دانلود رایگان مقاله مطالعه عددی در گرد و غبار با مفاهیم برآورد بودجه گرد و غبار جهانی

abstract

The estimates of the contribution of dust devils (DDs) to the global dust budget have large uncertainties because the dust emission mechanisms in DDs are not yet well understood. In this study, a large-eddy simulation model coupled with a dust scheme is used to investigate DD dust entrainment. DDs are identified from the simulations using various threshold values for pressure drop and vorticity in the DD center. A vortex-tracking algorithm is presented, which automatically detects and tracks vortices based on different pressure drop and vorticity criteria. The results show that DD dust lifting can be largely explained by convective turbulent dust emission. DD dust entrainment varies strongly between individual DDs even for similar atmospheric conditions, but the maximum emissions are determined by atmospheric stability. By relating DD emission and counts to the Richardson number, we propose a new and simple method to estimate regional and global DD dust transport.

8. Discussion and conclusions

In this paper, dust devil (DD) dust transport was investigated using LES. Our results show that the DD number density, n, decreases with Ri2 for negative Ri. n is smaller in our study than in the studies of Ohno and Takemi (2010) and Raasch and Franke (2011). The reason is likely the higher horizontal resolution used in the latter studies. For example, Raasch and Franke (2011) identi- fied twice as many DDs when using a 1 m horizontal resolution in their simulations rather than a 2 m grid resolution. Additionally, we excluded short tracks and connected tracks if pressure minima were found in the intermediate locations, thereby reducing our n. The order of magnitude of pressure drop, turbulent wind speed, and duration of the detected DDs are in agreement with those observed in the field (e.g. Sinclair, 1969; Metzger et al., 2011; Lorenz et al., 2015). With a mean value of hdi ¼ 86 m, the diameters in our study are somewhat larger than that of observed DDs (Balme and Greeley, 2006; Lorenz, 2011), with one reason likely being the different definition used to determine DD diameter. Another reason is probably an underestimation of the number of small DDs in our study. Our results show that instantaneous shear stresses in DDs are sufficiently large to aerodynamically lift dust particles as has been observed by Balme et al. (2003). Surface dust fluxes of about 100  103 lg m2 s1 and 2 m PM20 dust concentrations of 100  103 lg m3 have been obtained in our study, smaller or on the lower end of those observed in laboratory experiments and in the field. Neakrase and Greeley (2010) measured total mass removals of 105  1011 lg m2 s1 for 2 lm clay in their laboratory vortex generator experiments, much larger than the fluxes obtained here. In the field, Metzger et al. (2011) measured PM10 concentrations of 103  104 lg m3 at 2 m height, close to those obtained in our study for the more intensive DDs.