The Evolution Of Mesoscale Vortex And Moisture In The Formation Of A Tropical Cyclone

The formation of tropical cyclones is the result of the interaction between multiscale weather systems.Due to the lack of observation and the involved interactions between systems of multiple scales,tropical cyclogenesis remains an mysterious phenomenon in the research of tropical meteorology.In this work,a high-resolution model was carried out to simulate the formation of tropical cyclone “Megi”(2010)to gain insight into the key mesoscale process occurring in this process,including the formation and intensification of the mesoscale vortex,the water vapor evolution of the disturbance and their impacts on the formation of “Megi”.The high-resolution results from the model showed that a midlevel mesoscale vortex accompanying the cyclonic vorticity preceded the genesis of “Megi”.The intensity of the mesoscale vortex then fluctuated with the subsequent appearance of deep convection,stratiform and shallow convection.Both the midlevel convergence associated with the stratiform,and the upward vorticity transport related to the deep intense convection played a role in the formation of the mesoscale vortex.Though the former played a leading role,the latter could not be neglected.The weakening of the mesoscale vortex was mainly associated with the divergence related to the shallow convection.It is found that the mesoscale vortex played a dual role in the formation of Megi.On the one hand,the formation of the mesoscale vortex was accompanied with stratifrom precipitation,which induced the surface divergence and weakened the surface cyclonic circulation.On the other hand,the coupled cold core below the mesoscale vortex destabilized the atmosphere,facilitating the development of the convection and thus the formation of Megi.The sensitivity experiment further showed that the development of the mesoscale vortex retarded the genesis of Megi.Similar to the development of the mesoscale vortex,the analysis indicated that the column water vapor of pre-Megi's disturbance increased with fluctuation.When shallow convection dominated,the water vapor content of the disturbance increased significantly.Further analysis showed that both the surface evaporation from the ocean and the horizontal water vapor convergence from the atmosphere contributed to the increase of the water vapor,and the contribution from the latter largely exceeded the former.When the pre-Megi's disturbance was occupied with deep intense convection,the system desiccated a little because the consumption associated with the deep intense convection was a little larger than moisture supplied by the ocean and the atmosphere.However,the decrease of water vapor was smaller than the increase in the shallow convection phase.Throughout the whole genesis process,the water vapor from the ocean could not be neglected,though its magnitude was relatively much smaller than that from the surrounding atmosphere.In addition,the WISHE mechanism was also necessary to the genesis of Megi.The idealized experiments further indicated that with the same amount of column water vapor,the initial disturbance with higher mid-level humidity developed much faster than that with higher low-level humidity.With higher mid-level humidity,deep intense convection burst much earlier,which further moistened the middle atmosphere that inhibited the downward motion and the associated detrimental impacts on the boundary layer.This facilitated the aggregation of the deep intense convection near the center of the disturbance and made the disturbance intensify rapidly.