Environmental influences on the intensity changes of tropical cyclones over the western North Pacific

Abstract

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.

An Ensemble Approach to Investigate Tropical Cyclone Intensification in Sheared Environments. Part II: Ophelia (2011)

“…The greater tilting of the horizontal vorticity was a result of the correlation between upward motion and the horizontal vorticity due to the upper-tropospheric trough. From this perspective, the outcome of a TC-trough interaction depends not only on the size and strength of the PV associated with the trough, as stated by Hanley et al (2001), but also on the interaction between convection and the trough-induced shear as suggested by Shu et al (2014). This conclusion was also supported by sensitivity experiments in which AHW forecasts with perturbed initial conditions had greater MSLP changes in response to variability of the downshear convection than in response to variability of the upper-tropospheric trough.…”

“…Oftentimes a moderately sheared environment stems from an upper-tropospheric trough in close proximity to a TC (e.g., Hanley et al 2001; Davis and Bosart 2002;Molinari et al 2006;Shu et al 2014). In a composite study of TC-trough interactions in the North Atlantic, Hanley et al (2001) found that TCs are more likely to intensify than weaken after interacting with uppertropospheric troughs.…”

“…Hanley et al (2001) explained that TC intensification was favored in situations of smaller and weaker upper-tropospheric potential vorticity (PV) disturbances than in the cases of TC weakening due to the weaker shear and stronger divergence aloft. However, the composite study of western North Pacific TCs of Shu et al (2014) showed that uppertropospheric troughs can inhibit intensification by promoting dry-air intrusion and reducing convective activity. Although Shu et al (2014) restricted their study to TCs near the western North Pacific subtropical high, their results suggest that upper-tropospheric troughs could provide a mechanism for dry-air entrainment, which is a known detrimental effect of vertical wind shear (Riemer et al 2010;Munsell et al 2013;Tang and Emanuel 2012).…”

“…However, the composite study of western North Pacific TCs of Shu et al (2014) showed that uppertropospheric troughs can inhibit intensification by promoting dry-air intrusion and reducing convective activity. Although Shu et al (2014) restricted their study to TCs near the western North Pacific subtropical high, their results suggest that upper-tropospheric troughs could provide a mechanism for dry-air entrainment, which is a known detrimental effect of vertical wind shear (Riemer et al 2010;Munsell et al 2013;Tang and Emanuel 2012). These contradicting results warrant further investigation to discern the role of uppertropospheric troughs on TC intensity changes.…”

Does strong vertical wind shear certainly lead to the weakening of a tropical cyclone?

“…Apart from looking at the magnitude of VWS, the direction of VWS has also found to be important for TC genesis and intensity. Some studies show that easterly shear is more favorable to TC intensification than westerly shear (Ritchie and Frank 2007, Zeng et al 2010, Shu et al 2014. The beta-induced northwesterly shear within the inner core of the TC could partly offset the easterly shear and thereby results in weaker VWS and favors TC intensification.…”