Large-amplitude Gravity Research Articles

Intermittency of gravity wave potential energies and absolute momentum fluxes derived from infrared limb sounding satellite observations

Abstract. Atmospheric gravity waves contribute significantly to the driving of the global atmospheric circulation. Because of their small spatial scales, their effect on the circulation is usually parameterized in general circulation models. These parameterizations, however, are strongly simplified. One important but often neglected characteristic of the gravity wave distribution is the fact that gravity wave sources and, thus, the global distribution of gravity waves are both very intermittent. Therefore, time series of global observations of gravity waves are needed to study the distribution, seasonal variation, and strength of this effect. For gravity wave absolute momentum fluxes and potential energies observed by the High-Resolution Dynamics Limb Sounder (HIRDLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) limb sounding satellite instruments, we investigate the global distribution of gravity wave intermittency by deriving probability density functions (PDFs) in different regions as well as global distributions of Gini coefficients. In the stratosphere, we find that intermittency is strongest in mountain wave regions, followed by the polar night jets and by regions of deep convection in the summertime subtropics. Intermittency is weakest in the tropics. A better comparability of intermittency in different years and regions is achieved by normalizing observations by their spatially and temporally varying monthly median distributions. Our results are qualitatively in agreement with previous findings from satellite observations and quantitatively in good agreement with previous findings from superpressure balloons and high-resolution models. Generally, momentum fluxes exhibit stronger intermittency than potential energies, and lognormal distributions are often a reasonable approximation of the PDFs. In the tropics, we find that, for monthly averages, intermittency increases with altitude, which might be a consequence of variations in the atmospheric background and, thus, varying gravity wave propagation conditions. Different from this, in regions of stronger intermittency, particularly in mountain wave regions, we find that intermittency decreases with altitude, which is likely related to the dissipation of large-amplitude gravity waves during their upward propagation.

On the High Winds in the Tianshan Grand Canyon in Northwest China: General Features, Synoptic Conditions, and Mesoscale Structures

This work studies the characteristics of high winds in the Tianshan Grand Canyon (TGC), Northwest China by using surface wind observations at six automatic weather stations during 2017–2018. Three high wind indices are examined, namely, high wind hour (HWH), station high wind event (SHWE) and regional high wind event (RHWE). SHWE denotes persistent high winds of more than 3 h at an individual station while RHWE is defined for the southern TGC as a whole. High winds are mainly northwesterly/westerly, occurring predominantly in southern TGC. HWHs most often occur in spring and summer, while winter and spring HWHs possess the strongest intensity. The occurrence of HWHs exhibits an apparent diurnal variation, most frequent in the afternoon and evening while least from mid-night to early morning. SHWEs and RHWEs are prone to take place in spring, possessing relatively long lifetime and strong intensity. According to composite analysis of the 0.25° FNL reanalysis, favorable synoptic conditions are obtained for the persistent high winds in spring. Of great importance is the mid-tropospheric trough in the mid-high latitudes located to the north (i.e., upstream) of the TGC. The cold advection behind the trough cools and stabilizes the mid-lower troposphere, while the post-trough sinking motion causes a surface high that accelerates the low-level northwesterly wind. The synoptic thermal and dynamical forcing help set an upstream flow of moderate Froude number such that the mesoscale orographic flow traversing the TGC is in the nonlinear high-drag regime. Large-amplitude gravity waves are excited, with the isentropes descending abruptly over the lee slope which resembles internal hydraulic jump. Downslope windstorms are produced at the expense of flow potential energy as the subcritical upstream flow is transitioned to supercritical at the TGC peak. These findings have important implications on the formation and prediction of high winds in the TGC.

Deep mass redistribution prior to the 2010 Mw 8.8 Maule (Chile) Earthquake revealed by GRACE satellite gravity

Subduction zones megathrust faults constitute a considerable hazard as they produce most of the world's largest earthquakes. However, the role in megathrust earthquake generation exerted by deeper subduction processes remains poorly understood. Here, we analyze the 2003 – 2014 space-time variations of the Earth's gravity gradients derived from three datasets of GRACE geoid models over a large region surrounding the rupture zone of the Mw 8.8 Maule earthquake. In all these datasets, our analysis reveals a large-amplitude gravity gradient signal, progressively increasing in the three months before the earthquake, North of the epicentral area. We show that such signals are equivalent to a 60km3 water storage decrease over 2 months and cannot be explained by hydrological sources nor artefacts, but rather find origin from mass redistributions within the solid Earth on the continental side of the subduction zone. These gravity gradient variations could be explained by an extensional deformation of the slab around 150-km depth along the Nazca Plate subduction direction, associated with large-scale fluid release. Furthermore, the lateral migration of the gravity signal towards the surface from a low coupling segment around −32.5∘ North to the high coupling one in the South suggests that the Mw 8.8 earthquake may have originated from the propagation up to the trench of this deeper slab deformation. Our results highlight the importance of observations of the Earth's time-varying gravity field from satellites in order to probe slow mass redistributions in-depth major plate boundaries and provide new information on dynamic processes in the subduction system, essential to better understand the seismic cycle as a whole.

Structural controls on the gold mineralization at the eastern margin of the North China Craton: Constraints from gravity and magnetic data from the Liaodong and Jiaodong Peninsulae

The eastern block of the North China Craton (NCC) has been completely decratonized and it hosts numerous early Cretaceous gold deposits dated at about 130–120 Ma. The Jiaodong Peninsula in the easternmost NCC is the largest gold producing district in China, and it hosts more than 150 gold deposits with known gold resources of over 5000 tonnes (t). Also located at the eastern margin of NCC, the Liaodong Peninsula is poorly explored at depths greater than 800 m, with no large concealed gold deposit being discovered yet. In this paper, we carefully analyzed the gravity and magnetic characteristics of the ore-controlling structures in both provinces, including faults, Precambrian basement metamorphic rocks and the Mesozoic granitic intrusions. The deep regional faults are characterized by linear gravity gradient zones or elongated positive magnetic anomalies. The Precambrian basement metamorphic rocks usually have large-amplitude gravity and magnetic highs, while the lithological contacts of Mesozoic granite intrusions are commonly defined by gravity lows or narrow magnetic high anomalies. Interpretations from gravity and magnetic signatures predict two concealed lithological contacts between Mesozoic granites and Precambrian metamorphic rocks beneath Liaodong and Jiaodong.The spatial locations of known gold deposits are then correlated with the concealed structures inferred from the gravity and magnetic data in our paper. Their spatial relationship suggests that the gold mineralization is mainly controlled by oblique regional faults along the structural and lithological contacts between Mesozoic granites and Precambrian metamorphic rocks. Additionally, the structural intersections of different ore-controlling faults, especially the hydrothermal alteration zones at the contacts between Precambrian basement metamorphic rocks and the Mesozoic granites, are most favorable for the gold mineralization in the Liaodong and Jiaodong areas. Many large gold deposits are documented along a deep-seated, but concealed, structure beneath the Jiaodong gold district, while the Qingchengzi ore field and its vicinity in the Liaodong Peninsula still remain underexplored, despite of having a very similar structural setting, and deserve further investigation.

Insights on the tectonic styles of the Red Sea rift using gravity and magnetic data

Global topography, bathymetry, magnetic, and gravity data were analyzed to study the nature of the Red Sea crust to infer its tectonic regime. The Red Sea is a classic example of incipient ocean basin formation after the breakup of a continent. We used spectral analysis of the magnetic and gravity data and two-dimensional (2D) forward modelling of the gravity data to image crustal sources within the Red Sea. The magnetic data were also analyzed to determine the Curie point depth, which was then used to estimate heat flow values within the Red Sea. Profiles of bathymetry, gravity, magnetic, and heat flow data were constructed across the southern, central and northern portions of the Red Sea to determine crustal structural differences between these regions. Residual and regional gravity anomaly maps determined from the spectral analysis and 2D gravity forward models indicate that there is a large amplitude gravity maxima over the main axial trough within the central and southern Red Sea caused by a combination of partially molten material intruding into the crust and the formation of a new oceanic crust. However, the central portion of the Red Sea based on the residual gravity anomaly map is a transition zone where the oceanic crust is discontinuous and is separated by inter-trough zones. In the northern portion of the Red Sea, the discontinuous residual gravity maxima and high frequency magnetic anomalies can be correlated with isolated deeps implying the last stage of the continental rifting and beginning of seafloor spreading.

Nonlinear Simulations of Gravity Wave Tunneling and Breaking over Auckland Island

Large-amplitude gravity wave oscillations were observed directly above and in the lee of the Southern Ocean’s Auckland Island (50.88S, 166.18E) within the mesospheric airglow and sodium layers at ;78–83 km altitudes during research flight RF23 of the Deep Propagating Gravity Wave Experiment (DEEPWAVE)

The Role of Gravity Wave Breaking in a Case of Upper-Level Near-Cloud Turbulence

Abstract An observed turbulence encounter that occurred outside a mesoscale convective system over the central United States on 3 June 2005 is investigated using observations and high-resolution numerical modeling. Here, the mechanisms associated with the observed moderate-to-severe turbulence during the evolution of this convective system are examined. Comparison between aircraft-observed eddy dissipation rate data with satellite and radar shows that a majority of turbulence reports are located on the south side and outside of a nocturnal mesoscale convective system (MCS), relatively large distances from the active convective regions. Simulations show that divergent storm-induced upper-level outflow reduces the environmental flow on the south side of the MCS, while on the north and northwest side it enhances the environmental flow. This upper-level storm outflow enhances the vertical shear near the flight levels and contributes to mesoscale reductions in Richardson number to values that support turbulence. In addition to the role of the MCS-induced outflow, high-resolution simulations (1.1-km horizontal grid spacing) show that turbulence is largely associated with a large-amplitude gravity wave generated by the convective system, which propagates away from it. As the wave propagates in the region with enhanced vertical shear caused by the storm-induced upper-level outflow, it amplifies, overturns, and breaks down into turbulence. The location of the simulated turbulence relative to the storm agrees with the observations and the analysis herein provides insight into the key processes underlying this event.

Retrieval of intrinsic mesospheric gravity wave parameters using lidar and airglow temperature and meteor radar wind data

Abstract. We analyse gravity waves in the upper-mesosphere, lower-thermosphere region from high-resolution temperature variations measured by the Rayleigh lidar and OH temperature mapper. From this combination of instruments, aided by meteor radar wind data, the full set of ground-relative and intrinsic gravity wave parameters are derived by means of the novel WAPITI (Wavelet Analysis and Phase line IdenTIfication) method. This WAPITI tool decomposes the gravity wave field into its spectral component while preserving the temporal resolution, allowing us to identify and study the evolution of gravity wave packets in the varying backgrounds. We describe WAPITI and demonstrate its capabilities for the large-amplitude gravity wave event on 16–17 December 2015 observed at Sodankylä, Finland, during the GW-LCYCLE-II (Gravity Wave Life Cycle Experiment) field campaign. We present horizontal and vertical wavelengths, phase velocities, propagation directions and intrinsic periods including uncertainties. The results are discussed for three main spectral regions, representing small-, medium- and large-period gravity waves. We observe a complex superposition of gravity waves at different scales, partly generated by gravity wave breaking, evolving in accordance with a vertically and presumably also horizontally sheared wind.

A Numerical Investigation on Tidal and Gravity Wave Contributions to the Summer Time Na Variations in the Midlatitude E Region

AbstractThe Na density variations in the E region have been studied over the past few decades. Although considerable progress in understanding and in modeling the metal layer observations has been made, Na density features above 100 km have yet to be explained. Various studies have linked them to the Na+ variations, a major reservoir for Na in E region. But the lack of comprehensive modeling investigations and of wind and temperature observations prevents further understanding on this important ion‐neutral coupling topic. In this study, we conduct a numerical simulation on the summer time Na density behavior in the midlatitude E region, where both the ion density and the neutral atmosphere are modulated by tidal and gravity waves. Simulation results show good agreement with Na lidar measurements and reveal that atmospheric waves can transport Na upward to generate Na layers and variations in E region considerably. The vertical wind component of the large amplitude tidal wave can extend the Na layer above 120 km into the thermosphere. The simulation also demonstrates that the modulation of large amplitude gravity (GW) wave can generate small‐scale sporadic Na layers (Nas) in the E region. Finally, eddy diffusion enhancement in the GW saturation process can significantly alter the Nas spatial and temporal structures.

Atmospheric Conditions during the Deep Propagating Gravity Wave Experiment (DEEPWAVE)

This paper describes the results of a comprehensive analysis of the atmospheric conditions during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) campaign in austral winter 2014. Different datasets and diagnostics are combined to characterize the background atmosphere from the troposphere to the upper mesosphere. How weather regimes and the atmospheric state compare to climatological conditions is reported upon and how they relate to the airborne and ground-based gravity wave observations is also explored. Key results of this study are the dominance of tropospheric blocking situations and low-level southwesterly flows over New Zealand during June–August 2014. A varying tropopause inversion layer was found to be connected to varying vertical energy fluxes and is, therefore, an important feature with respect to wave reflection. The subtropical jet was frequently diverted south from its climatological position at 30°S and was most often involved in strong forcing events of mountain waves at the Southern Alps. The polar front jet was typically responsible for moderate and weak tropospheric forcing of mountain waves. The stratospheric planetary wave activity amplified in July leading to a displacement of the Antarctic polar vortex. This reduced the stratospheric wind minimum by about 10 m s−1 above New Zealand making breaking of large-amplitude gravity waves more likely. Satellite observations in the upper stratosphere revealed that orographic gravity wave variances for 2014 were largest in May–July (i.e., the period of the DEEPWAVE field phase).

Dynamics of Orographic Gravity Waves Observed in the Mesosphere over the Auckland Islands during the Deep Propagating Gravity Wave Experiment (DEEPWAVE)

Abstract On 14 July 2014 during the Deep Propagating Gravity Wave Experiment (DEEPWAVE), aircraft remote sensing instruments detected large-amplitude gravity wave oscillations within mesospheric airglow and sodium layers at altitudes z ~ 78–83 km downstream of the Auckland Islands, located ~1000 km south of Christchurch, New Zealand. A high-altitude reanalysis and a three-dimensional Fourier gravity wave model are used to investigate the dynamics of this event. At 0700 UTC when the first observations were made, surface flow across the islands’ terrain generated linear three-dimensional wave fields that propagated rapidly to z ~ 78 km, where intense breaking occurred in a narrow layer beneath a zero-wind region at z ~ 83 km. In the following hours, the altitude of weak winds descended under the influence of a large-amplitude migrating semidiurnal tide, leading to intense breaking of these wave fields in subsequent observations starting at 1000 UTC. The linear Fourier model constrained by upstream reanalysis reproduces the salient aspects of observed wave fields, including horizontal wavelengths, phase orientations, temperature and vertical displacement amplitudes, heights and locations of incipient wave breaking, and momentum fluxes. Wave breaking has huge effects on local circulations, with inferred layer-averaged westward flow accelerations of ~350 m s−1 h−1 and dynamical heating rates of ~8 K h−1, supporting recent speculation of important impacts of orographic gravity waves from subantarctic islands on the mean circulation and climate of the middle atmosphere during austral winter.

What lies beneath: geophysical mapping of a concealed Precambrian intrusive complex along the Iowa–Minnesota border

Large-amplitude gravity and magnetic highs over northeast Iowa are interpreted to reflect a buried intrusive complex composed of mafic–ultramafic rocks, the northeast Iowa intrusive complex (NEIIC)...

Geophysical study of the San Juan Mountains batholith complex, southwestern Colorado

One of the largest and most pronounced gravity lows over North America is over the rugged San Juan Mountains of southwestern Colorado (USA). The mountain range is coincident with the San Juan volcanic field (SJVF), the largest erosional remnant of a widespread mid-Cenozoic volcanic field that spanned much of the southern Rocky Mountains. A buried, low-density silicic batholith complex related to the volcanic field has been the accepted interpretation of the source of the gravity low since the 1970s. However, this interpretation was based on gravity data processed with standard techniques that are problematic in the SJVF region. The combination of high-relief topography, topography with low densities, and the use of a common reduction density of 2670 kg/m3 produces spurious large-amplitude gravity lows that may distort the geophysical signature of deeper features such as a batholith complex. We applied an unconventional processing procedure that uses geologically appropriate densities for the uppermost crust and digital topography to mostly remove the effect of the low-density units that underlie the topography associated with the SJVF. This approach resulted in a gravity map that provides an improved representation of deeper sources, including reducing the amplitude of the anomaly attributed to a batholith complex. We also reinterpreted vintage seismic refraction data that indicate the presence of low-velocity zones under the SJVF. Assuming that the source of the gravity low on the improved gravity anomaly map is the same as the source of the low seismic velocities, integrated modeling corroborates the interpretation of a batholith complex and then defines the dimensions and overall density contrast of the complex. Models show that the thickness of the batholith complex varies laterally to a significant degree, with the greatest thickness (∼20 km) under the western SJVF, and lesser thicknesses (<10 km) under the eastern SJVF. The largest group of nested calderas on the surface of the SJVF, the central caldera cluster, is not correlated with the thickest part of the batholith complex. This result is consistent with petrologic interpretations from recent studies that the batholith complex continued to be modified after cessation of volcanism and therefore is not necessarily representative of synvolcanic magma chambers. The total volume of the batholith complex is estimated to be 82,000–130,000 km3. The formation of such a large felsic batholith complex would inevitably involve production of a considerably greater volume of residuum, which could be present in the lower crust or uppermost mantle. The interpreted vertically averaged density contrast (–60 to –110 kg/m3), density (2590–2640 kg/m3), and seismic expression of the batholith complex are consistent with results of geophysical studies of other large batholiths in the western United States.

Flow–topography interactions in shallow-water turbulence

Topography and external time-varying flows such as tides can have a profound influence on atmospheric and oceanic fluid motion. The effects of topography on turbulence have been little studied, and until recently those of tides have been overlooked. Our lack of understanding of these effects is a serious impediment to predicting flows both in the atmosphere and in the oceans. As a first step toward modelling realistic tidal oscillations, this study focuses on how an initially balanced flow containing a distribution of potential vorticity anomalies develops and becomes unbalanced (by emitting gravity waves) as a result of interactions with topography in the presence of an oscillating flow, here in the form of pure inertial oscillations (at the Coriolis frequency). We investigate a large parameter space spanned by the Burger number, the maximum amplitude of topography, and the amplitude of the flow oscillations. Comparisons are made with a time-independent and spatially-homogeneous steady flow in order to better understand the role played by flow oscillations. The most persistent large-amplitude gravity wave activity occurs for large-amplitude flow oscillations and for large topography. The latter produces large potential vorticity and depth anomalies which remain locked to the topography, thereby allowing gravity waves to be continually generated. By contrast, for a steady flow, large potential vorticity and depth anomalies are advected away from topographic features and so gravity waves are only generated at early times.

A Numerical Study of Clear-Air Turbulence (CAT) Encounters over South Korea on 2 April 2007

Abstract On 2 April 2007, nine cases of moderate-or-greater-level clear-air turbulence (CAT) were observed from pilot reports over South Korea during the 6.5 h from 0200 to 0830 UTC. Those CAT events occurred in three different regions of South Korea: the west coast, Jeju Island, and the eastern mountain areas. The characteristics and possible mechanisms of the CAT events in the different regions are investigated using the Weather Research and Forecasting model. The simulation consists of six nested domains focused on the Korean Peninsula, with the finest horizontal grid spacing of 0.37 km. The simulated wind and temperature fields in a 30-km coarse domain are in good agreement with those of the Regional Data Assimilation and Prediction System (RDAPS) analysis data of the Korean Meteorological Administration and observed soundings of operational radiosondes over South Korea. In synoptic features, an upper-level front associated with strong meridional temperature gradients is intensified, and the jet stream passing through the central part of the Korean Peninsula exceeds 70 m s−1. Location and timing of the observed CAT events are reproduced in the finest domains of the simulated results in three different regions. Generation mechanisms of the CAT events revealed in the model results are somewhat different in the three regions. In the west coast area, the tropopause is deeply folded down to about z = 4 km because of the strengthening of an upper-level front, and the maximized vertical wind shear below the jet core produces localized turbulence. In the Jeju Island area, localized mixing and turbulence are generated on the anticyclonic shear side of the enhanced jet, where inertial instability and ageostrophic flow are intensified in the lee side of the convective system. In the eastern mountain area, large-amplitude gravity waves induced by complex terrain propagate vertically and subsequently break down over the lee side of topography, causing localized turbulence. For most of the CAT processes considered, except for the mountain-wave breaking, standard NWP resolutions of tens of kilometers are adequate to capture the CAT events.

The severe thunderstorm of 4 October 2007 in Mallorca: an observational study

Abstract. During the afternoon of 4 October 2007, a thunderstorm swept across the Island of Mallorca from southwest to northeast. Strong straight-line winds (up to 30 m/s) and heavy rain (rates up to 100 mm/h) were registered accompanying the storm. Tornadoes with an estimated intensity of F2–F3 developed nearby the city of Palma, severely affecting industrial installations. One person was killed by the impact of heavy debris while more than 10 million € in damages were attributed to the event in the industrial area only. The observed evolution of temperature, humidity, wind and pressure, as well as the sequence of radar images, reveal that a squall line was initially organized over the sea and then moved north-eastwards at an estimated speed of around 80 km/h. This paper presents an analysis of the event from an observational point of view. The aim of the study is to contribute to the characterization of these rare events in the Western Mediterranean by analyzing the observational information available for this particular extreme event. The diagnosis is aimed at helping forecasters to identify this kind of organized deep convective events and being able to issue timely warnings. The synoptic scenario shows warm and moist advection at low levels over Balearics and an upper-level trough over mainland Spain. This situation is known to be prone to deep convection in Mediterranean Spain in autumn. Radiosonde ascents from Murcia and Palma show convective instability at mid levels that can conduce to develop convection if appropriate ascents occur. A plausible lifting mechanism to trigger convection is attributed to large amplitude gravity waves, registered as short-period pressure oscillations by surface barographs.

Mass transfer enhancement by gravity waves at a liquid–vapour interface

Experimental results on mass transfer enhancement by large amplitude gravity waves at a liquid–gas/vapour interface are presented. The waves are sub-harmonically excited in a circular cylinder that is partially filled with liquid, by oscillating the cylinder in the direction normal to the liquid surface. The lowest asymmetric sloshing mode (1, 1) as well as the axisymmetric mode (0, 1) are considered in the limit of large fluid depth approximation and for wave amplitudes that include breaking. The fluids used are low viscosity and low surface tension liquids of low boiling point temperatures. In the mass transfer experiments the lower part of the test cell is filled with cold liquid and the upper part with gas, generally vapour, at a temperature above the saturation temperature. When the interface is at rest and the gas is vapour, the pressure decrease due to condensation is small. In the presence of large amplitude sloshing the condensation rate is large and the pressure decreases rapidly and substantially. A model is developed that expresses the pressure variation in terms of a Jacob number, interfacial temperature gradient and an effective diffusion coefficient. The effective dimensionless diffusion coefficient is the relevant similarity parameter and is determined in the experiments. In Appendix A results are presented for conditions of evaporation in the presence of a non-condensable gas.

Propagation and reflection of gravity waves in a meridionally sheared wind field

Although the properties of gravity waves propagating in a vertically sheared flow have been extensively studied, the effects of a horizontally sheared wind on gravity wave propagations were seldom reported. In this paper, according to the linear theory, the characteristics of gravity wave reflection in a meridionally sheared zonal background wind field are discussed, which are evidently different from those of wave reflected by a vertically sheared flow. By numerical simulations, we present the whole process of a gravity wave packet reflection in a meridionally sheared wind. When the wave reaches the reflecting level, the zonal disturbance velocity in the evanescent region shows an evanescent wave configuration, which is consistent with the Airy function form predicted by the linear theory; while the meridional disturbance velocity exhibits rather different wave structures. The wave number spectra of zonal and meridional disturbance velocities also show different characteristics in the reflection process. The energy center of the wave packet is reflected in a position nearer than the reflecting level predicted by the linear theory. While the wave propagates along (against) the meridionally sheared wind, the meridional perturbation kinetic energy and total wave energy decrease (increase), whereas, the zonal perturbation kinetic energy increases (decreases); and an energy exchange between the wave potential and wave kinetic energies can be observed. Earth rotation has a slight influence on the energy transfer between the wave and the background flow. If the wind shear beyond the reflecting level isn't strong enough, part components of the incident wave may steadily penetrate across the reflecting level, and yields a transmission wave. A large amplitude gravity wave propagating in a meridionally sheared wind can obviously induce a mean flow, which strengthens slightly the wave reflection, indicating that the transmission rate slightly decreases with the increasing amplitude of the incident wave. This differs from the reflection of waves in a vertical sheared wind field, in which the mean flow induced by the large amplitude wave significantly enhances the wave transmission effect. In the meridionally sheared flow, the transmission rate seems to mainly depend on the sheared wind and the parameters and spectral components of the incident waves.

Regional gravity analysis of Burkina Faso: Implications for the location of metallic ore deposits

Gravity data were analyzed in conjunction with available geological data to determine the origin of observed gravity anomalies and their possible relationship to metallic ore deposits. The gravity data analysis included the construction of a Bouguer gravity anomaly, isostatic residual gravity anomaly and enhanced horizontal gravity gradient maps, and two and one-half dimensional gravity models. The isostatic residual gravity anomaly field could be broken down into five distinct regions based on anomaly amplitude, trend and wavelength. The analysis of these regions showed that both the Birimian and granitoid provinces consist mainly of a series of short wavelength gravity maxima and minima with a few large scale anomalies which suggests that the subsurface geology is more complicated than is currently known. Two gravity models roughly oriented north-south also implied this complicated subsurface geology and showed that most source bodies have depths up to 5 km. The known base metal deposits occur on the edge of small-scale gravity maxima within the Birimian province with the exceptions of the deposits within the Bouroum-Yalogo belt which occur next to a large amplitude gravity maximum related to an ultramafic complex.

Can trapped gravity waves be relevant for severe foehn windstorms? A case study

This study presents high-resolution numerical simulations of an Alpine south foehn case (14-16 November 2002) during which extremely strong surface winds occurred in several regions of the Alps. The specific focus is on a storm event in the upper Isar Valley in the Bavarian Alps, where gusts estimated to about 45 m s -1 caused substantial damage to buildings and vegetation. For this event, the simulations indicate that the extreme low-level wind speeds were related to a large-amplitude trapped gravity wave, contrasting previous findings that violent surface winds are usually related to vertically propagating gravity waves, especially in the presence of low-level wave breaking, or to shooting hydraulic flow. According to the model results, gravity waves were excited over the mountain ranges adjacent to the valley and propagated towards the valley axis due to their three-dimensional dispersion characteristics. Wave trapping was caused by a deep neutral layer in the upper troposphere. The low-level wind maximum formed beneath the wave trough, reflecting the dynamical structure of trapped waves for which wind and temperature perturbations are in phase. Comparison of the model results with surface wind measurements shows that the simulated flow evolution is consistent with reality except for a minor timing error. A sensitivity test in which the neutral layer was removed exhibits significantly weaker surface winds in the region of interest, suggesting that wave trapping made an important contribution to this windstorm event.