SGP Introduction

The Atmospheric Radiation Measurement (ARM) Program is a multi-laboratory, interagency program that was created in 1989 with funding from the U.S. Department of Energy (DOE). The ARM Program is part of DOE's effort to resolve scientific uncertainties about global climate change with a specific focus on improving the performance of general circulation models (GCMs) used for climate research and prediction by improving the parameterizations of clouds and radiations used in GCMs. These improved models will help scientists better understand the influences of human activities on the earth's climate.

In pursuit of its goal, the ARM Program has established and operates field research sites, called Cloud and Radiation Testbeds (CARTs), in three climatically significant locales (Southern Great Plains - SGP, Tropical Western Pacific, North Slopes of Alaska). Scientists collect and analyze data obtained over extended periods of time from large arrays of instruments to study the effects and interactions of sunlight, radiant energy, and clouds on temperatures, weather, and climate.

The SGP CART site consists of in situ and remote-sensing instrument clusters arrayed across nearly 50,000 square miles in Oklahoma and Kansas. ARM Science Team members are now using the information obtained from the SGP to improve cloud and radiative models and parameterizations that are embedded within GCMs.

The decision to locate the CART site in the Southern Great Plains region can be partially attributed to the pre-existence of a number of other national and state observing networks in the region, all of which possess observing platforms that add value to the goals of the ARM Program and provide for mutually beneficial activity. Among these are the Oklahoma Mesonet, NOAA Wind Profiler Network, the WSR-88D radar array, and various watershed study areas of the USDA. The site also contains the following desirable attributes: relatively homogeneous geography, widely variable cloud types and surface flux properties, and large seasonal variation in temperature and specific humidity.

Deployment of the first instrumentation at the SGP site occurred in the spring of 1992, just 24 months after the program was approved. The Central Facility site of the CART, between Lamont and Billings, Oklahoma, was dedicated in November 1992. Additional instrumentation and data processing capabilities have been incrementally added in the succeeding years. We continue to add, and/or remove, instrumentation as the scientific needs of the program evolve. The University of Oklahoma, through the SGP Site Scientist Team, helps provide scientific oversight for operations of the SGP CART Site. The Oklahoma Climatological Survey is part of the Site Scientist Team, providing educational outreach support.

ARM seeks to better determine and document the above interactions between radiation, clouds, and aerosols, in order to build better numerical schemes as input to GCMs. The way it has chosen to do this is by collecting measurements of the highest quality over a long period of time, to allow scientists from across the world to use them to build better models (shown at right is the radiometer calibration facility).

The ARM Program has developed a coherent approach to climate research. Formerly, measurements were made in field campaigns lasting only a few weeks. Investigators made their own independent observations. Locations and sponsorship varied. The data generated often were fragmented and inconsistent and sometimes were inaccessible. Dissemination of the data to the scientific community often took years. Under the ARM Program, a firm foundation of continuous baseline measurements is supplemented with periods of more frequent measurements at the same site (intensive observation periods), in an area that should provide the widest possible range of meteorological conditions for model testing. A consistent body of data of known quality is processed and disseminated in near-real time.

Specialized instruments are the core of the CART facility. The instruments, which provide a foundation of continuous observations, are deployed across Kansas and Oklahoma at locations selected to provide the spatial and temporal scales required. These instruments measure solar radiation, meteorological conditions, and clouds, allowing interpretation of radiative transfer within and between atmospheric columns every 30 minutes.

The continuous observations at the CART site are supplemented by intensive observation periods, when the frequency of measurements is increased and special measurements are added to address specific research questions. During such periods, nearly a gigabyte of data (one billion bytes) is generated daily. Both during intensive observation periods and at other times, scientists bring their own specialized instruments to the CART site, typically for several weeks.

The ARM Program uses computers to obtain calibrated data of known quality from the field instruments. These data are distributed to scientists quickly enough for use in near-real-time computer simulations. This speed allows researchers to test the predictions of their models while weather patterns are developing, greatly decreasing the time required to refine and validate models.

Clouds may absorb sunlight and convert it to heat, raising temperatures. However, clouds may also reflect and scatter sunlight and infrared radiation, decreasing temperatures. Uncertainty about the relative strengths of these effects contributes greatly to the overall uncertainty that limits the effectiveness of general circulation models. Models need a much more accurate description of cloud formation and dissipation and the way clouds interact with sunlight and infrared radiation. The research community is seeking answers to several specific questions about clouds and climate: How is cloudiness distributed? What determines the distribution of cloudiness? How do clouds directly affect the atmosphere? What roles do the effects of clouds play in regulating or maintaining the present climate? How do clouds influence climate change?

The ARM Program scientists seek to understand clouds and radiative transfer better so that climate models can be refined to predict trends for longer periods of time. The information generated will help to answer our questions about whether our climate is truly changing and to what extent. Like clouds, greenhouse gases (water vapor, carbon dioxide, and others) can also absorb radiation and warm the atmosphere. The warming these gases cause can alter their own abundances and distributions. Along with the effects of clouds, feedback interactions like this contribute to the complexity of modeling weather and climate.

The ARM Program deploys specialized remote sensing instruments in a fixed location to gather atmospheric data of unprecedented quality, consistency, and completeness. More than 30 instrument clusters have been placed around the CART site, at the central facility and at boundary and extended facilities. The locations for the instruments were chosen so that the measurements reflect conditions over the typical distribution of land uses within the site.

The locations of the instrument clusters were selected both to provide the spatial and temporal scales required and to sample conditions over representative land use types. Many of the instruments are the only one of their kind or the first of their kind. This uniqueness sets the CART site apart as a world-class facility.

Solar radiation includes visible, ultraviolet, and near-infrared light. The absorption, transmission, and reflection of these components of solar radiation depend on the wavelength of the light. When scientists can better understand the effects of radiation in Earth's atmosphere, they can make significant progress on the question of whether and to what extent human activity is altering climate. The instruments at the CART site were selected to characterize these effects of radiation. In some cases, new instruments had to be developed.

Radiometric observations include measurements of direct and scattered radiant energy from the sun, as well as the infrared energy emitted from Earth. The ARM Program uses several types of conventional, broadband solar and infrared radiometers. Pyranometers measure the total solar radiation coming from the sky or reflected from the surface. Pyrgeometers measure the total infrared radiation coming from the sky or emitted from the surface. Pyrheliometers measure the direct-beam solar radiation. Filtered, rotating shadowband radiometers detect total, direct, and diffused solar radiation in selected wavebands, and can infer aerosol optical depth.

The atmospherically emitted radiance interferometer (Univ. of Wisconsin) is a key instrument for studies of radiative transfer. Used to measure infrared radiances with high accuracy and spectral resolution, this instrument is essential for experiments on the effects of greenhouse gases, clouds, and fine particles (aerosols) on atmospheric transmission, absorption, and emission of infrared radiation. In addition, specialized radiometers are used to examine the spectral distribution of ultraviolet, visible, and near-infrared radiation. The AERI data can be used for: 1) evaluation of line-by-line radiative transport codes; 2) detection/ quantification of cloud effects on ground-based measurements of infrared spectral radiance and; 3) calculation of vertical atmospheric profiles of temperature and water vapor. Continuous AERI thermodynamic profiles were able to identify the breakdown of the convective "cap" on May 3, 1999 shortly before the large Oklahoma tornado outbreak occurred.

Cloud observations are desirable, because clouds play an important part in cooling and heating the Earth's atmosphere. Some of the instruments at the site are used to observe clouds for coverage, type, and physical properties (water or ice content, thickness, etc.). Whole-sky imagers provide a view of the entire sky as seen from the surface. They generate continuous estimates of the fraction of the sky covered by clouds, as well as measurements of the heights of cloud bases and the dimensions of clouds.

Aerosol characterization can be accomplished by taking measurements at 10 meters (33 feet) above the ground to document the size, distribution, and concentration of atmospheric particles. Lidar systems such as the Raman lidar provide data on the relative distribution of aerosols in a vertical column of the atmosphere overhead. This lidar is an active, ground-based laser remote sensing instrument that measures vertical profiles of water-vapor mixing ratio and several cloud- and aerosol-related quantities. Lidar (light detection and ranging) is the optical analog of radar, using pulses of laser radiation to probe the atmosphere.

Wind, temperature, and humidity greatly influence radiation and cloud processes in the atmosphere as a function of height. Knowledge of the state of the atmosphere is needed to interpret radiometric data and model results. The Radar Wind Profilers use microwave radar to record vertical changes in wind speed and direction. The profilers use high-frequency radar for measurements in the lower atmosphere and low-frequency radar for upper-atmosphere applications. The 915-MHz Doppler radar wind profiler provides information on mean wind speed in the lowest few kilometers of the atmosphere. When the profiler is operated in conjunction with pulsed sounds, the resulting radio acoustic sounding system (RASS) can provide information on temperature as a function of height. Like the wind profilers, the RASS uses two types of radar, one for low-level and one for high-level observations. Depending on atmospheric conditions, the larger 50-MHz radar wind profiler and RASS system can be used for observations to greater heights (up to seven kilometers), but with less vertical resolution.

The 60-meter (197-foot) tower and the surface flux station are used to collect information on wind speed and direction, temperature, humidity, and solar and infrared radiation in the lowest part of the atmosphere. The surface flux station is also used to record the movement of heat and moisture between the ground and the atmosphere.

Water vapor in the atmosphere gives off very small but detectable amounts of microwave energy. Microwave radiometers that point vertically help infer the amounts of water vapor and liquid integrated in a column of atmosphere. They are used to observe sky radiance at 23.8 GHz (gigahertz) and 31.4 GHz, the unique absorption bands of water vapor in Earth's atmosphere.

Energy balance Bowen ratio or eddy correlation stations are used to measure the rates of heat and moisture exchange between the surface of the Earth and the atmosphere at selected locations. Conventional meteorological stations measure temperature, humidity, wind speed and direction, precipitation rate, snow depth, and atmospheric pressure at the surface.

Finally, radiosondes (pictured earlier) use helium balloons to carry lightweight instrument packages to altitudes of about 20 kilometers (12.5 miles). These 285-gram (10-ounce) packages (including battery) radio information on temperature, humidity, and wind direction and speed to computers at the central facility. Technicians release radiosondes on balloons as frequently as eight times per day. Typically, the balloons travel horizontally about 130 kilometers (80 miles) before they burst and parachute safely back to Earth.

Scientific Milestones

In return for its investment of nearly $40 million annually for 10 years in the ARM Program, DOE is realizing benefits in the form of significant scientific milestones. The unique facilities at the CART site are attracting scientific participants from many other agencies, who come to use the site for research that cannot be performed elsewhere. More than a hundred major experiments were conducted in the site's first three years of operation. The following significant technical milestones have been met:

The ARM Program is generating a long-term set of spectral radiation and meteorological data of unprecedented accuracy for use in testing climate-related hypotheses.

Measurements from the CART site are being used by many different agencies (NASA, NOAA, DoD, USDA, and others) to validate models and satellite data.

Algorithms developed from ARM data are being used to improve general circulation models by agencies including the Goddard Fluid Dynamics Laboratory, the Goddard Space Flight Center, the British Meteorological Office, and the European Centre for Medium-Range Weather Forecasts (and soon, NCEP).

Use of data from the atmospherically emitted radiance interferometer has reduced the uncertainty in estimates of incoming longwave radiation fluxes by 75%.

Support from the ARM Program has led to the development of a rapid radiative transfer module for use in climate models that has significantly improved agreement with results from line-by-line radiative transfer models.

Introduction to the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains Cloud and Radiation Testbed (CART) Site
July 21, 1999
Randy Peppler, SGP Associate Site Scientist