Modeling Impacts on the Stratocumulus-to-Cumulus Transition Associated with Southern Africa Biomass Burning Outflow Constrained by ARM Observations

Abstract

Global climate models continue to struggle with accurately simulating aerosol-cloud interactions and spatial distributions of low clouds, particularly over the open ocean. Representing such changes in cloud regimes are crucial toward climate models accurately assessing global mean cloud radiative forcing in Earth’s evolving climate. The objective of this proposal is to quantify changes in the stratocumulus-to-cumulus transition associated with biomass burning aerosol outflow across the southeast Atlantic utilizing model simulations, Earth system satellites and surface observations via an Atmospheric Radiation Measurement mobile facility at Ascension Island. A better understanding of how biomass burning emissions influence the transition within the Atlantic will improve model representation of cloud microphysical processes in addition to Earth’s radiation budget, which feedback directly on to the West African monsoon.

This project aims to address the following research questions: (1) Can climate models accurately represent surface and mid-tropospheric conditions associated with increased/decreased aerosol transport (e.g., horizontal/vertical aerosol distribution and low-cloud fraction)? (2) What are the underlying mechanisms as to which biomass burning aerosols modulate the stratocumulus-to-cumulus transition near Ascension Island? and (3) Do possible changes in cloud radiative properties feedback on to the larger scale meridional monsoonal flow? Toward addressing these research questions, seventeen months of ground-based observations encompassing two separate burning seasons (2016-2017) at Ascension Island in addition to Earth system satellites will be analyzed to identify clean or polluted scenes and associated meteorological conditions. A regionally refined version of the Community Atmosphere Model driven with chemistry (CAM-Chem) and specified dynamics will be operated to determine aerosol radiative direct effects by calling on the radiation diagnostics at each time step. Aerosol microphysical effects will be studied using the Weather Research and Forecasting Model with turbulence resolving capability based on a large-eddy simulation (WRF-LES). WRF-LES will be used with an altered version of the Thompson aerosol-aware cloud physics to represent the spatial and temporal variability of aerosol number concentrations from surface observations at Ascension Island for a more realistic representation of aerosol-cloud interactions compared to using prognostic values.

We anticipate the following outcomes. (1) Thorough analysis of meteorological conditions associated with clean and polluted conditions near the observation site. (2) Through a series of high-resolution simulations, quantify changes in the stratocumulus-to-cumulus transition associated with an increase or decrease of smoke transport and distinguish changes between aerosols and meteorology. (3) Improve model accuracy for simulating low clouds over the open southeast Atlantic Ocean. Particularly, we aim to quantify how aerosols influence boundary-layer dynamics (turbulence, moisture fluxes) and radiative perturbations.