The Characteristics of Mesoscale Convective Systems (MCSs) over East Asia in Warm Seasons

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1 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2012, VOL. 5, NO. 2, The Characteristics of Mesoscale Convective Systems (MCSs) over East Asia in Warm Seasons LI Jun 1, WANG Bin 1,2, and WANG Dong-Hai 3 1 State Key Laboratory of Numerical modeling for Atmospheric Science and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China 2 Center for Earth System Science, Tsinghua University, Beijing , China 3 State Key Laboratory of Severe Weather, Chinese Academy of Meteorology, Beijing , China Received 13 July 2011; revised 1 September 2011; accepted 19 September 2011; published 16 March 2012 Abstract Mesoscale convective system (MCS) cloud clusters, defined using an objective recognition analysis based on hourly geostationary infrared satellite data over East Asia during the warm seasons of (except 2004), were investigated in this study. The geographical pattern of MCS distribution over East Asia shows several high-frequency centers at low latitudes, including the Indo-China peninsula, the Bay of Bengal, the Andaman Sea, the Brahmaputra river delta, the south China coastal region, and the Philippine Islands. There are several middle-frequency centers in the middle latitudes, e.g., the central-east of the Tibet Plateau, the Plateau of west Sichuan, Mount Wuyi, and the Sayan Mountains in Russia; whereas in Lake Baikal, the Tarim Basin, the Taklimakan Desert, the Sea of Japan, and the Sea of Okhotsk, rare MCS distributions are observed. MCSs are most intensely active in summer, with the highest monthly frequency in July, which is partly associated with the breaking out and prevailing of the summer monsoon in East Asia. An obvious diurnal cycle feature is also found in MCS activities, which shows that MCSs are triggered in the afternoon, mature in the evening, and dissipate at night. MCS patterns over East Asia can be characterized as small, short-lived, or elongated, which move slowly and usually lead to heavy rains or floods. Keywords: Mesoscale Convective System, infrared satellite data, distribution, monthly variation, diurnal cycle Citation: Li, J., B. Wang, and D.-H. Wang, 2012: The characteristics of Mesoscale Convective Systems (MCSs) over East Asia in warm seasons, Atmos. Oceanic Sci. Lett., 5, Introduction China is among the minority of countries that are the worst hit by natural disasters, with a long history of frequent catastrophes every year. Among all the various natural disasters, floods resulting from heavy rainstorms are one of the most important calamities that threaten China's national economy and human life and property. Mesoscale Convective Systems (MCSs) under certain weather conditions are blamed for heavy rainfall over East Asia. To investigate the laws of the activity and distribution of MCSs in East Asia, it is necessary to improve Corresponding author: WANG Bin, wab@lasg.iap.ac.cn the accuracy of strongly convective weather forecasts. Studies on MCS climatologies over East Asia are also useful for understanding the feature of the atmospheric water cycle under the background of the East Asia monsoon. Due to the small spatial scale and the short duration of MCS, geostationary satellites or radars with high spatiotemporal resolution are among the most effective monitoring tools. Benefiting from the large scope of exploration and infrared (IR) remote sensing by geostationary satellites that operate day and night, IR satellite cloud imagery is widely used to investigate MCSs over a large scope and a long period of time. Since the first man-made satellite was launched in the 1960s, creative works on how to best exploit satellite cloud imagery have been done by Fujita et al. (1968, 1969, 1975). Studies on MCSs have been conducted worldwide after Maddox (1980) brought forward the concept of Mesoscale Convective Complexes (MCCs) in Climatologies addressing large populations of MCSs have been developed in North and South America, Africa, India, Europe, and Australia (Maddox, 1980; Velasco and Fritsch, 1987; Desbois et al., 1988; Augustine and Howard, 1988, 1991; Miller and Fritsch, 1991; Laing and Fritsch, 1993a, b, 1997; Arnaud et al., 1992; Hodges and Thorncroft, 1997; Gray and Marshall, 1998; Machado et al., 1998; Carvalho and Jones, 2001; Morel and Senesi, 2002). However, studies of MCS activities over the East Asian continent remain scarce. To understand the nature and behavior of MCSs, we aimed to analyze them over East Asia for a 12-year period ( , except 2004) using an objective technique. Based on surface station observations and satellite cloud imagery data, the activity laws of MCS in the East Asian area during warm seasons are further presented in this paper. The remainder of the paper is organized as follows: in Section 2, the data and methods we used are described; some results are presented in Section 3; and a summary is included in Section 4. 2 Data and methods Based on the theory of pattern recognition and matching in the field of digital image processing technology, a system to track convective clouds has been developed that automatically recognizes and tracks MCSs (Li, 2010).

NO. 2 LI ET AL.: THE CHARACTERISTICS OF MCS OVER EAST ASIA 103 Through subjective and objective tests, the tracking system was evaluated and was demonstrated to be credible.

2 NO. 2 LI ET AL.: THE CHARACTERISTICS OF MCS OVER EAST ASIA 103 Through subjective and objective tests, the tracking system was evaluated and was demonstrated to be credible. Due to the scarcity of objective and deterministic criteria, it remains difficult to make comparisons among various statistical results regarding MCSs. For this reason, an effort was made to propose objective and deterministic criteria of MCSs over Eastern Asia, in which an MCS was defined as a contiguous cold cloud shield with an extent of at least 5000 km 2, an IR TBB (Temperature of Black Body) lower than 227 K and a duration of no less than three hours. An MCS was recognized by the following three steps: (1) Singling out cloud clusters with a top IR TBB lower than 227 K; (2) Removing those with an extent less than 5000 km 2 ; (3) Tracking those with a life-time longer than three hours. Convective weather systems usually occur in warm seasons, e.g., from March to September. The data used here were hourly IR TBB images of geostationary satellites from the Geostationary Meteorological Satellite 5 (GMS-5) and FengYun 2 series Geostationary Meteorological Satellite C (FY-2C). GMS-5 was launched on 18 March 1995 and was suspended at the end of FY- 2C was launched on 19 October We used the imagery data of GMS-5 from 1996 to 2003 and FY-2C from 2005 to 2008 in warm seasons. The two sets of data have a horizontal resolution of 12 kilometers 0.1 degrees in longitude and latitude, respectively. Both datasets provide measurements once an hour. It is inevitable that some data are missed during the receiving or transferring processes, and thus, the data sets are incomplete. Figure 1 shows the yearly and monthly integral ratio of satellite data. The average integral ratio is 79.5%. Simply put, FY-2C has a better integral ratio than GMS-5. A total of IR digital TBB cloud images of geostationary satellites during the seasons from March to September over the past 12 years (from 1996 to 2008, except 2004) were used in this study. 3 Results 3.1 Geographical location and seasonal distribution In this study, MCSs over Eastern Asia were detected from the data sets, which were used to establish an information database of MCSs occurring over Eastern Asia in the past 12 years. Based on the database, the features of MCS activity over Eastern Asia were statistically analyzed and were of statistical significance due to the sufficient number of samples. Figure 2 shows the overall distribution of all these MCSs. Several high-frequency centers are observed in low latitudes, including the Indo- China peninsula, the Bay of Bengal, the Andaman Sea, the Brahmaputra river delta, the South China coastal region, and the Philippine Islands. There are several middle-frequency centers in the middle latitudes, e.g., the central-east part of the Tibet Plateau, the Plateau of west Sichuan, Mount Wuyi, and the Sayan Mountains in Russia. Rare MCS distributions are exhibited in Lake Baikal, the Tarim Basin, the Taklimakan Desert, the Sea of Japan, and the Sea of Okhotsk. The monthly distribution of MCSs (Fig. 3a) shows that July was the most active month in terms of MCS development (21% of the whole database). In addition, 60% of the MCSs were recorded in June, July, and August, which indicates summer was the most intense period of MCS Figure 1 Integral ratio of satellite data: (a) yearly and (b) monthly.

3 104 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 5 Figure 2 The overall spatial distribution of MCSs over East Asia (percentage, %). activity. Summer was also the time when the convective season over East Asia was more marked. The hourly detection of MCS position displayed a southward displacement from March to May during spring and a northward displacement from May to August during summer. Finally, the MCS exhibited a southward retreat in September, as Fig. 3b demonstrates. In March and April, almost all MCSs were situated over land, and they occurred in the northern periphery of the Tibetan Plateau. The centers of MCS activity then moved southeastward in May and September. This movement was associated with the breaking out and retreat of the summer monsoon, which can cause an increase in convection activity over East Asia. The most frequently active regions in summer were found to be the Indo-China peninsula, the Bay of Bengal, the Andaman Sea, the South China coastal region, the Philippine Islands (where the summer monsoon may be responsible for MCS development), and the inland area (central-east of the Tibet Plateau, the Plateau of west Sichuan, and Mount Wuyi). The MCS's location of activity shifted between south and north. However, a rare MCS distribution was exhibited in Lake Baikal, the Tarim Basin, the Taklimakan Desert, the Sea of Japan, and the Sea of Okhotsk, even in the MCS active months. The reason for such a large area with rare MCS distribution is currently complex and obscure. Figure 3b shows the monthly active frequency distribution of MCS, which indicates that MCSs were most active during summer (from June to August). MCSs were mainly active in the northern periphery of the Tibetan Plateau. With the breaking out of the summer monsoon in May, the active regions shifted from the north of the Tibetan Plateau to the south of East Asia and the Indo-China peninsula, the Bay of Bengal, the Andaman Sea, the South China coastal region, and the Philippine Islands. When the summer monsoon advanced northward and prevailed over Eastern Asia in June, July, and August, the active regions were evidently enlarged and formed several high-frequency centers, as mentioned above. Following the southward retreat of the summer monsoon in September, the MCS active regions returned to the south. 3.2 Morphological characteristics of MCS The statistical distribution of the maximum areal extent of MCSs is plotted in Fig. 4a. The median maximum area reached km 2 ; 25.68% and 50.2% of the MCSs detected did not surpass km 2 and km 2 at any stage, respectively, showing that there were commonly small MCSs over East Asia. MβCS (MCS of β scale), a kind of MCS with slow movement, a typical lifetime of less than six hours and an area less than km 2, may partly develop into an MαCS (MCS of α scale) under certain weather conditions. The average area of an MCS in East Asia is km 2, and the average lifetime is 4.5 hours. The median values for average area (41382 km 2 ), duration (3.84 h), and ellipticity (0.55) show that a large number of MCSs are small, short-lived, and in elongated forms. A quasi-direct ratio correlation between the scale of an MCS and its lifetime was also found, and the ratio of increase is near 6.4 km h 1. The ellipticity at the time of maturation (with maximum areal extent) was chosen to represent the approximate shape of MCSs throughout their life cycle. Figure 4b shows the distribution of ellipticity (simple e-distribution), which presents a median value of The e-distribution is close to the normal distribution, and the average ellipticity is Ten percent of the e-distribution is elongated, presenting values below 0.31, probably due to the proximity of cold fronts. In general, MCSs crossing the East Asian territory tend to have elongated forms, as 75% of them have an ellipticity below Dynamical properties of MCSs The speed at which MCSs move is an important parameter because the speed is related to the occurrence of floods. The speed and size of the system, together with the average rainfall rate, can be used to estimate the total precipitation. Thus, for similar average rainfall rates and

NO. 2 LI ET AL.: THE CHARACTERISTICS OF MCS OVER EAST ASIA 105 Figure 3 (a) MCS monthly occurrence distribution, and (b) the monthly spatial distribution of MCSs over East Asia (percentage, %).

4 NO. 2 LI ET AL.: THE CHARACTERISTICS OF MCS OVER EAST ASIA 105 Figure 3 (a) MCS monthly occurrence distribution, and (b) the monthly spatial distribution of MCSs over East Asia (percentage, %). spatial sizes, a slow MCS may lead to greater flood damage than a more rapidly moving system. MCSs with speeds below 15 km h 1 are commonly characterized as quasi-stationary MCSs, which may lead to serious hazards due to the possibility of producing intense floods. The distribution of mean speed shows that 54% of MCSs moved at speeds below 50 km h 1, which are considered slowly moving MCSs (Fig. 5a). The average velocity is 55.8 km h 1, and the median velocity is 48 km h 1. Solar radiation is the essential energy source for global

5 106 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 5 Figure 4 (a) Area distribution at maturation time (units: 10 4 km 2 ), and (b) ellipticity distribution at maturation time (the dashed line is the curve of normal distribution). atmospheric motion, but only a few short wavelengths of solar radiation are directly absorbed by the atmosphere. The solar radiation energy is mostly absorbed by the global surface, and the energy is then transferred to the atmosphere by means of surface long wave radiation and turbulence transportation. Thus, this energy supply takes on an evident diurnal variation trait consistent with the atmospheric boundary layer's diurnal cycle. Affected by this energy budget, the evolution of an MCS's initiation, maturation, and vanishing must also take on a similar diurnal cycle trait. For instance, an MCS reaches its initiation peak during the period from afternoon to dusk of local solar time, the maturation peak around dusk, and then the vanishing peak after dusk, but before midnight. This process is the result of the surface long wave radiation and turbulence transportation usually reaching their top at 13 or 14 o'clock in the afternoon, when convection usually tends to evolve into MCS. Then, MCS can be developed toward maturation before sunset and gradually vanish after sunset, but before midnight. The MCS's initiation, maturation, and vanishing all reach their valley floor from 8 to 11 o'clock in the morning because the atmospheric energy cumulates during this period and the convection is under development. In addition, the MCS's initiation, maturation, and vanishing all reach a hyposummit before sunrise, which might be explained by the long-wave radiation from the cloud top resulting in a cooling effect or by the large thermal capacity of sea water resulting in the MCS's diurnal cycle over the sea. The detailed reason will be further analyzed in the future. Figure 5 (a) MCS mean speed distribution (units: km h 1 ) (the dashed line is the curve of normal distribution), and (b) the diurnal variation of MCSs over East Asia (percentage, %). 4 Summary and discussion An objective MCS tracking system was applied to identify and characterize MCSs for the period over East Asia. A total of MCSs, providing enough samples to prove the significance of statistical results, were detected from March to September, mostly in the convective warm season. There were several high-frequ ency centers in low latitudes and several middle-frequency centers in middle latitudes, while there were some regions with few MCS activities. The hourly detection position displayed southward and northward displacements and was accompanied by the surge and retreat of the summer monsoon. Summer was the most intense period of MCS activity, with a peak in July. The median values for average area (41382 km 2 ), duration (3.84 h), and ellipticity (0.55) show that a large number of MCSs were small, short-lived, and in elongated forms. The mean speed distribution displays an average velocity of 55.8 km h 1 and a median velocity of 48 km h 1. Of the total MCSs, 54% moved slowly at speeds below 50 km h 1, and they usually caused heavy rains or floods. Affected by the energy budget diurnal variation, the evolution of an MCS's initiation, maturation, and vanishing must also take on a typical diurnal cycle trait.

6 NO. 2 LI ET AL.: THE CHARACTERISTICS OF MCS OVER EAST ASIA 107 Acknowledgements. This study was supported by the National Basic Research Program of China (973 Program, Grant No. 2011CB309704), the Ministry of Finance of China and the China Meteorological Administration for the Special Project of Meteorological Sector (Grant No. GYHY(QX) ), and the National Natural Science Foundation of China (Grant No ). References Arnaud, Y., M. Desbois, and J. Maizi, 1992: Automatic tracking and characterization of African convective systems on Meteosat Pictures, J. Appl. Meteor., 31(5), Augustine, J. A., and K. W. Howard, 1988: Mesoscale convective complexes over the United States during 1985, Mon. Wea. Rev., 116(3), Augustine, J. A., and K. W. Howard, 1991: Mesoscale Convective Complexes over the United States during 1986 and 1987, Mon. Wea. Rev., 119(7), Carvalho, L. M. V., and C. Jones, 2001: A satellite method to identify structural properties of mesoscale convective systems based on the maximum spatial correlation tracking technique (MASCOTTE), J. Appl. Meteor., 40(7), Desbois, M., T. Kayiranga, B. Gnamien, et al., 1988: Characterization of some elements of the Sahelian climate and their interannual variations for July 1983, 1984 and 1985 from the analysis of METEOSAT ISCCP data, J. Climate, 1(9), Fujita, T. T., D. L. Bradbury, C. Murino, et al., 1968: A study of mesoscale cloud motions computed from ATS-1 and terrestrial photographs, in: Satellite and Mesometeorology Research Project, University of Chicago, Chicago, 25pp. Fujita, T. T., E. W. Pearl, and W. E. Shenk, 1975: Satellite-tracked cumulus velocities, J. Appl. Meteor., 14(4), Fujita, T. T., K. Watananbe, and T. Izawa, 1969: Formation and structure of anticyclones caused by large-scale cross equatorial flows determined by ATS-1 photographs, J. Appl. Meteor., 8(4), Gray, M. E. B., and C. Marshall, 1998: Mesoscale convective systems over the UK, , Weather, 53(11), Hodges, K. I., and C. D. Thorncroft, 1997: Distribution and statistics of African mesoscale convective weather systems based on the ISCCP meteosat imagery, Mon. Wea. Rev., 125(11), Laing, A. G., and J. M. Fritsch, 1993a: Mesoscale convective complexes in Africa, Mon. Wea. Rev., 121(8), Laing, A. G., and J. M. Fritsch, 1993b: Mesoscale convective complexes over the Indian monsoon region, J. Climate, 6(5), Laing, A. G., and J. M. Fritsch, 1997: The global population of mesoscale convective complexes, Quart. J. Roy. Meteor. Soc., 123(538), Li, J., 2010: Study on the Characteristics of the Mesoscale Convective Cloud Clusters Occurred in East Asia during Warm Seasons (in Chinese), Ph. D. dissertation, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 154pp. Machado, L. A. T., W. B. Rossow, R. L. Guedes, et al., 1998: Life cycle variations of mesoscale convective systems over the Americas, Mon. Wea. Rev., 126(6), Maddox, R. A., 1980: Mesoscale convective complexes, Bull. Amer. Meteor. Soc., 61(11), Miller, D., and J. M. Fritsch, 1991: Mesoscale convective complexes in the Western Pacific region, Mon. Wea. Rev., 119(12), Morel, C., and S. Senesi, 2002: A climatology of mesoscale convective systems over Europe using satellite infrared imagery. I: Methodology, Quart. J. Roy. Meteor. Soc., 128(584), Velasco, I., and J. M. Fritsch, 1987: Mesoscale convective complexes in the Americas, J. Geophys. Res., 92(D8),

NOTES AND CORRESPONDENCE. Seasonal Variation of the Diurnal Cycle of Rainfall in Southern Contiguous China

NOTES AND CORRESPONDENCE. Seasonal Variation of the Diurnal Cycle of Rainfall in Southern Contiguous China 6036 J O U R N A L O F C L I M A T E VOLUME 21 NOTES AND CORRESPONDENCE Seasonal Variation of the Diurnal Cycle of Rainfall in Southern Contiguous China JIAN LI LaSW, Chinese Academy of Meteorological