Impact of inertia-gravity waves (IGW) generated in the upper troposphere on precipitation events and the interaction of both phenomena
Final Report Abstract
During the evolution of poleward breaking Rossby waves warm and moist subtropical air is advected into the North-Atlantic/ European sector. For such events strong upper-tropospheric jets streaks are formed at the air mass boundaries. At the same time, the moist air makes the occurrence of convection and associated precipitation events (PE) very likely: Goal1: We followed the hypothesis, that the jet-generated sub-synoptic inertia-gravity waves (IGW) which modulate the lower tropospheric air and give rise to convection events. In numerical simulations with the Fifth-generation Mesoscale Model (MM5) these processes were identified. The resulting patterns in surface wind, cloudiness and precipitation have been validated with ground-based observations from automated weather stations, radars and satellites. Hence, observational evidence was added to the process of the convective amplification of jet-generated inertia gravity waves. Goal 2 and 3: Documentation of one case of IGW and PE coupling with many independent observational dataset was presented. Similarity of observed structures at the meso-alpha scale suggested this link which was further supported with MM5 simulations. In comparison between dry and moist MM5 simulations we could identify an increase of IGW amplitudes by convection. In process studies we showed that IGWs may trigger convection through a local change in stability. It was important to investigate details of numerical diffusion in order to choose an optimal model resolution for mesoscale simulations. Goal 4: The diagnosis of model results identified the Lagrangian wind speed tendency and the condensational heating as the relevant forcings for jet- and convection-generated IGWs. Comparing four field campaigns with different intense upper tropospheric jets and convective activity we could demonstrate, that the proposed empirical parameterisation predicts the correct magnitude of the diagnosed inertia-gravity waves. Such a parameterisation could lead to more realistic simulation of such waves in global circulation models which do not explicitly resolve the scales of convective activity (see NG3). Three new ideas were born based on our previous project work resulting into three new goals: New Goal 1: Based on a case study and three similar events, we establish the hypothesis that with a known climatology of poleward Rossby wave breaking events a parameterisation of the formation or occurrence of high and thin cirrus clouds seems to be possible. In future this would be the basis for climate change study in determining the radiative role of high and thin cirrus clouds during changing RWB events in mid-latitudes in order to quantify their regional contribution to the radiative budget especially to differentiate between the Atlantic and Pacific sector. That this difference exists was also a main result shown in the next objective. Further in this context of RWB events IGW play not an important role because the generation in the exit region of the induced jet streak is too far away from the location of Cirrus appearance near the centre of the anticyclone. New Goal 2: The water vapor transports in different flow configurations (here induced by zonal ozone changes in the stratosphere) depend mainly on Rossby wave activity or baroclinic wave life cycles and mean water vapor gradients which are produced by the large-scale flow. That means it determines the regional moisture concentration as the reservoir for precipitation. We found significant changes of this reservoir and of convective PE in a coupled atmosphere-ocean GCM. The induced changes of convective PE mirror the Rossby wave activity and spontaneously generated jet-induced IGWs in those regions of strong changes under coupled condition. Further the feedbacks of these processes on the large-scale flow should be analyzed in future as well as the link to the occurrence of high and thin Cirrus. Furthermore we analyzed satellite data for a comparison of the Lagrange transport (3 dim. Brewer-Dobson circulation) in determining the large-scale structure of water vapor in the upper troposphere and lower stratosphere. But first of all (next objective) we want to improve our understanding of the link between RWB events, jet generated IGWs and moisture processes. New Goal 3: Using the condensational heating, an empirical parameterization function for convection-generated IGWs was presented. Application to four field campaigns showed the prediction of the correct order of magnitude. This way we contributed to a better modeling of this type of IGWs extending the work of other researchers. The proposed parameterization can be implemented into general circulation models with spare resolution leading to more realistic PE. As a result of convective activity the ageostrophic motions in the jet exit and around fronts were amplified. That means time and place were set by the baroclinic wave related jets and fronts, the intensity is determined by convection. Hence, the Weather Research Forecasts (WRF) simulations confirmed the conceptual picture. For the first time, a parameterization containing three generation processes is presented (jet, front and convection). The test with the WRF model data was successful. The systematic study of appearance of IGWs in baroclinic waves revealed new information on their structure (classification of wave packets) and energy (new parameterization).
Publications
- Impact of upper-level jet-generated inertia-gravity waves on surface wind and precipitation. Atm. Chem. Phys. Disc., 7, 15,873 - 15,909, 2008
Zülicke, Ch., and D. Peters
- 2010. Mesoscale patterns of wind and precipitation due to inertia-gravity waves in model simulations and observational networks. (Sixth Study Conference on BALTEX, Miedzyzdroje at Wolin, Poland, 14-18 June 2010); International BALTEX Secretariat Publication No. 46 (June 2010; ISSN 1681- 6471) 86 – 87
Zülicke, Ch.
- On the upper tropospheric formation and occurrence of very high and thin cirrus clouds during anticyclonic poleward Rossby wave breaking events. Tellus, 62A, 228-242, 2010
Eixmann, R., D. Peters, Ch. Zülicke, M. Gerding, and A. Dörnbrack
(See online at https://doi.org/10.1111/j.1600-0870.2010.00437.x)
