The observations from TOGA COARE show that the population of deep convection are strongly modulated by the ACTIVE and SUPPRESSED phases of intraseasonal oscillation. During the active phase, the interaction amoung deep convection, air-sea fluxes, and the large-scale circulation (e.g., enhanced low-level westerlies and convergence, etc.) appears to be a positive feedback process. During the suppressed phase, such positive feedback is overwhelmed by the inhibition of organized deep convective systems, even though the bulk instability measures (e.g. CAPE) and SST are at their maximum. The interactive process between convection (not well-organized, mostly small, short-lived cloud systems) and their environment during the suppressed phase is not well-understood. The objective of this study is to investigate the large-scale control and convective feedback processes in the two distinct large-scale regimes using the PSU/NCAR nonhydrostatic mesoscale model version 5 (MM5). MM5 include both explicit resolved clouds with ice microphysics and parameterized subgrid-scale clouds. The model is initialized with the ECMWF global analysis, NMC SST analysis, and COARE soundings. A nested-grid model domain is used to include a realistic large-scale flow pattern over a ~5000km X 2500km region and a ~2500km X 1500km inner domain where the structure of mesoscale convective systems can be explicitly resolved.
Two sets of MM5 simulations have been performed for 29 Nov-4 Dec 1992 (suppressed) and 20-26 Dec 1992 (active) periods. We found that (1) during the active period, the total precipitation exceeds local evaporation as expected. The large-scale moisture convergence into the model domain accounts for a large portion of total precipitation. The cloud population is dominated by organized mesoscale and super convective systems. They tend to dry the lower troposphere. (2) During the suppressed period, the total precipitation is very close to the local evaporation from ocean surface. The cloud systems are mostly small, isolated, relatively shallow cumulonimbus. The low-mid troposphere is relatively dry compared to the active periods. The lateral mixing of clouds in the dry environment reduces the strength of the deep convective clouds and also moisten its environment at the same time. The two distinct convective feedbacks in redistributing tropospheric moisture could have important implication to the interactions between convection and large-scale circulation on the intraseasonal scale. (3) Convectively induced gravity waves are evidently play an important role (in addition to convective cold pool) in triggering new convection in both suppressed and active periods. The model sensitivity to different air-sea flux calculations including the COARE flux algorithm by Fairall et al. for the two distinct regimes will be discussed.