Synoptic characteristics of precipitation over the HEIFE area in Northwestern China are investigated. The results show that rainfall in the HEIFE area is caused by disturbance on the polar frontal troughs, and the rain area covers the whole HEIFE area of 200 x 100km. However, the amount of rainfall depends on the altitude, e.g. 600mm/year in the Qilian Mountains at 3000mASL and only 100mm/year or less at the bottom flat of the valley called "Hexi Corridor" at around 1300mASL.During summer, the water vapor content is rather large and stratification of the air over the desert shows potential instability. However, because of the high lifting condensation level (LCL) and the free convection level (LFC) over the desert, isolated cumulus convections do not develop, in spite of the large buoyancy induced by high ground temperatures. As a result, rainfall over the HEIFE area occurs when the disturbances on the polar frontal trough move into the area. The total amount of rainfall observed in the lower land of the HEIFE area, however, seems to be significantly reduced due to the evaporation of raindrops within the deep and dry boundary layer. Meanwhile, in winter, the water vapor content is quite small and stability is nearly neutral, which accounts for the low precipitation in this area.

The e¤ect of the horizontal component f H of the planetary vorticity on the symmetric stability of zonal flow is investigated using the linearized Boussinesq equations on the f -plane. It is shown that, as in the case of neglecting f H , the stability under full-component Coriolis force is determined by the sign of the potential vorticity. It is also revealed that even in such a generalized situation, the movement associated with the symmetric instability can be decomposed into two independent motions, i.e., the buoyancy oscillation (or instability) modified by the Coriolis force and the inertial oscillation (or instability) modified by the buoyancy. The squared product of their frequencies remains proportional to the potential vorticity of the zonal flow. Meanwhile, the horizontal component of the planetary vorticity is found to exhibit both stabilizing and destabilizing e¤ects, although there is a wide range of stable regions that are not a¤ected by f H . The existence of f H also causes an asymmetry such that the stability changes depending on the sign of the vertical shear of the zonal flow, even if the Richardson number and the dimensionless relative vorticity are maintained constant.

An elliptical eye was observed in Typhoon Herb during its passage around the Sakishima Islands located in the southwest of the Okinawa Islands in Japan on 30–31 July 1996. Later analysis showed that this elliptical eye had a structure with wavenumber 2 in the radar echo distribution, rainfall field, and pressure field. It is argued here that this structure is likely caused by a dynamic instability arising from radial shear in the tangential flow. To examine this possibility, a linear eigenvalue problem was set up using the asymmetric balance model in which the basic flow was based on the observed surface wind in Typhoon Herb, and the stability of the basic flow for both two- and three-dimensional perturbations was examined. Although for large Rossby numbers, the asymmetric balance model is not formally valid for higher azimuthal wavenumber perturbations, results certainly show that an unstable mode appears with the perturbation with wavenumber 2. In particular, the largest growth rate of the perturbation with wavenumber 2 is shown to occur for the three-dimensional perturbation. The eigenmode structure and rotation period of this three-dimensional perturbation with wavenumber 2 are comparable with the observations. These results suggest that barotropic instability due to the radial shear of tangential flow is relevant to the formation of the elliptical eye of Typhoon Herb.

The geometry and behavior of the polygonal eye of Typhoon Sinlaku (T0216) are presented both objectively and quantitatively based on the ground-based radar data by using two types of spectral analyses (i.e., Fourier analysis and time-space cross-spectral analysis). The analyses reveal that the azimuthal wavenumber-2 perturbation is dominant even when the apparent shape of the eye seems pentagonal. Then the wavenumber-5 perturbation dominates second in most of the analytical period although it becomes most dominant during the typhoon's passage over Okinawa Island, Japan. The counterclockwise phase shift is seen not only in these perturbations but also in, at least, the wavenumber-4, -5, and -6 perturbations. Except for the wavenumber-2 perturbation, the angular velocities are around 1308 h 21 and almost coincide with each other. The radial wind profile estimated from the surface observational data suggests the existence of a ring-shaped high-vorticity region enclosing the core of the typhoon. This implies that the formation of the pentagonal eye can be attributed to wave breaking caused by barotropic instability of the circular flow constituting this typhoon. Such a possibility is also discussed by performing the linear stability analysis based on the nondivergent barotropic vorticity equation.