气候模式中云的次网格结构对全球辐射影响的研究

A study of the effect of sub-grid cloud structure on global radiation in climate models.

利用一种用于大尺度天气、气候模式的随机云产生器(SCG)和独立气柱近似(ICA)辐射算法,研究了次网格云的水平结构以及垂直重叠结构对全球辐射场的影响。比较了水平非均匀云(IHCLD)和水平均匀云(HCLD)的辐射场差异以及云的最大-随机重叠(MRO)和一般重叠(GenO)的辐射场差异。结果显示,与HCLD相比,IHCLD一方面可增加地面净短波辐射通量,纬向平均最大值(约1 W/m^2)和次大值(约0.6 W/m^2)分别位于高纬度低云密集地区和对流旺盛的热带地区;另一方面可增加大气顶的净长波辐射通量,纬向平均最大值(0.3 W/m^2)出现在热带地区。不同的重叠结构对短波和长波辐射收支也有很大的影响,MRO和GenO的短波辐射通量差异在热带辐合带最大,达到30—40 W/m^2,在高纬度低云带的纬向平均也可达到5 W/m^2左右;长波辐射通量差异具有相似的地区分布,但量值相对较小。不同重叠结构可以造成大气上下层的辐射加热率差异,影响大气热力层结。云的水平和垂直结构对有云区域辐射收支的影响将改变大气热力、动力状况以及水汽条件,从而影响模拟的气候系统的演变。文中采用单向云-辐射计算,排除了与气候系统其他过程复杂的相互作用,从而使其结果具有一定的普适性,可为不同大尺度模式进行次网格云辐射参数化提供参考。

A recently developed stochastic sub-grid cloud generator（SCG） and independent column approximation（ICA） are used to study the effect of horizontal heterogeneity and vertical overlap of clouds on global radiative fields.Results of experiments IHCLD（In-homogeneous Clouds） and HCLD（Homogeneous Clouds） show that horizontally inhomogeneous clouds lead to both larger net shortwave flux at surface（FSNS） and larger net longwave flux at the top of atmosphere（FLNT）.The largest（about 1 W/m2 ） and second largest（about 0.6 W/m2 ） increases in FSNS occur at the higher latitude where low clouds dominant and in the tropics where convection movement is quite active,respectively.However,the largest increases in FLNT（about 0.3 W/m2 ） occur in the tropics due to tremendous high clouds.Experiments GenO（General Overlap） and MRO（Maximum-Random Overlap） suggest a stronger signal of impact on the shortwave and longwave radiation budget.Regionally,the most significant differences of FSNS reach up to 30 - 40 W/m2 in ITCZ regions,as well as a zonal mean difference of about 5 W/m2 at the higher latitudes with large amount of low clouds.There are similar spatial distributions for the differences of FLNT,but with smaller value.Overlap-assumption shift can also trigger opposite changes to radiative heating rate in the upper and lower atmospheric layers,thus influences the thermodynamic structure of the atmosphere.The impacts of cloud horizontal and vertical structure on radiation budget will surely carry over to the thermodynamic, dynamic status and water vapor conditions of the atmosphere,and consequently change the evolution of modeled climate. The off-line cloud-radiation calculation makes the results generally applicable,so as to provide valuable information when parameterize cloud-radiation process in different models.