Aerosol influence on the pre-monsoon rainfall mechanisms over North-East India: A WRF-Chem study

The importance of aerosols and absorbing aerosols in influencing weather is well recognized. This study is carried out over North-East (NE) India during the pre-monsoon season, during which the region experiences the highest atmospheric heating due to high loading of absorbing aerosols. The region, also a biodiversity hotspot, has observed a decreasing rainfall trend in this season. Previous aerosol-rainfall studies over India have mostly focused on the monsoon season, but pre-monsoon rainfall also is an important rainfall component for this region. This study includes the results of a modelling study carried out with WRF-Chem model to investigate how aerosols impact the pre-monsoon rainfall and their mechanisms over this region. The influence of black carbon (BC) on radiation, clouds, rainfall and rainfall characteristics has also been investigated. The study shows that aerosols induce changes in cloud amounts which relates to rainfall changes. The semi-direct effect was found to be responsible for cloud reduction below 2 km altitude, thus affecting mainly lower-level clouds. Aerosol radi-ative feedbacks affected rainfall according to the height of the terrain in the region and tended to increase and decrease rainfall in general over low and high-terrain areas, respectively. Aerosol effects suppressed rainfall for more duration than it caused enhancement. The effect of increasing black carbon emissions showed a consistent increase in moisture in the region but also a consistent reduction in clouds, the changes in the spatial distribution of which was also related to the rainfall changes. BC caused suppression of lower intensity and enhancement of higher intensity rainfall. These results help to understand better the effects of aerosols on pre-monsoon rainfall distributions and the mechanisms involved.