Synoptische, diagnostische und numerische Untersuchungen der Intensitätsänderungen von tropischen Wirbelstürmen und Monsuntiefs

The intensity changes of an Australian monsoon depression and typhoons Flo (1990) and Ed (1990) are investigated using operational analyses, diagnostic studies, and numerical model calculations. The analyses are based on gridded data from the Australian Bureau of Meteorology’s (BMRC’s) Tropical Analysis System, the European Centre for Medium-Range Weather Forecasts (ECMWF) data archive, and the 1990 Tropical Cyclone Motion experiment (TCM-90). The TCM-90 analyses are provided by the Japan Meteorological Agency and the National Centers for Environmental Prediction and incorporate typhoon boguses of the Geophysical Fluid Dynamics Laboratory and the Japan Meteorological Agency. The monsoon depression developed over northwestern Australia in February 1994. After remaining quasi-stationary for several days after its formation in the monsoon trough, the depression drifted slowly southwestwards along the coast of Western Australia and eventually weakened. The evolution is exemplified by time-height cross-sections of various kinematic quantities as well as the apparent source of potential vorticity, which has been taken into account in the PV equation for the first time. A prominent feature of these diagnostic fields is the impact of convective and diabatic processes on the development. The structure of the depression is very similar to those of the few monsoon depressions that have been documented over the Indian subcontinent. The formation of the depression coincided with the passage of an upper-level middle-latitude trough and a low-level anticyclone to the south, but the extent to which the genesis was influenced by these disturbances is unclear. The equation for the mean tangential wind in a storm-following cylindrical coordinate system is applied to investigate the intensity changes of tropical cyclones and monsoon depressions. In this equation, the symmetric, the resolved asymmetric and the subgrid-scale contributions are fully separated. The tangential wind changes can be interpreted in terms of absolute vorticity flux, angular momentum flux, PV flux or the Eliassen-Palm flux-divergence. The corresponding eddy fluxes are compared, and the impact of convection on the intensity changes as well as the interaction between a monsoon depression and upper-level troughs and ridges is investigated. Analyses of observational data for a monsoon depression that formed near the coast of northwestern Australia in February 1994 show that all terms in the tangential wind equation are important, except for the drift term. These important terms include above all the subgrid-scale terms which are presumed to be largely due to processes associated with deep convection. In the lower and middle troposphere, the main terms contributing to the intensification of the monsoon depression are the radial absolute vorticity fluxes (cyclonic inward fluxes of relative and planetary vorticity; or the corresponding terms that include the Eliassen-Palm fluxes, the radial PV-fluxes, or the angular momentum fluxes) and the vertical advection of tangential wind. In the upper troposphere, the cyclonic tangential wind change can be attributed mainly to the relative vorticity fluxes (anticyclonic outward fluxes of relative vorticity), the vertical advection of tangential wind, and the subgrid-scale contributions. The weakening tendency of the upper-level anticyclone associated with these effects and the channelled, widespread outflow are related to the strengthening of the monsoon depression. The inward movement of the eddy PV-flux-maxima in the upper troposphere reflects the approach of PV anomalies associated with passing mid-latitude troughs. We hypothesize that the convection near the monsoon depression is enhanced in front of the approaching PV anomalies by the reduction of static stability and by the vacuum cleaner effect, and that this influences the intensification of the depression. As a check on the robustness of the findings, the results using BMRC analyses are compared with those using ECMWF analyses. Despite the high diurnal variability, the patterns of the daily-mean symmetric, resolved-asymmetric and unresolved-asymmetric contributions agree relatively well, although the figures from the higher resolution ECMWF analyses show more details with more distinct peaks. The intensity changes of typhoons Flo and Ed, which were strongly influenced by deep convection, have been investigated using the Europa- and the Deutschland-Modell of the Deutscher Wetterdienst. The sensitivity of model forecasts to different initial conditions, typhoon boguses, model resolution and physical parameterizations has been examined. The typhoons Flo and Ed developed east of Guam and drifted northwest and westward, respectively. During the observation period, they were embedded in the monsoon trough and were about 1700 km apart. After 15 September 1990, Flo intensified rapidly to supertyphoon strength and recurved towards Japan. Before weakening over the coast of Vietnam, Ed undertook an unusual, southwestward drift. The effects of the large-scale and subgrid-scale asymmetries on the intensity of Typhoon Flo were not significant despite the nearby TUTT-cell and could not explain the intense deepening phase after 15 September. An inward or outward drift of maximal or minimal absolute eddy-vorticity fluxes was not analysed. The predictions of the asymmetry effects give useful hints about the kind and strength of the effects independently of the grid-resolution and the typhoon bogus incorporated in the start analysis. As in the case of the Australian monsoon depression, the vertical advection of momentum and the subgrid-scale effects are not negligible. For the two typhoons, the Europa- and the Deutschland-Modell showed a modest forecast skill for intensity change. The initial data fields, the incorporated boguses, the model resolution, and the physical parameterizations have a significant influence on the intensity forecasts. The boundary values barely affect the intensity prediction. Compared to operational forecast models, the two models have good skill in predicting typhoon tracks. The track prediction is nearly independent of the grid resolution and the cumulus, radiation and diffusion parameterization applied. Variations in track arise primarily from different boundary conditions and initial analyses including the typhoon boguses used. The model resolution, the initial analysis used, and the typhoon bogus incorporated have nearly no influence on the distribution and size of the rain areas. However, the typhoon bogus used and the model resolution strongly affect the amount of core region precipitation. The intensity of the model typhoons is correlated with the precipitation intensity and the strength of convection. Averaged over the entire typhoon area, the differences in the amount of rain for the two model resolutions is small, however.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Ludwig-Maximilians-Universität

Year

2000

Author dc:creator

• Hell, Reinhard Michael

Repository record source_url

https://edoc.ub.uni-muenchen.de/316/

OAI identifier oai:identifier

oai:edoc.ub.uni-muenchen.de:316