Diagnosing the possible dynamics controlling Sahel precipitation in the short-range ensemble community atmospheric model hindcasts
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The actual dynamics and physical mechanisms affecting the Sahel precipitation pattern and amplitude in the climate models remain under debate due to the inconsistent drying and rainfall variability/pattern among them. We diagnose the boreal summer rainfall pattern in the Sahel and its possible causes using short-range ensemble hindcasts based on NCAR community atmospheric model with the local ensemble transform Kalman filter (CAM-LETKF) data assimilation. The CAM-LETKF assimilation was conducted using 64 ensemble members with an assimilation cycle of 6-h. By comparing the superior and inferior groups within these 64 ensembles, we confirmed the influence of the Atlantic in the West Sahel rainfall (a robust feature in the ensembles) and a severe model bias resulting from erroneously modeled locations and magnitudes of low-level Sahara heat low (SHL) and African easterly jet (AEJ). This bias is highly related to atmospheric jet dynamics as shown in recent studies and local wave instability triggered mainly by the boundary-layer temperature gradient and amplified by land–atmosphere interactions. In particular, our results demonstrated that more accurate divergence and convergence fields resulting from improved SHL and AEJ in the superior groups enabled more accurate rainbelt patterns to be discerned, thus improving the ensemble mean model hindcast prediction by more than 25 % in precipitation and 16 % in temperature. We concluded that the use of low-resolution climate models to project future rainfall in the Sahel requires caution because the model hindcasts may quickly diverge even the same boundary conditions and forcings are applied. The model bias may easily grow up within a few months in the short-range CAM-LETKF hindcast, let along the free model centennial simulations. Unconstrained future climate model projections for the Sahel must more effectively capture the short-term key boundary-layer dynamics in the boreal summer to be credible regardless model dynamics and physics.
KeywordsLand–atmosphere interaction Global climate model evaluation Sahel precipitation Local ensemble transform Kalman filter (LETKF) Model uncertainty
Constructive comments and suggestions from the three anonymous reviewers are really appreciated. The author would also like to thank Dr. Junjie Liu for providing the ensemble model configurations and Prof. Inez Fung for the initialization of this project. The author acknowledges the computing resource of NERSC, Lawrence Berkeley National Laboratory, USA and National Center for High Performance Computing, Taiwan. The Climate Research Unit (CRU) TS3.1 dataset is available online at http://www.cru.uea.ac.uk/data. NCAR is sponsored by the U. S. National Science Foundation (NSF). Y. H. Tseng was partially supported by the NSF Earth System Model (EaSM) Grant 1419292 (EaSM-3: Collaborative Research: Quantifying Predictability Limits, Uncertainties, Mechanisms, and Reagional Impacts of Pacific Decadal Climate Variability). Y.-H. Lin and M.-H. Lo were supported by the Ministry of Science and Technology, Taiwan (Grant of MOST 103-2111-M-002-006).
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