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An Atmospheric Radiation and Cloud Station in the Tropical Western Pacific

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An Atmospheric Radiation and Cloud Station in the Tropical Western Pacific An Atmospheric Radiation and Cloud Station in the Tropical Western Pacific J. H. Mather,* T. P. Ackerman,* W. E. Clements/ F. J. Barnes/ M. D. Ivey,# L. D. Hatfield @ and R. M. Reynolds& ABSTRACT The interaction of clouds and radiation is a particularly difficult issue in the study of climate change. Clouds have a large impact on the earth's radiation budget but the range of spatial and temporal scales and the complexity of the physical processes associated with clouds made these interactions difficult to simulate. The Department of Energy's Atmospheric Radiation Measurement (ARM) program was established to improve the understanding of the interac- tion of radiation with the atmosphere with a particular emphasis on the effects of clouds. To continue its role of pro- viding data for the study of these interactions, the ARM program deployed an Atmospheric Radiation and Cloud Station (ARCS) in the tropical western Pacific. This site began operation in October 1996. The tropical western Pacific is a very important climatic region. It is characterized by strong solar heating, high water vapor concentrations, and active convection. The ARCS is equipped with a comprehensive suite of instruments for measuring surface radiation fluxes and properties of the atmospheric state and is intended to operate for the next 10 years. The ARCS is an integrated unit that includes a data management system, a site monitor and control system, an external communications system, redundant electrical power systems, and containers that provide shelter for the equipment as well as work space for site operators, technicians, and visiting scientists. The dataset the ARCS produces will be invaluable in studying is- sues related to clouds and radiation in the Tropics. The site is located in Manus Province, Papua New Guinea, at 2.060°S, 147.425°E, 300 km north of the island of New Guinea. Two more ARCS are planned for deployment across the tropi- cal Pacific. 1. Introduction fluxes and properties of the atmospheric state and is intended to operate for the next 10 years. The dataset The Department of Energy's (DOE's) Atmo- it produces will be invaluable for studying issues re- spheric Radiation Measurement (ARM) program has lated to clouds and radiation in the Tropics. The ob- deployed an Atmospheric Radiation and Cloud Sta- jectives of the ARM program are to relate surface tion (ARCS) in the tropical western Pacific. The tropi- radiative fluxes to the atmospheric composition and cal western Pacific (TWP) ARM site began operation use the observed relationships to develop parameter- in October 1996. The ARCS is equipped with a broad izations of the interaction of clouds and radiation for range of instruments for measuring surface radiation use in general circulation models (U.S. Department of Energy 1996). The first ARM site is located in the U.S. *The Pennsylvania State University, University Park, Pennsyl- southern Great Plains at Lamont, Oklahoma, and at vania. an associated grid of extended facilities. This site has +Los Alamos National Laboratory, Los Alamos, New Mexico. been operating since 1992. The southern Great Plains #Sandia National Laboratory, Albuquerque, New Mexico. region was chosen as the initial ARM site because it ^Pacific Northwest National Laboratory, Richland, Washington. is geographically homogeneous and it experiences a &Brookhaven National Laboratory, Upton, New York. wide range of cloud types and climate conditions Corresponding author address: Dr. James Mather, Department of Meteorology, The Pennsylvania State University, 503 Walker (Stokes and Schwartz 1994). The TWP ARCS is the Building, University Park, PA 16802. first component of the second ARM site and the first E-mail:[email protected] in a remote region. The tropical western Pacific and In final form 18 December 1997. the north slope of Alaska, which will follow as the Bulletin of the American Meteorological Society 627 Unauthenticated | Downloaded 10/09/21 09:48 PM UTC third ARM region of study, were chosen because they of clouds and their effect on radiation are not well represent a wide range of climatic conditions. understood, but also because of the disparity in spa- The TWP is a very important climatic region. It is tial scales between these processes and the coarse reso- characterized by strong solar heating, high water va- lution of climate models. To improve this situation, por concentrations, and active convection. The sea measurements of cloud distributions and the radiation surface temperature in the TWP is exceptionally high budget are required. over a very large area; consequently, this region is Climate data from the tropical Pacific are scarce often referred to as the warm pool. The region under- largely because of the general inaccessibility of the goes little seasonal variability but it does exhibit fluc- region. However, the ARM program is not the first tuations on a variety of timescales including a strong climate research program to study this area. The diurnal cycle, the 40-50-day Madden-Julian oscilla- World Meteorological Organization organized an tion (Madden and Julian 1994), and the interannual early study of tropical convection in 1974. The Glo- El Nino-Southern Oscillation (Battisti and Sarachik bal Atmospheric Research Program's Atlantic Tropi- 1995). While the Madden-Julian oscillation and cal Experiment (GATE) was designed in response to El Nino are confined to the tropical Pacific, these the need to characterize tropical convection in com- cycles have been linked to variability in precipitation puter models. The goal of GATE was to improve un- around the globe. These links between the Tropics and derstanding of tropical convection and its role in midlatitudes are not well understood and, conse- global circulation. GATE involved intensive observa- quently, have focused considerable interest on tropi- tions of convection off the west coast of Africa using cal climate variability, particularly in the context of a combination of satellites, radar, aircraft, and upper- global climate change. air soundings (Houze and Betts 1981). An important signature of tropical intra- and Following GATE, the World Meteorological Or- interannual oscillations is the frequency and spatial ganization began the World Climate Research distribution of convection. During the active phase of Programme. The objectives of this program were to the Madden-Julian oscillation, convection over the determine the extent to which climate change could tropical oceans is very deep and widespread with con- be predicted and the extent of human impact on cli- vective towers extending to the tropical tropopause. mate. One of the programs sponsored by the World The cirrus outflow layers associated with these tow- Climate Research Programme was the Tropical Ocean ers cover a large fraction of the warm pool region and Global Atmosphere (TOGA) program. One of the (Prabhakara et al. 1993). Both the convective towers main objectives of the TOGA program was to improve and the cirrus have large impacts on radiative heating understanding of the El Nino-Southern Oscillation of the atmospheric column and the ocean surface. phenomenon. The TOGA program was designed to During the suppressed phase, deep convection and the improve the predictability of the tropical ocean- associated cloudiness is much less frequent. A promi- atmosphere coupled system and to design an opera- nent feature of the El Nino-La Nina cycle is the change tional observing system for the tropical Pacific in the longitudinal distribution of deep convection. (Manton 1991). During the La Nina phase of the Southern Oscillation, A major experiment called the Couple Ocean- the convection is centered over the western Pacific, but Atmosphere Response Experiment (COARE) was or- during the El Nino phase, the region of most active ganized under the TOGA program (Webster and Lucas convection shifts to the east. 1992). The primary goals of TOGA COARE included Climate models attempting to predict the response understanding the processes that couple the ocean and of the earth's climate to a forcing such as a doubling the atmosphere and the processes that organize con- of the carbon dioxide concentration must first accu- vection in the western Pacific warm pool region. The rately represent the distribution of clouds and their intensive part of TOGA COARE was carried out from effect on solar and infrared radiation in the present November 1992 to March 1993 and included obser- climate. Having done this, it may then be possible to vations of convection, sea surface temperatures rain- anticipate how cloud properties and their subsequent fall, and fluxes of heat, moisture, and momentum. effect on the radiation budget will change given some In cooperation with the TOGA COARE experi- external forcing. There is considerable uncertainty in ment, the ARM program sponsored the Pilot Radia- current climate models in these areas, partly because tion Observation Experiment (PROBE) (Renne et al. many physical processes controlling the distribution 1994). PROBE was meant to provide information for 628 Vol. 79, No. 4, April 1998 Unauthenticated | Downloaded 10/09/21 09:48 PM UTC the design of a permanent, remote ARM station and, ability. Climate data from the Manus site are shown at the same time, provide useful radiation data for in Fig. 2. There is no seasonal variation in the surface TOGA COARE (Waliser et al. 1996; Long 1996). air temperature and only a weak suggestion of a cycle Using the lessons learned from PROBE, the first in the rainfall. The mean of the daily maximum tem- ARCS has been designed, built, and de- ployed. This paper describes the ARCS and its components and will present some initial observations. 2. ARM in the tropical western Pacific Candidate locations for the first TWP site had to be within the warm pool re- gion. This can roughly be defined as the region bounded in longitude by 120°- 180°E and in latitude by 10°N and 10°S of the equator.
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