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

Abstract

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 (0.7μm), 3.40 (1.1μm), 3.80 (1.7μm), 4.32 (2.6μm), 4.67 (4.6μm), and 4.53 (7.0μm), respectively, and maximum n was 4.67 for the 4.6μm 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.