Abstract
Abstract. Dynamical and microphysical processes in pyroconvective clouds in mid-latitude conditions are investigated using idealized three-dimensional simulations with the Active Tracer High resolution Atmospheric Model (ATHAM). A state-of-the-art two-moment microphysical scheme building upon a realistic parameterization of cloud condensation nuclei (CCN) activation has been implemented in order to study the influence of aerosol concentration on cloud development. The results show that aerosol concentration influences the formation of precipitation. For low aerosol concentrations (NCN = 200 cm−3), rain droplets are rapidly formed by autoconversion of cloud droplets. This also triggers the formation of large graupel and hail particles, resulting in an early onset of precipitation. With increasing aerosol concentration (NCN = 1000 cm−3 and NCN = 20 000 cm−3) the formation of rain droplets is delayed due to more but smaller cloud droplets. Therefore, the formation of ice crystals and snowflakes becomes more important for the eventual formation of graupel and hail, which is delayed at higher aerosol concentrations. This results in a delay of the onset of precipitation and a reduction of its intensity with increasing aerosol concentration. This study is the first detailed investigation of the interaction between cloud microphysics and the dynamics of a pyroconvective cloud using the combination of a high-resolution atmospheric model and a detailed microphysical scheme.
Highlights
Deep convection induced by vegetation fires is one of the most intense forms of atmospheric convection
Precipitation reaching the surface is used to calculate the rain rate. This indicates that the dynamical evolution of a strongly polluted pyroconvective cloud is limited in the beginning, but more sustainable, pointing to a cloud lifetime effect resulting from the higher aerosol loading and the reduced precipitation (Lohmann and Feichter, 2005)
– a first investigation has been presented of the influence of aerosol particles on the evolution of a pyroCb using a realistic description of the activation of cloud droplets and realistic aerosol number concentrations
Summary
Deep convection induced by vegetation fires is one of the most intense forms of atmospheric convection. The extreme cloud dynamics with high updraft velocities up to 20 m s−1 (Trentmann et al, 2006; Rosenfeld et al, 2007; Reutter et al, 2009) already at the cloud base, high water vapour supersaturation up to 1% (Reutter et al, 2009) and high number concentration of aerosol particles freshly emitted by the fire up to 105 cm−3 (Andreae et al, 2004; Reid et al, 2005) represent a particular setting for aerosol–cloud interactions These clouds, known as pyrocumulus or pyrocumulonimbus (pyroCb) (Fromm et al, 2010), can occur anywhere in the world where there is sufficient fuel density to produce enough heat to trigger convection, but are most frequently observed in boreal forests (Nedelec et al, 2005; Rosenfeld et al, 2007) and tropical forests (Andreae et al, 2004). The height of the pyroconvective cloud top depends strongly on the background meteorology
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