CMA-MESO三维变分同化系统2 m相对湿度资料同化研究

为了提高CMA-MESO （China Meteorological Administration Mesoscale model）（原GRAPES）三维变分同化系统中2 m相对湿度资料的应用效果，改善模式中相对湿度的分析和降水预报效果，分析了2015年6—8月T639（T639L60全球中期数值预报系统，0.28125°×0.28125°）分析场低层相对湿度和2 m相对湿度之差与表征稳定度的理查森数（Ri）的关系，发现二者有很好的相关，Ri＜0时，模式低层相对湿度与2 m相对湿度的差异较小，基本在同化观测误差范围内。依据该统计结果，对CMA-MESO同化系统中2 m相对湿度同化方案进行优化，Ri＜0时，将观测站地形低于模式地形的2 m相对湿度观测由观测站高度改为模式最低层高度进行同化，形成新的2 m相对湿度同化方案，旨在解决2 m相对湿度资料同化时模式地形高度与观测站高度不同的影响。2018年7月CMA-MESO三维变分同化系统（3DVar）个例和连续试验结果显示:新的2 m相对湿度同化方案同化分析资料数量明显增加，且08时多于20时（北京时），新增观测点新息向量（背景减观测）与周围原有观测新息向量保持基本一致，分析残差偏差和均方根误差减小，降水预报效果明显改善。新2 m相对湿度同化方案通过提高观测站低于模式地形高度的观测资料合理应用，从而改善了3 km模式系统同化分析和预报效果。 

Assimilation of 2 m relative humidity observations in CMA-MESO 3DVar system

In order to improve the relative humidity analysis and forecast of the CMA-MESO (China Meteorological Administration Mesoscale model) 3DVar (3-Dimensional Variational) data assimilation system at 3 km resolution, a new observation operator for 2 m relative humidity observations and the tangent linear as well as its adjoint have been coded, tested, and then introduced into the CMA-MESO 3DVar operational assimilation system. The T639 (T639L60 Global Medium Range Forecast System, 0.28125°×0.28125°) analysis of results for three months (1 June to 31 August, 2015) shows that the difference in relative humidity between 2 m and the lowest model level is a function of the Richardson number. Therefore, 2 m relative humidity can be directly compared to relative humidity at the lowest model level. The observation operator assumes that relative humidity is constant in unstable surface layer. The results show that more 2 m relative humidity observations can be used in the 3DVar assimilation system with this scheme, and the relative humidity analysis is more close to observations. The EST (Equitable Threat Score) value of summer (1—31 July, 2018) experiments is higher when 2 m relative humidity observations are assimilated using this scheme. The assimilation of surface relative humidity observations seems to be improved.