1 Impact of Grid Nudging Parameters on 2 Dynamical Downscaling during Summer over 3 Chinese Mainland 4

The grid nudging technique is often used in regional climate dynamical 12 downscaling to make the simulated large-scale fields consistent with the driving fields. In this 13 study, we focused on two specific questions about grid nudging: (1) which nudged variable 14 had a larger impact on the downscaling results and (2) what was the “optimal” grid nudging 15 strategy for each nudged variable to achieve better downscaling result during summer over 16 the Chinese mainland. To solve this queries, 41 3-month long simulations for the summer of 17 2009 and 2010 were performed using the Weather Research and Forecasting model (WRF) to 18 downscale National Centers for Environmental Prediction (NCEP) Final Operational Global 19 Analysis (FNL) data to a 30-km horizontal resolution. The results showed that nudging 20 horizontal wind or temperature had significant influence on the simulation of almost all 21 conventional meteorological elements; nudging water vapor mainly affected the 22 precipitation, humidity, and 500 hPa temperature. Moreover, the optimum nudging scheme 23 varied with simulated regions and layers. As a whole, the optimal nudging time was one hour 24 or three hours for nudging wind, three hours for nudging temperature, and one hour for 25 nudging water vapor. The optimal nudged level was above the planetary boundary layer for 26 almost every nudged variable. 27