geomagnetic field for 0 3 ka 2 a new series of time varying global models 1 steadily increasing numbers of archeomagnetic and paleomagnetic data for the holocene have allowed development of temporally continuous global spherical harmonic models of the geomagnetic field extending present and historical global descriptions of magnetic field evolution the current work uses various subsets of improved data compilations details of which are given in a companion paper by donadini et al 2009 and minor modifications of standard modeling strategies using temporally and spatially regularized inversion of the data and cubic spline parameterizations for temporal variations to produce five models with enhanced spatial and temporal resolution for 0 3 ka spurious end effects present in earlier models are eliminated by enforcing large scale agreement with the gufm1 historical model for 1650 1990 a d and by extending the model range to accommodate data older than 3 ka age errors are not considered as a contribution to data uncertainties but are included along with data uncertainties in an investigation of statistical uncertainty estimates for the models using parametric bootstrap resampling techniques we find common features but also significant differences among the various models indicating intrinsic uncertainties in global models based on the currently available holocene data model cals3k 3 based on all available archeomagnetic and sediment data without a priori quality selection currently constitutes the best global representation of the past field the new models have slightly higher dipole moments than our previous models virtual axial dipole moments vadms calculated directly from the data are in good agreement with all corresponding model predictions of vadms these are always higher than the spherical harmonic dipole moment indicating the limitations of using vadms as a measure of geomagnetic dipole moments

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

mechanisms of interannual variability of the southwest united states summer rainfall maximum abstract the mid summer rainfall singularity of the southwest united states principally arizona exhibits marked variations on interannual and decadal time scales examination of the synoptic mechanisms involved in these variations is undertaken here in particular associations between the rainfall the dominant latitude of the summertime mid tropospheric subtropical ridge str over the southwest united states and the sea surface temperatures ssts of the equatorial and north pacific region are documented the analysis utilizes a composite approach for sets of extreme years chosen on the basis of the rainfall and circulation anomalies it is found that northward southward displaced seasonal str is associated with wetter drier summers in arizona further these extremes have tended to follow winters characterized by positive negative phases of the pacific north america pna teleconnection pattern the latter association arises at least in part from the memory imparted to the atmosphere by th

simulation of a north american monsoon gulf surge event and comparison to observations 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 studied but their detailed evolution is still unclear to address this several convection permitting simulations are performed over the core monsoon region for the 12 14 july 2004 gulf surge event this surge event occurred during the north american monsoon experiment which allows for extensive comparison to field observations a 60 h reference simulation is able to reproduce the surge event capturing its main characteristics speed and direction of motion thermodynamic changes during its passage and strong northward moisture flux while the ti

relationships between soil moisture and summer precipitation over the great plains and the southwest 1 the relationships between evaporation e and summer precipitation p over the united states are examined using the observed precipitation data set the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis and the ncep regional spectral model rsm simulations the composites of p anomalies based on soil moisture and e anomalies indicate that the relationships between e and p are regionally dependent the e anomalies over the great plains are associated with large p anomalies located north of the e anomalies and areas downstream along the path of the great plains low level jet gpllj the impact of e anomalies on p over the southwest is small and is only recognized during the strong moisture surge events from the gulf of california the vertically integrated meridional moisture fluxes qv associated with the gpllj are stronger and more persistent than the fluxes associated with the low level jet from the gulf of california to the southwest gcllj the e anomalies over the great plains are stronger and more persistent than the e anomalies over the southwest therefore the e anomalies over the great plains have a better chance for the local feedback and for the changes in moisture flux convergence to take effect the impact of e anomalies on p over the southwest is small and only recognized during strong moisture surge events from the gulf of california both the surge events and e anomalies over the southwest persist for less than one week the time window for the large e anomalies to occur during surge events is narrow therefore the net impact is small

diurnal boundary layer development over sloping terrain abstract a one dimensional prognostic model of the atmospheric boundary layer coupled to a surface energy budget is described which utilizes a profile formulation non local for exchange coefficients in the daytime convective boundary layer and an exchange coefficient scheme based on local richardson number for the stable nocturnal boundary layer this combined local and non local model is used to simulate the full diurnal cycle of the boundary layer behavior of days 33 34 of the wangara experiment results of the simulation indicate that these simplified exchange coefficient schemes simulate the mean features of the boundary layer as well as the more complex and computationally expensive higher order closure models this diurnal boundary layer formulation is incorporated into a two dimensional mesoscale model employing a terrain following coordinate system to examine boundary layer behavior over sloping terrain similar to that of the great plains of the united states particular emphasis is placed on th

diurnal variation and horizontal extent of the low level jet over the northern gulf of california abstract this note describes aspects of the diurnal variation and geographical location of the summertime low level jet over the gulf of california based on wind soundings made at two sites near the northern end of the gulf during a two week summer field program in 1995 the low level wind observations from puerto penasco a site along the northern shore of the gulf showed a strong diurnal cycle decreasing in amplitude to near 0 by the 1800 m altitude a weaker but deeper diurnal cycle was observed at mexicali an inland site some 200 km west northwest of puerto penasco the strongest winds were observed at the coastal site the observations suggest that the strongest southerly winds in the northern gulf lie closer to the eastern shore of the gulf

regional simulation of the low level monsoon winds over the gulf of california and southwestern united states using a fine scale 10 20 km nested regional modeling system synoptic variations in climatological summertime low level wind fields over the gulf of california and the southwestern united states are described under appropriate synoptic conditions strong surge events can develop these are characterized by low level southerly flow over the entire gulf of california with southerly winds extending into arizona california and southern nevada vertically these southerly winds are present through the bottom 1 2 km of the atmosphere southerly flow is persistent throughout the day with some local diurnal cycling over the foothills of the sierra madre occidental and northwestern mexico under nonsurge conditions the simulated low level winds have a significantly different geographic structure nighttime southerly flow is limited to the northeastern gulf and small portions of southwestern arizona flow over the central and southern gulf is northerly with weak variable winds over the foothills of the sierra madre occidental during the day southerly winds are present over the central and northern gulf of california extending into southwestern arizona however this southerly wind pattern does not support continuous flow from the mouth of the gulf into northwestern mexico instead there is a westerly component associated with predominantly upslope flow over the foothills of the sierra madre occidental and the sonora desert

padm2m a penalized maximum likelihood model of the 0 2 ma palaeomagnetic axial dipole moment summary we present a new time varying model for palaeomagnetic axial dipole moment padm for the past 2 myr and compare it with earlier virtual axial dipole moment vadm reconstructions which have been based on stacking and averaging scaled relative palaeointensity records the padm is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood pml approach to recover a cubic b spline representation of axial dipole field variations on million year timescales the pml method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average vadm reconstructions we apply the pml method to a set of 96 032 published data 1800 palaeointensities from igneous rocks 3300 archaeointensities and 86 relative palaeointensity time series of variable lengths and resolutions two models are discussed padm2mp is a trial model based on a subset of the nine longest available sedimentary records padm2m uses a comprehensive data set 76 records 81 446 data 10 records were eliminated and is our preferred model padm2m has a lower mean than existing vadm reconstructions but shows similarities in long period variability some differences in timing amplitude and resolution of certain features can be attributed to variations in age assignments others result from our more comprehensive data set and a reduction in bias attributable to pml modelling padm2m has an average axial dipole moment over 0 2 ma of 5 3 1022 am2 with a standard deviation of 1 5 1022 am2 the brunhes chron average 6 2 1022 am2 is higher than for earlier epochs of matuyama 4 8 1022 am2 as seen in some previous studies the power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 myr 1 we see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour

model dynamics of summertime low level jets over northwestern mexico using a fine scale nested regional modeling system the diurnal forcing of summertime low level winds over the gulf of california and northwestern mexico is investigated on diurnal timescales simulated nocturnal low level jets develop over the northern portion of the gulf the foothills of the sierra madre occidental and parts of southern arizona the southerly component of the nocturnal jet is the result of a geostrophic balance involving the coriolis force and a cross gulf pressure gradient force associated with nighttime slope cooling over the elevated sierra madre occidental additionally horizontal temperature gradients over the sloped orography produce vertical variations in this cross gulf pressure gradient force generating the jet like vertical shear in wind components above the nocturnal boundary layer frictional effects are responsible for producing shear in the wind profiles below the boundary layer this balance is distinctly different from the inertial balance that is believed to be responsible for the low level jet over the great plains region of the united states daytime winds are part of a directly driven wind field forced by horizontal pressure gradients associated with slope heating up the sierra madre occidental and sea land temperature gradients north of the gulf during synoptically forced surge events a similar diurnal cycle in low level flow is still present however the local thermal forcing appears to be superimposed upon the large scale synoptic forcing resulting in weaker up slope flow during the day and stronger along slope flow at night

surges over the gulf of california during the mexican monsoon abstract northward surges of relatively cool moist maritime air from the tropical pacific into the southwestern united states occur via the gulf of california every summer during the mexican monsoon season these surges advect large amounts of moisture northward and promote increased convective activity in arizona mesoscale model output from 32 successive 24 h simulations are used to examine both the large scale and mesoscale features associated with surge events model data are compared to high resolution aircraft measurements on two days showing clearly that the model is capable of reproducing many of the detailed structures found in the observations results from a large scale analysis indicate that strong surges are produced in the model when a midlatitude trough is in the proper phase relationship with a tropical easterly wave weak surges occur when this relationship is missing

the marine atmospheric boundary layer jet in the coastal ocean dynamics experiment the vertical and horizontal structure of the atmospheric boundary layer near the northern california coast was investigated during spring upwelling conditions as part of the 1981 coastal ocean dynamics experiment two daytime aircraft flights were devised to measure mean and turbulent quantities for 25 km tracks along and across the continental shelf from 30 to 1200 m altitude the pacific high offshore and an inland thermal low characterized the synoptic situations and equatorward winds resulted which produced intense ocean upwelling the wind profile was found to be jet shaped maximum wind speeds were 25 m s 1 and 16 m s 1 at 400 and 100 m respectively in the two flights above and below the heights of the jet speed maxima the wind speed decreased rapidly sharp density inversions existed at the levels of the peak wind speeds in one flight the inversion also sloped down toward the coast turbulence was effectively confined below the inversions strong zonal temperature gradients above the inversion between the hot land and cool marine air caused a thermal wind for which the wind speed decreased with height below the inversion turbulent stress divergence was large and effective in transferring the horizontal momentum of the wind to the sea slowing the wind there was also a small thermal wind below the inversion but opposite to the one above owing to the well mixed air temperature following the sea surface temperature which was cold at the coast and relatively warmer offshore owing to the upwelling these effects explain the jet shaped wind profile the mean momentum equations show that an acceleration toward the coast can balance the pressure gradient coriolis and turbulent stress divergence forces and there were some observations to support this complete balance was not obtained for the turbulent kinetic energy budget inferred dissipation rates were too large

paleointensity estimates from historic and modern hawaiian lava flows using glassy basalt as a primary source material abstract published paleointensity estimates derived from lavas extruded in known fields are highly variable and rarely recover the expected field strength within an accuracy of better than 10 inconsistent results on modern volcanic rocks lend even greater uncertainty to intensity experiments performed on lava flows emplaced during periods of unknown geomagnetic field strength the majority of published paleointensity data are collected from the slowly cooled massive centers of lava flows where the magnetic grains are more likely to be multi domain and produce non ideal experimental results glassy volcanic material found on subaerial lava flow tops and in sub aqueous and subglacial environments however is rapidly cooled and therefore most likely of all volcanic materials to behave as single domain particles demanded by neel theory we present a new paleointensity study of historic and modern hawaiian lavas and test the viability of subaerially emplaced glassy basaltic material as an accurate recorder of magnetic field intensity six of eight lava flows sampled on the big island of hawaii 1843 1859 1935 1950 1960 1990 c e produce well behaved arai plots and recover an average intensity to within 2 7 t of the expected field strength or better than 8 accuracy we apply very strict selection criteria including a minimum of three specimens per site to prevent extraneous field estimates from affecting the final results individual volcanic glass results from the 1960 c e lava flow have a much lower variance than published data from the same volcanic unit glassy materials should therefore be collected wherever possible as they allow recovery of geomagnetic field strength with unprecedented accuracy

influence of spatial resolution on diurnal variability during the north american monsoon abstract diurnal variability is an important yet poorly understood aspect of the warm season precipitation regime over southwestern north america in an effort to improve its understanding diurnal variability is investigated numerically using the fifth generation pennsylvania state university psu ncar mesoscale model mm5 the goal herein is to determine the possible influence of spatial resolution on the diurnal cycle the model is initialized every 48 h using the operational ncep eta model 212 grid 40 km model analysis model simulations are carried out at horizontal resolutions of both 9 and 3 km overall the model reproduces the basic features of the diurnal cycle of rainfall over the core monsoon region of northwestern mexico and the southwestern united states in particular the model captures the diurnal amplitude and phase with heavier rainfall at high elevations along the sierra madre occidental in the early afternoon that shifts to lower elevations along the west slopes in the evening a

scale analysis of marine winds in straits and along mountainous coasts abstract the complicated wind regimes in straits which develop in response to different large scale pressure fields are investigated by scale analysis of the equations of motion adjustment of the mass and motion fields in straits o los km in width is governed by four nondimensional numbers separate along and cross strait rossby numbers a strait drag coefficient and a stratification parameter which relates the internal rossby radius of deformation to the width of the strait the wind field is in approximate geostrophic balance with an imposed cross channel pressure gradient an along channel pressure gradient is primarily balanced by ageostrophic acceleration of the wind field down the axis of the strait the gap wind vertical motion and the accompanying horizontal divergence in the near surface wind field can be large even for moderately stable stratification as a consequence there may be particularly abrupt transitions of the surface wind field at the exits of straits where there is a rapid c

the lower atmosphere over the gulf of california automated meteorological observations at coastal and island stations instrumented aircraft flights and coastal soundings were made between july 1983 and june 1985 to define the variations of the lower atmosphere over the northern half of the gulf of california during the two important climatic seasons the mid latitude winter and the subtropical summer a marine layer is well defined over water during both seasons but dissipates within a few kilometers inland the winter large scale pressure field is dominated by the great basin high over the southwestern united states modulated by upper level synoptic activity it causes 3 to 6 days events of northwesterly winds 8 12 m s 1 directed along the gulf s axis which are coherent over basin scales and bring cool dry desert air over the gulf the vertical scale of these winds is 700 1000 m over land on the western side of the gulf close to the height of the baja california mountains but only 100 400 m over water defined by the wintertime inversion cross gulf winds related to topographic effects during upper level trough passages over baja california are particularly intense in the northwestern portion of the gulf the winter marine layer is defined by a 1 to 4 5 c temperature inversion and a dew point temperature of 6 11 c a moisture content of 6 8 g kg 1 weak subsidence reduces dew point values to 4 c above the inversion winter winds above the marine layer are coherent 0 8 across the width of the gulf the summer large scale pressure field is dominated by a thermal low over the southwestern united states and drives weak 2 5 m s 1 southeasterly winds also directed primarily along the gulf which are less spatially uniform than winter winds air temperature and moisture content are rather constant and topographic effects are absent the summer marine layer is on the order of 200 300 m thick with dew point temperatures of 26 28 c 21 24 g kg 1 and capped by a weak temperature inversion 1 2 c over water aloft the dew point temperature is 17 21 c and winds are weak and uncorrelated across the width of the gulf during june and early july 1984 four week long pulses of southeasterly winds 10 m s 1 support the existence reported previously of moisture surges from the gulf into the sonoran desert a dynamical analysis of subdiurnal motions shows that the cross gulf momentum balance is geostrophic the along gulf balance is ageostrophic with the along gulf pressure gradient opposed by friction at the surface these results are consistent with a scale analysis of the equations of motion in the marine layer over the gulf of california

a case study of the great plains low level jet using wind profiler network data and a high resolution mesoscale model abstract a detailed case study of one complete episode of a typical summertime great plains low level jet llj using data collected by the noaa wind profiler demonstration network is presented the high temporal and spatial resolution of the data from the profiler network permits a much more detailed picture of the great plains llj than is possible from previous studies of this phenomenon a three dimensional mesoscale numerical model is also used to simulate the episode and to provide information on the physical mechanisms responsible for the initiation evolution maintenance and decay of the llj the position and width of the jet core as well as the diurnal variation of wind speed and direction inside the jet core are well predicted by the model the analysis and modeling suggest that the diurnal oscillation of horizontal pressure gradient over sloping terrain is secondary to the inertial oscillation mechanism resulting from the release of frictional constraint in the evening and throughout the nigh

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

pyroclastic geology of the rhyolitic volcano of la primavera mexico explosive eruptions of this large quaternary rhyolitic volcano have produced voluminous pumice fall deposits and ignimbrites a stratigraphie study reveals more than 30 fall deposits totalling more than 90 km3 which have come from many different vents some are of plinian type and one is among the largest known in the world the main ignimbrite rio caliente is of intra plinian type in all 140 km3 of rhyolitic material were erupted during the past 105 years of which 50 km5 are dispersed well outside the volcano the dense rock equivalent volume is 60 km3 of which fall deposits ignimbrites and lava bodies comprise 45 25 and 30 respectively the average output rate of 0 06 km3 per century is nearly an order of magnitude less than for the most productive rhyolitic volcanoes known a circular area 10 km across in the centre of la primavera contains updomed lacustrine ashes and associated sediments including a remarkable giant pumice bed which probably accumulated in a caldera lake la primavera has no record of historical eruptions and hot springs are the only signs of present activity but the updoming is thought to be due to the uprise of a new acid pluton beneath the volcano and future eruptions are probable

paleointensity applications to timing and extent of eruptive activity 9 10 n east pacific rise placing accurate age constraints on near axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges geomagnetic paleointensity of submarine basaltic glass sbg holds promise for placing quantitative age constraints on near axis flows in one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful sbg paleointensity estimates from 189 near axis 4 km sites at the east pacific rise 9 10 n paleointensities range from 6 to 53 t and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field samples within and adjacent to the axial summit trough ast have values approximately equal to or slightly higher than the present day samples out to 1 3 km from the ast have values higher than the present day and samples farther off axis have values lower than the present day the on axis samples 500 m from the ast provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique results from samples collected near a well documented eruption in 1991 1992 suggest there may be a small negative bias in the paleointensity estimates limiting resolution of the dating technique possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain anomalously high magnetic unblocking temperatures leading to a small cooling rate bias and or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories paleointensity remains useful in approximating age differences in young flows and a clear along axis paleointensity contrast near 9 50 n is suggestive of a 150 200 year age difference paleointensity values of off axis samples are generally consistent with rough age interpretations based on side scan data furthermore spatial patterns in the paleointensity suggest extensive off axis flow emplacement may occur infrequently with recurrence intervals of 10 20 kyr results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows with flow size linked to time between eruptions