20 February 2012

Worldwide Water in the Balance: Tropical Clouds Hold Clues for the Global Water Cycle


To study the wellspring of atmospheric water, you have to start with tropical clouds. Scientists at Pacific Northwest National Laboratory showed that global climate models are not accurately depicting the true depth and strength of tropical clouds that have a strong hold on the general circulation of atmospheric heat and the global water balance. Their analysis points to the need for model improvements to project water cycle changes in the 21st century. The research was published in the Journal of Climate.


Researchers Drs. Samson M. Hagos and L. Ruby Leung, atmospheric scientists at PNNL, surveyed tropical divergence in three global climate models, three global reanalyses (models corrected with observational data), and four sets of field campaign soundings. Their survey uncovered significant uncertainties in current climate projections of the intensity and vertical structure of the low-level convergence of moisture to and upper-level divergence of heat away from the tropics. These current uncertainties are also reflected in future climate projections by these models.

The research team divided the total circulation providing moisture for precipitation into three parts: deep-divergent, shallow-divergent, and non-divergent. They found that in the tropics and subtropics, deep divergent circulation is the largest contributor to net precipitation, or total precipitation minus evaporation. Further, all global circulation models studied portray this process as deeper and stronger than what is observed in field measurements.

“Inter-model differences in the present day climate simulations inevitably carry over to future projections of climate change,” said Hagos, lead author of the study. “Identifying these uncertainties provides a strong basis for model improvements.”


Emphysema, asthma, and lung cancer. These are just a few of the respiratory illnesses that are associated with exposure to ozone and air pollution particles. In most affluent regions of the world, air pollution is regulated to reduce the risk to human health and natural ecosystems caused by these airborne toxins. These air pollutants also alter present and future climate patterns, so making realistic scenarios of these emissions is important for making realistic projections of future climate change. Understanding the many factors that influence a region’s air pollution levels helps scientists project future levels and climate change.

“This work was a foundational reference case for the recently released RCP4.5 model scenario, one of four scenarios that will be used by modeling groups around the globe to make realistic projections of future climate change,” said Dr. Steven J. Smith, scientist at the Joint Global Change Research Institute, a partnership between PNNL and the University of Maryland, and lead research author. The four Representative Concentration Pathway scenarios are described in a special issue of Climatic Change.

Researchers are working on evaluating the co-benefit of climate policy in terms of reduced pollutant levels.

This work was funded by the Department of Energy’s Office of Science and the Environmental Protection Agency. The work was performed by Dr. Steven J. Smith and Page Kyle of JGCRI, and Dr. J. Jason West of the University of North Carolina.

Steven J Smith