A suite of numerical experiments were conducted to investigate the sensitivity of the tropical cyclone (TC) motion—western Pacific subtropical high (WPSH) intensity relationship to initial storm size. Two TC cases, Songda (2004) and Megi (2010), were studied. It was found that with the increase of initial storm size, the main body of the WPSH tends to withdraw eastward and the TC tends to turn northward earlier. The involved physical mechanism was investigated. Rather than the change of the beta effect due to storm size change, it is the change of the geopotential height in the TC outer region that is critical for the different TC tracks between the sensitivity experiments. Due to increase of the initial storm size, the inflow mass flux entering the TC region increases, leading to a significant decrease in 500 hPa geopotential height in the TC outer region after 2–3 day integration. As a result, the simulated intensity of the WPSH over its fringe close to the TC decreases notably when the WPSH edge is within the TC outer region. Such a decrease leads to a break of WPSH. Subsequently, the TC turns northward toward the break of the subtropical high. This further weakens the intensity of the WPSH over the region close to the TC. The result helps us better understand the relationship between the TC track and WPSH intensity. It also indicates that a proper representation of initial storm size is important for realistic prediction of TC track and the change of the WPSH.

The sensitivity of the North Pacific storm track to midlatitude oceanic frontal strength within the Kuroshio/Oyashio Extensions is investigated by applying artificially changed meridional sea surface temperature (SST) gradients in the Weather Research Forecasting model version 3.4. The result of sensitivity experiments further confirms the close relationship between the storm track activity and meridional SST gradient; i.e., the storm track activity can be intensified as a response to increases in the oceanic frontal strength. In order to better understand the mechanism for the storm track intensification due to increased SST gradient, velocity‐temperature correlation and local energetics are analyzed. The result indicates that the enhancement of the meridional SST gradient leads to amplitude magnification of eddy meridional velocity and temperature and their phase consistency, suggesting that synoptic‐scale eddies tend to approach the optimum structure for the baroclinic energy conversion, which is mainly responsible for the SST front‐induced enhancement of storm track activity. In order to estimate the impact of the oceanic front on the maintenance of the near‐surface baroclinicity, further investigation is made by the composite analysis. With the increase in oceanic frontal strength, the near‐surface baroclinicity experiences a slow but strong restoration. The increase in the meridional SST gradient results in the intensification in the cross‐frontal differential sensible heat flux, which can more effectively offset the relaxing effect of the transient eddy poleward heat transport.

A suite of semiidealized numerical experiments are conducted to investigate the sensitivity of tropical cyclone (TC) intensity to changes of sea surface temperature (SST) over different radial extents. It is found that the increase of inner SST within the range 1.5–2.0 times the radius of maximum wind (RMW), defined as the effective radius (ER), contributes greatly to the increase of TC intensity and the reduction of TC inner‐core size, whereas the increase of outer SST (defined as SST outside the ER) reduces TC intensity and increases TC inner‐core size. Further analysis suggests that the effects of SST inside and outside the ER on TC intensity rely on the factors that influence the TC development. As the SST increases inside the ER, more surface enthalpy flux enters the TC eyewall and less enters the outer spiral rainbands. This will decrease the RMW, leading to a smaller eyewall radius where strong latent heating is released. As a result, the central pressure of the TC deepens with stronger radial pressure gradient. Meanwhile, the difference between SST and upper tropospheric temperature increases. All factors above contribute to TC intensification as the inner SST increases. The opposite happens as the SST increases outside the ER. How TC intensity responds to the change of the entire SST depends on the competitive and opposite effects of inner and outer SST. Moreover, understanding the mechanisms is vital to the forecast of variations in TC intensity and inner‐core size when a TC comes across an ocean cold or warm pool.

This study focuses on the decadal variability of tropical cyclone (TC) activity over the South China Sea (SCS) since the 1970s and its possible cause behind. It is found that TC activity over the SCS experiences a significant decadal change around the mid‐1990s. Compared to the period from the 1970s to the early 1990s, the number of TCs formed in the SCS remarkably increases from the mid‐1990s through the 2000s. In particular, this change of TC genesis is closely related to a decadal shift in atmospheric intraseasonal variability (ISV) that occurred in 1994. The ISV on the 30–60 days time scale over the SCS has been increasing since the mid‐1990s, and the increased TC frequency after 1994 is attributed primarily to the active convection induced by the enhancement of the SCS ISV. In addition, the TC activities before the mid‐1990s are mostly confined within the SCS basin. However, more TCs form over the SCS and move northeastward since the mid‐1990s and finally enter the East China Sea and the Philippine Sea. Anomalies of westerly over the northern SCS after 1994 are responsible for the northeastward moving of TCs.

The influence of urban clusters in the lower reaches of Yangtze River Valley (YRV) on a summertime heavy precipitation event is evaluated based on ensemble simulations using the Weather Research and Forecasting (WRF) model. Results show that control runs with realistic representation of urban areas perform better than the sensitivity runs with urban areas replaced by cropland, suggesting that the urban canopy physics in the surface layer, and hence realistic urban land‐use classification, is crucial for urban‐induced precipitation modelling. Precipitation starts later but is stronger due to the urban effects. Urbanization leads to two adjacent belts of increasing and decreasing precipitation to the north of the Yangtze River. These urban‐induced variations in precipitation are attributed to the impact of urban clusters in the lower reaches of the YRV, which act like a single large (non‐continuous) city to slow down the precipitation process due to its building barrier effect. Such an impact leads to more precipitation in the north of the precipitation field but provides less moisture for precipitation in the south. The variations of turbulent kinetic energy and vorticity that resulted from the effects of urban clusters show upward propagation during the precipitation event, suggesting that the urbanization impact may be transferred from surface to upper atmosphere, which could exert important influences on precipitation processes.