Collaborative Research: EaSM-3: Understanding the Development of Precipitation Biases in CESM and the Superparameterized CESM on Seasonal to Decadal Timescales

用于研究年代际气候变率和预测的最新全球模式的偏差问题一直是气候科学领域的一个长期问题。热带降水、环流和海表温度（SST）的偏差会影响季节到年际、年代际和长期气候变化的模拟。此外，在这一努力下，三名博士生将接受培训，从事气候建模的职业生涯，为国家科学劳动力的能力作出贡献。该项目的目标是减少社区地球系统模型（CESM）和一个新版本的CESM的热带降水偏差，其中包括超参数化云方案，SP-CESM。热带降水偏差被归因于积云参数化不佳、海洋耦合反馈、海洋涡动混合不足、陆-气相互作用以及低云不足的远程影响。通过观察模拟的前十年中偏差的初始发展，将误差的来源分开：积云参数化误差在几天到几周内发展，局部海洋耦合误差在几个月内发展，中纬度低云偏差的远程效应可能需要数年才能影响热带降水。这种初始降雨和SST偏差的发展将用于测试CESM和SP-CESM中云微物理参数化的改进，以及CESM中积云参数化的变化。影响南半球中纬度低云覆盖的变化将是一个焦点。目标是在保持良好的厄尔尼诺/南方涛动变率的同时，大幅度减少两种模式在初始多年预测和长期预测中热带海温和降水的偏差。

The issue of bias in state-of-the-art global models that are used to study decadal climate variability and prediction has been a long-standing problem in the field of climate science. Biases in tropical precipitation, circulation, and sea-surface temperature (SST) affect simulations of seasonal to interannual, decadal, and longer-time climate variations. Additionally, under this effort three doctoral students will be trained for careers in climate modeling, contributing to national scientific workforce capabilities.The goal of this project is to reduce tropical precipitation biases in Community Earth System Model (CESM) and a new version of the CESM that incorporated the superparameterized cloud scheme, SP-CESM. Tropical precipitation biases have been attributed to poor cumulus parameterizations, ocean coupling feedbacks, ocean eddy mixing deficiencies, land-atmosphere interaction, and remote effects of insufficient low clouds. Sources of error will be separated by looking at the initial development of the biases over the first decade of simulations.Cumulus parameterization errors develop within days to weeks, local ocean coupling errors develop in months, and the remote effect of midlatitude low cloud biases may take years to affect tropical precipitation. This initial rainfall and SST bias development will be used to test improvements to the cloud microphysical parameterizations in CESM and SP-CESM, as well as changes to the cumulus parameterization in CESM. Changes affecting Southern Hemisphere midlatitude low cloud cover will be a focus. The goal will be to significantly reduce the tropical SST and precipitation bias in both models for both initialized multiyear forecasts and long-term projections while maintaining good ENSO variability.