SGER: Upgrading Community Atmosphere Model (CAM) Cloud Macrophysics

气候模型是一个复杂的软件系统，其各个部分必须正确地协同工作。这对于它的大气“湿物理”参数化来说是一个特别的挑战--湍流、积云对流、云形成、云-气溶胶相互作用和降水，因为这些过程在比气候模式网格单元小得多的长度尺度上紧密地相互作用。社区大气模式(CAM)是世界领先的社区开发的气候模式的大气组成部分。几个研究小组的协调努力，松散地组织成一个CAM微物理倡议，正在彻底改造大多数潮湿物理参数。他们的目标是减少CAM的云和降水偏差，并使全球能够可信地模拟人为产生的气溶胶对云的影响。在这些努力中，我们发现了用于“云宏观物理”的CAM公式中的一些重要缺陷。“云宏观物理”是一种确定云量分数以及在每个模式网格单元中凝结和蒸发的液体和冰云粒子质量的参数。这些缺点涉及云分数和云凝结计算之间的不一致，这往往导致“空云”(没有云粒子的云区)，特别是在浅积云对流区域。在高纬度地区，CAM云预测对调用不同参数化的顺序的任意更改很敏感。这项研究的目标是实现和测试一套详细的代码更改，这些更改将：(1)更好地处理积云和层状云之间的相互作用，改善有有限云分数但没有凝结的空云问题。(2)确保云宏观物理在适当的状态下运行。(3)使用更合理的闭合来计算网格单元云区的凝结/蒸发速率。(4)使用云量和相对湿度之间的关系，类似于其他主要气候中心使用的方案。这项研究必须在未来几个月内完成，以便及时进行评估，以便纳入下一代社区气候系统模式4(CCSM 4)，该模式将于2009年初冻结，并应用于政府间气候变化专门委员会(气专委)第五次评估。模型的改变将提高在CAM中处理云过程的内部一致性。它们将允许所有潮湿过程有助于在每个网格单元内实现一致和物理上可实现的云凝结的次网格空间分布。所有更改都是基于已出版文献中的建议。更广泛的影响。许多NCAR、能源部和大学科学家使用CCSM作为他们的主要气候模型。这项研究将为在微物理、积云对流和湿湍流参数化方面进一步改进代码提供坚实的基础，这些参数是产生人类诱导的气溶胶对云和气候的影响的尖端模拟以及对21世纪气候变化的更好预测所必需的。这些变化将可用于模式模拟，以支持气专委第五次评估，对云-气溶胶反馈的更好模拟正在成为该评估的一个重要推动力。

A climate model is a complex software system whose parts must work properly together. This is a particular challenge for its atmospheric 'moist physics' parameterizations - turbulence, cumulus convection, cloud formation, cloud-aerosol interaction and precipitation, because these processes tightly interact on length scales much smaller than a climate model grid cell. The Community Atmosphere Model (CAM) is the atmospheric component of the world's leading community-developed climate model. Coordinated efforts by several research groups, loosely organized into a CAM Microphysics Initiative, are completely revamping most of the moist physics parameterizations. Their aim is to reduce CAM's cloud and precipitation biases and to enable credible global simulation of the effect of human-produced aerosol on clouds. During these efforts, some important shortcomings have been found in the CAM formulation for 'cloud macrophysics'- the parameterizations that determine the fractional cloud cover and the masses of liquid and ice cloud particles that condense and evaporate in each model grid cell. These shortcomings involve inconsistencies between the calculation of cloud fraction and cloud condensate that often result in 'empty clouds' (cloud area with no cloud particles), especially in regions of shallow cumulus convection. In high-latitude regions, CAM cloud predictions are sensitive to arbitrary changes to the order of calls to different parameterizations. The goal of this research is to implement and test a set of detailed code changes that will: (1) Better treat the interaction between cumulus and stratiform cloud cover, ameliorating issues of 'empty clouds' with finite cloud fraction but no condensate.(2) Ensure that the cloud macrophysics is operating on an appropriate state. (3) Use a more justifiable closure to calculate the rate of condensation /evaporation in the cloudy part of grid cells. (4) Use a relationship between cloud fraction and relative humidity that is analogous to schemes used by other leading climate centers. This research must be completed within the next several months in order to be evaluated in time to be included in the next generation Community Climate System Model 4 (CCSM 4), to be frozen in early 2009 and applied to the Intergovenmental Panel on Climate Change (IPCC) fifth assessment.Intellectual merit. The model changes will improve the internal consistency of the treatment of cloud processes in CAM. They will allow all moist processes to contribute to a consistent and physically realizable subgrid spatial distribution of cloud condensate within each grid cell. All changes are based on recommendations from the published literature. Broader Impacts. Many NCAR, DOE and university scientists use the CCSM as their primary climate model. This research will provide a solid basis for further code improvements in the microphysics, cumulus convection, and moist turbulence parameterizations that are needed to produce a cutting-edge simulation of human-induced aerosol effects on clouds and climate and better projections of 21st century climate change. These changes will be ready to use for model simulations in support of the 5th IPCC Assessment, for which better simulation of cloud-aerosol feedbacks is emerging as an important thrust.