Turbulent flux measurements by the eddy correlation method were performed on board a ship in the tropical western Pacific during the summer MONEX field phase. The turbulent fluxes of momentum, sensible heat, and latent heat were estimated for three different stages, -the disturbed period influenced by the ITCZ, the equatorial doldrums, and the undisturbed period. The average Bowen ratio was 0.068 for the undisturbed period and 0.136 for the disturbed period. The Bowen ratio also varied significantly during the passage of a local convective disturbance. The reduced bulk transfer coefficients were 1.04×10-3 for momentum, 1.50×10-3 for sensible heat, and 0.91×10-3 for latent heat. These bulk transfer coefficients were mostly consistent with other results obtained over the tropical oceans. The drag coefficient was almost constant in the moderate wind speed range (1.5<U<8m/s). In unstable conditions, σw/u*, σu/u*, σT/T*, and σq/q* were well expressed by the universal functions of z/L predicted by the Monin-Obukhov similarity theory. The proportional constants, Cu, Cw, CT, and Cq were 2.37, 1.78, 1.82, and 1.36, respectively, and neutral values were σu/u*=2.53 and aw /u* =1.68, respectively. The u spectrum displayed the -2/3 fall-off for f>0.2, and agreed well with Kaimal's unstable limit for f<0.2. The w spectrum had the expected -2/3 fall-off, and also showed a marked bimodal shape. However, the whole shape agreed approximately with Kaimal's unstable limit. The temperature spectrum had a plateau between 0.2<ƒ<2.0, and agreed well with Kaimal's unstable limit for ƒ<0.2. The humidity spectrum agrees approximately with Kaimal's temperature spectrum, and was similar to the horizontal velocity spectrum. A flatter shape was apparent in the uw cospectrum, and the wT cospectrum exhibited a plateau for 0.2<ƒ<1. The wq cospectrum approximately followed Kaimal's generalized form of the wT cospectrum for f<2.
In East Asia, a quasi-stationary frontal zone called the Baiu frontal zone (BFZ) forms during the early summer and provides nearly as much precipitation as the Intertropical Convergence Zone (ITCZ). Since the BFZ has several characteristics different from both the ITCZ and polar frontal zones, Ninomiya (1984) proposed the BFZ should be classified as a subtropical frontal zone. Using mainly ten-day mean data, we compare the BFZ around Japan with the subtropical portions of the South Pacific Convergence Zone (SPCZ) and the South Atlantic Convergence Zone (SACZ), which are significant precipitation zones in the summer Southern Hemisphere. These three precipitation zones are shown to have several common features as the following. The BFZ, the SPCZ, and the SACZ (hereafter referred to as the SPZs, the Subtropical Precipitation Zones) commonly form along the subtropical jet on the eastern side of a quasi-anchored trough, which lies to the northeast (in the Northern Hemisphere) or southeast (in the Southern Hemisphere) of the localized active convection of the tropical monsoon. The rainfall amount in the SPZs attains -400 mm/month when they are active. All of the SPZs are characterized by convergence zones with an interior thick moist layer and baroclinic zones with an upper subtropical jet. They are also characterized as poleward boundaries of the moist tropical or monsoon airmass associated with a low-level large gradient of moisture mixing ratio. Since the evaporation rate is much smaller than the precipitation rate along the SPZs, high rainfall in the SPZs is maintained by the convergence of two types of moisture currents in the SPZs. One is eastward along the SPZ and the other is along the northwestern (in the N.H.) or the southwestern (in the S.H.) periphery of the subtropical high. The latter transports moisture evaporated under the western part of the subtropical high. Generation of convective instability by the differential advection process is found along the SPZs and maintains active convection along the SPZs. Since the SPCZ and the SACZ have several unique characteristics different from the ITCZ and polar frontal zones but similar to the BFZ, it is concluded that all of the SPZs can be classified as subtropical frontal zones.