Abstract
The boreal summer of 2008 was unusual for the Indian monsoon, featuring exceptional heavy loading of dust aerosols over the Arabian Sea and northern-central India, near normal all-India rainfall, but excessive heavy rain, causing disastrous flooding in the Northern Indian Himalaya Foothills (NIHF) regions, accompanied by persistent drought conditions in central and southern India. Using the NASA Unified-physics Weather Research Forecast (NUWRF) model with fully interactive aerosol physics and dynamics, we carried out three sets of 7-day ensemble model forecast experiments: (1) control with no aerosol, (2) aerosol radiative effect only and (3) aerosol radiative and aerosol-cloud-microphysics effects, to study the impacts of aerosol-monsoon interactions on monsoon variability over the NIHF during the summer of 2008. Results show that aerosol-radiation interaction (ARI), i.e., dust aerosol transport, and dynamical feedback processes induced by aerosol-radiative heating, plays a key role in altering the large-scale monsoon circulation system, reflected by an increased north-south tropospheric temperature gradient, a northward shift of heavy monsoon rainfall, advancing the monsoon onset by 1–5 days over the HF, consistent with the EHP hypothesis (Lau et al. in Clim Dyn 26(7–8):855–864, 2006). Additionally, we found that dust aerosols, via the semi-direct effect, increase atmospheric stability, and cause the dissipation of a developing monsoon onset cyclone over northeastern India/northern Bay of Bengal. Eventually, in a matter of several days, ARI transforms the developing monsoon cyclone into meso-scale convective cells along the HF slopes. Aerosol-Cloud-microphysics Interaction (ACI) further enhances the ARI effect in invigorating the deep convection cells and speeding up the transformation processes. Results indicate that even in short-term (up to weekly) numerical forecasting of monsoon circulation and rainfall, effects of aerosol-monsoon interaction can be substantial and cannot be ignored.
Highlights
The Northern India and Himalayan foothills (NIHF) region is an essential component of the Indian monsoon (Gadgil et al 2003)
Strong anomalous cooling of the lower troposphere was found over the Indo-Gangetic Plain (20°–30°N), and anomalous warming above with maximum upper troposphere warming anchored over the Tibetan Plateau (Fig. 2c)
Using the NASA Unified Physics Weather Research Forecast (NU-WRF) model, we have conducted numerical forecast experiments to investigate the dynamics of aerosol-monsoon interaction over the complex topography of the NIHF region, under realistic initial and boundary conditions
Summary
The Northern India and Himalayan foothills (NIHF) region is an essential component of the Indian monsoon (Gadgil et al 2003). An increasing number of studies have suggested that absorbing aerosols (mainly desert dusts and carbonaceous aerosols) can affect the interannual and intraseasonal variability of the Indian monsoon rainfall (Lau et al 2006; Lau and Kim 2006; Lau 2014; Manoj et al 2011; Hazra et al 2013; Vinoj et al 2014; Kim et al 2015; D’Errico et al 2015; Sanap and Pandithurai 2015, and many others) Based on these studies, Lau (2016) has proposed a new paradigm arguing that natural aerosol, dust and carbonaceous aerosols from natural sources should be considered an essential component of an intrinsic aerosol-monsoon weather and climate system. The experiments will be conducted based on a case study of the 2008 Indian monsoon
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