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

热带辐合区(ITCZ)是一个狭窄的深对流云和暴雨带，横跨赤道以北的热带海洋。它的名字来源于来自南北半球的信风的汇合，这促进了上升的运动和云的形成，在那里风聚集在一起。大多数气候模型的一个不幸特征是，它们产生了一个虚假的次要ITCZ，与真正的ITCZ平行，横跨赤道以南的太平洋。这种“双重ITCZ”偏向至少二十年来一直是模式模拟的一个持续特征，通常被归因于对流云表示的不足。对流云太小，不能在典型的气候模式网格上进行显式模拟，因此必须通过参数化方案间接表示。最近，该项目的首席研究人员(PI)对常用的张麦克法兰(ZM)对流参数化方案进行了改进，大大减少了共同体地球系统模式(CESM，特别是CESM1)模拟中的双重ITCZ偏差。这个项目的目的是解释为什么这些变化减少了偏差，并更普遍地确定了大气和海洋机制，通过这些机制，对流表示的不足导致了二次ITCZ的形成。最基本的是，深层对流是对大气柱中对流可用势能(CAPE)积聚的响应，云上升气流中的热能垂直交换去除了CAPE，并使柱稳定在进一步的垂直运动中。但是，过于严格地执行这一过程的对流参数化往往会产生频繁而持久的弱对流，而不是现实世界中更零星和更强烈的对流。修订后的ZM方案用动态CAPE(DCAPE)取代了CAPE，这意味着在行星边界层(PBL，直接感受到表面影响的大气最低处或两公里)上方产生的CAPE。在物理上，dCAPE的使用假定边界层中产生的CAPE，特别是由地面加热和蒸发产生的CAPE，主要被PBL中的浅对流消耗，因此不会直接引发深层对流。这一公式已被证明在陆地上产生了更真实的日对流周期。ZM方案的其他变化涉及云动力学的重新呈现，特别是卷吸，即云内潮湿空气与云周围较干燥的环境空气的混合。卷吸的变化可以对模拟的环流和气候产生实质性的影响，这并不令人惊讶，但这种影响很难预测和解释。许多实验被用来检验云变化对双重ITCZ偏差的影响。其中一些涉及单独查看参数化中的每个变化，以了解其对改善偏差的贡献。其他人则着眼于海-气相互作用在减少偏差方面的影响。初步工作表明，在虚假的ITCZ区域模拟的海面温度(SST)相对较低，这是违反直觉的，因为对流通常发生在较暖的SST上。因此，PI假设由于北部更冷的SST的非局地效应，对流在较冷的SST上被促进，并进行了施加SST和地面风应力的模式实验来检验这一假设。ITCZ偏差既是一个实际问题，也是一个科学问题，因为这种偏差影响了大多数用于在面对气候变化时提供决策信息的模型。了解模拟对流的性质如何造成偏差，也将有利于世界各地的研究界，因为他们依赖气候模型作为了解气候系统的工具。对CESM的改进有一个直接的好处，因为它是世界上使用最广泛的气候模型。此外，该项目还支持一名博士后研究员，从而发展这一领域的科学队伍。该项目还为本科生提供暑期实习机会。这一奖励反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.