ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
An Idealized Numerical Study on the Evolution Mechanism of a Long-life Convective-Scale Updraft in Outer Rainband of Sheared Tropical Cyclone
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Abstract:
This study investigates the evolution mechanism of a long-life convective-scale updraft in outer rainband of numerically simulated sheared tropical cyclone (TC). The updraft originates on the downshear right quadrant of outer rainband within a sheared TC with a lifespan of 2.5-hour. This updraft undergoes two intensification processes and displays complex evolutionary characteristics with two peaks in vertical mass flux. Results show that strong localized vertical wind shear and low-level high-value equivalent potential temperature are the main favorable environmental factors maintaining an updraft long life cycle. The strengthening and weakening of neighboring convective cells lead to different responses to updraft intensity by adjusting the variation of local equivalent potential temperature. The vertical momentum budget suggests that an updraft is growing when it is dominated by positive buoyancy pressure gradient acceleration and positive thermal buoyancy, yet there are differences between the two strengthening mechanisms. In the first intensification stage, the development of neighboring convective cells cause an increase in the equivalent potential temperature at lower levels. Moreover, the increase in updraft tilt and the latent heating lead to a significant increase in thermal buoyancy, resulting in a larger vertical velocity. In the early second intensification stage, the occurrence and development of new convective cells in the vicinity of the focused updraft induce an increase in localized equivalent potential temperature. Subsequently, however, the mature and dissipation of these neighboring convective cells lead to downward motion stronger and localized equivalent potential temperature decrease, resulting in smaller thermal buoyancy and smaller vertical velocity. Analogous to the weakening mechanism of convective cells in mid-latitudes, during the weakening phase, the focused updraft exhibits a decrease in tilt, and then force a downdraft directly beneath it. This downdraft and downdrafts of neighboring convective cells carry low-value equivalent potential temperature toward the lower layers to form a surface cold pool. Consequently, thermal buoyancy tends to decrease with decreasing equivalent potential temperature at lower layers, which suppress the growth of the focused updraft. In addition, the negative contribution of water loading is harmful to the development of the focused updraft. The imbalance between thermal buoyancy, buoyancy pressure gradient acceleration, and water loading constitute the primary physical mechanism responsible for the prolonged evolution of the updraft. However, a tilted updraft structure can also influence its own development.
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