Testing the performance of state-of-the-art dust emission schemes using DO4Models field data

K Haustein,L Menut,R Washington,R G Bryant,J King,G Wiggs,D S G Thomas,F D Eckardt

doi:10.5194/gmd-8-341-2015

Abstract

Abstract. Within the framework of the Dust Observations for Models (DO4Models) project, the performance of three commonly used dust emission schemes is investigated in this paper using a box model environment. We constrain the model with field data (surface and dust particle properties as well as meteorological parameters) obtained from a dry lake bed with a crusted surface in Botswana during a 3 month period in 2011. Our box model results suggest that all schemes fail to reproduce the observed horizontal dust flux. They overestimate the magnitude of the flux by several orders of magnitude. The discrepancy is much smaller for the vertical dust emission flux, albeit still overestimated by up to an order of magnitude. The key parameter for this mismatch is the surface crusting which limits the availability of erosive material, even at higher wind speeds. The second-most important parameter is the soil size distribution. Direct dust entrainment was inferred to be important for several dust events, which explains the smaller gap between modelled and measured vertical dust fluxes. We conclude that both features, crusted surfaces and direct entrainment, need to be incorporated into dust emission schemes in order to represent the entire spectra of source processes. We also conclude that soil moisture exerts a key control on the threshold shear velocity and hence the emission threshold of dust in the model. In the field, the state of the crust is the controlling mechanism for dust emission. Although the crust is related to the soil moisture content to some extent, we are not as yet able to deduce a robust correlation between state of crust and soil moisture.

Highlights

We test the emission schemes over a range of shear velocities and quantify the differences with observations (Sect. 4.2). This is followed by an exploration of separate box model components in an attempt to diagnose model–observation differences in emission (Sect. 4.3)
Comparing the emission flux obtained with the size distribution given by this fixed category and the observed size distribution, we find that the resulting model saltation flux is significantly reduced in the case of the fixed category
The performance of current state-of-the-art dust emission schemes has been tested against observational data retrieved during the 2011 DO4Models field campaign in Botswana

Summary

Introduction

Atmospheric mineral dust is the dominant aerosol species in terms of mass (Andreae, 1996; Textor et al, 2006), yet it is one of the major sources of uncertainty in the climate system (Forster et al, 2007; Boucher et al, 2013) despite recent efforts to reduce these uncertainties from a remote sensing (Ginoux et al, 2010, 2012; Ashpole and Washington, 2012; Brindley et al, 2012), physico-chemical (Redmond et al, 2010; Formenti et al, 2011), or modelling point of view (Huneeus et al, 2011; Knippertz and Todd, 2012; Klose and Shao, 2012). Key elements of model dust emission schemes are largely based on empirical data from wind tunnel experiments. Their emitted dust loadings have often been tuned to match global (Pérez et al, 2011; Huneeus et al, 2011) or regional (Laurent et al, 2006; Heinold et al, 2009; Haustein et al, 2012) satellite or in situ dust data (Holben et al, 1998; Remer et al, 2002; Kahn et al, 2005) rather than attending to the efficacy of the emissions in key regions. The smooth threshold friction velocity, the saltation flux as well as the vertical emission flux are functions of the size distribution and chemical composition of the soil particles (Kang et al, 2011)

Objectives

Results

Conclusion