Role of Aerosols in Modulating Cloud Liquid Water During Active-Break Cycle of Indian Summer Monsoon: A Modeling Study

Indian summer monsoon exhibits large variability in the intraseasonal time scale, called the active-break cycle. The active-break cycle is associated with variations in precipitation, cloud liquid water (CLW), winds. In the central Indian region, the CLW in the break-to-active transition phase of monsoon is significantly higher than in the active-to-break transition phase (Bhattacharya et al 2014). This change in the CLW and rain efficiency may be due to indirect effect of aerosol that loads in the break phase.

To verify this hypothesis the Weather Research and Forecasting Model coupled with Chemistry (WRF/Chem) was run in cloud resolving mode (with no chemistry) over central India with two different model setup. The first set of simulations does not allow aerosols to change with rainrate (called ‘control'). In the second simulation, the rate of change of aerosol number concentration with precipitation intensity was introduced in the model based on Aerosol Robotic Network (AERONET) observations (called ‘modified'). It has been seen that the modified simulation has higher amount of CLW in the break-to-active phase as compared to the active-to-break phase, consistent with previous observation study. This increase in CLW in break-to-active phase is associated with increase in droplet number concentration on account of availability of more cloud condensation nuclei in the break phase. The greater aerosol loadings reduce the rain efficiency in the building phase of active cycle and increase the rain rates in the later half.

Reference: Variability of Cloud Liquid Water and Ice over South East Asia from TMI Estimate. Anwesa Bhattacharya, Arindam Chakraborty and V. Venugopal. Climate Dynamics;May2014, Vol. 42 Issue 9/10, p2411