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

在一年中的大部分时间里，东太平洋赤道上的降水反差很大，赤道以北的热带辐合带(ITCZ)有强对流降水，而赤道以南只有云层和毛毛雨。造成降水对比的几个因素：在ITCZ中，地面风的辐合提供了抬升，低大气稳定度促进了对流云，高海面温度(SST)通过蒸发提供了水汽，而相反的条件则保持在南部。但从2月左右开始，赤道以南又出现了第二个ITCZ，从2月到4月，东太平洋出现了双重ITCZ，赤道南北约5度处有对流降雨。这种次级ITCZ在南半球的出现为理解ITCZ的成因提供了机会，特别是产生从不利的深对流条件转变为有利的深对流条件的动力和热力学机制。本文研究了东南太平洋次级ITCZ的成因，特别是信风逆温(TWI)，这是一个将潮湿的大气边界层与其上方更温暖、更干燥的自由对流层分开的浅层。热带气旋的高度稳定性直接抑制了ITCZ形成所需的对流，反而促进了遮蔽东南太平洋的层积云(SC)云层，产生了冷的海温，进一步抑制了对流。由于边界层风通常由较冷的海温吹向较暖的海温，冷的海温也会阻止边界层风的辐合，从而抑制ITCZ的形成。该项目考虑了降低东南太平洋地区热带气旋稳定性的因素，特别是北半球ITCZ边界层湿静力能量的输送，以及由于稳定性降低可能导致层云覆盖的减少，这可能导致海温变暖。进一步的工作利用动量收支分析来理解从地面风辐散到辐合的变化，其中一个想法是横跨TWI的下行动量输送起到了重要作用。这项工作是通过分析观测数据集和气候模型模拟，以及使用共同体大气模型(CESM)的大气成分模型为该项目执行的专门模拟进行的。通过支持和培训研究生，这项工作具有更广泛的教育影响。此外，该项目通过科罗拉多州立大学的NSF支持的本科生研究体验(REU)网站和大学大气研究公司运营的大气研究与科学重大机会(SOARS)计划，部分支持在夏季进行研究的本科生。研究结果对气候模型的开发很有意义，因为大多数气候和地球系统模型都存在双重的ITCZ偏差，即南半球的ITCZ全年大部分时间都在太平洋的大部分地区持续存在。这个奖项反映了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。

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.