The Impact of Typhoon Danas ( 2013 ) on the Torrential Rainfall Associated with Typhoon Fitow ( 2013 ) in East China

When typhoon Danas (2013) was located at northeast of Taiwan during 6–8 October 2013, a torrential rainfall brought by typhoon Fitow (2013) occurred over the east of China. Observations show that the rainband of Fitow, which may be impacted by Danas, caused the rainfall over north of Zhejiang. 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.


Introduction
Extreme rainfall is responsible for a variety of societal impacts, including flash flooding that can lead to damage, injury, and fatalities [1].Tropical cyclones (TCs) are often heavy rain producers [2].Thus, it is of great interest to accurately predict extreme rainfall caused by TCs.However, heavy rainfall (including TCs rainfall) interweaves multiscale nonlinear interactions among different physical processes and weather systems [3][4][5].Such interactions include environmental moisture transport and binary TC (BTC) interaction [6].
Binary tropical cyclones (BTCs) can interact with each other when they are close enough [7][8][9][10].Their interaction depends on the distance of two TCs, the differences in TC size, intensity, and the variations in the currents [11].Based on the results of numerical experiments, Shin et al. [10] suggested that the critical separation distance of binary vortices is slightly less than twice the radius at which the relative vorticity of one vortex becomes zero.Concerning the observations of binary tropical cyclones and realistic flow patterns surrounding tropical cyclones, studies by Carr III et al. [12] and Carr III and Elsberry [13] proposed four conceptual models of track-altering binary tropical cyclones that occurred in the Pacific Ocean.
If there is another typhoon near the area of disaster, the effects of binary tropical cyclone interaction make the process of precipitation become much more complicated.Studies [14,15] showed that BTC processes may associate with torrential rainfall in favorable environment conditions.Wu et al. [14] found that tropical Storm Paul (1999) plays an important role in impeding the movement of Rachel, thus becoming one of the key factors in enhancing the rainfall amount in southern Taiwan.Xu et al. [15] found that Goni (2009, 08 W) transported a large amount of moisture and energy into Morakot (2009, 09 W).The interaction between Goni and Morakot accounts for about 30% of precipitation over Taiwan.
In this study, we discussed the role of the circulation associated with Danas (2013) played in the extreme rainfall caused by Fitow (2013).Specifically, the purpose of this paper is to quantify the distant effects of typhoon Danas on the extreme rainfall brought by rainband of Fitow in the east of China on 8-10 October 2013.In Section 2, we described the   model configuration and design of numerical experiments used in this study.We presented the results of numerical simulations in Section 3. Finally we summarize conclusions in Section 4.

Overview of 8-10 October 2013 Torrential Rainfall
Figure 1 shows 500 hPa geopotential height and wind from the National Centers for Environmental Prediction (NCEP) FNL analysis.It shows a South Asian anticyclone over the Tibetan Plateau in southwestern China, and a west wind trough extended from north of China to Sichuan province (Figure 1(a)).In the mid-latitudes over the Japan Sea, there was a subtropical anticyclone.During the typhoon Fitow landfall over east of China (Figures 1(b), 1(c), and 1(d)), subtropical anticyclone moved to east.Warm and moisture of typhoon interacted with cold air after the westerly trough.This condition was favorable for the development of convection and result of rainfall.

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Typhoon Fitow hit north of Fujian province during 6-8 October 2013 and produced extreme rainfall and brought about catastrophic flash flooding to Zhejiang province.The observed 48 h rainfall is shown in Figure 2. The extreme rainfall areas are mainly located in the coast and north of Zhejiang province.The exceptional rainfall with a record amount of 700 mm (northeast of Zhejiang) exceeded the 60year recurrence.The southeast coast of mainland China experiences several hits of landfalling typhoons every year [15].However, the amount of rainfall over Zhejiang (especially, north of Zhejiang) brought by Fitow is quite rare.Thus, it is of great interest to explore the possible mechanism responsible for the unusual heavy rainfall.
Radar mosaic reflectivity (Figure 3) shows a quasistationary rainband, which was nearly in east-west direction over 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 the north rainband, there was another rainband occurring over the south of Zhejiang province.The inner echo band was also composed of a lot of echo centers.Inside the two radar echo bands, there were a few echo blocks extending to the eyewall along the radial, which corresponded to the connecting spiral rainband.
Studies have shown that rainbands of a TC moving slowly outward along the radial [16][17][18] may remain stationary with new cells forming on the upwind edge [19,20].This process also occurred to the rainbands of typhoon Fitow (Figure 3).The north rainband remained stationary over Zhejiang province.However, the south rainband moved further to the south when typhoon Fitow was to the southwest.The two rainbands were maintained by a different source of moisture.The north rainband was sustained by the east flow for transporting warm and moisture water to the region, with new convective cells repeat initiated in the upwind of rainband.However, the south rainband of Fitow was maintained by both east flow and typhoon circulation associated with typhoon Danas.The north quasistationary rainbands began to decay, as typhoon Danas was moving further to the southwest.
The rainfall brought by typhoon Fitow is similar to the case in typhoon Wipha (2007, 13 W) [21,22], but with larger amount and more severe disaster.Besides the favorable environments similar to Wipha, what other factor contributed to the extreme rainfall caused by Fitow?

Model Description and Experimental Design
The WRF model version 3.4.1 [23] was utilized here at convection-permitting resolutions to simulate the extreme rainfall event.The model is initiated at 1800 UTC 5 October 2013, but the domain D03 is activated at 6 h later.Figure 4 shows the domain and topography for two experiments.The model horizontal spacing is 30 km, 10 km, and 3.3 km for d01, d02, and d03.Sizes of model grids are 375 × 246, 238 × 160, and 394 × 286, respectively.30 sigma levels were defined with the model top at 100 hPa.The model physical options include the Thompson microphysics scheme [24], the YSU planetary boundary layer scheme [25], the Kain-Fritsch cumulus parameterization scheme [26,27], the Noah land surface model [28,29], the rapid radiative transfer model [30] longwave, and the Dudhia shortwave radiation scheme [31].The cumulus parameterization scheme was not applied to the finest (3.3 km) grid domain to explicitly resolve the convective rainfall.
Control (CTRL) and sensitive experiments (No Danas) were performed to investigate the impact of Danas on the extreme rainfall.In the control experiment, the initialized condition was from NCEP-NCAR reanalysis data (1 ∘ × 1 ∘ ) [32], while, in the sensitive experiment, the Danas vortex in the reanalysis data is removed.The method to remove a vortex in the analysis field is the tropical cyclone (TC) bogussing scheme in the ARW-WRF [23].The scheme can remove an existing tropical storm and was used to remove typhoon Danas vortex in No Danas experiment.

Result and Discussion
We first examine the structure of the rainband and its rainfall in CTRL experiment.Figure 5 presents the simulated radar reflectivity (DBZ) from CTRL experiment.In the CTRL experiment, there are two rainbands over the north and south of Zhejiang province, respectively, in good agreement with observation (Figure 5(a)).The north rainband extended outward of typhoon.New convective cells repeat initiated at upwind and move along rainband (Figures 5(b) and 5(c)).However, the simulated north rainband develops west of the location and more intense in northwest of Zhejiang province of the observed rainband (Figures 5 and 4).
The simulated 48 h accumulated rainfall of CTRL is shown in Figure 7(a).The distribution and intensity of simulated rainfall is nearly the same as what was observed (Figures 7(a) and 2).The maximum rainfall in CTRL experiment is 550 mm, compared with 570 mm that was observed.There are broad regions of the north and coast of Zhejiang exceeding 300 mm.The horizontal distribution of simulated 48 h accumulated precipitation is also similar to that of the observation, but the location is incorrect to the southwest.In good agreement with the simulated distribution of the rainbands, there is a westward and southward displacement of the simulated precipitation and overestimate over northwest of Zhejiang province.Figure 11 presents time series of hourly area averaged rainfall, from the CMORPH-Gauge merged data and the CTRL and No Danas experiments.Compared with observation, the CTRL experiment underpredicted the total rainfall in the north Zhejiang province; the underprediction occurred mainly during 1500 UTC 6 October-0000 UTC 8 October 2013, namely, the time of the heaviest rainfall.In spite of the underestimation of the peak rainfall 6 Advances in Meteorology intensity of Fitow, the simulated in CTRL experiment show the reasonable evolution and distribution of rainfall.
Figure 6 shows the simulated track of CTRL, during 0000 UTC 6 October-0000 UTC 7 October 2013, China Meteorological Administration-Shanghai Typhoon Institute (CMA-SH).The model simulated the intensity of typhoon Fitow reasonably well at the first 12 h of integration but considerably underpredicted the weakening rate of typhoon Fitow (Figure 6(a)).On the other hand, the simulated track of Saomai is nearly the same as the observed track  over the ocean.After Fitow landfall, there is a westward and southward displacement of the simulated precipitation (Figure 6(b)).
Figure 8 compares CTRL and No Danas simulated moisture flux (vector) and specific humidity (shaded, unit: g/Kg) 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 bridge Fitow and Danas, which result in more moisture brought to north of Zhejiang (Figure 8(a)).On the other hand, the No Danas experiment shows only two narrow moisture bands connecting to typhoon Fitow in the south of Zhejiang province (Figure 8(b)), and the high values (>14 g/Kg) of moisture over Zhejiang province are confined mainly around typhoon Fitow, and there is not a band of high moisture (>14 g/Kg) over the north of Zhejiang.The results indicate that one of the significant differences between the two experiments was the moisture transported by typhoon Advances in Meteorology  Danas.Previous research has shown that low-level moisture from the ocean can produce more rainfall [27,33].
Prior to interaction between binary typhoon Danas and Fitow, the rainbands distribution in the two experiments was nearly identical (Figures 5(a) and 9(a)).This agreement suggests that removal of typhoon Danas had little impact on formation of north-rainband farther west previous to the arrival of the typhoon Danas-related moisture [34].As in the CTRL experiment, two rainbands appear over the north and south of Zhejiang province about 1900 UTC 06 October.However, the north rainband disappeared and is displaced with little scattered convective cells, and its convection has little area of stratiform rainfall brought with it.During 1900 UTC 06 to 0100 UTC 07, the differences between the two simulations became even better defined, as convective cells repeated initiation on upwind and move alone the rainband in the CTRL experiment, but this process did not happen in No Danas.Accordingly, by 0100 UTC 07 (Figures 5(d) and 9(d)), there is still a rainband over the north of Zhejiang province in the CTRL simulation, while it disappeared in No Danas.As a result, the extreme rainfall of the north of Zhejiang occurs in the CTRL experiment, but not in No Danas.
Much of the reason for these differences in the maintenance of the north rainband can be attributed to the eastward transport of moisture associated with Danas. Figure 10 shows cross sections along A1-A2 (Figures 10(a) and 1(c)) and B1-B2 (Figures 10(b) and 10(d)) at 0000 UTC 7 October 2013.As can be seen, there are convective cells lining from northwest to southeast along the north rainband.The radar reflectivity greater than 35 DBZ extended from surface to ∼8 Km.On the contrary, in the No Danas simulation, the development of convective cells ceases.This suggests that typhoon Danas provided extra moisture for rainband and allowed it to persist for a longer period.
The importance of the moisture becomes clear.In the CTRL simulation, there is a seemingly unlimited supply of moisture to the north rainband of Fitow, whereas the No Danas experiment has a less source of moisture.Consequently, new convection continues to initiate on the western side of rainband in the CTRL, as more moist air is transported into the area.As a result of the rainband moving slowly over the north of Zhejiang province, this allows more rainfall to accumulate at the north of Zhejiang province.On the other hand, once the instability from nearby sources is released in the No Danas experiment, the development of rainband ceases and weakens.These results indicate that the moist air from east coast of China played an important role in moisture convergence and the formation of north rainband regardless of whether Danas existed and that the Danas-related moisture provided an additional source of fuel for this rainband and allowed the convection along the rainband to persist for a longer period.
In summary, the No Danas experiment shows that the extra moisture associated with typhoon Danas caused about 2 times of the maximum precipitation from the outer rainbands of typhoon Fitow over the north of Zhejiang province-the maximum 48 h total in the No Danas run was ∼220 mm compared with more than 500 mm in the CTRL experiment and observation (Figures 1, 3, and 10).This indicates about 55% reduction in total precipitation from the CTRL to No Danas.Namely, extra moisture from the typhoon Danas led to more than 55% enhancement of the total rainfall over the north of Zhejiang province.Thus, the interaction between typhoon Fitow and Danas during 1900 UTC 06 and 0400 UTC 07 October (Figure 11) Advances in Meteorology    transported extra moisture to the north of Zhejiang province and resulted in the maintenance of north rainband of typhoon Fitow.Consequently, this process made the greatest difference in the precipitation happen at the north of Zhejiang province.

Conclusion
The ARW-WRF model is used to investigate the impact of typhoon Danas (2013) over the western North Pacific on torrential rainfall produced by typhoon Fitow (2013) over east of China.In this case, when typhoon Danas was located in northeast of Taiwan, torrential precipitation occurred far to the west over Zhejiang province of east China and its coastal area.In CTRL simulation, the model reasonably reproduced the major features of typhoon Fitow rainband and the torrential rainfall over Zhejiang province.To investigate the contribution of typhoon Danas to the rainfall in Zhejiang province, a sensitive experiment (No Danas) was performed in which the typhoon Danas vortex was removed.As a result of absence of typhoon Danas, rainfall over the north of Zhejiang associated with north rainband of typhoon Fitow rapidly disappeared, indicating that typhoon Danas played an important role in the rainfall produced by typhoon Fitow for the case studied.
The major process involved in the effects of typhoon Danas is through the enhanced westward moisture transport into the north rainband of typhoon Fitow.As a result, typhoon Danas played an important role in the torrential rainfall in the north of Zhejiang province, although the typhoon Danas was more than 1000 km to the east over northeast of Taiwan.

Figure 6 :
Figure 6: (a) Minimum SLP (central sea level pressure (hPa)) and (b) the track of typhoon Fitow from CMA best track data and the WRF model simulation in control experiment.

Figure 10 :
Figure 10: Cross sections along A1-A2 of vertical motion (contour, m s −1 ) 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.