Aerosol Indirect Effect on Long-lasting Mesoscale Convective Systems: A modeling study

The atmospheric aerosols serving as cloud condensation nuclei (CCN) can modulate both cloud droplet sizes and cloud life spans (the indirect effect of aerosols). Both the observations and cloud modeling studies have provided abundant evidences of the indirect effect of aerosols for shallow cloud types at relatively clean environment, e.g., marine stratus/stratocumulus. However, for the deep precipitation systems such as the mesoscale convective systems (MCS) formed in relatively unstable environment, the impact of increasing aerosol concentration becomes uncertain. Will the cloud dynamics dominate? Will the change of CCN concentrations produce any significant change in the cloud and precipitation structure? If it does, can it be detected by exiting observations? This presentation tries to answer these questions using a cloud-resolving model with an explicit bin microphysical scheme.

The Goddard Cloud Ensemble (GCE) Model with the Hebrew University Cloud Model (HUCM)'s explicit cloud microphysics package is used in this study. The explicit microphysical scheme resolves the size distributions of water drops, hail, graupel, snow aggregates, three ice species (dendrites, plates and columns), as well as the aerosols using different mass bins. The activations of CCN are calculated using the physical principles, making the model a perfect tool to study the aerosol indirect effects. Three cases are simulated in this study: a mid-latitude continental MCS (PRE-STORM, June, 1985), a tropical maritime MCS (TOGA COARE, February, 1992), and a tropical sea breeze convection (CRYSTAL, July, 2001). It is shown through model sensitivity tests that increasing aerosol concentrations may change both the microphysical and dynamical structures of deep convective systems. The sensitivities of aerosol as CCN are also highly case dependent, ranging from reducing surface rainfall in the mid-latitude MCS case to enhancing the convection in the tropical case. The detailed analyses of model outputs reveal some of the mechanisms and the complexity of the aerosol-microphysics-dynamics interactions.