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

国家大气研究中心社区地球系统模型 (NCAR CESM) 中双热带辐合带 (ITCZ) 偏差的面向过程调查

基本信息

批准号：
2054697
负责人：
Guang Zhang
金额：
$ 68.04万
依托单位：
University of California-San Diego Scripps Inst of Oceanography
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2021
资助国家：
美国
起止时间：
2021-07-15 至 2024-06-30
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=2054697&HistoricalAwards=false
关键词：
Process oriented Investigation Double InterTropical

项目摘要

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.

热带辐合带（ITCZ）是一条狭窄的深对流云和暴雨带，横跨赤道以北的热带海洋。它因北半球和南半球信风的汇聚而得名，信风会促进风聚集处的上升运动和云的形成。大多数气候模型的一个不幸特征是，它们会生成一个虚假的次要 ITCZ，与赤道以南太平洋对面的真实 ITCZ 平行。这种“双 ITCZ”偏差至少二十年来一直是模型模拟的一个持续特征，通常归因于对流云表示的缺陷。对流云太小，无法在典型的气候模型网格上明确模拟，因此必须通过参数化方案间接表示。最近，该项目的主要研究者（PI）对常用的Zhang-McFarlane（ZM）对流参数化方案进行了改进，大大减少了社区地球系统模型（CESM，特别是CESM1）模拟中的双ITCZ偏差。该项目的目标是解释为什么这些变化减少了偏差，并更普遍地确定对流表示的不足导致形成次级 ITCZ 的大气和海洋机制。从根本上说，深层对流的发生是为了响应大气柱中对流可用势能 (CAPE) 的积累以及云上升气流中热能的垂直交换消除了 CAPE 并使大气柱稳定以进一步垂直运动。但是对流参数化过于严格地执行这一过程，往往会产生频繁且持续的弱对流，而不是现实世界中看到的更加零星和强烈的对流。修改后的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层最低一公里或两公里）之上产生的CAPE。物理上，dCAPE的使用假设PBL中产生的CAPE，特别是由表面加热和蒸发产生的CAPE，大部分被PBL中的浅对流消耗，因此不会直接引发深层对流。这种公式已被证明可以产生更真实的陆地对流昼夜循环。 ZM 方案的其他变化涉及云动力学的表示，特别是夹带的表示，即云内潮湿空气与云周围干燥环境空气的混合。毫不奇怪，夹带的变化会对模拟的环流和气候产生重大影响，但这种影响很难预测和解释。进行了许多实验来检查云变化对双 ITCZ 偏差的影响。有些涉及分别查看参数化的每个变化，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对减少偏差的影响。初步研究表明，虚假 ITCZ 区域的模拟海面温度 (SST) 相对较冷，这是违反直觉的，因为对流通常发生在较温暖的海表温度上。因此，PI 假设，由于北方更冷的海表温度的非局部效应，对流在更冷的海表温度上得到促进，并进行了施加海表温度和表面风应力的模型实验来检验这一假设。ITCZ 偏差是一个实用的模型。这既是一个令人担忧的问题，也是一个科学问题，因为这种偏见影响着大多数用于应对气候变化决策的模型。了解模拟对流的特性如何造成偏差也将有利于全世界的研究界，因为他们依赖气候模型作为理解气候系统的工具。 CESM 的改进有直接的好处，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，从而培养该领域的科学队伍。该项目还为本科生提供暑期实习机会。该奖项反映了 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。