Modified Climate Model Better Replicates Global Rainfall

Bhattacharya, A., Pacific Northwest National Laboratory

Cloud-Aerosol-Precipitation Interactions

Cloud-Aerosol-Precipitation Interactions

Song X, GJ Zhang, and JF Li. 2012. "Evaluation of Microphysics Parameterization for Convective Clouds in the NCAR Community Atmosphere Model CAM5." Journal of Climate, 25(24), 10.1175/jcli-d-11-00563.1.

Rainfall in the tropics.

Rainfall in the tropics.

By improving an existing, sophisticated, global climate model, scientists can now simulate cloud and rainfall more accurately.

Supported by the U.S. Department of Energy’s Atmospheric System Research program, a research team from the Scripps Institution of Oceanography and NASA’s Jet Propulsion Laboratory developed a technique to realistically replicate how microphysical processes within clouds impact regional rainfall. In other words, the group found a way to better depict the process of producing rain from clouds and how aerosols—dust-sized particles in the atmosphere–affect atmospheric convection and the formation of rain-bearing clouds.

In a paper published in the Journal of Climate, the researchers show that their modification in the Community Atmosphere Model version 5 or CAM5 resulted in approximately a 40% improvement in depicting regional rainfall in the tropics.

The climate science community has long recognized that the original CAM5 lacked an acceptable method to replicate physical processes by which aerosols modify THE microphysical properties of thunderstorm clouds.

When the number of aerosols in thunderstorm clouds increases, the total number of water (or ice) droplets also increases, but the size of each droplet is reduced. Known as ‘indirect effects’, these changes impact the way the droplets coalesce into raindrops that in turn, depending on other meteorological conditions, can either reduce or increase regional rainfall.

So far, including these details in a global climate model has posed numerical challenges. At the same time, though, the difficulty in representing these ‘indirect effects’ underscores a critical need to incorporate such microphysical processes into climate models. This becomes more obvious as observations make it clear that in polluted parts of the world, there is a distinct relationship between aerosol loading, cloud formation, and rainfall patterns.

In their paper, the researchers also show that their improved CAM5 model more accurately replicated the geographical distribution of rainfall, which observations show is not uniform. In their version of the model, summer rainfall increases significantly in the western tropical Pacific, South China Sea, and the Bay of Bengal but decreases over the maritime continents, the Arabian Peninsula, and the Arabian Sea.

“The scheme describes microphysical processes in convection well,” writes Xiaoliang Song, the lead author of the paper, indicating the close agreement between model results and actual observations.

The inclusion of microphysical interactions in the modified CAM5 also reduced bias in the global mean precipitation simulations by 16%.These were all goals that the original CAM5 aimed to achieve, but so far had not been able to.

However, work is far from complete, and improving the representation of microphysical processes in global models continues to be a critical tool for the climate science community to understand rainfall changes in the future. “We will pursue this topic further in the near future,” concluded Song, indicating firm plans to continue investigating the impacts of aerosols on cloud, rainfall, and climate.