ARM Program Achieves Milestone in Global Cloud Properties Research

Revercomb, H. E., University of Wisconsin, Madison

Radiation Processes

Radiative Processes

Knuteson, R.O., Best, F.A., Dedecker, R.G., Feltz, W.F., Revercomb, H.E., and Tobin, D.C., 2004: "10 Years of AERI Data from the DOE ARM Southern Great Plains Site," In Proceedings from the Fourteenth ARM Science Team Meeting, U.S. Department of Energy,Washington, D.C.

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From the unassuming farmlands of north-central Oklahoma comes a milestone for the global climate research community. March 2004 marked the 10-year anniversary for an instrument that now holds the prestigious distinction of providing the longest set of continuous atmospheric interferometer data ever—anywhere. Using this robust data set, the climate modeling community is making unprecedented advances in reducing uncertainties in longwave radiation theory and improving scientific understanding of the radiative effects of clouds and water vapor.

Scattered among 55,000 square miles of mostly cattle pasture and wheat fields, the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Program operates a heavily instrumented research facility, called the Southern Great Plains (SGP) site. This was the first research site established (in 1992) by the ARM Program for the purpose of studying clouds and water vapor and their impact on the energy balance of our atmosphere. In 1994, an Atmospheric Emitted Radiance Interferometer, or AERI, began operating at the SGP.

The AERI (Figure 1) measures the absolute longwave radiance emitted directly downwards by the atmosphere at high spectral resolution. The AERI is comprised of an interferometer, radiance detectors, two calibration black-bodies, a Stirling cooler, and sophisticated optics, all of which are computer controlled for automated operation. Every 10 minutes, it completes its cycle of measuring atmospheric radiance, viewing the calibration bodies, and storing data. The result isn't difficult to imagine: spectral measurements obtained every 10 minutes for 10 years equals a whole lot of data (see Figure 2)! This unique data set is being used for many purposes: long-term comparisons with model output to validate and improve atmospheric radiation models, retrieval of temperature and humidity structure in the lower atmosphere, determination of the effects of clouds on the surface infrared radiation balance, and in the retrievals of the microphysical properties of the overhead clouds.

The AERI measures and records the radiance emitted by the atmosphere at 5100 wavelengths from 3.3 to 19 μm every 10 minutes, see Figure 2. Spectral measurements at the core of the CO2 absorption band (upper panel) are directly proportional to the air temperature just above the instrument. It operates, therefore, in this spectral region, as a very sophisticated thermometer. The annual cycle of temperature, as well as daily extremes are clearly seen. Spectral measurements in the atmospheric window region (bottom panel) are heavily influenced by clouds. In clear conditions, radiance values (expressed here as a radiation temperature) are very low; in cloudy conditions, particularly for low thick clouds, the radiance values are high, although always less than the values in the upper panel. The measurements in the lower panel, then, are a 10-year record of the occurrence of cloud at the SGP and the effect of that cloud on the thermal infrared radiation budget.

Due to the successful deployment of the first AERI at SGP's Central Facility near Lamont, Oklahoma, four more AERI's were installed at each of the SGP boundary facilities, which stretch across the border into Kansas. These instruments were deployed in 1998. They are linked by the Multiple AERI Data System, which allows the status of each instrument (some of which are 200 miles from the Central Facility) to be monitored remotely. Results from this network are used to improve weather forecasting capability, as well as for climate research.