Rain Reevaporation, Boundary Layer–Convection Interactions, and Pacific Rainfall Patterns in an AGCM

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Abstract
Sensitivity experiments with an atmospheric general circulation model (AGCM) show that parameterized rain reevaporation has a large impact on simulated precipitation patterns in the tropical Pacific, especially on the configuration of the model’s intertropical convergence zone (ITCZ). Weak reevaporation leads to the formation of a “double ITCZ” during the northern warm season. The double ITCZ is accompanied by strong correlation between precipitation and high-frequency vertical motion in the planetary boundary layer (PBL). Strong reevaporation leads to a better overall agreement of simulated precipitation with observations. The model’s double ITCZ bias is reduced. At the same time, correlation between high-frequency (periods < 15 days) vertical motion in the PBL and precipitation is reduced. Experiments with modified physics indicate that evaporative cooling by rain near the PBL top weakens the coupling between precipitation-related heating and vertical motion in high-frequency motions. The stren... Abstract Sensitivity experiments with an atmospheric general circulation model (AGCM) show that parameterized rain reevaporation has a large impact on simulated precipitation patterns in the tropical Pacific, especially on the configuration of the model’s intertropical convergence zone (ITCZ). Weak reevaporation leads to the formation of a “double ITCZ” during the northern warm season. The double ITCZ is accompanied by strong correlation between precipitation and high-frequency vertical motion in the planetary boundary layer (PBL). Strong reevaporation leads to a better overall agreement of simulated precipitation with observations. The model’s double ITCZ bias is reduced. At the same time, correlation between high-frequency (periods < 15 days) vertical motion in the PBL and precipitation is reduced. Experiments with modified physics indicate that evaporative cooling by rain near the PBL top weakens the coupling between precipitation-related heating and vertical motion in high-frequency motions. The stren...

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