography

geomorphic and land cover identification of dust sources in the eastern great basin of utah u s a abstract this study identifies anthropogenically disturbed areas and barren playa surfaces as the two primary dust source types that repeatedly contribute to dust storm events in the eastern great basin of western utah u s a this semi arid desert region is an important contributor to dust production in north america with this study being the first to specifically identify and characterize regional dust sources from 2004 to 2010 a total of 51 dust event days deds affected the air quality in salt lake city ut modis satellite imagery during 16 of these deds was analyzed to identify dust plumes and assess the characteristics of dust source areas a total of 168 plumes were identified and showed mobilization of dust from quaternary deposits located within the bonneville basin this analysis identifies 4 major and 5 secondary source areas for dust in this region which produce dust primarily during the spring and fall months and during moderate or greater drought conditions with a palmer drought index pdi of 2 or less the largest number of observed dust plumes 60 of all plumes originated from playas ephemeral lakes and are classified as barren land cover with a silty clay soil sediment surface playa surfaces in this region undergo numerous recurrent anthropogenic disturbances including military operations and anthropogenic water withdrawal anthropogenic disturbance is necessary to produce dust from the vegetated landscape in the eastern great basin as evidenced by the new dust source active from 2008 to 2010 in the area burned by the 2007 milford flat fire this fire was the largest in utah s history due to extensive cover of invasive cheatgrass bromus tectorum along with drought conditions however dust mobilization from the milford flat burned area was limited to regions that had been significantly disturbed by post fire land management techniques that consisted of seeding followed by chaining or tilling of the soil dust storms in the eastern great basin negatively impact air quality and transportation in the populated regions of utah this study details an improved forecasting protocol for dust storm events that will benefit transportation planning and improve public health

impact of subgrid scale orography on equatorial angular momentum budget and the cold surges in a general circulation model abstractthe dynamical relations between equatorial atmospheric angular momentum eaam equatorial mountain torques and cold surges are analyzed in a general circulation model gcm first the authors show that the global eaam budget is well closed in the gcm much better than in the ncep ncar reanalysis they then confirm that the equatorial torques due to the tibetan plateau the rockies and the andes are well related to the cold surges developing over east asia north america and south america respectively for all these mountains a peak in the equatorial mountain torque components precedes by few days the development of a cold surge confirming that the cold surge s preconditioning is dynamically driven by large scale mountains the authors also analyze the contribution of the subgrid scale orography sso parameterizations and find that they contribute substantially to the torques in experiments where these parameterizations are almost entirely reduced over a given massif the authors find th

the effect of the continental boundary layer on the dynamics of fronts in a 2d model of baroclinic instability ii surface heating and cooling the effect of surface heating and cooling on the structure and evolution of the surface front that develops in the finite amplitude eady wave is investigated spatially uniform short wave radiative heating of the surface is weakly frontolytic although it appears this result may be sensitive to the way in which the boundary layer turbulence is parametrized the cross front potential temperature gradient is weakened principally by the differential heating produced by horizontal variations in the depth of the mixed layer and to a smaller extent by horizontal variations in the surface sensible heat flux the direct effect of enhanced daytime turbulent stresses is to retard the winds in the boundary layer reducing the convergence in the cross front plane while the indirect effect is to produce a strong cross isobaric flow towards the trough thereby increasing the convergence the net effect is to reduce the rate of frontogenesis near the surface front during the day the front is oriented vertically in the boundary layer because of the strong mixing nocturnal cooling through long wave radiative emission from the surface is strongly frontogenetic at night as the turbulence decays in the trough ahead of the surface front air parcels accelerate down the pressure gradient towards the centre of the trough producing low level cross front surges on both sides of the trough axis this enhanced nocturnal convergence rapidly strengthens the cross front potential temperature gradient and tilts the front westward with height such nocturnal frontogenesis is especially intense when it follows strong daytime turbulent mixing copyright 2005 royal meteorological society

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