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Klaus P. Hoinka

Abstract

Numerical solutions of a hydrostatic mesoscale model for stratified flow over topography are compared with analytical results and observational data. The numerical model was found to reproduce well phase and amplitude of linear and nonlinear waves provided by analytical models. The model was effective in simulating observed chinook associated with an intense mountain wave. In a nonchinook case the model was successful in reproducing main features of the observed velocity distribution associated with an elevated region of blocking. The observed and simulated momentum flux profiles are almost identical. The comparisons of the model results with observations and analytical results demonstrate the overall ability of the model to realistically simulate mountain wave flow. In a series of numerical experiments we have investigated how far the steepness of the waves depends on stratospheric wind structure and on orography. Our simulations suggest that there is a bifurcation line between the linear and nonlinear regime of flow depending on the magnitude of the stratospheric wind. Tests with an asymmetric orography show significant increase in wave amplitude up to the magnitude which was observed. Finally, two bora‐type flows, a cyclonic and one anticyclonic, are simulated and show the expected structure in the wind and temperature fields.

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Klaus P. Hoinka

Abstract

Statistics of the global tropopause pressure are evaluated for the period between 1979 and 1993. The analysis is based on gridded data as provided by the ECMWF reanalysis project. The thermal and dynamical definitions of the tropopause are applied in order to derive the surface separating the troposphere from the stratosphere. The dynamical tropopause is evaluated for threshhold values of potential vorticity of ±1.6, ±2.5, and ±3.5 potential vorticity units. Positive (negative) values are applied for the Northern (Southern) Hemisphere. The thermal tropopause is determined using the lapse-rate criteria. Frequency distributions of the tropopause pressure are shown above continental, ocean, polar, midlatitude, and tropical areas. Trends in the tropopause pressure are evaluated and compared with observed trends in total ozone.

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Klaus P. Hoinka

In the present study foehn clearance is described and compared with strong downslope windstorms like the foehn and chinook. The comparison is based on surface and satellite data taken during foehn north of the Alps in southern Bavaria and during chinook east of the Rocky Mountains. The mesoscale features of foehn clearance are shown in terms of temperature, humidity, cloudiness, and wind. A statistic of the occurrence of foehn and chinook is used to estimate the frequency of foehn clearance.

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Klaus P. Hoinka

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This study presents global statistics of tropopause parameters for a 15-yr period (1979–93). The parameters are height, temperature, potential temperature, mixing ratio of water vapor, and zonal, meridional and vertical wind. The global tropopause is derived from ECMWF reanalysis data by applying the thermal and dynamical definitions of the tropopause. The tropopause climatology evaluated from the ECMWF reanalysis data is compared with those provided by radiosonde and satellite data. The meridional and zonal variations in the mean tropopause height and temperature reflect the global jet stream structure. The seasonal variation of tropical tropopause temperatures is characterized by a minimum during Northern Hemispheric summer and a corresponding winter maximum. The tropopause mixing ratio of water vapor attains a minimum in the Tropics that increases gradually poleward. The maximum of the relative humidity occurs in the Tropics above the Pacific and Indian Oceans. There are no global trends in the height and temperature of the tropopause but a decrease of about 10% per decade is evaluated for the mixing ratio of water vapor. The negative trend is particularly strong above both polar regions.

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Joseph Egger and Klaus-P. Hoinka

Abstract

The atmosphere induces changes of the earth’s angular momentum via orographic and frictional torques and, in particular, via torques exerted by the equatorial bulge. While the torques leading to fluctuations of the axial component of angular momentum and to corresponding changes of the length of the day have been investigated intensively in the past, this is not so for the torques linked to the equatorial components of global angular momentum. Here, the orographic torques that affect the equatorial components of global angular momentum and contribute to polar motion are discussed. The torques linked to the equatorial bulge are considered as well. European Centre for Medium-Range Weather Forecasts reanalysis data are used to investigate the torques both in an inertial and an earth fixed frame. The basic statistical characteristics of the mountain torques are derived for various regions on the globe. It is found that a large fraction of the torque exerted by the bulge is due to the presence of orography. Moreover, mountain torques are not negligible when compared to those exerted by the bulge. Examples of atmospheric flow patterns linked to large torques at the Tibetan plateau are given.

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Klaus P. Hoinka and Dietrich Heimann

Abstract

The channeling effect of the Pyrenees on a cold front is illustrated using high resolution surface data. Satellite data support the analysis of the surface data and show that the surface front is trapped to a significant degree in the vicinity of the mountains.

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Günther Zängl and Klaus P. Hoinka

Abstract

The polar and subpolar tropopause in both hemispheres is investigated using the ECMWF Reanalysis (ERA) data from 1979 to 1993 and radiosonde data from 1989 to 1993. Both the thermal and the dynamical criteria are applied to each dataset. The tropopauses derived from the radiosonde data are used to validate the ERA-derived tropopauses and to investigate the sharpness of the tropopause. The validation reveals that the ERA data are well suited for the determination of the tropopause. A comparison between the thermal and the dynamical tropopause shows a very good agreement except for polar winter, and there is clear evidence that the dynamical criterion is more appropriate in winter.

The results show that the annual cycle of the polar tropopause can be classified into three different patterns. A single wave with a tropopause pressure maximum in winter and a minimum in summer is typical for the subpolar parts of eastern Siberia and North America. A double wave with pressure maxima in spring and autumn and minima in summer and winter is found above northern Europe, western Siberia, and generally at high Arctic latitudes. Finally, Antarctica exhibits a reversed single wave with a pressure maximum in summer and a minimum in winter. Tropopause temperatures are generally highest in summer and lowest in winter, but the amplitude of their annual cycles shows distinct differences. It is lowest in those regions where a single pressure maximum in winter is present and largest in the Antarctic. A comparison between the tropopause pressure and the temperatures in 500 and 100 hPa reveals that the tropopause pressure is closely related to the temperature difference between 500 and 100 hPa. A large temperature difference corresponds to a low tropopause pressure and a small temperature difference to a high tropopause pressure. The sharpness of the tropopause, that is, the change in vertical temperature gradient across the tropopause, is found to be highest in summer and lowest in winter. Its annual cycle and its regional differences are primarily determined by the mean temperature gradient above the tropopause because it varies much more strongly than the gradient below the tropopause.

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Joseph Egger and Klaus P. Hoinka

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Given a flow domain D with subdomains D1 and D2, piecewise potential vorticity inversion (PPVI) inverts a potential vorticity (PV) anomaly in D2 and assumes vanishing PV in D1 where boundary conditions must be taken into account. It is a widely held view that the PV anomaly exerts a far-field influence on D1, which is revealed by PPVI. Tests of this assertion are conducted using a simple quasigeostrophic model where an upper layer D2 contains a PV anomaly and D1 is the layer underneath. This anomaly is inverted. Any downward physical impact of PV in D2 must also be represented in the results of a downward piecewise density inversion (PDI) based on the hydrostatic relation and the density in D2 as following from PPVI. There is no doubt about the impact of the mass in D2 on the flow in the lower layer D1. Thus results of PPVI and PDI have to agree closely. First, PPVI is applied to a locally confined PV anomaly in D2. There is no far-field “response” in D1 if stationarity is imposed. Modifications of boundary conditions lead to “induced” flows in D1 but the results of PPVI and PDI differ widely. This leads to a simple proof that there is no physical far-field influence of PV anomalies in D2. Wave patterns of the streamfunction restricted to D2 are prescribed in a second series of tests. The related PV anomalies are obtained by differentiation and are also confined to D2 in this case. This approach illustrates the basic procedure to derive PV fields from observations which excludes a far-field response.

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Klaus P. Hoinka and Günther Zängl

Abstract

In this note the impact of the vertical coordinate system on upper-tropospheric and lower-stratospheric structures is studied using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Two different simulations are compared. One uses the original sigma coordinate, and the second one uses a modified coordinate system having smoother coordinate surfaces in the free atmosphere. Fields of atmospheric variables, such as wind and temperature, show only a weak signal related to the vertical coordinate system. However, diagnostic quantities involving horizontal and vertical derivatives react very sensitively to the vertical coordinate. The results indicate that, in the presence of steep topography, a meaningful computation of the potential vorticity (PV) field in the tropopause region is possible with the modified coordinate system only. This is mainly because disturbances in the horizontal wind field that appear at first sight unimportant induce large errors in the relative vorticity field with the original coordinate system. In addition, the simulation with the original coordinate indicates a spurious moisture transport across the tropopause above the orography. On the other hand, the impact of the coordinate system on the structure and the amplitude of orographic gravity waves turns out to be quite small.

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Klaus P. Hoinka and Hans Volkert

The German Front Experiment, 1987 (GFE87) is a field experiment using aircraft, radar, surface, and upper-air observations to determine the influence of the European Alps on cold fronts. Measurements are concentrated in the Rhine Valley and in the Alpine foreland south of Munich. The observation period will be between October and December 1987, concurrently with the British and French joint field effort Mesoscale Frontal Dynamics Project (MFDP) and Fronts ‘87. The background and the setup of GFE are described.

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