Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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CERES and Surface Radiation Budget Data for BALTEX
G. Louis Smith, Bruce A. Wielicki and Paul W. Stackhouse
Langley Research Centre, NASA, Hampton, Virginia 23681, USA; g.l.smith@larc.nasa.gov
1. Introduction
The Clouds and Earth Radiant Energy System (CERES)
project has the objective of measuring shortwave and
longwave radiances from the Earth, from which the fluxes of
reflected solar radiation and of Earth emitted radiation at the
"top of the atmosphere" (TOA) can be computed. In
addition, other data are used with these measurements to
compute the radiation fluxes at the surface and at levels in
the atmosphere. These results will provide information about
the time-varying geographical distribution of heating by the
Sun and cooling of the Earth by outgoing longwave
radiation (Wielicki et al., 1996). Thus, CERES data products
are important to the Global Energy and Water Experiment
(GEWEX). As the premier regional project of GEWEX,
BALTEX has as a primary objective to understand better the
energy budget over the Baltic Sea Basin, which drives the
regional weather and climate. CERES data products will be
a valuable resource for scientists investigating processes
over the BALTEX region. This paper gives a brief overview
of CERES and description of the data products.
2. CERES Overview
There are two CERES instruments on the Terra and two on
the Aqua spacecraft. Both Terra and Aqua are in Sunsynchronous
orbits with an altitude of 705 km. Terra crosses
the Equator north-bound at 2230 hours and Aqua crosses the
Equator north-bound at 1330 hours local time (Smith et al.,
2004). Thus, every day CERES flies over the BALTEX
region near noon and at two times each night. On each
spacecraft, one CERES instrument scans in a cross-track
mode in order to map the radiation field geographically and
the second CERES instrument scans in azimuth as well as in
nadir angle so as to provide data from which improved
models of the anisotropy of the radiation fields have been
derived (Loeb et al.; 2003, 2004). The second CERES
instrument can also be used in a programmable azimuth
mode to support field programs. The spatial resolution of the
CERES measurements is 20 km near nadir.
CERES instruments began operating on the Terra spacecraft
in February 2000 and on the Aqua spacecraft in June 2002
(Smith et al., 2003), so that the data record covers the
BRIDGE Extensive Observing Periods and Coordinated
Observational Periods, as fig. 1 shows.
Figure 1. Time line of BRIDGE and CERES.
3. CERES Data Products
Because of the wide variety of application of radiation
budget data, from local process studies to climate studies,
there are a number of data products (Wielicki et al., 1998).
The first product of scientific use is the ES-8, which
provides data for each CERES measurement in terms of
radiance, the location of the pixel on Earth in terms of
latitude and longitude, the zenith and azimuth angles of
the spacecraft from the Earth location, the type of scene
(land, ocean, cloud, etc.), and the shortwave and longwave
fluxes at TOA.
The fluxes from the individual measurements are averaged
over a 2.5 o latitude-longitude grid, and then averaged over
the diurnal cycle (by use of a model of the diurnal cycle)
to produce a daily map of radiation fluxes for the globe.
These daily values form the ES-9 product. The daily
radiation flux values are next averaged over a month to
produce monthly-mean grid values of radiation fluxes,
which are archived as ES-4. These maps are useful for
climate studies.
For some applications of CERES data to BALTEX, it may
be desirable to begin with the individual measurements
from the ES-8 and to grid them e.g. on the REMO grid.
The ES-4, -8 and –9 data products are computed in the
same manner as were the data products for the Earth
Radiation Budget Experiment (ERBE) and are directly
comparable to ERBE results.
Since the improved radiation anisotropy models have been
developed, they are now used to compute improved values
of fluxes. These models require additional information
about the scene, which is derived from MODIS (Moderate
Resolution Imaging Spectroradiometer) instruments,
which are also on the Terra and Aqua spacecraft. Data
from these instruments are used to compute cloud fraction,
cloud top height, optical depth, water content and phase of
the droplets. CERES measurements together with this
information are used to compute the radiation flux at TOA
and the surface for each CERES footprint. This data
product is called the Single Scanner Footprint (SSF).
Instantaneous TOA and surface fluxes from the SSF are
averaged over a quasi-equal angle grid with 1 o resolution
at the Equator. These values are archived as a product,
(SFC) which contains daily values for one month.
Monthly average gridded values are then computed as the
SRBAVG product
The SSF is also used with meteorological data to compute
the radiation fluxes within the atmosphere at 500 mb and
the tropopause. This information and the cloud properties
are archived as the Clouds and Radiative Swath (CRS).
Next these results are gridded (FSW) and averaged over a
month to give monthly-mean maps (AVG).
The CERES scanners can be programmed to operate with
any prescribed azimuth as a function of time. Thus, as the
spacecraft flies over a given point, e.g. a location for a
field mission, the instrument will rotate in azimuth as it
scans, producing multiple measurements of the location.
This feature has been used to support a number of projects
(Szewczyk and Priestley; 2003, 2004)