Simulation of 210Pb and 7Be scavenging in the tropics by the LMDz general circulation model

A new formulation of aerosol scavenging by convective precipitations, consistent with the Emanuel convective mass-flux scheme, is implemented into the Laboratoire de Météorologie Dynamique general circulation model, LMDz. The aerosols 210Pb and 7Be are used as inert tracers to evaluate LMDz performance in terms of atmospheric transport in the tropics. Wind fields are calculated over 2007 in a regular grid with a resolution of 1.875° in longitude, 1.24° in latitude and 40 vertical levels. The first part of the paper addresses the sensitivity of LMDz simulations to convective schemes and to scavenging parameterizations. Results are analyzed at two tropical stations and one mid-latitude station by comparing simulated aerosol concentrations with available data, collected at surface stations. On a daily scale, the observed variations of concentrations are poorly reproduced by any considered model at both tropical stations. Nevertheless, fluctuations at timescales longer than a few days may be captured over periods of a few weeks to a few months by the new formulation. cond part of this paper focuses on the new implemented parameterization of convective scavenging. The objective is to interpret mismatches of simulated concentrations with observed data and to determine which transport mechanisms are responsible for peaks of 7Be concentrations recorded at tropical stations. Typical meteorological situations in Polynesia are analyzed in view of convective transport and scavenging of 7Be. In particular, the sensitivity to the location of the South Pacific Convergence Zone is examined on concentrations in Polynesia. Results demonstrate that 7Be plumes are brought downward in the middle atmosphere by large-scale subsidence associated to the subtropical jet, in a 35°–25°S band. At surface, 7Be plumes are associated either to shallow convection or to deep convection with moderate precipitations. Within moderate convective events, surface concentrations are enhanced by surface release due to re-evaporation of precipitations. Most 7Be surface plumes reaching the Polynesian stations are produced locally in the neighborhood of stations and advected over one or two days by surface large-scale winds. The largest mismatches may be attributed to uncertainties in the simulation of precipitations within deep convective cloud systems that develop along troughs.