Process-oriented Investigation of Double InterTropical Convergence Zone (ITCZ) Biases in National Center for Atmospheric Research Community Earth System Model (NCAR CESM)

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The intertropical convergence zone (ITCZ) is a narrow band of deep convective clouds and heavy rain that extends across the tropical oceans just north of the equator. It gets its name from the convergence of the trade winds from the Northern and Southern Hemispheres, which promotes rising motions and cloud formation where the winds come together. An unfortunate feature of most climate models is that they generate a spurious secondary ITCZ, running parallel to the real ITCZ across the Pacific ocean just south of the equator. This "double-ITCZ" bias, which has been a persistent feature of model simulations for at least two decades, is generally ascribed to deficiencies in the representation of convective clouds. Convective clouds are too small to simulate explicitly on typical climate model grids and thus they have to be represented indirectly through parameterization schemes. Recently the Principal Investigators (PIs) of this project made improvements to the commonly used Zhang-McFarlane (ZM) convective parameterization scheme which substantially reduced the double-ITCZ bias in simulations of the Community Earth System Model (CESM, in particular CESM1). The goal of this project is to explain why these changes reduced the bias, and to indentify more generally the atmospheric and oceanic mechanisms through which inadequacies in the representation of convection lead to the formation of a secondary ITCZ.Fundamentally, deep convection occurs in response to the build-up of convective available potential energy (CAPE) in an atmospheric column, and the vertical exchange of thermal energy in cloud updrafts removes the CAPE and stabilizes the column to further vertical motions. But convective parameterizations which enforce this process too strictly tend to produce frequent and persistent weak convection instead of the more sporadic and intense convection seen in the real world. The revised ZM scheme replaces CAPE with dynamic CAPE (dCAPE), meaning CAPE generated above the planetary boundary layer (PBL, the lowest kilometer or two of the atmosphere which directly feels the effect of the surface). Physically, the use of dCAPE assumes that the CAPE generated in the PBL, in particular the CAPE generated by surface heating and evaporation, is largely consumed by shallow convection in the PBL and thus does not directly initiate deep convection. This formulation has been shown to produce a more realistic diurnal cycle of convection over land. Other changes to the ZM scheme involve the respresentation of cloud dynamics, in particular the representation of entrainment, meaning the mixing of moist air within clouds with the drier environmental air surrounding the cloud. It is not surprising that changes in entrainment can have substantial effects on simulated circulation and climate but the effects are difficult to anticipate and explain.A number of experiments are conducted to examine the effects of cloud changes on the double-ITCZ bias. Some involve looking at each change in parameterization separately to understand its contribution to ameliorating the bias. Others look at the effects of air-sea interactions in reducing the bias. Preliminary work shows that the simulated sea surface temperature (SST) in the region of the spurious ITCZ is relatively cold, which is counterintuitive since convection usually occurs over warmer SST. The PIs thus hypothesize that convection is promoted over the colder SSTs due to the non-local effect of even colder SSTs to the north, and model experiments with imposed SSTs and surface wind stress are performed to test this hypothesis.The ITCZ bias is a practical concern as well as a scientific problem, as the bias affects most models used to inform decision-making in the face of climate change. An understanding of how the properties of simulated convection contribute to the bias would also benefit the worldwide research community which depends on climate models as tools for understanding the climate system. Improvements to CESM have a direct benefit as it is the most widely used climate model in the world. In addition, the project supports a postdoctoral fellow, thereby developing the scientific workforce in this area. Summer internships for undergraduate are also provided through the project.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.