A Study of Assimilation of Surface Observational Data in Complex Terrain Part III: Comparison Analysis of Two Methods on Solving the Problem of Elevation Difference between Model Surface and Observation Sites

Elevation difference between surface observation sites and numerical model surface is a big problem of Surface Observation Data Assimilation (SODA). In Part_I (Xu et al, 2007a), results have shown that the elevation difference between observation sites and numerical model surface can impact the results of SODA. And better results can be obtained when surface observation data are assimilated into a numerical model with temperature observation reduced from actual station elevation to model terrain height. In Part_II (Xu et al, 2007b), better results also can be obtained when surface observation data are assimilated into a numerical model with Terrain Error of Representativeness (TER), which is related to the elevation difference between model surface and observation sites. In this paper, the two methods to solve negative influence of the difference between observation station and model elevation are analyzed and compared, by making three-month experiments in WRF_3DVAR. One is that the TER is added into the surface observation error, another is that temperature observation is reduced, via the temperature lapse rate, from actual station elevation to model terrain height. Results show that, when the difference between observation station and model elevation increases to a certain value, effects of the former method are close to the analysis field of other data assimilation, while the latter method is more robust when the value of temperature lapse rate is accurate. The latter method is sensitive to the value of temperature lapse rate, and the sensitivity decreases when radiosonde data are also assimilated. The effects of the two methods are the same when the model surface is close to observation sites, while the former method is better when radiosonde data are also assimilated. The scheme of Guo Yongruns surface data assimilation (Guo et al., 2002) with TER is the best scheme in this paper. The SODA scheme can improve the results of rainfall simulation. The surface data assimilation approach without consideration of the difference between observation station and model elevation is not suitable to be used.