How vertical wind shear affects the rapid intensification of Typhoon Jangmi (2008)

Typhoon Jangmi was the strongest recorded storm in all of 2008 (reaching a sea-level pressure of 905 hPa). The asymmetric winds accompanying Jangmi were highly variable, which resulted in several episodes of stark changes in vertical wind shear. There were also numerous periods of rapid intensity change, which, in general, are notoriously hard to model with accuracy. Tropical cyclone (TC) asymmetries appear to affect storm intensity and are often ascribed to environmental vertical wind shear characteristics. This study uses the Weather Research and Forecasting Model (WRF) at high resolution to simulate the storm's response to the sudden changes in wind shear. This is in an effort to improve the modeling of TC intensity change through a better understanding of TC processes. Jangmi's response to changes in shear is examined in the context of two main theories on how TC intensity often reacts to shear: 1) increased levels of shear cause a balanced response in the mid-levels which leads to a cooling down-shear and a warming up-shear, which should decrease TC intensity due to increased core stability and 2) high levels of shear reduce TC intensity by venting critical equivalent potential temperature (theta-e) from the upper-level core. First, the height at which core theta-e values are most important to Jangmi's intensity is determined. Next, to ascertain the effect of shear on these theta-e levels, the asymmetries associated with the differences in shear are analyzed. To see how these asymmetries affect the distribution of theta-e, the radial and vertical winds are evaluated over time. Next, the changes in theta-e values accompanying shear magnitude changes are gauged as to their distribution relative to the TC core. It is found that Jangmi's minimum sea-level pressure is very closely related to upper-level, near core values of theta-e and that the venting hypothesis describes much of shear's effect on Jangmi's intensity changes.