Biomass Burning Aerosol Effects on Clouds and Precipitation

Overview

During the dry season in South America, biomass burning events inject a large amount of aerosols—mainly black carbon and organic carbon—into the atmosphere. The radiative and microphysical effects of aerosols can impact local weather and climate. We have simulated a multi-day biomass burning event to investigate the effects that the resulting aerosols have on clouds and precipitation in the dry season of South America.

Project Details

Emission sources and atmospheric conditions strongly affect the inhomogeneous distribution of aerosols, both horizontally and vertically in the troposphere. Fully coupled models are required to model these distributions and accurately capture the chemical and meteorological effects of aerosols.

We use the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) to perform simulations of a biomass burning event at 36-kilometer (km) and 4-km resolutions. WRF-Chem is a fully coupled meteorology-chemistry-aerosol mesoscale model that simulates the varying thermodynamic conditions, aerosol concentrations, and chemical species simultaneously.

Results and Impact

Our results show the main effect of biomass burning aerosols reduce the diurnal amplitude of convection by decreasing clouds and precipitation in the afternoon and increasing them at night. Daily averages indicate that the afternoon decrease in clouds and precipitation is greater than the nighttime increase. This modulation is primarily driven by the radiative effects of the aerosols (their absorption and scattering of sunlight) reducing clouds during the day. The aerosol's microphysical effects (providing cloud condensation nuclei) counteract the radiative effects at night, but to a lesser degree.

Our 36-km and 4-km simulations indicate that the biomass burning reduces surface precipitation by 5% and 3% respectively, and reduces the amplitude of the diurnal convection cycle by 11% and 5% respectively.

Another finding is that aerosols lifted by deep convection can significantly warm the upper troposphere through their local radiative heating effects. This results in significant moistening of the upper troposphere, potentially affecting the transport of water vapor from the troposphere to the stratosphere.

Role of High-End Computing Resources

A WRF-Chem model can capture regional cloud variations better than uncoupled models. However, the computational time required for the coupled model is much greater than for the uncoupled model. Supercomputing resources at NASA's Jet Propulsion Laboratory allow us to run the high-resolution, fully coupled model simulations in a large enough domain for an adequate time period.