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.