Looking at Clouds from Both Sides Now

Hank Hogan

Clouds, as Joni Mitchell sang, get in the way. They block incoming sunlight and outgoing radiation, playing key roles in weather and in the transfer of heat in the atmosphere. They also influence optical communications and affect the availability of solar power. For many applications and areas of research, clouds are more than whimsical ice cream castles in the air.

For the big picture on clouds, the best platform is a satellite that offers a global view from above. At times, however, researchers need ground-based sensors that peer upward to provide localized, continuous data with high resolution. Looking up also reveals cloud bottoms, something that can’t be done with a satellite. But there’s a problem with this approach.

“There are quite a few very nice visible-sky imaging instruments, which work really well during day, and sometimes during night, but which do so differently during day and night,” explained Joseph A. Shaw, an associate professor of electrical and computer engineering at Montana State University in Bozeman.

Shaw now has a solution in the form of the infrared cloud imager. The instrument measures clouds consistently, without regard to the presence or absence of the sun.

Originally created for a joint US-Japan research project studying the Arctic atmosphere, the device consists of a box containing a microbolometer-based infrared camera with a spectral response of 8 to 14 µm and a 320 × 240 array of 50-µm-square pixels. The camera faces a gold mirror, which rotates so that the detector captures the thermal emission from one of three scenes.

The first scene is of the sky as seen through a rectangular hatch in the box, which shuts during rain or snow. The other two views are of blackbody sources, objects that emit a temperature-dependent spectrum that follows theoretical predictions. One of the blackbody sources is kept near 50 °C, and the other floats at the ambient temperature of the interior of the box, typically near 20 °C.

The researchers employ these blackbody sources to calibrate the camera. Shaw noted that other instruments have used the technique but that the infrared cloud imager’s calibration is performed with a twist -- through arrays that contain gain and offset terms for each individual pixel.

A potential source of error is the contrast between a cloud and the surrounding air. This becomes less likely when clouds are thin, high and cold. The contrast also decreases when the overall water vapor content in the air rises. To correct for this, the researchers measure water vapor content using either a microwave radiometer or a balloon-borne in-strument package that sends data back after launch.

As a result of the calibration and correction, in tests reported in the July 12 issue of Optics Express, the infrared device produced images of the sky under a variety of conditions that agreed with readings from other instruments. Importantly, the instrument demonstrated the capacity to perform reliable and robust cloud detection regardless of changing atmospheric conditions or the time of day.

Shaw said that there are plans to expand the angular field of view of the infrared cloud imager. Another goal is to simplify water vapor correction so that the device will no longer require the simultaneous deployment of a microwave radiometer. Even without such improvements, he sees the cloud spotter as having a number of potential uses.

“We are pursuing ideas presently for using the infrared cloud imager technique to study link-availability statistics on Earth-space optical communication paths, which are very heavily impaired by clouds,” he said.