Of the so-called atmospheric greenhouse gases, water vapor is the most effective in producing surface-atmospheric warming. Water-vapor concentrations increase rapidly over the tropical oceans when sea surface temperatures (SSTs) begin to rise (as in global warming scenarios, for example). This effect is greatest over the huge "warm pools" of the Pacific Ocean. The water- vapor content of the atmosphere above the ocean increases by about 15—20% for each 1% of increase in SST. These increasing concentrations of water vapor trap more and more heat, which causes the ocean's surface temperature to rise even further, thus creating a "super-greenhouse effect." Unchecked, this feedback mechanism would result in runaway warming. This is not what is observed, however; even in the "warm pool," SSTs never exceed 304 K (31ºC). This suggests that some kind of "thermostat" might exist. Furthermore, deep intensive convection over the tropical ocean—with cloud tops reaching altitudes of 18 to 20 km—only when SSTs exceed about 300 K. These observations raise two central questions:
It has been argued that cooling by evaporation from the ocean surface provides such a mechanism.
However, observations from space and from the atmospheric boundary layer indicate that this process is not sufficient. Rather, it may be the very high and cold cirrus clouds, streaming from tropical thunderstorms, stretching over large areas of the Pacific, and reflecting the incoming solar radiation that, in fact, act as a thermostat (see Figure 1).

The purpose of the Central Equatorial Pacific Experiment (CEPEX) is to investigate this mechanism. The overall scientific goal of CEPEX is to establish the respective roles of cirrus radiative effects and surface evaporation in limiting maximum surface temperature in the equatorial Pacific.
Direct in-situ measurement of radiation fluxes, cirrus microphysics, evaporation rates, and water- vapor distributions must be obtained over a range of SSTs, from regions where SST is just below the convection threshold temperature to regions where SST exceeds it. Accordingly, the CEPEX experiment domain will encompass the transition (with respect to SST) region from the central equatorial Pacific to the tropical south Pacific or the tropical western Pacific "warm pool."
The primary experimental objectives of CEPEX are to:
Multiple platforms (surface, airborne, and space-borne) will be employed to achieve these objectives (see Figure 2). Observations from high-altitude aircraft above and below the cirrus will be used to estimate the albedo of cirrus and the radiation energy converging into the cirrus, as well as the water-vapor distribution above and below the cirrus, the horizontal gradient of cirrus radiative heating, and the microphysical causes for the brightness of the cirrus. Observations from a low-level aircraft and a ship will be used to estimate evaporation from the sea surface and its relationship to SST gradients and how the cirrus regulates solar energy flux to the sea surface. In addition, upsondes launched from the ship and islands, dropsondes launched from aircraft, surface buoys, satellite cloud data, and island surface meteorological and radiation sensors will complete the CEPEX composite observing system (see Figure 3).

CEPEX will be conducted in March 1993 with an operations base in Fiji, immediately following the TOGA-COARE study of the western tropical Pacific Ocean, and will take advantage of many of COARE's observing systems, including several critical ones that will remain in place during the CEPEX field phase. Data from the TOGA (Tropical Ocean Global Atmosphere)- COARE field phase will provide information essential to CEPEX (i.e., understanding the most important forcing mechanisms for maintenance of the warm pool). CEPEX will contribute to the interpretation of TOGA-COARE results by providing coverage for an extended period and over a larger area and by focusing on the thermodynamic cloud forcing mechanism for the regulation of the ocean warm pool.

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