Abstract
Current climate models commonly overestimate precipitation over the Tibetan Plateau (TP), which limits our understanding of past and future water balance in the region. Identifying sources of such models’ wet bias is therefore crucial. The Himalayas is considered a major pathway of water vapor transport (WVT) towards the TP. Their steep terrain, together with associated small-scale processes, cannot be resolved by coarse-resolution models, which may result in excessive WVT towards the TP. This paper, therefore, investigated the resolution dependency of simulated WVT through the central Himalayas and its further impact on precipitation bias over the TP. According to a summer monsoon season of simulations conducted using the weather research forecasting (WRF) model with resolutions of 30, 10, and 2 km, the study found that finer resolutions (especially 2 km) diminish the positive precipitation bias over the TP. The higher-resolution simulations produce more precipitation over the southern Himalayan slopes and weaker WVT towards the TP, explaining the reduced wet bias. The decreased WVT is reflected mostly in the weakened wind speed, which is due to the fact that the high resolution can improve resolving orographic drag over a complex terrain and other processes associated with heterogeneous surface forcing. A significant difference was particularly found when the model resolution is changed from 30 to 10 km, suggesting that a resolution of approximately 10 km represents a good compromise between a more spatially detailed simulation of WVT and computational cost for a domain covering the whole TP.
Highlights
The Tibetan Plateau (TP), the highest and most extensive highland in the world, is considered the water tower of Asia (e.g., Xu et al 2008) because it hosts the headwaters of many major Asian river systems (Su et al 2016)
As revealed from the only two available rain gauges within D3, WRF2 significantly reduces precipitation bias compared to the coarser-resolution simulations
Our results indicate that a coarser resolution can lead to stronger summertime water vapor transport (WVT) crossing the Himalayas, which in turn may cause the wet biases in precipitation over the TP
Summary
The Tibetan Plateau (TP), the highest and most extensive highland in the world, is considered the water tower of Asia (e.g., Xu et al 2008) because it hosts the headwaters of many major Asian river systems (Su et al 2016). Regional climate models (RCMs) with a coarse resolution do not reduce the wet bias (referred to positive bias in precipitation ) much (e.g., Ji and Kang 2013; Maussion et al 2014; Gao et al 2015; Ma et al 2015). The computational errors of pressure gradient force at the lowest model levels induced by the terrain-following coordinate (Danard et al 1993) and the decoupling between advection and condensation processes (Codron and Sadourny 2002) are noticed as probable factors contributing to the overestimation of orographic precipitation. Due to the coarse resolution, climate models cannot resolve the steep terrain (like the Himalayas) and fail to capture regional-scale processes, which may affect the simulation of water vapor transport (WVT) and precipitation
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