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April 1999

INDOEX studies aerosols over the Indian Ocean

How much do sulfate aerosols cool the climate? To help find the answer, NCAR sent researchers, instruments, and the NCAR/NSF C-130 research aircraft to the $25-million Indian Ocean Experiment (INDOEX), now wrapping up several months of observations.

INDOEX is based in the Republic of the Maldives, an archipelago southwest of India's southern tip. There, NCAR scientists have been working alongside more than 70 researchers from a dozen nations to observe the tropical oceans and atmosphere. The highly instrumented C-130 was based at Malé airport, which occupies its own island in the archipelago.

UOP's Joint Office for Science Support (JOSS) has been overseeing operations from the INDOEX support office for logistics and data management, headed by Dick Dirks. "The government of the Maldives has been extraordinarily cooperative and accommodating," Dick says. "They're deeply concerned with climate change research." With a top elevation of only around two meters (six feet) above sea level, the Maldives are vulnerable to rising sea levels due to climate change.

Jeff Kiehl. (Photo by Carlye Calvin.)

According to Jeff Kiehl (Climate and Global Dynamics Division, or CGD), one of the principal investigators for INDOEX, "In the future, pollution in the tropics will increase, so we'd better understand it now. The chemistry in the tropics is severely undersampled." The Indian subcontinent and surrounding nations are rich sources for many kinds of aerosols, including those produced from industrial and auto emissions, biomass burning, and soil dust. With Asia's population rising at a dramatic rate, the amount of sulfur dioxide released into the atmosphere is expected to increase.

Jeff helped design INDOEX to ensure that the project collects the data needed to advance global climate modeling. Some physical and chemical processes in the earth system are so complicated that modelers cannot simulate each detailed step. "With INDOEX data, we can actually test the way we treat aerosols in computer models against observations," he explains.

The observation region is downwind of the Indian subcontinent during the spring and extends into the pristine Southern Hemisphere. With the weather typically pleasant at this time of year (calm, with little rain), the investigators were sampling both polluted and clean air in clouds and clear sky.

"It's a natural laboratory for studying direct and indirect effects of aerosols," says Andy Heymsfield (Mesoscale and Microscale Meteorology Division, or MMM), another INDOEX researcher. The direct effect of aerosols is the scattering that occurs when solar radiation bounces off particles in clear air. The indirect effects have to do with sulfates' interactions with clouds.

Globally, aerosols are an important source of nuclei around which cloud droplets can condense, and in the tropics they are the chief source. The more cloud condensation nuclei, the brighter the cloud, that is, the more solar radiation reflected back into space before it reaches the earth's surface. This radiative effect is what makes clouds, and the indirect effects of aerosols, so important in climate change research. However, indirect effects are now so little understood that estimates in global climate models vary from almost no effect to more than enough cooling to offset global warming from greenhouse gas increases.

"A host of changes in cloud physical and microphysical properties is lumped under the term 'indirect effects,' " says CGD scientist Bill Collins. He's using satellite measurements to estimate optical depths in clouds and the sizes of aerosol droplets.

The Maldives archipelago runs in a north-south line from about 800 kilometers (500 miles) southwest of the tip of the Indian subcontinent to the equator. The personnel and observing systems have been spread across several islands, with all transport by boat. Resources included five aircraft, two research ships, and a host of ground-based systems. Satellite data were used for forecasting weather, monitoring the motion of pollutants, and measuring radiation at different altitudes. This combination of ground, airborne, and satellite data is expected to vastly increase scientists' understanding of the nature and scope of aerosols' indirect effects.

Andy and MMM colleague Greg McFarquhar studied some of the cloud microphysical changes using a scanning aerosol backscatter lidar developed at NCAR and mounted on the C-130. From the aircraft the lidar can characterize cloud top and bottom. The scientists observed the optical depth of clouds in both clear and cloudy aerosol-laden skies.

INDOEX is coordinated by the Center for Clouds, Chemistry and Climate (C4), an NSF Science and Technology Center at the Scripps Institution of Oceanography. Serving as chief scientists are Paul Crutzen, director of the Max Planck Institute for Chemistry and a 1995 Nobel laureate, and C4 director Veerabhadran Ramanathan. •Anatta, Carol Rasmussen, and Bob Henson

On the Web:
The INDOEX field catalog can be found at
Joint Office for Science Support - INDOEX PAGES

Notes from the Maldives

INDOEX project director Bruce Gandrud (ATD) reported in mid-March that the experiment was proceeding well:

The biggest surprise to everyone has been the degree of pollution which we routinely see in the low levels of the Northern Hemisphere atmosphere above the Indian Ocean on almost every research flight. There is a clearly distinguishable and remarkable increase in visibility as you pass through the ITCZ [intertropical convergence zone] into the Southern Hemisphere. This transition is also clearly shown by many of the sensors on the aircraft and ship platforms.

The next challenge will be to get all of the data reduced, submitted in final form, and interpreted. This will be a unique data set that will be used in many ways to better understand the climate system as well as for other areas of atmospheric research. There are plans for INDOEX special sessions, which will be announced soon, at scientific meetings in the United States and Europe.

We have all enjoyed the interactions with the Maldivian people enormously. One of the daily highlights each of us experiences is the dhoni (boat) ride from Malé to the airport and back.

Steve Williams (JOSS) elaborates on t0he experience of carrying out a field project in the middle of the Indian Ocean:

There are about 1,200 islands here. No one knows [the exact number] for sure because they disappear and surface periodically! About 300 of these are inhabited, and 70 of these 300 are resort islands. The government leases the islands to resort companies, which account for the main income of the country. The most populous island is the capital, Malé (just under one square mile). There are 75,000 people on this island, with the remainder of the 250,000 population in villages on the 200-plus smaller islands. To say it's crowded is an understatement!

We are located near the equator (4°N), so the days are all about the same length. It's funny to see all the TV satellite dishes pointing straight up. And the time zone is exactly 12 hours ahead of [Boulder time].

The Maldives is a pretty liberal Moslem country. You hear the call to prayer five times per day. Stores generally shut down for 20 minutes, and some people go to the local mosques to pray. The first call to prayer is around 4:30 a.m.! That one usually wakes up the roosters, who begin crowing, so there isn't much sleeping in. Actually, with the heat and humidity, it's a good time at sunrise to get out for a walk around the island, while it's cool. The highs each day (regardless of season) are in the low to mid 90s [Fahrenheit] with lows around 80. This is the dry season before the onset of the monsoon, so I've seen no rain since I've been here. •

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Edited by Bob Henson, bhenson@ucar.edu

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