Influence of Three Dimensional Thermal Radiation on Cloud Droplet Growth

太阳和热辐射驱动天气和气候，并强烈影响云的形成。然而，三维辐射对云的影响仍然知之甚少，也没有进行系统的研究。由于精确的三维辐射传输的复杂性和计算成本，辐射在当今的云解析模型中经常被忽略，或者至多使用忽略水平辐射传输的平面平行一维近似来处理。更好地了解云中发生的物理过程将改进数值天气预报和气候模式中的云参数化，从而改进天气和气候预报。提出了一种快速计算云分辨模型三维热辐射传输的方法。该方法首次允许详细研究三维辐射对云动力学和微物理的影响。首次研究三维热辐射对云动力学的影响是在提出者的博士论文中进行的。接下来的步骤是研究三维热辐射对云微物理的影响，这是拟议研究的目的。云侧和云顶的辐射冷却可以显著加速液滴的生长。云侧冷却是一种三维效应，不能用一维辐射传输近似来解释。辐射对液滴生长的影响可能会弥补现有液滴生长理论的空白：扩散液滴生长在大约10微米的液滴尺寸上明显减慢。然而，只有当液滴的最小尺寸为20微米时，才会发生以下碰撞和聚并过程。对于所提出的研究，需要一个云粒度解析微物理模型（bin微物理模型）。托管组（位于博尔德的NOAA）已经开发了这样一个模型（TAU云微物理代码），并且已经使用它很多年了。在本研究中，上述3D热辐射传输求解器将被实现到包含bin微物理的云解析模型中（例如主机组使用的System of Atmospheric Modeling (SAM)）。在第二步，模拟将建立并执行，以解决三维热辐射如何影响云滴生长或降水发展的问题。最后，将对模拟结果进行评估，并对三维辐射与无辐射或一维辐射模拟的影响进行研究。

Solar and thermal radiation drive weather and climate and strongly affect cloud formation. However, 3D radiation effects on clouds are still poorly understood and have not been investigated systematically. Due to the complexity and the computational costs of accurate 3D radiative transport, radiation is often neglected in today's cloud resolving models or, at best, treated using a plane-parallel 1D approximation which neglects horizontal radiation transport. A better understanding of the physical processes taking place in clouds will improve cloud parameterizations in numerical weather prediction and climate models and thus improve weather and climate prediction. A fast method to calculate 3D thermal radiative transfer in cloud resolving models was developed by the proposer. This method allowed for the first time to study 3D radiation effects on cloud dynamics and microphysics in detail. First studies of the effects of 3D thermal radiation on cloud dynamics were made within the PhD thesis of the proposer. The next consequent step is to investigate the effect of 3D thermal radiation on cloud microphysics which is the aim of the proposed study. Radiative cooling at cloud sides and cloud top could speed up droplet growth significantly. The cloud side cooling is a 3D effect which cannot be accounted for with 1D radiative transfer approximations. It is possible that the radiative effect on droplet growth could close the existing gap in droplet growth theory: Diffusional droplet growth slows down considerably at about 10 micron droplet size. However, the following process of collision and coalescence only takes place when droplets have a minimum size of 20 micron. For the proposed study, a cloud particle size resolving microphysical model (bin microphysics model) is needed. The hosting group (at NOAA, Boulder) has developed such a model (TAU Cloud Microphysical Code) and has been working with it for many years. For the proposed study, the above mentioned 3D thermal radiative transfer solver will be implemented into a cloud resolving model including bin microphysics (e.g. System of Atmospheric Modeling (SAM), used by the hosting group). In a second step, simulations will be setup and performed to address the questions how 3D thermal radiation affects cloud droplet growth or the development of precipitation. Finally, the simulation results will be evaluated and the effects of 3D radiation in comparison to simulations with no-radiation or 1D-radiation have to be investigated.