Reply on RC1

Abstract

Dust particles larger than 20 µm in diameter (0.2 μm < D < 100 µm) have been regularly observed to remain airborne during long-range transport. In this work we extend the parameterization of mineral dust cycle in the GOCART-AFWA dust scheme of WRFV4.2.1, to include also such coarse and giant particles. The initial particle size distribution in our parameterization is based on observations over desert dust sources and the Stokes’ drag coefficient has also been updated to account for dust particles of all sizes (Re < 105). The new code is applied to simulate dust transport over Cape Verde during the August 2015 AER -D campaign. Model results are evaluated using both airborne dust measurements and the CALIPSO-LIVAS pure dust product. The results show that the modeled lifetimes of the coarser particles are shorter than those observed. Various processes are proposed to explain such inaccuracies, such as the electric field inside dust plumes and non-spherical aerodynamics. Additional sensitivity runs are performed by artificially reducing the settling velocities of the particles to compensate for such underrepresented processes in the model. Our simulations show that particles with diameters of 5–17 μm and 40–100 μm are better represented assuming 80 % reduction in settling velocity (UR80) while particles at the range 17–40 μm are better represented in the UR60 scenario. The overall statistical analysis shows that the UR80 experiment presents the closest agreement with the airborne in situ measurements both in Cape Verde and over the sources. The UR80 experiment improves also the vertical distribution of dust in the model, as compared to the CALIPSO-LIVAS pure dust product. Further research is requested in order to understand the physical processes behind the reduction of settling velocity.