Wet bias of summer precipitation in the northwestern Tibetan Plateau in ERA5 is linked to overestimated lower-level southerly wind over the plateau

Abstract

The Tibetan Plateau (TP), also called the Third Pole, is considered to be “the world water tower”. The northwestern TP (NWTP), which has an average elevation higher than 4800 m, is an arid region where the summer precipitation is largely overestimated by the ERA5 global reanalysis product. We hypothesize that this wet bias is mainly caused by unrealistic lower-level winds that trigger strong convection over the region; it can be reduced by using a high-resolution regional climate model with a large domain that allows realistically representing interactions between the Westerlies and Asian summer monsoons. Here, downscaling using the Weather Research and Forecasting (WRF) model driven by ERA5 was conducted with a large domain (8°‒50° N, 65°‒125° E) at 9 km for the period 1979‒2019 (WRF9km). Precipitation values from WRF9km and ERA5 were evaluated against satellite observations; compared with ERA5, WRF9km captured the climatological summer precipitation over the NWTP with a much-reduced wet bias. The ERA5 overestimation is mainly caused by excessive convective precipitation, likely linked to strong vertical motions over the NWTP induced by an overestimated lower-level southerly wind.

Reducing Winter Precipitation Biases Over the Western Tibetan Plateau in the Model for Prediction Across Scales (MPAS) With a Revised Parameterization of Orographic Gravity Wave Drag

“…In winter, WVT to the TP mainly comes from the westerly wind belt (X. Liu et al., 2020). Changes in atmospheric circulation can affect the WVT which is critical for the precipitation over the TP (e.g., Lin et al., 2018; Ou et al., 2023). Figure 9a shows the distribution of water vapor flux (WVF) over the TP and in surrounding areas in winter of 2000–2019 in the EXP_KD05 experiment.…”

“…Ou et al. (2023) and P. Zhou et al. (2022) revealed that the wet biases over the TP in the ERA5 global reanalysis can be alleviated by WRF dynamical downscaling simulation at a gray‐zone resolution (9 km), owing to the more realistic representation of low‐level southerlies and WVT.…”

“…Lin et al (2018) also found that increasing the resolution of the Weather Research and Forecasting (WRF) model can reduce the wet biases over the TP, because of a better representation of the orographic drag of complex terrain which can reduce the horizontal winds and thus the water vapor transport (WVT) toward the TP. Ou et al (2023) and P. Zhou et al (2022) revealed that the wet biases over the TP in the ERA5 global reanalysis can be alleviated by WRF dynamical downscaling simulation at a gray-zone resolution (9 km), owing to the more realistic representation of low-level southerlies and WVT. Recently, there is a growing interest in the simulation of rainfall over the TP using convection-permitting modeling (CPM) which explicitly resolves deep convection.…”

Mesoscale convective systems in the third pole region: Characteristics, mechanisms and impact on precipitation

“…The current literature lacks studies with a focus on the link between MCS-associated precipitation, water availability for agriculture and flooding in the river basins around the TP, even though there exist several refined multi-decadal simulations that might be useful for this purpose (Table 4). These include, for instance, the High Asia Refined analysis (HAR; Maussion et al, 2014;Wang et al, 2021c) with a horizontal grid spacing of 10 km and a WRFbased downscaling product with a horizontal grid spacing of 9 km (Ou et al, 2020;2023; Table 4). In addition, a CORDEX Flagship Pilot study named "Convection-Permitting Third Pole" (Prein et al, 2022) has been launched and endorsed by WCRP-CORDEX to coordinate the high-resolution modeling work focusing on MCSs and precipitation over the TP and its downstream regions.…”

Synergistic Impact of Complex Topography and Climate Variability on the Loss of Microclimate Heterogeneity in Southeast Asia

“…The fifth-generation ECMWF atmospheric reanalysis of the global climate was downscaled to provide a high-resolution (3 km) temperature and precipitation data set over Southeast Asia using the Weather Research and Forecasting Model version 3.7.1. This data set has been proven to be highly reliable and meets high accuracy requirements (Ou et al, 2020(Ou et al, , 2023.…”