利用常规气象观测、地面加密自动站、ERA5（0. 25°×0. 25°）逐小时再分析及全球地形资料，对2020年 11月 18-19日（过程 1）、2021年 11月 7-8日（过程 2）内蒙古东南部两次回流型极端特大暴雪天气的特征和积雪深度差异成因进行对比分析，为提升内蒙古暴雪及其影响预报能力提供一定参考。结果表明：（1）两次过程发生的季节、强降雪落区和24 h累计降水量接近，但过程1影响范围更广，雪强更强且出现了罕见的冻雨天气，过程2持续时间更长，积雪更深，11个站积雪深度突破历史极值。（2）过程1和 2均是在 500 hPa高空槽（涡）、700 hPa西南急流、925 hPa东北急流共同作用下，暖湿空气沿低层冷垫爬升，产生显著锋生，中层锋区与低层东北回流叠加出现特大暴雪天气。（3）过程1低层冷空气强度偏弱、持续时间偏短，中层西南气流强度更强且厚度更厚，导致中层大气更暖、更湿，暖层位势厚度深厚，中低层大气云中液态水含量明显高于过程 2，有利于融化层建立，雪花或冰晶在融化层融化，前期为冻雨，后期以湿雪形式落至地面；过程2受冷涡影响，系统配置更为深厚，低层大气相对更冷，冷垫更强且持久，无融化层，温度条件适合在空中形成片状雪花，落地后以干雪为主，更有利于形成较大积雪。（4）降雪初期，过程1近地面气温在-1 ℃以上，地表温度在0 ℃左右，湿雪落至地面后不能快速凝结，随着近地面气温降低易形成冻雨；过程2近地面气温为-9 ℃，地表温度-3 ℃，近地面风速更弱，有利于较干的大雪花落至地面后形成积雪。

By using conventional meteorological observations，ground densified automatic stations，ERA5 （0. 25° ×0. 25°）hourly reanalysis and global topographic data，the differences between the characteristics and causes in snow depth of two extreme heavy snowfall events under the return flow pattern in southeastern Inner Mongolia on November 18-19，2020（Process 1）and November 7-8，2021（Process 2）were compared and analyzed，which can provide the certain references for improving the forecasting ability of heavy snowfall and its impacts in Inner Mongolia. The results show that：（1）The two processes occurred in the same season，with simi‐ lar areas of heavy snowfall and 24-hour cumulative precipitation. Process 1 had a wider influence range，stronger snow intensity and rare freezing rain weather，while Process 2 lasted longer and had deeper snow accumulation， with 11 stations snow depth breaked the historical records.（2）Both Process 1 and Process 2 were caused under the effect of 500 hPa upper-level trough（vortex），700 hPa southwest jet stream，and 925 hPa northeast jet stream. Warm and humid air climbed along the cold air mass at the lower level，producing significant frontogenesis，and the combination of the mid-level front area and the low-level northeast reflux results in the extra large snowfall weather.（3）The intensity of the cold air at the lower level in Process 1 was weaker，the duration was shorter，while the intensity and thickness of the southwest airflow at the mid-level were stronger，resulting in a warmer and more humid mid-level atmosphere and deeper warm layer. The content of liquid water in the clouds at the mid and lower levels was significantly higher than that in Process 2，which conducived to the establishment of a melting layer. Snowflakes or ice crystals melt in the melting layer. In the early stage，it is freezing rain， and in the later stage，it falls to the ground in the form of wet snow. In Process 2，affected by the cold vortex， the system configuration was deeper，the lower-level atmosphere was relatively colder，the cold air mass was stronger and more persistent，which has no melting layer. The temperature conditions are suitable for the formation of flaky snowflakes in the air. After landing，it is mainly dry snow，which is more conducive to the formation of a larger snow depth.（4）At the initial stage of the snowfall，the near-surface temperature in Process 1 was above -1 ℃，and the surface temperature was around 0 ℃. Wet snow could not quickly freeze when it fell to the ground，and it was easy to form freezing rain with the near-surface temperature decreased. In Process 2，the nearsurface temperature was -9 ℃ and the surface temperature was -3 ℃. The near-surface wind speed was weaker， which was conducive to the formation of snow accumulation when relatively dry large snowflakes fell to the ground.