Cloud–aerosol coupled index in estimating the break phase of Indian summer monsoon

Abstract

The Indian summer monsoon (ISM) is largely influenced by intra-seasonal variability like break and active phases of monsoon. In the present study, different cloud and aerosol parameters are considered and analyzed to formulate a cloud–aerosol coupled index (CACI) that can aid in forecasting the break phase of ISM. The method of principal component analysis is implemented to identify the significant cloud and aerosol parameters during break and active phases of ISM. The threshold ranges of each parameter are evaluated by using the normal probability density function. The result reveals that for break phase, the significant parameters are cloud water path (CWP), cloud optical depth, aerosol index, zonal wind (ZW), and meridional wind (MW) at 850 hPa pressure level whereas for active phase, the parameters found to be important are aerosol optical depth, CWP, ZW, and MW at 850 hPa pressure level. The significantly correlated (p < 0.05) parameters are taken for formulating CACI. The results show that no such significant correlation is possible with the parameters during active phase of ISM. The CACI is thus formulated to forecast the break phase of ISM. The result shows that the CACI is capable of forecasting the break phase of Indian summer monsoon over central India with 88 % certainty and 10 days lead time. The result is validated with the observations.