Zx. Pu et Sa. Braun, Evaluation of bogus vortex techniques with four-dimensional variational data assimilation, M WEATH REV, 129(8), 2001, pp. 2023-2039
The effectiveness of a four-dimensional variational data assimilation (4DVA
R) technique for creating "bogus" vortices in numerical simulations of hurr
icanes is evaluated in this study. A series of numerical experiments is con
ducted to generate initial vortices for Hurricane Georges and Bonnie (1998)
in the Atlantic Ocean by assimilating bogus sea level pressure and wind in
formation into a mesoscale numerical model (MM5). Several different strateg
ies are tested for investigating the sensitivity of the initial vortex repr
esentation to the type of bogus information.
While some of the results in this study confirm conclusions made in previou
s studies, some significant differences are obtained regarding the role of
bogus wind data in creating a realistic bogus vortex. In contrast with prev
ious studies in which the bogus wind data had only a marginal impact on cre
ating a realistic hurricane, this study concludes that the wind information
is very important because 1) with assimilation of only bogus sea level pre
ssure information, the response in wind field is contained largely within t
he divergent component, with strong low-level convergence leading to strong
upward motion near the center; and 2) with assimilation of bogus wind data
only, an expected dominance of the rotational component of the wind field
is generated. In this latter case, the minimum pressure is also adjusted si
gnificantly, although the adjusted sea level pressure does not always match
the actual hurricane minimum pressure. The generated vortex offers a smoot
h start to the forecast and leads to a significant improvement in the forec
ast. Only when both the bogus sea level pressure and wind information are a
ssimilated together does the model produce a vortex that represents the act
ual intensity of the hurricane and results in significant improvements to f
orecasts of both hurricane intensity and track.
As the 4DVAR experiments are performed with relatively coarse horizontal gr
id resolution in this study, the impact of vortex size on the structure of
the initial vortex is also evaluated. The authors find that when the scale
of the specified bogus vortex is smaller than that which can be resolved by
the model, the assimilation method may result in structures that do not co
mpletely resemble observed structures in hurricanes. In contrast, when the
vortex is sufficiently large for it to be resolved on the horizontal grid,
but not so large as to be unrealistic, more reasonable hurricane structures
are obtained.