Analysis of tropical storm formation and observational impact using ensemble data assimilation and high-resolution numerical simulations

Presentation Type: 
Andrew Penny (Naval Postgraduate School)
Joshua P. Hacker (NCAR/RAL, Boulder, CO, USA)
Patrick A. Harr (Naval Postgraduate School, Monterey, CA, USA)

During the THORPEX Pacific-Asian Regional Campaign (TPARC) / Tropical Cyclone Structure-2008 (TCS-08) field experiment in the western North Pacific, a non-developing tropical disturbance (TCS025) was closely monitored during five aircraft missions. An analysis of in situ observations, which included data from GPS dropwindsondes, flight-level instrumentation, and the Electra Doppler Radar (ELDORA) onboard the Naval Research Laboratory (NRL) P-3, revealed that the circulation was asymmetric and misaligned in the vertical. The circulation structure of TCS025, in combination with vertical wind shear and mid-level low-equivalent potential temperature air, limited convection and kept the TCS025 disturbance from developing further. In the majority of operational forecasts and high-resolution research simulations initialized from operational analyses, TCS025 developed into a tropical cyclone as the simulated vortex was aligned in the vertical.

To further assess the role of vortex misalignment in the vertical for TCS025, in situ observations were assimilated using the ensemble Data Assimilation Research Testbed (DART) system in conjunction with the Weather Research and Forecasting (WRF) numerical model. After assimilation of the first set of ELDORA observations, the low-level circulation of the ensemble mean was relatively weak and displaced a considerable distance northward of the mid-level circulation, which was consistent with observations.

Following the assimilation period of the ELDORA observations, the 96-member ensemble was integrated forward without further data assimilation (DA), and a wide range of development scenarios resulted. In this study, ensemble simulations that forecast development of the TCS025 disturbance are compared and contrasted with those that more correctly forecast non-development. Additionally, ensemble DA experiments and forecasts were also conducted without ELDORA observations to examine the impact of these data on the circulation structure and simulated evolution of TCS025. The differences in initial conditions and the convective characteristics that followed are examined for the DA experiments with and without ELDORA observations.

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