使用理想气候模型对于早期火星大气中水冰云增暖效应的研究
丁峰
地质证据表明火星在距今30～40亿年前存在着多次时间尺度长于100年的利于湖泊形成的气候态。这些早期温暖的气候无法仅用二氧化碳和水汽的温室效应来解释。近期，针对地表水有限分布的气候态，由高空水冰云造成的早期火星变暖机制被提出。但是云和降水的微物理参数化仍然是类地行星（包括现代地球）气候模拟的主要不确定因素之一。这里我们开发了针对类地行星大气的理想三维湿大气环流模式，在其中使用了简化的云微物理方案来约束物理参数空间中高空水冰云的潜在变暖效应。在我们的大气环流模式中，云汇项以一为常量的时间尺度表征，可将其解释为大气中云粒子存在的时间尺度。模拟结果显示随着云粒子生存时间尺度的变化，出现了寒冷和温暖两种不同的气候态。全球极端寒冷的气候在云粒子生存尺度为15天时急剧转变为温暖的气候态，这种剧烈的气候转变源于冰面升华，云冰路径和大气温室效应之间的强正反馈循环。当我们在数值模拟中改变地表冰盖分布，地表二氧化碳气压和行星倾角时，这种剧烈的气候转变的行为都会存在并且大都发生于云粒子具有约为10天的生存尺度。我们的研究结果表明，古代火星大气条件下云微物理（例如，云冰粒径谱分布，云冰转换成雪的时间尺度，雪粒径谱分布）的理论或实验研究对于量化早期火星大气中高层水冰云的变暖效应至关重要。
Constraining the Warming Effect of High-altitude Water Ice Clouds on Early Mars in a 3D Moist General Circulation Model with a Simplified Cloud Microphysical Scheme
Geologic evidence suggests >102-yr-long lake-forming climates persisted on Mars 3-4 Ga. These early warm climates cannot be explained by the greenhouse effect of CO2 and water vapor alone. Recently, a warming mechanism for early Mars based on high-altitude water ice clouds was proposed, for situations where the surface water inventory is limited and far away from tropical regions. However, microphysical representations of clouds and precipitation remain one of the main uncertain factors for climate modeling of terrestrial planets, including present-day Earth. Here we use a three-dimensional moist general circulation model (3D moist GCM) with a simplified cloud microphysical scheme to constrain the potential warming effect of high-altitude water ice clouds in a physical parameter space. In our GCM, the cloud sink term is characterized by a constant timescale that can be interpreted as the conversion timescale from clouds to precipitation. Two distinct climate regimes emerge as the conversion timescale of cloud particles is varied in the GCM simulations, separated by the time scale of ~10 days when the global climate dramatically jumps from a cold state to a warm state. We show that this dramatic climate transition results from a strong positive feedback loop among surface evaporation, cloud mass, and the atmospheric greenhouse effect, and develop a toy model with such positive feedback to reproduce the GCM simulations. The behavior of dramatic climate transition is robust as we vary the surface ice distribution, surface CO2 pressure, and the obliquity of the planet in the GCM simulations. Our findings suggest that theoretical or experimental studies on cloud microphysics (e.g., cloud radii, conversion timescales) in ancient Mars' atmospheric conditions are crucial for quantifying the warming effect of high-level water ice clouds.
Left panel: Global annual mean surface temperature as a function of the lifetime of cloud particles τc in SP (black solid) and EQ (red dashed) simulations with standard parameters. Right panels: Surface water ice distributions in the SP and EQ simulations. Brown shading represents land.
