A diagnostic study on heavy rainfall induced by landfalling Typhoon Utor (2013) in South China: 2. Postlandfall rainfall

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1 PUBLICATIONS RESEARCH ARTICLE This article is a companion to Meng and Wang [2016] doi: /2015jd Key Points: ISOs played important roles in reviving Typhoon Utor inland Monsoonal surges contribute to initiation of MCSs Positive feedback between rainfall and TC circulation Correspondence to: Y. Wang, yuqing@hawaii.edu Citation: Meng, W., and Y. Wang (2016), A diagnostic study on heavy rainfall induced by landfalling Typhoon Utor (2013) in South China: 2. Postlandfall rainfall, J. Geophys. Res. Atmos., 121, 12,803 12,819, doi: / 2015JD Received 13 DEC 2015 Accepted 4 OCT 2016 Accepted article online 6 OCT 2016 Published online 12 NOV American Geophysical Union. All Rights Reserved. A diagnostic study on heavy rainfall induced by landfalling Typhoon Utor (2013) in South China: 2. Postlandfall rainfall Weiguang Meng 1,2 and Yuqing Wang 3 1 Institute of Tropical and Marine Meteorology/Key Laboratory of Regional Numerical Weather Prediction, CMA&GD, Guangzhou, China, 2 State Key Laboratory on Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China, 3 International Pacific Research Center and Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, University of Hawai i atmānoa, Honolulu, Hawaii, USA Abstract In this part, mechanisms responsible for the maintenance of Typhoon Utor (2013) after landfall and its associated heavy rainfall in South China were investigated with methods including piecewise potential vorticity (PV) inversion. The focus was on the monsoonal influence and the interaction between Utor and the mesoscale convective systems (MCSs) embedded in the southwesterly monsoon flow. The results show that after landfall Utor underwent coalescence with the cyclonic monsoon gyres, related to the quasi-biweekly oscillation (QBWO) and the Madden-Julian oscillation (MJO). The QBWO cyclonic gyre, which better developed and coalesced with the tropical cyclone (TC) vortex, enhanced the large-scale southwesterly monsoon flow southeast of the TC, providing favorable condition for initiation of convection and organization of MCSs in the eastern outer rainband of the TC. Latent heat release in the MCSs provided a positive feedback to enhance the TC circulation and the southwesterly monsoon flow, slowing down the decay of Utor and sustaining heavy rainfall. Piecewise PV inversion confirmed that the nonlinear balanced flow inverted from the PV anomaly associated with latent heating in MCSs led to an increase of the low-level southwesterly flow south of the TC by over 4 m s 1, which contributed notably to the formation of the lowlevel jet and moisture convergence in the eastern outer rainband. It is suggested that the positive feedback between the outer rainband MCSs and the southwesterly monsoon flow is a major mechanism responsible for the maintenance of Typhoon Utor after its landfall and the associated postlandfall heavy rainfall over South China. 1. Introduction Most tropical cyclones (TCs) cause heavy rainfall mainly near the coastal region during landfall. Some TCs can bring widespread heavy rainfall and severe flooding events after they move farther inland and lead to deadly and destructive disasters [Chen and Ding, 1979]. This is particularly true when the remnant of a landfalling TC lasts a long time wandering inland. Because the remnant of a landfalling TC can only maintain its intensity and result in rainfall reinforcement under some special environmental conditions and with complex multiscale interactions, to accurately forecast such an event is always very challenging [Jones et al., 2003; Chen et al., 2010]. The well-known August 1975 ( 75.8 ) severe flooding that occurred in China due to the rainfall reinforcement by Typhoon Nina (1975) [Chen and Ding, 1979] is one of the notable disaster events caused by the remnant of a landfalling TC. Though it weakened to a tropical storm before making landfall in southeastern China and caused little damage during its landfall period, Nina produced widespread heavy rainfall as the remnant of the storm moved farther inland in Henan Province. The heaviest rainfall was recorded at a total of 1631 mm with 830 mm falling in a 6 h period, leading to the collapse of reservoirs and flash flooding and causing enormous losses of life and property. A more recent notable rainfall reinforcement event associated with the remnant of a TC inland over China was caused by Tropical Storm Bilis (2006), which caused sustained torrential rains, flooding, debris flows, and mudslides, leading to 843 fatalities and extensive property damage in northern South China [Gao et al., 2009]. The most recent event was Typhoon Utor (2013), which was another long-lasting landfalling TC that caused heavy inland rainfall. After it made landfall, Utor moved slowly first northward and then turned southwestward, wandering around southern South China for about 4 days. In addition to the heavy rainfall that occurred during its landfall period on 14 August and was MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,803

2 investigated in part 1 of this study, Utor brought sustained heavy rainfall and severe flooding to most parts of southern Hunan Province and central eastern Guangdong Province in the subsequent 3 days. In Huidong, a county located over the southeastern coast of Guangdong Province, mm rainfall was observed in 24 h, breaking the daily rainfall record in the province. It is our interest in this study to examine the physical mechanisms responsible for the postlandfall sustained heavy rainfall associated with Typhoon Utor. Over decades, continuous efforts have been made to investigate the heavy rainfall induced by landfalling TCs [Bosart and Carr, 1978; Rodgers et al., 1994; Rogers et al., 2003; Chan et al., 2004; Atallah and Bosart, 2003; Atallah et al., 2007; Chen et al., 2006; Gao et al., 2009; Chien and Kuo, 2011]. Most previous studies have revealed that heavy rainfall events associated with landfalling TCs often involve complex scale interactions, with the rainfall magnitude and distribution greatly affected by the nonlinear interactions among different physical processes and with other weather systems. In a literature review, Chen et al. [2010] summarized various mechanisms that are responsible for heavy rainfall related to landfalling TCs. They are ample moisture transport, extratropical transition, TC interaction with monsoon surges, activities of mesoscale convective systems (MCSs) within the TC circulation, land surface processes and/or topographic effects, etc. As the most important factor, moisture transport or moisture supply for landfalling TC rainfall has been given special attention. Chen et al. [2010] pointed out in their review that some studies have revealed that heavy rainfall in China associated with a landfalling TC usually occurs with a strong moisture transport channel prevalently from southeast of the TC, while such a channel is always absent in the case with weak rainfall. Numerical model sensitivity simulations confirmed that if such a moisture transport channel was artificially cut off, the simulated rainfall would be significantly reduced. Since the East Asian summer monsoon possessed both copious amounts of moisture and energy favorable for deep convection, the effect of monsoonal surges on TC rainfall has been widely examined in previous studies. An investigation of rainfall reinforcement associated with postlandfall TCs over the mainland China by Dong et al. [2010] revealed that rainfall reinforcements for TCs with westward tracks are mainly due to the interaction between cloud clusters in a monsoonal surge and the TC circulation while those with northward tracks are mainly attributed to the interactions between the TC and a westerly trough to the north. Tropical Storm Bilis (2006) was such a TC with westward-southwestward track after its landfall and a typical storm interacting with the monsoonal surge. Gao et al. [2009] analyzed mechanisms responsible for the rainfall associated with Bilis and confirmed that the heavy rainfall, especially that which occurred at the later stage, was mainly produced by the interaction between Bilis and the southwesterly monsoonal surge. The East Asian summer monsoon often experiences low-frequency variabilities related to the Madden-Julian oscillation (MJO) and the quasi-biweekly oscillation (QBWO), or the so-called intraseasonal oscillations (ISOs). The MJO reflects the large-scale coupling between atmospheric circulation and tropical deep convection [Zhang, 2005], characterized by an eastward propagation of enhanced large-scale convection mainly initiated over the tropical Indian Ocean. The QBWO can be viewed as the high-frequency part of tropical intraseasonal variability [Zhang, 2005; Waliser, 2006], which describes tropical convective anomalies that occur regionally in association with the summer monsoon activity. Two types of QBWO could be classified according to their propagations [Kikuchi and Wang, 2009]: the westward and eastward propagating modes. The QBWO in the Asian-Pacific summer monsoon region usually originates from equatorial western Pacific and manifests as a westward propagating mode. Most studies have investigated the relationship between the ISOs and the activity of TCs. Some have shown that sudden track changes of TCs over the western North Pacific are often related to the interaction of the TC circulation with the ISOs [Harr and Elsberry, 1991, 1995; Chen and Huang, 2009; Ko and Hsu, 2006, 2009; Ge et al., 2010; Wu et al., 2011; Liang et al., 2011; Ling et al., 2016]. The ISOs are also found to have great impacts on the activity of Asian and East Asian summer monsoon [Krishnamurti and Subrahmanyam, 1982; He, 1990; Li et al., 2001], which could influence both amount and distribution of rainfall associated with a landfalling TC. Ge et al. [2010] and Hong et al. [2010] investigated the heavy rainfall event associated with Typhoon Morakot (2009), a landfalling TC that caused record-breaking rainfall and led to great flooding and landslides in southern Taiwan Island. They found that both the coalescence of the TC circulation with the low-frequency cyclonic gyre and the monsoonal surges played important roles in enhancing the heavy rainfall. They concluded that the rainfall-induced condensational heating, by further increasing the low-level southwesterly monsoonal flows or monsoonal surges and transporting more moisture to the rainfall areas, contributed greatly to the rainfall enhancement. Wu et al. [2011] also examined the rainfall MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,804

3 event associated with Typhoon Morakot (2009) and found that the coalescences of the TC circulation with the low-frequency cyclonic gyres of QBWO and MJO might lead to an increase in the southwesterly monsoonal flows to the southeast of the TC. As a result, the westward movement of the TC slowed down significantly, leading to the increased residence time of the TC over the vicinity of Taiwan Island after landfall. This was considered the main mechanism responsible for the extensively heavy rainfall induced by Typhoon Morakot over southern Taiwan Island. In addition to emphasizing the impacts of monsoonal surges and their interaction with the TC circulation, some studies have noticed the role of cloud clusters or MCSs embedded within the monsoonal flows in enhancing the rainfall associated with a landfalling TC [Chen et al., 2010; Chien and Kuo, 2011; Lee et al., 2011, 2012; Chen et al., 2014a, 2014b]. In a study investigating long-lasting rainbands associated with the interaction between TCs and the East Asian summer monsoon, Chen et al. [2014a] identified two types of such rainbands: one is the Outer Mesoscale Convective System (OMCS) defined by Lee et al. [2012], and the other is defined as the Enhanced Rainband (ERB). According to Lee et al. [2012] and Chen et al. [2014a], an OMCS is related to a long-lived and cold-topped linear convective system that has a thick and large stratiform precipitation region in the outer region of a TC, while an ERB is manifested as a form of transformation of the TC principal rainband to have a larger cold cloud shield and longer duration due to an interaction with the monsoon environment. Since the MCSs or cloud clusters embedded in a rainband are the main precipitation systems closely related to rainfall enhancement associated with a landfalling TC, more attention should be given to the understanding of possible mechanisms responsible for these systems. In part 1 [Meng and Wang, 2016], we have shown that in addition to heavy rainfall occurred at landfall, Typhoon Utor (2013) brought sustained heavy rainfall in the subsequent 3 days as it moved farther inland. We also mentioned that interactions between the TC circulation and the intensification of southwesterly monsoonal flow should contribute greatly to the TC vortex maintenance and the occurrence of inland heavy rainfall. In this part, we will focus on the postlandfall rainfall, especially on the third rainfall episode defined in part 1. The impacts of MJO and QBWO on the intensification of southwesterly monsoon flows will be investigated. In particular, the activity of MCSs embedded within the monsoon flows and their interactions with the TC remnant will be examined in some details. The rest of the paper is organized as follows. Section 2 introduces data and methodology used in the study. Section 3 describes the synoptic situation related to the postlandfall stage of Typhoon Utor and presents analysis on the roles of low-frequency variabilities of the monsoon flows including MJO and QBWO and the activity of MCSs in maintaining the circulation of Utor and the associated heavy rainfall. In section 4, the dynamical response to latent heating released from the heavy rainfall is examined based on the nonlinear balanced piecewise PV inversion technique to demonstrate the importance of diabatic heating and circulation feedback to the maintenance of Utor inland and to the triggering of convective MCSs. The major results are summarized in section Data and Methodology The atmospheric data used in this study are mainly the European Centre for Medium-Range Weather Forecasting (ECMWF) ERA-Interim reanalysis data ( ecmwf.int/datasets/data/interim-full-daily) as used in part 1. We retrieved and used the data with the horizontal resolution of in both latitude and longitude. To identify the activity of MCSs associated with the monsoon flow and their effects on the landfalling TC, the hourly Japan MTSAT (Multifunctional Transport Satellite) infrared (channel 1, IR1) images data ( were also used in the analysis. The best track TC data from China Meteorological Administration were used to show the observed TC positions. To examine the scale interactions between Utor and the East Asian summer monsoon, we used the Lanczos filter [Duchon, 1979] to subtract the flow patterns on the two intraseasonal time scales as mentioned above. Similar to some previous studies [Li and Zhou, 2013; Ling et al., 2016], a band-pass filter with a day period and a band-pass filter with a day period were used, respectively, to obtain the MJO-scale and QBWO-scale flows. Note that filtering for the day MJO is more justifiable than for the day QBWO since the latter might be affected by the TC circulation. In particular, by comparing the temporal filtering results with and without the removal of TC vortices, Hsu et al. [2008] found that TCs could contribute significantly to ISOs in vorticity fields over the western North Pacific. In our analysis, we applied the temporal MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,805

4 Figure 1. (a) The mean geopotential height at 500 hpa (contours, gpm), wind (vectors, m s 1 ), and hpa thickness (shading, gpm) averaged in the 3 day period from 0000 UTC on 15 August to 0000 UTC on 18 August (b) The mean geopotential height at 850 hpa (contours, gpm) and moisture flux (vectors with magnitude shaded, g kg 1 ms 1 ) averaged in the 3 day period from 0000 UTC on 15 August to 0000 UTC on 18 August The average location of TC during this period is marked with typhoon symbols. filter for the original field without a removal of the TC circulation. This was definitely not clean enough since the QBWO might be partially contributed by the TC circulation. Although the removal of the TC circulation could be ideal, the coalescence of Utor with the QBWO cyclonic gyre in the weakening stage of Utor (see section 3.2 below) makes the removal of the TC circulation a problem. We considered that for the Utor case the contribution by the TC circulation to the ISOs should not be as significant as that reported in Hsu et al. [2008] because the TC circulation was relatively weak after it made landfall and moved inland. Note that the scale separation approach used in our analysis was mainly to help understand scale interactions. Even though it is incomplete, it is still helpful, as done in Hong et al. [2010] and Wu et al. [2011]. Finally, to understand the role of latent heating in sustaining the circulation of Utor after landfall and the enhancement of southwesterly monsoon flow, the nonlinear balanced piecewise PV inversion technique developed by Davis and Emanuel [1990] and Davis [1992a, 1992b] was employed with the data degraded from ECMWF s ERA-Interim reanalysis data as mentioned above. 3. Monsoon Evolution and the Activity of MCSs 3.1. Synoptic Situation During Utor s Postlandfall Period To reveal the overall feature of synoptic-scale circulation after Utor made landfall, we show in Figure 1a the geopotential height at 500 hpa and the geopotential thickness between 500 and 1000 hpa averaged during the 3 day period between 0000 UTC on 15 August and 0000 UTC on 18 August. It is clearly seen that after Utor made landfall, it moved slowly first northward and then toward southwest (refer to part 1) and was located south of the westward extending western North Pacific subtropical ridge. The geopotential thickness between 500 and 1000 hpa indicates that the subtropical ridge was in conjunction with deep warm air, while no evident frontal or baroclinic zone existed between the TC and the midlatitude westerly systems. At 850 hpa (Figure 1b) strong moisture flux prevailed to the south southeast in the TC circulation. This is consistent with previous findings in landfalling TCs with heavy rainfall [Chen et al., 2010] and suggests that the enhancement of monsoon flow to the south southeast of Utor could provide abundant moisture and other favorable conditions, such as convective instability, for initiation of deep convection and heavy rainfall to the south southeast in Utor s circulation over South China. As we can see from Figure 1b, the strong southwesterly moisture flux at 850 hpa was not restricted to the southeastern quadrant of Utor; it presented as a conveyor belt covering a large area extending westward to the central Indochina Peninsula and the Bay of Bengal. Note that over the northwestern South China Sea (SCS) and south of the TC vortex, the geopotential thickness between 500 and 1000 hpa indicates the existence of a region with a broad layer of warm air associated with the southwesterly monsoonal flow (Figure 1a). The warm advection could be expected over downstream of these areas as well, providing lifting MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,806

5 Figure 2. The wind fields at 850 hpa (streamlines, vectors with magnitude shaded, m s 1 ) (a, c, e) on the MJO time scale and (b, d, f) on the QBWO time scale at 0000 UTC on 15 (Figures 2a and 2b), 16 (Figures 2c and 2d), and 17 (Figures 2e and 2f) August Typhoon symbols mark the TC center at the given time. condition favorable for deep convection downstream in the TC circulation. These strongly suggest that after Utor made landfall and moved farther inland, interactions between the TC and the intensification of southwesterly monsoon flow should be the major mechanism responsible for the postlandfall heavy rainfall. The environmental vertical wind shear (see part 1) had little contribution to the inland rainfall distribution since the vertical wind shear did not change its direction Monsoon Variability Figure 2 displays the wind fields at 850 hpa on the MJO and QBWO time scales, at 0000 UTC on 15, 16, and 17 August. It is clearly shown that after landfall of Utor, the MJO cyclonic gyre had a wide scale, elongated in the zonal direction, and occupied most parts of South China, the northeastern SCS, and the western Northwest Pacific (Figures 2a, 2c, and 2e). There were two cyclonic centers in the large MJO cyclonic gyre, with the more distinct one to the east over the ocean east of Taiwan Island and the weaker west one over the southern coast of South China. Utor was located slightly to the west of the weaker cyclonic gyre center and in places MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,807

6 predominantly controlled by the northerly flow of the MJO cyclonic gyre. This indicates that the MJO contributed to the slow movement and southwestward track turning of Utor as discussed in part 1, but it had little effect on the enhancement of southwesterly flow south of Utor since there was no significant enhancement of southwesterly flow to the south of the MJO gyre. The wind fields at 850 hpa on the QBWO time scale (Figures 2b, 2d, and 2f) also showed two cyclonic gyres over the corresponding areas. The east one was located farther east to Taiwan Island than the east MJO cyclonic gyre, and the west one was over South China. The west QBWO cyclonic gyre developed better and had a horizontal scale comparable to that of the TC (cf. Figure 1b). Utor was well coalesced with the west QBWO cyclonic gyre. Because the west QBWO gyre remained almost stationary in this period, Utor moved slowly over south China (see part 1) within the QBWO gyre. Note that the west QBWO cyclonic gyre could be partially contributed by the circulation of Utor. Nevertheless, the low frequent ISOs and their associated propagation of tropical convective anomalies had their own spatial and time scales and should have their own entities even without the TC though the TC might have enhanced the ISOs in the Utor case (for a general discussion, see Hsu et al. [2008]). Therefore, wesimply consider thatthe ISOsprovidedthelarge-scaleforcingto thetcinour followingdiscussion. As it coalesced well with the TC circulation, the QBWO cyclonic gyre might lead to the enhancement of southwesterly monsoonal flow to the south of Utor and also the maintenance of Utor s circulation (Figure 2). As we can see from Figures 2b, 2d, and 2f, the QBWO-scale westerly-southwesterly winds to the south of Utor were generally over 4 m s 1, which increased steadily as the TC remained inland. The unfiltered wind speed south of Utor was generally greater than 12 m s 1 during this period (Figure 3b), which met the typical low-level jet (LLJ) criterion and should be largely contributed by the intensified QBWO flow. The coalescence processes of the TC circulation with both the MJO and QBWO gyres and their influence on TC tracks and the enhancement of southwesterly monsoonal flow have been emphasized in previous studies [Carr and Elsberry, 1995; Ge et al., 2010; Wu et al., 2011]. Our analysis above suggests that the two time scale ISOs might have different impacts on Utor after its landfall. The MJO gyre played a role in affecting the motion of Utor, while the QBWO gyre contributed greatly to the enhancement of southwesterly monsoon flow and heavy rainfall in the eastern outer rainband of Utor over South China, through its coalescence with the TC circulation and steadily increasing southwesterly flow. Figure 3 presents the time-longitude cross sections, respectively, for cloud top temperature from the MTSAT IR1 images and the filtered and unfiltered winds at 850 hpa. The cross section for cloud top temperature was created along 23.5 N in order to better illustrate convective activity over the major rainfall area in the eastern outer rainband of the TC, while other cross sections were taken along 22 N, roughly the southern boundary of the major rainfall area over South China to trace the change of southwesterly flow. As shown in Figure 3a, starting from the early morning on 15 August, convection to the east of the TC center (around E) was more active than that to the west of the TC center. Convective cloud clusters were organized in the longitudes between 113 and 117 E and became more and more active, with cloud top temperature decreased to below 52 C as Utor moved slowly inland. Three convective episodes occurred with cloud top temperature below 72 C, two at night on, respectively, 15 and 16 August, and one from early morning through almost the whole day of 17 August. This indicates that heavy rainfall in the eastern outer rainband of Utor was mainly induced by deep convective systems. Note that accompanied by these convective systems were strong southwesterly monsoon surges. The evolution of the unfiltered winds at 850 hpa (Figure 3b) indicates that accompanied by each convective episode mentioned above, the monsoon southwesterly flow experienced a strengthening with the wind speed over 12 m s 1. This strongly suggests that the monsoon southwesterly LLJ is very important to the development of those convective episodes, which in some way is similar to that found in the development of midlatitude MCSs [Maddox, 1983; Cotton et al., 1989]. Figures 3c and 3d present the evolution of the lower tropospheric winds associated with the two ISO modes (MJO and QBWO). As mentioned earlier, most coastal areas of South China west of 111 E or west of the TC center were dominated by the MJO northerly winds. Although the MJO wind speed over these areas increased steadily throughout the dissipation of Utor, it did not exceed 3 m s 1 before Utor dissipated. The MJO winds were even weaker in the area east of 111 E. In contrast, the QBWO winds were much stronger around Utor. In particular, after its landfall, Utor was located near the center of the QBWO cyclonic gyre. As seen in Figure 3d, the QBWO winds east of 111 E were generally greater than 4 m s 1 and consistent with the unfiltered southwesterly monsoon flow. As mentioned earlier, the QBWO should contribute to the MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,808

7 Figure 3. Time-longitude cross sections of (a) TBB (the cloud top black body temperature, shading, C) along the latitude of 23.5 N, (b) unfiltered wind, (c) filtered MJO wind, and (d) filtered QBWO wind at 850 hpa along the latitude of 22 N. Wind speeds in Figure 3b are shaded every 2 m s 1 and in Figures 3c and 3d are shaded every 0.5 m s 1. Typhoon symbols indicate the longitude of TC position. strengthening of southwesterly monsoon flow southeast of Utor and thus play an important role in the associated heavy rainfall Activities of MCSs Deep convection in the monsoon environment is often organized into an MCS, a complex thunderstorm with larger scale and longer duration than an individual thunderstorm, which usually brings abundant rainfall to the affected areas. In fact, the convective episodes shown in Figure 3a were directly related to the activities of MCSs. Figures 4 and 5 present hourly MTSAT-IR1 cloud images for MCSs occurring at night on 15 and 16 August, corresponding to the first two episodes with cloud top temperature below 72 C in Figure 3a. Except for convection near the inner core region of Utor, there were two more intense convective cloud bands to the southeast in the outer core region of Utor when it was approximately centered at E, 24.3 N (Figure 4) and a much larger MCS to the southwest in the outer region of Utor. The southwest MCS covered most areas in the Beibu Gulf in the northwest SCS and the coastal inland region with large cloud shields and could be regarded as an OMCS as defined by Lee et al. [2012]. Since this OMCS was generally located steadily southwest of Utor, it mainly contributed to rainfall in the coastal areas of southwestern Guangdong and southeastern Guangxi Province in rainfall region I as defined in part 1 (Figure 4). Rainfall in region II in the border areas of Guangxi, Guangdong, and Hunan Provinces was mainly caused by the inner MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,809

8 Figure 4. Satellite imageries at 1 h intervals from (a) 1900 UTC on 15 August to (d) 2200 UTC on 15 August The TBB data are from MTSAT IR1. The shading scale is in degree centigrade. Typhoon symbols mark the TC center at 0000 UTC on 16 August. core convection, which was mostly initiated in the southeastern quadrant of Utor (Figure 4), and then developed and stretched northward into southern Hunan Province (Figure 5). The two outer cloud/rain bands to the southeast of Utor and the associated consecutive development of MCSs were responsible to the postlandfall heavy rainfall in Region III, which we will focus on. One band was along the central coast of South China and extended northward to the central part of Guangdong Province, and the other band was in the coastal region of SCS. As shown in Figure 4a, at 1900 UTC on 15 August convection was initiated within the cloud band along the central coast of South China and became more and more organized and finally evolved into an MCS over the Pearl River Delta (PRD) in the next 2 h (Figures 4b and 4c). By 2200 UTC on 15 August, the MCS became mature with convective clouds extending northeastward (Figure 4d) and evolved into a mesoscale convective complex (MCC) as defined by Maddox [1980, 1983]. MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,810

9 Figure 5. Same as Figure 4, except for times from (a) 2100 UTC on 16 August to (d) 0000 UTC on 17 August Typhoon symbols mark the TC center at 0000 UTC on 17 August. The MCS occurring at night on 16 August (Figure 5) showed quite similar features to those at night on 15 August. At about 2100 UTC on 16 August (Figure 5a), the MCS was also initiated within an outer rainband in the central coastal region of south China and rapidly intensified into an MCS about 2 h later (Figures 5b and 5c). As the MCS matured at 0000 UTC on 17 August (Figure 5d), it was located almost at the same area as the previous one centered at about 114 E, 23.5 N. The similar distribution of cloud top temperature below 52 C indicates that this MCS also met the sizeand shape criteria for an MCC.Note that the extended outer rainband in the central coastal regionofsouthchinashowedmost featuresliketheerbdefinedbychenetal.[2014a],suggestingthatthe interaction between the MCSs and the monsoon flows should have played some roles in the rainfall enhancement. The consecutive occurrence of two MCSs described above contributed greatly to rainfall in region III. These MCSs tended to be initiated near the coast of South China in the PRD region, implying that the complex land-sea contrast and/or perhaps the urban land surface process over these areas might have contributed MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,811

10 Figure 6. Horizontal distributions of the 24 h mean vertically integrated apparent heat source hq 1 i (contours, 10 3 Wm 2 ) and horizontal winds at 850 hpa (vectors with magnitude shaded, m s 1 ) averaged (a) from 1200 UTC on 15 August to 1200 UTC on 16 August and (b) from 1200 UTC on 16 August to 1200 UTC on 17 August Typhoon symbol in Figure 6a marks the TC center at 0000 UTC on 16 August and in Figure 6b marks the TC center at 0000 UTC on 17 August. to the initiation of convection and the organization of the MCSs. Because of the coarse spatial and temporal resolutions of the data here, we could not examine the details of convective initiation processes, which is a topic that we are working on using high-resolution cloud-resolving atmospheric model, and the results will be reported separately in due course. With active convection, on the one hand, MCSs usually directly brought copious rainfall to the affected areas, and, on the other hand, by releasing condensational heating, they could produce a feedback to enhance the large-scale and TC-scale circulations in which they were embedded. Figure 6 shows the horizontal distribution of the 24 h averaged, vertically integrated apparent heat source hq 1 i [Yanai et al., 1973] and the large-scale circulation at 850 hpa. During the 24 h periods ending at 1200 UTC, respectively, on 16 and 17 August, the averaged maximum hq 1 i was mainly located east of the TC center in the strong large-scale southwesterly monsoon flow or the eastern outer rainband region with active MCSs. Since latent heating is one of the most dominant terms in Q 1, maximum in hq 1 i east of the TC center was mainly contributed by latent heat release from convection in the MCSs. To further understand how the interaction between the TC circulation and the MCSs embedded in the outer rainband contributed to the sustaining heavy rainfall after landfall of Utor, the effect of diabatic heating on the large-scale circulation is analyzed in the next section based on piecewise PV inversion approach. Note that corresponding to the southwestern large cloud shields (or the OMCS mentioned above), no apparent heat source center was detected, suggesting that the OMCS had no apparent heating effect on the large-scale circulation. We found that this OMCS showed features similar to the OMCS associated with Typhoon Fengshen discussed in Chen et al. [2014b], with a large cloud shield to the south southwest of the TC and southwesterly monsoon flows to the south. However, radar (figure not shown) and automatic weather station observations (cf. Figures 3b 3d in part 1) showed that convective rainfall associated with the OMCS in the Utor case was not as rich as that associated with the OMCS found in Fengshen. This explains why no apparent heat source center was detected with it. 4. Results From Piecewise PV Inversion A widely used approach to investigate the dynamical effect of diabatic heating on atmospheric circulation is the potential vorticity (PV ~ q) framework (PV is defined as a quantity which is proportional to the product of the absolute vorticity and the gradient of potential temperature. The Ertel PV used in our analysis is given as q ¼ 1 ρ η θ, where η is the absolute vorticity vector, ρ is air density, and θ is potential temperature.). As discussed in Hoskins et al. [1985], one of the main tenets of the PV thinking is the conservation principle of PV for adiabatic, frictionless motion, while with diabatic processes, latent heating becomes the PV source. MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,812

11 Figure 7. (a) Differences in wind field (stream lines) and geopotential height (contours at intervals of 4 gpm) at 850 hpa between 1800 UTC on 15 August and 0000 UTC on 16 August 2013 and PV perturbations (all q, shading at every 0.3 PVU, 1 PVU = 10 6 Km 2 kg 1 s 1 ) at 0000 UTC on 16 August. (b) The east-west vertical cross sections of PV perturbations (all q, shaded at every 0.3 PVU) and the associated inverted geopotential height (contours at interval of 2 gpm) across the TC center at 0000 UTC on 16 August Therefore, PV can be used as an indicator of the impact of latent heating released in MCSs on the atmospheric dynamical field. Another fundamental principle of PV is its invertibility, which allows one to quantify the balanced dynamical fields, such as geopotential height, wind, and temperature, associated with a given PV anomaly. To quantify the effect of latent heating released in the MCSs in the monsoon flows as discussed in section 3, we performed PV diagnosis using the piecewise PV inversion technique developed by Davis and Emanuel [1990] and Davis [1992a, 1992b] (refer to their papers for details) Piecewise PV Diagnosis Figure 7 presents an example of heavy rainfall associated with MCSs that might impose a considerable influence on the large-scale circulation. Figure 7a gives the PV perturbations (q, shaded areas) at 850 hpa at 0000 UTC on 16 August and the associated 6 h changes in wind and geopotential height fields. The perturbation PV was calculated as the deviation from the time mean PV averaged in a 10 day period from 11 to 20 August 2013 using the ERA-Interim reanalysis data. The 6 h difference fields in wind and geopotential height were their differences between 1800 UTC on 15 August and 0000 UTC on 16 August. As we can see from Figure 7a, the maximum perturbation PV at 850 hpa was well collocated with the TC center. A region of positive PV perturbation stretching southeastward was associated with the MCSs southeast of the TC center as discussed earlier. A negative center of geopotential height difference with a value below 4 gpm was accompanied with a convergent cyclonic wind difference field to the east of the TC center and corresponded well with the center of diabatic heating (Figure 6a) associated with the MCSs. This indicates that the perturbation PV was mainly diabatically generated. This can be further seen from the vertical cross section of perturbation PV across the TC center shown in Figure 7b. The maximum perturbation PV over 1 potential vorticity unit (PVU) was located in the middle to lower troposphere below the level of maximum convective heating (figure not shown) and slightly to the east of the TC center. The inverted geopotential height difference (see a description below) from the perturbation PV shows negative values below 20 gpm. This low-pressure anomaly can lead to low-level convergence and help maintain the cyclonic TC circulation. This in turn could contribute to heavy rainfall and the release of diabatic heating, suggesting that heavy rainfall associated with the MCSs could have a notable impact on the maintenance of Utor inland. As mentioned earlier, due to the lack of diabatic heating source, no apparent PV perturbations associated with the OMCS was found to the southwest of the TC center. In contrast to the eastern ERB, which was accompanied with an anomalous convergent cyclonic wind field, the OMCS to the southwest was embedded in an MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,813

12 Figure 8. The east-west vertical cross sections of the nonlinear balanced geopotential height (contours at interval of 2 gpm) inverted from q components related to (a) lb, (b) mu, (c) ms, and (d) ul across the TC center at 0000 UTC on 16 August The associated q components are shaded at every 0.05 PVU in Figures 8a and 8b and at every 0.3 PVU in Figures 8c and 8d. environment controlled by the low-level divergent flow difference (Figure 7a). Since the region to the southwest of the TC was occupied by west-to-northwesterly flow (cf. Figure 6), which was relatively dry, the lack of sufficient moisture supplies might be the main reason why the OMCS in our case produced little precipitation, quite different from the OMCS associated with Typhoon Fengshen discussed by Chen et al. [2014b]. To further quantify the impact of various PV perturbations, the piecewise PV inversion developed by Davis and Emanuel [1990] and Davis [1992a, 1992b] under the nonlinear balanced assumption [Charney, 1955] was conducted. Note that to ensure numerical convergence of the piecewise PV inversion, the high horizontal resolution ERA-Interim data were degraded to horizontal resolution of 1 1 in the region of N, E first. As mentioned earlier, a 10 day mean flow was used as the reference state to define the PV anomalies. Nineteen vertical pressure levels from 1000 to 100 hpa with an interval of 50 hpa were used in the PV inversion. Potential temperatures averaged between 950 and 1000 hpa and between 100 and 150 hpa were used for the lower and upper boundary conditions, respectively. Homogeneous Dirichlet conditions were applied to the lateral boundaries. Similar to that in Davis [1992b] and Chen et al. [2008], the effect of PV perturbation was partitioned into four components: (1) lower boundary effect (lb) defined as all perturbation PV at 975 hpa, (2) dry processes in the middle to lower troposphere (mu) with all positive perturbation PV between 400 and 950 hpa with relative humidity RH < 70%, (3) latent heat release (ms) in the middle to lower troposphere with all positive perturbation PV between 400 and 950 hpa with RH 70%, and (4) upper tropospheric effect (ul), which includes all perturbation PV in the layer between 125 and 400 hpa. According to Davis [1992b] and Chen et al. [2008], the 70% threshold for RH was a reasonable value to separate different contributions of dry and moist dynamics to the piecewise PV inversion as used in previous studies. The PV inversion described above was performed for the 4 day period from 0000 UTC on 14 August to 0000 UTC on 18 August. A typical example of the results is given in Figure 8 for 0000 UTC on 16 August, which shows the east-west cross sections across the TC center for geopotential height perturbation attributed to each of the four PV anomalies defined above. Note that the total geopotential height perturbation corresponding to the four components is shown in Figure 7b. We can see clearly that the lower boundary MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,814

13 Figure 9. Time series of perturbation relative vorticity at 850 hpa (10 5 s 1, curve ec ), nonlinear balanced relative vorticity inverted from all q (curve all ) and from its four components (curves ul, ms, mu, and lb ) all averaged in a radius of 600 km from the TC center from 0000 UTC on 14 August to 0000 UTC on 18 August The curve ec was calculated based on the perturbation wind field from the 10 day mean winds in a similar way to the calculation of perturbation PV from ERA-Interim reanalysis. effect (lb) induces positive geopotential height perturbations reaching over +12 gpm at the low levels but decaying rapidly with height (Figure 8a). The geopotential height perturbations associated with the dry processes in the middle to lower troposphere (mu) are negligible at all levels (Figure 8b). The negative geopotential height perturbation with a maximum value below 20 gpm over the TC center in Figure 7b is mainly contributed by the latent heat release (ms) in the middle to lower troposphere (Figure 8c). The upper tropospheric effect (ul) mainly produces negative geopotential height perturbations with the maximum negative value about 8 gpm at the upper levels, showing little effect below (Figure 8d). These results indicate that the total geopotential height perturbation over the TC center in Figure 7b is produced predominantly by the latent heat release. Since MCSs were the main precipitating systems during this period, latent heat release in the active MCSs should contribute significantly to the maintenance of Typhoon Utor after its landfall. Figure 9 shows the time series of perturbation vorticity at 850 hpa averaged within the 600 km radius from the TC center computed, respectively, from the ERA-Interim reanalysis data and from the inverted perturbation wind field from the total perturbation PV and the four components. The perturbation vorticity from the ERA-Interim data was calculated based on the perturbation wind field from the 10 day mean winds in a similar way to the calculation of perturbation PV. It shows that the observed perturbation vorticity decreased steadily after Utor made landfall at about 0600 UTC on 14 August. Nevertheless, from 1200 UTC on 15 August, the decrease in the observed vorticity stopped and even started to increase from 1200 UTC on 16 August until the TC dissipated. The inverted perturbation vorticity from the PV perturbation (all) shows a similar tendency to that observed except that the inverted perturbation vorticity is smaller and decreased more slowly than the observed vorticity before 0000 UTC on 16 August. The lower inverted values are mainly due to the lateral boundary conditions used in the relatively small domain for the PV inversion because the background vorticity associated with the summer monsoon trough and its ISOs were ignored. Nevertheless, the dominant contribution to all by ms (Figure 9) further suggests that during the postlandfall period, the weakening of Utor substantially slowed down and even shortly reintensified due to the positive feedback from the latent heat release in the organized MCSs. This will be further discussed in the next subsection Dynamical Response to Latent Heating Since the aforementioned MCSs were active during nighttime on 15 and 16 August, their influence on the large-scale circulation can be understood by analyzing the associated dynamical features at 0000 UTC on 16 August and 0000 UTC on 17 August, respectively. Figure 10 presents the nonlinear balanced geopotential height and wind fields at 850 hpa inverted from the middle to lower tropospheric perturbation PV (ms), which mainly resulted from latent heat release as indicated in section 4.1. At 0000 UTC on 16 August (Figure 10a), the perturbation geopotential height and wind fields inverted from ms showed the low pressure system (with the maximum negative perturbation geopotential height over 28 gpm) with a cyclonic circulation representing Typhoon Utor. To the south southeast of the TC center, the inverted wind filed from ms showed a maximum west-southwesterly wind up to 4 m s 1, which strengthened the basic southwesterly monsoon flow southeast of the TC center. This led to the enhanced moisture convergence and favored MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,815

14 Figure 10. The nonlinear balanced geopotential height (contours, gpm) and horizontal wind (vectors with magnitude shaded, m s 1 ) at 850 hpa inverted from the middle to lower tropospheric q related to latent heat release (ms) at (a) 0000 UTC on 16 August and (b) 0000 UTC on 17 August deep convection and heavy rainfall to occur southeast of the TC center. The inverted perturbation wind field at 850 hpa (Figure 10a) can be considered as the simple Rossby wave response to the off-equatorial midtropospheric heating [Gill, 1980]. Namely, heating associated with heavy rainfall induced the low-level cyclonic circulation centered a little to the northwest of the heating with the enhanced low-level southwesterly flow to the south. At 0000 UTC on 17 August (Figure 10b), the inverted nonlinear balanced geopotential height and wind fields at 850 hpa associated with latent heat release (ms) showed features similar to those at 0000 UTC on 16 August and also displayed the maximum westerly south southeast of the TC center. Since the PV inversion was based on the nonlinear balanced dynamics, it is interesting to examine how the balanced and unbalanced dynamics contributed to the observed circulation. Figure 11 shows the nonlinear balanced geopotential height and wind fields at 850 hpa inverted from the total PV ~ q under the framework of nonlinear balanced dynamics of Charney [1955] at 0000 UTC on 16 August and 0000 UTC on 17 August, respectively, together with the associated unbalanced flow and its divergence fields. Here the unbalanced flow was simply obtained by subtracting the balanced flow from the ERA-Interim reanalysis wind field. At both 0000 UTC on 16 August and 0000 UTC on 17 August, the inverted nonlinear balanced geopotential height at 850 hpa showed a close resemblance to that in the ERA-Interim reanalysis (not shown), with its associated winds well presenting the storm-scale circulation of Typhoon Utor. However, because of the nonlinear balance constraint, the inverted TC circulation was a little bit weaker than that in the ERA-Interim reanalysis. This may be explained by the nonlinear balanced assumption which retained only the rotational component of the flow in the piecewise PV inversion technique. Similar result was also found in previous studies on Mei-yu front rainstorm in the subtropical environment [e.g., Chen et al., 2008]. Nevertheless, the similarity between the inverted results and the reanalysis strongly suggests that the nonlinear balanced assumption together with the piecewise PV inversion can capture the major dynamical processes reasonably well. The unbalanced (or divergent) flow presents a convergent flow with its maximum lay east to the TC center at both given times. The unbalanced convergent flow was closely related to diabatic heating in the inner core region of the TC and collocated with the MCSs discussed earlier. Note that the unbalanced flow became considerably weaker in the TC circulation on 17 August (Figure 11d) than that on 16 August. We consider that the resolution might not be high enough to resolve precipitation-related unbalanced flow in some given times. Nevertheless, this strongly suggests that the unbalanced flow contributed to the heavy rainfall in the MCSs and in turn it was strengthened/maintained by latent heat released in convection in the MCSs. This positive feedback was a key to the maintenance of the circulation of Typhoon Utor after landfall and the associated heavy rainfall. 5. Summary In this part, we first analyzed the evolution of monsoon variability and the episodes of MCSs during the postlandfall period of Typhoon Utor (2013). We then conducted piecewise PV inversion analysis to investigate the mechanisms that contributed to the maintenance of the postlandfall TC circulation and the associated MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,816

15 Figure 11. The nonlinear balanced geopotential height (contours, gpm) and horizontal wind (vector with magnitude shaded, m s 1 ) at 850 hpa inverted from the total perturbation PV at (a) 0000 UTC on 16 August and (b) 0000 UTC on 17 August 2013, with the corresponding unbalanced wind (vectors with magnitude shaded, m s 1 )defined as the difference in winds between the original ERA-Interim reanalysis winds and the inverted balanced winds, and the associated divergence (contours, 10 5 s 1 ) at 850 hpa at (c) 0000 UTC on 16 August and (d) 0000 UTC on 17 August sustained heavy rainfall over south China. The analyses focused on the influence of the East Asian summer monsoon flow and the interaction between the embedded MCSs and the TC. The intraseasonal variability of the southwesterly monsoon flow was examined to understand the monsoonal influence on Typhoon Utor after landfall. The filtered wind field indicates that Utor underwent a coalescence process with the cyclonic gyre associated with the QBWO after it made landfall over southern South China. The QBWO cyclonic gyre, which better coalesced with the TC vortex, enhanced the large-scale southwesterly monsoon flow to the southeast of the TC, providing favorable environmental condition for convection to develop and organize into MCSs within the outer rainband to the east of the TC. Utor was located in the western part of the MJO cyclonic gyre, and the predominant northerly flow steered Utor to slow down its northward movement and then to turn southwestward. In sharp contrast to the cyclonic QBWO gyre, the cyclonic MJO gyre showed little effect on the enhancement of the southwesterly monsoon flow south of the TC center while it contributed significantly to the track of Utor. Note that because of the coalescence of the TC circulation with the QBWO cyclonic gyre in the dissipating stage of Utor, the actual role of Utor in the inland heavy rainfall is hard to separate from the QBWO. Time-longitude cross sections of the satellite cloud top temperature revealed that after Utor made landfall, convection to the east of the TC center was more active than that to the west. Beginning from 1200 UTC on 15 August, three convective periods (episodes) with cloud top temperature below 72 C were identified in the eastern outer rainband of the TC: two occurred at the nighttime on both 15 and 16 August, and one MENG AND WANG HEAVY RAINFALL INDUCED BY TYPHOON UTOR 12,817