The dust particles act as a "heat pump" drawing heat from the sun and surface, and attracting moisture from the Gulf of California and the Gulf of Mexico. Without dust, rainfall would be up to 40 percent less over Arizona, New Mexico and Texas. | Creative Commons photo by Jared
The Perfect Recipe for a Monsoon:
- Dust leads to more heat in the atmosphere (in this case, 45 percent more).
- Increased heat in the atmosphere leads to stronger winds, which attracts more moisture.
- More moisture means more rain (in this case, 40 percent more).
Give it some respect. Dust is more than something to be brushed off the furniture. It's also a powerful heat carrier.
Researchers at the Pacific Northwest National Laboratory (PNNL) found that dust kicked up from the United States Southwest's desert floor by seasonal winds attracts and stores heat, and may even fuel a monsoon.
What the locals call the summer rainy season, scientists call the North American Monsoon. The word monsoon triggers images of torrential rainfall, but it's actually a package deal of changes in wind, atmospheric pressure, and precipitation caused by a seasonal shift in the difference between land temperature and the temperature of nearby oceans.
In June, July and August, temperatures in northern Mexico and the U.S. Southwest climb. As heat rises from the surface, low-level winds shift and draw moisture from nearby oceans. PNNL scientists found that dust lofted from local deserts boosts atmospheric heat. The dust particles act as a "heat pump" drawing heat from the sun and surface, and attracting moisture from the Gulf of California and the Gulf of Mexico. The dust effect significantly increases summer rainfall in a region that gets 70 percent of its annual precipitation in just three months. Without dust, rainfall would be up to 40 percent less over Arizona, New Mexico and Texas.
It's the perfect recipe for a monsoon: dust leads to more heat in the atmosphere, which leads to stronger winds, which attracts more moisture, which leads to more rain.
The PNNL researchers studied both solar energy and heat radiating from the Earth, as well as other effects of dust. They combined data from several ground-based instrument networks that measure the chemical make-up of atmospheric particles with satellite data that measure optical properties of dust. That allowed them to gauge how well the sun's energy moves through the atmosphere over the region.
Next, they compared 15-year simulations with and without dust emissions using a weather forecasting model that includes atmospheric chemistry. The 15-year simulations meant they averaged out naturally occurring yearly climate variations. The model studies revealed the dust effect increases by 45 percent the concentration of atmospheric particles near the deserts, causing the increased precipitation.
"We found that dust heated the lower atmosphere and strengthened low-level north-south winds," said Dr. Chun Zhao, lead author and climate scientist at PNNL. "Strengthening the winds leads to as much as a 40 percent increase in precipitation in the monsoon. This is a significant effect that has not been well understood."
Their study, a first on the North American Monsoon, shows that the heat pump effect is consistent with how dust acts in West African and Asian monsoon regions. PNNL researchers are the first to investigate this effect in a dry region of the United States where water resources are limited and populations are rising. How dust contributes to monsoonal rainfall is important in a region where most climate models predict that rainfall will decrease.
The research, which was reported in the journal Atmospheric Chemistry and Physics, used computing resources at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. For more information, see "Desert Dust Intensifies Summer Rainfall in U.S. Southwest."
