Improved tropical modes of variability in the NCEP Climate Forecast

A new stochastic multi-cloud model (SMCM) convective parametrization, which mimics the interactions at sub-grid scales of multiple cloud types, is incorporated into the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) model (referred to as CFSsmcm hereafter) in lieu of the pre-existing simplified Arakawa-Schubert (SAS) cumulus scheme. A detailed analysis of the tropical intra-seasonal variability (TISV) and convectively coupled equatorial waves (CCEW), in comparison with the original (control) model and with observations, is presented here. The last 10-years of a 15-year long climate simulation are analyzed. Significant improvements are seen in the simulation of the Madden-Julian oscillation (MJO) and most of the CCEWs as well as the Indian summer monsoon (ISM) intra-seasonal oscillation (MISO). These improvements appear in the form of improved mechanisms and physical structure of these waves. This can be regarded as a validation of the central idea behind the SMCM according to which organized tropical convection is based on three cloud types namely, the congestus, deep and stratiform cloud decks that interacts with each other and form a building block for multiscale convective systems. An adequate account for the dynamical interactions of this cloud hierarchy thus constitutes an important requirement for cumulus parameterizations to succeed in representing atmospheric tropical variability. SAS fails to fulfill this requirement evident in the unrealistic mechanisms and structures of the major intra-seasonal modes simulated by CFSv2 as documented here. 18