An analysis of tropical storm formation using ensemble data assimilation and high-resolution numerical simulations

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 vertical misalignment of the circulation in the presence of vertical wind shear and mid-level low-θ_e air limited convection and kept the TCS025 disturbance from developing further. In operational and research simulations with high-resolution numerical models, TCS025 developed into a tropical cyclone as the simulated vortex was aligned in the vertical. In these simulations, initial conditions were constructed using operational global analyses.

To examine the impact of vortex misalignment in the vertical, 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 the assimilation of the first set of ELDORA observations, the low-level circulation of the ensemble mean was displaced considerably northward of the mid-level circulation, which was consistent with observations. A high-resolution numerical simulation initialized from the ensemble mean still over-developed the TCS025 disturbance, but development was delayed 24 h relative to simulations without data assimilation.

After assimilation of the first set of ELDORA observations, the full 96-member ensemble was also integrated in time without further data assimilation, 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. The differences in initial conditions and the convective characteristics that followed are examined.