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Towards improving convection parameterization and the MJO in next-generation climate models

Tropical biases remain a significant problem in global atmosphere models, even at horizontal resolutions of 20-50 km. In addition to mean state errors, another glaring deficiency is the general absence of the 30-60-day Madden-Julian oscillation (MJO), which modulates the frequency of tropical cyclone genesis over several basins and interacts with the lower-frequency El-Nino Southern-Oscillation. It is thought that such errors stem mainly from deficiencies in convection parameterization, but the precise nature of these deficiencies remains unclear. In order to address such tropical problems, we propose to run and analyze a suite of short-term [O(10-day)], high-resolution weather hindcasts, focusing on a 40-day period of enhanced MJO activity during the Year of Tropical Convection (YoTC) when special observational and assimilated datasets are available. The hindcasts will be performed using 4 different high-resolution atmospheric models (GEOS-5, CAM5, HiRAM, and WRF) as part of a multi-institutional collaborative research effort. The goal is to see how simulations of the MJO and other high-impact weather phenomena, as well as the mean state, are affected by either i) increases in model resolution (going from 50- down to 5-km horizontal grid spacing) or ii) the use of a “superparameterization” of convection at 50-km horizontal grid spacing. Hindcasts will also be generated with each of the models’ convection schemes turned off, to see how the various schemes tend to improve (or degrade) their respective model’s performance at high resolution. We hypothesize that models with more realistic convective processes will do better at simulating the MJO, so our diagnosis of model output will include both process-level and global-scale aspects, and will compare these in order to test this hypothesis. Improved understanding of this convective process-global performance relationship will serve the overall goal of improved ability to simulate convection variability and the MJO in models used to predict changes in regional-scale climate and high-impact weather for the decades to come.

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