Observations and modeling of the 9 April 2009 convective 'brown' cloud near Riyadh, Saudi Arabia

Aircraft measurements taken within a relatively weak convective cell, flanking a thunderstorm complex that was propagating northeastward near Riyadh, Saudi Arabia on 9 April 2009, indicated total cloud droplet concentrations (CDCs; 3-50 µm diameters) of 800 -1200 cm-3 in the mid-troposphere, a marked departure from the typical CDCs of ~ 400 cm-3 that were observed elsewhere in the thunderstorm complex (1300-1320 UTC 9 April) at comparable altitudes. Based on this information, other aircraft analysis, plus visual photographic evidence, we have hypothesized that the high droplet number concentrations were the result of ingestion of a large amount of dust particles by the cloud.

In this paper, we examine in detail the microphysical and aerosol characteristics of this case, both in isolation and contextually compared to other convective clouds observed by aircraft during roughly similar environmental conditions during late March and early April of 2009. We also conduct a back trajectory analysis of airflows related to the convection on April 9, both using the HYSPLIT model as well as a suite of multiscale simulations using the WRF modeling system applied to a hierarchy of nested domains. We further analyze the output from the WRF simulations, which utilize an innermost domain capable of simulating storm scale dynamics, thermodynamics and cloud microphysics, to better understand the differences in cloud and aerosol properties observed between the weak cell and the main convective complex. This analysis will include preliminary results from simulations incorporating basic aerosol treatments within the WRF chemistry modules. These latter simulations include aerosol-cloud microphysical-radiative interactions and should provide a robust simulation of the different CDCs and mean diameters observed by the aircraft, and allow us to partially verify or refute our hypothesis.