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SOLA, 2010, Vol. 6A, 025−028, doi:10.2151/sola.6A-007 25

Characteristics of in the of Numerically Simulated Sidr

Nasreen Akter1 and Kazuhisa Tsuboki1, 2 1Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan 2Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan

(Figs. 1b and 1c). The cell indicated in Fig. 1b is ~116 km away Abstract from the radar, and its local altitude is ~1 km. Also, no sounding data are available. The observed data are not sufficient for inves- Cyclone Sidr had an intense rainband east of the cyclone tigation of the detailed structure and intensity of the convective center. The rainband exhibited two strong convective lines (band cells along the band. axes) composed of convective cells. To study the characteristics of Therefore, the present study aims to clarify the characteristics the convective cells, a simulation was performed at 1-km resolu- of the convective cells in the rainband of Cyclone Sidr over the tion by using a -resolving model. In both band axes, some sea and the environmental factors affecting cell development. For cells showed the characteristic structure of a . Supercells this purpose, a high-resolution simulation was performed using within the outer axis had stronger updraft, more intense precipita- the Cloud-Resolving Simulator (CReSS) model (Tsuboki tion, and a longer lifetime than those of the inner axis. The values and Sakakibara 2007). of the CAPE and helicity are different in the strong vertical shear environment of the inner and outer axes. A large moisture flux was present throughout the on the east side of the outer 2. Numerical simulation and verification axis. On the other hand, the upper level west of the inner axis was relatively dry. These are additional factors affecting the differ- The high-resolution simulation was carried out using a ences in cell characteristics in the two band axes. horizontal grid spacing of 1 km. The horizontal grid number was 903 × 1206. The computational domain was 85.69°E−94.78°E 1. Introduction

Cyclone Sidr is a category-4-equivalent cyclone of the Indian Ocean, which originated in the and made over Bangladesh on November 15, 2007. It had a prominent rainband east of the center, which was associated with heavy - fall and severe flooding as well as damaging winds. The intense of the rainband was determined by its constituent convective cells. Previous studies have found supercells in (Spratt et al. 1997; Lee et al. 2008; Eastin and Link 2009). They have a deep, continuously rotating updraft that is not interrupted by a downdraft. Supercells found in the rainband are mostly the mini-supercell type. Mini-supercells are smaller and short-lived, having shallow (MCs) (Burgess et al. 1995; Suzuki et al. 2000), in contrast to typical midlatitude super- cells (Moller et al. 1994). They are characterized by maximum reflectivities of less than 50−55 dBZ and echo tops < 10 km (NWS 2004). McCaul and Weisman (1996) proposed that in a hurricane environment, weak low-level cold pools restrict surface vorticity development, resulting in shallow supercells. It is evident from the literature that environmental characteristics, such as strong low-level , moderate storm-relative environmental he- licity (SREH), and moderate to large CAPE, are favorable for the development of supercells in the rainband at the time of landfall or over land (McCaul 1991; McCaul and Weisman 1996; Baker et al. 2009). McCaul and Weisman (2001) suggested that the CAPE and shear are important determinants of the morphology and intensity of convective cells. However, studies of the characteristics of cells in the rainbands and their environment over the ocean are still limited. Moreover, during the approach of Cyclone Sidr, only a single Doppler radar at a range of 440 km was operating over the entire coast. Its horizontal resolution in Doppler mode is 625 m, and the azimuthal resolution is roughly 2.5 km. Supercell-like structures (weak hook-shaped echoes and positive-negative velocity cou- plets) are observed with much data noise in the velocity signature Fig. 1. (a) Plan position indicator (PPI) display of radar reflectivity at 0° elevation for 1000 UTC November 15, 2007 with the observed track (solid red line). (b) Reflectivity and (c) Doppler velocity, respectively, indicated in (a). (d) Simulated rain rate (shading) at 1000 UTC and track (solid red Corresponding author: Nasreen Akter, Hydrospheric Atmospheric Research −2 −1 Center, Nagoya University, Nagoya 464-8601, Japan. E-mail: akter@rain. line). Arrows represent surface moisture flux vectors (kg m s ). Dashed hyarc.nagoya-u.ac.jp. ©2010, the Meteorological Society of Japan. lines indicate the two band axes. Oval shows the north cluster. 26 Akter and Tsuboki, Characteristics of Supercells in Cyclone Sidr and 13.97°N−25.05°N. The height of the domain was 30 km with into the north cluster (Fig. 1d). For this purpose, three isolated stretched grid spacing from 100 m at the surface to 564 m at the convective cells of the inner axis (a, b, and c) and three of the top level. The cloud microphysical process used to predict the outer axis (d, e, and f) were selected. Cells a, d, and e are marked mixing ratios of cloud water, rain, cloud ice, , and in Fig 1d. The results of an inner-axis cell (a) and an outer-axis and the number densities of cloud, ice, snow, and graupel was the cell (d) are presented in detail as representative cells. The proper- bulk cold rain type. The subgrid-scale turbulence was parameter- ties of other cells are summarized later. ized using 1.5-order closure with turbulent kinetic energy. The The time sequences of cells a and d are shown in Fig. 2a. Cell radiation process was considered in calculating the heat balance of d is more intense and larger than cell a. Figures 2b and 2c show the ground surface. The references for the physical parameterized the vertical component of vorticity at 0950 UTC for cell a (cross schemes we used were cited in Tsuboki and Sakakibara (2007). section at 90.17°E) and cell d (at 90.58°E), respectively. In both The initial and boundary conditions were provided by the six- cases, positive and negative vorticities are present in the cells. hourly data of the Japan Meteorological Agency-Global Spectrum Both cells have strong positive vorticity on the east side and weak Model (JMA-GSM) output. The JMA-Merged satellite and in-situ negative vorticity on the west. A strong updraft is associated with data Global Daily Sea Surface (MGDSST) data were the positive vorticity, and a weak downdraft is associated with the used for the lower boundary condition in the ocean region. The negative vorticity. Inner-axis cell a has a weaker pair and period of the simulation experiment was 24 h, from 0000 UTC weaker vertical velocity than outer-axis cell d. November 15 to 0000 UTC November 16, 2007. As indicated in Figs. 3a and 3b, the mixing ratio of precipita- Cyclone Sidr was simulated well in the experiment. The result tion (sum of rain, snow, and graupel) of cell a is ~9 g kg−1 and is verified by radar data. Figures 1a and 1d show the observed extends up to ~8 km in height. The downdraft is very weak (~ and simulated rainfall distributions, respectively, at 1000 UTC −1 m s−1). In cell d (Figs. 3c and 3d), the horizontal precipitation November 15, along with the cyclone track. Both the simulated structure shows a bean-shaped pattern with the intense updraft and and observed rainbands exhibit two band axes (strong vorticity of an MC. The strong updraft results in a large mixing lines). The outer axis contains isolated cells, and the inner axis is ratio of precipitation (> 10 g kg−1), which extends up to ~16 km. mostly composed of isolated cells as well as cluster or line types The downdraft is ~−5 m s−1 in the cell. In both cases, the updraft of convection. The radar reflectivity image (Fig. 1a) shows that is separated from the downdraft, and cell-relative flows approach the rainfall intensity in the outer axis is greater than that in the cells from the east below 2 km in height. inner axis. Similar differences in convection intensity are also The characteristics of the six simulated cells are summarized found in the simulation (Fig. 1d). in Table 1. The outer-axis cells contain MCs at higher levels with vorticity > 0.014 s−1. The dimensions, updraft, downdraft, and pre- cipitation intensities are larger than those of inner-axis cells. The 3. Convective cells lifetime of a cell, which is defined as the period from formation to dissipation or merging into the north cluster, is ≥ 2 h for outer-axis Convective cells were formed and were arranged along low- cells, whereas it is less than 1.5 h for inner-axis cells. The heights level convergence lines. After forming, they moved north-north- of inner-axis cells are ≤ 10 km, whereas those of outer-axis cells westward. Some cells dissipated and others merged. To examine are ≥ 14 km. A boundary of 2 × 10−3 s−1 vorticity contours is the basic characteristics of the convective cells, we investigated considered to mark the height and diameter of an MC. Cell dimen- isolated cells from formation to dissipation or until they merged sions are confined to the precipitation mixing ratio of 2 g kg−1.

Fig. 3. Cell a: (a) horizontal cross section of precipitation mixing ratio Fig. 2. (a) Time sequences of cells a and d. Shading indicates mixing ratio (shading; g kg−1) at 1.2 km in height at 0950 UTC. Solid black and dashed of rain (g kg−1), and solid contours are the updraft (5 m s−1 intervals) at red contours are the updraft (2 m s−1 intervals) and downdraft (1 m s−1 1.2 km in height. (b) North-south cross section of vertical component of intervals), respectively. Arrows represent the cell-relative horizontal ve- vorticity (shading; s−1), updraft (black contours; 5 m s−1 intervals), and locity. (b) East-west cross section (at 20.23°N) of the mixing ratio and downdraft (red contours; 2 m s−1 intervals) for cell a at 0950 UTC. (c) velocity vector in the section. Arrows are vectors of cell-relative zonal and Same as (b) but for cell d at 0950 UTC. Red lines in (a) indicate the cross vertical components of velocity. Cell d: (c) and (d) same as 3a and 3b at sections of the cells. 0950 UTC except for east-west cross section at 20.13°N. SOLA, 2010, Vol. 6A, 025−028, doi:10.2151/sola.6A-007 27

Table 1. Summary of cell characteristics.

Inner-axis cells Outer-axis cells Properties a b c d e f

Max. vorticity (s−1) 0.010 0.008 0.009 0.014 0.017 0.019 Max. updraft (m s−1) 16.3 14.4 18.0 32.2 27.7 31.5 Max. downdraft (m s−1) −4.6 −3.4 −6.1 −0.9 −9.8 −9.7 Max. cell length (km) ~15 ~14 ~17 ~28 ~25 ~32 Max. cell height (km) 10 9 10 16 14 16 Max. precip. (g kg−1) 11.2 8.4 12.1 16.4 13.9 18.4 MC height (km) 5−8 5−7 4−10 9−10 6−10 7−11 MC diameter (km) 7−9 4−6 7−8 7−11 6−11 6−12 Lifetime (min) 90 60 70 120 170 120 Cell speed (m s−1) 30.8, 24.9, 40.4, 26.9, 29.5, 30.7, and dir. 2.2° 19.2° 2.5° 4.9° 357° 343°

Table 2. Environmental parameters of cells.

Inner-axis cells Outer-axis cells Environmental parameters a b c d e f

SBCAPE (J kg−1) 547 470 610 1257 865 1000 0−6-km total shear 33.3 16.8 36.7 36.9 33.8 42.7 (m s−1) Fig. 4. Hodographs for (a) inner-axis cells and (b) outer-axis cells. 0−6-km bulk shear 21.3 7.5 22.2 22.1 19.8 28.4 Numbers indicate the height in km. Closed circles in each color indicate (m s−1) the cell motion vector for each color hodograph. BRN 3 13 3 5 4 3 0−3-km SREH (m2 s−2) 148 5 124 226 124 294 Bunkers (2002). The moderate CAPE, large shear, and moderate SREH are favorable for typical supercell formation (Weisman and Klemp 1982; Davies-Jones et al. 1990) in the outer-axis region. In contrast, inner-axis cells were shorter-lived and less intense. 4. Environment for convective cells Considering their MC intensities and dimensions, they may be referred to as shallow or mini-supercells (Burgess et al. 1995). Environmental fields, such as surface-based (SB) CAPE, They have a relatively low CAPE, which gives the same result as wind profile (hodograph), bulk Richardson number (BRN), and that for mini-supercells explained by Wicker and Cantrell (1996). SREH, were investigated during the formation of each cell. The It is worth mentioning that mini-supercells show slightly smaller low-level around the cell is considered to be the environ- vorticity (≥ 8 × 10−3 s−1), which is similar to that of the offshore ment for an individual cell; we calculated the environmental mini-supercell observed in (Eastin and Link 2009). parameters averaged in the region of 0.5° × 0.5° for each cell. Here, the BRN is less than that for a typical supercell simulated They are summarized in Table 2. The average CAPE of inner-axis in unidirectional shear (Weisman and Klemp 1982). However, cells is almost half of that of outer-axis cells. The hodographs in the present case demonstrates the evolution of a supercell with a Fig. 4 indicate strong clockwise vertical shear for all cells in both strong clockwise wind shear. axes. The ratios of the SBCAPE to the shear (BRN) are 3−13. To Furthermore, we examined additional environmental factors calculate the BRN, we used the SBCAPE. SREH values for outer- to explain the difference in the supercell characteristics of the axis cells are larger (cell e has a slightly lower value) than those of inner and outer axes. No significant difference in the surface tem- inner-axis cells. All the environmental parameters of cell b are less perature appears between the two band axes (Fig. 5a). Both have than those of the others, because cell b formed within a relatively cloud-scale weak cold pools, and their horizontal spreading is con- weak southwesterly with small vertical shear far from the cyclone fined within each band region. A noticeable difference was found center. in the environmental field between the west and east sides of the rainband. The southerly was intense throughout the troposphere on the east side of the outer axis, and it transported 5. Discussion more moisture than that on the west side (Fig. 1d). This results in a larger surface moisture flux in the outer-axis region than in the Since the vorticity values of the six cells are almost 0.01 s−1 or inner-axis region (Fig. 5b). In addition, the upper level west of the larger, and their structures show the characteristics of supercells, inner axis was dry (50% relative humidity) above 6 km, whereas we consider all cells in Table 1 to be supercells. The MC intensity, the east side contains 90% humidity up to 10 km in height (Fig. lifetime, precipitation amount, and structure of cells, as well as 5c). The difference in moisture field was also validated by a satel- their environmental parameters, are different for the inner- and lite observation of precipitable water (not shown). Convective- outer-axis cells. The outer-axis cells are intense, longer-lived, scale downdrafts that were resolved in the simulation transported and higher, having a bean-shaped structure with strong MCs. the upper dry air to the surface and decreased the humidity around According to the definition of Moller et al. (1994), they are similar the inner-axis cells (Fig. 5d). Both the large horizontal moisture in structure to the classic or high-precipitation type of supercell at flux and the upper-level high humidity are additional favorable midlatitudes. For these cells, the CAPE is ~1000 J kg−1, which is a environmental factors for the more intense supercells in the outer- moderate value (Bluestein 1993). The vertical shear is larger than axis region. the threshold of the bulk (10−15 m s−1) and total (20−25 m s−1) shear for the development of a typical supercell indicated by 28 Akter and Tsuboki, Characteristics of Supercells in Cyclone Sidr

Kyoto University and Dr. F. Murata at Kochi University for pro- viding the radar data. The numerical simulations were performed using the supercomputer at Nagoya University.

References

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