Detailed structure of lows and frontal systems simulated with fine grid resolution

Figure 1 shows the liquid water path, column BCA mass and wet deposition flux of BCA around a low and the frontal systems accompanying the low. The structures of the low and frontal systems simulated by the model were highly dependent on the grid resolution (Fig. 1a,d,g). The structure of the vortex around (50°N, 150°E) became clear and the horizontal scale of the cold fronts decreased when the grid resolution was fine. In the results with fine grid resolution, the vortex around the low involved large amounts of BCA that were carried northward by a low (Fig. 1b). This is consistent with the results of a regional model24; the filamentary structure of the vortex around lows was clearly reproduced by the fine grid resolution and BCA transport by the low was larger than was suggested using a coarse grid resolution.

Figure 1 Differences in the structure of a low and frontal system and black carbon aerosol (BCA) transport for model simulations using different grid resolutions. (a,d,g) Example of frontal systems and lows visualised by the vertically accumulated mass of liquid water (liquid water path: LWP) (g m−2) at 21UTC on 20111124 over Japan; (b,e,h) black carbon aerosol (BCA) transport by the lows and frontal systems shown by the vertically accumulated BCA (column BCA) (mg m−2); and (c,f,i) wet deposition flux of BCA at the same time. (a–c), (d–f) and (g–i) indicate the results of the simulation at 3.5-, 14- and 56-km resolution, respectively. The mapping of the figures was created using the Grid Analysis and Display System (GrADS)51 version 2.1.a1. Full size image

In addition to the components of a frontal system itself, the contrast between cloud and cloud-free areas (i.e., the detailed structure of the clouds) around the systems were well resolved using the fine grid resolution. The detailed structure of clouds reproduced a small-scale contrast between the areas in which wet deposition occurred actively and the areas where wet deposition was inactive (Fig. 1c). The BCAs in the cloud-free area remained in the atmosphere and were readily incorporated into the vortex around the lows. On the other hand, the contrast was obscure and the cloud area was broadly extended in the simulation with a coarse grid resolution (Fig. 1g). This broad area of cloud effectively removed the BCA by wet deposition (Fig. 1i). The ratio of the cloud free area around lows and frontal systems, defined as the ratio of the number of grids whose LWP was smaller than 1 g m−2 to the total number of grids with lows and frontal systems (see the ref. 27 for details of the extraction of the systems), increased from 23.30, 42.25 and 51.44% at grid resolutions of 56, 14 and 3.5 km, respectively. As a result of the differences in the cloudy area, the precipitation amount averaged over all lows and frontal systems decreased with the finer grid resolutions to 3.434, 3.389 and 3.178 mm day−1, respectively. The small cloud area and low precipitation led to a small number of grids in which wet deposition was active. The amount of aerosol removed by wet deposition decreased at finer grid resolutions.

The difference in the cloud and cloud-free areas at different resolutions is supported by the results of a previous study, which showed that a better representation of convection, clouds and aerosol–cloud interaction processes of a cloud resolving model resulted in a better performance than that of a model with coarser grid resolution. A previous study underestimated the BCA using a 10-km grid resolution24; it was speculated that this underestimation (O (10 km)) was derived from the underestimation of background aerosol. However, our results indicated that simulations with a kilometre order (O (1 km)) grid resolution can further reduce the underestimation of BCA in the Arctic.

The differences referred to above resulted in a large dependency of the distribution of BCA upon the grid resolution and transport by the vortex decreased with coarsening grid resolution (Fig. 1b,e,h). These differences in the transport of BCA in lows and frontal systems were common for almost all lows simulated by the model; large amounts of BCA were shown to be transported to the Arctic inside cyclonic circulations around frontal areas.

The dependency of the BCA transport by lows and frontal systems on the grid resolution also led to a difference in BCA mass concentrations over the Arctic (Fig. 2). At low altitudes, with fine grid resolution, BCA from the Eurasian continent (Europe and Russia) was effectively carried to the Arctic (Fig. 2a). In contrast, with coarse grid resolution, transport was low in this region (Fig. 2c). This transport corresponded to low-level transport in the polar front region21, which was located further south in winter than in other seasons22. Using fine grid resolution, above the boundary layer, a large amount of the BCA was transported to the Arctic regardless of the longitude (Fig. 2d–f). This indicates that BCA emitted from continents was effectively carried up by the convection around lows and frontal systems and thus reached the Arctic. Due to the dependency of simulated BCA transport on the grid resolution, the BCA concentrations increased in the Arctic with increasing grid resolution. As a result, the ratio of the vertically accumulated mass concentration of BCA (column BCA) in the Arctic to global column BCA mass loading simulated by the 3.5-km resolution model was 4.2-times higher than that simulated by the 56-km resolution model (Table 1).

Table 1 Fraction of BCA in the Arctic, mass flux of black carbon aerosol (BCA) to the Arctic and the contribution of lows and frontal systems (LF) to the mass flux. Full size table

Figure 2 Differences in black carbon aerosol (BCA) transportation to the Arctic for model simulations with three different horizontal resolutions. (a–f) Mixing ratio of BCA at (a–c) z = 845 m (where z is the height) and (d–f) z = 6250 m averaged over the final 10 days of the simulation and (g–i) temporal evolution of zonally accumulated column BCA mass concentration simulated at (a,d,g) 3.5-, (b,e,h) 14- and (c,f,i) 56-km grid resolutions, respectively. The mapping of the figures was created using the Grid Analysis and Display System (GrADS)51 version 2.1.a1. Full size image

The northward mass flux across 60°N and the upward mass flux of BCA by lows and frontal systems (see Supplementary Information for details of the calculations of mass flux) systematically increased at finer grid resolutions over the whole layer (Fig. 3). This indicated that the upward and poleward transport of BCA was enhanced by reproducing the detailed structure of lows and frontal systems. The statistical analysis of the sensitivity of the resolution to BCA transport was also shown quantitatively in the poleward mass flux of BCA across the 60°N latitude line (Table 1). The mass flux gradually increased with a finer grid resolution. In addition to the absolute value of the mass flux, the contribution of lows and frontal systems to the transport of BCA gradually increased with increasing grid resolution. These results all indicate that realistic simulations of the detailed structures of lows and frontal systems increased the simulated amounts of BCA transported from the continents in the Northern Hemisphere by lows and frontal systems.

Figure 3 Vertical profile of the northward and upward black carbon aerosol (BCA) mass flux by lows and frontal systems. The vertical profile of the (a) zonally accumulated poleward BCA mass flux across 60°N and (b) horizontally averaged upward BCA mass flux around lows and frontal systems, simulated with (red) 3.5-km, (green) 14-km and (blue) 56-km horizontal grid resolution and averaged during the final 10 days of the simulation. The whiskers of each plot show the range from 5th to 95th percentile. The method used to extract lows and frontal systems were based on the previous study27. Full size image

Regional variability of BCA transport

The total northward BCA mass flux across 60°N including lows and frontal systems, over all vertical layers, systematically increased at finer grid resolutions (Fig. 4a) and the profile had two peaks. The peak in the lower layer corresponded to low-level transport21 which was clear in the Europe–Siberia region (0°E-120°E: Fig. 4b). BC emitted from the Siberian region could not be carried into the upper layer due to a strong inversion (called the polar dome28) and therefore the dependency was largest in the lower layer in Siberia than in other regions. The peak in the upper layer mainly originated from the flux in the Asian region (120°E-60°W: Fig. 4c) and North America (60°W-0°E: Fig. 4d), which was derived from BCA being elevated by the lows and frontal systems. From these analyses we concluded that BCA transport to the Arctic over the upper (lower) layer was enhanced mainly by the increased transport from the Asian and North America (Siberian) regions generated by reproducing the detailed structure of lows and frontal systems. Such an analysis is one merit of using a global scale model.

Figure 4 Regional variability of the northward black carbon aerosol (BCA) mass flux. Zonally accumulated vertical profile of the northward BCA mass flux across 60°N over (a) the whole globe, (b) Europe and Siberia (0°E-120°E), (c) Asia (120°E-60°W) and (d) North America (60°W-0°E), simulated with (red) 3.5-km, (green) 14-km and (blue) 56-km horizontal grid resolution and averaged during the final 10 days of the simulation. The whiskers of each plot show the range from the 5th to 95th percentile. Full size image

Comparison between the model and observations

A comparison between the model and in situ measurements showed that these increased simulated levels of BCA transport contributed to a reduction in the underestimation of BCA in the Arctic (Fig. 5). BCA levels were well reproduced regardless of the resolution, except for Arctic area. In most of the observation sites (except for the Arctic), the BCA level simulated by the model with a fine grid resolution was closer to the observation (Table 2) and the difference in the BCA level among the different resolutions was included in the range of the variability (Fig. 5d–f).

Table 2 Comparison between surface observations and the model for several regions. Full size table

Figure 5 Comparison between surface black carbon aerosol (BCA) simulated by the model and that observed by in situ measurements. (a–c) Model-derived mass concentrations of surface BCA averaged over the final 10 days of the simulation and (filled circle) those observed by Interagency Monitoring of Protected Visual Environment (IMPROVE52), China Atmosphere Watch Network (CAWNET53), Canadian Aerosol Baseline Measurement website (CABM) and European Supersites for Atmospheric Aerosol Research (EUSAAR) and (d–f) the scatter plot between model-derived BCA mass and observed surface BCA mass. The open circles, triangle, square and closed circle in (d–f) represent results for the observation site in North America, the Arctic, China and Europe. The thin and thick grey lines in (d–f) show the range from minimum to maximum and the 25th to 75th percentile. All data were averaged for November 2011, except for data from CAWNET which were monthly averaged data for November 2006 and 2007. Details of the observation data are provided in Supplementary Information. The mapping of the figures was created using the Grid Analysis and Display System (GrADS)51 version 2.1.a1. Full size image

The surface BCA levels in the Arctic differed markedly between the low- and high-resolution models (Table 2), with surface BCA concentrations simulated using the fine grid resolution close to those measured at Arctic observation sites. In addition, the difference between the model with the finest grid resolution (3.5 km) and the observation was smaller than that between the model with 14-km resolution and the observation (Table 2). Hence, simulations using the global-scale kilometre-order model enabled reproduction of lows and frontal systems and such simulations reduced the underestimation of BCA in the Arctic.

As well as a comparison of the surface BCA level, the comparison of vertical profiles of BCA is important because current GCMs generally overestimate BCA in the upper layer4. The vertical profile indicated that the BCA level simulated with a fine grid resolution was large in all vertical layers (Fig. 6), which is consistent with the vertical profile of the BCA flux (Fig. 4). Unfortunately, in situ measurements of the vertical profile were not conducted on the targeted date (November 2011). In the future, we should conduct experiments targeting the date when the aircraft measurements were conducted to assess the vertical distribution of BCA. However, to understand the model performance, an assessment of the vertical profile through a comparison with the results of previous aircraft measurement is meaningful, even though the measurements were conducted in different seasons.

Figure 6 Vertical profile of the black carbon aerosol (BCA). Vertical profile of (a) BCA mass concentration and (b) mixing ratio of BCA averaged over the Arctic region, simulated with (red) 3.5-km, (green) 14-km and (blue) 56-km horizontal grid resolution and averaged during the final 10 days of the simulation. The whiskers of each plot show the range from the 5th to 95th percentile. Full size image

Based on a previous observational study5, the surface BCA level is at a minimum in summer. After the summer, the surface BCA level gradually increases to a maximum in spring. Although surface observation may be representative of the BCA level over the upper layer29, BCA over the upper layer displays a similar seasonal cycle to the surface according to several aircraft measurements (Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models of Climate Chemistry, Aerosols and Transport: POLARCAT30, Arctic Research of the Composition of the Troposphere from Aircraft and Satellites; ARCTAS31, Aerosol, Radiation and Cloud Processes affecting Arctic Climate: ARCPAC32, Polar Airborne Measurements and Arctic Regional Climate Model Simulation Project: PAMARCMiP33 and High-Performance Instrumented Airborne Platform for Environmental Research Pole-to-Pole Observations: HIPPO34,35). The BCA level simulated with the coarse resolution was much lower than in the aircraft measurements listed above, but the order of the BCA level simulated by the finest resolution was closer to the aircraft measurement (several to several tenths ng m−3) (Fig. 6). However, our model did not show the reduction in the BCA level above the 10-km layer that was observed in spring (ARCTAS campaign) and in fall (HIPPO campaign). From these analyses, we can infer that as in current GCMs, our model also overestimated the BCA level above the boundary layer36. To improve the vertical distribution of BCA, a more detailed understanding of the microphysical processes of BCA, as well as a finer grid resolution, are required.