Summer Convective Afternoon Rainfall Simulation and Projection Using WRF Driven by Global Climate Model. Part I: Over Taiwan

Abstract

This paper is the first of a two-part study that investigates summer convective afternoon rainfall (CAR) activity over the East Asian continent through simulation and projection. We focus on the CAR activity over Taiwan in Part I. In Part II, the changes in CAR activity over South China and Luzon are examined. Using the WRF (Weather Research and Forecasting) model driven by two super-high resolution global climate models: HiRAM (High Resolution Atmospheric Model) and MRI (Meteorological Research Institute Atmospheric General Circulation Model), this study evaluates the performance of models (HiRAM, MRI, and WRF) used to simulate the summer CAR activity over Taiwan. The evaluations focus on the spatialtemporal variations in CAR activity during two time periods: the present-day (1979 -2003, historical run) and the future (2075 -2099, RCP8.5 scenario). For the present-day simulations, analyses indicate that both HiRAM and MRI cannot accurately simulate the temporal evolution of diurnal rainfall. This timing shift problem can be fixed after dynamical downscaling using WRF. The WRF dynamical downscaling approach also helps generate a more realistic CAR amount simulation over western Taiwan, where most residents live. In future projections all models predict that a significant decrease in CAR amount will occur over southwest Taiwan. This decrease in CAR amount is suggested due to the decrease in CAR frequency, not a change in the CAR rate. All models suggest that the cause of decrease in CAR frequency over southwest Taiwan is weaker local afternoon surface wind convergence and thermal instability to suppress CAR genesis.

Summer Convective Afternoon Rainfall Simulation and Projection Using WRF Driven by Global Climate Model. Part II: Over South China and Luzon

“…2d), the results show that both the diurnal rainfall over South China and Luzon tend to reach their maximum at 17 LT. This feature is similar to that revealed over Taiwan (Huang et al 2016). In view of previous studies (e.g., Kikuchi and Wang 2008), one frequently adopted method to illustrate major diurnal rainfall features over a larger domain is to apply an Empirical Orthogonal Function (EOF) analysis on the 3-hourly rainfall.…”

“…Finally, we examine what causes the projected changes in CAR frequency over South China and Luzon. In view of part I of this two-part investigation (Huang et al 2016), it is suggested that the changes in daytime surface wind convergence and thermal instability are the two major factors that modulate the changes in CAR frequency over Taiwan. To know if this suggestion can be applied to explain the changes in CAR frequency over South China and Luzon, we examined the projected changes in daytime surface wind convergence (Figs.…”

“…Results are presented in section 3 followed by a conclusion given in section 4. Note, Huang et al (2016) (Mizuta et al 2012)], suggesting the WRF dynamical downscaling approach is not very sensitive to driving forcing. From the preliminarily comparison of WRF-HiRAM (shown later) and WRF-MRI's (not shown) simulated CAR activity, this study also found that the former is very similar to the latter for both South China and Luzon.…”

“…It will be shown later that the WRF dynamical downscaling approach suggested by Huang et al (2016) for simulating Taiwan's diurnal rainfall is also suitable for South China and Luzon's diurnal rainfall simulations. Another objective of this study is to project future changes in summer convective afternoon rainfall (CAR) activity over South China and Luzon.…”

“…Among these studies, Huang et al (2013) recently adopted a similar dynamical downscaling approach to obtain more realistic simulations of diurnal precipitation change over South China, and suggested that a regional model setup suitable for diurnal rainfall simulation over South China might not be suitable for simulating diurnal rainfall over other East Asian sub-regions. Huang et al (2016) used the WRF (Weather Research and Forecasting Model; Skamarock et al 2008) driven by two super high resolution global climate models and noted that using WRF dynamical downscaling of global model data helped to generate a more realistic diurnal rainfall simulation over Taiwan. However, as inferred from Huang et al (2013), it is possible that Huang et al's (2016) suggested WRF dynamical downscaling approach might not perform well in the simulations of diurnal rainfall variation over South China and Luzon (i.e., the two continental areas closest to Taiwan; marked in Fig.…”