Radar Analysis on the Interaction between Southwesterly Monsoonal Flow and Circulation Associated with Typhoon Morakot (2009)

Abstract

This study investigates the mesoscale interaction of typhoon circulation and southwesterly flow during the passage of Typhoon Morakot (2009) over Taiwan using radar observations. Single Doppler radar analysis characterized typhoon features with remarkably distant rainbands, and identified the strong southwesterly monsoonal flow with a maximum speed of more than 45 m s -1 in the southern flank of distant rainbands. Dual-Doppler synthesized winds elucidated a confluent mechanism on the northern side of the rainbands with a maximum convergence over 1.5 × 10 -3 s -1 . Velocity azimuth display (VAD) winds showed the intensification of southwesterly flow at low levels. The southwesterly monsoonal flow initiated about 6 hrs before typhoon landfall, and then became prominently involved with typhoon circulation. Also, the radial component with respect to the typhoon center was enhanced and became comparable with the tangential one (~30 m s -1 ) about 7.5 hrs after landfall. The variation in intensity of radial components can be regarded as a unique precursor for the extension of influence of the southwesterly monsoonal flow onto the typhoon circulation. Furthermore, the strong convergence showed that the interaction between the southwesterly flow and typhoon circulation might contribute to the development of rainbands, as well as the intensification of the inward radial flow embedded within typhoon.

Dual-Doppler Radar Investigation of a Convective Rainband during the Impact of the Southwesterly Monsoonal Flow on the Circulation of Typhoon Morakot (2009)

“…Therefore, they can offer different areas of overlapping scan coverage, which are available for dual-Doppler radar wind synthesis. Wei et al (2012) have discussed that the long baseline between Chiku and Kenting radar would smooth the synthesized wind field due to coarse sampling. The relatively short baseline between the Makung and Chiku radar, however, can improve the horizontal resolution of synthesized wind to 1.5 km.…”

“…Hsu et al (2010) learned that the SW flow in the Typhoon Morakot case supplied abundant moisture and formed a large-scale convective region, which was much larger than the size of the typhoon vortex. Wei et al (2012) demonstrated that Typhoon Morakot was embedded within a wide low pressure zone, which slowly moved westward under the extension of the subtropical ridge. The low pressure system introduced the intense SW flow from the Bay of Bengal reaching to the east of Taiwan carrying abundant moisture (Fig.…”

“…Hong et al (2011) concluded that Typhoon Morakot was associated with a large-scale convection region with monsoon circulation of a different time scale in the tropical western North Pacific. By analyzing the velocity azimuth display (VAD) of Doppler radar network observation, Wei et al (2012) found that the southwesterly (SW) monsoonal flow, which exceeded 40 m s −1 , was more intense than the circulation of Typhoon Morakot (approximately 35 m s −1 ). The strong environmental flow impinged on the typhoon circulation and intensified the radial inward component.…”

A Decade after Typhoon Morakot (2009): What Have We Learned about Its Physics and Predictability?

“…As Morakot was impacting Taiwan, the strong southwesterly flow converged with the northwesterly flow around the TC at low levels over the southern Taiwan Strait to form a primary rainband that was highly asymmetric with deep convection especially to the south of the TC center, as shown in Fig. 5 and many observational studies (e.g., Chen et al 2010;Jou et al 2010;Wei et al 2012). While this persistent, asymmetric rainfall pattern was also consistent with the northerly vertical wind shear (VWS) of about 5 m s 1 (Corbosiero and Molinari 2002;Chien and Kuo 2011;Wang et al 2012), the inner core of Morakot was weak and the rainbands were largely collocated with the outer circulation far from the center (Fig.…”

“…Wang et al (2015) studied the convective-scale processes and interaction between the updrafts of convective cells inside this rainband and the background VWS. This VWS was associated with a westerly low-level jet (LLJ) of about 45 m s 1 near 1 km in height (also Wei et al 2012), and thus had a hairpin shape in the hodograph. Below the LLJ, the interaction would produce negative (positive) dynamic pressure perturbations (pd') ahead and east (behind and west) of convective updrafts, as is typically found in midlatitudes (Rotunno and Klemp 1982;Klemp 1987).…”