The China Meteorological Administration (CMA) and the National Centers for Environmental Prediction (NCEP) reanalysis datasets are employed to examine the large-scale characteristics of rapidly intensifying western North Pacific tropical cyclones (TCs). The results show that of all 27 581 samples for the period 1970–2007, 85%, 65%, and 47% of all tropical depressions (TDs), tropical storms (TSs), and typhoons (TYs), respectively, intensify. Of the 1214 TCs, 18%, 70%, 30%, and 10% of all tropical cyclones, supertyphoons, severe typhoons, and typhoons, respectively, underwent rapid intensification (RI) at least once during their lifetime. Three kinds of cases—RI, slow change in intensity (SC), and rapid decay (RD)—during the period 1982–2007 are used to analyze the large-scale conditions associated with them. The comparison shows that the RI cases tend to occur farther south and east than the non-RI cases. In addition, the RI cases have a more westerly component of motion and intensify more during the preceding 12 h than do the non-RI cases. For the non-RI cases, the SC cases tend to have a lower initial intensity and a lower speed of motion than do the RD cases. Also, the RI cases are farther from their maximum potential intensity and develop in warmer water, lower vertical shear, and more easterly upper-tropospheric flow than do the non-RI cases. The probability of RI for TCs is estimated by using the rapid intensify index (RII) developed in this study for the western North Pacific basin. The verification based upon the cross validation shows that the RII is skillful relative to climatology.

How the Saharan air layer (SAL) affects tropical cyclone intensity in the North Atlantic Ocean is an issue in debate. A composite study of 274 cases from 37 named tropical cyclones that formed during the period 2005–2007 is conducted using AIRS relative humidity between 600–700 hPa. Typically the dry SAL air is first observed within 1000 km north of the tropical cyclone center and then intrudes southward and towards the inner region of tropical cyclones along the cyclonic flow. This study provides evidence that the SAL can affect tropical cyclone intensity in both favorably and unfavorably manners by intensifying tropical cyclones when it is first found mostly in the northwest quadrant and then weakening tropical cyclones when its dry air intrudes within 360 km of the tropical cyclone center, mostly in the southwest and southeast quadrants. It appears that the SAL is favorable for the initial development of tropical cyclones but unfavorable for their subsequent intensification.

How the rainfall characteristics of landfalling tropical cyclones (TCs) over China change with the dry-air intrusion is explored through analyzing Tropical Rainfall Measuring Mission (TRMM) and environmental fields. It is found that the rainfall area of landfalling TC is positively correlated to the midlevel environmental relative humidity: the larger the surrounding relative humidity before TC landfall, especially in the southern quadrants of the TC, the larger the rainfall coverage at landfall. Even when situated in a dry environment, the TC may produce severe concentrated rainfall with stronger intensity than in a moist environment. Results show that interaction between synoptic environment and TC is essential for influencing rainfall distribution for landfalling TCs over China. As a TC moves with northward component under two subtropical highs and westerly trough, it is under the influence of significant dry-air intrusion, which results in limited rainfall area. The increasing northwesterly vertical wind shear that is nearly opposite to the TC movement, on the one hand enhances the upshear-side subsidence, which offsets the friction-induced ascent ahead of TC. On the other hand, it strengthens the downshear-side updraft with the corporation of increasing synoptic convergence and results in severe asymmetric rainfall there. When a TC moves under the influences of enhanced subtropical high and monsoonal southwesterlies, it is under the moist environment that causes a larger rainfall area. Influenced by the weaker vertical wind shear with a similar direction as the TC movement, the rainfall distributes relatively symmetrically with heavy rain over the downshear-friction convergence overlapping region at landfall.When a TC makes landfall, its surrounding condition changes dramatically. First of all, the underlying surface changes from ocean to land. Previous observational studies have found that TC rainfall generally has a maximum ahead of the TC center, due to surface friction-induced lower level convergence induced by different magnitudes of radial momentum advection in different quadrants of the TC. Shapiro (1983) used a simple slab boundary layer model to analyze the effect of storm movement on boundary frictional convergence against the observations of radar and wind for Hurricanes Frederic (1979) and Allen (1980) and found that for slow-moving storms, the convergence was located in the front right quadrant, while for fasting-moving storms, it was concentrated more directly ahead of TC. A long-term statistical study also showed that the rainfall maximum tends to locate in the front-to-front-right quadrant of the track, especially in the western North Pacific (WNP;Chen et al., 2006).As the most active basin for TC genesis, the WNP has a favorable environment with warm sea surface temperature and high relative humidity (RH; e.g., Gray, 1968). In addition, a climatological weather systemthe western North Pacific subtropical high (WNPSH)-affects East Asia during the highly active TC season over the WNP. Observ...

Abstract. The influence of environmental conditions on the intensity changes of tropical cyclones (TCs) over the western North Pacific (WNP) is investigated through examination of 37 TCs during 2000-2011 that interacted directly with the western North Pacific subtropical high (WNPSH). Comprehensive composite analysis of the environmental conditions is performed for two stages of storms: one is categorized as intensifying events (maximum wind speed increases by 15 kn over 48 h) and the other is categorized as weakening events (maximum wind speed decreases by 15 kn over 48 h). Comparison of the composite analysis of these two cases show that environmental conditions associated with the WNPSH play important roles in the intensity changes of TCs over the WNP. When a TC moves along the southern periphery of the WNPSH, the relatively weaker easterly environmental vertical wind shear helps bring warm moist air from the south and southeast to its southeast quadrant within 500 km, which is favorable for the TC to intensify. However, when a TC moves along the western edge of the WNPSH, under the combined influences of the WNPSH and an upper-level westerly trough, a strong westerly vertical shear promotes the intrusion of dry environmental air associated with the WNPSH from the north and northwest, which may lead to the inhibition of moisture supply and convection over the western half of the TC and thus its weakening. These composite results are consistent with those with additional geographic restrictions, suggesting that the dry air intrusion and the vertical wind shear (VWS) associated with the WNPSH, indeed affect the intensity changes of TCs over the WNP beyond the difference related solely to variations in geographical locations. The average sea surface temperature (SST) of 27.6 • C for the weakening events is also lower than an average of 28.9 • C for the strengthening events, but remains above the critical value of 27 • C for TC intensification, suggesting that the SST may be regarded as a less positive factor for the weakening events.

Abstract. The influence of environmental conditions on the intensity changes of tropical cyclones (TCs) over the western North Pacific (WNP) is investigated through examination of 37 TCs during 2000–2011 that interacted directly with the western North Pacific subtropical high (WNPSH). Comprehensive composite analysis of the environmental conditions is performed for two stages of storms: one is categorized as intensifying events (maximum wind speed increases by 15 kts over 48 h) and the other is categorized as weakening events (maximum wind speed decreases by 15 kts over 48 h). Comparison of the composite analysis of these two cases show that environmental conditions associated with the WNPSH play important roles in the intensity changes of TCs over the WNP. When a TC moves along the southern edge of the WNPSH, the relatively weaker easterly environmental vertical wind shear helps bring warm moist air from the south and southeast, which is favorable for the TC to intensify. On the other hand, when a TC moves along the western edge of the WNPSH, under the combined influences of the WNPSH and an upper-level westerly trough, a strong westerly vertical shear promotes the intrusion of dry environmental air associated with the WNPSH from the north and northwest, which may lead to the inhibition of moisture supply and convection over the west half of the TC and thus its weakening. The average sea surface temperature (SST) of 27.8 °C for the weakening events is also lower than an average of 28.9 °C for the strengthening events, but remains above the critical value of 27 °C for TC intensification, suggesting that the SST may be regarded as a less positive factor for the weakening events.