Parcel model and three-dimensional simulations of pyro-convective clouds: from CCN activation to precipitation

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 w=20 m s^-1) already at the cloud base, high water vapor supersaturations (up to 1%), and high number concentrations of aerosol particles freshly emitted by the fire (up to NCN=100,000 cm^-3) represent a unique setting for aerosol-cloud interactions.

A crucial step in the microphysical evolution of a convective cloud is the activation of aerosol particles to form cloud droplets. The activation process affects the initial number and size of cloud droplets, and can thus influence the evolution of the convective cloud and the formation of precipitation. The main parameters determining the initial number and size of cloud droplets are the number, size and hygroscopicity of aerosol particles available at the cloud base as well as the updraft velocity. To investigate the influence of these parameters under the conditions of pyro-convection, we performed numerical simulations using a cloud parcel model with a detailed spectral description of cloud microphysics, including different Köhler model approaches for hygroscopic growth. The results can be classified into three regimes depending on the ratio between updraft velocity , w, and aerosol number concentration, NCN,: (1) an aerosol-limited regime (high w/ low NCN), (2) an updraft- limited regime (low w/ high NCN) and (3) a transitional regime (intermediate w/NCN). The results suggest that the variability in the initial cloud droplet number concentration in (pyro-) convective clouds is mostly dominated by the variability of updraft velocity and aerosol particle number concentration.

Building upon a realistic parameterization of CCN activation derived from the parcel model activation study, the cloud resolving 3D model ATHAM was used to investigate the dynamical and microphysical processes of pyro-convective clouds in 2- and 3-dimensional simulations. A state-of-the-art two-moment microphysical scheme was implemented in order to study the influence of the aerosol concentration on the development of pyro-convective clouds. Here, we present results from idealized model simulations using the US standard atmosphere. The results show that the aerosol concentration influences the formation of rain. For low aerosol concentrations (NCN = 1000 cm^-3) rain is rapidly formed by warm microphysical processes while for high aerosol concentrations (NCN = 60000 cm^-3) the ice phase is more important for the formation of rain. This leads to a delay of the onset of precipitation for more polluted atmospheres. In addition, we find that the amount of heat emitted by the fire has a significant impact upon the development and cloud top height of pyro-convective clouds.