Better Resolved Orography Improves Precipitation Simulation Over the Tibetan Plateau in High-Resolution Models

Regarded as the Asian Water Tower, the Tibetan Plateau (TP) collects atmospheric precipitation from a vast area of land and feeds into major rivers that sustain the livelihood of billions of people in East, South and Central Asia. It is critical to reasonably simulate the hydrological cycle over the TP in order to assess future climate risks to agriculture, water resources and ecosystem services. To address the chronic wet biases over the TP in state-of-the-art climate models, we have compared 12 high-resolution (HR) climate models (25–50 km) to their corresponding low-resolution versions (100–200 km) with respect to the 1979–2014 climatology. It is found that the HR models consistently reduce about half of the wet biases over the TP, mainly from better resolved orography. The wet biases are reduced by 41% over the northern and western TP, mainly contributed by decreased frequency of light precipitation (0.1–10 mm day⁻¹), which is attributed to reduced evaporation because of weakened surface wind by raised orography. The most significant reduction of biases (53%) rising from decreased frequency of mid-heavy precipitation (10–50 mm day⁻¹), appears over the southern and eastern TP, on the leeside of elevated orography where steeper orography enhances rain shadow effect by stronger downward motion along the sharper slope, while partly compensated by air column convergence due to vertical stretching of the downward flow for potential vorticity conservation. This study highlights the importance of surface processes and resolving complex orography in simulating precipitation and large-scale hydrology around the TP which potentially benefits the future hydrological projection.