Convective episodes near the intertropical discontinuity in summertime West Africa: representation in models and implications for dust uplift

This study focusses on the production of mesoscale convective systems (MCSs) close to the intertropical discontinuity (ITD), including their associated dust uplift processes and representation in global and high-resolution, limited-area models. Findings from this work include a detailed description of the synoptic-scale meteorology important for the formation of a large, rare Saharan MCS and the spectacular dust plume which it created in June 2010. Results are presented from a high resolution simulation of this event using the Weather Research and Forecasting (WRF) model. Important dynamical processes which govern the triggering and development of the MCS are discussed as well as the mechanisms for dust uplift with which it is associated. Also highlighted from the simulation is the strong dependence of simulations on initial conditions and the disagreement at particular times between some operational analysis and reanalysis products (referred to here as (re)analysis products). (Re)Analyses are ostensibly representing the atmosphere at the same time and with the same observations as each other. Despite this, disagreement with respect to low-level moisture distribution between (re)analyses is shown to be large at times. Disagreement is as a result of different representations of the West African monsoon (WAM) flow and is greatest during the retreat after a northward excursions. It is also found that extreme disagreement events are linked to the occurrence of rainfall and anomalously high aerosol optical depth (AOD) values north of the zonal-mean ITD (ITD©). The seasonal patterns of rainfall in the Sahara and disagreement between (re)analysis products are shown to be similar, suggesting a link between the occurrence of convective storms and the representation of the West African Monsoon. There is also a spatio-temporal connection between anomalous rainfall events and anomalously high AOD values. Analysis of the synoptic-scale meteorology reveals a statistically significant 925 hPa geopotential dipole present during extreme rainfall events. This has been used to produce a preliminary version of an anomalous rainfall in the Sahara (ARS) index.