Observations of mesoscale and boundary-layer scale circulations affecting dust transport and uplift over the Sahara

J H Marsham,C M Grams,W M F Grey,B T Johnson,D J Parker,A N Ross

doi:10.5194/acp-8-6979-2008

Abstract

Abstract. Observations of the Saharan boundary layer, made during the GERBILS field campaign, show that mesoscale land surface temperature variations (which were related to albedo variations) induced mesoscale circulations. With weak winds along the aircraft track, land surface temperature anomalies with scales of greater than 10 km are shown to significantly affect boundary-layer temperatures and winds. Such anomalies are expected to affect the vertical mixing of the dusty and weakly stratified Saharan Residual Layer (SRL). Mesoscale variations in winds are also shown to affect dust loadings in the boundary layer. Using the aircraft observations and data from the COSMO model, a region of local dust uplift, with strong along-track winds, was identified in one low-level flight. Large eddy model (LEM) simulations based on this location showed linearly organised boundary-layer convection. Calculating dust uplift rates from the LEM wind field showed that the boundary-layer convection increased uplift by approximately 30%, compared with the uplift rate calculated neglecting the convection. The modelled effects of boundary-layer convection on uplift are shown to be larger when the boundary-layer wind is decreased, and most significant when the mean wind is below the threshold for dust uplift and the boundary-layer convection leads to uplift which would not otherwise occur. Both the coupling of albedo features to the atmosphere on the mesoscale, and the enhancement of dust uplift by boundary-layer convection are unrepresented in many climate models, but may have significant impacts on the vertical transport and uplift of desert dust. Mesoscale effects in particular tend to be difficult to parametrise.

Highlights

Mineral dust uplifted from deserts is an important component of Earth’s climate system
The convective boundary layer (CBL) depth during B301 was similar to B302 (1400 to 1800 m compared with 1200 to 1700 m, see Sects. 3.1 and 3.3) and so this shows a larger scale of organisation in the boundary-layer convection than for B302, for a similar CBL depth
Results from low level flights made during the GERBILS field campaign have been used to demonstrate the effects of mesoscale land surface temperature (LST) anomalies on the virtual potential temperatures and winds in the boundary layer and mesoscale wind variations on dust loadings

Summary

Introduction

Mineral dust uplifted from deserts is an important component of Earth’s climate system. Mesoscale circulations generated by Shortwave radiation) field campaign, which took place in such land surface variations may affect low-level winds June 2007, aimed to understand the differences between an6d98s0o the uplift oJf.dHe.sMertardsuhsatm. We expect land surface variations in desert regions to affect surface fluxes and so 2 Data, model and methods used induce mesoscale circulations (Segal and Arritt, 1992). Low albedo regions are expected to locally increase the rate of growth of the CBL into the SRL, A large array of instruments was present on the FAAM and so affect the vertical transport of dust between the CBL BAe146 aircraft during GERBILS and all data presented and SRL. This was in the middle or upper CBL (see Sect. 3.3)

The COSMO and LEM models

Cospectral analysis

Results

Cospectral analysis of data from flight B302

Conclusions