2009 Volume 87 Issue 4 Pages 615-633
Azimuthal wavenumber-one asymmetric structures in the inner-core region of typhoon are examined primarily using the operational mesoscale analyses from the Japan Meteorological Agency (JMA) for the four typhoon seasons from 2004 to 2007. The results show that both the cold and high specific humidity anomalies at low-tomiddle-tropospheric levels tend to be located in the downshear-left quadrant (looking down at the direction of environmental vertical wind shear), consistent with some theoretical predictions. In addition, the vertical tilt of typhoon vortex is directed predominantly downshear to downshear-left with a magnitude being roughly proportional to shear strength, in agreement with previous numerical case studies.
In the present study, the wavenumber-one structures of near-surface winds are also investigated using both theoretical and statistical approaches. In the theoretical approach, a set of analytical formulae that relate the asymmetries of radial and tangential winds to convective asymmetry in the eyewall region are derived by applying a scaling argument to the result from a numerical simulation of Typhoon Chaba (2004). The formulae predict that the maximum in storm-relative tangential wind occurs 90° azimuthally downwind of the enhanced updraft region. The statistical approach using the mesoscale analysis data reveals that the azimuthal location of tangential wind maximum relative to storm direction depends strongly on the directional difference between shear and storm motion, under relatively strong shear conditions, and the wind maximum, contrary to the majority of cases, tends to occur to the left of motion in cases where the directional difference is very small. Considering the strong dependence of convective asymmetry in the inner-core region on the shear, the results are in line with expectations from the analytic theory.