In the upper troposphere and lower stratosphere in mid-latitudes, especially in winter, propagating Rossby waves are observed which determine the dynamics, transport and exchange in the tropopause region. A classification shows 4 typical cases of downstream Rossby wave-breaking in regions of weak wind. Inertia-gravity waves (IGW for short here), with a relatively small horizontal wavelength (~300-900 km) can be stimulated by Rossby wave breaking. They propagate regionally under certain conditions both horizontally and vertically and can play a significant role in exchange processes of the upper troposphere and lower stratosphere, this being not yet well understood.
Hence, the study of the forcing and propagation of inertia-gravity waves and the associated exchanges in the upper troposphere and lower stratosphere in connection with breaking Rossby waves is central to this project.
- To investigate breaking Rossby waves in their vertical structure in connection with the polar vortex and to determine the connection with inertia-gravity waves by field experiments
- To analyse and model the forcing mechanisms and propagation of inertial gravity waves for typical cases of Rossby wave breaking in order to understand the dominating processes
- To study the mixing and exchange processes between the troposphere and stratosphere caused by inertial gravity waves, to quantify the associated exchanges.
Processes will thus be studied which play a key role in determining the nature of exchanges between the troposphere and stratosphere, as well as in horizontal transports in the lower stratosphere. In particular, the coupling between the upper levels of the troposphere and the stratosphere through dynamical processes, as well as the exchange of trace gases, will be observed.
State of research and own work:
Especially low temperatures, a precondition for active heterogeneous chemistry resulting in ozone destruction (Teitelbaum und Sadourny, 1998), are created through poleward propagation of planetary waves. Inertia-gravity waves in particular, for example caused by mountains in Scandinavia, can alter the mean temperature locally by some 5 K to 10 K either up or down (Dörnbrack et al., 1999), which in the latter case aids the generation of PSCs. IGWs with periods of a few hours, horizontal wavelengths of 300-900 km and vertical wavelengths of 1-4 km are observed in various regions of the earth (Sato et al., 1997, Thomas et al., 1999, Herzog et al., 1999). These are connected with a number of different forcing mechanisms, such as ageostrophic adjustment, orographic forcing or jet instability.
Radiosondes with high resolution (with GPS winds) and ozone sondes (whereby ozone can be considered as a passive tracer under certain conditions), in addition to continuous measurements with radar (wind vectors) and Lidar (temperature) are necessary for estimating periods, wavelengths and energy propagation. The preparation of climatic charts is especially difficult since no optimal classification of events relating to the potentially enormous number of different synoptic situations has been made, apart from some exceptional cases.
O‘Sullivan und Dunkerton (1995) simulated type LC1 Rossby wave breaking in a modeling study and showed that inertia-gravity waves are formed through ageostrophic adjustment downstream of the jet stream. Similar results were found in the frontal model of Reeder und Griffiths (1996) and confirmed by Lidar measurements with backward trajectory analyses of the airflow by Herzog et al. (1999). Further open questions are how other types of breaking Rossby waves (Esler und Haynes, 1999) and their horizontal and vertical structure influence the forcing and propagation of IGW and how IGW change the mixing and exchange processes between troposphere and stratosphere?
Breaking Rossby waves are a frequent phenomenon in wintertime in both hemispheres (Norton, 1994, Peters & Waugh, 1996, Appenzeller et al., 1996). Poleward breaking Rossby waves, which have been intensively studied in our group (Peters & Waugh, 1996, Bartels & Peters, 1996, Peters & Waugh, 1999) are connected to ozone mini-hole events (Peters et al., 1995, James et al., 1997). The winters during the period 1990-93 were analysed with contour advection calculations of Ertel´s PV on the 330K isentrope, to define breaking event types P2, poleward and downstream, and P1, poleward and upstream (Peters & Waugh, 1996). With the help of a contour dynamics model and based on model simulations of a simplified GCM, a theoretical basis for Rossby wave breaking was laid. The exchange between troposphere and stratosphere was computed and it was shown that tropospheric air in particular is mixed into the stratosphere during P2 events (Bartels & Peters, 1996). Following on from this work, the pilot study LEWIZ (set a link www.iap-kborn.de/lewizdpe.htm) was prepared with the goals of measuring and studying IGWs in connection with a poleward downstream breaking Rossby wave (type P2). It is anticipated that coordinated measurements of high resolution radiosondes (every 3 hours), ozone sondes (every 12 hours) with VHF-radar (wind vectors in high temporal and vertical resolution in the upper troposphere and lower stratosphere) and Lidar (temperature at night) will be carried out over Kühlungsborn over a 3-day period in each case. The choice of starting dates for the campaign will be made by consulting 5-day forecasts of the German Weather Service.
The main task during the first year is the development of a MM5 modeling study centered over Kühlungsborn. With this model, those processes will be studied in more detail which have been observed during the LEWIZ pilot campaigns. The latter will have already been carried out as a field experiment at IAP during winters (1999/2000 & 2000/2001), involving employing coordinated radio- and ozone sondes measurements in association with radar- and lidar measurements during poleward breaking Rossby wave events. As a follow-up to this experiment, the field experiment LEWIZ will be prepared for carrying out during the winter (2001/2002) to complete a set of chosen Rossby wave breaking events. In the second year, since sufficient numbers of breaking events will then be available, process studies will be carried out with MM5, the collected observational data and finally ECMWF analyses in order to investigate the generation and propagation of IGW. Still further, the LEWIZ campaign will be prepared and carried out during winter (2002/2003 & 2003/2004), i.e. once again employing coordinated measurements using radio- and ozone sondes in association with radar- and lidar measurements to extend the 4 chosen types of Rossby wave breaking events. In the third year, the process studies with Rossby wave breaking types LC1 (equatorward upstream) and LC2 (equatorward downstream) will be continued, whereby the primary aspect will be the study and quantification of mixing and exchange processes. A comparison with the ERA 40 data will allow general climatologically statements to be made.
Prospects for success and anticipated results:
The project will combine the experiences of our theoretical working group on the subject of breaking Rossby waves with the experiences of the radar- and lidar group in measuring and diagnosing gravity waves. A VHF radar and a Rayleigh/Temperature Lidar (Alpers et al., 1999) are both continually in operation at the IAP under suitable weather conditions and darkness. Another VHF radar (Hoffmann et al., 1999) is run by the IAP in northern Norway, such that comparisons are fruitful. Experiences with ozone- and radiosonde starts have also been gained through participation in several Match-campaigns (Schulz et al., 1999). The DWD have already made their 5 day forecasts of meteorological fields in the upper atmosphere available to us, allowing us to determine the respective campaign starts in advance. ECMWF data will also be made available to us, which is of particular interest to this project since these analyses have been produced at a higher vertical and horizontal resolution since this spring. Further experiences will be gained from the pilot campaign LEWIZ. The MM5 code is available and, given the modeling experience of our group, it should be possible to implement it on a fast computer. Several research trips and conference visits abroad are planned to secure quality of results and for scientific discussions in an international context. Co-operation with Dörnbrack, DLR, (MM5), Wernli, Switzerland (storm tracks), Vaughan, UK (VHF Radar), Kirkwood, Sweden (VHF Radar) should continue to be built up for scientific exchange of experience. Further, the EC-project METRO will give us experiences which can be brought into the project.
Under these circumstances, it would appear probable to us that qualitative and quantitative findings can be made with regards the generation and propagation of IGW and their effect on exchange processes associated with breaking Rossby waves, such that a high quality solution to the scientific questions can be gained.