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Special Session 5: Ocean-atmosphere interaction, multi-scale climate variability and their implication for biogeochemical processes |
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A stochastic multicloud parameterization scheme for the GCM
Tuesday 10th @ 1525-1545
Room 1
Qiang Deng* , College of Ocean and Earth Sciences, Xiamen University
Boualem Khouider, Department of Mathematics and Statistics, University of Victoria
Andrew Majda, Courant Institute, New York University
R.S. Ajayamohan, Center for Prototype Climate Modeling, New York University Abu Dhabi
Presenter Email: qdeng@xmu.edu.cn
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| Coarse-grid GCMs are typically used for climate research. The effect of unresolved small-scale processes on the large-scale flow is taken into account by the parameterization scheme, which is still one of the most challenging problems in climate modelling research. Despite the continued research efforts, coarse-resolution GCMs still represent poorly the Madden-Julian Oscillation (MJO). A new look into the cumulus parameterization problem was provided through the multicloud model, based on a judicious representation of the dynamics associate with three cloud types (congestus, deep and stratiform), which are observed to characterize organized tropical convective systems. To capture the missing variability, in the stochastic multicloud model (SMCM), a lattice model is designed for the cloud area fractions. In our work, for the first time, SMCM is coupled to a GCM. At first, with an idealized uniform SST setting, the stochastic model drastically improves the results of the deterministic model and clear MJO-like structures with many realistic features are successfully reproduced. Then, it is used to demonstrate the importance of stratiform heating for the organization of convection on planetary and intraseasonal scales, with a warm pool structure of SST placed at the equator. With higher stratiform heating, the model produces low-frequency and planetary-scale MJO-like wave disturbances, while parameters associated with lower to moderate stratiform heating yield mainly synoptic-scale convectively coupled Kelvin-like waves. Rooted from the stratiform instability, it is conjectured that the strength and extent of stratiform downdrafts are key contributors to the scale selection of convective organizations perhaps with mechanisms that are in essence similar to those of mesoscale convective systems. The study is also extended to the asymmetric forcing cases by placing the warm pool forcing north of the equator mimicking the migration of the ITCZ during boreal summer, to understand the impact of changes in stratiform heating (SH) on the monsoon dynamics. It is shown that the mean monsoon circulation and convection over the monsoon trough (MT) is sensitive to SH. Northeastward propagating monsoon intraseasonal oscillations (MISO) prevail when SH is strong, while low-pressure systems (LPS)-like disturbances characterize the MT variability when SH is weaker. While the strength of the MT increases with the SH, its westward extent is inversely proportional to the SH, which is consistent with the prevalence of westward moving LPS in the weak SH regime. Only in the purely LPS regime do the background vorticity and zonal wind profiles over the MT are consistent with observations. |
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