Analysis of Causation of Asymmetric Precipitation Associated with Severe Typhoon Damrey

DOI:10.16555/j.1006-8775.2014.04.004

Vol.20 No.4 JOURNAL OF TROPICAL METEOROLOGY December 2014

Article ID: 1006-8775(2014) 04-0323-11 ANALYSIS OF CAUSATION OF ASYMMETRIC PRECIPITATION ASSOCIATED WITH SEVERE TYPHOON DAMREY

1, 2 3 1 4 XU Xiang-chun (许向春) , YU Yu-bin (于玉斌) , WANG Shi-gong (王式功) , LI Xun (李勋)

(1. Department of Atmospheric Science, Lanzhou University, Lanzhou 730000 China; 2. Hainan Institute of Meteorological Science, Haikou 570203 China; 3. China Meteorological Administration, Beijing 100081 China; 4. Hainan Meteorological Observatory, Haikou 570203, China)

Abstract: Severe typhoon Damrey moved across Hainan Island from 00:00 UTC 25 September to 00:00 UTC 27 September in 2005 and gave rise to a significant rain process during its 48-h passage. The precipitation intensity on the southern part of the island is stronger than that on the northern, showing obvious asymmetric distribution. Using Tropical Rainfall Measuring Mission (TRMM) data, the associated mesoscale characteristics of the precipitation were analyzed and the formation of asymmetric rainfall distribution was investigated in the context of a subsynoptic scale disturbance, vertical wind shear and orographic factors. The results are shown as follows. (1) The subsynoptic scale system provided favorable dynamic conditions to the genesis of mesoscale rain clusters and rainbands. (2) The southern Hainan Island was located to the left of the leeward direction of downshear all the time, being favorable to the development of convection and leading to the asymmetric rainfall distribution. (3) Mountain terrain in the southern Hainan Island stimulated the genesis, combination and development of convective cells, promoting the formation of mesoscale precipitation systems and ultimately resulting in rainfall increase in the southern island. Key words: typhoon storm flood; numerical simulation; Severe Typhoon Damrey; causes of formation of asymmetric precipitation CLC number: P444 Document code: A

1 INTRODUCTION 1.6 and 2.5 times as strong as that forced by the divergence of the improved Q moist vector and the Asymmetry characterizes the precipitation of rainfall intensity as a result of land-surface friction tropical cyclone (TC) circulation, especially obvious [1] forcing is even 2 to 3 times as large as that produced after landfall (Chen and Meng ). Factors responsible by topographic uplifting. In their study of the for asymmetric distribution of TC rainfall are quite [2] structural change in the precipitation of landfall TCs complicated. According to Rogers et al. , its using the Tropical Rainfall Measurement Mission contributors include the terrain, changes in the (TRMM) and radar data from the Hong Kong attributes of the underlying surface during landfall, Observatory, He et al.[6] discovered that after landfall the movement of the TC and the interaction between the ratio of convective rainfall to the total rainfall the TC and its ambient airflow. Results from many [3] [4] somewhat decreases and the spirally distributed specialists, (e.g., Cangialosi and Niu et al. ) have rainfall over land is not as clear as that at sea, due to shown that terrain substantially alters the intensity and changes in the attributes of the underlying surface. distribution of TC rainfall via such mechanisms as Having diagnostically studied the dynamic factor that land surface friction and topographic uplifting. With [5] resulted from wavenumber-one, asymmetric structure an improved moist Q vector, Yue performed of precipitation occurring during the landfall of Billis diagnostic analysis and quantitative computation of (2006), Shi et al.[7] showed that vertical wind shear terrain-based forcing for the asymmetric rainfall of (VWS) has much greater effect on the formation of Typhoon Haitang (2005) and pointed out that the asymmetric rainfall structure than the terrain, and rainfall intensity forced by terrain factors is between changes in the attributes of the underlying surface

Received 2013-08-07; Revised 2014-08-20; Accepted 2014-10-15 Foundation item: National Science Backbone Project (2013BAK05B03); National Natural Science Foundation of China (40765002); Special Science Project for Public Welfare Industries (Meteorological Sector) (GYHY200906002) Biography: XU Xiang-chun, Senior Engineer, M.S., primarily undertaking research on tropical cyclones. Corresponding author: XU Xiang-chun, e-mail: [email protected] 324 Journal of Tropical Meteorology Vol.20 during landfall and the speed at which the storm September to 00:00 UTC 27 September 2005, the moves, and the intrusion of cold air are also playing storm brought severe rainfall to the island that was important roles in the genesis of rain. According to Lv asymmetrically distributed, much more in the south et al.[8], the asymmetrically distributed rain of than in the north. It is different from the case studied Typhoon Aere (2004) can be attributed to the in Chan and Liang[15] and Tuleya and Kurihara[16], circulation pattern and water vapor source typical of which indicated that severe rainfall occurred to the twin typhoons. In Ding et al.[9], the break-up of a right of the moving direction. It may be due to the fact spiral rainband around the point of landfall, which that the case in question is an island-transit typhoon: it resulted from not only the terrain but also the stayed over the land surface of the island for only 13 interaction between the upper-level TC circulation hours and the rain amount for the whole life cycle of and mid-level systems, increased the asymmetric rain the storm was well contributed by that resulting from of Typhoon Haitang (2005). the enhancement of the spiral cloud band as Damrey The asymmetric rainfall varies from one landfall left the island and re-entered the sea. In this work, the TC to another, rain intensity and distribution also TRMM data was first used to study the mesoscale changes with the same TC when it experiences features of this process of asymmetric precipitation. different stages of evolution, and the role of factors Then, it applied the NCEP reanalysis in diagnostic causing asymmetric rain varies from TC to TC analysis of the background field of subsynoptic scale (Cangialosi[3]; Niu et al.[4]; Yue[5]; He et al.[6]; Shi et disturbance taking place in meso- and fine-scale al.[7]; Lv et al.[8]; Ding et al.[9]; Yuan et al.[10]; Deng et convection systems. Finally, the next-generation WRF al.[11]; Wang et al.[12]). However, meso- and model was used to simulate the process. On the basis small-scale convective systems are essential in TC of successful simulation, a terrain-sensitive precipitation, judging from the observational fact. experiment was designed to study, together with the Small convective systems on the eyewall and the model output, the effect of VWS and the topographic enhancement of spiral rainbands during landfall have features of the land and mountain ranges in the island, been much focused in recent TC research. Analyzing in an attempt to reveal some possible causes high-resolution simulations of Typhoon Rananim responsible for the asymmetric distribution of the rain (2004), Li et al.[13] found that small-scale convective as the typhoon passes islands. ascending motion happens immediately after environmental inflow converges with outflows from a 2 DATA AND METHODOLOGY vortex, or when the inflow is being checked by the vortex itself to cause convergence. Small may they be 2.1 Data source and computation methods in spatial proportion, the fine-scale convection plays an important role in the transportation of mass in the In this work, the TRMM 3B42 rainfall retrieval eyewall. In Chen et al.[14], the spatial distribution is data (http://daac.gsfc.nasa. Gov /TRMM_DP studied intensively of the spiral convective cloud band /01_Data_Products), whose temporal resolution is 3 of Typhoon Matsa (2005) during landfall, with the hours, is used to study the mesoscale characteristics of finding that the cloud band had significant asymmetric rainfall. enhancement during the landfall, with the minimum For the subsynoptic disturbance field, the data for brightness temperature having an average drop of 20.2 analysis is extracted from the NCEP reanalysis at 1 °C and the wind speed of the outer spiral cloud band °×1 ° spatial resolution using the Shapiro nine-point having an average increment of 2.0 m/s and maximum filter operator. For any given physical quantity F, it can be denoted as F = FF* + ′ , in which F * is the increment of 4.1 m/s. In the 48 hours in which [17] Typhoon Damrey passed the Hainan Island from synoptic part and F′ is the subsynoptic part (Li ). 00:00 Coordinated Universal Time (UTC) 25 The nine-point filter operator is expressed by 2 * SS(1− ) S Fij,=+F ij ,() FFFFF i+ 1, j +++− i − 1, j ij , + 1 ij , − 144 ij , +() FFFF i ++ 1, j 1 + i −+ 1, j 1 + i +− 1, j 1 + i −− 1, j 1 − F ij , (1) 24 where the response function is circular field between two vertical layers at 200 hPa 2 and 850 hPa, with a radius five degrees of ⎛⎞2 π RSn(),12sin=−⎜⎟ S . If S = 0.5, n = 2, we have R longitude/latitude from the center, by the expression ⎝⎠n presented below: = 0. Therefore, by smoothing operation with the filter 55⎧ ⎫⎧⎫ operator (1), we obtain the disturbance that filters out 11⎪UUii−−11++⎪⎪⎪ VV ii UAVA==∑∑⎨ ⎬⎨⎬ii， (2) wavelengths twice as long as the gridpoint interval. A AAii==11⎪⎩⎭22⎪⎪⎪ ⎩⎭ disturbance field that is free of wavelengths and twice where the is the regional mean, ￣ the as long as the gridpoint interval can be obtained by subtracting a post-filtering smoothed field from the azimuthal mean, and Ai the circular area at a radius original field. For the VWS, the computation follows of 1 longitude/latitude. For the azimuthal mean, 360 ° the circular shear method in Hanley et al.[18] for a is equally divided into 15 °. As the original data is on

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No.4 XU Xiang-chun (许向春), YU Yu-bin (于玉斌) et al. 325 the Cartesian coordinates that centers on the typhoon, it is interpolated into 24 equal parts on the cylindrical coordinate by performing bilinear interpolation and running arithmetic averaging for the 24 values of the circular areas with corresponding radius.

2.2 Design of the simulation scheme The simulation scheme uses the mesoscale regional model WRF V2.2 that takes as the initial field the NCEP global final reanalysis data available every six hours at a resolution of 1 °×1 °. 35 (a) layers are set in the vertical and the model top is set at obsSLP simSLP obsVMX simVMX 50 hPa. No bogus scheme is used in the simulation. 1005 60 The area to be simulated centers on (110.0 °E, 19.0 995 50 °N) and is two-way nested: the coarse domain has 133 985 975 40 )

×100 gridpoints at intervals of 12 km while the fine /s m

965 30 ( domain has 283×211 gridpoints at intervals of 4 km. SLP(hPa) 955 VMX The simulation is initiated at 18:00 UTC 23 20 945 September 2005 for 78-h integration till 00:00 UTC 935 10 27 September, giving model output every hour. In the 925 0 analysis below, the numerical data are all from the computation of the gridpoint field of the fine domain -50 -44 -38 -32 -26 -20 -14 -8 -2 4 10 16 22 28 (4 km×4 km). The physical processes used in the relative time coarse and fine domain of the model include the YSU (b) planetary scheme, Kain-Fritsch cumulus convection Figure 1. The observed and simulated location of the eye (a) parameterization, Ferrier microphysics scheme, and intensity near it (b) for Typhoon Damrey from 18:00 UTC RRTM longwave radiation scheme and Dudhia 23 to 00:00 UTC 27 September 2005. The abscissa in (b) is for shortwave radiation scheme. Four sets of terrain the time relative to landfall and 0 indicates the time of landfall. sensitivity experiments were designed. Scheme 1 sets Probably due to the lack of oceanic observations, the underlying surface of the Hainan Island as the the simulated intensity is weaker than the observation ocean, Scheme 2 sets it at 0 m above sea level, prior to the landfall on the island. As the resolution of scheme 3 sets it half as much as the real height above the NCEP initial field used by the model is not high sea level, and Scheme 4 twice as much. Except for the enough, the initial intensity of Damrey is not terrain height, the sensitivity experiments use the described accurately (so that in the initial model field, same model parameters as those in the control. the minimum sea level pressure is 10 hPa higher and maximum wind speed is 5 m/s smaller than the 2.3 Verification of the simulation results observation). Despite the weaker-than-reality intensity, The track and intensity objectively located by the the simulated trend of intensity evolution is consistent Central Observatory of China every three hours are with the observation so that it reproduces the two used as the observation. Fig. 1 gives the track and shifts in intensity as Damrey weakened after landfall intensity of Damrey simulated by the WRF model. and kept the intensity as it re-entered the sea. For the The simulated typhoon track is a line connecting the 48 hours around the point of landfall, the time this geometric centers of the minimum enclosed pressure work focuses on, the simulated intensity is 10 hPa contours of the surface pressure field in the model smaller than the observation in terms of average error. output. For the 24 hours before the landfall on the It is comparable with the intensity deviations island through the re-entry of the sea, the simulated determined when studying the causation of track differs from the observed one by less than 20 km precipitation (Niu et al.[4]; Yue[5]; He et al.[6]; Shi et at the largest deviation. It shows that the simulation al.[7]; Lv et al.[8]). does a good job in reproducing the deviating track of Figure 2 gives the rainfall amount observed at Damrey that changed from a southwestward to a national basic stations for the whole lifecycle of westward direction prior to the landfall. Damrey (Fig. 2a) and that simulated over accumulated sections of time (Fig. 2b) between 00:00 UTC 25 and 00:00 UTC 27 September. Generally speaking, the model is successful in simulating the rainfall over the entire lifecycle of the storm, which is stronger in the south than in the north of the island, with generally

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326 Journal of Tropical Meteorology Vol.20 consistent area of rain but small difference in circulation structure, an usual dynamic coordinate magnitude. The simulated center of severe system is used in this work to study the fields of precipitation is 50 km more southward than that of the physical quantities (Chen et al.[14]). The TC is always observation and the extreme of rain amount is 73 mm at the center of the studied area, i.e. its coordinates are larger than the observation. One of the possible causes at (0, 0). The numerals of the abscissa (ordinate) are for the deviation is the large difference in resolution for the distance from the eye in the unit of longitude between the observational data and model output: the (latitude). One longitude (latitude) is equivalent to spatial resolution is around 100 km for the national about 110 km. 24 hours before the landfall, the basic stations while being only 4 km for the model periphery circulation is generally of a symmetrically result. circular pattern and severe rainfall centers on the eastern side (Fig. 3a). During the landfall, a 20 convective cloud system develops asymmetrically so that precipitation is much stronger in the south than in the north 4 to 6 hours after the landfall on the island. 500 19.5 400 On the southern side of the rain, the area of severe 2 300 rainfall with the rainrate ≥10 mm/h is about 3000 km 200 in a circular form, showing features of a mesoscale 19 100 rainband (Fig. 3b). The rainrate is generally ≤2 mm/h on the northern side. When Damrey leaves the island 18.5 to head for the sea, the destroyed structure of the inner spiral rainband is restored so that convective cells 109 109.5 110 110.5 111 develop intensely on it and cloud clusters of severe (a) convection gradually form an arc-like pattern (Fig. 3c) that is about 50 km in width and about 300 km in length, a typical mesoscale pattern. Nine hours after the entry onto the sea, severe convective cells develop on the outer spiral rainband that eventually merge to form an arc-shaped north-south oriented mesoscale rainband that strides across Hainan Island, with the width being about 30 km and the length about 400 km. It is longer than that forming on the inner spiral rainband. As shown in the analysis, the two mesoscale rain centers are generally consistent, which are over the mountains in the southern island and near the

(b) southern and northern coastlines. The rainrates are Figure 2. The observed (a) and simulated (b) rainfall amount above 15 mm/h but reduce substantially in the part for Typhoon Damrey in its passage over Hainan Island from extended to the sea (Fig. 4d). It is obvious that it is 00:00 UTC 25 to 00:00 UTC 27 September. Units: mm. closely related to the enhancement of heavy rain, which results from the uplift of southerly flows of the 3 MESOSCALE CHARACTERISTICS OF TC circulation by the windward side of mountains, ASYMMETRIC PRECIPITATION and the mesoscale convergence of the flow field, which is caused by changes in the underlying surface According to the TRMM observations, the near the coast. asymmetric precipitation, being stronger in the south than in the north, is mainly from three severe 4 ANALYSIS OF THE CAUSATION OF mesoscale raining systems, which is a mesoscale rain ASYMMETRIC RAIN DISTRIBUTION cluster that formed on the eyewall at the initial time after the landfall of Damrey, a mesoscale rainband 4.1 Subsynoptic disturbances that formed on the inner spiral rainband when it is 4.1.1 FLOW FIELD OF THE DISTURBANCES moving off the island to the sea, and a mesoscale rainband that formed on the outer spiral cloud band As the genesis of mesoscale systems is more after it is over the sea. They usually last three to four closely linked with the flowfield of subsynoptic hours. Fig. 3 gives the TRMM-observed distribution disturbances, this work uses the nine-point filter of 3-hour mean rate of rainfall for the three mesoscale operator to separate the subsynoptic disturbances from systems as Damrey affected the island. In view of the the large-scale flowfield provided in the NCEP fact that the TC is a dynamic system under the reanalysis, with attempts to discuss the dynamic background of the general circulation and has its own factors for the genesis and evolution of the three

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No.4 XU Xiang-chun (许向春), YU Yu-bin (于玉斌) et al. 327 mesoscale rain systems. Fig. 4 shows the evolution of rainband. Nine hours after the entry to the sea, the the flow field at 925 hPa. 24 hours prior to the landfall, center of the disturbance circulation once again the center of the disturbance, which is at sea then, coincides with the cyclone center and the area of almost coincides with that of the circulation (Fig. 4a). intense airflow convergence in the southeast of the The flow field is circularly and symmetrically cyclone center now moves to the southern mountains distributed near the disturbance center and there is a and coast of the island. Under favorable topographic circular area of flow convergence and a circular area effect, convective cells are intrigued on the outer of airflow convergence exists at the periphery, about spiral cloud band and eventually converge to form a 300 km from the circulation center. Four hours after mesoscale rainband. During the passage through the the landfall (Fig. 4b), the disturbance center is island, the disturbance center deviates from the gradually separating from the storm center and the circulation center at each of the levels from 925 hPa flow field is adjusting near the circulation center, to 200 hPa but in different locations. At the middle about 50 km to the southeast of the circulation center, and lower levels below 500 hPa, the disturbance resulting in the enhancement of the converging center is to the southeast of the cyclone center but the airflow at the southeast periphery of the circulation former is to the southwest of the latter. The output of center. It is consistent with the location where the mesoscale modeling also exhibits similar cloud cluster of severe convection forms 4 to 6 hours characteristics. For instance, the location of the after the landfall (Fig. 3b). As Damrey heads west cyclone center simulated 4 hours after the landfall is over the island, the centre keeps rotating in the 20 km more southward than the observation (Fig. 1a), disturbance flow field, increasing the distance from which is close to the location of the subsynoptic the cyclone center. As it leaves the island (Fig. 4c), disturbance center determined by filtering. As shown the disturbance center moves to a place about 80 km in a comprehensive analysis, the subsynoptic southeast to south of the circulation center, where a disturbance system provides favorable conditions for convergence zone of intense airflow follows the eye three severe mesoscale rains. Good consistence is to move west to the southern land and coast of the found between the mesoscale rain cluster and the island over which the convergence coincides with the disturbance center and between the mesoscale inner spiral cloud band to provide dynamic rainband and the line of airflow convergence of the background for the development of a mesoscale disturbance.

(a) (b)

Figure 3. Three-hour average rainrate during the passage of Damrey through the island (Units: mm/h). The abscissa (ordinate) value is for the distance of longitude (latitude) from the eye. (a): 22-24 hours before landfall; (b): 4-6 hours after landfall; (c): 0-2 hours after entering the sea; (d): 9-11 hours after entering the sea.

4.1.2 VERTICAL VELOCITY OF THE DISTURBANCE A well-defined structure of wavetrain can be

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328 Journal of Tropical Meteorology Vol.20 discerned from the radial, vertical cross section of the drops to the level of 700 hPa and below while the vertical velocity of the disturbance through the eye, speed increases to 6.5×10-3 hPa/s, while the northern which is determined from filtering the NCEP center drops to the level of 850 hPa and below while reanalysis (figure omitted). During the island visit, the speed increases to 3.0×10-3 hPa/s. As Damrey Damrey is always stronger in the south than in the leaves the island for the sea, the ascending motion north in both the height to which the ascending flow weakens on both sides and the field of vertical reaches and the vertical velocity. Two hours prior to velocity gradually returns to the pattern and intensity the landfall, descending motion prevails in the eye it once has before the landfall. During the landfall, the while ascending motion dominates from the eyewall vertical motion increases much more in the southern to the areas 200 km away to the south and north. On side than in the northern side, indicating that the the southern side, the vertical velocity centers on 500 surface friction and topographic uplift due to massive -3 hPa at an ascending speed of 2.0×10 hPa/s. The mountain ranges in the central and southern part of the ascending flow extends to 200 hPa. On the northern island may be playing a key role in assisting the side, the vertical velocity centers on 850 hPa at a low-level airflow to ascend. Next, more work will be speed of 1.5×10-3 hPa/s and the ascending flow done on a numerical experiment on topographic reaches the height of 300 hPa. Four hours after the sensitivity. landfall, the southern centre of the ascending motion

(a) (b)

Figure 4. Flow field of the 925-hPa disturbance during the passage of Damrey through the Hainan Island. The abscissa (ordinate) values are for the longitude (latitude) distance from its eye. (a) 24 hours before landfall; (b) 4 hours after landfall; (c) 3 hours after leaving the island; (d) 9 hours after entering the sea.

4.2 Vertical wind shear condensed particles formed during the ascent are being transported downstream via advection to cause As shown in a number of observational studies in intense rain on the left side in the shear direction. recent years (Marks et al.[19]; Franklin et al.[20]; Figure 5 gives the temporal evolution of the Corbosiero and Molinari[21]), the convection and rain magnitude of VWS and leeward change of the shear inside the circulation area tend to concentrate on the for Damrey as it passes the Hainan Island, which are left side leeward of the VWS in the ambient plotted based on the output from the control surrounding. When the shear occurs, the convective experiment. 20 hours prior to the landfall, the VWS is updraft to its left is strengthened while that to the right 3.3 m/s and gradually increases to 6 m/s at landfall as is weakened. According to the work of Zhang et al.[23] it approaches the land. The shear magnitude is quite on its mechanism, forcing produced by the shear in close to 6.5 m/s and 7.5 m/s, two values of maximum TCs results in a secondary circulation that is made up VWS at TC landfalls statistically determined in Xu[24]. of ascending motion in the shear direction and After the landfall, the shear increases in a way that is descending motion against the shear direction. Then, different from that of TCs making landfall on the with the rapid counterclockwise rotation of the vortex,

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No.4 XU Xiang-chun (许向春), YU Yu-bin (于玉斌) et al. 329 continent. Throughout the passage over the island, the island and heading for the sea, it changes Damrey decreases persistently in terms of VWS and counterclockwise from north to south. Comparing the even keeps a small value of 3 m/s some time off the areas that receive the rain also indicates that the island island at sea. It can be seen that the magnitude of the passage is accompanied by areas of severe rain VWS is important in keeping the intensity and shifting from the southeast of the eye to the southwest structure of Damrey but has no immediate role in and then back to the southeast again. The left leeward affecting the intensity and distribution of the rain. The side of the vertical shear is conducive to the distribution of rainfall is more closely connected with development of convection, which corresponds well the downshear evolution of the shear. Before Damrey with the area of severe rain as observed by TRMM. makes landfall and after it leaves the island for the sea, Staying most of the time over land in the southern part the downshear remains at south steadily and changes of the island during the island passage, the leeward little. It changes drastically after its passage over the side of the shear plays an important role in the genesis island. At landfall and during the initial time of of the asymmetric rain that is stronger in the south movement over the island, the downshear direction than in the north of the island. changes clockwise from south to north. Upon leaving

Figure 5. Temporal evolution of the magnitude of vertical shear and the direction of the downshear during the landfall. The box indicates where the island passage is.

4.3 Effect of the terrain on Hainan. When the underlying surface is the ocean, the TC is compact in structure with vertical motion The uplifting due to mountainous terrain under centers stripe-distributed over the sea in the southeast some conditions is usually the driver for the quadrant of the eye. Spiral cloud bands develop mesoscale convection after the landfall of TCs (Chen vigorously but with small, ill-defined convective cells and Meng[1]). As shown in a number of studies (e.g. inside (Fig. 6a). With the underlying surface changed Shi[25]), terrain determines the spatial distribution of to land and the above-sea-level terrain height set at 0 rain and is one of the factors that must be taken into m, the vertical motion is so affected that the center of account in studying the causation of asymmetric rain the vertical velocity moves from the sea to the land resulting from landfalling TCs. For Hainan Island, the southwest of the eye (Fig. 6), which shows that land general terrain is higher in the central part than in the friction increases the converging and ascending parts that surround it, with a main mountain range motion to alter the distribution of convection. With lying across the central to southern part of the island. the above-sea-level terrain height changed to 1/2 the Wu Zhi Mountain is the highest in the island with the real size (Fig. 6c), the ascending motion expands to a main peak at 1867 m and there are six other much wider area with the stripe-shaped area of mountains higher than 1500 m. Next, four sets of vertical motion split into a number of meso- and terrain sensitivity experiment are designed to study small-scale convective cells (which correspond to the response of mesoscale clusters and rainbands as different centers of vertical motion). When real terrain well as accumulated amount of lifecycle rain to the is used (Fig. 6d), these stripe-shaped cells merge into land and large-sized mountains in an attempts to shed lump-shaped ones of larger size and are located from light on the effect of terrain on the distribution of TC the mountains in the southern island to the coast, rainfall. which is close to the location of the mesoscale rain clusters (which are also lump-shaped) observed by 4.3.1 RESPONSE OF MESOSCALE RAIN CLUSTERS TO TERRAIN TRMM (Fig. 3b). When the terrain increases to twice Figure 6 gives the distribution of vertical velocity the real height, convection distributes much like the field at 700 hPa in both the control and sensitivity real terrain but with larger intensity (figure omitted). experiments four hours after the landfall of Damrey

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(a) (b)

Figure 6. 700-hPa vertical velocity field of ω four hours after the landfall of Damrey. Units: 10-3 hPa/s. (a) Scheme 1; (b) Scheme 2; (c) Scheme 3; (d) Control.

4.3.2 RESPONSE OF MESOSCALE RAINBAND TO TERRAIN absence of terrain height the difference in marine and land underlying surface has the most significant effect In order to see how the terrain affects the around the coast, which is consistent with the result mesoscale rainband, Fig. 7 gives the radar echo for the given in the preceding section. With the increase of lowest model layer (32 m) from the output of the above-sea-level terrain to half the actual height (Fig. control and sensitivity experiments nine hours after 7c), the small convection cells on the rainband Damrey is at the sea again. When the underlying increase from three to five over the land, with two of surface is the ocean (Fig. 7a), it does not comprise the the most intense cells located over the mountains in TC structure with evenly distributed and circularly the central part and coastal area in the south of the structured radar echo that gets through the eyewall island, indicating that terrain height is playing a where an outer spiral rainband extends outward from relatively large role in stimulating the development of the eye, all the way from the central land part of the the meso- and fine-scale convective systems. With island to the Indochina Peninsula to the southwest. real terrain (Fig. 7d), the convective cells develop The convection cells on the rainband are not more vigorously over the mountain range in the especially active and areas of intense echo are over central and southern of the island, and some of these the land in the southern island and marine area to the cells gradually merge to form a large, organized and south. Neither the marine nor the underlying surface stripe-shaped cell, resulting in the heavier rainfall in of land has any effect on the spiral rainband. When the southern than in the northern part of island, a the underlying surface is set at 0 m above sea level pattern consistent with the mesoscale rainband (Fig. 7b), the rainband begins to break off and the observed by TRMM (Fig. 4d). With the marine part is much weakened, with the part over the above-sea-level height increased to twice the real land split into three lump-shaped cells of convection. value (figure omitted), the echo distributes in a way In the southern part of the island, the area of echoes similar to the real terrain: an intense center is still over higher than 40 dbz increases, showing that with the the central mountainous area but the mesoscale

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No.4 XU Xiang-chun (许向春), YU Yu-bin (于玉斌) et al. 331 feature of the rainband is less significant, showing signs of a rain cluster that has more intense echoes.

(a) (b)

Figure 7. Radar echoes of the lowest model layer nine hours after Damrey re-enters the sea. Units: dbz. Other captions are the same as in Fig. 6.

4.3.3 RESPONSE OF PRECIPITATION TO TERRAIN rain shifting from the southern coast to the area of higher levels above sea level in the central and Figure 8 gives the rainfall amount accumulated southern inland part of the island, which is consistent over the entire lifecycle of Damrey as it affects the with the surface observation in both the value and Hainan Island during the control and sensitivity distribution. When the terrain is made twice the real experiments. When the underlying surface is set as the height (Fig. 8d), the rainfall increases by as much as ocean and the above-sea-level height is set as land at 0 200 mm in the south but remains unchanged in the m (Fig. 8a and 8b), the rain is weakened with the north as compared to the case with real terrain. As central value dropped by about 100 mm but without shown in a comprehensive analysis, both the change much difference in distribution. However, as the land in underlying surface and the increase of terrain is raised to half the real height above sea level (Fig. height have significant effect on the precipitation, 8c), the rainfall increases by 20 to 50 mm in the which is consistent with the results verified by northern part of the island and the accumulated researchers on the amplifying effect of mountain rainfall is close to the observation in the northern part ranges on rainfall (Hanley et al.[18]). However, when at surface except for a few locations (Fig. 2a). terrain height is set between 0 m and 1000 m (up to Changes of accumulated rainfall over the southern half the real size), the rain increment is not significant. land area are not significant. When the Large-sized mountain ranges above 1000 m have above-sea-level height is set at real size, this area significant effect on the amount of rain. receives much more rain (Fig. 2b), with the center of

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(a) (b)

Figure 8. Accumulated rain amount from the output of the sensitivity experiments between 00:00 UTC 25 September and 00:00 UTC 27 September 2005. Units: mm. Other captions are the same as that of Fig. 6.

5 RESULTS AND DISCUSSION Damrey passes the Hainan Island, and the structure of its circulation. At the initial phase of the landfall, the With the data of TRMM, the mesoscale features homogeneous underlying surface (the ocean) changes of an asymmetric, intense rain that occurred as Severe to an asymmetric structure that comprises both the Typhoon Damrey (Coded 0518) passed Hainan Island ocean and the land. It stimulates the change in the were studied. The causation was investigated through disturbance field, whose center deviates towards the diagnostic analysis of subsynoptic disturbance center of Damrey circulation to set a right background systems, computation of physical quantities from the of mesoscale converging circulation for the mesoscale output of numerical simulation and sensitivity rain cluster on the south of the eyewall at the early experiments with terrain: time of the landfall. In the meantime, the change in (1) The intense rain of interest is made up of three the disturbance field also helps the structure of storm mesoscale processes: a mesoscale rain cluster that circulation change. The leeward direction of formed on the eyewall at the initial time after the downshear in the Damrey circulation undergoes landfall of Damrey, a mesoscale rainband that formed dramatic change from south to north. The left side of on the inner spiral rainband when it is moving off the the vertical wind downshear is conducive to the island to the sea, and a mesoscale rainband that superimposition between the area of developing formed on the outer spiral band after it is over the sea. convection and the center of disturbance circulation, They usually last three to four hours. The center of the which work together to facilitate the development of intense rain is located over the central mountains and the mesoscale rain cluster. When Damrey is off the southern coast, constituting to a pattern of stronger island and over the sea again, the leeward direction of rainfall in the south than in the north. downshear changes once again, from north to south (2) The mechanism in which mesoscale rain this time, due to the change in the underlying surface. systems form is closely correlated with the changes in Likewise, the left side of the vertical wind downshear the subsynoptic disturbance systems, which result is favorable for the development of secondary from changes in the attributes of underlying surface as circulation that superimposes with the area of airflow

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Citation: XU Xiang-chun, YU Yu-bin, WANG Shi-gong, et al. Analysis of causation of asymmetric precipitation associated with severe typhoon Damrey. J. Trop. Meteorol., 2014, 20(4): 323-333.

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