Chapter 4


Introduction The fundamental design requirement is to determine, from observations, how the atmospheric greenhouse effect, water-vapor distribution, cloud radiative forcing, evaporation, and surface solar and long-wave fluxes vary from the warm ocean (SST > 300 K) with fully developed convection, to the colder ocean (SST < 300 K), where convection is marginal or entirely suppressed. Observations will be used to examine how the extended convective-cirrus anvil cloud system influences the evaporative and radiative fluxes at the sea surface as well as the water-vapor concentrations and radiative fluxes in the troposphere and lower stratosphere.

The entire Pacific Ocean between 20ºS and 20ºN is the ideal region in which to observe these changes. The supporting platforms, especially the satellites, will observe most of this region. However, the in-situ observations from the primary platforms are necessarily limited to a much smaller region and to selected temporal sampling, between about 160ºE and 160ºW and between 4ºS and 4ºN. This smaller region captures the transition from convectively disturbed conditions (SST > 300 K) west of the dateline or south of 4ºS (the south Pacific convergence zone) to a regime of suppressed convection (SST < 300 K) east of the dateline.

Observations have to be taken across an SST gradient to observe the link between SST and the relevant parameters. TOGA-COARE observations will characterize the western Pacific warm pool climatology. By orienting CEPEX observing tracks parallel to the equator and east from the warm pool, the probability of observing gradients in SST, evaporation, and radiative fluxes will be maximized.

Parameters, Resolution, and Accuracy

The required observations to meet the primary and supporting objectives of CEPEX fall under the following broad categories:

A detailed listing of the parameters is given in Tables 3 to 5. These tables prescribe the required spatial resolution, absolute and relative accuracies, and specifics of the observational platform for each of the measurements. The specific objective of each measurement is defined in the last column.

Required Observing Platforms

The various platforms required to meet CEPEX objectives are shown schematically in Figure 2. As shown in Figure 19, the measurements fall into four distinct categories:

The CEPEX composite observing system (Figure 3) consists of both primary and supporting platforms. Primary platforms have been especially designed and commissioned by the CEPEX investigators to meet the primary objectives of the experiment. Supporting platforms include operational systems (e.g., satellites and island stations) and special platforms commissioned under the auspices of the TOGA-COARE field program (e.g., moorings and aircraft observations of evaporation and radiation fluxes in the warm pool).

Table 3 , Table 4 , Table 5

Primary Platforms

Supporting Platforms

The aircraft, by themselves, cannot meet the spatial and temporal sampling accuracy. For example, the measurements made by the high-altitude aircraft along a path will be insufficient to estimate the heating rates of a cloud system, such as a cluster. We will use satellite measurements to get the synoptic perspective and aircraft measurements for in-situ truth. Several supporting platforms will be needed; these include

Supporting Data Sets from TOGA-COARE

CEPEX will use the following observations taken during TOGA-COARE:

TOGA-COARE flight plans prescribe high-altitude (50,000—65,000 ft; 15—19.5 km) ER-2 flights and medium-altitude (33,000—39,000 ft; about 10—12 km) DC-8 flights in January and February 1993. TOGA-COARE investigators plan to use these flights, in part, to make cirrus radiation observations over the heart of the western Pacific warm pool (private communication, T. Ackerman, R. Lukas, F. Valero, and P. Webster). The COARE domain extends from 140ºE to 180º (dateline) and from 10ºS to 10ºN. COARE will also be determining the monthly to seasonal average of evaporation in the warm pool, using an armada of aircraft. These evaporation measurements are critical to obtaining the CEPEX objectives.

Intercomparison Flights

The absolute accuracy of measurements is very critical for achieving the CEPEX objectives. A very effective procedure for assessing the absolute accuracy of the measured quantities is to intercompare measurements taken from independent instruments within one platform and from identical or independent instruments from different platforms. Intercomparison flights should be made at the beginning, middle, and end of the observing period. The intercomparison flights should include all three aircraft, the R/V Vickers, and the Japanese Geostationary Meteorological Satellite (GMS) data collection periods. Whenever possible, such flights should also overfly the moorings and be collocated with the overpasses of one of the four NOAA polar orbiters.

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