Power law relation between size-resolved vertical dust flux and friction velocity measured in a fallow wheat field

• The power n in the relationship between vertical dust flux and friction velocity depends on particle size.
• The n values were 2.54 (0.7 μm), 3.40 (1.1), 3.80 (1.7), 4.32 (2.6), 4.67 (4.6), and 4.53 (7.0).
• A shorter average time such as 1-min mean is applicable for dust flux measurement.
• Particle size distributions of dust concentration at different heights had a similar shape.

Size-resolved dust flux measurements are important for estimation of concentration and physical properties of dust particles in the atmosphere and their impacts on cloud and radiation transfer. In this study, we evaluate the power law relationship between size-resolved dust flux, Fair,d, and friction velocity, u∗. During the Japan–Australia Dust Experiment, size-resolved dust fluxes were estimated with the gradient method by using data from optical particle counters and profile measurements of wind speed and air temperature in a dry, non-crusted fallow wheat field in Australia. When a power law is fitted to the u∗–Fair,d relation, i.e., Fair,d–u∗n, the coefficient of determination R2 is largest for n = 4. However, when a power law is fitted separately to data of each particle size range, n is found to depend on the particle size with values of 2.54 (0.7 μm), 3.40 (1.1 μm), 3.80 (1.7 μm), 4.32 (2.6 μm), 4.67 (4.6 μm), and 4.53 (7.0 μm), respectively, and maximum n was 4.67 for the 4.6 μm diameter. This suggests that the Fair,d–  u∗n relationship is not universal, and embedded in n is the nature of soil aggregation. For practical purposes, n should be determined separately for each particle size range when estimating size-resolved dust fluxes.

There has been a continuing debate on the validity of the power–law relationship between dust emission flux, Fair,d, and friction velocity u∗. In the Japan–Australia Dust Experiment (JADE), size-resolved dust fluxes were estimated by measurements of dust concentration gradient and other surface and meteorological parameters. The JADE data suggest that a Fair,d–u∗n relationship with n = 4 best describe the overall dependence of dust emission on friction velocity. However, if a power law is fitted to the size-resolved dust fluxes, as the graph shows, n is be particle size dependent, ranging from 2.54 to 4.67. The Fair,d–u∗n relationship with n = 4 is thus not universal, and embedded in n is the nature of soil aggregation.Figure optionsDownload as PowerPoint slide