Studies On Characteristics And Possible Causes Of Deep Convective Systems Over The Asian Monsoon Region

Deep convective systems (DCSs) transport water vapor and pollutants vertically to upper troposphere, some even penetrate tropopause and transport water vapor and pollutants into stratosphere directly, which are vital in terms of global energy exchange, hydrological cycle and stratosphere-troposphere exchanges. Some intense DCSs associated with heavy rain, strong winds, hails and lightning, resulting in a serious impact on people’s lives. The Asian monsoon region is an important pathway for water vapor and pollutants entering the stratosphere, due to the physical interaction of the Tibetan Plateau and the Asian summer monsoon. TRMM data is valuable for analyzing3-D structure and intensity of DCSs over tropical and sub-tropical regions because of the simultaneous observations of its onboarded multi-sensors. Based on TRMM data and NCEP Climate Forecast System reanalysis data, this thesis studied the spatiotemporal distributions, convective properties and structure features of DCSs with20dBZ echo top height exceeding14km over the world, especially the Asian monsoon region. Finally, a dynamic condition as well as transportation of water vapor and convective available potential energy (CAPE) for the forming of DCSs near southern slope of the Himalayas were investigated. The main conclusions are as follows:1) DCSs and lightning are both mainly distributed over the tropical continental regions, e.g., central Africa, northern South America and maritime continent between the Asia and the Oceania. It was found that the distributions of DCSs are different significantly in terms of convective depth (expressed by cloud top height and20dBZ echo top height) and convective intensity (represented by40-dBZ echo top height, lightning flash rate). DCSs with stronger convective intensity preferred to locate in the mid-latitude regions, e.g., southern North America, southern South America and the Asian monsoon region, except over Central Africa. While, DCSs with higher cloud top are mainly distributed over the northern India Ocean-maritime continent-western equatorial Pacific and central Africa, followed by South America, which is consistent with the distribution of convective available potential energy. Distribution of DCSs over the Tibetan Plateau-Asian monsoon region showed a significant particularity.2) The distribution of DCSs over the Asian monsoon region showed significant regional features and closely related to the Asian summer monsoon. DCSs are mainly distributed in south of20°N and concentrated near the India’s east coast during the pre-monsoon season (March to May), further move markedly to mid-latitude area and most frequent near the Himalayan foothills in monsoon (June to September). DCSs over the Tibetan Plateau is weak in convective intensity and small in horizontal size, but occurred more frequent than over central and eastern China. While, intensity of DCSs over central and eastern China is more intense. Convective intensity of oceanic DCSs is weak, but the cloud top height is tallest and horizontal size is larger than continental DCSs significantly. DCSs over the terrestrial Asian monsoon region showed obvious diurnal variation, mainly appeared in the afternoon until midnight, while DCSs over the Tibetan Plateau are more concentrated in the afternoon until evening, consistent with the solar radiation. There is no obvious diurnal variation over the ocean. The diurnal variation of DCSs over Maritime Continent is similar with that over the mainland.3) DCSs and intense DCSs (IDCSs, with40-dBZ echo tops exceeding10km) occur more frequently over the continental regions than over the ocean. About23.0%of total DCSs develop into IDCSs in the SSH, followed by the TP (20.8%) and the SAMR (15.3%), and the least over the ocean (2.2%). The average20-dBZ echo-top height of IDCSs exceeds16km asl and9%of them even exceeds18km asl. Although most of both DCSs and IDCSs occur between April and October, DCSs have a peak in August, while IDCSs have a peak in May. The cloud top heights of DCSs increase from the Tibetan Plateau, to the Himalayan foothills nearby, the south Asian monsoon region, and to the ocean. Oceanic DCSs is tallest in cloud top but convective intensity is weak. The most intense DCSs are more concentrated near the Himalayan foothills, and the horizontal size is larger than that over the south Asian monsoon region (terrestrial). DCSs over the Tibetan Plateau occur frequently and can grow up to upper troposphere easier than adjacent lower regions due to its higher elevation and strong sensible heat flux in boreal summer, but its convective intensity is weaker.4) The distribution of IDCSs (the1000most intense DCSs) is closely related to the establishment of the water vapor transport passage, which flows from the Bay of Bengal along the Himalayas to northwest and reaches the westernmost in monsoon. IDCSs mainly occur over continental regions with moderate humidity (6-16g kg-1) and accompanied by a strong moisture gradient. Moreover, the environment with convective inhibition about-60J kg-1is conducive to the formation of IDCSs. Wind speed over the ocean is weak before the onset of Indian Summer Monsoon (March to May), impacted by the oceanic moisture, the specific humidity and convective available potential energy (CAPE) over north of the Bay of Bengal is larger than other continental regions, as a result, IDCSs mainly located in eastern SSH. During the onset of Indian Summer Monsoon (May and June), wind speed begin to increase and the water vapor transport passage is formed and extends to the middle of the SSH, then, IDCSs more evenly distributed throughout the Himalayan foothills. During the Indian Summer Monsoon (June to September), strong southwest wind transport water vapor from the Bay of Bengal to the westernmost SSH, which ultimately lead to IDCSs concentrated over the concave indentation region in the westernmost SSH.