海洋浅对流云雨滴形成的微物理和动力机制的数值模拟研究

Shallow maritime cumulus clouds are one of the most prevalent cloud types in the tropical atmosphere. As one principal component of the Hadley cell, trade wind cumulus clouds play an important role in the global circulation. The importance of their precipitation has been increasingly realized for its impacts on marine boundary layer dynamics and cloud organization. The warm rain process is responsible for much of the precipitation in the Tropics and contributes to precipitation formation in many midlatitude convective clouds. However, there still has no aggreement on the warm rain formation mechanism. Shallow maritime cumulus clouds can produce rain in 15-20 min. The slow diffusional growth of large cloud droplets and the narrow spectra of cloud droplets in adiabatically ascending cloud parcels make it difficult to interpret the broad spectra of cloud droplets and rapid warm rain formation of in-situ observations. The existence of ultragiant condensation nuclei, turbulent, and inhomogeneous entrainment of unsaturated air into clouds has been applied to account for the formation of large cloud droplets. Homogeneous entrainment of unsaturated air is one possible mechanism to explain the existence of small cloud droplets at the cloud boundaries. However, in the core of cumulus clouds, the existence of broad spectra of cloud droplets and low liquid water content and temperatures compared with parcel theory calculations has puzzled us for a long time.?????.The role of ultra-giant aerosols, turbulence and entrainment mixing in warm rain formation under broad spectra of cloud droplets may be different from our current understanding. The aim of my research is to discover the relative importance of these three factors in warm rain formation of shallow maritime cumulus clouds in a real dynamical field. Numerical simulations compared with in-situ observations will be conducted to figure out the role of these three factors in autoconversion and accretion processes of cloud droplets. We have taken the first step in this direction to study the cumulus cloud spectra with the aid of a large eddy simulation (LES) version of the Advanced Research WRF (ARW) model coupled with high resolution spectral (bin) microphysics. We will focus on the composite case of the rain in cumulus over the ocean (RICO) field experiment. The following processes will be elucidated in our future studies.1..The relationship between giant and ultragiant condensation nuclei and the autoconversion rate of cloud droplets to embryonic raindrops. 2..Effects of air turbulence on autoconversion rate of cloud droplets to embryonic raindrops. 3..Effects of the vertical shear of horizontal mean wind on the autoconversion of cloud droplets to embryonic raindrops.

海洋浅对流云是热带大气一种大量发生的云系。海洋浅对流积云的暖雨形成过程决定这类云系所产生降水量的多少, 并且暖雨的形成过程对浅对流积云发展后期冰云的形成起着极其重要的作用。我们通过建立高分档的微物理模型，利用三维WRF模型来研究雨滴胚胎的形成过程。从而搞清楚以下科学问题：（1）对流初期上升气流的强弱与大核和巨核对云滴增长速度的影响。上升气流的大小决定云底过饱和度的大小以及核化的云滴浓度， 而上升气流的强弱可能影响着大核和巨核浓度，研究对流初期的动力和热力扰动与雨滴胚胎的形成的关系。（２）在三维对流空间，空气湍流对半径小于20微米的云滴粒子并合的影响。也就是研究涡流消散率和雷诺数在空间的不同分布条件下，最有利于雨滴胚胎生长的空间位置。（３）积云侧边界的挟卷对积云云滴谱的影响。从微物理的角度讲，挟卷后的混合有均质混合和异质混合两种可能对云滴谱产生影响的机制。我们关注的是动力过程对云滴谱的影响。

项目组针对海洋性浅对流云暖雨形成的滴谱拓宽问题进行了研究，把分档云微物理方案耦合到中尺度数值模式WRF的大涡模块LES中。在分档微物理方案中，采用了二阶量的方法计算云滴粒子的凝结增长和碰并增长，能够抑制数值扩散的问题。对气溶胶方案进行了改进，利用一维分档模式的结果，分析了半径较大的气溶胶活化后的云滴粒子的半径尺度，气溶胶活化后不再被清空，增加了伴随云滴碰并和雨滴沉降过程的气溶胶的碰并和沉降。分析了外场观测试验RICO的雷达飞机等资料，利用理想廓线和RICO试验实际观测的廓线进行了数值模拟，讨论了大核巨核、湍流碰并、夹卷混合、风切变和初始扰动等对于海洋性浅对流云雨滴形成的影响。通过分析得到一些结果，分析对比大粒子的位置和夹卷非均匀混合的位置，可以发现大粒子出现的位置高于夹卷非均匀混合的位置，同时大粒子出现的时间要早于造成夹卷混合的涡旋环流形成的时间，可以认为夹卷非均匀混合对于雨胚的出现没有起到主要作用。湍流碰并对云滴粒子的加速增长并不明显，湍流碰并对雨滴胚胎粒子的形成没有起到主要作用。大滴粒子出现的快慢与过饱和度直接相关，主要与垂直上升气流的强弱相关。通过项目组成员的努力，建立了三维高时空分辨率考虑云和气溶胶相互作用的分档暖云模式，为今后进行研究云微物理方案参数化方法及积云对流参数化方法等工作提供了基础。通过对气溶胶方案的改进，为后面研究积云侧边界夹卷混合及气溶胶二次活化问题和气溶胶的增长、沉降及气溶胶的湿清除问题提供了基础。通过分析观测和模拟结果，认为在海洋性浅对流云雨滴的形成中，除了要考虑大核巨核、湍流碰并和夹卷混合机制在暖雨形成过程中的作用外，上升气流的大小是最需要考虑和关注的影响因子。因此，在以后研究我国大陆地区积云的暖雨形成时，需要进行飞机对积云动力场的观测研究。