Observed Rainfall Asymmetry in Tropical Cyclones Making Landfall Over China

JANUARY 2015 Y U E T A L . 117

Observed Rainfall Asymmetry in Tropical Cyclones Making Landfall over China

ZIFENG YU College of Atmospheric Sciences, and Paciﬁc Typhoon Research Center, Nanjing University of Information Science and Technology, Nanjing, and Shanghai Typhoon Institute, and Laboratory of Typhoon Forecast Technique, China Meteorological Administration, Shanghai, China

YUQING WANG Department of Meteorology, and International Paciﬁc Research Center, University of Hawai‘i at Manoa, Honolulu, Hawaii

HAIMING XU College of Atmospheric Sciences, and Paciﬁc Typhoon Research Center, Nanjing University of Information Science and Technology, Nanjing, China

(Manuscript received 25 November 2013, in ﬁnal form 7 August 2014)

ABSTRACT

In this study, the rainfall asymmetries in tropical cyclones (TCs) that made landfall in the Hainan (HN), Guangdong (GD), Fujian (FJ), and Zhejiang (ZJ) provinces of mainland China and Taiwan (TW) from 2001 to 2009 were analyzed on the basis of TRMM satellite 3B42 rainfall estimates. The results reveal that in landfalling TCs, the wavenumber 1 rainfall asymmetry shows the downshear to downshear-left maximum in environmental vertical wind shear (VWS), which is consistent with previous studies for TCs over the open oceans. A cyclonic rotation from south China to east China in the location of the rainfall maximum has been identiﬁed. Before landfall, the location of the rainfall maximum rotated from southwest to southeast of the TC center for TCs making landfall in the regions from HN to GD, TW, FJ, and ZJ. After landfall, the rotation became from southwest to northeast of the TC center from south China to east China. It is shown that this cyclonic rotation in the location of the rainfall maximum is well correlated with a cyclonic rotation from south China to east China in the environmental VWS between 200 and 850 hPa, indicating that the rainfall asymmetry in TCs that made landfall over China is predominantly controlled by the large-scale VWS. The cyclonic rotation of VWS is found to be related to different interactions between the midlatitude westerlies and the landfalling TCs in different regions. The results also indicate that the axisymmetric (wavenumber 0) component of rainfall generally decreased rapidly after landfall in most studied regions.

1. Introduction the 24-h rainfall of 1062 mm at Linzhuang in the Henan province of mainland China caused by landfalling Ty- Landfalling tropical cyclones (TCs) often bring very phoon Nina (7503) in 1975 (Chen et al. 2004). Torrential heavy rainfall to the affected region (Chen and Ding rainfall associated with landfalling TCs is one of the 1979). Many extreme rainfall events are related to most devastating natural disasters in the coastal regions landfalling TCs around the world (Tao 1980; Rappaport of China, inﬂicting huge losses in property and human 2000). Examples of the record-breaking heavy rainfall lives (Zhang et al. 2009). The regions that suffer most events include the 24-h rainfall of 1248 mm at Baixin in are largely determined by the distribution of rainfall in Taiwan caused by Typhoon Gloria (6312) in 1963 and TCs. Therefore, the spatial distribution of rainfall in a landfalling TC is of particular interest to meteorolo- Corresponding author address: Zifeng Yu, Shanghai Typhoon gists because of its relevance to the rainfall forecasts. Institute, 166 Puxi Road, Xujiahui, Shanghai 200030, China. The rainfall distribution in a TC can be decomposed E-mail: [email protected] into a wavenumber 0 (or an azimuthal mean) component

DOI: 10.1175/JAMC-D-13-0359.1

Ó 2015 American Meteorological Society Unauthenticated | Downloaded 10/05/21 07:42 AM UTC 118 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 54 and a series of lower-wavenumber components (Lonfat Chen et al. (2006) investigated the effects of VWS and et al. 2004). The distribution of rainfall in a landfalling TC storm motion on TC rainfall asymmetries based on is often controlled not only by storm motion and vertical composite analyses. They found that the overall TC wind shear (VWS) for TCs over oceans (Lonfat et al. rainfall asymmetry depended on the juxtaposition and 2004; Chen et al. 2006), but also by other factors such as relative magnitude of the storm motion and the envi- surface conditions (including terrains and coastlines) ronmental VWS over the ocean. Subsequent multicase (Bender et al. 1987; Chang et al. 1993; Yeh and Elsberry analyses of the shear-induced TC asymmetry using the 1993a,b; Linetal.1999, 2002, 2005, 2006; Wu and Kuo satellite-based precipitation estimates have further con- 1999; Wu 2001; Wu et al. 2002; Yu et al. 2010; Yu and Yu firmed the relationship between the spatial distribution of 2012). Therefore, although the rainfall in a landfalling TC TC rainfall and the environmental VWS (Ueno 2007; tends to be dominated by the wavenumber 0 component Wingo and Cecil 2010; Hence and Houze 2011), namely, in general, a significant spatial variability can be induced the downshear-left preference of the rainfall maximum. by various dynamical and thermodynamic processes. TC translation speed becomes an important factor in the Previous studies have shown that rainfall asymmetries low VWS environment. Recently, Reasor et al. (2013), in a TC are affected by the environmental VWS (Marks based on a multicase composite of the asymmetric TC 1985; Merrill 1988; Jones 1995, 2000a,b, 2004; Wang and structure derived from airborne Doppler radar mea- Holland 1996a; DeMaria 1996; Frank and Ritchie 1999, surements, reconfirmed that the impact of storm motion 2001), the planetary vorticity gradient (Peng and Williams on the eyewall convective asymmetry was secondary 1990; Wang and Holland 1996b,c; Bender 1997), the storm compared to the findings from previous studies using motion (e.g., Bender 1997; Peng et al. 1999), the friction- lightning and precipitation data. induced asymmetric boundary layer convergence in a Most of previous studies have focused on the rainfall moving TC (e.g., Shapiro 1983), the asymmetric environ- asymmetries in TCs over oceans. In a numerical study mental moisture distribution (Dunion and Velden 2004), under idealized conditions, Chan and Liang (2003) showed and also the convectively coupled vortex-Rossby waves in the precipitation maximum to the left-front quadrant of the eyewall (Chen and Yau 2001; Wang 2002a,b). a TC when it approached a coastline prior to its landfall. Lonfat et al. (2004) analyzed the global rainfall dis- Chan et al. (2004) investigated the asymmetric distribution tribution in TCs based on the satellite data deduced of convection in TCs making landfall in south China and from the Tropical Rainfall Measuring Mission (TRMM) showed that convection was generally enhanced to the rain estimates. Their results showed the relationships west of the TC center before landfall, namely to the left- between the TC intensity, the geographical location, and front of the storm, which was consistent with the mod- the rainfall asymmetry relative to the storm motion. eling results of Chan and Liang (2003). Shu et al. (2012) However, the storm motion alone could not fully explain investigated the spatial distribution of precipitation in the basin-to-basin variability in the rainfall asymmetry landfalling TCs affecting China using the Geostationary and thus other mechanisms, such as the environmental Meteorological Satellite-5 infrared brightness tempera- VWS, that affect the asymmetric rainfall distribution ture (GMS5-TBB) estimated-rainfall dataset during the have also been investigated intensively (e.g., Rogers period of 2001–09. Their results showed that the envi- et al. 2003; Chen et al. 2006; Lonfat et al. 2007). The ronmental VWS was an important factor contributing to rainfall asymmetry associated with VWS and storm the down-shear rainfall maximum. motion in Hurricane Bonnie (in 1998) was studied based The primary goal of this study is to diagnose the im- on a high-resolution numerical simulation by Rogers pacts of environmental VWS and storm motion on the et al. (2003). Their results suggested that the combination asymmetric rainfall distribution in TCs making landfall of shear and storm motion explained well the rainfall over south and east China through motion-relative and asymmetry in Hurricane Bonnie. The accumulated rain- shear-relative composite analysis of precipitation fields. fall was distributed symmetrically across the track of the We use 9-yr (2001–09) TRMM satellite rainfall esti- storm when the shear was strong and across the track, and mates to examine the differences in the rainfall asym- it was distributed asymmetrically across the track of the metry in TCs making landfall in different regions and storm when the shear was weak and along the track. their detailed changes during landfall. We will also in- Corbosiero and Molinari (2003) observed the maximum vestigate how the rainfall asymmetry is related to envi- occurrence of lightning downshear-left in TCs near the ronmental VWS and storm motion. The details of data coastal regions, similar to the cross-track shear in Hurri- used and the analysis methods are described in section 2. cane Bonnie (1998) described by Rogers et al. (2003). The asymmetric rainfall distributions in landfalling TCs Their results showed that the environmental VWS largely are discussed in section 3. Factors affecting the rainfall controls the asymmetric convective activity in TCs. asymmetry in landfalling TCs, such as the storm motion

Unauthenticated | Downloaded 10/05/21 07:42 AM UTC JANUARY 2015 Y U E T A L . 119 and VWS, are examined in section 4. Major results are which are available from the Shanghai Typhoon summarized in section 5. Institute. b. Rainfall analysis method 2. Data and analysis methods The TC rainfall distribution was analyzed using the a. Data Fourier decomposition. The spatial asymmetries were Whereas rain gauge data are routinely available, they created by binning rainfall estimates in 10-km-wide are sparse in many important regions even over land. annuli from the TC center to about 500-km radius, which Therefore, this complicates the analysis of the TC rain- is the same method as used in Lonfat et al. (2004) and fall asymmetries during landfall. The weather radar Chen et al. (2006). First, in each annulus, the wave- network could provide relatively good spatial and tem- number 1 Fourier coefficients were computed using all poral coverage, but a TC could be captured only if the TC rain-rate estimates (Boyd 2001): center is very close to the radar site but the spatial cov- 5 å u 5 å u erage is still very limited. Problems associated with the a1 [Ri cos( i)] and b1 [Ri sin( i)], inter-radar calibration and blockage by mountains also i i limit the accuracy and capability of radars in rainfall es- where Ri is each of the individual rain-rate estimates and timation. Therefore, the satellite precipitation estimates ui is the phase angle of the estimate relative to either the are widely used to document precipitation characteristics, storm motion or the VWS vector. The wavenumber 1 in particular over the open oceans and coastal and rainfall asymmetric component M1 can be represented by mountainous areas where few surface observations exist. Chen et al. (2013b) evaluated the capability of TRMM 5 u 1 u M1 [a1 cos( ) b1 sin( )]. satellite retrievals in capturing the precipitation features in landfalling TCs. They found that, overall, the TRMM Note that M1 was not divided by the mean rain rate R 3B42 estimates the heavy rain in TCs more accurately calculated over the entire annulus as used in Lonfat et al. over ocean than over land. Previous studies (Jiang et al. (2004). We calculated the variations of 6-hourly ampli- 2008a,b; Yu et al. 2009; Chen et al. 2013a) have also tudes of the wavenumber 0 through the wavenumber 4 shown that the TRMM 3B42 dataset could give quite rea- rainfall components and their amplitudes relative to the sonable rainfall patterns in landfalling TCs when compared sum of the wavenumber 0–4 rainfall rates within the with the gauge data or radar estimates. Therefore, in this radius of 500 km from the TC center for 24 h prior to, at study, the TRMM 3B42 data described in Yu et al. (2009) the time of, and 24 h after landfall. The wavenumber 2–4 were used to analyze the spatial distribution of rainfall in asymmetric components were computed similarly. TCs from 24 h prior to landfall to 24 h after landfall. When the shear-relative rainfall asymmetries were The wind fields from the National Centers for Envi- concerned, the rainfall asymmetry was composited rel- ronmental Prediction–National Center for Atmospheric ative to the VWS vector over each TRMM observation. Research (NCEP–NCAR) global reanalysis data (Kalnay When the motion-relative rainfall asymmetries were et al. 1996) were used to calculate the large-scale envi- concerned, the rainfall asymmetry was composited rel- ronmental VWS. This is different from the work of Chen ative to the motion vector over each TRMM observa- et al. (2006) who used the developmental datasets from tion. The direction of storm motion at each TRMM the Statistical Hurricane Intensity Prediction Scheme observation was calculated by linear interpolation from (DeMaria and Kaplan 1999) and the Statistical Typhoon the two closest best-track records to the observational Intensity Prediction Scheme (Knaff et al. 2005). The time. For both the shear-relative and the motion-relative horizontal resolution of the NCEP–NCAR reanalysis rainfall asymmetries, the phase maximum represents the data used in this study is 18318. location of the largest rainfall asymmetry. The larger the The environmental VWS was defined as the differ- asymmetry amplitude is, the more the variability in ence in vector winds averaged within a 500-km radius the spatial rainfall distribution may be explained. This from the TC center between 200 and 850 hPa (V200 2 can also help characterize the relative importance of the V850) at 6-h intervals, which is also different from that individual effects of VWS and TC motion. used in Chen et al. (2006). They defined the VWS as the difference between the 200- and 850-hPa (V200 2 V850) 3. Rainfall distributions in landfalling TCs winds averaged in an annular region between 200- and 800-km radii from the TC center, at 12-h intervals. In To provide an overview on landfalling TCs in differ- addition, the TC positions were extracted from the best- ent regions in China, we first examined the tracks of track data of the China Meteorological Administration, landfalling TCs during 2001–09 in five regions including

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FIG. 1. Tracks of TCs that made landfall in different regions of China during 2001–09: (a) HN, (b) GD, (c) TW, (d) FJ, and (e) ZJ. Numbers of TCs making landfall in these regions are listed in (a)–(e). (f) The names and locations of the ﬁve regions.

Hainan (HN), Guangdong (GD), Taiwan (TW), Fujian (tropical storm intensity) at 24 h prior to landfall were (FJ), and Zhejiang (ZJ), namely from south China to included as landfalling TC cases in the analysis. The east China (Fig. 1). There were 14, 22, 20, 14, and maximum rainfall was located around the TC center, 8 landfalling TCs in these regions, respectively, in the nine generally south of the TC center in all regions 24 h prior years. Figure 2 shows the average rainfall distributions to and at the time of landfall. Compared to the situation in TCs 24 h prior to, at the time of, and 24 h after landfall prior to and at the time of landfall, the rainfall intensity in the ﬁve regions. Note that only TCs with the maxi- was signiﬁcantly reduced within 24 h after landfall ex- 2 mum sustained 10-m wind speed larger than 17.2 m s 1 cept for TCs affecting Hainan. Note that this rainfall

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FIG. 2. Average rainfall distributions (mm) in landfalling TCs in different regions of China during 2001–09: (a) HN, (b) GD, (c) TW, (d) FJ, and (e) ZJ. The x and y axes are distance (3 0.18)fromtheTC center (origins). Stage (I) is 24 h prior to, stage (II) is at the time of, and stage (III) is 24 h after landfall. The color scale indicates the amplitude of the average 6-h rainfalls (mm).

Unauthenticated | Downloaded 10/05/21 07:42 AM UTC 122 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 54 intensity decrease after landfall still needs further veri- than that from 24 h prior to landfall to 24 h after landfall fication by gauge rain data or radar rainfall estimates in in ours, they also found a decrease in the axisymmetric the future since satellite algorithms hardly detect the rainfall after landfall. Note that this result still needs fur- light rain rates associated with stratiform rain in regions ther verification because of the limited accuracy of satel- with complex terrains over land (Negri and Adler 1993; lite rain estimates over land, as we mentioned earlier. Kubota et al. 2009; Shige et al. 2013). Similar to the Figure 5 shows the amplitudes of the wavenumber 0–4 rainfall intensity, the location of the rainfall maximum rainfall components. There were no large differences in also changed somewhat from region to region. the amplitudes of the wavenumber 0–4 components for The wavenumber 1 rainfall asymmetry in TCs gives TCs making landfall in different regions except for an a better illustration for the location of the rainfall max- obvious increase in the amplitude of the wavenumber imum. Figure 3 presents the wavenumber 1 rainfall 0 component before landfall in TW and ZJ (Figs. 5c,e). asymmetry in TCs for the five regions. The maximum in The amplitudes of the wavenumber 0–4 rainfall com- the wavenumber 1 rainfall asymmetry was generally lo- ponents were almost unchanged in HN. However, there cated south of the TC center in all landfalling TCs prior was a gradual decrease in rainfall from 24 h prior to to landfall. Interestingly, the maximum in the rainfall landfall to 24 h after landfall in both GD and FJ. asymmetry rotated cyclonically from south China to east China, namely, from HN to GD, TW, FJ, and ZJ. The 4. Factors affecting the rainfall asymmetry in maximum in the rainfall asymmetry was located south- landfalling TCs west of the TC center in HN, south of the TC center in a. An overview of the environmental settings GD and TW, south and southeast of the TC center in FJ, and southeast of the TC center in ZJ. After landfall, this The rainfall asymmetries in TCs may arise from the cyclonic rotation of rainfall maximum still existed. Except environmental VWS, the storm motion, the interaction in HN where the rainfall maximum was always located with the upper-tropospheric synoptic systems, and the southwest of the TC center, the rainfall maximum rotated nonuniform surface characteristics (Shapiro 1983; cyclonically for landfalling TCs in other regions after Marks 1985; Merrill 1988; Jones 1995, 2000a,b, 2004; landfall, a feature similar to that prior to landfall. Wang and Holland 1996a; DeMaria 1996; Frank and To see more clearly the evolution of the rainfall asym- Ritchie 1999, 2001; Kepert and Wang 2001; Yu et al. metry in landfalling TCs in different regions, the 6-hourly 2010; Yu and Yu 2012). Some studies (Kepert 2002, amplitudes of the wavenumber 0–4 rainfall components 2006a,b; Chan and Liang 2003; Chan et al. 2004; Wong relative to the total amplitude of the wavenumber 0–4 and Chan 2006, 2007) also showed that the land–sea components were computed within the radius of 500 km contrast would produce highly asymmetric structures in from the TC center for 24 h prior to, at the time of, and for a landfalling TC, which can cause the asymmetric dis- 24 h after landfall. As shown in Fig. 4, the amplitude of the tribution of rainfall in both the eyewall and in spiral wavenumber 0 (axisymmetric component) rainfall was rainbands. Nong (2000) argued that outer spiral rain- generally about 0.4–0.6 of the total amplitude, which bands induced by the land–sea contrast may play a cer- means that the axisymmetric component explains about tain role in inducing the asymmetric rainfall distribution. half of the variance of the total rainfall. The amplitude of In an idealized simulation, Chen and Yau (2003) found the wavenumber 1 component was about 0.2, while the that the asymmetric structure in a landfalling TC was amplitudes of the wavenumbers 2, 3, and 4 components accompanied by the highly asymmetric distribution of were all less than 0.2 but larger than the amplitude of any rainfall. They showed that the boundary layer frictional higher wavenumber components. This is consistent with convergence and associated convection produced a low- the conclusion from previous studies that the axisym- level positive potential vorticity band ahead of the TC metric component is dominant on average and the prior to landfall. In addition, in many landfalling TC wavenumber 1 component is dominant in the asym- cases, the rainfall asymmetry could be affected consid- metric rainfall distribution (Marks 1985; Marks et al. erably by the interaction of the TC with the midlatitude 1992). Another obvious feature in Fig. 4 is the large westerlies. It is found that the strength of the TC– decrease of the amplitude of the wavenumber 0 rainfall extratropical flow interaction (the phasing between the TC component after landfall in most regions. Wu et al. and the extratropical flow) would amplify the North Pacific (2013) analyzed the asymmetric distribution of pre- Ocean flow following the TC recurvature (Archambault cipitation in TCs making landfall along east China coast et al. 2013) and that the change in TC intensity was posi- using reflectivity data collected from coastal Doppler tively correlated with the change in the upper-tropospheric radars from 8 h prior to landfall to 6 h after landfall. angular momentum export (Chan and Chan 2013). Though the time span in their analysis was much shorter Kitabatake and Fujibe (2009) made composite analyses

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FIG.3.AsinFig. 2, but for the wavenumber 1 rainfall asymmetry (mm) as a function of the distance from the storm center. The color scale indicates the amplitude of the asymmetry relative to distance from the TC center, and the axes indicate full degrees.

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FIG. 4. The 6-hourly amplitudes (3 100%) of the wavenumber 0, 1, 2, 3, and 4 asymmetric components of TC rainfall relative to the total rainfall in regions (a) HN, (b) GD, (c) TW, (d) FJ, and (e) ZJ, during the period from 24 h prior to landfall to 24 h after landfall.

to show that the average TC structure was related to the were generally more than 108 latitudes to the north with environment such as a trough in the midlatitude westerly, the subtropical high immediately to the north of the TC the subtropical high, or another TC. circulation. By the time of landfall, TCs moved closer to To understand how the large-scale environment in- the westerly troughs and ridges to the north. About 24 h fluences the rainfall asymmetry in landfalling TCs over after landfall, TCs were located west of the western China, we first examined the 500-hPa geopotential Pacific subtropical high in FJ and ZJ and were directly height and moisture flux. As shown in Fig. 6, overall, affected by the westerlies to the north, with the westerly prior to landfall, the midlatitude westerlies at 500 hPa trough impinging onto the TC circulation. However,

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FIG.5.AsinFig. 4, but for the 6-hourly wavenumber 0, 1, 2, 3, and 4 asymmetric components (mm).

TCs making landfall in HN, GD, and TW were still far in the dataset) occurred (figure not shown), except for away from the midlatitude westerlies and thus no direct TC Chebi (2001) making landfall in FJ and TC Haima trough interaction occurred. (2004) making landfall in ZJ within 24 h after landfall. As it moved northward, a landfalling TC might ex- This is different from those cases in Japan where most perience an extratropical transition (ET). Loridan et al. landfalling TCs entered the midlatitude westerlies and (2014) found that 67% of cases that experienced ETs often experienced ET processes, as analyzed in Loridan around Japan exhibited strong winds on both sides of the et al. (2014). In addition, we found that the maximum moving cyclone but the maximum was often located to wind speed was always to the right of the TC motion the left of the motion vector. In this study, we examined vector (figure not shown). The maximum water vapor the possible transitioning phase of each storm making flux around the TCs was also to the right (Fig. 6). The landfall in FJ and ZJ by using the IBTrACS (Knapp water vapor flux between the midlatitude trough and the et al. 2010). It is found that few cases that experienced landfalling TC became stronger as the landfalling loca- ‘‘ET’’ (‘‘nature’’ flag information, indicating that the tions changed from south China (HN, GD, and TW) to storm was fully transitioned into an extratropical storm east China (FJ and ZJ).

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FIG. 6. The 500-hPa geopotential height (solid contours; gpm) and moisture ﬂux [color 2 shading;g(hPacms) 1] ﬁelds 24 h prior to, at the time of, and 24 h after landfall for TCs making landfall in regions HN, GD, TW, FJ, and ZJ. The x and y axes are distance (8) from the TC center (origins).

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FIG.7.AsinFig. 6, but for the 300-hPa potential temperature (solid contours; K) and vertical wind 2 shear (color shading; m s 1).

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FIG. 8. Probability distribution of motion directions for the landfalling TCs in the ﬁve regions during the period from 24 h prior to landfall to 24 h after landfall.

Figure 7 shows the potential temperature and high If the TC recurvature is defined as a change in the TC 2 VWS (.20 m s 1) fields for the landfalling TCs in the motion heading from westward to northward or north- five regions. In general, the dense potential temperature eastward (Archambault et al. 2013), the recurvature of contoured areas (baroclinic zone) to the north were TCs within 24 h after landfall was not frequent in the five accompanied by a high VWS band, which corresponded regions that we analyzed. to the midlatitude westerly systems (Fig. 6). The mid- Figure 9 shows the motion-relative wavenumber 1 latitude baroclinic zone and the high VWS band were rainfall asymmetry in the five regions. During the pe- too far to directly interact with the landfalling TCs in riod from 24 h prior to landfall to 24 h after landfall, HN, GD, and TW, which means that the landfalling TCs the dominant rainfall asymmetries for HN were to the would be hardly affected by the midlatitude westerly left of the motion vector, much more than to the front, systems in these regions. But for the landfalling TCs in and those for GD, TW, and FJ were to the left or even FJ and ZJ, consistent with the enhancement of water to the rear of the TC center. The largest rainfall vapor flux channels, the higher VWS band along with asymmetry was located in the rear of the TC center the existing baroclinic zone was much closer to the TCs, for ZJ prior to landfall, while it rotated cyclonically to especially after landfall (Fig. 7), indicating direct in- the right-front after landfall. Therefore, the rainfall teraction of the landfalling TCs with the midlatitude asymmetries in landfalling TCs relative to the TC systems. motion had large variability among different regions, Given the above large-scale environmental set- suggesting that the rainfall asymmetries could not be tings, the rainfall asymmetries, in particular its well explained by the TC motion for TCs making wavenumber 1 component, in landfalling TCs can be landfall in China. considered as being forced by either the TC motion or c. Effect of vertical wind shear the large-scale environmental VWS or both. There- fore, in the following discussion, we will focus on the Previous studies (Willoughby et al. 1984; Marks et al. effects of both the TC motion and the large-scale 1992; Franklin et al. 1993; Gamache et al. 1997; Corbosiero VWS. and Molinari 2002; Black et al. 2002) have already shown that large-scale VWS has a significant impact on the de- b. Effect of TC motion velopment of convective asymmetries in TCs. Figure 10 During the period from 24 h prior to landfall to 24 h shows the changes of VWS between 200 and 850 hPa av- after landfall, over 50% of TCs that made landfall in eraged within 500 km from the TC center for the land- HN, GD, TW, and FJ were moving west-northwestward falling TCs in the five regions from 24 h prior to landfall to with the highest probability of 73% for TCs making 24 h after landfall. At the time of 24 h prior to landfall, landfall in HN, while about 50% of TCs making landfall the averaged 850-hPa zonal wind U was about 20.7, 1.0, 2 in ZJ were mostly moving north-northwestward (Fig. 8). 20.3, 0.4, and 24.9 m s 1, and the averaged 850-hPa Overall less than 10% of landfalling TCs in China meridional wind V was about 1.5, 3.0, 2.9, 4.1, and 2 moved north-northeastward or east-northeastward, ex- 4.2 m s 1, respectively, for HN, GD, TW, FJ, and ZJ. cept for slight above 19% of TCs making landfall in FJ. Namely, TCs making landfall over China were generally

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FIG. 9. The wavenumber 1 rainfall asymmetry (mm) relative to the storm motion. The storm motion vector is aligned with the positive y axis (upward). The x and y axes are distance (8)from the TC center (origins). Stage (I) is 24 h prior to, stage (II) is at the time of, and stage (III) is 24 h after landfall. The color scale indicates the amplitude of the asymmetry relative to the storm motion.

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FIG. 10. Changes in the averaged zonal wind (U; 2 2 ms 1) and meridional wind (V;ms 1) within 500 km of the storm center for TCs making landfall in (a) HN, (b) GD, (c) TW, (d) FJ, and (e) ZJ. The open and solid circles represent the nine points (at every 6 h) starting at 24 h prior to and ending at 24 h after landfall, respectively. The black solid (dashed) lines are wind changes at 200 (850) hPa. The green (red) dashed lines represent the vertical wind shear at 24 h prior to (after) landfall.

2 steered by the southerly–southeasterly winds in the 20.5, 1.2, and 3.4 m s 1 in the corresponding regions. lower troposphere and moved northwestward, mainly Therefore, the landfalling TCs were inﬂuenced by because of the large-scale steering ﬂow and partly be- easterly shear for HN, GD, TW, and FJ and westerly cause of the so-called beta drift resulting from advection shear for ZJ 24 h prior to landfall (Fig. 10). of the earth’s vorticity by the TC circulation (Fiorino By the time of 24 h after landfall, the averaged 2 and Elsberry 1989; Wang and Holland 1996b,c). The 850-hPa U was 0.4, 0.7, 0.0, 1.1, and 23.7 m s 1, and the 2 averaged 200-hPa U was 26.8, 25.6, 25.2, 23.6, and averaged 850-hPa V was 3.3, 2.0, 1.5, 2.5, and 3.1 m s 1, 2 21.1 m s 1, and the averaged 200-hPa V was 21.1, 21.7, the averaged 200-hPa U was 24.9, 22.1, 20.9, 1.5, and

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FIG. 11. Probability distribution of vertical wind shear for the landfalling TCs in the ﬁve regions during the period from 24 h prior to landfall to 24 h after landfall.

2 2 3.3 m s 1, and V was 21.7, 1.2, 0.8, 2.6, and 7.0 m s 1, regions were mostly embedded in the deep southeasterly respectively, for HN, GD, TW, FJ, and ZJ. In compar- flow southwest of the western Pacific subtropical high. ison with the TCs 24 h prior to landfall, the TCs 24 h Figure 13 shows the shear-relative wavenumber 1 after landfall were more often influenced by the westerly rainfall asymmetry in TCs making landfall in the five trough and ridge in the midlatitudes, as already seen in regions. The rainfall maximum for all regions was Fig. 6. Generally, the mean VWS was northeasterly mainly downshear to downshear-left whenever 24 h before landfall, and decreased with time during landfall. prior to landfall, at the time of landfall, or 24 h after The exceptions were a slight weakening of VWS with landfall. Therefore, in contrast to TC motion, VWS time for HN and a strengthening of the mean VWS and should be the primary controlling factor on the rainfall also a changing from westerly shear to southwesterly asymmetry in landfalling TCs, consistent with the results VWS for ZJ. for TCs over the oceans in previous studies (Chen et al. In addition to the mean VWS discussed above, we also 2006; Ueno 2007; Wingo and Cecil 2010; Hence and examined the probability density functions (PDFs) of Houze 2011). Note that the weak VWS for TW would VWS in all individual regions. As shown in Fig. 11, about suggest weak shear-relative asymmetries, but that was 90%, 87%, 69%, and 67% of the landfalling TCs were not the case. The postlandfall wavenumber 1 rainfall influenced by easterly shear in HN, GD, TW, and FJ, asymmetries were much larger for HN and TW than for and the landfalling TCs in ZJ were most frequently the other regions (seen also from Fig. 5). This is prob- influenced by southwesterly shear. Note that only about ably due to the fact that more TCs making landfall on 25% of TCs making landfall in ZJ were influenced by the two islands were back over the water 24 h later. Since easterly shear and about 75% of TCs were influenced by the VWS showed a cyclonic rotation from south China westerly shear. to east China, a similar cyclonic rotation appeared in the The PDFs of VWS within 24 h after landfall were also location of the rainfall maximum. The cyclonic rotation examined for all regions (Fig. 12). It is found that the of VWS was related to the large-scale settings, which led positive values of U200 2 U850 and V200 2 V850 to different interactions between the midlatitude west- gradually increased when the landfalling location erlies and the landfalling TCs in different regions, as changed from south China to east China. Finally, the discussed in section 4a. landfalling TCs in ZJ were still mainly influenced by southwesterly shear. But for landfalling TCs in TW and 5. Conclusions FJ, the highest probability of U200 2 U850 and V200 2 V850 was near 0, especially at 24 h after landfall, in- In this study, an effort has been made to document the dicating that the TCs 24 h after landfall in TW and FJ rainfall asymmetries in landfalling TCs over China be- were subject to very weak VWS effect. This is mainly tween 2001 and 2009 based on the TRMM precipitation due to the fact that the TCs making landfall in these estimates and to understand their forcing mechanisms

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21 FIG. 12. Probability distribution of vertical wind shear (m s ) for the landfalling TCs within 24 h after landfall in the ﬁve regions: (a) HN, (b) GD, (c) TW, (d) FJ, and (e) ZJ.

using the NCEP–NCAR global reanalysis data. Con- The amplitude of the wavenumber 0 rainfall component sistent with the TCs over oceans, the large-scale VWS increased prior to landfall for TCs approaching TW and predominantly controls the wavenumber 1 rainfall ZJ and then decreased after landfall, while it showed little asymmetry with the rainfall maximum downshear to change for landfalling TCs in HN but gradually decreased downshear-left of the TC center in the landfalling TCs. in GD and FJ prior to landfall. Nevertheless, a large de- The results show that the asymmetric rainfall maximum crease in the amplitude of wavenumber 0 rainfall was displayed a cyclonic rotation for TCs making landfall from commonly observed in most of the regions. HN to GD, TW, FJ, and ZJ. Prior to landfall, TCs making Results from the analyses of 500-hPa geopotential height landfall in HN, GD, TW, and FJ had the rainfall maxi- and water vapor ﬂux indicate that the water vapor ﬂux mum southwest and south of the TC center, while TCs between the midlatitude trough and the landfalling TC making landfall in ZJ had the rainfall maximum southeast became stronger when the landfalling location changed of the TC center. After landfall, the rainfall maximum from south China (HN, GD, and TW) to east China (FJ rotated from southwest to northeast of the TC center from and ZJ). The environmental VWS was easterly for TCs south China to east China. The amplitudes of the wave- making landfall in south China, while it was mainly west- number 0–4 rainfall asymmetries showed different evo- erly for TCs making landfall in ZJ. This is mainly associated lutions among TCs making landfall in different regions. with the interactions between the midlatitude westerlies

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FIG. 13. As in Fig. 9, but relative to the vertical wind shear. The shear vector is aligned with the positive y axis (upward). The color scale indicates the amplitude of the asymmetry relative to the vertical wind shear.

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