Title: Dynamical sensitivity analysis of tropical cyclogenesis: a barotropic mode in the eastern Pacific

Authors: Brett Hoover (University of Wisconsin)
Michael Morgan (University of Wisconsin)

Tropical cyclones (TCs) in the eastern Pacific often form on the cyclonic shear-side of a persistent zonal jet extending from the surface to the lower troposphere; the existence of this jet implies the possibility for barotropic energy conversion as a mechanism for the formation of the ambient low-level vorticity necessary for tropical cyclogenesis. Such a scenario provides for the possibility a 'barotropic mode' in eastern Pacific tropical cyclogenesis that is distinct from a more traditional cyclogenesis process wherein convection and diabatic heating play a more central role. One may be prompted to ask - is there any evidence of a barotropic mode in eastern Pacific tropical cyclogenesis?

An adjoint model is employed to compute sensitivity of tropical cyclone intensity to perturbations of the model initial conditions. Of 24 cases of tropical cyclogenesis in the eastern Pacific between 2008-2009, at least half display structures in sensitivity to lower tropospheric winds which may indicate the potential for barotropic growth, while a third display no such structures. Storm-centered composites of sensitivity gradients and basic states for 'barotropic' and 'non-barotropic' cases are physically interpreted. 'Barotropic' cases tend to appear when the low-level jet is "thinnest" in the meridional direction and contains the most shear. Optimal perturbations to winds and temperatures for increasing TC intensity are also computed and binned according to the possibility for barotropic growth. Finally, National Hurricane Center forecast error statistics are grouped into 'barotropic' and 'non-barotropic' bins to show that cases with the possibility for barotropic g rowth tend to be more accurately forecast at 48 hours, and a hypothesis is provided for the observed differences in predictability of 'barotropic' and 'non-barotropic' cases.

GMAO Head: Michele Rienecker Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: May 27 2011