circulations and rainfall on the lee side of the island of hawaii during harp using aircraft data and portable automated mesonet pam data during the hawaiian rainband project harp 11 july 24 august 1990 the circulation on the lee side of the island of hawaii is analyzed the largest temperature deviations 2 3 k and negative mixing ratio deviations 2 2t o2 4gk g 21 are found in the northwestern and southwestern leeside areas of the island because of the descending trade winds aloft in the lee of low 2 km mountain ridges and tops rainfall in these areas is much less compared to the central leeside coast e g kona coast in these areas the sea breeze duration is short compared to the kona coast because of the presence of opposing trade winds the wind speed is greater because of larger land sea thermal contrast during the stronger trade wind days adiabatic descent in the lee is more significant and the lcl is higher as a result in the northern and southern leeside areas localized trade wind rain showers driven by local circulations occur mainly under weak trades in the kona area the mean surface winds are calm behind the massive mountains with a daytime upslope onshore and nighttime downslope offshore flow along the axis of the return flow in the large wake off the kona coast the water vapor content is higher than the ocean upstream possibly caused by the low level convergence between the counterrotating eddies along the wake axis along the kona coast the rainfall in the afternoon hours continues in the early evening and reaches the daily maximum after the onset of the land breezes possibly because of the lifting of stronger moist return flow by the land breeze front under stronger trade winds the return flow would be stronger because of stronger island blocking more rainfall occurs there when trades are stronger

a case study of trade wind rainbands and their interaction with the island induced airflow abstract a case study of trade wind rainbands observed on 22 august 1990 during the hawaiian rainband project is presented it shows that the interaction between the morning rainbands and the island induced airflow is important for the evolution of the rainbands in the early morning of 22 august there are two convective periods 0400 0600 and 0700 0900 hst hawaii standard time on the windward side of the island of hawaii for both periods preexisting rain cells are observed in the trade wind flow at least 40 km upstream of the island and move westward toward the island at night and in the early morning the offshore flow opposes the trade winds resulting in a convergent region over the area immediately upstream of the island as the first group of rain cells 0400 0600 hst moves toward the island the low level convergent airflow provides a favorable kinematic background for the enhancement of the coming rain cells these rain cells merge in the convergent zone and become a well defined rainband

the effects of the island of hawaii on offshore rainband evolution abstract the diurnal cycle of airflow on the island of hawaii and the structure of the low level flow separation line between the island induced offshore flow and incoming trade winds are reasonably well understood this study examines the formation and organization of nocturnal and early morning offshore hawaiian rainbands the flow patterns associated with these rainbands and their relationship to the flow separation line this investigation uses the complete radar and aircraft dataset from the 1990 hawaiian rainband project the formation and evolution of nocturnal and early morning rainfall on the windward side of the island were found to be related to four major factors a the existence or absence of trade wind cloud patches arriving from the northeast pacific b the effect of island blocking on the trade wind flow as it approaches the island c the low level convergence and thermal and moisture stratification near the flow separation line and d the position of the flow separation line con

core processes earth s eccentric magnetic field earth s magnetic field is characterized by a puzzling hemispheric asymmetry calculations of core dynamo processes suggest that lopsided growth of the planet s inner core may be part of the cause

a multiwinter analysis of channeled flow through a prominent gap along the northern california coast during caljet and pacjet abstract experimental observations from coastal and island wind profilers aircraft and other sensors deployed during the california land falling jets experiment of 1997 98 and the pacific land falling jets experiment of 2000 01 2003 04 were combined with observations from operational networks to document the regular occurrence and characteristic structure of shallow 400 500 m deep cold airstreams flowing westward through california s petaluma gap from the central valley to the coast during the winter months the petaluma gap which is the only major air shed outlet from the central valley is 35 50 km wide and has walls extending at most a modest 600 900 m above the valley floor based on this geometry together with winter meteorological conditions typical of the region e g cold air pooled in the central valley and approaching extratropical cyclones this gap is predisposed to generating westward directed ageostrophic flows driven by along gap pressure differences two case studies and a fiv

pressure perturbations and upslope flow over a heated isolated mountain abstract surface and upper air data collected as part of the cumulus photogrammetric in situ and doppler observations cupido experiment during the 2006 monsoon season around the santa catalina mountains in southeast arizona are used to study the diurnal variation of the mountain scale surface convergence and its thermal forcing the thermal forcing is examined in terms of a horizontal pressure gradient force which is derived assuming hydrostatic balance the mountain is 30 km in diameter 2 km high and relatively isolated the environment is characterized by weak winds a deep convective boundary layer in the afternoon and sufficient low level moisture for orographic cumulus convection on most days the katabatic divergent surface flow at night and anabatic convergent flow during the day are in phase with the diurnal variation of the horizontal pressure gradient force which points toward the mountain during the day and away from the mountain at night the daytime pressure deficit over the mo

empirical evidence for latitude dependence and asymmetry of geomagnetic spatial variation in mainland china spatiotemporal geomagnetic variation is a significant research topic of geomagnetism and space physics generated by convection and flows within the fluid outer core latitude dependence and asymmetry as the inherent spatiotemporal properties of geomagnetic field have been extensively studied we apply and modify an extension of existing method hidden markov model hmm which is an efficient tool for modeling the statistical properties of time series based on ground magnetic measurement data set in mainland china first we find the parameters of hmm can be used as the geomagnetic statistical signature to represent the spatiotemporal geomagnetic variations for each site the results also support the existence of the geomagnetic latitude dependence more apparently furthermore we provide solid empirical evidence for geomagnetic asymmetry relying on such ground magnetic measurement data set

bottom up control of geomagnetic secular variation by the earth s inner core temporal changes in the earth s magnetic field known as geomagnetic secular variation occur most prominently at low latitudes in the atlantic hemisphere that is from 90 degrees east to 90 degrees east whereas in the pacific hemisphere there is comparatively little activity this is a consequence of the geographical localization of intense westward drifting equatorial magnetic flux patches at the core surface despite successes in explaining the morphology of the geomagnetic field numerical models of the geodynamo have so far failed to account systematically for this striking pattern of geomagnetic secular variation here we show that it can be reproduced provided that two mechanisms relying on the inner core are jointly considered first gravitational coupling aligns the inner core with the mantle forcing the flow of liquid metal in the outer core into a giant westward drifting sheet like gyre the resulting shear concentrates azimuthal magnetic flux at low latitudes close to the core mantle boundary where it is expelled by core convection and subsequently transported westward second differential inner core growth fastest below indonesia causes an asymmetric buoyancy release in the outer core which in turn distorts the gyre forcing it to become eccentric in agreement with recent core flow inversions this bottom up heterogeneous driving of core convection dominates top down driving from mantle thermal heterogeneities and localizes magnetic variations in a longitudinal sector centred beneath the atlantic where the eccentric gyre reaches the core surface to match the observed pattern of geomagnetic secular variation the solid material forming the inner core must now be in a state of differential growth rather than one of growth and melting induced by convective translation

atmospheric flow over a succession of nearly two dimensional ridges and valleys observations of neutral static stability flow in a nearly two dimensional valley are presented upstream and downstream of the valley are a succession of similar ridges and valleys the valley sides have a slope of about 32 to the horizontal and when the wind is nearly perpendicular to the valley there is a marked flow separation the observations of mean flow and turbulence are compared with numerical simulations of flow over periodic two dimensional ridges and valleys

thermochemical flows couple the earth s inner core growth to mantle heterogeneity the uppermost 100 km of the earth s inner core is subject to an east west divide seismic waves travel more rapidly and are attenuated more severely in the eastern than in the western hemisphere and the west is more anisotropic that is seismic waves travel at different speeds in different directions than the eastern hemisphere the origin of this hemispherical dichotomy has remained enigmatic aubert et al now show that a model incorporating thermochemical convection and dynamo action can account for these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle seismic waves sampling the top 100 km of the earth s inner core have revealed that the eastern hemisphere is seismically faster more isotropic and more attenuating than the western hemisphere it is now shown that a single model of thermo chemical convection and dynamo action can account for all these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle seismic waves sampling the top 100 km of the earth s inner core reveal that the eastern hemisphere 40 e 180 e is seismically faster1 2 more isotropic2 3 and more attenuating4 than the western hemisphere the origin of this hemispherical dichotomy is a challenging problem for our understanding of the earth as a system of dynamically coupled layers previously laboratory experiments have established that thermal control from the lower mantle can drastically affect fluid flow in the outer core5 which in turn can induce textural heterogeneity on the inner core solidification front6 the resulting texture should be consistent with other expected manifestations of thermal mantle control on the geodynamo specifically magnetic flux concentrations7 8 in the time average palaeomagnetic field9 10 over the past 5 myr and preferred eddy locations11 in flows imaged below the core mantle boundary by the analysis of historical geomagnetic secular variation12 here we show that a single model of thermochemical convection and dynamo action can account for all these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle the main feature of this thermochemical wind is a cyclonic circulation below asia which concentrates magnetic field on the core mantle boundary at the observed location and locally agrees with core flow images this wind also causes anomalously high rates of light element release in the eastern hemisphere of the inner core boundary suggesting that lateral seismic anomalies at the top of the inner core result from mantle induced variations in its freezing rate

the implications of long lived asymmetry of remanent magnetization across the north pacific fracture zones article i nfo large marine magnetic anomalies accompany the pacific fracture zones fzs for thousands of kilometers although the origin of these anomalies is poorly understood their underlying magnetization contrasts should reflect the temporal record of crustal accretion as well as geomagnetic field variations here we present an analysis of archival and newly collected magnetic anomaly profiles measured across three fzs from the north pacific cretaceous quiet zone 120 6 to 83 ma that are characterized by a remarkably uniform shape forward and inverse modeling indicate that these anomalies arise from remanent magnetization with enhanced remanence located on one side of each fz along the entire studied area a comparison of geochemical and magnetic data from active ridge discontinuities and transform faults suggests that elevated iron content near segment ends is likely responsible for the observed anomalies in the cretaceous quiet zone as well a more complex magnetization setting is observed where the fzs contain multiple faults there the simple model of one sided enhancements is only partly valid comparison between 3d forward modeling of the quiet zone magnetization and the calculated magnetization contrasts found across the pioneer and pau fzs suggests that the intensity of the geomagnetic field during the cretaceous superchron had less than 50 percent variability about its average value no major trends in the strength of the geomagnetic field during the superchron are observed the presence of long duration 30 m y zones of enhanced magnetization along the young old sides of the pioneer pau fzs both left stepping requires some long lived asymmetry in crustal construction processes near ridge transform intersections although the underlying mechanism that controls this long lived asymmetry remains unclear absolute plate motions might explain this asymmetry shorter period few m y variations in the amplitudes of the enhancements probably result from oscillations in crustal construction

evolution of downslope flow under strong opposing trade winds and frequent trade wind rainshowers over the island of hawaii abstract the evolution of downslope flow on the windward side of the island of hawaii during 7 8 august 1990 is investigated this period is characterized by atypical strong 11 m s 1 upstream trade winds and frequent nocturnal rainshowers in the late afternoon the onset of downslope flow first occurs on the upper slope which is frequently dominated by orographic clouds with mean weak surface winds on the windward slopes because of island blocking the downslope flow onset occurs after the slope surface becomes negatively buoyant along the hilo coastal areas that are well exposed to decelerating trade wind flow the offshore flow onset there occurs much later 2 h than the hawaiian rainband project harp mean as the drainage front moves slowly from the windward lowlands toward the coast against the atypical strong incoming trade wind flow the downslope flow onset along the coast occurs after a band of trade wind rainshowers has moved over the coast these rainshowers produce evaporatively coole

measurements of the flow structure around ailsa craig a steep three dimensional isolated hill observations of neutral flow around an isolated island are presented the island has a fairly smooth shape with slopes of about 30 and a horizontal section which is roughly elliptical with axis ratio 1 5 the flow field 4 m above the surface shows a speed up around the sides and over the summit with reverse flow on the lee slope measurements with an instrumented aircraft of the mean flow in the wake of the island reveal a single very powerful trailing vortex with axis close to the upstream wind direction there are indications that the single vortex was a consequence of the asymmetry caused by the island s elliptical shape measurements of fluctuating quantities with a tethered balloon system show marked changes in the components of the turbulence energy in the flow around the side of the hill

the north atlantic oscillation in the ncep ncar reanalysis abstract the upper air circulation characteristics of the north atlantic oscillation nao are studied from the 1958 97 national centers for environmental prediction national center for atmospheric research reanalysis with regard to interannual variability and long term trends the low high phase of the nao is defined by small large values of the ponta delgada azores minus akureyri iceland surface pressure difference for the fields of 200 mb topography divergence and divergent flow 500 mb vertical motion 1000 mb topography and total wind and sst during january differences are computed between ensembles of years of extremely low minus high nao phase and are tested for statistical significance in the low nao phase topographies throughout the tropospheric column stand anomalously high low in the subpolar subtropical domains ascending motion and upper tropospheric divergent outflow from the realm of the icelandic low broadly southward and convergent inflow and subsidence on the polewa

the cretaceous tertiary boundary ktb transition in ne brazil at 7800 km from yucatan the cretaceous tertiary boundary ktb transition of the poty quarry ne brazil is the most distant locality with published accounts of chicxulub impact tsunami deposits impact spherules and ir anomaly new investigations based on sedimentology biostratigraphy mineralogy and geochemistry fail to confirm these reports latest maastrichtian planktic foraminiferal zones cf1 below an erosive and bioturbated disconformity and early danian zone p1a 1 above indicate a short hiatus with the ktb clay zone p0 the ir anomaly and the characteristic negative 13 c excursion missing the disconformity coincides with the globally recognized latest maastrichtian sea level fall above the disconformity an upward fining micro conglomerate with abundant reworked cretaceous foraminifera sub angular phosphate clasts calcitic and phosphatic spheroids along with an early danian zone p1a 1 assemblage is interpreted as a gravity flow deposit common spheroids throughout the late campanian maastrichtian appear to be chamber infillings of the benthic foraminifer dentalina alternata minor ir anomalies in thin clay layers of zone pla and no evidence of the chicxulub impact reveal that the poty quarry section remains a very important example of the complex global environmental and sea level changes observed in kt sequences from north america to central america that are commonly misinterpreted as impact tsunami events supplementary materials data for trace elements are available at www geolsoc org uk sup18585

occurrence of upslope flows at the pico mountaintop observatory a case study of orographic flows on a small volcanic island 1 upslope flows caused by mechanical forcing in strong synoptic winds or by buoyant forcing driven by solar heating under weak synoptic winds can influence the air composition at mountaintop observatories using meteorological and trace gas measurements at the pico nare observatory on pico mountain azores islands north atlantic ocean the frequency and impact of such orographic flows on a small volcanic subtropical island was examined to determine the origin of mechanically lifted air upstream kinetic energy was balanced against potential energy gained during uplift sheppard s model mechanically forced upslope flow is most important during october through april when the calculated probability of observing marine boundary layer mbl air at the observatory near the summit ranges from 35 to 60 per month in contrast lower synoptic wind speeds and a more stable lower free troposphere during may september result in a reduced frequency of mbl impacts 20 buoyant upslope flows buf were quantified through meteorological measurements on the mountain slope in summer 2004 diurnal cycles of wind direction on the mountain slope consistent with daytime upslope and nighttime downslope flow were found on 24 of the days during late june july and august 2004 buoyant forcing can also occur in the presence of moderate synoptic winds resulting in enhancement of the mechanically forced upslope flow on the windward side of the mountain such conditions were found on 15 of the summer days in 2004 however on buf days the specific humidity at the mountaintop was significantly smaller than on the slope indicating turbulent mixing during ascent or vertical decoupling of air masses impacts of buf or a mixture of buoyant and mechanical upslope flow on o3 or nitrogen oxides mixing ratios at the mountaintop station were rare or extremely small and no significant diurnal cycle of o3 expected if daytime buf of mbl air occurred regularly was present midday increases in isoprene concentrations a nonmethane hydrocarbon nmhc expected to be emitted from vegetation more than 700 m below the pico nare station were found on 54 of the days during may august 2005 no corresponding increase in n butane used for heating and cooking at sea level residences was detected suggesting that the air did not originate from as low as sea level these results indicate that the latitude size and topography of pico island combine to prevent frequent transport of mbl air to the pico nare station in the summer

geomagnetic field hemispheric asymmetry and archeomagnetic jerks abstract we investigate the origin of the so called archeomagnetic jerks detected in the french archeomagnetic record over the past three millennia although only very large scale global archeomagnetic field models are currently available we show that the occurrence of archeomagnetic jerks is intimately linked to what we define as most eccentric events i e periods of time when a simple description of the geomagnetic field in terms of an eccentric dipole reveals the center of this eccentric dipole to strongly move away from the earth s center from the behavior of the much better known historical field we interpret the evolution of the center of the eccentric dipole as reflecting the production and gathering of flux patches at the core mantle boundary within preferential hemispheres archeomagnetic jerks would thus correspond to episodes of maximum geomagnetic field hemispheric asymmetry such most eccentric events could also provide an explanation for some of the properties previously reported in the long term paleomagnetic field

flow during the evening transition over steep alpine slopes a field campaign the slope experiment near la fouly self 2010 was conducted to monitor the evening transition of slope flows on clear sky days from july to september 2010 in a narrow valley of the swiss alps a steep west facing slope with inclinations ranging from 25 to 45 was instrumented from 1900mto 2200mabove sea level detailed measurements were made along a linear transect of the slope with two turbulence towers two weather stations five surface temperature measurement stations and a tethered balloon system the present study focuses on nine exemplary convective days characterized by weak synoptic flow and clear sky conditions during which thermal circulations prevail the analysis of the observational data shows that topographic shading triggers the evening transition the topographic configuration around the experimental site results in a sharply defined shading front propagating up slope causing a sudden decrease in incoming short wave radiation on the order of several hundreds of wm 2 within a few minutes the slope surface rapidly responds to the advancing shading front in some cases reductions in surface temperatures of some 10 c in less than 10 min are observed this is rapidly followed by an early evening calm period with very small turbulent kinetic energy tke 0 05m2 s 2 and extremely light wind speeds 0 5ms 1 when the inertia driven upslope flow is fully stopped by the katabatic acceleration a shallow local drainage flow forms and reaches a quasi equilibrium 1 5 h after the local sunset an analysis of the tke budget close to the surface shows that the buoyancy flux is much greater than the shear production in the last hours before the local sunset possibly due to valley curvature effects copyright c 2012 royal meteorological society

a laboratory model for convection in earth s core driven by a thermally heterogeneous mantle thermal convection experiments in a rapidly rotating hemispherical shell suggest a model in which the convection in earth s liquid outer core is controlled by a thermally heterogeneous mantle experiments show that heterogeneous boundary heating induces an eastward flow in the core which at a sufficiently large magnitude develops into a large scale spiral with a sharp front the front separates the warm and cold regions in the core and includes a narrow jet flowing from the core mantle boundary to the inner core boundary the existence of this front in the core may explain the pacific quiet zone in the secular variation of the geomagnetic field and the longitudinally heterogeneous structure of the solid inner core

terrain induced turbulence over lantau island 7 june 1994 tropical storm russ case study numerical simulations of terrain induced turbulence associated with airflow over lantau island of hong kong are presented lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level this research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of chek lap kok which is located to the lee of lantau intensive ground and aerial observations were taken from may through june 1994 during the lantau experiment lantex this paper focuses on flow associated with the passage of tropical storm russ on 7 june 1994 during which severe turbulence was observed the nature of the environmental and topographic forcing on 7 june 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction high resolution numerical simulations initialized using local sounding data were performed using the clark model the simulation results indicate that gravity wave dynamics played a very minor role in the flow distortion and generation of turbulence as a result of this flow regime relatively high vertical and horizontal resolution was required to simulate the mechanically generated turbulence associated with tropical storm russ results are presented using a vertical resolution of 10 m near the surface and with horizontal resolutions of both 125 and 62 5 m over local nested domains of about 13 24 km on a side the 125 m model resolution simulated highly distorted flow in the lee of lantau with streaks emanating downstream from regions of sharp orographic gradients at this resolution the streaks were nearly steady in time at the higher horizontal resolution of 62 5 m the streaks became unstable resulting in eddies advecting downstream within a distorted streaky mean flow similar to the 125 m resolution simulation the temporally averaged fields changed little with the increase in resolution however there was a three to fourfold increase in the temporal variability of the flow as indicated by the standard deviation of the wind from a 10 min temporal average overall the higher resolution simulations compared quite well with the observations whereas the lower resolution cases did not the high resolution experiments also showed a much broader horizontal and vertical extent for the transient eddies the depth of orographic influence increased from about 200 m to over 600 m with the increase in resolution a physical explanation using simple linear arguments based on the blocking effects of the eddies is presented the nature of the flow separation is analyzed using bernoulli s energy form to display the geometry of the separation bubbles the height of the 80 m2 s22 energy surface shows eddies forming in regions of large orographic gradients and advecting downstream tests using both buoyancy excitation and stochastic backscatter to parameterize the underresolved dynamics at the 125 m resolution are presented as well as one experiment testing the influence of static stability suppressing turbulence development all these tests showed no significant effect implications of these results to the parameterization of mechanically induced turbulence in complex terrain are discussed

analyses of shocked quartz at the global k p boundary indicate an origin from a single high angle oblique impact at chicxulub abstract the precise cause and timing of the cretaceous paleocene k p mass extinction 65 ma ago remains a matter of debate many advocate that the extinction was caused by a meteorite impact at chicxulub mexico and a number of potential kill mechanisms have been proposed for this although we now have good constraints on the size of this impact and chemistry of the target rocks estimates of its environmental consequences are hindered by a lack of knowledge about the obliquity of this impact an oblique impact is likely to have been far more catastrophic than a sub vertical one because greater volumes of volatiles would have been released into the atmosphere the principal purpose of this study was to characterize shocked quartz within distal k p ejecta to investigate whether the quartz distribution carried a signature of the direction and angle of impact our analyses show that the total number maximum and average size of shocked quartz grains all decrease gradually with paleodistance from chicxulub we do not find particularly high abundances in pacific sites relative to atlantic and european sites as has been previously reported and the size distribution around chicxulub is relatively symmetric ejecta samples at any one site display features that are indicative of a wide range of shock pressures but the mean degree of shock increases with paleodistance these shock and size distributions are both consistent with the k p layer having been formed by a single impact at chicxulub one site in the south atlantic contains quartz indicating an anomalously high average shock degree that may be indicative of an oblique impact with an uprange direction to the southeast 45 the apparent continuous coverage of proximal ejecta in this quadrant of the crater however suggests a relatively high impact angle of 45 we conclude that some of the more extreme predictions of the environmental consequences of a low angle impact at chicxulub are probably not applicable

forcing of flow reversal along the windward slopes of hawaii abstract hawaii is an island approximately 4 km high the lower portion of which is immersed in an easterly trade wind layer that is typically 2 km deep blockage of the trade wind combined with diurnal thermally driven circulations leads to a general flow stagnation along the windward slopes and to a reversal of flow at night this westerly downslope flow is confluent with the incoming trade wind and usually extends offshore from observations in the hawaiian rainband project this flow is examined to determine if it is primarily driven by blocking dynamics or thermal forcing it is determined that the westerly downslope flow is principally a thermally driven circulation and that it closely resembles a density drainage current this flow is normally initiated by evaporative cooling principally from orographic rainfall but perhaps also from evapotranspiration under less cloudy and precipitation free conditions downslope flow can be initiated and maintained by radiative cooling of the land surface b

possible links between long term geomagnetic variations and whole mantle convection processes the earth s internal magnetic field varies on timescales of months to billions of years the field is generated by convection in the liquid outer core which in turn is influenced by the heat flowing from the core into the base of the overlying mantle much of the magnetic field s variation is thought to be stochastic but over very long timescales this variability may be related to changes in heat flow associated with mantle convection processes over the past 500 myr correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late mesozoic era beginning about 180 myr ago simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability such as occurred between the middle jurassic and middle cretaceous periods may have been triggered by a decrease in core mantle boundary heat flow either globally or in equatorial regions this decrease in heat flow could have been linked to reduced mantle plume head production at the core mantle boundary an episode of true polar wander or a combination of the two

airflow within major alpine river valleys under heavy rainfall summary this study documents the aire ow within major alpine river valleys in the presence of precipitation the analysis seeks a conceptual understanding of the effect of rain on the valley e ow ground based and airborne doppler radar and surface data from the mesoscale alpine programme show that during persistent rainy periods the subsidence caused by melting and evaporation of precipitation particles contributes to the formation of a down valley e ow this down valley eow opposes moist southerly e ow aloft which is lifted over the topographic barrier and in which precipitation particles are formed

mantle driven geodynamo features effects of compositional and narrow d anomalies abstract lower mantle heterogeneity could cause deviations from axial symmetry in geodynamo properties global tomography models are commonly used to infer the pattern of core mantle boundary heat flux via a linear relation that corresponds to a purely thermal interpretation of lower mantle seismic anomalies ignoring both non thermal origins and non resolved small scales here we study the possible impact on the geodynamo of narrow thermal anomalies in the base of the mantle originating from either compositional heterogeneity or sharp margins of large scale features a heat flux boundary condition composed of a large scale pattern and narrow ridges separating the large scale positive and negative features is imposed on numerical dynamos we find that hot ridges located to the west of a positive large scale core mantle boundary heat flux anomaly produce a time average narrow elongated upwelling a flow barrier at the top of the core and intensified low latitudes magnetic flux patches when the ridge is located to the east of a positive core mantle boundary heat flux anomaly the associated upwelling is weaker and the homogeneous dynamo westward drift leaks precluding persistent intense low latitudes magnetic flux patches these signatures of the core mantle boundary heat flux ridge are evident in the north south component of the thermal wind balance based on the pattern of lower mantle seismic tomography masters et al 2000 we hypothesize that hot narrow thermal ridges below central asia and the indian ocean and below the american pacific coast produce time average fluid upwelling and a barrier for azimuthal flow at the top of the core east of these ridges below east asia and oceania and below the americas time average intense geomagnetic flux patches are expected

evolution of katabatic flow on the island of hawaii on 10 august 1990 abstract the data collected on 10 august 1990 from the hawaiian rainband project were analyzed to study the evolution of katabatic flow results from this study show that for a relatively dry case thermal forcing is important to account for the onset evolution depth and offshore extension of the katabatic flow on the windward side of the island of hawaii where the mean winds are weak because of island blocking during the evening transition the initiation of katabatic flow on the windward slopes was mainly driven by the diurnal heating cycle at the windward coast the wind shift was caused by the arrival of the drainage front from the windward lowlands after the arrival of the drainage front at hilo slopewise instability was generated because of rain showers along the coast and diminishing orographic clouds and showers on the slopes along the coast because of the reduced radiative cooling due to cloud cover and vertical mixing associated with rains a warm period was observed at hilo on the slop

cloud patterns lee of hawaii island a synthesis of satellite observations and numerical simulation 1 standing well above the trade wind inversion hawaii island maximum elevation 4 2 km splits the northeast trade winds and induces a westerly reverse flow in the wake satellite observations and regional model simulations are used to investigate circulation effects on lee cloud formation during summer over the island the cloud distribution is consistent with orographic induced vertical motions over the lee ocean our analysis reveals a cloud band that extends southwestward over a few tens of kilometers from the southwest coast of the island this southwest lee cloud band is most pronounced in the afternoon anchored by strong convergence and maintained by in situ cloud production in the upward motion such an offshore cloud band is not found off the northwest coast an asymmetry possibly due to the coriolis effect on the orographic flow off the kona coast the dynamically induced westerly reverse flow keeps the wake cool and nearly free of clouds during the day along the kona coast clouds are blown offshore from the island by the easterly trades in the afternoon in a layer above the reverse flow deprived of in situ production these afternoon kona coast clouds dissipate rapidly offshore at night the offshore land valley breezes converge onto the onshore reverse flow and a cloud deck forms on and off the kona coast bringing nighttime rain as observed at land stations to illustrate the circulation effect lee cloud formation is compared between tall hawaii and short kauai oahu islands which feature the flow around and flow over regimes respectively effects of trade wind strength on the leeside cloudiness are also studied

monitoring the monsoon in the himalayas observations in central nepal june 2001 abstract the monsoon himalayan precipitation experiment mohprex occurred during june 2001 along the south slopes of the himalayas in central nepal radiosondes were launched around the clock from two sites one in the marsyandi river basin on the eastern footslopes of the annapurna range and one farther to the southwest near the border with india the flights supported rainfall and other hydrometeorological observations including surface winds from the marsyandi network that has been operated in this region since the spring of 1999 the thermodynamic profiles obtained from the soundings support the observed nocturnal maximum in rainfall during the monsoon with total column moisture and instability maximized just before rainfall peaks coinciding with the appearance of a monsoon depression over central india the onset of the monsoon in this region was characterized by a weeklong weakening of the upper level westerlies and an increase in moisture and convective instability the vertical structure of

multiple factors in the origin of the cretaceous tertiary boundary the role of environmental stress and deccan trap volcanism a review of the scenarios for the cretaceous tertiary k t boundary event is presented and a coherent hypothesis for the origin of the event is formulated many scientists now accept that the event was caused by a meteorite impact at chicxulub in the yucatan peninsula mexico our investigations show that the oceans were already stressed by the end of the late cretaceous as a result of the long term drop in atmospheric co2 the long term drop in sea level and the frequent development of oceanic anoxia extinction of some marine species was already occurring several million years prior to the k t boundary the biota were therefore susceptible to change the eruption of the deccan traps which began at 66 2 ma coincides with the k t boundary events it erupted huge quantities of h2so4 hcl co2 dust and soot into the atmosphere and led to a significant drop in sea level and marked changes in ocean temperature the result was a major reduction in oceanic productivity and the creation of an almost dead ocean the volcanism lasted almost 0 7 m y extinction of biological species was graded and appeared to correlate with the main eruptive events elements such as ir were incorporated into the volcanic ash possibly on soot particles this horizon accumulated under anoxic conditions in local depressions and became the marker horizon for the k t boundary an oxidation front penetrated this horizon leading to the redistribution of elements the eruption of the deccan traps is the largest volcanic event since the permian triassic event at 245 ma it followed a period of 36 m y in which the earth s magnetic field failed to reverse instabilities in the mantle are thought to be responsible for this eruption and therefore for the k t event we therefore believe that the k t event can be explained in terms of the effects of the deccan volcanism on an already stressed biosphere the meteorite impact at chicxulub took place after the onset of deccan volcanism it probably played a regional rather than a global role in the k t extinctions

regional modeling of the geomagnetic field in europe from 6000 to 1000 b c we have developed a first low degree regional geomagnetic model for the european continent valid for the period 6000 1000 b c from a selected compilation of sedimentary and archeomagnetic data the scha dif 8k model this model provides information about both direction declination and inclination and intensity of the earth s magnetic field by connecting it with our previous model scha dif 3k valid from 1000 b c to 1900 a d and the igrf we furnish continuous geomagnetic field information for the last 8000 years in europe it has been developed using the revised spherical cap harmonic analysis in 2 dimensions technique r scha2d and using the norm of the earth s magnetic field to constrain the inversion problem the size of the cap is 22 and the maximum degree of the expansion is 2 the linearization problem was solved by using the truncated taylor s series applied to the expressions representing the relationship between the declination inclination and intensity data and the cartesian components of the geomagnetic field we used the geocentric axial dipole gad field as our initial or reference field for time we used the classical sliding overlapping window method the size of the window was set to 100 years shifted by 50 years we compared the model s prediction with the input data with the global cals7k 2 model and with new independent data the regional model shows a better fit to the input and to the independent data than the global model especially in terms of intensity and agrees with the virtual axial dipole moment given by other studies for the last 8000 years the european geomagnetic field has recorded rapid changes or archeomagnetic jerks the average field for the last 8000 years in europe is indistinguishable from the gad field

on the dynamics of hawaiian cloud bands comparison of model results with observations and island climatology abstract this paper presents a detailed comparison study of three dimensional model results with an aircraft wind field mapping for the island of hawaii model runs were initialized using an aircraft sounding from 1 august 1985 and detailed predictions from the model are compared with observations from that day the strength and location of the upwind convergence zone were well simulated as well as the strong deflection and deceleration of the flow around the island and the geometry and location of the upstream cloud bands the good agreement between the model results and observations supports the results of our previous study in which we show that the flow pattern and associated cloud processes around the island of hawaii can be understood by considering the flow of a stably stratified fluid around a large three dimensional obstacle model runs with different wind directions showed that increasing northerly tradewind flow resulted in the band clouds moving closer to the shore line and the large scale

diurnal cycle of convective instability around the central mountains in japan during the warm season abstract convective rainfall often shows a clear diurnal cycle the nighttime peak of convective activity prevails in various regions near the world s mountains the influence of the water vapor and convective instability upon nocturnal precipitation is investigated using a numerical model and observed data recent developments in gps meteorology allow the estimation of precipitable water vapor pwv with a high temporal resolution a dense network has been established in japan the gps analysis in august 2000 provides the following results in the early evening a high gps pwv region forms over mountainous areas because of the convergence of low level moisture which gradually propagates toward the adjacent plain before midnight a region of convection propagates simultaneously eastward into the plain the precipitating frequency correlates fairly well with the gps pwv and attains a maximum value at night over the plain the model also provides similar characteristics in the diurnal cycles of rainfall an

the role of offshore convergence on coastal rainfall during tamex iop 3 abstract the role of offshore convergence on the coastal rainfall maximum along the northwestern coast of taiwan is analyzed based on tamex taiwan area mesoscale experiment data and numerical experiments using the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 from a case study during tamex iop 3 moderate rainfalls 40 mm h 1 are observed along the northwestern coast of taiwan associated with the arrival of three rainbands in succession during 1000 1600 lst these rainbands form over the taiwan strait with northeast southwest or north south orientation they intensify off the northwestern coast there is a tendency for the most intense echoes to align in a northeast southwest orientation off the northwestern coast an orographically enhanced convergence zone with a northeast southwest orientation occurs in the area where the deflected southerly flow converges with the prevailing southwesterly flow that is modified by the storm

numerical simulations and observations of airflow through the alenuihāhā channel hawaii abstractduring the summer sustained winds in the alenuih h channel hawaii may exceed 20 m s 1 with higher gusts the advanced research weather research and forecasting model is used to diagnose airflow in the hawaiian coastal waters high resolution 2 km runs are performed for july 2005 covering the alenuih h channel and nested in a 6 km state domain under normal trade wind conditions 7 8 m s 1 winds at the channel entrance are 1 2 m s 1 faster than upstream due to the convergence of the deflected airflows by the islands of maui and hawaii and accelerate through the channel due to along gap pressure gradients and lower pressure in the wakes of both islands the acceleration is accompanied by descending airflow 9 cm s 1 in the exit region with lowering of the trade wind inversion deceleration occurs downstream of the channel exit with a rapid change from sinking motion to rising motion 3 cm s 1 under normal or strong trade wind conditions the flow is subcritical froude number fr

numerical simulations of airflow and cloud distributions over the windward side of the island of hawaii part ii nocturnal flow regime abstract numerical experiments are performed with the fifth generation mesoscale model to study the evolution of island airflow thermodynamic fields and clouds over the island of hawaii at night the model has successfully simulated the major observed features associated with the nocturnal flow regime these include the formation of nocturnal inversion development gradual offshore extension and deepening of the katabatic flow shifting of the overall cloudy areas from the windward slopes to the ocean and generation of clouds within the simulated offshore convergence zone furthermore it is also shown that rain evaporative cooling affects the depth strength and offshore extension of the katabatic flow in the early evening the nocturnal cooling provides the land sea thermal contrasts for the development of the simulated downslope flow on the windward slopes with continued nocturnal cooling and rain evaporation the simulated katabatic flow extends toward the coast the simulated convergence zone

diurnal variation of surface airflow and rainfall frequencies on the island of hawaii abstract the high resolution portable automated mesonet data from the hawaiian rainband project are used to document the circulation over the entire island of hawaii for the first time it is shown that the surface airflow and rainfall occurrences over the island are strongly modulated by the diurnal heating cycle most areas over the island show daytime upslope and nighttime downslope components separated by morning and evening transitions on the windward slope the onset of the downslope upslope wind during the evening morning transition starts on the slopes and progresses downward the effects of island blocking are also evident on the windward slopes of mauna kea and mauna loa the mean winds are weak 1 m s 1 due to flow deceleration flow splitting occurs in the hilo bay area as the trade winds are forced to move around the island on the lee side the trade winds are absent for stronger trade wind days the island blocking is more significant with a higher surface pressure 0 2 0 5 hpa on

boundary layer updrafts driven by airflow over heated terrain abstractthis study presents linear and nonlinear scalings for boundary layer ascent forced by airflow over heated terrain and compares them to results from corresponding high resolution numerical simulations close agreement between theory and simulation is found over most of the parameter space considered including variations in background winds boundary layer stability mountain height and diabatic heating rate as expected the linear and nonlinear scalings perform best for linear and nonlinear flows respectively for a convective boundary layer the scalings accurately predict vertical motion for all flows considered including those that extend well into the nonlinear regime thus these scalings may ultimately help to improve the parameterization of subgrid orographic ascent in large scale models the vertical velocity scalings are less accurate for mechanically blocked flows in stable boundary layers for which a simple vertical displacement scaling is superior although the scalings do not tre

tropical island convection in the absence of significant topography part i life cycle of diurnally forced convection abstract diurnally forced convection was observed over the tiwi islands north of the australian continent as part of the maritime continent thunderstorm experiment immature peninsula scale 5 15 km sea breezes were observed to initiate moist convection early each day principally through convergence that results from the confluence or collision of peninsula breeze fronts convection initiated by peninsula scale breezes usually fails to organize beyond a small cluster of cells and dissipates as a local event mature island scale 100 km breezes develop by late morning and subsequently play a pivotal role in the forcing and evolution of organized convection the initiation of mesoscale convective systems mcss is observed to be a direct consequence of breeze front collisions for only 20 of the days on which organized convection develops this is referred to as type a forcing and it occurs when normal convective development is delayed or otherwise suppressed type a forcing is nature s backup mech

numerical simulations of island scale airflow over maui and the maui vortex under summer trade wind conditions abstract the fifth generation pennsylvania state university ncar mesoscale model mm5 coupled with the noah land surface model lsm is employed to simulate island scale airflow and circulations over maui county hawaii under summer trade wind conditions during july august 2005 the model forecasts are validated by surface observations with good agreement in this study it is shown that a previously known closed circulation over the central valley of maui or the maui vortex represents the northern cyclonic vortex of the dual counter rotating vortices in the lee of haleakala which extend up to the base of the trade wind inversion with a westerly reversed flow 2 m s 1 at low levels the northern cyclonic vortex is more pronounced than the southern anticyclonic vortex the asymmetric structure of the dual vortices is related to the shape of haleakala and the flow deflection by the west maui mountains the maui vortex has a relatively narrow east west extent in the lowest levels especially at nigh

the cretaceous tertiary boundary event in ecuador reduced biotic effects due to eastern boundary current setting a multidisciplinary study of a new cretaceous tertiary k t boundary section near guayaquil ecuador reveals an unusually cool water low diversity planktic foraminiferal fauna and a high diversity radiolarian fauna similar to those found in southern high latitude k t sequences despite the fact that this section was deposited near the cretaceous equator the k t boundary is located by planktic foraminifera within a narrow interval bounded by last appearances of tropical cretaceous species and first appearances of tertiary species including parvularugoglobigerina eugubina as in southern high latitudes there is no major mass extinction of either planktic foraminifera or radiolarians at this level a major radiolarian faunal discontinuity occurs some 6 m higher in the section within foraminiferal zone plc some 300 500 kyr after the k t event 13c values from bulk carbonates show both high and low latitude characteristics similarly to low latitudes there is a 3 negative 13c excursion at the k t boundary which is generally interpreted as a major decrease in primary productivity but unlike the low latitudes recovery occurs within a few thousand years as compared with 300 500 kyr and suggests rapid nutrient influx from the antarctic region via a current similar to the humboldt current today similarly to high latitude k t sequences a negative 13c shift occurs in the early danian zone plb about 300 kyr after the k t boundary sedimentologic and mineralogic data indicate a late maastrichtian with relatively low biogenic quartz and high carbonate followed by increasing biogenic quartz 50 and decreasing carbonate 5 during the early danian this suggests intensified atmospheric and oceanic circulation and upwelling off ecuador during the early danian the k t transition is marked by increased volcanic activity continental erosion and terrigenous influx but this also occurs in the early danian zone at the p1ap1b zonal transition and is thus not unique to the k t boundary we suggest that the catastrophic biotic effects normally observed at the k t boundary in low latitudes are greatly reduced or absent in the eastern equatorial pacific because this region was dominated then as now by upwelling and current transport of nutrient rich waters from the antarctic ocean as a result the biotic patterns are characteristic of southern high latitudes whereas the 13c pattern combines ameliorated low latitude effects with predominantly high latitude trends

near k t age of clastic deposits from texas to brazil impact volcanism and or sea level lowstand near k t boundary clastic deposits from texas mexico haiti guatemala and brazil often described as impact generated tsunami deposits are stratigraphically below well defined k t boundary horizons and appear not to be causally related to the k t boundary event stratigraphic evidence indicates that their deposition began during the last 170 200 kyr of the maastrichtian coincident with a major eustatic sea level lowstand that lowered sea level by as much as 70 100 m clastic deposition ended a few tens of thousands of years before the k t boundary during a rapidly rising sea level the presence of glass in clastic deposits in haiti northeastern mexico and yucatan suggests that the sea level lowstand coincided with a time of major volcanism or pre k t boundary bolide impact

numerical simulations of local winds over steep orography in the storm over north norway on october 12 1996 some power pylons 30 m high broke down in a storm over north norway in october 1996 the pylons were situated in a short narrow valley 3 km long surrounded by mountains reaching 600 850 m the pylons fell in a direction opposite to the main southwesterly winds and it is likely that the damage was caused by wind gusts of more than 50 m s 1 high resolution simulations with the nonhydrostatic numerical model memo have been made to investigate the local winds initial conditions have been taken from successful limited area synoptic scale predictions of the damaging storm for upstream southwesterly wind leeside bluff separation was simulated in the valley the recirculation filled the valley in a flow organized like a corkscrew down the valley in the bottom of the valley the recirculation formed a return jet with a maximum wind speed between 17 and 20 m s 1 in the different simulations and with a considerable component toward the separation line at the ground downstream from the mountaintops on the southern side of the valley the axis of the corkscrew was found at 0 4 of the valley height a veering of the initial upstream wind direction of 20 changed the flow to be steered up the valley with a shallow recirculation confined to the southern side of the valley only the turbulence in the recirculation area was found to be large and it is argued that the wind gusts in the southwesterly case might reach values 3 times the mean wind speed

air flow over a two dimensional hill studies of velocity speed up roughness effects and turbulence wind tunnel measurements have been made of the streamwise mean and turbulent velocities over a rough bell shaped two dimensional hill with height h and maximum slope 0 26 placed in a neutrally stable boundary layer of thickness 10 h and roughness length zo 0 02 h close agreement is found between the mean velocity and predictions obtained from taylor s 1977 computational model and jackson and hunt s 1975 analytical linearized model for locations at or upwind of the hill top but not in the wake examination of the models shows that the shear stresses are only important in an inner region close to the hill surface so that as suggested by jackson and hunt 1975 the perturbation to the mean flow outside this region is essentially inviscid the theory shows that over very rough surfaces such as wooded or urban terrain the height of this inner region can be of the same order as the height of the roughness elements so that in our experiments no measurements could be made in this region in a second experiment flow over a smooth hill on a rough surface was studied the additional increase of wind speed over the hill top can be estimated by assuming a linear superposition of the velocity changes produced by the changes in elevation and in surface roughness in this case rough to smooth in the lee of a hill however the change in roughness significantly alters the flow with flow separation being suppressed and here a linear superposition is not appropriate finally we consider why observed changes in turbulence structure close to the surface differ from those well above the surface calculations of these changes based on the simple theoretical arguments of equilibrium shear layers and rapidly distorted turbulent flows agree well with turbulence measurements in wind tunnels and in the field

trade wind rainfall near the windward coast of hawaii a dynamically based mesoscale climatology of rainfall and other data are examined for typical trade wind conditions near hawaii relative distributions of rainfall are deduced from radar reflectivity data for a 4000 km 2 region over the windward island and the upstream ocean the cumulative rainfall data suggest three zones of forcing that are characteristic of 1 the regional ocean 2 upstream island induced divergence and 3 flow reversal near and over the island it is confirmed that most intense rainband amplifications occur over a mesoscale convergence line that separates the easterly trade winds from an island induced westerly flow this flow separation line resembles a classical gravity current when positioned over the windward island and near shore the domain scale cumulative rainfall is statistically associated with the strength of island blocking as defined by a froude number fr when fr is in the ordinary range 0 3 the island maximum is five times greater than the oceanic average when fr is elevated 0 3 the rainfall maximum increases to 10 times the oceanic average overall the windward island maximum is seven times the upstream oceanic average increased rainfall is more strongly correlated with increased wind speed than with a reduction in dry static stability both component variables of fr unlike rainfall amount the position of the island maximum is relatively insensitive to fr except in the hours surrounding sunrise in the upstream divergence zone 20 45 km offshore rainfall is double the oceanic background it is speculated that the cause is blocking related there are very strong diurnal variations in rainfall amount and distribution both over the island and the upstream ocean windward island rainfall is at a maximum in the hours surrounding midnight when breeze and blocking forcings are fully cooperative in the coastal region seemingly unrelated oceanic rainfall exhibits a strong nocturnal maximum the temporal phase of which may be influenced by wind speed a deep minimum in domain scale rainfall occurs near noon

palaeomagnetic field intensity variations suggest mesoproterozoic inner core nucleation the earth s inner core grows by the freezing of liquid iron at its surface the point in history at which this process initiated marks a step change in the thermal evolution of the planet recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the earth s core resulting in estimates of the timing of inner core nucleation ranging from less than half a billion to nearly two billion years ago recent inner core nucleation high thermal conductivity requires high outer core temperatures in the early earth that complicate models of thermal evolution the nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner core nucleation to be estimated directly previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements by the lack of an effective means of assessing their reliability and by shorter timescale geomagnetic variations here we examine results from an expanded precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria our analysis provides intensity based support for the dominant dipolarity of the time averaged precambrian field a crucial requirement for palaeomagnetic reconstructions of continents we also present firm evidence for the existence of very long term variations in geomagnetic strength the most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1 5 billion years ago this observation is most readily explained by the nucleation of the inner core occurring during this interval the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the earth