一个统计低云方案及其在大气环流模式中应用初探

A STATISTICAL LOW-LEVEL CLOUD SCHEME AND ITS TENTATIVE APPLICATION IN A GENERAL CIRCULATION MODEL

文中利用湍流耗散特征时间尺度和湍流垂直扩散系数对湍流二阶距进行参数化 ,将一个统计云方案与一阶湍流闭合方案进行耦合。基于数值试验 ,在不同的相对湿度、温度垂直梯度、以及湍流耗散特征时间尺度条件下 ,对该方案云量模拟能力的分析 ,发现该统计云方案对其采用的参数及湍流耗散特征时间尺度敏感。基于该数值模拟分析 ,修改了该统计云方案 ,并结合其他边界层积云参数化方案 ,给出了一个基于统计的低云参数化方案。将其初步应用于NCARCCM 3后发现 :该方案可以显著增强CCM 3对副热带低云的模拟能力 ,可以合理地模拟出大洋东部大陆西岸冷海域低云量大值中心 ,显示出该方案对于改进大气环流模式低云参数化具有潜在的应用前景。

In this study, a statistical cloud scheme is first introduced and coupled with a 1st-order turbulence scheme with 2nd-order turbulence moments parameterized by the timescale of the turbulent dissipation and the vertical turbulent diffusion coefficient. Then the ability of the scheme to simulate cloud fraction at different relative humidity, vertical temperature profile, and the timescale of the turbulent dissipation is examined by numerical simulation. It is found that the simulated cloud fraction is sensitive to the parameter used in the statistical cloud scheme and the timescale of the turbulent dissipation. Based on the analyses, the introduced statistical cloud scheme is modified. By combining the modified statistical cloud scheme with a boundary layer cumulus scheme, a new statistically-based low-level cloud scheme is given and tentatively applied in NCAR CCM3. It is found that the simulation of low-level cloud fraction is markedly improved and the centers with maximum low-level cloud fractions are well simulated in the cold oceans off the western coasts with the new statistically-based low-level cloud scheme applied in CCM3. It suggests that the new statistically-based low-level cloud scheme has a great potential in general circulation model for improving the low-level cloud parameterization.