Global Effects of SuperParameterization on Hydro-Thermal Land

Wednesday, December 13, 2017 - 13:40
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Conventional General Circulation Models (GCMs) in the Global Land–Atmosphere Coupling Experiment (GLACE) tend to produce overly strong Land–Atmosphere coupling (L–A coupling) strength. We investigate the effects of cloud SuperParameterization (SP) on L–A coupling on timescales longer than the diurnal where it has been previously shown to have a strong effect. Using the Community Atmosphere Model v3.5 (CAM3.5) and its SuperParameterized counterpart SPCAM3.5, we conducted experiments following the GLACE and Atmospheric Model Intercomparison Project (AMIP) protocols. On synoptic-to-subseasonal timescales, SP significantly mutes hydrologic L–A coupling on a global scale, through the atmospheric segment. But on longer seasonal timescales, SP does not exhibit detectable effects on hydrologic L–A coupling. Two regional effects of SP on thermal L–A coupling are also discovered and explored. Over the Arabian Peninsula, SP strikingly reduces thermal L–A coupling due to a control by mean regional rainfall reduction. Over the Southwestern US and Northern Mexico, however, SP remarkably enhances the thermal L–A coupling independent of rainfall or soil moisture. We argue that the cause may be a previously unrecognized effect of SP to amplify the simulated Bowen ratio. Not only does this help reconcile a puzzling local enhancement of thermal L–A coupling over the Southwestern US, but it is also demonstrated to be a robust, global effect of SP over land that is independent of model version and experiment design, and that has important consequences for climate change prediction.

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