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Version 3.8.1 of the Advanced Research Weather Research and Forecasting Model (WRF-ARW; Skamarock et al., 2008) was used to simulate the 26−27 January 2017 snowburst event at convection-appropriate resolutions. The model contained only one domain, which included all of Northeast China and the surrounding regions (34°−54°N, 110°−138°E). The model center was located at 45°N, 124°E, and the horizontal grid spacing was 4 km. The number of horizontal grid points was 641×561, and the terrain-following vertical coordinate, sigma, had 51 vertical levels from the surface to 50 hPa, with more vertical levels near the surface and fewer levels aloft (Laprise, 1992). The Community Atmospheric Model (CAM) scheme (Collins et al., 2004, NCAR Tech Note) was employed for longwave and shortwave radiation; in addition, the Pleim-Xiu Land Surface Model (PX LSM) (Pleim and Xiu, 1995; Xiu and Pleim, 2001) and the Asymmetrical Convective Model, version 2 (ACM2) planetary boundary layer (PBL) (Pleim, 2007) were used. The precipitation process in the model was represented by the Morrison double-moment scheme (Morrison et al., 2009). The hourly fifth global climate reanalysis produced by European Centre for Medium-Range Weather Forecasts (ERA5/ECMWF) with a horizontal grid spacing of 0.25°×0.25° and a vertical resolution of 37 levels was used to initialize the model and produce lateral boundary conditions at 3-h intervals. The experiment was initialized at 1200 UTC on 25 January 2017 and integrated for 24 h. History files were generated once an hour.
On the basis of the above control simulation with full terrain (denoted as "CTL"), one idealized experiment is further conducted (denoted as "TRNP"). All configurations in the TRNP experiment are the same as the CTL experiment except that all the terrain height greater than 200 m in the Changbai Mountains are set to be 200 m to remove the effects of Changbai Mountains in TRNP (200 m is the mean elevation of the plains in Northeast China). Through this simple sensitivity experiment, one can basically separate the component of snowfall that was attributed solely to the cold-front and thus ascertain to what extent the orography enhanced the snowfall. Additionally, the physical processes associated with the cold front and the orography can be distinguished and roughly separated.
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Figure 3 illustrates the simulated output of cumulative liquid-equivalent precipitation for both the CTL and TRNP experiments for comparative purposes. Since little precipitation fell after 1200 UTC on 26 January within Northeast China, only the precipitation from 0000 to 1200 UTC is shown. Figures 3a-c were obtained by interpolating the station observations into a set of 0.1°×0.1° grids. Due to the interpolation, the location of BLJ center in Fig. 3a is a little different from that in Fig. 1. Figures 3a-f can be used to validate the performance of the simulation while Figs. 3d-i can be used to analyze the enhancement of the snowfall due to the complex terrain.
Through a comparison of Figs. 3a and 3d, the CTL simulation reproduced the observed snowfall very well, including the detailed banded structure of the precipitation and the three snowfall centers within the snowband. As shown in Fig. 3d, both the Jilin center and the Benxi center were encompassed by narrow small-scale bands (approximately 300 km long and 30 km wide) within the overall snowband; the Jilin center even exhibited a double-banded structure. Notably, the BLJ center was quite evident in the simulation results, as it exhibited a cellular precipitation structure that extended beyond China. The much larger extent of BLJ in the simulation compared to that in the observation was likely due to the sparseness of observation stations in the mountains and the lack of observations outside of China (Fig. 1).
In addition to the spatial distribution of the snowband, its temporal evolution was also effectively simulated, as shown in Figs. 3b-c and Figs. 3e-f. During the observational period, snowfall was most abundant during 0600−1200 UTC; the circumstances were the same in the simulation, although the simulated precipitation in the Benxi center appeared earlier than the observed precipitation. Nevertheless, Fig. 3 still validates the accuracy of the model in simulating the snowband. The solid performance of the model implies that the physical processes associated with the snow band were simulated reliably, especially regarding both the mesoscale and small-scale processes that cannot be resolved by reanalysis data.
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To evaluate the role of orography in producing the snow burst, snowfall amounts and distributions from the CTL and TRNP experiments are compared. From Figs. 3d-f and 3g-i, the snowfall area produced by the cold-frontal snowband does not change much after removing the downstream terrain of the Changbai Mountains. However, the snowfall intensity is significantly influenced. In Fig. 3g, all three snowfall centers in Jilin and Liaoning provinces are greatly reduced, especially the Benxi Center in Liaoning Province. Comparison between Figs. 3e-f and 3h-i indicates that the 0600−1200 UTC snowfall contributes to the difference between Figs. 3d and 3g, a time when the snowband was just becoming active over Changbai Mountains. This indicates that the orography of the Changbai Mountains plays an important role in inducing the snow burst or creating heavy snowfall centers.
To quantitatively evaluate to what extent orography effects enhance the snowfall, Table 1 calculates the numbers of grids within the snowband region for different snowfall intensities. The results of Table 1 are consistent with Fig. 3d-i in that the snowfall area greater than 3 mm (12 h)−1 does not appreciably change in the CTL and TRNP experiments. However, when comes to the > 6 mm and > 9 mm snowfall areas, the orographic effects are quite evident. They contribute 49.7% to the 12-h snowfall greater than 6 mm compared to 26.8% of the cold front, and contributes 66.1% to the 12-h snowfall greater than 9 mm compared to 7.4% of the cold front.
0000−1200 UTC snowfall CTL TRNP Orographic effects (CTL−TRNP) > 3 mm 51301 48452 2849 > 6 mm 13599 6845 6754 > 9 mm 3382 1147 2235 Table 1. Number of grids within the snowband region (36°−46°N, 120°−130°E) for different snowfall intensities
In the following section, we will use the TRNP experiment to analyze the mechanism that supports the snowband development associated with the cold front. Then, by comparing CTL and TRNP experiments, the orographic effects or the interaction between the cold-frontal snowband and orography that intensifies the snowband can be more clearly observed.