Reduction of Pacific Double‐ITCZ Bias by Convection Parameterization in NCAR CESM2.2.

The impact of convective closure on the double‐ITCZ bias in the NCAR CESM2.2 is investigated in this study. The standard CESM2.2 simulates a remarkable double‐ITCZ bias in the central and eastern Pacific, especially in boreal winter and spring. Modifications to the closure in convection parameterization scheme greatly reduce the double‐ITCZ bias in all seasons, demonstrating that convection parameterization can substantially influence the double‐ITCZ bias in CESM2.2. Further analyses suggest that convection parameterization can modulate the tropical atmosphere‐ocean feedback processes, through which it influences the SST in the southern ITCZ region and hence the double‐ITCZ bias. The changes in the upper ocean temperature advection induced by modified convective closure plays important roles in reducing the warm SST bias and double‐ITCZ precipitation bias in the southern ITCZ region. The modified convective closure improves the low‐level cloud and shortwave cloud radiative forcing in the southeastern Pacific. However, surface heat flux plays only a limited role in reducing warm SST bias and double ITCZ bias because the impacts of shortwave radiation changes are largely canceled by changes in longwave radiation and latent heat flux. Plain Language Summary: Despite decades of model development, the large‐scale distribution of precipitation simulated by most of coupled global climate models (GCMs) still displays a remarkable double‐intertropical convergence zone (ITCZ) bias over the tropical Pacific Ocean. Observations show asymmetrical distribution of precipitation with respect to the equator in the central and eastern Pacific with one intense ITCZ precipitation band north of the equator all year‐round and a much weaker zonal rain band south of the equator (southern ITCZ) only in boreal spring. However, coupled GCMs tend to simulate overly strong southern ITCZ rain band, resulting in double‐ITCZ bias in annual mean precipitation. In this study, we investigate the impact of convection parameterization scheme on the double‐ITCZ bias in the NCAR Community Earth System Model, version 2.2 (CESM2.2). The standard CESM2.2 simulates a remarkable double‐ITCZ bias in the central and eastern Pacific. Modifications to the closure in convection parameterization scheme greatly reduce the double‐ITCZ bias, demonstrating that the convection parameterization can substantially influence the double‐ITCZ bias in CESM2.2. Further analyses suggest that the convection parameterization can modulate the tropical atmosphere‐ocean feedback processes, through which it influences the upper ocean temperature advections, sea surface temperature, and hence precipitation in the southern ITCZ region. Key Points: Modifications to the closure in convection parameterization scheme greatly reduce the double‐ITCZ bias in all seasons in CESM2.2Convection scheme can modulate the tropical atmosphere‐ocean feedback processes, through which it influences the SST and double‐ITCZ biasesSurface heat flux changes play a limited role in reducing the SST and double‐ITCZ biases [ABSTRACT FROM AUTHOR]

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