NASA's RapidScat to Unveil Hidden Cycles of Sea Winds

Credit: JPL/NASA

Ocean waves, the hot sun, sea breezes, the right combination makes a great day at the beach.

A different combination makes a killer hurricane.

The complex interactions of the ocean and the air above it that can create such different outcomes are not yet fully known.

Scientists would especially like to understand the role that the daily heat of the sun plays in creating winds.

In a few months, NASA will send an ocean wind-monitoring instrument to a berth on the International Space Station.

That unique vantage point will give the International Space Station Rapid Scatterometer (ISS-RapidScat), the ability to observe daily (also called diurnal) cycles of wind created by solar heat.

Winds contribute to motion in the ocean on every scale, from individual waves to currents extending thousands of miles.

They affect local weather as well as large-scale, long-term climate patterns such as El Niño.

Across the tropical Pacific, winds help or hinder local economies by allowing nutrient-rich water to well up from the ocean depths, nourishing marine life to the benefit of coastal fisheries, or blocking its upwelling.

Since the hours of daylight are totally predictable, you might expect their influence on winds to be equally obvious. But that's not the case.

According to Sarah Gille, an oceanographer at Scripps Institution of Oceanography, San Diego, "There's an enormous amount of diurnal wind variation between 30 degrees north and south of the equator, and we don't understand the timing. It's clear that the winds aren't just triggered every day at noon [when the sun is highest]."

Scatterometer observations from satellites have proven invaluable for understanding ocean winds.

A scatterometer is a type of radar that bounces microwaves off Earth's surface and measures the strength and direction of return signals.

The more uneven the surface, the stronger the return signals. On the ocean, higher winds create larger waves and therefore stronger return signals.

The return signal also tells scientists the direction of the wind, because waves line up in the direction the wind is blowing.

The reason spaceborne scatterometers haven't helped much with the specific question of daily wind cycles has to do with their orbits.

All modern instruments have been in sun-synchronous orbits, in which a satellite is always oriented at the same angle relative to the sun.

In this type of orbit, a satellite passes over every location at the same fixed times, for example, 6 a.m. and 6 p.m. over the equator.

The resulting data can't throw much light on the question of how winds develop over the course of a day.

More information: For more information about ISS-RapidScat, visit: winds.jpl.nasa.gov/missions/RapidScat/