An examination of a non-developing tropical disturbance in the western North Pacific using high-resolution numerical simulations and data assimilation

One of the major issues in tropical meteorology is the uncertainty as to whether a tropical cloud cluster will eventually develop into a tropical cyclone. Based on the well-known environmental conditions related to ocean condition, atmospheric dynamics, and atmospheric thermodynamics, it is clear that most tropical cloud clusters will not develop into a tropical cyclone. Therefore, it is important to understand the primary processes that determine whether a cloud cluster under mostly favorable conditions may not develop.

During the THORPEX Pacific-Asian Regional Campaign (TPARC) and Tropical Cyclone Structure-2008 programs (TCS-08), a tropical disturbance (TCS025) was forecast to develop and was closely observed during five aircraft reconnaissance missions. However, similar to the outcome for the majority of such systems, TCS025 failed to intensify. This provided for an unprecedented dataset of a non-developing system, which included airborne Electra Doppler Radar (ELDORA) dual Doppler radar.

An in-depth examination of observations revealed that TCS025 failed to develop due to vertical wind shear and misalignment of the circulation structure in the vertical. A multi-physics ensemble using the Weather Research and Forecasting (WRF) model is employed to expand upon the observational findings. Simulations that unrealistically develop TCS025 exhibit exaggerated convective precipitation processes and improved circulation alignment. In addition, ensemble data assimilation experiments that incorporated aircraft and radar data are used to improve the model initial conditions such that they were more representative of the weak, vertically misaligned circulation that was evident in the observations. Comparisons among simulations with and without data assimilation indicate that the improved initial conditions delayed the onset of over-development of TCS025 by 24 h.