Characteristics of tropical cyclogenesis predictability: Perspectives from T-PARC/TCS08

It has been suggested in a number of previous studies that tropical cyclogenesis may be sensitive to aspects of large-scale forcing as well as mesoscale dynamics that organize areas of deep tropical convection. In this study, we explore the hypothesis that tropical cyclogenesis is sensitive to small perturbations to the basic properties of the background state through organized mesoscale convection and synoptic-scale forcing. The recently developed adjoint and tangent linear models for the atmospheric portion of the nonhydrostatic Coupled Atmosphere/Ocean Mesoscale Prediction System (COAMPS) are used to explore the mesoscale sensitivity of tropical cyclogenesis to the initial state. A unique aspect of this system is that an exact adjoint to the explicit microphysics has been developed. The forward, adjoint and tangent linear models are applied at horizontal resolutions ranging from 20-40 km and are used to explore predictability issues for several tropical cyclones and non-developing storms in the Western Pacific including during the recent THORPEX Pacific Asian Regional Campaign (T-PARC) and the ONR Tropical Cyclone Structure-08 (TCS08) experiments, which took place in fall 2008.

During T-PARC/TCS08, COAMPS guidance was produced twice daily using the adjoint and tangent linear models over the Northwestern Pacific. An adaptive response function region was used to target favorable areas for tropical cyclogenesis and development. Real-time COAMPS-adjoint forecasts with lead times of 36 h, 48 h, and 72h were executed twice daily during T-PARC/TCS08 using a horizontal resolution of 40 km. The characteristics of the COAMPS sensitivity patterns such as preferred altitudes, comparison of wind field to temperature and moisture field sensitivities, and sensitivity to the sea surface temperature will be summarized. Preliminary data-denial experiment results will be presented and the implications for observing system design on the mesoscale will be discussed.

Results indicate that 24-h forecasts of tropical cyclone formation in the Western Pacific are very sensitive to the initial state. The adjoint-based sensitivity fields indicate highly structured patterns in the wind, thermal, and microphysical fields that project on to the model simulated deep convection, which ultimately influences the intensification rate. Relatively small basic state perturbations based on the adjoint calculations on the order of observational errors (1 m/s, 1 K) lead to rapid growth rates in the near-surface horizontal velocity of more than 10 m/s and 6 hPa deepening rate of the central pressure over 24 h. Implications of the adjoint-based sensitivity fields for the predictability of tropical cyclone formation will be addressed.