January 1, 2011 - Atmospheric gravity waves over the Arabian Sea

Atmospheric gravity waves over the Arabian Sea

At times, remote sensing instruments orbiting above the earth can capture phenomena that would otherwise be invisible. For example, in areas of sunglint, disturbances of the ocean surface can stand in bold relief. On December 23, 2010, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite captured a striking image of a semi-circle of atmospheric gravity waves over the Arabian Sea. The visible wave patterns do not come from waves of water; they are created when the waves in the atmosphere touch the surface of the ocean.

Gravity waves form in the atmosphere when there is a disturbance in the air layer, such as when lighter, warmer air moves upward (buoyancy) and then gravity pulls it back down. This creates an oscillation, or wave pattern, that ripples outwards. In the trough of the wave (the low point), the air touches the surface of the ocean, causing the water to be roughened. The area between these troughs remains smooth.

This phenomenon of roughened and smooth water can be subtle and difficult to discern, except in areas of sunglint. Sunglint occurs when the Sun reflects off the surface of the ocean at the same angle that a satellite sensor is viewing the surface. In a sunglint area, smooth ocean water reflects a large amount of sunlight, creating a silvery, mirror like appearance. Roughened water reflects less light back to the sensor, so the water appears dark. In sunglint, atmospheric gravity waves become visible as bands of sliver and blue.

In the southeast section of the image, swirls of turquoise mark the outflow of sediment from the Gulf of Kutch. This inlet, on the western coast of India, is known for extreme tides, which can carry large amount of sediment. To the north, a haze covers the Indus River Valley, and appears to extend over the Arabian Gulf.

Image Facts
Satellite: Terra
Date Acquired: 12/24/2010
Resolutions: 1km ( B), 500m ( B), 250m ( B)
Bands Used: 1,4,3
Image Credit: Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC