Helmut Kraus

A complete sea-breeze circulation cell derived from aircraft observations

Using an instrumented aircraft, a complete sea-breeze circulation cell was sampled from its offshore end to the sea-breeze front over land and up to a height of approximately 1000 m AMSL. Many of its typical features can be resolved in detail mainly by cross-sectional analysis. These are: the diverging onshore flow in the lower layers over the sea the convergence over land at the seabreeze front and the connected cross-frontal circulation, a distinct return flow in the upper layers, the propagation of the seaward end of the cell over water, as well as the propagation of the front over land, the baroclinicity as a driving mechanism of the cell and also budget and frontogenesis terms.

An observational aircraft-based study of sea-breeze frontogenesis

In the summer of 1988/89 flights were carried out in the Coorong coastal area of South Australia to investigate sea-breeze fronts. The flights yielded data sets of the structure of the fronts in the cross-frontal direction with a spatial resolution of approximately 3 m. The study is focused on the budgets of sensible and latent heat in the vicinity of the front and on frontogenesis/frontolysis processes which are closely related to budget considerations.The frontogenesis relationships and the budgets were established on a 2 km length scale by low-pass filtering of the space series. As the wind components were measured with high accuracy, all processes which determine frontogenesis could be evaluated and are displayed in x,z-cross-sections: these are the confluence, shear and diabatic effects, all of which play a role in ∂q/∂x-, ∂q/∂z-, ∂θ/∂x- as well as ∂θ/∂z-frontogenesis. A detailed analysis is given for two different states of frontal development. The presented results shed much light on the governing physical processes in the frontal region with strong emphasis on the effects of confluence-generated updrafts, on shear instabilities causing bulges and clefts in the frontal surface as well as producing the elevated frontal head, and on processes related to differential heating and moistening.

The structure of the subpolar inversion-capped ABL

SummaryThe problem of polar and subpolar inversions is attacked. Thus a literature review of the exploration of the polar lower atmosphere is presented together with an attempt to classify these inversions, according to the participating processes, such as subsidence, advection and radiation. These physical processes are incorporated into a simple inversion model, giving the heights of inversion and cloud base, the radiative cooling rates and the vertical profiles of the thermodynamic quantities and the turbulent fluxes. The model is applied to a period with subsidence inversion over the Norwegian Sea during summer. The case study shows a good agreement between the results of the steady-state model and the mean observed thermodynamic structure of the atmospheric boundary layer.ZusammenfassungDie Erforschung der polaren und subpolaren atmosphärischen Grenzschicht wird in einer Literaturübersicht dargestellt. Anhand der bei der Entstehung beteiligten Prozesse wie großskaliges Absinken, Advektion und Strahlung wird eine Einteilung der Inversionen vorgenommen. Mit einem einfachen Modell, das diese Prozesse berücksichtigt, wird eine sommerliche Inversionslage über der Norwegischen See untersucht. Das Modell liefert die Höhen von Inversion und Wolkenbasis, die Strahlungsabkühlung und die Vertikalprofile der thermodynamischen Zustandsgrößen und der turbulenten Flüsse. Die Modellergeb nisse zeigen eine gute Übereinstimmung mit den Beobachtungen.

A nocturnal low level jet during PUKK

A nocturnal low-level jet characterized by a distinct inertial oscillation lasting from around sunset until sunrise the next day was observed during the coastal experiment PUKK. The phenomenon appeared over an area of at least 40 km in diameter; it thus had a mesoscale extension. We were able not only to observe this fine case but also to simulate it by an integrated dynamical PBL model.

Low-level jet phenomena described by an integrated dynamical PBL model

A time-dependent integrated dynamical boundary-layer model is used to study various features of the nocturnal low-level jet (LLJ). The basic concept is that of Thorpe and Guymer (1977). It is extended by entrainment processes, advection and an equation for the upper height of the turbulent layer. Applications show the role of the energy terms and how the LLJ changes considerably under synoptic forcing processes of variable geostrophic wind.

Orographical modification and large scale forcing of a cold front

SummaryThe development of a cold front influenced by orography and large scale forcing is examined with a two-dimensional meso-scale model. The model is based on the primitive equations and uses the hydrostatic and anelastic approximations. Gradients of the basic flow and temperature field in the third dimension are taken into account during the simulations. Low diffusive numerical schemes and radiation boundary conditions reduce the numerical errors to an acceptable minimum for a two day simulation and avoid reflections at the upper and lateral boundaries. Frontogenetical forcing is included in the simulations by specifying either a vertically sheared or horizontally convergent basic zonal flow field. Model runs with an idealized cold front were carried out over flat terrain and in the presence of a bell shaped mountain ridge.The simulations show a weakening of the cold front on the windward side of the mountain ridge and a strong reintensification on the leeward side relative to the control runs without topography. Analysis of frontogenesis terms demonstrates the importance of convergence in the ageostrophic circulation and of along-front temperature advection for the development of the cold front. The strong intensification of the cold front on the leeward side of the mountain ridge can only partly be explained by superposition with the mountain induced wave. It is mainly caused by ageostrophic deformation forcing in the strong downward flow of this wave.The results also show that the cold front passage over the mountain ridge is not a continuous process. The formation of a new frontal structure on the leeward side of the mountain ridge, well separated from the primary one, is observed while the initial cold front still exists in the upslope region. Generally nonlinear interactions between the mountain wave and the cold front are the important mechanisms to explain these phenomena.

The role of radiation in an inversion-capped planetary boundary layer

In an inversion-capped planetary boundary layer (PBL), the structure of the turbulent fluxes as well as the height of the inversion are determined by the interaction of turbulent mixing in the PBL, large-scale subsidence above the PBL and radiational cooling. Here the sensitivity of the inversion height and of the turbulent fluxes due to radiational processes is investigated with the aid of a three-layered model for a well mixed PBL. For an example of the Trade-Wind region, the inversion height (i.e., the difference between surface pressure and pressure at the inversion level) varies between 46 and 257 mb and the surface flux of moist static energy between 417 and 99 W m-2, if the (mean) radiative net flux divergence for both the inversion and the well-mixed layer is changed over a reasonable range of values. None of the parameterization schemes existing in the literature is able to describe these radiational effects in an appropriate way. This is due to the fact that these parameterizations are either not or not flexibly enough linked to the thermodynamical model parameter. Therefore the demand for an adequate parameterization of the radiational influence in a well-mixed PBL under a subsidence inversion is obvious.

On the energetics of the urban canopy layer

Abstract With the help of two boundary layer experiments in Bonn (F.R.G.) in July 1982 and February 1983 two case studies of the energetics of a mid-latitude city under fair weather conditions were evaluated. The temperature and humidity distributions and the sensible and latent heat fluxes could be derived for typical urban and rural sites from a combination of theoretical considerations and measurements. Here only the data from the summer experiment are studied in detail. The parallel measurements of the urban temperature and humidity fields and of the turbulent fluxes of sensible and latent heat allow the interpretation of the urban heat island via the processes which cause the phenomenon. In this paper special emphasis is given to the parameterization of the urban heat storage, the exchange of energy between the canopy layer and the boundary layer, and the interaction of mesoscale advection and local heating of the UCL ( U rban C anopy L ayer) and RCL ( R ural C anopy L ayer).

Energy fluxes in an alipine valley

SummarySurface energy fluxes were measured in the Dischma-valley near Davos, Switzerland at 1970 m from August 6th to August 15th, 1980. They are part of a case study for the microscale atmospheric conditions of an alpine valley. The universal function ϕH could be derived from the whole set of profile measurements of temperature, humidity and wind speed. It corresponds to the relation which was found over homogeneous terrain.ZusammenfassungDie Energieflüsse wurden im Dischma-Tal in der Nähe von Davos in 1970 m Höhe vom 6. bis 15. August 1980 gemessen. Sie waren ein Teil einer Sonderuntersuchung mikroskaliger atmosphärischer Bedingungen in einem alpinen Tal. Die allgemeine Funktion ϕH konnte von einem Satz von Profilmessungen der Temperatur, der Feuchtigkeit und der Windgeschwindigkeit abgeleitet werden. Sie entspricht der Beziehung, die auch über homogenem Terrain gefunden worden ist.

Frontal substructures within the planetary boundary layer

A two-dimensional mesoscale model, extended by a TKE closure for the subgrid-scale terms and coupled with a soil model, is used to investigate the role of the Planetary Boundary Layer (PBL) for the development and the substructures of two different types of cold fronts. The effects of turbulent friction, large-scale (geostrophic) forcing and the diurnal variation of the terms of the surface energy balance (SEB) equation on the frontal development are studied by 10 different model runs. The ageostrophic cross-frontal circulation in the lowest two kilometres of a cold front results from friction as well as from large-scale forcing. The first one dominates the PBL processes and causes a special boundary-layer structure, which becomes apparent through the existence of seven characteristic zones defined for the x-z cross sections of potential temperature. The arrangement of these characteristic zones depends on the sense of rotation of the frictionally induced part of the ageostrophic circulation and hence on the direction of the along-front jet within the boundary layer. The daytime increase of the terms of the SEB equation for a midlatitude midsummer case leads to a strong enhancement of the frictionally induced cross-frontal circulation. The arrangement of the seven characteristic zones, however, is approximately conserved.