A number of observational and modeling studies have shown a tendency for typhoon strength vortices to develop upward motion and produce precipitation, particularly in the eyewall, on the downshear to downshear-left side of the tropical cyclones (TCs). However, the directional relationships obtained from the observational studies have been mostly confined to the TC cases in the Atlantic basin. Furthermore, little evidence has been presented so far for the relationship in magnitude, between shear and rainfall asymmetry.
In the former part of the present study, the observational analysis on TC rainfall asymmetries is extended to the western North Pacific TCs in 2004, using the two types of rain-rate data, the Radar-AMeDAS precipitation data, and satellite-based rainfall estimates, such as TMI and AMSR-E rain rates. It is well demonstrated from the analysis that rainfall in the inner-core region of a TC tends to occur on the downshear to downshear-left side, irrespective of data type used and latitudes where TCs are located. However, as far as the relationship between shear and storm motion is concerned, a sharp contrast is found between low and middle latitudes. In middle latitudes TCs have a tendency to move to the left of the shear, consistent with previous studies, while in low latitudes they tend to move to the right of the shear. The contrasting shear-relative storm heading between the two latitudes is attributed to the difference in vertical structure of the ambient wind.
In the latter part of the study, to explore the quantitative relationship between shear and rainfall asymmetry, a formula for the shear-induced vertical motion is derived from the thermal wind balance equation for TC-like vortices. The formula states that the shear-induced vertical motion should be a function not only ofshear magnitude, but also of vortex strength. To validate the formula a set of idealized numerical experiments are conducted, with realistic wind profiles, in which the initial environmental winds are specified from the 6-hourly JMA global analyses for two major typhoon cases in 2004. It is found from the numerical study that the magnitude of wavenumber-one vertical motion, predicted by the formula, is much more strongly correlated with that of model-produced rainfall asymmetry, than the shear alone, suggesting that the vortex strength is one of the main factors determining the magnitude of shear-induced rainfall asymmetry. The results from the idealized simulations also suggest that vortex tilt would have only a minor contribution to the rainfall asymmetry in the inner-core region, at least for well-developed TCs.