Chapter 4 | | OBSERVATIONAL REQUIREMENTS |
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:
- Radiation. The required measurements include solar and long-wave radiation fluxes at the
sea surface and at altitudes corresponding to the tropopause, the cirrus anvil top and base, and the
top of the boundary layer.
- Water vapor, temperature, and winds. The required measurements include SST and
vertical profiles of temperature, winds, and water-vapor concentrations from the sea surface to
about 25 km.
- Evaporative and sensible heat fluxes from the sea surface.
- Cirrus microphysical parameters. These include size distribution of ice crystals, particle
shape, and cloud-top and cloud-base altitudes.
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:
- Flight-level measurements. Observations of radiation, water-vapor concentrations,
microphysics, and evaporation are required at specific altitudes. These have to be made from
aircraft platforms and, hence, are referred to as flight-level measurements.
- Vertical profiles. Temperature and water-vapor concentrations from the surface to about
25 km in altitude are best obtained from sondes deployed from aircraft or launched from ship and
island stations. Some additional profiles will be obtained by aircraft.
- Surface measurements. Observations of SST and radiation fluxes at the sea surface are
best obtained from an aircraft within the boundary layer, from a ship, and from available
moorings.
- Satellite measurements. In order to relate and characterize the measurements from these
platforms in terms of synoptic, convective, and cirrus cloud systems, space-borne observations of
radiances, cloud properties, and water vapor are required.
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
- Aircraft: The most important questions raised in the primary objectives will be investigated
via a series of flights collecting data at the following altitudes:
- Radiation and water-vapor concentration above the cirrus tops, cloud-top altitude,
and cloud optical depths at an altitude of approximately 18 km (60,000 ft) by the
NASA (National Aeronautics and Space Administration)/Ames ER-2 aircraft.
- Radiation and water vapor below the cirrus base at an altitude of approximately 14
km (44,000 ft), or below, by a Learjet aircraft operated by Aeromet, Inc.
Dropsondes deployed from the Learjet will measure vertical profiles of temperature
and horizontal winds, as well as water-vapor concentration between the cirrus base
and the sea surface. In addition, cloud-base and cloud-top altitudes as well as ice-
crystal size and shape, from the -30ºC level to the maximum altitude the Learjet can
achieve, will be examined.
- Evaporation from the surface, SST, lower troposphere water-vapor concentration,
and near-surface solar insolation at an altitude of 100—10,000 ft (0.03—3) by the
NOAA P-3 aircraft.
In order to estimate the radiative energy converging into the anvils, the ER-2 and the Learjet will
fly simultaneously and in stacked formation to the greatest extent possible.
- Ship: In-situ measurements of SST, sea surface solar insolation, downward long-wave
flux, and precipitation will be made from the Research Vessel Vickers, operated by NOAA and the
University of Southern California. Upsondes launched from the ship will measure vertical profiles
of temperature, winds, water vapor, and ozone concentration from the surface to about 25 km.
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
- polar and geostationary satellites for solar and infrared radiation cloud properties, column
water vapor, and their diurnal variations
- moorings for SST, solar radiation, precipitation, and boundary-layer humidity and wind
observations
- island stations for temperature and humidity soundings, vertical velocities, and radiation
measurements
Supporting Data Sets from TOGA-COARE
CEPEX will use the following observations taken during TOGA-COARE:
- evaporative fluxes in the warm pool
- radiative cooling rates and column greenhouse effect in clear regions, either during
convectively suppressed conditions or in descending branches of neighboring clouds
- microphysical properties and radiative heating rates within anvil clouds in the warm pool
- temperature, winds, and water-vapor soundings
- SST
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.
Back to Table of Contents