High-Resolution Modeling of Typhoon Morakot (2009): Vortex Rossby Waves and Their Role in Extreme Precipitation over Taiwan

Abstract

A high-resolution nonhydrostatic numerical model, the Advanced Regional Prediction System (ARPS), was used to simulate Typhoon Morakot (2009) as it made landfall over Taiwan, producing record rainfall totals. In particular, the mesoscale structure of the typhoon was investigated, emphasizing its associated deep convection, the development of inner rainbands near the center, and the resultant intense rainfall over western Taiwan.Simulations at 15-and 3-km grid spacing revealed that, following the decay of the initial inner eyewall, a new, much larger eyewall developed as the typhoon made landfall over Taiwan. Relatively large-amplitude wave structures developed in the outer eyewall and are identified as vortex Rossby waves (VRWs), based on the wave characteristics and their similarity to VRWs identified in previous studies.Moderate to strong vertical shear over the typhoon system produced a persistent wavenumber-1 (WN1) asymmetric structure during the landfall period, with upward motion and deep convection in the downshear and downshear-left sides, consistent with earlier studies. This strong asymmetry masks the effects of WN1 VRWs. WN2 and WN3 VRWs apparently are associated with the development of deep convective bands in Morakot's southwestern quadrant. This occurs as the waves move cyclonically into the downshear side of the cyclone. Although the typhoon track and topographic enhancement contribute most to the recordbreaking rainfall totals, the location of the convective bands, and their interaction with the mountainous terrain of Taiwan, also affect the rainfall distribution. Quantitatively, the 3-km ARPS rainfall forecasts are superior to those obtained from coarser-resolution models.

A Numerical Study on the Formation and Maintenance of a Long‐Lived Rainband in Typhoon Longwang (2005)

“…A prominent feature of VRWs is their spiral shape resulting from strong tangential wind shearing effect (Wang, ). The existence of VRWs in TCs has been confirmed by observational analysis (e.g., Corbosiero et al, ; Reasor et al, ) and numerical simulations (e.g., Chen & Yau, ; Franklin et al, ; Hall et al, ; Wang, , , ). VRWs are often coupled with convection and can affect TC intensity and the geometry of the eyewall (Chen & Yau, ; Nikitina & Campbell, ; Wang, ).…”

“…Furthermore, the coupling between the PV band and vertical velocity band indicates a strong feedback between the dynamics and thermodynamic factors (e.g., Chen & Yau, ; Hall et al, ; Wang, , ; Yu & Chen, ), as suggested by L18. Specifically, the positive perturbation of the PV wave first triggered convection via Ekman pumping.…”

“…The background PV gradient was also able to support the propagation of VRWs (Figures c and d). Note that a VRW‐induced inner rainband after landfall has also been found in Typhoon Morakot (2009) by Hall et al ().…”

“…Because the wavenumber‐1 PV signal lagged behind the wavenumber‐1 vertical velocity (Figure d1) and both showed no obvious cyclonic propagation (Figures d2–f2), they were unlikely to be related to VRW. Instead, this wavenumber‐1 signal corresponded with the deep‐layer VWS (Figure ), which could induce convection downshear or downshear left of a TC (e.g., Hall et al, ; Li et al, ; Wang & Holland, ; Xu & Wang, ).…”

Evaluation of Version-7 TRMM Multi-Satellite Precipitation Analysis Product during the Beijing Extreme Heavy Rainfall Event of 21 July 2012

“…Typhoon Morakot in Taiwan in August 2009 [1][2][3], severe floods in Pakistan in 2010 [4] and in Beijing in July 2012 [5,6], and Hurricane Sandy [7] in northeast U.S. in October 2012 have shown that the precipitation system tends to develop into heavier storms with a longer dry period between two precipitation events [8]. On 21 July 2012, a 60-year return period extreme storm hit Beijing and adjacent regions, triggering flash floods in the urban area and landslides in the mountainous region.…”