Why does global numerical weather prediction model fail to accurately forecast 21.7 Zhengzhou extreme precipitation?

Global numerical weather prediction models failed to forecast extreme precipitation

Extreme precipitation has significant social and economic impacts, including threats to human life, disruptions to infrastructure, and economic losses. On 20 July 2021, an extreme precipitation event occurred in Zhengzhou City, North China, with an hourly rainfall of 201.9 mm that broke the record of hourly rainfall in mainland China. Unfortunately, even the state-of-the-art global NWP models, such as the European Centre for Medium-Range Weather Forecasts (ECMWF), failed to accurately predict this event. The predicted rainfall was mainly located over the Taihang Mountains (i.e., to the west of Zhengzhou city) and the rainfall intensity was much lower.

 

The critical role of complex terrain in extreme precipitation forecast

To investigate the influence of complex terrain on the “21.7” Zhengzhou extreme precipitation, the research team conducted a series of numerical experiments using the Model for Prediction Across Scales (MPAS), with particular interest in the parameterization of orographic gravity wave drag (OGWD). The horizontal resolution in the MPAS model is set to 15 km, which is comparable to that of leading global NWP models.

The results show that OGWD due to unresolved complex terrain can substantially affect both the location and intensity of extreme precipitation. The OGWD acts to decelerate the low-level easterly winds, which enhances the blocking effect of model-resolved orography on the low-level mesoscale vortex that produces the extreme precipitation. Consequently, the low-level vortex stays to the east of the Taihang Mountains, leading to enhancement of moisture convergence and thus extreme precipitation in Zhengzhou (Fig. 1a).

When the subgrid OGWD is omitted, the low-level mesoscale vortex moves westward and climbs over the Taihang Mountains, leading to a decrease in the rainfall intensity and northwestward shift of the rainfall center (Fig. 1b). These forecast biases resemble that found in operational global NWP models. The findings reveal the importance of multiscale orography through the interaction between model physics (i.e., parameterized OGWD) and dynamics (i.e., flow blocking by resolved terrain) in determining the location and intensity of extreme precipitation. Additional sensitivity experiments with different configurations of model physics (e.g., cumulus convection, planetary boundary layer, and radiation) confirm the robustness of the results.

 

Accurate representation of complex terrain effect remains challenging

This study demonstrates that the OGWD owing to complex terrain can exert great influences on the extreme precipitation (taking the 21.7 Zhengzhou event for example) predicted by global NWP models. However, accurately representing the effects of complex terrain is still challenging which is a large source of model uncertainties. Thus, to improve the forecasting ability of extreme precipitation, especially in regions of complex terrain, deeper understanding of OGWD dynamics, more observations to constrain their parameterization, and new technologies like machine learning to combine the two together are in high demand. This will also strengthen the foundation for efficient risk assessment and adaptive decision-making.

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