二流—四流球谐函数谱展开累加辐射传输方案在全球气候模式中的应用

Application of Two-Four Stream Spherical Harmonic Expansion Approximation in a Global Climate Model

本文首先构建了二流—四流球谐函数谱展开累加辐射传输的新方案,然后将其应用于国家气候中心第二代大气环流模式BCC_AGCM2.0.1的新版本中,并与模式中原有的Eddington累加方案进行了比较。由于新方案本质上是单层Eddington近似方案在四流上的推广。因此新方案在计算精度上要优于原方案。通过在全球气候模式中的应用与比较,本文发现新方案对气候模拟会产生比较大的影响。在晴空条件下,新方案计算的在南纬30°到60°区间、北大西洋东北部以及非洲北部的撒哈拉沙漠区域的地表向下年平均短波辐射通量要小于原方案结果,最大差别可以达到3.5 W/m2;同时,新方案计算的在南纬30°到60°区间和北大西洋东北部的大气顶向上年平均短波辐射通量要大于原方案结果,最大差别达到3 W/m2。在有云大气情况下,新方案计算的地表向下年平均短波辐射通量要小于原方案结果,并随着纬度的增加,新旧两种方案的差别逐渐变大,在南北极时达到最大5.5 W/m2;同时,新方案计算的在赤道区域的大气顶的年平均短波向上辐射通量要小于原方案结果,最大差别为2.5 W/m2,而在南北纬30°到60°区间,新方案计算的在大气顶的年平均短波向上辐射通量则要大于原方案结果,最大差别为1.5 W/m2。新方案计算的年平均短波加热率普遍高于原方案结果,特别是在800 h Pa到地表之间的低层大气以及50 h Pa到100 h Pa的高层大气,最大差别可达0.03 K/d。因此,新方案有助于改善全球气候模式中普遍存在的赤道平流层中下层的温度冷偏差现象。

In this study,a new scheme for the radiative transfer algorithm,called the two-four stream spherical harmonic expansion approximation,is built and applied in the new version of the Beijing Climate Center atmospheric general circulation model （BCC AGCM2.0.1）.It is then compared with the original Eddington approximation scheme.Because this new scheme expands the Eddington approximation to solve radiative transfer through the atmosphere,it has better accuracy.We found the new scheme to have a great effect on climatic simulation.In a clear sky,the new scheme reduces the shortwave downward radiative flux in the surface in the 30°-60°S regions,in the Northeast Atlantic,and in the Sahara desert,with the largest reduction being 3.5 W/m2.Meanwhile,it increases the shortwave upward radiative flux at the top of the atmosphere （TOA） in the 30-60°S regions and in the Northeast Atlantic,with the largest increase being 3 W/m2.For all-sky cases,the new scheme reduces the shortwave downward radiative flux,and the difference between the two schemes becomes larger with increasing latitude.The largest difference reaches 5.5 W/m2 in the two polar regions.The new scheme also reduces the shortwave upward radiative flux at the TOA in the tropics,with the largest difference being 2.5 W/m2,but increases this flux in the 30°-60°S regions,with the largest difference being 1.5 W/m2.Moreover,the new scheme increases the shortwave heating rate within the atmosphere generally,especially for the levels between 800 hPa and the surface and between 50 and 100 hPa where the largest difference reaches 0.03 K/d.Therefore,the new scheme is useful in global climate modeling for improving the so-called temperature cold bias phenomena generally existing in the lower parts of the stratosphere above the tropics.