Assimilating Surface Observations in a Four-Dimensional Variational Doppler Radar Data Assimilation System to Improve the Analysis and Forecast of a Squall Line Case Xingchao CHEN 1, 2 ,Kun ZHAO 1 ( ),Juanzhen SUN 2 ,Bowen ZHOU 1 ,Wen-Chau LEE 2 1 Key Laboratory for Mesoscale Severe Weather/MOE and School of Atmospheric Science, Nanjing University, Nanjing 210046, China 2 National Center for Atmospheric Research, Boulder, Colorado 80301, USA Adv. Atmos. Sci.,2016,33(10),1106-1119.DOI:10.1007/s00376-016-5290-0