J. Egger

Fronts and orography

SummaryTheoretical work on the impact of orography on cold fronts is reviewed and related to observations. This paper contains the following sections:1.Introduction2.Passive scalar fronts3.Shallow water fronts4.Fronts in deformation flows5.Orographic jets6.Cold-air damming7.Föhn and fronts8.Fronts in valleys9.Cold surges10.Lee cyclogenesis11.Orographic rain and fronts12.Pressure drag13.Numerical simulations14.Analysis

Dynamical aspects of ozone mini-hole formation

SummaryA case study and statistical evaluations provide evidence that so-called ozone mini-hole events over Europe, where a rapid drop of total ozone is followed by complete recovery after a few days, are due to the northeascward motion of patches of air with low total ozone content. These patches appear to originate in subtropical latitudes. They correlate well with minima of potential vorticity near the tropopause. Contour dynamics is invoked to explain some basic features of the deformation and northward motion of the mini-holes as well as the related large-scale flow structures.

Thermally Induced Flow in Valleys with Tributaries Part II: Response to Cooling

SummaryFlow in long and deep main valleys with tributaries is studied for constant surface heating switched on att=0. The valley flows are obtained from a numerical model which combines slope wind layer equations with equations for the valley flow off the slopes. Much simpler linear models are used for the intepretation of the model results. If there are no sidevalleys an up-valley wind regime evolves in the main valley after the switch-on of the heating which protrudes towards the head. It is shown that the topographic amplification factor which captures the geometry of the valley and stratification are important factors in determining the intensity of the along-valley flow. However the up-valley winds are also quite sensitive to the specification of the boundary conditions at the upper end of the slope wind layers. If sidevalleys are added strong inflow to these tributaries is found only if their topographic amplification factors are larger than that of the main valley. This flow into the tributaries is mainly balanced by downward motion on top of the main valley but flow entering through the mounth of the main valley can contribute as well. Tributaries can induce flow in the main valley long before the main valleys own up-valley wind regime has reached the location of the tributary.

Modification of fronts by the Alps: Simulations and numerical experimentation

SummaryAn isentropic limited area model is used to simulate and investigate the frontal passages of 3 May and 8 October 1987. It is demonstrated that a southward outbreak of air with high potential vorticity on 3 May 1987 affected the propagation of the front to the north of the Alps and the related formation of an orographic jet. Moreover, the outbreak plays a crucial role in the genesis of a lee cyclone. On the other hand, no such outbreak occurred on 8 October 1987 and it is shown that the propagation of the front near the ground was hardly affected by the flow at upper levels.

Simple models of the valley-plain circulation Part I: Minimum resolution model

SummaryA minimum resolution model of thermally induced valley-plain circulations is presented. Slope winds and cross-valley circulations are treated as subgrid motions and are incorporated via aK-ansatz. Using this model one can determine the dependence of the valley-plain circulation on the valleys geometry, on the effectivity of the subgrid motions and on stratification. The relative importance of advective processes is assessed and the response to heating with a diurnal cycle is examined.ZusammenfassungEin Modell mit extrem niedriger Auflösung wird auf das Problem der thermisch induzierten Ausgleichsströmung zwischen einem Gebirgstal und einer vorgelagerten Ebene angesetzt. Hangwinde und die zugehörigen Zirkulationen in der Talatmosphäre werden im Modell als subskalige Bewegungen angesehen und durch einenK-Ansatz erfaßt. Das Modell erlaubt es, Aussagen über die Abhängigkeit der Talwinde zum Zuschnitt des Tals, von der Wirksamkeit der subskaligen Bewegungen und von der Schichtung zu machen. Die Bedeutung der Temperaturadvektion läßt sich abschätzen. Ferner wird die Reaktion der Zirkulation auf Antriebe mit Tagesgang untersucht.

Altai-Sayan lee cyclogenesis: Numerical simulations

SummaryIn this paper we compare and analyse two cases of lee cyclogenesis observed near the Altai-Sayan massif (14–16 April 1988; 3–5 March 1986). The interpretation of the events is based on numerical simulations with an isentropic primitive equation model, where runs without orography are compared to those with realistic orography. The runs with mountains are successful in depicting the observed developments, so that the no-mountain experiments can be assumed to provide further information on the dynamics of both cases.The case of 14–16 April 1988 is dominated by rapid cyclogenesis and cut-off low formation aloft linked to the southward motion of a potential vorticity maximum towards the Altai-Sayan massif. Cyclone formation would have occurred without mountains as well. However, cyclogenesis at low levels is enhanced by the presence of the mountains through blocking of the cold air and corresponding deformation of the surface cold front. So this is a case of orographically modified cyclogenesis whereas the cyclogenesis of 3–5 March 1986 is induced by the mountain. In that case a broad and almost steady trough moves eastward over the Altai-Sayan region. Cyclone formation is absent in the no-mountain run. However, both cases exhibit some similarities with respect to the low-level developments induced by the mountains. A conceptual model is presented in order to further elucidate the key features of both cases.

A note on the dynamics of the quasi-stationary Kunming front

SummaryThe quasi-stationary Kunming front is oriented parallel to the north-south running major mountain chains of south-western China and normal to the upper-level westerlies. The front curves towards the east off the mountain. We hypothesize that the shape of the Kunming front is due to ageostrophic flow along the mountains which is induced by westerly flow in the cold air and at upper levels. Eventually the front must become stagnant. This conjecture is supported by numerical experiments with simple models.

Cold front propagation in the Loisach valley: Simulations with a simple model

SummaryA simple, basically two-dimensional multilevel model is used to simulate the passage of a cold front in the Loisach valley as described by Müller and Sladkovic (1990). It is found that the front moves essentially like a density current on its way from the mouth of the Loisach valley towards Garmisch. However, a quasi-stationary state is attained soon after the front passed the branching point of the valley near Garmisch where the cold air is then flowing east- and westward. This flow state is characterized by downward motion in the Loisach valley proper and rapid ascent above Garmisch. This circulation becomes less intense as the valleys are filled with cold air. The agreement of model results and observations is satisfactory given the limitations of the model.

Simple models of the valley-plain circulation Part II: Flow resolving model

SummaryIn extension of the work presented in the first part of this paper a simple two-level grid point model for valley-plain flow is described where basic horizontal flow structures can be resolved. Except for the increased horizontal resolution this flow resolving model (FR-model) bears strong resemblance to the MR-model discussed in the first part.The FR-model is mainly used to study the profiles of wind and temperature along the valley axis. To better understand the valley flows in the FR-model the linear theory of forced flow in infinitely wide valleys is presented. This theory predicts a characteristic length of decay of the response with distance to the valley mouth for steady-state flow. If the forcing has a diurnal cycle the response shows the characteristics of damped wave motion. At the mouth there is a jump of the temperature.It is shown that the valley flow in the FR-model can be understood qualitatively on the basis of the linear theory. It is demonstrated that the FR-model is capable of reproducing the gross characteristics of observed flows near the mouth of valleys.ZusammenfassungIn Fortführung der Modellentwicklung, wie sie im ersten Teil dieser Arbeit vorgetragen wurde, wird ein Gitterpunktmodell mit zwei Schichten in der Vertikalen (FR-Modell) für die Ausgleichströmung zwischen Ebene und Vorland vorgestellt, das horizontale Strömungsstrukturen wesentlich besser aufzulösen gestattet, als dies im MR-Modell von Teil I möglich war. Mit Ausnahme der Auflösung in der Horizontalen stimmen beide Modelle ansonsten fast exakt überein.Das FR-Modell wird hauptsächlich zum Studium der Strömungs- und Temperaturprofile am Talausgang und im Tal benutzt. Die Modellresultate werden mit den Aussagen einer linearen Theoric für ein sehr breites Tal verglichen. Es ergibt sich eine gute qualitative Übereinstimmung. Insbesondere findet man für stationäre Talwinde in Modell und Theorie eine Übergangszone am Talausgang mit starken Temperaturgradienten und einer exponentiellen Abnahme der Windintensität ins Tal hinein. Hat der Antrieb einen Tagesgang, so reagiert das Windsystem mit einer Verzögerung und dem exponentiellen Abfall überlagert sich eine Wellenbewegung.Das FR-Modell ist in der Lage, den nächtlichen Strahlstrom, wie er an Talausgängen beobachtet wird, in Grundzügen nachzubilden.

Überschreitung einer breiten Gebirgsbarriere durch Rossby-Wellen

ZusammenfassungIn einem barotropen, quasigeostrophischen Modell der Atmosphäre wird das Verhalten von Zyklonen und Antizyklonen beim Überqueren einer breiten Gebirgsbarriere, wie sie etwa die Rocky Mountains darstellen, untersucht. Bei anhaltender Westströmung bildet sich über dem Gebirge ein Keil hohen Luftdruckes aus. Die Störungen, die das Gebirge zu überschreiten suchen, folgen der Grundströmung, und ihre Zugbahn beschreibt somit einen antizyklonalen Bogen. Doch ist keinerlei Einfluß der Orographie auf ihre Amplitude oder auf ihre Phasengeschwindigkeit in der ursprünglichen Zugrichtung festzustellen.SummaryA barotropic, quasigeostrophic model is used to study the crossing of large mountain barriers by cyclones and anticyclones. The westerly basic flow is forced to form a stationary ridge over the mountains. The cyclones and anticyclones follow the streamlines of the basic flow curving anticyclonically on crossing the mountains, but there is no influence of the orography on their amplitude or their phase velocity inx-direction.