15 JUNE 2016 W U E T A L . 4383 Meteorological Regimes of the Most Intense Convective Systems along the Southern Himalayan Front XUEKE WU College of Atmospheric Sciences, Lanzhou University, Lanzhou, China XIUSHU QIE Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, and Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, China TIE YUAN AND JINLIANG LI College of Atmospheric Sciences, Lanzhou University, Lanzhou, China (Manuscript received 8 December 2014, in final form 18 January 2016) ABSTRACT Based on 16 years of Tropical Rainfall Measuring Mission (TRMM) data and NCEP Climate Forecast System Reanalysis data, the most intense convective systems (ICSs) along the southern Himalayan front (SHF) are studied using the multivariate techniques of principal component analysis in T mode and k-means cluster analysis. Three clusters, classified according to the near-surface fields of wind, specific humidity, convective available potential energy, and convective inhibition, correspond to the premonsoon (March– May), the establishment of the monsoon (late May–early June), and the Indian summer monsoon itself (June– September), respectively. The location of ICSs along the SHF is closely related to the establishment of the transport passage from the eastern SHF to the northwestern SHF along the Himalayas. During the pre- monsoon, the southwesterly wind is weak and moist air from the Bay of Bengal is transported to the eastern SHF, where ICSs are densely distributed. The oceanic southwesterly wind is enhanced and the transport passage extends to the central SHF during the monsoon establishment period, when ICSs distribute over the whole SHF homogeneously. The southwesterly wind is the strongest and the transport passage extends to the westernmost SHF after the monsoon is established, when ICSs mainly concentrate over the concave in- dentation region. Backward trajectory analysis confirms that, besides the local environment, the moisture transport from the Arabian Sea (17%) and the Bay of Bengal (9%) are two important long-range transport pathways for the summer monsoon ICSs at the western end of the SHF. 1. Introduction and encounters the Himalayas, the low-level warm and moist flow is obstructed by the steep mountain barrier The Tibetan Plateau is the largest and highest and then capped by warm and dry air from the Iranian or mountain barrier in the world. During boreal summer, Tibetan Plateaus (Sawyer 1947; Houze et al. 2007), strong sensible heating leads to an upward motion over which could prevent the premature release of instability the plateau and the Himalayan southern slope, which and lead to the accumulation of unstable energy. drives the surrounding surface air to converge toward Together with the strong topographic lifting by the Hi- the plateau, like a sensible-heat-driven air pump (Wu malayas, the low-level warm and moist flow is lifted to et al. 2007). When the air mass flows from the Arabian become saturated and breaks through the stable layer, Sea and the Bay of Bengal over the Indian subcontinent so intense convective systems frequently occur in this region (Houze et al. 2007; Romatschke et al. 2010; Qie Corresponding author address: Dr. Xueke Wu, College of At- et al. 2014). mospheric Sciences, Lanzhou University, Lanzhou 730000, China. The southern Himalayan front (SHF), especially its E-mail: [email protected] western and eastern ends, is also the region known to DOI: 10.1175/JCLI-D-14-00835.1 Ó 2016 American Meteorological Society Unauthenticated | Downloaded 10/06/21 03:57 AM UTC 4384 JOURNAL OF CLIMATE VOLUME 29 yield the most vigorous convection on Earth (e.g., Qie Sea meets the dry air flowing off the Afghan plateau et al. 2003; Liu and Zipser 2005; Zipser et al. 2006; Liu (Houze et al. 2007), the midlevel dry air provides a et al. 2007; Qie et al. 2014). Some of the most frequent capping inversion that prevents the release of convec- lightning activity occurs at the westernmost tip of tive instability until the air is lifted along small foothills the SHF as seen by the Tropical Rainfall Measuring of the northwestern Himalayas (Medina et al. 2010). Mission (TRMM), where lightning density exceeds Severe convection is more likely to occur in regions 2 2 70 flashesÁkm 2Áyr 1 (Cecil et al. 2014), almost the same as with a large low-layer moisture gradient—namely, that in central Africa. Compared with deep convective drylines (Weston 1972; Wu et al. 2013). The compre- systems [DCSs; with 20-dBZ echo-top height exceeding hensive study of DCSs (Qie et al. 2014) shows that the 14 km; unless otherwise stated, all heights in the paper intensity of deep convection over the SHF is the most are above mean sea level (MSL)], intense convective intense, followed by the deep convection over the Indian systems (with 40-dBZ echo-top height exceeding 10 km) subcontinent. This is consistent with the result obtained occur more frequently and densely along the southern from CloudSat and the Cloud–Aerosol Lidar and Infrared slope of the Himalayas than over the adjacent regions Pathfinder Satellite Observations (CALIPSO)data(Luo (e.g., the Tibetan Plateau and the Indian subcontinent; et al. 2011). Qie et al. 2014). The average 20-dBZ echo-top height of The location of the most extreme convection is closely intense convective systems can reach 16 km, and some related to the unique topography of the region and its even exceed 18 km (Qie et al. 2014). Meanwhile, the interaction with southwest monsoon winds. Houze et al. anticyclonic circulation over the Asian monsoon region, (2007) and Romatschke et al. (2010) showed that the one of the largest upper-level anticyclones on Earth terrain plays an important role in releasing and en- (named the South Asian high or Tibetan high), is an hancing the convection. The moisture sources of con- important pathway for water vapor and pollutants to vective systems (e.g., deep convection and rainstorm) in enter the stratosphere (Park et al. 2007; Randel et al. the concave indentation have been discussed widely in 2010; Bian et al. 2012). This means that the intense recent years. For example, Houze et al. (2007) and convective systems along the SHF also play an impor- Medina et al. (2010) showed that convective systems tant role in the stratosphere–troposphere exchange. with intense radar echoes (e.g., 40-dBZ echo top ex- The TRMM Precipitation Radar (PR) (Kummerow ceeding 10 km or 40-dBZ echo area exceeding 1000 km2) et al. 1998, 2000) data provide a unique opportunity to tend to occur near the western indentation. The low- study the vertical structure of convection in remote re- level flow is moistened over the Arabian Sea and heated gions of complex terrain (Cecil et al. 2002; Nesbitt and by the sensible heat flux when it passes over the Thar Zipser 2003). The radar echo maximum heights (Zipser Desert, and the convective system is triggered by the et al. 2006), horizontal structures (Hirose and Nakamura orographic lifting when the low-level flow arrives at the 2004), and both vertical and horizontal structures Himalayan foothills. Afterward, the studies of rain- (Houze et al. 2007; Romatschke et al. 2010) of convec- storms over Pakistan (Houze et al. 2011) and India tion in the region have been analyzed. Deep convection (Rasmussen and Houze 2012) showed that low- to is more likely to occur over the east coast of the Indian midlevel air is brought into the region along the Hima- subcontinent during the premonsoon and move north- layan foothills by the southeasterly flow. Recently, ward to the SHF during the monsoon (Romatschke et al. based on 14 years of TRMM observational data and 2010; Romatschke and Houze 2011; Wu et al. 2013; Qie NCEP reanalysis data, Qie et al. (2014) confirmed that et al. 2014). However, it should be noted that the con- the occurrence of deep convection over the westernmost vective systems with different intensity show a different southern slope of the Himalayas is also closely corre- seasonal variation over the southern Himalayan slope— lated to the establishment of moisture transport passage for example, the maximum occurrence frequency of along the foothills of the Himalayas from the Bay of deep convective systems appears in August, while that Bengal, in addition to that from the Arabian Sea directly of intense deep convective systems appears in May (Qie (Houze et al. 2007; Medina et al. 2010). However, the et al. 2014). On a diurnal scale, convective systems form trajectory fraction from the Bay of Bengal versus from preferentially in the evening over land, as near-surface the Arabian Sea for forming intense convective systems moist flow is capped by dry air aloft (Romatschke et al. (ICSs) over the concave indentation region is still 2010). Studies of the monsoon convection (during not clear. June–September) in 2002 and 2003 showed that deep To better understand the physical processes that de- and wide intense convective cores over the north- termine the occurrence of intense convection along the western Indian subcontinent tend to occur where the SHF, this study investigates the meteorological regimes low-level moist layer of monsoon air from the Arabian for the onset of intense convection along the SHF using Unauthenticated | Downloaded 10/06/21 03:57 AM UTC 15 JUNE 2016 W U E T A L . 4385 16 years of TRMM observational data and the NCEP Climate Forecast System (CFS) Reanalysis data. Data and methods are described first, and then the temporal variation and geographical distribution of the most ICSs over the domain of interest are investigated. Further, the atmospheric environment and possible causes of ICSs under three clusters are discussed in detail based on multivariate analysis methods.