Responses of extreme convective storms in the southern Himalayan front to the Indian summer monsoon

Abstract

South Asian monsoon convection in the southern Himalayan front (SHF) plays an important role in the entry of South Asia pollutants to the Qinghai-Tibetan Plateau and even to the stratosphere. Combining the 16-yr TRMM observations and Indian summer monsoon (ISM) index, the responses of extreme convective storms to the ISM are investigated. The results showed that the top height of convective storms increases gradually while their convective intensity weakens with the evolution from pre-monsoon to monsoon season. The establishment period of ISM (late May to early June) is found to be an important period for convective storms in the SHF. Not only does the frequency of storms occur more frequently, but also the convective intensity of storms is stronger compared with pre-monsoon and monsoon seasons. The ISM is proved to have an important impact on the formation of convective storms in northwestern inland Indian subcontinent, dominated by the water vapor supply, rather than the ‘juicy’ eastern SHF. Cloud top height of storms is more sensitive to the strength of ISM than the convective intensity. The water vapor transportation originated from the Bay of Bengal have more significant contribution in convective storm formation there than that from Arabian Sea. In stronger ISM years, the southerly meridional wind component over the Bay of Bengal is stronger, which forces more water vapor to be transported to further inland areas along the Himalayas. Together with the warm center and anticyclone in upper troposphere, resulting in convective storms in the westernmost of SHF occur more frequently and can grow up to higher top heights.