Collaborative Research: The Relationship between the Trade Wind Inversion Layer and the Seasonal Development of the Southeast Pacific Inter-Tropical Convergence Zone (ITCZ)

在一年的大部分时间里，东太平洋赤道地区的降水对比强烈，赤道以北的热带辐合带（ITCZ）有强对流降水，而南部只有云量和毛毛雨。有几个因素促成了降雨对比：在ITCZ，地面风辐合提供了抬升，低大气稳定性促进了对流云，高海温（SSTs）通过蒸发提供水分，而南部则相反。但从2月左右开始，赤道以南出现了第二个ITCZ，从2月到4月，东太平洋出现了双ITCZ，在赤道以北和以南5度左右有对流降雨。因此，南半球次级ITCZ的出现为了解ITCZ的成因提供了一个机会，特别是产生深层对流从不利条件向有利条件转变的动力和热力学机制。这里进行的研究考察了东南太平洋次级ITCZ的成因，特别关注了信风逆温（TWI），这是一个浅层，将潮湿的大气边界层与其上方温暖干燥的自由对流层分开。TWI的高稳定性直接抑制了ITCZ形成所必需的对流，反而促进了遮蔽东南太平洋的层积云（Sc）云层，产生了进一步阻碍对流的冷海温。由于边界层风通常从较冷的海温吹向较暖的海温，冷海温也通过抑制边界层风辐合来抑制ITCZ的形成。该项目考虑了降低东南太平洋TWI稳定性的因素，特别关注了来自北半球ITCZ的边界层湿静态能量的输送，以及由于稳定性降低而可能导致的层云覆盖减少，这可能导致海温变暖。进一步的工作使用动量预算分析来理解从地面风辐散到辐合的变化，其中一种观点是，跨TWI的向下动量输送起着重要作用。这项工作是通过分析观测数据集和气候模式模拟，以及利用社区大气模式、社区地球系统模式（CESM）的大气成分模式为该项目进行的专门模拟来进行的。通过对研究生的支持和培训，这项工作具有更广泛的教育影响。此外，该项目支持本科生在夏季进行研究，部分通过美国国家科学基金会支持的科罗拉多州立大学本科生研究经验（REU）网站和由大学大气研究公司运营的大气研究与科学重大机遇（SOARS）计划。研究结果对气候模式的发展具有重要意义，因为大多数气候和地球系统模式都存在双重ITCZ偏差，其中南半球ITCZ在太平洋大部分地区持续大部分时间。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

For most of the year there is a strong rainfall contrast across the equator in the eastern Pacific, with heavy convective precipitation in the intertropical convergence zone (ITCZ) north of the equator but only cloud cover and drizzle to the south. Several factors contribute to the rainfall contrast: in the ITCZ surface wind convergence provides uplift, low atmospheric stability promotes convective clouds, and high sea surface temperatures (SSTs) supply moisture through evaporation, while opposite conditions hold to the south. But starting around February a second ITCZ develops south of the equator and from February to April the eastern Pacific features a double ITCZ, with convective rainfall around 5 degrees north and south of the equator. The appearance of this secondary ITCZ in the Southern Hemisphere thus provides an opportunity to understand ITCZ genesis, in particular the dynamic and thermodynamic mechanisms that produce the transition from unfavorable to favorable conditions for deep convection.Research conducted here examines the genesis of the secondary ITCZ in the southeast Pacific with a particular focus on the trade wind inversion (TWI), a shallow layer which separates the moist atmospheric boundary layer from the warmer and drier free troposphere above it. The high stability of the TWI directly inhibits the convection necessary for ITCZ formation and instead promotes the stratocumulus (Sc) cloud deck which shades the southeast Pacific, producing cold SSTs which further discourage convection. Cold SSTs also inhibit ITCZ formation by discouraging boundary layer wind convergence, as boundary layer winds generally blow from colder to warmer SST. The project considers the factors which reduce the stability of the TWI in over the southeast Pacific, looking in particular at the transport of boundary layer moist static energy from the Northern Hemisphere ITCZ, as well as the possible reduction of stratus cloud cover due to reduced stability, which could lead to warmer SSTs. Further work uses analysis of momentum budgets to understand the change from surface wind divergence to convergence, one idea being that downward momentum transport across the TWI plays an important role. The work is conducted through analysis of observational datasets and climate model simulations, as well as specialized simulations performed for the project using the Community Atmosphere Model, the atmospheric component model of the Community Earth System Model (CESM).The work has educational broader impacts through the support and training of a graduate student. In addition, the project supports undergraduate students who conduct research during the summers, in part through the NSF-supported Research Experience for Undergraduates (REU) site at Colorado State University and the Significant Opportunities in Atmospheric Research and Science (SOARS) Program run by the University Corporation for Atmospheric Research. Research results are of interest for climate model development as most climate and earth system models suffer from a double ITCZ bias, in which a Southern Hemisphere ITCZ persists for most of the year over much of the Pacific ocean.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.