65th IHC Booklet/Program (pdf - 4.9MB) - Office of the Federal ...

Diagnosing initial condition sensitivity of Typhoon Sinlaku (2008) and Hurricane Ike
(2008)
William A. Komaromi, Sharanya J. Majumdar and Eric D. Rappin
(wkomaromi@rsmas.miami.edu)
Rosenstiel School of Marine and Atmospheric Science, University of Miami
An examination of the sensitivity of numerical simulations of tropical cyclone (TC) track
to perturbations in the initial conditions has been conducted for Typhoon Sinlaku (2008) and
Hurricane Ike (2008). Balanced perturbations of different amplitude, spatial scale and location
were created via a modification of local relative vorticity at a chosen location and depth. For
each TC, 5-day Weather Research and Forecasting (WRF) model track predictions are compared
to an unperturbed (“control”) simulation. The control simulations replicate forecast errors
evident in the operational models specific to the chosen TCs, including a premature recurvature
in the forecast for Sinlaku and a landfall too far south along the Texas coast for Ike.
In the case of Typhoon Sinlaku (2008), the premature recurvature in the control
simulation is found to be corrected by a variety of initial perturbations, including the weakening
of an upper-level low directly to its north and the weakening of a remote shortwave trough in the
mid-latitude storm track. It is suggested that one or both of the shortwaves may have been
initialized too strongly. In the second case, Hurricane Ike (2008) initialized four days prior to its
landfall in Texas, the forecasts were not sensitive to most remote perturbations. The primary
corrections to the track of Ike arose from a weakening of a mid-level ridge directly to its north,
and the strengthening of a shortwave trough in the mid-latitudes. For both Sinlaku and Ike,
targets selected objectively by the Ensemble Transform Kalman Filter (ETKF) were often but
not always consistent with the most sensitive regions found in this study.
The methodology employed by this study may be used retrospectively by the research
and operational communities to offer suggestions on the weather features in which the initial
analysis required improvement, via a combination of improved modelling, observational
coverage or data assimilation. The diagnostic technique may also be used to improve
understanding of objective sensitivity methods, such as Singular Vectors and the ETKF, and
their limitations.
Poster Session – Page 23