First, a series of three-dimensional and a four-dimensional ensemble-variational hybrid (3DEnVAR and 4DEnVar) data assimilation experiments are performed to assimilate data from High-Definition Sounding System (HDSS) Dropsondes, collected for Hurricane Joaquin (2005). It is found that the experiments with the assimilation of the HDSS dropsonde observations capture well the intensity changes during the rapid weakening (RW) of Hurricane Joaquin. Compared with 3DEnVAR, 4DEnVar leads to better assimilation results and subsequent forecasts and thus offers a set of simulations to diagnose the processes associated with the RW of Hurricane Joaquin.
Further diagnoses show that a drastic increase in the vertical wind shear (VWS, with a magnitude of 12 m s-1) is found before the RW. This high VWS is persistent during the 0-12 h period of RW, inducing changes in the vortex structure of Hurricane Joaquin through dry air intrusion in the mid-level and the mixing of the upper-level warm core. The conveyance of low air from above into the boundary layer occurs at the same time, resulting in depressed values in the inflow layer and reduced eyewall values through the updraft. As a consequence, downdrafts replace the high air with low air in the boundary layer, leading to the weakening of inflow in the boundary layers. When Hurricane Joaquin moves over an area where the SSTs are below 28oC within the hurricane inner core during the 18-30 h period of RW, the cold SSTs significantly inhibit latent and sensible heat release within the hurricane inner core and its vicinity, thus resulting in the continuous weakening of Hurricane Joaquin.
Additional experiments are also conducted to assimilate dropsondes, atmospheric motion vectors (AMVs), and tail Doppler radar (TDR) data for several other hurricane case studies during the TCI experiment. Details will be presented at the conference.