This paper presents a diagnosis of the geostrophically forced secondary circulation and the connected frontogenetical processes of mid-latitudinal cold fronts. Mesoscale isentropic analyses in three dimensions and the numerical solution of suitable Sawyer-Eliassen-equations are the basis of the study. It represents part of the investigation of the DFG (German Science Foun-dation) - programme 'Fronts and Orography'. The analyses are based mainly on rawinsonde data, which were frequently recorded during the field phase of the 'German Front Experiment 1987' (FRONTEX).
The three most prominent fronts of the measuring campaign which included a total of five events, exhibited frontogenetical processes in the upper and middle region of the troposphere during their propagation over Western and Central Europe. A direct secondary vertical circulation is induced to counteract frontogenetical processes by adiabatic effects and to limit growth of the front. Additionally, in some cross sections another direct but smaller cell appears ahead of the front in the upper troposphere. During the eastward displacement of the fronts cross sections from north to south and from northwest to southeast show frontolytic effects in the frontal region, which can be found some hours later in cross sections from west to east as well. With the onset of the frontolysis, the sense of rotation of the secondary circulation changes from direct to indirect. In comparison, the two other frontal events are much weaker. Consequently frontogenetical effects are correspondingly weaker and in one case weaken even further during the displacement towards the eastern parts of Central Europe. The corresponding secondary circulation appears with lower intensity, too and no change of sense of rotation is found.
Furthermore, the described forced circulations confirm the postfrontal sinking processes, which were anticipated by cross sections of relative humidity of two frontal events over Western Europe. This is in agreement with the results of Browning and Monk (1982).
A comparison between different approximations of the secondary circulation equations shows that ageostrophic advections pointing in the direction of the current, can be neglected in the underlying equations, while the other component of the horizontal ageostrophic advection plays a main role in the secondary circulation. A complete quasigeostrophic approximation in which all the ageostrophic advection terms are left out, yields only qualitatively useful results.
Due to a large number of rawinsonde measurements available at 00 and 12 UTC, prominent frontal structures and stronger components of the secondary circulation and of the frontogenetic processes are more easily identified than at times when data from fewer stations are available.
|
|
Last Modified: Sun Jun 13 12:36:42 2004 |
|