Surface Pressure Field Research Articles

Patterns of atmospheric excitation functions of polar motion from high‐resolution regional sectors

In this paper regional values of equatorial components of the atmospheric excitation function of polar motion, which are proportional to the equatorial components of angular momentum (AAM), were computed at high spatial resolution in 3312 equal‐area sectors from the surface pressure fields of the NCEP‐NCAR reanalyses in order to improve our knowledge of patterns of atmospheric excitation functions of polar motion. The inverted barometer (IB) model of oceanic isostatic adjustment is applied to readjust the effective atmospheric pressure fields. To identify the influence of different geographic regions of the atmosphere on the excitation of polar motion, correlations and covariances between these regional excitations values and either the global geodetic excitation function, determined from the geodetic observational data, or the atmospheric excitation functions, equivalent to the sum of such excitations in all regions, were computed in different spectral bands, from subseasonal to interannual period ones. The covariances show the regional contribution to the global excitation, whereas the correlations show an overall similar relationship. Furthermore, these analyses determine regional sources of polar motion excitation by surface pressure variations in the atmosphere in different spectral bands. The broad regions over Eurasia and North America were identified as principal atmospheric excitation sources for polar motion. The results revealed that atmospheric excitation over land regions in the northern middle latitudes, especially over Asia, is exceptionally strong, a point generally noted in previous studies, though without identifying detailed features. Over the Himalayan Mountains the covariance of local with global atmospheric excitation in the annual band forms a small minimum surrounded by larger values in a ring‐like formation. For other terms, maxima of atmospheric excitation have a longitudinal formation mostly across the Eurasian continent. The atmospheric excitation of polar motion over the ocean is weak when subject to the IB correction, but it contains nevertheless a clear annual signal and is highly correlated with the annual oscillation of the global geodetic excitation function of polar motion.

Greenland Ice Sheet Elevation Change in Winter and Influence of Atmospheric Teleconnections in the Northern Hemisphere

The relationship between the variability of the surface elevation of the Greenland Ice Sheet (GIS) in winter and sea level pressure is identified through analysis of data from satellite-borne radar altimeters, together with meteorological data fields during 1993−2005. We found that both the North Pacific Oscillation (NPO) and the North Atlantic Oscillation (NAO), the two major telecon- nection patterns of the atmospheric surface pressure fields in the Northern Hemisphere, significantly influence the GIS winter elevation change. Further, it is suggested that the NPO may affect the GIS accumulation by influencing the NAO, particularly by changing the intensity and loca- tion of the Icelandic Low.

High-frequency non-tidal ocean loading effects on surface gravity measurements

SUMMARY We model atmospheric and non-tidal oceanic loading effects on surface gravity variations, using global surface pressure field provided by the European Centre for Medium-range Weather Forecasts (ECMWF), and sea surface height from the Toulouse Hydrodynamic Unstructured Grid Ocean model (HUGO-m) barotropic ocean model. We show the improvement in terms of reduction of variance of 15 different superconducting gravimeters of the worldwide Global Geodynamics Project (GGP) network, compared to the classical inverted barometer assumption. We also study two storm surges over the Western European Shelf in 2000 and 2003. We compare the HUGO-m sea surface height variations to various tide gauges measurements as well as the induced loading effects to the computations of Fratepietro et al., using the Proudman Storm Surge model, for the Membach (Belgium) station. The agreement between modelled ocean loading and gravity observations is largely improved when using a global atmospheric loading correction, compared to the classical local approach. The remaining discrepancies are mainly due to hydrological loading contributions.

Frequency-dependent atmospheric pressure admittance of superconducting gravimeter records using least squares response method

Atmospheric pressure is one of the largest environmental noise sources in surface gravity observations. This effect is usually removed by two different methods that are based on the availability of pressure data and their temporal and spatial distribution. The first method is based on a physical approach by either convolving the global surface (2D) pressure field with an appropriate Green’s function or by using a 3D spherical model for attraction determination. The second method determines a transfer function between atmospheric pressure and gravity, known as barometric pressure admittance. In this paper, we focus on the second method by adopting an alternative approach for the determination of the pressure admittance that is based on the least-squares (LS) product spectrum of the atmospheric pressure and gravity time series. It represents a smooth gravity response to air pressure fluctuations in specific frequency bands (frequency-dependent admittance) with a magnitude of about 0.293 μ Gal/mbar in the low-frequency band and 0.443 μ Gal/mbar in the high-frequency band. This transfer function is derived from a 1-year data set recorded at the Canadian Superconducting Gravimeter Installation (CSGI; Cantley, Canada). The results show that the frequency-dependent pressure admittance reduces the quadratic norm of the gravity residuals by 95.4% while the physical approach correction which is based on the global atmospheric data reduces it by 92.2%. In addition, the frequency-dependent admittance shows an obvious improvement in the coherence spectrum.

Uncertainties in atmospheric surface pressure fields from global analyses

Operational analyses from the U.S. National Centers for Environmental Prediction (NCEP) and the European Centre for Medium‐Range Weather Forecasts (ECMWF) provide, among other quantities, atmospheric surface pressure. This field is especially useful for data processing and interpretation of altimeter and gravity satellite missions like Jason‐1 and GRACE (Gravity Recovery and Climate Experiment). Quantitative use of this pressure field, however, requires knowledge of its uncertainties. To assess these uncertainties, we compare NCEP and ECMWF analyses of surface pressure to an extensive set of surface barometric observations for the period 2001–2005, both land‐ and ocean‐based, as well as to each other. Differences between each analysis and the observations are quite similar, indicating that both analyses generally fit the station observations equally well. Such differences, including both bias and variance about the bias, are largest over parts of the Southern Ocean, and over Asia, central Africa, and Antarctica. These bias and variance differences between the NCEP and ECMWF surface pressure fields change only slightly over the period of study. More importantly, though, such differences are substantially smaller than those found by previous studies for an earlier period (1993–1995), suggesting recent improvements in both analyses. The largest differences are found over high latitudes, particularly over Antarctica, and in the respective winter hemisphere. Much of the differences occur on rapid timescales, though with a significant portion at timescales longer than 1–2 months. The potential impact of such uncertainties on de‐aliasing and interpreting sea level and gravity data from altimeter and GRACE missions is discussed.

LES prediction of wall-pressure fluctuations and noise of a low-speed airfoil

The wall-pressure fluctuations and noise of a low-speed airfoil are computed using large-eddy simulation (LES). The results are compared with experimental measurements made in an open-jet anechoic wind-tunnel at Ecole Centrale de Lyon. To account for the effect of the jet on airfoil loading, a RANS calculation is conducted in the full wind-tunnel configuration, which provides velocity boundary conditions for the LES in a smaller domain within the potential core of the jet. The flow field is characterized by an attached laminar boundary layer on the pressure side and a transitional and turbulent boundary layer on the suction side. The predicted unsteady surface pressure field shows reasonable agreement with the experimental data in terms of frequency spectra and coherence in the trailing-edge region. In the nose region, characterized by unsteady separation and transition to turbulence, the wall-pressure fluctuations are highly sensitive to small perturbations and difficult to predict or measure with certainty. The LES, in combination with the Ffowcs Williams and Hall solution to the Lighthill equation, also predicts well the radiated trailing-edge noise. A finite-chord correction is derived and applied to the noise prediction, which is shown to improve the overall agreement with the experimental sound spectra.

An Evaluation of Scatterometer-Derived Oceanic Surface Pressure Fields

Abstract Oceanic surface pressure fields are derived from the NASA Quick Scatterometer (QuikSCAT) surface wind vector measurements using a two-layer similarity planetary boundary layer model in the midlatitudes and a mixed layer planetary boundary layer model in the tropics. These swath-based surface pressure fields are evaluated using the following three methods: 1) a comparison of bulk pressure gradients with buoy pressure measurements in the North Pacific and North Atlantic Oceans, 2) a least squares difference comparison with the European Centre for Medium-Range Weather Forecasts (ECMWF) surface pressure analyses, and 3) a parallel spectral analysis of the QuikSCAT and ECMWF surface pressure fields. The correlation coefficient squared between scatterometer-derived pressure fields and buoys is found to be R2 = 0.936. The average root-mean-square difference between the scatterometer-derived and the ECMWF pressure fields ranges from 1 to 3 hPa, depending on the latitude and season, and decreases after the assimilation of QuikSCAT winds in the ECMWF numerical weather prediction model. The spectral components of the scatterometer-derived pressure fields are larger than those of ECMWF surface analyses at all scales in the midlatitudes and only at shorter wavelengths in the tropics.

Searching for resemblance between large-scale sea level pressure patterns leading to “intense” precipitation events over Italy

How strong is the resemblance between two states of the atmosphere leading to the same outcome? In the following pages this question was addressed by studying the resemblance of the surface pressure fields, obtained by NCEP reanalyses, corresponding to “intense” precipitation events over 5 Italian climatic areas. Such “intense” precipitation events were selected by means of 45 daily precipitation records covering the period from 1951 to 2000.

A physical basis for the interannual variability of rainfall in the Sahel

AbstractA major factor in rainfall variability over Sahelian West Africa is the latitudinal location of the tropical rainbelt. When it is displaced abnormally far northward, the Sahel experiences a wet year. An anomalous southward displacement results in drought. In this paper we examine the question of what controls the location during the boreal summer, hypothesizing that inertial instability plays a role. An analysis of surface pressure and temperature fields, wind fields, divergence and vertical motion show that the criteria for inertial instability are satisfied in wet Augusts but not in dry ones. The key determinant appears to be the surface pressure gradient between the continent and the equatorial Atlantic. When this is large, inertial instability results in the development of a low‐level westerly jet. The presence of this jet enhances the horizontal and vertical shear, and displaces the African Easterly Jet northwestward. Associated with this situation is strong vertical motion over the Sahel and subsidence over the Guinea Coast, producing dry conditions over the latter. The result is a rainfall dipole, one of two major modes of variability over West Africa. Important factors include sea surface temperatures (SSTs) in the equatorial Atlantic and pressure in the South Atlantic. The first of these factors suggests a link with the Atlantic Niño mode of tropical Atlantic variability; while the second suggests a possible link with the Pacific and the extratropical South Atlantic. Overall, our study relates the well‐known SST influence on Sahel rainfall to atmospheric dynamics over the continent. Copyright © 2007 Royal Meteorological Society

Remote sensing of surface pressure on Mars with the Mars Express/OMEGA spectrometer: 2. Meteorological maps

Surface pressure measurements help to achieve a better understanding of the main dynamical phenomena occurring in the atmosphere of a planet. The use of the Mars Express OMEGA visible and near‐IR imaging spectrometer allows us to tentatively perform an unprecedented remote sensing measurement of Martian surface pressure. OMEGA reflectances in the CO2 absorption band at 2 μm are used to retrieve a hydrostatic estimation of surface pressure (see companion paper by Forget et al. (2007)) with a precision sufficient to draw maps of this field and thus analyze meteorological events in the Martian atmosphere. Prior to any meteorological analysis, OMEGA observations have to pass quality controls on insolation and albedo conditions, atmosphere dust opacity, and occurrence of water ice clouds and frosts. For the selected observations, registration shifts with the MOLA reference are corrected. “Sea‐level” surface pressure reduction is then carried out in order to remove the topographical component of the surface pressure field. Three main phenomena are observed in the resulting OMEGA surface pressure maps: horizontal pressure gradients, atmospheric oscillations, and pressure perturbations in the vicinity of topographical obstacles. The observed pressure oscillations are identified as possible signatures of phenomena such as inertia‐gravity waves or convective rolls. The pressure perturbations detected around the Martian hills and craters may be the signatures of complex interactions between an incoming flow and topographical obstacles. Highly idealized mesoscale simulations using the WRF model enable a preliminary study of these complex interactions, but more realistic mesoscale simulations are necessary. The maps provide valuable insights for future synoptic and mesoscale modeling, which will in turn help in the interpretation of observations.

Atmospheric effects of the total solar eclipse of 4 December 2002 simulated with a high‐altitude global model

The atmosphere's response to the total solar eclipse of 4 December 2002 is studied using a prototype high‐altitude global numerical weather prediction model (NOGAPS‐ALPHA). Local reductions in solar ultraviolet (UV) radiation during the eclipse are estimated using astronomical calculations of umbral and penumbral surface trajectories and observed solar limb darkening at ∼200–300 nm. In NOGAPS‐ALPHA these UV eclipse shadows yield stratospheric radiative cooling rate footprints peaking near 27 K day−1, a value 2–3 times larger than assumed in previous modeling. Difference fields between NOGAPS‐ALPHA runs with and without this eclipse forcing reveal vertically deep middle atmospheric responses, with three‐dimensional horizontal structures very similar to the large‐scale “bow‐wave” response first proposed by Chimonas (1970). Such structure appears clearly only at later times when total eclipses have abated and gravity waves generated in the stratosphere have had time to propagate vertically. Bow‐wave amplitudes and direct thermal cooling responses are both small (≲1 K for temperature and ≲2–3 m s−1 for horizontal winds), contradicting some rocketsonde measurements that suggest much larger responses near 50–60 km altitude. We also find clear evidence of a bow‐wave‐like response in the model's surface pressure fields, with an amplitude ∼0.1–0.5 hPa, while surface air temperatures in NOGAPS‐ALPHA show ∼4 K cooling over Africa during the eclipse. Both findings are consistent with surface atmospheric data acquired during previous eclipse passages.

Surface deformation due to loading of a layered elastic half-space: a rapid numerical kernel based on a circular loading element

This study is motivated by a desire to develop a fast numerical algorithm for computing the surface deformation field induced by surface pressure loading on a layered, isotropic, elastic half-space. The approach that we pursue here is based on a circular loading element. That is, an arbitrary surface pressure field applied within a finite surface domain will be represented by a large number of circular loading elements, all with the same radius, in which the applied downwards pressure (normal stress) is piecewise uniform: that is, the load within each individual circle is laterally uniform. The key practical requirement associated with this approach is that we need to be able to solve for the displacement field due to a single circular load, at very large numbers of points (or ‘stations’), at very low computational cost. This elemental problem is axisymmetric, and so the displacement vector field consists of radial and vertical components both of which are functions only of the radial coordinate r. We achieve high computational speeds using a novel two-stage approach that we call the sparse evaluation and massive interpolation (SEMI) method. First, we use a high accuracy but computationally expensive method to compute the displacement vectors at a limited number of r values (called control points or knots), and then we use a variety of fast interpolation methods to determine the displacements at much larger numbers of intervening points. The accurate solutions achieved at the control points are framed in terms of cylindrical vector functions, Hankel transforms and propagator matrices. Adaptive Gauss quadrature is used to handle the oscillatory nature of the integrands in an optimal manner. To extend these exact solutions via interpolation we divide the r-axis into three zones, and employ a different interpolation algorithm in each zone. The magnitude of the errors associated with the interpolation is controlled by the number, M, of control points. For M = 54, the maximum RMS relative error associated with the SEMI method is less than 0.2 per cent, and it is possible to evaluate the displacement field at 100 000 stations about 1200 times faster than if the direct (exact) solution was evaluated at each station; for M = 99 which corresponds to a maximum RMS relative error less than 0.03 per cent, the SEMI method is about 700 times faster than the direct solution.

Warm and Dry Spells (WDS) in Austral Winter over Central South America

Abstract. The horizontal and vertical structure of unusually warm and dry spells (WDS) over the central parts of South America during the winter and post-winter months (JJAS) are studied. During WDS the mean temperature and humidity anomalies over central Brazil are about +4.1°C and −13.2%, respectively. The mean duration of WDS is 11 days and their mean frequency is less than one per year during the months of JJAS. Apparently, WDS have no preference for the phase of ENSO. Widespread and persistent subsidence in the middle troposphere is observed in tropical Brazil during WDS, which renders the lower tropospheric air warm and dry. The negative anomalies of the specific humidity are observed to be associated with the subsidence regions. A strong, slow moving ridge in the eastern South Pacific and a low-pressure center in northern Argentina are important surface characteristics during the WDS. A more detailed investigation of two specific WDS events, a strong event (August–September 1999) and a moderate one (June 2002), shows a blocking-like situation in the 500-hPa geopotential and surface pressure fields in the Pacific. The South Atlantic subtropical high somewhat approaches the continent. Strong northerlies over the central and eastern parts of Brazil are also observed in the lower troposphere. During WDS the regional circulation acquires summertime characteristics, except for the absence of precipitation, and the circulation in the meridional plane is in the opposite sense from the Hadley circulation. A frontal system, supported by a 500-hPa trough, advances into central Brazil, causing the dissipation of the anomalous situation.

Precipitation rate spectra as dependent on dynamic forcing: application to probabilistic forecasting

Abstract. Occurrence frequencies, OF, of 12-h precipitation amounts, P, at stations in the former European USSR are displayed as dependent on dynamic forcing of vertical motions. The dynamic forcing is described by a "frontal parameter", F (calculated in the points of objective analysis grid), which depends on the surface pressure field curvature and on the baroclinicity in the lower half of the troposphere. The precipitation rate spectra for 4 seasons, calculated from a large sample of data (7 years, about 650 000 values of P for one season), show a monotonous OF growth of all ranges of P>1 mm/12 h with F increase. The growth is especially significant for heavy precipitation. As a result, F is shown to be an informative predictor of P spectrum or of probability of any given range of P. As a next step, two-dimensional spectra of precipitation occurrence frequency, as a function of F and LNB, that is, OF (F, LNB), are calculated, LNB being the level of neutral buoyancy at the gridpoint, an estimate of grid-scale convective instability. On this basis, an approach to probabilistic forecasting is suggested.

A hybrid method for vibroacoustics based on the radiative energy transfer method

A hybrid method is proposed to predict the noise radiated in the high-frequency range by a structure whose vibratory behavior is rather in the low-frequency range. This problem meets for instance when the structure radiates inside some closed acoustical spaces. The proposed approach is called hybrid as it couples two methods valid on different frequency bands respectively used for the structural and acoustical systems. Low-frequency structural approaches generally provide the velocity field on the vibrating surface while methods adapted to high-frequency acoustical studies are based on energy data: so the main difficulty for the coupling is a problem of data compatibility. In the proposed hybrid method, the radiated noise is predicted by using the radiative energy transfer method which is an energy boundary integral method basically equivalent to the known ray-tracing method for acoustic applications. Adequate equivalent energy sources are estimated on the surface of the vibrating structure from the surface pressure and velocity fields. These sources are then introduced in the integral equation of the radiative energy transfer method. The approach is applied to predict the radiation of a ribbed plate sandwiched between two absorbant panels: good agreement is observed with the results given by the boundary element method. Generally speaking, the hybrid method can be considered to apply the radiative energy transfer method for high-frequency acoustical predictions from the knowledge of the vibratory behavior of a structure.

Intraseasonal Atmospheric Variability in the Extratropics and Its Relation to the Onset of Tropical Pacific Sea Surface Temperature Anomalies

Abstract Previous research has shown that seasonal-mean boreal winter variations in the subtropical/extratropical sea level pressure and wind stress fields over the central North Pacific are significantly related to the state of the El Niño–Southern Oscillation (ENSO) 12–15 months later. Results presented in this note indicate that boreal winter ENSO events are also preceded by increased intraseasonal variance in the antecedent boreal winter atmospheric circulation patterns over the extratropical central North Pacific as well. Low (high) surface pressure anomalies associated with intraseasonal variability in this region are related to intraseasonal wind stress anomalies that represent a weakening (strengthening) of the trade winds over both the north and south subtropical/tropical Pacific. There is also a concurrent increase (decrease) in the central and eastern subtropical North Pacific sea surface temperatures that projects onto the seasonal-mean SST anomalies that precede mature ENSO events by 9–12 months. Overall these results suggest that similar to seasonal-mean subtropical surface pressure and wind stress fields, enhanced transient variability in the midlatitudes can subsequently induce changes in the atmospheric and oceanic structure of the tropical Pacific that may serve as a precursor to ENSO variability.

Teleconnection of IOD signal in the upper troposphere over southern high latitudes

Evidence has been found for the teleconnection of Indian Ocean Dipole mode (IOD) events in the southern high latitude sea surface pressure field, although the mechanisms that might lead to such far-reaching links remain unresolved. Based on the teleconnection pattern between IOD and the climate anomaly in the upper troposphere, we propose one such mechanism here: the energy propagation theory of the atmospheric planetary wave. Ray traces of the atmospheric planetary waves suggest that the energy propagation of the waves could be responsible for the teleconnection between IOD and tropospheric climate anomalies in southern high latitudes.

Precipitation and Cloud Structure in Midlatitude Cyclones

AbstractComposite mean fields and probability distribution functions (PDFs) of rain rate, cloud type and cover, cloud-top temperature, surface wind velocity, and water vapor path (WVP) are constructed using satellite observations of midlatitude cyclones from four oceanic regions (i.e., the North Pacific, South Pacific, North Atlantic, and South Atlantic). Reanalysis surface pressure fields are used to ascertain the locations of the cyclone centers, onto which the satellite fields are interpolated to give a database of ∼1500 cyclones from a two-year period (2003–04). Cyclones are categorized by their strength, defined here using surface wind speed, and by their WVP, and it is found that these two measures can explain a considerable amount of the intercyclone variability of other key variables. Composite cyclones from each of the four ocean basins exhibit similar spatial structure for a given strength and WVP. A set of nine composites is constructed from the database using three strength and three WVP ranges and is used to demonstrate that the mean column relative humidity of these systems varies only slightly (0.58–0.62) for a doubling in WVP (or equivalently a 7-K rise in sea surface temperature) and a 50% increase in cyclone strength. However, cyclone-mean rain rate increases markedly with both cyclone strength and WVP, behavior that is explained with a simple warm conveyor belt model. Systemwide high cloud fraction (tops above 440 hPa) increases from 0.23 to 0.31 as cyclone strength increases by 50%, but does not vary systematically with WVP. It is suggested that the composite fields constitute useful diagnostics for evaluating the behavior of large-scale numerical models, and may provide insight into how precipitation and clouds in midlatitude cyclones respond under a changed climate.

The disastrous storm of 4 November 1966 on Italy

Abstract. This is the first modeling reconstruction of the whole aspects (both meteorological and oceanographic) of the storm which hit Italy on 4 November 1966, producing 118 victims and widespread damages in Tuscany, at the northern Adriatic coast and in the north-eastern Italian Alps. The storm was produced by a cyclone which formed in the western Mediterranean and moved eastward towards Italy, reaching the Thyrrenian Sea, and then northward. The most peculiar characteristic of the storm has been the strong zonal pressure gradient and the consequent intensity and long fetch of the south-easterly sirocco wind, which advected a large amount of warm moist air, and determined exceptional orographic precipitation over Tuscany and the north-eastern Alps. The funneling of the wind between the mountain chains surrounding the Adriatic basin further increased the wind speed and determined the highest ever recorded storm surge along the Venetian coast. This study shows that present models would be able to produce a reasonably accurate simulation of the meteorological event (surface pressure, wind and precipitation fields, and storm surge level). The exceptional intensity of the event is not suggested by single parameters such as the sea level pressure minimum, the wind speed or the total accumulated precipitation. In fact, the precipitation was extreme only in some locations and the pressure minimum was not particularly deep. Moreover, the prediction of the damages produced by the river run-off and landslides would have required other informations concerning soil condition, snow coverage, and storage of water reservoirs before the event. This indicates that an integrated approach is required for assessing the probability of such damages both on a weather forecast and on a climate change perspective.

Experimental Investigation of Turbine Leakage Flows on the Three-Dimensional Flow Field and Endwall Heat Transfer

Within a European research project, the tip endwall region of low pressure turbine guide vanes with leakage ejection was investigated at DLR in Göttingen. For this purpose a new cascade wind tunnel with three large profiles in the test section and a contoured endwall was designed and built, representing 50% height of a real low pressure turbine stator and simulating the casing flow field of shrouded vanes. The effect of tip leakage flow was simulated by blowing air through a small leakage gap in the endwall just upstream of the vane leading edges. Engine relevant turbulence intensities were adjusted by an active turbulence generator mounted in the test section inlet plane. The experiments were performed with tangential and perpendicular leakage ejection and varying leakage mass flow rates up to 2%. Aerodynamic and thermodynamic measurement techniques were employed. Pressure distribution measurements provided information about the endwall and vane surface pressure field and its variation with leakage flow. Additionally streamline patterns (local shear stress directions) on the walls were detected by oil flow visualization. Downstream traverses with five-hole pyramid type probes allow a survey of the secondary flow behavior in the cascade exit plane. The flow field in the near endwall area downstream of the leakage gap and around the vane leading edges was investigated using a 2D particle image velocimetry system. In order to determine endwall heat transfer distributions, the wall temperatures were measured by an infrared camera system, while heat fluxes at the surfaces were generated with electric operating heating foils. It turned out from the experiments that distinct changes in the secondary flow behavior and endwall heat transfer occur mainly when the leakage mass flow rate is increased from 1% to 2%. Leakage ejection perpendicular to the main flow direction amplifies the secondary flow, in particular the horseshoe vortex, whereas tangential leakage ejection causes a significant reduction of this vortex system. For high leakage mass flow rates the boundary layer flow at the endwall is strongly affected and seems to be highly turbulent, resulting in entirely different heat transfer distributions.