Isentropic mixing properties in the stratosphere modeled by the forward calculation of an inverse model (TM5‐4DVAR) are evaluated against Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and Microwave Limb Sounder (MLS) observations. The isentropic mixing processes are separated into large‐scale stirring described by the “equivalent length” and small‐scale diffusion described by the diffusivity. Compared to the measurements, we find that the modeled stirring is not strong enough and that the small‐scale diffusivity is too large. TM5‐4DVAR produces excessive mixing‐induced poleward flux for stratospheric CH₄. The flux convergence presents negative biases in the tropics and positive biases in the polar regions. The biases cannot be reduced by improving the horizontal resolution only. Modeled isentropic mixing depends on the horizontal as well as the vertical resolution of the model. An increase in vertical resolution reduces numerical diffusion of the model in the vertical. The decreased vertical diffusion leads to reduction in the modeled isentropic diffusivity. Biases in modeled total column‐averaged mixing ratios of CH₄ are significant for both models with a coarse vertical resolution of 4° × 6° × 25 (and 2° × 3° × 25, 1° × 1° × 25) and an improved one of 1° × 1° × 40. They are estimated to be 7–14 and 3–7 ppb in the winter extratropics under the assumption that isentropic mixing is dominant over vertical transport on a time scale of 3 days. Correspondingly, resulted biases in inverted CH₄ surface emissions are estimated to be 0.5–1 and 0.2–0.5 mg/m²/hr, respectively, in the extratropics.

中文翻译：

---

平流层CH4模型模拟中的混合特性诊断

利用迈克尔逊干涉仪进行被动大气探测（MIPAS）和微波肢体探测（MLS）观测，对通过反演模型（TM5-4DVAR）的正向计算建模的平流层中的各向同性混合特性进行了评估。等熵混合过程分为“等效长度”描述的大规模搅拌和扩散率描述的小规模扩散。与测量结果相比，我们发现模拟的搅拌不够强，小范围的扩散率太大。TM5-4DVAR对平流层CH ₄产生过多的混合诱发的极向通量。通量会聚在热带呈现负偏压，在极地呈现正偏压。仅通过提高水平分辨率就无法减少偏差。建模的等熵混合取决于模型的水平和垂直分辨率。垂直分辨率的增加减少了模型在垂直方向上的数值扩散。减少的垂直扩散会导致模型的等熵扩散率降低。CH _4的建模总列平均混合比中的偏差这两种型号的垂直分辨率均为4°×6°×25（和2°×3°×25、1°×1°×25），改进后的分辨率为1°×1°×40，对两个型号来说都是非常重要的。假设在3天的时间尺度上等熵混合在垂直传输中占主导地位，则冬季温带生物估计为7-14 ppb和3-7 ppb。相应地，在温带地区，反向CH ₄表面排放的偏差估计分别为0.5-1和0.2-0.5 mg / m ² / hr。