coastal southerlies and alongshore surges of the west coast of north america evidence of mesoscale topographically trapped response to synoptic forcing abstract one of the most important warm season weather phenomena of the west coast of north america is the temporal transition from northerly to southerly flow within a few hundred kilometers offshore from the coastal mountains such wind shifts are often accompanied by cooler temperatures higher pressure and a change from nearly cloud free to low overcast conditions the more vigorous coastal transitions termed alongshore surges are characterized by an abrupt change in wind direction a sudden increase in wind speed to 15 m s 1 or more a precipitous temperature drop exceeding 10 c and a sharp rise in sea level pressure this paper presents two detailed case studies of topographically trapped coastal southerlies the strong surge event of 15 17 may 1985 and the far weaker case of 3 7 may 1982 it is shown that coastal southerlies and alongshore surges are controlled by the alongshore pressure gradients created by the synoptic scale flow at low levels and within approximately one rossby radius of th

models of coastally trapped disturbances during the spring and summer the climatological northerly flow along the u s west coast is occasionally interrupted by transitions to southerly flow that have a limited offshore scale and appear to be manifestations of marine layer flow that is rotationally trapped by the coastal mountains existing climatological and observational studies suggest that a synoptic scale offshore flow initiates these coastally trapped disturbances ctds using idealized simulations produced with a 3d nonhydrostatic model the authors find that an imposed offshore flow will produce ctds in idealized coastal environments the imposed flow first weakens the prevailing northerly flow in the marine layer and lowers the pressure at the coast the marine layer flow around this low pressure evolves toward geostrophic balance but is retarded as it encounters the coastal mountains to the south of the low and subsequently deepens the marine layer in this region the elevated marine layer then begins progressing northward as a kelvin wave and later may steepen into a bore or gravity current this progression being the ctd many observed features accompanying ctds are found in the numerical simulations including the formation of a mesoscale pressure trough offshore and deep southerlies in the ctd at the coast stability in the atmosphere above the marine layer can give rise to topographically trapped rossby waves and stronger ctd winds in these stable conditions propagation of wave energy away from the disturbance does not preclude strong quasi steady propagating ctds

large scale forcing of summertime monsoon surges over the gulf of california and the southwestern united states synoptic forcing of the gulf of california summertime low level wind field is described using a nested regional modeling system under appropriate synoptic conditions strong surge events develop that are characterized by persistent vertically extensive 1 2 km southerly flow extending along the entire gulf of california and into arizona california and even southern nevada these surge periods are initiated either by tropical cyclone activity to the south of baja california or by the westward propagation of lower tropospheric troughs from over the sierra madre occidental to the eastern pacific the flow over the gulf is primarily geostrophic and is associated with the presence of these low pressure centers to the south and west of baja california

mesoscale numerical modeling of meteorological events in a strong topographic gradient in the northeastern part of mexico a series of numerical experiments were carried out to study the effect of meteorological events such as warm and cold air masses on climatic features and variability of a understudied region with strong topographic gradients in the northeastern part of mexico we applied the mesoscale model mm5 we investigated the influence of soil moisture availability in the performance of the model under two representative events for winter and summer the results showed that a better resolution in land use cover improved the agreement among observed and calculated data the topography induces atmospheric circulation patterns that determine the spatial distribution of climate and seasonal behavior the numerical experiments reveal regions favorable to forced convection on the eastern side of the mountain chains eastern sierra madre and sierra de alvarez these processes affect the vertical and horizontal structure of the meteorological variables along the topographic gradient

numerical modeling of a coastal trapped disturbance part i comparison with observations abstract the coastal trapped disturbance ctd of 15 17 may 1985 represents an example of a strong mesoscale trapped event along the west coast of north america with abrupt transitions in many basic meteorological parameters in this study a comparison between observations and a numerical simulation of this event using the regional atmospheric modeling system rams is presented the model is shown to realistically reproduce ctd characteristics such as the coastal transition from northerly to southerly flow as a mesoscale coastal ridge of higher pressure with associated drops in marine layer temperature propagates northward along the west coast of north america simulated sea level pressure and temperature fields near the surface match well with observations especially at the synoptic scale the model realistically simulates mesoscale sea level pressure and 6 h pressure changes during the event the modeled hourly time evolution of sea level pressure and the southerly transitions at a series of coastal

sensitivity of coastally trapped disturbance dynamics to barrier height and topographic variability in a numerical model abstract the sensitivity of a coastally trapped disturbance ctd to topographic height is examined using simulations of the 15 18 may 1985 ctd these simulations include three with uniform topography in which the north american west coast mountains are represented by a three piece uniform ramp at the coast leading to a constant plateau inland and three with realistic topography in each trio of uniform and realistic topography simulations there is a control case in which the terrain height closely approximates reality and two variations in which the topography is multiplied everywhere by a topographic multiplication factor tmf of 1 5 and 0 5 to assess the sensitivity of the simulation to the barrier height average propagation speeds increased decreased by 15 20 with the increased decreased tmf it was found that the position of the ctd leading edge generally followed close behind a leading pressure trough minimum which propagated northward along the coast in a manner similar to a topographi

oxygen isotopes versus climap 18 ka temperatures a comparison from the tropical atlantic glacial interglacial differences in 18 o values of globigerinoides sacculifer have been mapped in the western and central tropical atlantic using data from surface and core sediments glacial interglacial differences in the western tropical atlantic if attributed to temperature changes only are at odds with temperature reconstructions performed by climap in the ceara rise region delta 18 o values exceeding 2 are recorded they can only be partly attributed to changes in the precipitation evaporation regime i e higher salinities and 18 o enriched precipitation an additional cooling of 1 2 c as compared to climap9s results is required to balance the 18 o record thus temperature changes from glacial to modern times amount to 2 3 c in the tropical atlantic off the northern coast of brazil

transcritical flows in the coastal marine atmospheric boundary layer abstract numerical solutions of shallow water flow in a variable width channel are computed to model the summertime marine atmospheric boundary layer off the u s west coast using an idealization of the coastline in the vicinity of point arena california as an example several steady state base flows are presented that are hydraulically transcritical these flows are strongly nonlinear weakly rotational and are forced by constant pressure gradients and damped by nonlinear bottom drag at the sea surface the transcritical base flows are supercritical in the vicinity of bends in the coastline but are subcritical to the north upstream and to the south downstream where the coastline is straight within the supercritical region orographic bends in the coastline produce expansion fans and compression jumps the same structures found in globally supercritical flows when the imposed pressure gradient forcing is increased the resulting base flow has a supercritical to subcritical transitional jump that

interaction of a warm season frontal system with the coastal mountains of the western united states part i prefrontal onshore push coastal ridging and alongshore southerlies abstract observational analyses and numerical simulations are used to investigate the interaction between a warm season frontal system trough and the complex terrain of the western united states prior to frontal landfall synoptically driven offshore flow was associated with the northward development of a thermal trough and a coastal pressure ridge and associated southerly winds moved northward along the coasts of northern california oregon washington and southwestern british columbia this coastal pressure ridge coupled with the subsequent weakening of the offshore flow resulted in the onshore push of cool marine air approximately 24 h before frontal passage the onshore surge of cool marine air into warm dry continental air associated with the thermal trough produced a mesofront that moved over the coastal and cascade mountains diagnosis of a realistic simulation of the event shows that both solar heating and adiabatic warming from downslope flow over the rockies and the cascade mountains are i

dynamics of a simulated north american monsoon gulf surge event abstractgulf surges are transient disturbances that propagate along the gulf of california goc from south to north transporting cool moist air toward the deserts of northwest mexico and the southwest united states during the north american monsoon they have been shown to modulate precipitation and have been linked to severe weather and flooding in northern mexico and the southwest united states the general features and progression of surge events are well documented but their detailed dynamical evolution is still unclear in this study a convection permitting simulation is performed over the core monsoon region for the 12 14 july 2004 gulf surge event and the dynamics of the simulated surge are examined initially convection associated with the tropical easterly wave precursor to tropical cyclone blas creates a disturbance in the southern goc on early 12 july this disturbance is a precursor to the gulf surge on 13 july and is a kelvin shock internal bore under the influence of rotation that diss

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

aircraft observations of a coastally trapped wind reversal off the california coast abstract the summertime marine atmospheric boundary layer off the california coast is normally characterized by northerly winds associated with the pacific high this pattern is occasionally disturbed by episodes of southerly winds and a finger of fog or low stratus adjacent to the coastline extending approximately 100 km offshore these events propagate northward along the coast with speeds between 5 and 12 m s 1 and have a life span of several days these occurrences have been referred to as coastally trapped wind reversals ctwrs coastally trapped disturbances or southerly surges the ctwr event of 22 25 june 2006 was explored by the university of wyoming king air research aircraft to document the physical characteristics of the wind reversal in an attempt to infer the forcing mechanisms responsible for the propagation two flights from 23 june are presented that are representative of the ctwr during its mature stage sawtooth maneuvers depict the ctwr vertical structure and isobaric legs directly

prefrontal wind shift lines in the plains of the united states abstract in the central united states cold fronts are sometimes preceded by a surface wind shift by several hours these wind shifts are examined to determine their origins and characteristics a climatology and a case study of an autumn prefrontal wind shift are presented data collected by the recently implemented oklahoma mesonet and a network of wind profilers permit finescale analyses of the autumn prefrontal wind shift it is found that prefrontal wind shifts are associated with both lee troughs and drylines the climatology reveals that in the central united states fronts are most frequently up to 60 of them preceded by wind shifts just east of the rockies where lee troughs and drylines originate this paper focuses on a case study of a prefrontal wind shift associated with a lee trough that forms when a front approaches the lee trough the prefrontal wind shift associated with the lee trough forms in the lee of the mountains but propagates away from the mountains when downslope flow decreas

a nonhydrostatic mesoscale simulation of the 10 11 june 1994 coastally trapped wind reversal abstract during the summer months the california coast is under the influence of persistent northwesterly flow several times each summer this regime is disrupted by coastally trapped wind reversals ctwr in which the northwesterly flow is replaced by southerlies in a narrow zone along the coast controversy exists as to the physical mechanisms responsible for initiation and maintenance of ctwrs while it is clear that coastal terrain is important in creating the trapped response the precise role played by terrain is unclear in the present study these issues are investigated using a nonhydrostatic mesoscale model to simulate the 10 11 june 1994 ctwr event the results show that the model successfully reproduces many of the observed features of this event including anomalous vertical structure involving the relatively shallow boundary layer with a warm nearly neutral layer above the northward propagation of southerly flow in advance of a tongue of coastal stratus fog and a substantial reduction i

orographically trapped coastal wind events in the pacific northwest and their oceanic response three ageostrophic orographically trapped wind events in the coastal zone of the pacific northwest are examined time series of wind speed wind direction air temperature sea level pressure ocean currents and ocean temperature are utilized to establish the general features of these events and to characterize the oceanic response it is suggested that these atmospheric events are highly nonlinear bores or gravity currents rather than quasi linear kelvin waves observed oceanic responses include a drop in sea surface temperature sst of 1 3 c offshore to approximately 150 km approximately the atmospheric rossby radius and a rise in sst of similar magnitude within 15 km of the shore a set of reduced gravity fully nonlinear single layer models is utilized to investigate the dynamics of the atmospheric features and their oceanic response and a possible air sea interaction the models reproduce several of the first order features of the air and sea phenomena such as the development of the atmospheric front and the wake of near inertial waves in the ocean

synoptic scale structure and the character of coastally trapped wind reversals abstract coastally trapped wind reversals that occur along the u s west coast have been described in numerous other studies the synoptic scale environment and the forcing of a coastally trapped kelvin wave are highly linked in the development of these wind reversals however not all wind reversals appear to behave like propagating kelvin waves and the analysis of coastal buoy observations for three years indicates that different types of disturbances occur both propagating disturbances and nonpropagating disturbances occur with similar frequencies while the synoptic scale characteristics associated with propagating and nonpropagating wind reversals are sometimes rather subtle several distinct differences occur that suggest a direct link between the coastal dynamics and the synoptic scale forcing synoptic forcing characterized by persistent low level offshore flow favors the development of propagating disturbances while weak nonsustained offshore flow characterizes nonpropagating disturbances the

numerical modeling of a coastal trapped disturbance part ii structure and dynamics abstract a detailed analysis of a simulation of a coastal trapped disturbance ctd using the colorado state university regional atmospheric modeling system rams is presented the ctd considered 15 18 may 1985 represents an example of a strong mesoscale trapped event with abrupt gravity current like transitions in many meteorological parameters and which was closely tied to the synoptic forcing propagation of this event along the west coast of north america occurred from initiation in the southern california bight baja california coastal region to the northern tip of vancouver island and the event appeared to have no difficulty in negotiating significant bends or gaps in the coastal mountains unlike some other events that have ceased or stalled near cape mendocino point arena and the mouth of the columbia river it is found that warm offshore flow ahead of the ctd and cool onshore flow in the southern california bight northern baja california coastal region both driven by the westward tracking

an observational and numerical study of an orographically trapped wind reversal along the west coast of the united states abstract observational analyses and a high resolution simulation using the pennsylvania state university ncar mesoscale model version 5 mm5 were used to describe the coastally trapped wind reversal of 19 21 july 1994 major findings include the following the event was initiated and controlled by changes in the synoptic scale flow and particularly by the development of offshore flow over the coastal terrain of oregon although synoptic control was dominant blocking of the inversion capped marine layer and mesoscale coastal pressure ridging were important components of the event the synoptic evolution associated with the trapped reversal was characterized by a shift from climatological near zonal 500 mb flow to high amplitude upper level ridging over the eastern pacific as this upper ridge moved northeastward the 850 mb flow over oregon became northeasterly and then southeasterly and 850 mb heights fell over western oregon the combination of falling heights aloft and increasing low level offshore

the papal front of 3 may 1987 a remarkable example of frontogenesis near the alps all available routinely collected surface data are used to document the development of the papal front which crossed southern germany on 3 may 1987 causing casualities and damage objective analyses on isentropic surfaces initialized with the european aerological data indicate that the development of the violent mesoscale front which exhibited squall line characteristics began when a distinct mid troposopheric anomaly of potential vorticity reached the north western rim of the alpine arc the forced uplift ahead of the associated shortwave trough combined with ageostrophic motions due to a jet streak at higher levels apparently provided a trigger for deep convection as was observed over the alpine foreland numerical simulations with a dry hydrostatic model indicate the relative importance of the synoptic scale forcing and the alpine orography for the generation of the papal front a westerly orographic jet over the foreland below crest height only develops in the simulation with full alps and is corroborated by the few mountain wind observations that are available a comparison with studies of frontal propagation along other mountain ranges reveals that the routine network in the alpine region and its surroundings is at least of similar density but so far hardly used for frontal case studies it is concluded that a series of routine data case studies could help to determine the variability of frontal progression along the alps and thus provide guidance when assessing the significance of single events particularly those sampled during special observation programmes

effects of coastal orography on landfalling cold fronts part i dry inviscid dynamics abstract in this study the interaction of cold fronts with idealized coastal terrain typical of the western united states and canada is considered two issues are examined first what are the factors that determine the strength of the coastal winds and second what are the orographic effects on the frontal evolution to address these issues the authors utilize a two dimensional boussinesq terrain following coordinate numerical model in which a uniform prescribed flow is forced to move over a plateau the resultant across mountain velocities are characterized by a zone of strongly decelerated flow upstream of the windward slope and a train of inertia gravity waves downstream a barrier jet oriented parallel to the mountain is produced by the coriolis force the variations of the magnitude of the upstream deceleration and the barrier jet over a wide range of froude numbers and rossby numbers are described steady linear theory applied to flow over a plateau shows that the upstream deceleration is dete

kelvin waves and internal bores in the marine boundary layer inversion and their relationship to coastally trapped wind reversals abstract detailed observations of a coastally trapped disturbance or wind reversal on 10 11 june 1994 along the california coast provide comprehensive documentation of its structure based on aircraft wind profiler radio acoustic sounding system and buoy measurements unlike the expectations from earlier studies based on limited data which concluded that the deepening of the marine boundary layer mbl was a key factor the 1994 data show that the perturbation was better characterized as an upward thickening of the inversion capping the mbl as the event propagated over a site the reversal in the alongshore wind direction occurred first within the inversion and then 3 4 h later at the surface a node in the vertical structure defined here as the altitude of zero vertical displacement is found just above the inversion base with up to 200 m upward displacements of isentropic surfaces above the node and 70 m downward displacements below although this is a single event it is shown that the vertic

aircraft observations and numerical simulations of the developing stage of a southerly surge near southern california abstractsummertime low level winds in the marine boundary layer off the california coast are predominantly from the north this pattern is interrupted periodically by southerly winds and low stratus that can propagate for hundreds of kilometers northward along the coast these events have been termed coastally trapped disturbances coastally trapped wind reversals and southerly surges their forcing has been the subject of extensive study and debate southerly surges remain difficult to forecast yet have a significant impact on coastal activities the beginning stage of a southerly surge on 16 june 2012 was explored during the precision atmospheric marine boundary layer experiment measurements of the horizontal wind and pressure field in the marine layer offshore from cape arguello hours prior to the onset of the surge were made using the university of wyoming king air research aircraft airborne measurements show that a horizontal pressure field is established with higher pressure to the south just pri

on the generation and propagation of the southern african coastal low the time height variability of the lower marine atmosphere during the eastwards propagation of coastal lows around the coast of southern africa is presented these coastal lows are shallow mesoscale disturbances that are trapped vertically by a strong subsidence inversion and horizontally through coriolis effects against the steep escarpment that fringes the subcontinent prior to the event warm offshore flow descending off this escarpment at about the inversion level provides an input of buoyancy which together with the cyclonic vorticity acquired during the descent acts to generate a coastal low rather than a high analysis of the data shows that coastal lows are characterized by a marked lowering of the inversion level a switch in the direction of the winds below the inversion and a drop in the near surface temperature which are all consistent with the hypothesis that the coastal low is an internal kelvin wave propagating in the marine layer good agreement between the observed propagation speed of the coastal low and the theoretical phase speed of such a wave reinforces this hypothesis a model of the vertical buoyancy advection into the marine atmosphere that results from the warm offshore flow is presented it is illustrated that an internal kelvin wave in the form of a coastal low results from this buoyancy advection comparisons are made between coastal low dynamics and those characteristics of other mesoscale coastally trapped disturbances which propagate in the marine layers of california and south eastern australia

a model investigation of the dynamics of a coastally trapped disturbance abstract numerical simulations of the 15 17 may 1985 coastally trapped disturbance ctd event along the west coast of north america are compared with the schematic model of ctd evolution developed by skamarock rotonno and klemp srk which was based upon more idealized simulations it is shown that the general evolution of the simulated may 1985 ctd is consistent with the srk schematic model it is further shown that secondary effects not contained in the srk simulations such as diurnal radiation variations and mesoscale topographic variations can account for the variable ctd initiation and propagation observed both in nature and in the present numerical simulations diurnal radiation variations coupled with differential heating of land and ocean appear to play an important role in setting up the alongshore temperature gradient necessary for ctd formation and evolution the modeled ctd is found to change dynamical characteristics from an initial kelvin wave bore similar to that discussed by ralph

wind stress curl forcing of the coastal ocean near point conception california near point conception california the atmospheric flow separates from the coast and a large wind stress curl results direct spatial wind field observations from 20 aircraft overflights in the spring of 1983 suggest that ekman pumping of on average 4 m day21 contributes to the local dynamics during strong and persistent upwelling events the curl driven ekman pumping reaches up to 20 m day21 a single complex empirical orthogonal function explains more than 72 of the spatial and temporal wind stress variance it reveals large wind stress curl at the center of the western santa barbara channel entrance this dominant mode correlates strongly r 2 5 0 79 with the wind stress observed at a moored buoy the location of largest ekman pumping in the ocean indicated by this mode coincides with the location of a laterally sheared cyclonic flow cold upwelled waters enter the santa barbara channel along its southern perimeter while warm and thus buoyant waters from the southern california bight exit the channel along its northern perimeter buoy wind stress and lateral current shear at 30 m depth correlate significantly at periods of about 3 5 and 6 days with phase lags of about 0 5 and 2 days respectively hydrographic observations from 1983 do not however indicate effects of ekman pumping on the internal mass field as winds vary in speed and direction at daily timescales this contrasts with 1984 observations that do indicate strong doming of isopycnals over the center of the channel at its western entrance winds prior to and during hydrographic observations in the spring of 1984 were both stronger and more steady than they were in 1983 cyclonic shears reach 0 4 f at the entrance of the santa barbara channel near point conception here f is the planetary vorticity the thermal wind balance explains the lateral shear of the alongchannel surface flow rather well

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

airborne observations of a catalina eddy summertime low level winds over the ocean adjacent to the california coast are typically from the north roughly parallel to the coastline past point conception the flow often turns eastward thereby generating cyclonic vorticity in the california bight clouds are frequently present when the cyclonic motion is well developed and at such times the circulation is referred to as a catalina eddy onshore flow south of the california bight associated with the eddy circulation can result in a thickening of the low level marine stratus adjacent to the coast during nighttime hours the marine stratus typically expands over a larger area and moves northward along the coast with the cyclonic circulation a catalina eddy was captured during the precisionatmosphericmarineboundarylayerexperimentinjuneof2012 measurementsweremadeofthe cloud structure in the marine layer and the horizontal pressure field associated with the cyclonic circulation using the university of wyoming king air research aircraft airborne measurements show that the coastal mountains to the south of los angeles block the flow resulting in enhanced marine stratus heights and a local pressure maximum near the coast the horizontal pressure field also supports a south north movement of marine stratus little evidence of leeside troughing south of santa barbara california was observed for this case implying that the horizontal pressure field is forced primarily through topographic blocking by the coastal terrain south of los angeles california and the ambient large scale circulation associated with the mean flow