NOAA Ground-Based Radiometers

Radiometers are passive instruments that receive energy signals, which are naturally emitted from sun. A radiometer antenna pointed upward into the air receives microwave and infrared emissions from the sun then by observing the absorption of the various elements of the atmosphere, Vapor, Liquid, Oxygen, the amount in the atmosphere can be found. Each constituent possesses a unique spectrum that corresponds exactly to its absorption spectrum. Radiometers "listen" at selected frequencies to best sort out the constituents and measure their abundances.

By monitoring microwave emissions from the zenith at appropriate frequencies near 20 or 23 and 31 GHz, it is possible to measure both the total atmospheric water vapor content and the cloud liquid water content over head; additional observations at 90 GHz allow very small amounts of liquid water to be detected. These path- integrated measurements provide information on the total amount of vapor and liquid along the radiometer's beam, but give no direct information about the range or altitude distribution of vapor or liquid in range along the beam. The infrared radiometers monitor thermal emissions near the wavelength of 11 microns. The measured IR brightness temperature is approximately equal to the physical temperature of cloud liquid at the base helping in the cloud type identification. The radiometers are also useful for estimating cloud droplet sizes (microwave) and ice particle sizes (IR) and mass contents from the ground when used in concert with millimeter-wave cloud radar, as in ETL's MMCR package. Winds just above the ocean surface can also be estimated from airborne microwave radiometer observations of surface reflections and emissions, which depend on the wind-caused roughness of the ocean surface.

Ground-Based Microwave Radiometers:

Ground-Based Infrared Radiometers in Operation:

Present Status:

Radiometer Schedule: