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Probing the Southwest’s Summer Rains

02.08.2004

From Mazatlán to Tucson, the National Center for Atmospheric Research (NCAR) is analyzing moisture-laden skies through September as part of the largest study yet of the North American Monsoon. Each year the midsummer arrival of quenching rains plays a vital role in dryland farming, ranching, and wildfire control across the southwest United States and northwest Mexico. The monsoon may also hold useful clues for predicting summer rainfall elsewhere in the United States.

“A long-term goal of the project is to produce forecasts of the monsoon’s onset with perhaps more than a week of lead time,” says NCAR’s David Gochis, one of the principal investigators for the North American Monsoon Experiment (NAME). "We’re exploring the limits of predictability.”

As with other monsoons around the world, the North American Monsoon develops in late spring and early summer as intensifying sunlight heats dry inland areas. The rising air across Mexico’s Sierra Madre and Central Plateau helps pull moisture from the Atlantic and Pacific oceans and the adjacent gulfs of California and Mexico, eventually triggering intense rains.

During June and July, the northward progress of intense summer heat pulls moisture in its wake, bringing showers and thunderstorms as far north as Colorado. However, the timing and strength of the monsoon varies from year to year, and its relationship to other weather features across North America is not well understood, according to Gochis. “We’re trying to link, on a continental scale, what have previously been viewed as somewhat disparate atmospheric systems,” says Gochis.

NAME is an eight-year study led by the National Oceanic and Atmospheric Administration (NOAA) and Mexico’s national weather service. The project’s major field campaign, which runs from July through September, involves scientists from more than 30 universities, laboratories, and agencies in the United States, Mexico, and Central America.

NCAR’s S-Pol Doppler radar is stationed near Mazatlán this summer for NAME. Elsewhere across northwest Mexico, three sets of profilers (upward-pointing radars) and radiometers will analyze winds and moisture aloft. Weather balloons, to be launched by undergraduate meteorology students from the United States and Mexico, will collect more detail on upper-level winds, temperature, and moisture. NCAR’s tools will join a host of others, including NOAA’s P-3 aircraft and a research vessel from the Mexican navy. NCAR’s parent organization, the University Corporation for Atmospheric Research, is helping with logistics for the sprawling field program. UCAR’s Joint Office for Science Support is hosting the NAME Project Office and maintaining the project’s online data catalog.

To prepare for NAME, Gochis oversaw the deployment of 100 rain gauges now being monitored by water managers and volunteers across Mexico’s Sierra Madre Occidental mountain range. He and his colleagues are investigating how the intense summer rains in this area modulate the central U.S. climate.

Meanwhile, the radar and upper-air data will help Steven Rutledge (Colorado State University), Richard Carbone (NCAR), and their CSU and NCAR colleagues to describe and understand the daily rains in and near the Sierra Madre Occidental and the cloud physics and dynamics that shape them. "All of this is to help us better represent rainfall processes in global weather and climate models," says Carbone.

In Tucson, Arizona, the monsoon’s arrival on July 8--five days later than average--was among the 12 latest in the last half-century. A late start to the monsoon, says Gochis, can stress water resources in the Southwest and reduce yields for dryland farmers in Mexico. "This is a great year to look at how we define the onset of the monsoon," says Gochis. "For a good portion of Mexico, the monsoon circulation was quite delayed in coming on."

NAME’s main goal this summer is to sample the atmospheric processes behind the monsoon in enough detail to create better computer models of its evolution. Over the next several years, NAME scientists will study possible links between the monsoon and neighboring weather regimes. The project is also strengthening ties between the U.S. and Mexican weather services, says NOAA’s José Meitín, a visiting scientist at UCAR and director of the NAME Operations Center in Mazatlán. "Both of the weather services are forecasting for NAME, and a combined assessment is being issued as the official forecast each day," Meitín says.

UCAR manages NCAR under primary sponsorship by the National Science Foundation. Opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.