P11 Overcome near-surface model-observation inconsistencies in "obs-nudging" FDDA

Wu, Yonghui, Yubao Liu, Linlin Pan, and Al Bourgeois, National Center for Atmospheric Research

Limited resolution NWP models, such as WRF, result in discrepancies between model terrain heights and the true surface heights (elevations) of many observation stations. Because of the existence of strong vertical gradients of most weather variables in the near surface and boundary layers, such height discrepancies often causes difficulties and inaccuracies in data assimilation for both surface and upper-air data, including radiosondes, and spoils model forecast verification as well. In this paper, terrain-height-adjustment algorithms are developed to overcome these problems with WRF “obs-nudging” FDDA. Surface observations and upper-air soundings are adjusted in the lower troposphere according to the physical consistency of the boundary layer thermodynamic and dynamic characteristics. Idealized (OSSE) experiments are set up to illustrate the issues and demonstrate the effectiveness of the proposed terrain-height-adjustment algorithms. Real weather case runs are conducted to study the performance of the algorithms. Verification results for both single observation experiments and integrated effect of assimilating full observations show obvious improvement data analysis at surface and in the lower-troposphere when using these terrain-height-adjustment algorithms.