Hindawi Publishing Corporation Advances in Meteorology Volume 2015, Article ID 383712, 11 pages http://dx.doi.org/10.1155/2015/383712

Research Article The Impact of Danas (2013) on the Torrential Rainfall Associated with (2013) in East

Hongxiong Xu1 and Bo Du2

1 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China 2ChinaMeteorologicalAdministrationMeteorologicalObservationCenter,Beijing100081,China

Correspondence should be addressed to Hongxiong Xu; [email protected]

Received 30 September 2014; Revised 20 January 2015; Accepted 20 January 2015

Academic Editor: Hann-Ming H. Juang

Copyright © 2015 H. Xu and B. Du. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

When typhoon Danas (2013) was located at northeast of during 6–8 October 2013, a torrential rainfall brought by typhoon Fitow (2013) occurred over the east of China. Observations show that the of Fitow, which may be impacted by Danas, caused the rainfall over north of . The Advanced Research version of the Weather Research and Forecast (ARW-WRF) model was used to investigate the possible effects of typhoon Danas (2013) on this rainfall event. Results show that the model captured reasonably well the spatial distribution and evolution of the rainband of Fitow. The results of a sensitivity experiment removing Danas vortex, which is conducted to determine its impact on the extreme rainfall, show that extra moist associated with Danas plays an important role in the maintenance and enhancement of the north rainband of Fitow, which resulted in torrential rainfall over the north of Zhejiang. This study may explain the unusual amount of rainfall over the north of Zhejiang province caused by interaction between the rainband of typhoon Fitow and extra moisture brought by typhoon Danas.

1. Introduction III et al. [12] and Carr III and Elsberry [13]proposedfour conceptual models of track-altering binary tropical cyclones Extreme rainfall is responsible for a variety of societal that occurred in the Pacific Ocean. impacts, including flash flooding that can lead to damage, If there is another typhoon near the area of disaster, injury, and fatalities [1]. Tropical cyclones (TCs) are often the effects of binary interaction make the heavy rain producers [2]. Thus, it is of great interest to process of precipitation become much more complicated. accurately predict extreme rainfall caused by TCs. However, Studies [14, 15] showed that BTC processes may associate heavy rainfall (including TCs rainfall) interweaves multiscale with torrential rainfall in favorable environment conditions. nonlinear interactions among different physical processes Wu et al. [14] found that tropical Storm Paul (1999) plays an and weather systems [3–5]. Such interactions include envi- important role in impeding the movement of Rachel, thus ronmental moisture transport and binary TC (BTC) interac- becoming one of the key factors in enhancing the rainfall tion [6]. amount in southern Taiwan. Xu et al. [15]foundthatGoni Binary tropical cyclones (BTCs) can interact with each (2009, 08 W) transported a large amount of moisture and other when they are close enough [7–10]. Their interaction energy into Morakot (2009, 09 W). The interaction between depends on the distance of two TCs, the differences in Goni and Morakot accounts for about 30% of precipitation TC size, intensity, and the variations in the currents [11]. over Taiwan. Based on the results of numerical experiments, Shin et al. In this study, we discussed the role of the circulation [10] suggested that the critical separation distance of binary associated with Danas (2013) played in the extreme rainfall vortices is slightly less than twice the radius at which the caused by Fitow (2013). Specifically, the purpose of this paper relative vorticity of one vortex becomes zero. Concerning is to quantify the distant effects of typhoon Danas on the theobservationsofbinarytropicalcyclonesandrealistic extreme rainfall brought by rainband of Fitow in the east of flow patterns surrounding tropical cyclones, studies by Carr China on 8–10 October 2013. In Section 2, we described the 2 Advances in Meteorology

584 586 586 574 574 588 ∘ 578 590 ∘ 578 590 40 N 592 40 N 582 592 584 582 594 582 584 586 ∘ ∘ 30 N 586 30 N 584 584 584582 586 586 578 584 578 586584 582

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Figure 1: 500 hPa geopotential height (contour, unit: dagpm) and wind (vector) at (a) 0000 UTC 06 October 2013, (b) 1200 UTC 06 October 2013, (c) 0000 UTC 07 October 2013, and (d) 1200 UTC 07 October 2013.

∘ 34 N

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Figure 2: The observed 48 h accumulated rainfall (unit: mm) ending at 0000 UTC 08 October 2013. Advances in Meteorology 3

∘ ∘ 32 N 32 N

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∘ ∘ 26 N 26 N ∘ ∘ ∘ ∘ ∘ ∘ 118 E 120 E 122 E 118 E 120 E 122 E (a) 16 UTC 06 (b) 19 UTC 06

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Figure 3: Radar mosaic reflectivity (DBZ) at (a) 16 UTC 06, (b) 19 UTC 06, (c) 22 UTC 06, and (d) 01 UTC07.

model configuration and design of numerical experiments FNL analysis. It shows a South Asian anticyclone over the used in this study. We presented the results of numerical Tibetan Plateau in southwestern China, and a west wind simulations in Section 3. Finally we summarize conclusions trough extended from north of China to Sichuan province in Section 4. (Figure 1(a)). In the mid-latitudes over the Sea, there was a subtropical anticyclone. During the typhoon Fitow 2. Overview of 8–10 October 2013 over east of China (Figures 1(b), 1(c),and1(d)), Torrential Rainfall subtropical anticyclone moved to east. Warm and moisture of typhoon interacted with cold air after the westerly trough. Figure 1 shows 500 hPa geopotential height and wind from This condition was favorable for the development of convec- the National Centers for Environmental Prediction (NCEP) tion and result of rainfall. 4 Advances in Meteorology

∘ ∘ ∘ ∘ ∘ ∘ Typhoon Fitow hit north of province during 6– 75 E 90 E 105 E 120 E 135 E 150 E ∘ 8 October 2013 and produced extreme rainfall and brought D01 40 N about catastrophic flash flooding to Zhejiang province. The ∘ observed 48 h rainfall is shown in Figure 2. The extreme 40 N rainfall areas are mainly located in the coast and north of ∘ 30 N Zhejiang province. The exceptional rainfall with a record D02 ∘ 03 amount of 700 mm (northeast of Zhejiang) exceeded the 60- 30 N D year recurrence. The southeast coast of expe- ∘ 20 N riences several hits of landfalling every year [15]. 20∘ However, the amount of rainfall over Zhejiang (especially, N north of Zhejiang) brought by Fitow is quite rare. Thus, it is of 10∘ great interest to explore the possible mechanism responsible N 10∘ for the unusual heavy rainfall. N Radar mosaic reflectivity (Figure 3) shows a quasistation- ∘ ∘ ∘ ∘ ∘ ary rainband, which was nearly in east-west direction over 90 E 100 E 110 E 120 E 130 E the north of Zhejiang province. Along the band, there were several echo centers of 45–55 dBz embedded in line, which corresponded to the north rainband of typhoon Fitow. Inside 500 1000 2000 3000 4000 5000 the north rainband, there was another rainband occurring 1500 2500 3500 4500 over the south of Zhejiang province. The inner echo band Figure 4: Topography (color-shaded, m) for the model domain. The wasalsocomposedofalotofechocenters.Insidethetwo outer box is d01 (30 km). The inner boxes are d02 (10 km) and d03 radar echo bands, there were a few echo blocks extending (3.3 km). to the eyewall along the radial, which corresponded to the connecting spiral rainband. Studies have shown that of a TC moving slowly The model physical options include the Thompson micro- outward along the radial [16–18] may remain stationary physics scheme [24], the YSU planetary boundary layer with new cells forming on the upwind edge [19, 20]. This scheme [25], the Kain-Fritsch cumulus parameterization process also occurred to the rainbands of typhoon Fitow scheme [26, 27], the Noah land surface model [28, 29], the (Figure 3). The north rainband remained stationary over rapid radiative transfer model [30]longwave,andtheDudhia Zhejiang province. However, the south rainband moved shortwave radiation scheme [31]. The cumulus parameteri- further to the south when typhoon Fitow was to the south- zationschemewasnotappliedtothefinest(3.3km)grid west. The two rainbands were maintained by a different domain to explicitly resolve the convective rainfall. source of moisture. The north rainband was sustained by Control (CTRL) and sensitive experiments (No Danas) the east flow for transporting warm and moisture water to were performed to investigate the impact of Danas on the the region, with new convective cells repeat initiated in the extreme rainfall. In the control experiment, the initialized ∘× ∘ upwind of rainband. However, the south rainband of Fitow condition was from NCEP-NCAR reanalysis data (1 1 ) was maintained by both east flow and typhoon circulation [32], while, in the sensitive experiment, the Danas vortex associated with typhoon Danas. The north quasistationary in the reanalysis data is removed. The method to remove rainbands began to decay, as typhoon Danas was moving a vortex in the analysis field is the tropical cyclone (TC) further to the southwest. bogussing scheme in the ARW-WRF [23]. The scheme can remove an existing tropical storm and was used to remove The rainfall brought by typhoon Fitow is similar to the typhoon Danas vortex in No Danas experiment. case in typhoon Wipha (2007, 13 W) [21, 22], but with larger amount and more severe disaster. Besides the favorable envi- ronments similar to Wipha, what other factor contributed to 4. Result and Discussion the extreme rainfall caused by Fitow? We first examine the structure of the rainband and its rainfall in CTRL experiment. Figure 5 presents the simulated radar 3. Model Description and Experimental Design reflectivity (DBZ) from CTRL experiment. In the CTRL experiment, there are two rainbands over the north and The WRF model version 3.4.1 [23]wasutilizedhereat south of Zhejiang province, respectively, in good agreement convection-permitting resolutions to simulate the extreme with observation (Figure 5(a)). The north rainband extended rainfall event. The model is initiated at 1800 UTC 5 October outward of typhoon. New convective cells repeat initiated at 2013,butthedomainD03isactivatedat6hlater.Figure4 upwind and move along rainband (Figures 5(b) and 5(c)). shows the domain and topography for two experiments. The However, the simulated north rainband develops west of the model horizontal spacing is 30 km, 10 km, and 3.3 km for d01, location and more intense in northwest of Zhejiang province d02, and d03. Sizes of model grids are 375 × 246, 238 × 160, of the observed rainband (Figures 5 and 4). and 394 × 286, respectively. 30 sigma levels were defined with The simulated 48 h accumulated rainfall of CTRL is the model top at 100 hPa. shown in Figure 7(a). The distribution and intensity of Advances in Meteorology 5

∘ (DBZ) ∘ (DBZ) 32 N 32 N

∘ ∘ 30 N 30 N

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∘ ∘ 26 N 26 N ∘ ∘ ∘ ∘ ∘ ∘ 118 E 120 E 122 E 118 E 120 E 122 E (a) 16 UTC 06 (b) 19 UTC 06

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10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80

(c) 22 UTC 06 (d) 01 UTC 07

Figure 5: Simulated radar reflectivity (DBZ) from CTRL experiment at (a) 16 UTC 06, (b) 19 UTC 06, (c) 22 UTC 06, and (d) 01 UTC07 October 2013. simulated rainfall is nearly the same as what was observed simulated precipitation and overestimate over northwest of (Figures 7(a) and 2). The maximum rainfall in CTRL experi- Zhejiang province. Figure 11 presents time series of hourly ment is 550 mm, compared with 570 mm that was observed. area averaged rainfall, from the CMORPH-Gauge merged There are broad regions of the north and coast of Zhejiang data and the CTRL and No Danas experiments. Compared exceeding 300 mm. The horizontal distribution of simulated with observation, the CTRL experiment underpredicted the 48h accumulated precipitation is also similar to that of the total rainfall in the north Zhejiang province; the under- observation, but the location is incorrect to the southwest. In prediction occurred mainly during 1500 UTC 6 October– good agreement with the simulated distribution of the rain- 0000 UTC 8 October 2013, namely, the time of the heaviest bands, there is a westward and southward displacement of the rainfall. In spite of the underestimation of the peak rainfall 6 Advances in Meteorology

∘ ∘ 30 N 34 N

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940 Figure 7: The simulated 48 h accumulated rainfall (unit: mm) end- ing at 0000 UTC 08 October 2013 from (a) CTRL and (b) No Danas. 930 0 3 6 9 12 15 18 21 24 Fcst (h)

CMA over the ocean. After Fitow landfall, there is a westward CTRL and southward displacement of the simulated precipitation (b) SLP of Fitow (Figure 6(b)). Figure 8 compares CTRL and No Danas simulated mois- Figure 6: (a) Minimum SLP (central sea level pressure (hPa)) and ture flux (vector) and specific humidity (shaded, unit: g/Kg) (b) the track of typhoon Fitow from CMA best track data and the WRF model simulation in control experiment. at 0100 UTC 7 October 2013. In the two experiments, high values of moisture over east of China are brought westward by the strong southeast flow (Figure 8). In the CTRL experiment, the Danas-related moisture exhibited spiral band-shaped intensity of Fitow, the simulated in CTRL experiment show bridge Fitow and Danas, which result in more moisture thereasonableevolutionanddistributionofrainfall. brought to north of Zhejiang (Figure 8(a)). On the other Figure 6 shows the simulated track of CTRL, during hand, the No Danas experiment shows only two narrow 0000 UTC 6 October-0000 UTC 7 October 2013, China moisture bands connecting to typhoon Fitow in the south Meteorological Administration- Typhoon Institute of Zhejiang province (Figure 8(b)), and the high values (CMA-SH). The model simulated the intensity of typhoon (>14 g/Kg) of moisture over Zhejiang province are confined Fitow reasonably well at the first 12 h of integration but mainly around typhoon Fitow, and there is not a band of high considerably underpredicted the weakening rate of typhoon moisture (>14 g/Kg) over the north of Zhejiang. The results Fitow (Figure 6(a)). On the other hand, the simulated indicate that one of the significant differences between the track of Saomai is nearly the same as the observed track two experiments was the moisture transported by typhoon Advances in Meteorology 7

∘ ∘ 34 N 34 N

∘ ∘ 30 N 30 N

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300 300

12 13 14 15 12 13 14 15

(a) CTRL (b) No Danas

Figure 8: Moisture flux (vector) and specific humidity (shaded, unit: g/Kg) at 0100 UTC 07 October 2013 from (a) CTRL and (b)No Danas.

Danas. Previous research has shown that low-level moisture provided extra moisture for rainband and allowed it to persist from the ocean can produce more rainfall [27, 33]. for a longer period. Prior to interaction between binary typhoon Danas and The importance of the moisture becomes clear. In the Fitow, the rainbands distribution in the two experiments CTRL simulation, there is a seemingly unlimited supply wasnearlyidentical(Figures5(a) and 9(a)). This agreement of moisture to the north rainband of Fitow, whereas the suggests that removal of typhoon Danas had little impact No Danas experiment has a less source of moisture. Conse- on formation of north-rainband farther west previous to the quently, new convection continues to initiate on the western arrival of the typhoon Danas-related moisture [34]. As in side of rainband in the CTRL, as more moist air is transported the CTRL experiment, two rainbands appear over the north into the area. As a result of the rainband moving slowly over and south of Zhejiang province about 1900 UTC 06 October. the north of Zhejiang province, this allows more rainfall to However, the north rainband disappeared and is displaced accumulate at the north of Zhejiang province. On the other with little scattered convective cells, and its convection has hand, once the instability from nearby sources is released little area of stratiform rainfall brought with it. During 1900 in the No Danas experiment, the development of rainband UTC 06 to 0100 UTC 07, the differences between the two ceases and weakens. These results indicate that the moist air simulations became even better defined, as convective cells fromeastcoastofChinaplayedanimportantroleinmoisture repeatedinitiationonupwindandmovealonetherainband convergence and the formation of north rainband regardless in the CTRL experiment, but this process did not happen in of whether Danas existed and that the Danas-related moisture No Danas. Accordingly, by 0100 UTC 07 (Figures 5(d) and provided an additional source of fuel for this rainband and 9(d)), there is still a rainband over the north of Zhejiang allowed the convection along the rainband to persist for a province in the CTRL simulation, while it disappeared in longer period. No Danas. As a result, the extreme rainfall of the north In summary, the No Danas experiment shows that the of Zhejiang occurs in the CTRL experiment, but not in extra moisture associated with typhoon Danas caused about No Danas. 2 times of the maximum precipitation from the outer Much of the reason for these differences in the mainte- rainbands of typhoon Fitow over the north of Zhejiang nance of the north rainband can be attributed to the eastward province—the maximum 48 h total in the No Danas run transport of moisture associated with Danas. Figure 10 shows was ∼220 mm compared with more than 500 mm in the cross sections along A1-A2 (Figures 10(a) and 1(c))andB1- CTRL experiment and observation (Figures 1, 3,and10). B2 (Figures 10(b) and 10(d)) at 0000 UTC 7 October 2013. As This indicates about 55% reduction in total precipitation canbeseen,thereareconvectivecellsliningfromnorthwest fromtheCTRLtoNoDanas. Namely, extra moisture from to southeast along the north rainband. The radar reflectivity the typhoon Danas led to more than 55% enhancement greater than 35 DBZ extended from surface to ∼8Km.On of the total rainfall over the north of Zhejiang province. the contrary, in the No Danas simulation, the development Thus, the interaction between typhoon Fitow and Danas of convective cells ceases. This suggests that typhoon Danas during 1900 UTC 06 and 0400 UTC 07 October (Figure 11) 8 Advances in Meteorology

∘ (DBZ) ∘ (DBZ) 32 N 32 N

∘ ∘ 30 N 30 N

∘ ∘ 28 N 28 N

∘ ∘ 26 N 26 N ∘ ∘ ∘ ∘ ∘ ∘ 118 E 120 E 122 E 118 E 120 E 122 E (a) 16 UTC 06 (b) 19 UTC 06

∘ (DBZ) ∘ (DBZ) 32 N 32 N

∘ ∘ 30 N 30 N

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10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80

(c) 22 UTC 06 (d) 01 UTC 07

Figure 9: Simulated radar reflectivity (DBZ) from No Danas at (a) 16 UTC 06, (b) 19 UTC 06, (c) 22 UTC 06, and (d) 01 UTC 07 October 2013. Advances in Meteorology 9

.5 11.0 0 1 0 11.0 10.0 0 0 0 10.0 9.0 0 9.0 0 0 8.0 1 3 8.0 0 1 7.0 1 1 0 2 7.0 0 0 0 .5 0 0 0 2 .5 6.0 0 0 6.0 0 0 0 1 0 0 1 0 0 5.0 0 0 5.0 0 2 0 2 1 0 1 0 Height (km) Height Height (km) Height 2 0 4.0 0 1 4.0 0 1 3 0 0 0 3.0 0 3 0 3.0 0 1 4 1 0 2.0 1 2 2.0 0 2 0 0 1.0 0 0 1.0 0 0 0 0 0.0 0.0 119.7N 120.3N 120.8N 121.4N 122.0N 119.7N 120.3N 120.8N 121.4N 122.0N z-wind component contours: −1.5 to 6 by .5 z-wind component contours: −1.25 to 2.5 by .25 (a) (b)

11.0 11.0 10.0 10.0 −.8 −0 −.1 9.0 9.0 .1 8 .1 . 2 .2 −.0 8.0 −0 8.0 . 1 −0 2.4 .2 . .1 7.0 7.0 2 .8 1.6 4 −0 1 . 1.6 . 1 .2 6.0 6.0 . .2 8 3.2 . .2 5.0 −0 5.0 .1 −0 −0 1.6 3 Height (km) Height 2.4 (km) Height . 4.0 4.0 .1 .8 2.4 −.0 −.1 −.0 3.0 3.0 .1 −0 −.0 −.0 .1 −0 .1 2.0 .8 2.0 −.0 1.0 −0 1.0 −.0 0.0 0.0 120.3N 120.5N 120.7N 120.9N 121.2N 120.3N 120.5N 120.7N 120.9N 121.2N z-wind component contours: −1.2 to 4.8 by .4 z-wind component contours: −.3 to .35 by .05

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 Reflectivity (DBZ) Reflectivity (DBZ) (c) (d)

−1 Figure 10: Cross sections along A1-A2 of vertical motion (contour, m s ) and radar reflectivity (shaded, DBZ) at 0100 UTC 07 October 2013 from (a) CTRL and (b) No Danas; (c) and (d) are the same as (a) and (b) but for cross section along B1-B2.

transported extra moisture to the north of Zhejiang province torrential rainfall over Zhejiang province. To investigate the and resulted in the maintenance of north rainband of contribution of typhoon Danas to the rainfall in Zhejiang typhoon Fitow. Consequently, this process made the greatest province, a sensitive experiment (No Danas) was performed differenceintheprecipitationhappenatthenorthofZhejiang in which the typhoon Danas vortex was removed. As a result province. of absence of typhoon Danas, rainfall over the north of Zhejiang associated with north rainband of typhoon Fitow 5. Conclusion rapidly disappeared, indicating that typhoon Danas played an important role in the rainfall produced by typhoon Fitow for The ARW-WRF model is used to investigate the impact of the case studied. typhoon Danas (2013) over the western North Pacific on The major process involved in the effects of typhoon torrential rainfall produced by typhoon Fitow (2013) over east Danas is through the enhanced westward moisture transport of China. In this case, when typhoon Danas was located in into the north rainband of typhoon Fitow. As a result, northeast of Taiwan, torrential precipitation occurred far to typhoon Danas played an important role in the torrential thewestoverZhejiangprovinceofeastChinaanditscoastal rainfall in the north of Zhejiang province, although the area. In CTRL simulation, the model reasonably reproduced typhoonDanaswasmorethan1000kmtotheeastover the major features of typhoon Fitow rainband and the northeast of Taiwan. 10 Advances in Meteorology

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