Simulation of Indian Summer Monsoon Intraseasonal Oscillations: Role of Ice Microphysics

The Indian summer monsoon (ISM) is a coupled climate system. A coupled forecast system involving a CGCM has vital role for seasonal forecast of ISM rainfall as well as for forecast of the monsoon active-break cycles.The realistic representation of cloud formation by global climate model (GCM) is one of the major attributions to the improved seasonal precipitation forecast of Indian summer monsoon (ISM). The effects of cloud microphysics (ice phase and water phase) in coupled climate model (e.g., CFS2) for the simulation of Indian summer monsoon is important, as deficiency in the implementation of its parameterization into a dynamical model may also limit its effects on the large-scale circulations. The coupled climate models have biases and low prediction skill in seasonal scale regarding monsoon intraseasonal oscillation (MISO) simulation. The effect of ice-microphysics in CGCM is important for both microphysics and radiation feedbacks. Therefore, better ice-phase microphysics is required in the new generation of climate forecast model, which may lead to improvements in monsoon simulation. Satellite observation indicates that over the ISM region, about 40–50% of rainfall events originate from melting of ice which indicates the greater role of ice processes. The dominant mixed-phase clouds over the same region are composed of ice crystals and liquid droplets.

In this study, models using only Convective Parameterization (CP) fail to comply with observations for capturing MISO and northward propagations of cloud during JJAS-Monsoon. The results of outgoing longwave radiation (OLR) and Hadley Circulation also explore the linkages between convection and dynamics through the distribution of heat source. The CP experiments cannot capture the convergence zone over central India and oceanic regions properly during the active and break phase of monsoon but the Convective Microphysics Parameterization(CMP) comply much better with observation in this aspect. The meridional scale and northward propagation over the ISM region are the unique features of the dominant MISO. In order to examine the role of ice-phase microphysics, the aspect of wave number and frequency structure, finite domain space-time spectra are examined.There is a significant difference in the power spectrum of filtered daily rainfall anomaly ( Lanczos filter of 20-100 days) over Extended Indian Summer Monsoon region (averaged over 60°E–110°E), Arabian Sea(averaged over 50°E–78°E), Bay of Bengal(averaged over 77°E–99°E), Indian Land Region(averaged over 68°E–98°E) Equitorial Indian Ocean(averaged over 40°E–105°E) and South Indian Ocean(averaged over 50°E–110°E) for different sensitivity experiments as compared to observation. For EIMR, it is seen that the observed maximum intensity of MISO occurs at wave number one with period of 40 days. The convective parameterization experiments simulate MISO with 40 days period with a higher meridional wave number (wave number 2) with very low intensity, whereas the CMP experiments simulate it with a slightly larger period with nearly same meridional wave number and intensity. For rest of the regions the CMP experiments succeed to capture the intensity and oscillation period better than that of CP experiments. This signifies that proper microphysics is essential for the realistic simulation space-time structure of the MISOs.

Keywords: MISO, Ice-Microphysics, Convective Parameterization, space-time spectra, convective microphysics parameterization