TCAR - Typhoon Committee

TCAR
CHAPTER 5 - RESEARCH FELLOWSHIP TECHNICAL REPORT
initialization scheme is used to predict TS track
and landfall when a TS occurs in the South China
Sea. Bogus vortex including both symmetric and
asymmetric components is constructed based on
the assumption that the storm motion is equal to
the vector sum of the large scale environmental
flow plus the vortex asymmetry (Smith and Ulrich,
1990; Smith, 1991; Smith and Weber, 1993; Weber
and Smith, 1995; Davidson and Weber, 2000).
Numerical experiments have been conducted
to select appropriate parameters in vortex
initialization scheme for correctly presenting
environmental flow, and the assumed vortex
structure in the South China Sea. Experiments
indicated that a scale of approximately 4 times the
radius of outermost closed isobar is appropriate
to preserve key components of the environmental
flow. Also, systematic northwest bias in forecasted
TC tracks caused by asymmetric component
of bogus vortex is corrected. Above mentioned
modifications brought significant improvement in
accuracy of TC (TC) track forecasts near Vietnam
coastline (Nguyen T. M. Phuong, 2003, 2004,
2005, 2006).
In CMA the limited area operational TS prediction
system had been used since 1996. In this system
the vortex initialization is similar to that developed
by Iwasaki et la (1987), i.e the bogus vortex is an
axi-symmetric, constructed by empirical methods
(Ma et la, 2007).
Since 2002 the second generation TS prediction
system, named GMTTP had been used
operationally. This is global spectral model version
T213L31 that is more advanced in both dynamics
and physics contents in comparison with the
limited area operational TS prediction system
(LTCM). Also, the vortex bogus scheme had been
improved by including asymmetric component in
the initial TC vortex. In new vortex initialization
scheme the bogus vortex consists of axisymmetric
component and asymmetric component. The first
is constructed as above mentioned ( similar to
Iwasaki et la 1987). The second is generated
from the analysis field around the TC that is
similar to the method described in Kurihara et la
(1993, 1995). The implementation of this bogus
scheme is as followings : (i) the axisymmetric
vortex is removed from T213L31 analysis field,
(ii) the axisymmetric vortex component is
generated, (iii) the asymmetric vortex component
is constructed, (iv) the axisymmetric and the
asymmetric components are added to form
asymmetric TC bogus vortex and (v) the bogus
vortex is inserted back to analysis field. The
flowchart of the second generation TS prediction
system is given in Fig.1. This asymmetric vortex
bogus scheme significantly improved accuracy
of CMA’s model TS track forecasts, especially in
the short ranges upto 72h. However, there exist
two kinds of systematic errors in the second
generation TS track prediction system : the first
is the westward deflection when a TS moved to
NE and the second is the N deflection when a TS
moved to W or NW due to many different factors
(Ma et la 2007).
Schematic depiction of the TC
prediction system at NMC/China
Fig . 1 . Flowchart of of the CMA’s second
generation TS prediction system (GTCMA) (Qu.
et la, 2009)
In 2006 in the CMA’s third generation TS track
prediction system a new TS initialization scheme
is build. This new scheme includes 3 procedures
(1) When TC occurs at first time, inserting an
appropriate vortex in the first guess fields, it is
called vortex formation.The appropriate vortex
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