S-Space College of Natural Sciences (자연과학대학) Dept. of Earth and Environmental Sciences (지구환경과학부) Theses (Ph.D. / Sc.D._지구환경과학부)
A Quasi-Stochastic Collection Model and Cloud and Precipitation Modeling
준확률 포착 모형과 구름 및 강수 모델링
- 자연과학대학 지구환경과학부
- Issue Date
- 서울대학교 대학원
- bin cloud microphysics ; WRF model ; clouds and precipitation ; aerosols ; hail climatology ; Mongolia ; improved quasi-stochastic collection model
- 학위논문 (박사)-- 서울대학교 대학원 : 자연과학대학 지구환경과학부, 2018. 2. 백종진.
- The evolution of cloud drop size distribution due to the collision-coalescence process is generally described by a quasi-stochastic model that solves the stochastic collection equation (SCE) in a deterministic way. In this study, an improved quasi-stochastic (IQS) model, which is derived by rigorously considering a finite model time step, is examined in the context of comparison with the normal quasi-stochastic (NQS) model. The IQS model allows a large collector drop to collide with a small collected drop more than one time in a model time step even if the collision probability is small. The number distribution of collector drops then follows the Poisson distribution with respect to the number of collisions. Using a box model that takes turbulence-induced collision enhancement (TICE) into account, it is found that large drops in the IQS model tend to have larger sizes than those in the NQS model and that the IQS model accelerates large-drop formation by a few minutes compared to the NQS model. The effects of the IQS model depend on the model time step and the shape of initial drop size distribution. The IQS model is incorporated into a detailed bin cloud microphysics scheme that is coupled with the Weather Research and Forecasting (WRF) model, and a single warm cloud is simulated under idealized environmental conditions. It is found that the onset of surface precipitation is accelerated in the IQS model.
The IQS model against the NQS model in precipitation prediction is evaluated. For this, a precipitation event observed over north central Mongolia on 21 August 2014 is simulated using the WRF model with a detailed bin cloud microphysics scheme. The surface precipitation amount is larger in the IQS model than in the NQS model, particularly over the strong precipitation region. The IQS model increases the mass contents of small drops and large drops due to multiple collisions. The increased large drops contribute to the increase in surface precipitation amount. The increased small drops are transported upward, which eventually leads to an increase in snow mass content. Deposition and riming in the IQS model occurs more actively, further increasing snow mass content. The increased snow mass content also contributes to the increase in surface precipitation amount through melting.
The impacts of aerosol loading on surface precipitation from mid-latitude deep convective systems are examined. For this, a precipitation case over north central Mongolia on 21 August 2014 is simulated with aerosol number concentrations of N0 = 150, 300, 600, 1200, 2400, and 4800 cm–3. The surface precipitation amount slightly decreases with increasing aerosol number concentration in the range of N0 = 150–600 cm–3, while it notably increases in the range of N0 = 600–4800 cm–3 (22% increase with eightfold aerosol loading). An attempt is made to explain why the surface precipitation amount increases with increasing aerosol number concentration in the N0 = 600–4800 cm–3 range. Higher aerosol number concentration results in more drops of small sizes. More drops of small sizes grow through condensation while being transported upward and some of them freeze, thus increasing the mass content of ice crystals. The increased ice crystal mass content leads to an increase in the mass content of small-sized snow particles largely through deposition, and the increased mass content of small-sized snow particles leads to an increase in the mass content of large-sized snow particles largely through riming. Also, more drops of small sizes increase the mass content of supercooled drops, which leads to an increase in the mass content of large-sized snow particles through riming. The increased mass content of large-sized snow particles resulted from these pathways contributes to a larger surface precipitation amount through melting and collision-coalescence.