This study attempts to understand contributions of ENSO and the boreal summer sea surface temperature anomaly (SSTA) in the East Indian Ocean (EIO) to the interannual variability of tropical cyclone (TC) frequency over the western North Pacific (WNP) and the involved physical mechanisms. The results show that both ENSO and EIO SSTA have a large control on the WNP TC genesis frequency, but their effects are significantly different. ENSO remarkably affects the east-west shift of the mean genesis location and accordingly contributes to the intense TC activity. The EIO SSTA affects the TC genesis in the entire genesis region over the WNP and largely determines the numbers of both the total and weak TCs. ENSO modulates the large-scale atmospheric circulation and barotropic energy conversion over the WNP, contributing to changes in both the TC genesis location and the frequency of intense TCs. The EIO SSTA significantly affects both the western Pacific summer monsoon and the equatorial Kelvin wave activity over the western Pacific, two major large-scale dynamical controls of TC genesis over the WNP. In general the warm (cold) EIO SSTA suppresses (promotes) the TC genesis over the WNP. Therefore, a better understanding of the combined contributions of ENSO and EIO SSTA could help improve the seasonal prediction of the WNP TC activity.

The 6-yr best-track data of tropical cyclones (TCs) in the western North Pacific are used to study the statistical features of TC size and radial wind profile. A TC size is defined as the azimuthal mean radius of 34-kt surface wind.On average, the TCs in the western North Pacific have a size of 203 km, and the size is larger for stronger TCs. Further analyses show that larger TCs tend to move faster than smaller ones, with a 23-24 km difference in size corresponding to a difference of about 10 km h −1 in moving speed. The TCs that recurve from westward to eastward moving have a mean size of 218 km, significantly larger than that of those without a turning point (179 km). Regional TC distributions demonstrate that the TCs affecting the Korean Peninsula and southwestern Japan have the largest mean size (250-280 km). There are also some large TCs affecting southern Philippines, while TCs over the South China Sea are generally small in size. Comparison of intensity and size of all TCs during their lifespan demonstrates that a TC tends to reach its maximum size 6 h after it reaches its maximum intensity, and the decrease rate of size during the weakening stage of a TC is much smaller than the increase rate of size during its developing stage. Thus, linear regression relations between TC size and intensity are established for its developing and weakening stages respectively, which can be used as a forecast tool for TC size.Features of TC radial wind profile are studied by analyzing a parametric wind model based on the radius data of 34-, 50-, and 64-kt surface winds. The results show that the shape parameter d most frequently takes the values of 0.3, 0.4, and 0.5. It generally increases (decreases) as the TC develops (weakens), implying a sharper (flatter) radial wind profile. Changes in d leads the tendency of intensity. The two parameters for the asymmetric model, namely p and q, are mostly 0.85-1.05 and 0-0.2, respectively, embodying the fact that the asymmetric component is generally much smaller than the symmetric component. The asymmetry in 34-kt surface wind is much stronger than that in 50-and 64-kt surface winds, with the maximum radius often in the northeast quadrant.

This study analyzes the fast-response (20 Hz) wind data collected by a multilevel tower during the landfalls of Tropical Storm Lionrock (1006), Typhoon Fanapi (1011), and Typhoon Megi (1015) in 2010. Turbulent momentum fluxes are calculated using the standard eddy-correlation method. Vertical eddy diffusivity Km and mixing length are estimated using the directly measured momentum fluxes and mean-wind profiles. It is found that the momentum flux increases with wind speed at all four levels. The eddy diffusivity calculated using the direct-flux method is compared to that using a theoretical method in which the vertical eddy diffusivity is formulated as a linear function of the friction velocity and height. It is found that below ~60 m, Km can be approximately parameterized using this theoretical method, though this method overestimates Km for higher altitude, indicating that the surface-layer depth is close to 60 m in the tropical cyclones studied here. It is also found that Km at each level varies with wind direction during landfalls: Km estimated based on observations with landward fetch is significantly larger than that estimated using data with seaward fetch. This result suggests that different parameterizations of Km should be used in the boundary layer schemes of numerical models forecasting tropical cyclones over land versus over the ocean.

Tropical cyclones (TC) consist of a large range of interacting scales from hundreds of kilometers to a few meters. The energy transportation among these different scales-that is, from smaller to larger scales (upscale) or vice versa (downscale)-may have profound impacts on TC energy dynamics as a result of the associated changes in available energy sources and sinks. From multilayer tower measurements in the lowlevel (,120 m) boundary layer of several landing TCs, the authors found there are two distinct regions where the energy flux changes from upscale to downscale as a function of distance to the storm center. The boundary between these two regions is approximately 1.5 times the radius of maximum wind. Two-dimensional turbulence (upscale cascade) occurs more typically at regions close to the inner-core region of TCs, while 3D turbulence (downscale cascade) mostly occurs at the outer-core region in the surface layer.

The effects of wind direction on variations in friction velocity with wind speed are studied under moderate (≥9 m/s) to strong (≥22 m/s) onshore wind conditions using 20‐Hz ultrasonic wind data from a coastal tower at three different heights. The effects of different averaging time intervals of 20, 10, 2, and 1 min on the variations are also investigated. Three typhoons passed by the tower during the 150 hr of observations. Regardless of wind direction, friction velocity increases with increasing wind speed, and linear regression shows that the rate of increase is ~50% less than previously reported. However, a leveling‐off or decrease in friction velocity with increasing wind speed is found under strong wind conditions using a bin‐averaged method. Wind direction affects the variations in friction velocity with wind speed. Friction velocity increases with increasing wind speed at a similar growth rate to that of previously published results, with a leveling‐off or decrease at wind speeds higher than 22 m/s when the wind blows roughly normal to the shoreline. Form drag induced by sea waves with longer wavelengths is suggested to be responsible for the effects of wind direction on the variations in friction velocity with wind speed. An averaging time interval of 1 min yields representative variations in friction velocity with wind speed using the observations described in this study.