Dust emission and transport in Northwest China: WRF-Chem simulation and comparisons with multi-sensor observations

In this study, the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) is employed to simulate a dust process in Northwest China during May 2018. The model's ability to simulate the dust process in Northwest China is firstly evaluated using various satellite-retrieved and observational data. The four-dimensional assimilation method is also used to optimize meteorological data and effectively improve the simulation of the dust process.

 

Fig. 1. Differences of wind field (unit: m/s) between the simulations (a-d: unassimilated; e-h: assimilated) and the observations at 03:00 UTC on 20–23 May.

The comparisons between the simulations based on five dust emission schemes within WRF-Chem and the observations show that, the Shao01 scheme overall has good performance in simulating the emission flux, the spatial pattern of source region, as well as the spatiotemporal variation of dust mass concentration, during this dust process. In comparison to Shao01, the GOCART AFWA and Shao04 schemes can also produce quite similar spatial pattern of dust source region, but tend to overestimate or underestimate dust emission and mass concentration. The Shao11 scheme fails to simulate the dust process since the importance of the fully disturbed particle size distribution is omitted. It is also noted that the GOCART scheme can well reproduce dust emission processes under weak wind erosion but underestimate dust emission flux under strong wind erosion. In addition, the GOCART scheme has produced some spurious emissions and thus blurred the distribution of dust source region.

Fig. 2. The averaged dust emission flux (unit: μg/m²/s) from the GOCART (a), GOCART AFWA (b), Shao01 (c), Shao04 (d) and Shao11 (e) schemes during 17–23 May.

Fig. 3. Variations of daily (a) and hourly (b) surface PM₁₀ concentrations, friction velocity (c) and 10 m wind speed (d) at the Turpan station during 17–23 May.

Northwest China is covered by mountains, basins, deserts and other landforms, thus the complex terrain is one of the key factors to influence the dust process over the region. Our study shows that after being emitted, the airborne dust transported toward the east and west. The dust to the east was diffused rapidly, but the portion toward the west was blocked and accumulated at the edges of the mountains and thus produced dust weather characterized by high dust concentration and long lifetime. The dust accumulated at the edges of the mountains could reach an altitude of more than 6 km due to wind and thermal effect, and finally arrive at Tibetan Plateau and eastern China.

Fig. 4. The simulated (with Shao01 scheme) dust (unit: μg /m³) transport path (a-c) during 20–23 May and vertical profiles of 38.5°N (d) and 85°E (g) at 05:00 UTC on 21 May, 36°N (e) and 95° E (h) at 06:00 UTC on 22 May and 36°N (f) and 98°E (i) at 05:00 UTC on 23 May.