Impact of Grid Nudging Parameters on Dynamical Downscaling during Summer over Mainland China

Abstract

Abstract:The grid nudging technique is often used in regional climate dynamical downscaling to make the simulated large-scale fields consistent with the driving fields. In this study, we focused on two specific questions about grid nudging: (1) which nudged variable has a larger impact on the downscaling results; and (2) what is the "optimal" grid nudging strategy for each nudged variable to achieve better downscaling result during summer over the mainland China. To solve these queries, 41 three-month-long simulations for the summer of 2009 and 2010 were performed using the Weather Research and Forecasting model (WRF) to downscale National Centers for Environmental Prediction (NCEP) Final Operational Global Analysis (FNL) data to a 30-km horizontal resolution. The results showed that nudging horizontal wind or temperature had significant influence on the simulation of almost all conventional meteorological elements, while nudging water vapor mainly affected the precipitation, humidity, and 500 hPa temperature. As a whole, the optimal nudging time was one hour or three hours for nudging wind, three hours for nudging temperature, and one hour for nudging water vapor. The optimal nudged level was above the planetary boundary layer for almost every nudged variable. Despite these findings, it should be noted that the optimum nudging scheme varied with simulated regions and layers, and dedicated research for different regions, seasons, and model configuration is advisable.

Impacts of Spectral Nudging Parameters on Dynamical Downscaling in Summer over Mainland China

“…Therefore, the nudging strength was uniformly set as 3 × 10 − 4 (nudging time of 1 h). Nudging above the PBL is a generally accepted scheme (Lo et al, 2008;Pohl and Crétat, 2014;Mai et al, 2017). Only the nudged variables and the cutoff wavelength setting are discussed in this study.…”

“…All observation data came from the China Meteorological Data Service Center website (http://data.cma.cn/ en) and underwent strict quality control. The locations of the observation stations are shown in the report by Mai et al (2017).…”

“…On the basis of this background information, our study focuses on the influence of nudged variables and cutoff wavelengths on spectral nudging over mainland China. Disasters are prone to occur in summer (Tang et al, 2009;Song and Tang, 2011;Hu et al, 2015;Wei et al, 2015;Mai et al, 2017), and there are many uncertainties in this research. Thus, the summer was selected as the simulated season.…”

“…For the nudged layers, previous studies (Lo et al, 2008;Pohl and Crétat, 2014;Mai et al, 2017) demonstrated that nudging above the planetary boundary layer (PBL) should be recommended. Both Lo et al (2008) and Pohl and Crétat (2014) compared the simulation performance obtained by nudging all atmospheric levels with that obtained by nudging only levels above the PBL.…”

“…The results showed that nudging above the PBL led to better results for some simulated meteorological fields. Mai et al (2017) explored different nudged level settings for grid nudging over mainland China and indicated that the regional climate simulation with nudging above the PBL showed better performance than other experiments with nudging above 850, 700, or 500 hPa. These studies demonstrated the advantages of nudging above the PBL.…”