Speaker: Associate Professor Koh Tieh Yong from SUSS

Title: "MULTI-SCALE CONVECTION IN THE TROPICS: MODELLING AND OBSERVATIONS"

Abstract: The Weather Research and Forecasting (WRF) model was used to dynamically downscale 27 years of the Climate Forecast System Reanalysis (CFSR) in a tropical belt configuration at 36 km horizontal grid spacing. A key parameter controlling the moisture reference profile in the Betts-Miller-Janjic convective adjustment scheme was adjusted and a Precipitating Convective Cloud scheme was developed to address the prior model deficiencies so as to obtain a good rainfall climatology as observed by the Tropical Rainfall Measuring Mission (TRMM). The representation of conventional and Modoki-type El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) in the model were confirmed by linear regression. Madden-Julian Oscillation (MJO) and Boreal Summer Intra-seasonal Oscillation (BSISO) were also well-simulated. However, WRF does not capture well the diurnal cycle of precipitation over the Maritime Continent, highlighting the crucial role of the TRMM dataset in the study of multi-scale interactions involving the diurnal cycle.

The WRF simulation shows that in the boreal summer, conventional ENSO modifies the MJO amplitude while Modoki-type ENSO and IOD impacts are MJO-phase dependent; in boreal winter, these inter-annual variations have little impact on MJO amplitude.

The TRMM observations show that in the Maritime Continent, a moderate El Nino event accentuates MJO’s influence on diurnal cycle amplitude (whether strengthening or weakening) with little change in phase; whereas a moderate La Nina event has the anti-symmetric effect of countering MJO’s influence on the diurnal cycle. A strong ENSO leads to non-linear impacts where the diurnal cycle’s amplitude tends to be enhanced with large changes in its phase across all MJO phases, regardless of the ENSO-phase.Paper 1, paper 2, paper 3