Aerosols are tiny particles suspended in the
air. The majority are derived from natural
phenomena such as volcanic eruptions, but it
is estimated that some 10–20% are generated
by human activities such as burning of
fossil fuels. The majority of aerosols form
a thin haze in the lower atmosphere and are
regularly washed out by precipitation. The
remainder are found in the stratosphere,
where they can remain for many months or
years. Scientists have yet to quantify
accurately the relative impacts on climate
of natural aerosols and those of human
origin, so as to reduce uncertainty about
how much aerosols are cooling Earth.
Predicting the rate and nature of future
climate change requires this clarification
of the processes involved.
As a consequence, the IPCC identifies
further information on aerosols as a
priority, highlighting a particular need for
additional systematic, integrated and
sustained observations which include the
spatial distribution of greenhouse gases and
aerosols. The Integrated Global Atmospheric
Chemistry Observations (IGACO) Theme of the
former IGOS Partnership aimed to provide a
framework ensuring continuity and spatial
comprehensiveness of the full spectrum of
atmospheric chemistry observations,
including the monitoring of atmospheric
composition parameters related to climate
change and environmental conditions. The
IGACO Theme Report was finalised in May 2004
and provides a comprehensive overview of
current and future satellite measurements
for tropospheric and stratospheric aerosols.
The report states, in particular, that
“satellite observations of aerosol optical
properties have progressed to a point where
they range from pre-operational to
operational, although there are
demonstration-mode instruments on a number
of research satellites”.
Reliable information on aerosols is also
required by applications outside the study
of the climate system. For example, accurate
and timely warnings of the presence of
airborne dust and ash – such as that arising
from desert dust clouds and volcanic
eruptions – are important to the safety of
airline operations. A worldwide volcanic ash
monitoring system, which is dependent on
satellite observations, is in place to
provide real time advice to pilots.
Measuring the distribution of aerosols
through the depth of the atmosphere is
technically difficult, particularly in the
troposphere.
Previously, techniques using instruments
such as AVHRR and ATSR were limited to
producing estimates of vertically integrated
total amounts, mainly over oceanic
regions.
Measurements over land are difficult (due to
persistent cloud cover and the high value, and
variability, of land surface reflectance), but
the new generation of multi-directional or
polarimetric instruments – such as AATSR and
MERIS (ended 2012), MODIS, and MISR, and
possibly APS on JPSS and 3MI on Post-EPS in
the future – offer better optical depth at
different frequencies, enabling aerosol
particle sizes, particularly over oceans, to
be inferred. The development of active
instruments such as ATLID and ALADIN, and
laser altimeter sensors, including ATLAS on
ICESat, should yield much improved measurement
capability. Since April 2006, Calipso has
flown a 3-channel lidar (designed specifically
to provide vertical profiles) and passive
instruments, orbiting in formation with Aqua,
Aura, Parasol (ending late 2013) and CloudSat
(the A-Train) to obtain coincident
observations of radiative fluxes and
atmospheric state. This comprehensive set of
measurements is essential for accurate
quantification of global aerosol and cloud
radiative effects.
Limb-sounding instruments such as ACE-FTS,
SCIAMACHY, and GOMOS principally provide data
on the upper troposphere and stratosphere with
high vertical resolution, but horizontal
resolution is relatively poor (typically of
the order of a few hundred km).
Current, long-term climatologies are based
upon AVHRR/3 on the NOAA and MetOp series of
low-Earth orbit satellites. These observations
will continue to provide estimates of total
column aerosol amounts over the ocean. AVHRR/3
is now replaced by a more capable visible and
infrared imager, called VIIRS, on the JPSS
series of satellites, starting with the Suomi
NPP mission launched in October 2011. VIIRS is
designed to acquire high-resolution
atmospheric imagery and generate a variety of
applied products, including some that give
information on atmospheric aerosols.
The CEOS response to the GCOS Implementation
Plan recognised that no operational aerosol
instruments measuring particle composition and
size/shape have been yet been flown and
efforts should be made to rectify this. It
encouraged
re-planning of the aerosol measurements
envisaged by APS/JPSS and consideration of
operational active sensing lidar (such as
CALIOP). CEOS committed to pursue the
following action: “CEOS agencies will
participate in replanning the APS instrument
removed from the planned payload of
[JPSS]”.
Essential Climate Variables: Upper Air
Temperature
With humidity, atmospheric temperature
profile data are a core requirement for
weather forecasting and are coordinated
within the framework of the Coordination
Group for Meteorological Satellites (CGMS).
The data are used for numerical weather
prediction (NWP), for monitoring
inter-annual global temperature changes, for
identifying correlations between atmospheric
parameters and climatic behaviour, and for
validating global models of the
atmosphere.
Upper air temperatures are a key dataset for
detection and attribution of tropospheric
and stratospheric climate change, measured
both by radiosondes and satellite
instruments. Temperatures measured by
high-quality radiosondes are an important
reference against which satellite-based
measurements can be calibrated. Upper air
temperatures are important for separating
the various possible causes of global
change, and are vital for the validation of
climate models.
Infrared (HIRS) and microwave sounders (MSU
and AMSU) have been providing atmospheric
profiles for almost 30 years. The microwave
data in particular have become key elements
of the historical climate record and
equivalent measurements need to be continued
into the future to sustain a long-term
record. The MSU radiance record is a primary
resource for this, providing essential
coverage over the oceans and data for
comparison and combination with radiosonde
data over land.
For global NWP, polar satellites provide
information on temperature with global
coverage, good horizontal resolution and
acceptable accuracy, but improvements in
vertical resolution are needed. Performance
in cloudy areas has been poor, but the
microwave measurements such as AMSU have
provided substantial improvements. As in the
case of humidity profiles, the Aqua, MetOp,
NOAA and JPSS missions offer comparable
improvements in vertical resolution for
measuring atmospheric temperature (using
AIRS+, AMSU-A, CrIS, HIRS, IASI, MSU).
For regional NWP, polar-orbiting satellites
provide information on temperature with
acceptable accuracy and good horizontal
resolution, but with marginal temporal
frequency and vertical resolution for
mesoscale prediction. Advanced radiometers
or interferometers planned for future
satellites should improve on the vertical
resolution and accuracy of current
radiometers. Geostationary satellites
provide frequent radiance data, but their
use over land is hindered because of the
difficulty in estimating surface emissivity.
In nowcasting, the temperature and humidity
fields are particularly useful for
determining atmospheric stability for
predicting precipitation type, the amount of
frozen precipitation, and convective
storms.
As with humidity profiles, nowcasting
predictions using atmospheric temperature
data benefit from hourly geostationary
infrared soundings (such as from the GOES,
MSG and FY-2 series – with these missions
now capable of providing such data at
15-minute intervals).
The combination of the HIRS/3 and AMSU
instruments on the NOAA and MetOp series
allows improved information, sufficient to
infer temperature within several thick
layers in the vertical. On the MetOp series,
IASI is used with other instruments to
deliver very precise sounding capacity.
IASI data assimilation has significantly
improved NWP forecasts. CrIS on the Suomi
NPP and the future JPSS series, which
replaces HIRS, is designed to enable
retrievals of atmospheric temperature
profiles at 1K accuracy for 1 km layers in
the troposphere. The GRAS instrument on
MetOp provides temperature information of
high accuracy and vertical resolution in the
stratosphere and upper troposphere (helping
to improve analyses around the tropopause)
using a GPS radio occultation technique. Its
information will thus be complementary to
that provided by the passive sounding
instruments on MetOp. China’s FY-2 series of
satellites (FY-2D, E & F), features
improved measurements from October 2004 with
the addition of new spectral channels to
their IVISSR instrument.
GPS radio occultation measurements provide
high vertical resolution profiles of
atmospheric refractive index that relate
directly to upper air temperatures. They
provide independent observations that can be
utilised to calibrate all other data.
Instruments such as GRAS and ROSA are being
flown on multiple low-orbit satellites (such
as SAC-C, SAC-D, Oceansat-2, Megha-Tropiques
and the COSMIC constellation). Systems need
to be developed for real time data exchange
and use, implemented into operational
meteorological data streams. Plans also need
to be made to ensure future radio
occultation instruments and platforms,
including on operational meteorological
satellites.
In response to the GCOS IP, CEOS undertook
to ensure continuity of GPS radio
occultation measurements with, at a minimum,
the spatial and temporal coverage
established by COSMIC by 2011. CEOS will
also continue efforts to exploit the
complementary aspects of radiometric and
geometric upper air determinations of
temperature and moisture.
The observations for water vapour
(atmospheric humidity) are a core
requirement for weather forecasting and are
largely dealt with in the framework of
CGMS.
A wide range of sensors is available to
measure column water vapour – microwave
imagers like SSM/I and traditional imagers
like AVHRR or MERIS on LEO platforms, and
GOES and SEVIRI on GEO platforms. Vertical
profiles are provided by microwave sounders
like SSM/T2, AMSU-B, HIRS/4 and MHS, by
hyperspectral infrared sounders like IASI
and AIRS, or by radio-occultation
observations provided by GRAS on MetOp or
ROSA on a variety of missions. These data
are supplemented by instruments on Aqua
(AIRS+, AMSU-A), Aura (HiRDLS, MLS, TES),
and the FY-3 series (MWHS), amongst
others.
All of these are being improved as
technology allows. In broad terms the
challenges are to improve vertical
resolution of observations and temporal
sampling, to overcome cloud problems and
improve the ability to process sounding data
over land. For instance, Suomi NPP (and the
forthcoming JPSS series) features the
combination of the CrIS interferometer and
ATMS sounder to derive accurate water vapour
profiles.
The 3-dimensional field of humidity is a key
variable for global and regional weather
prediction (NWP) models that are used to
produce short- and medium-range forecasts of
the state of the troposphere and lower
stratosphere. Polar satellites provide
information on tropospheric humidity with
global coverage, good horizontal resolution
and acceptable accuracy, but with poor
vertical resolution.
In the case of observations for regional NWP
models, polar and (mainly) geostationary
satellites provide estimates of total column
water vapour accurate to within 10–20%.
Enough information is collected to infer
moisture concentration within several thick
layers vertically, with good horizontal
resolution.
Vertical resolution is marginal for
mesoscale prediction, and the infrared
information is available only for cloud-free
fields of view. Despite this coarse vertical
resolution, the high temporal resolution of
the geostationary satellite observations
allows derivation of products like the
instability index for convective initiation,
which is used for nowcasting
applications.
Until recently, performance in cloudy areas
was poor, but the microwave measurements
from AMSU and MHS offer substantial
improvements. Geostationary infrared
soundings (e.g. by the GOES and INSAT
sounders and SEVIRI on MSG) are also helping
to expand coverage in some regions by making
measurements on repeat timescales of 15
minutes to one hour, thus creating more
cloud-free observations.
Over oceans, coverage is currently
supplemented by information on total column
water vapour from microwave imagers.
Satellite sounding data are difficult to use
over land, but progress in data
interpretation is expected in the near
future. Recent research has shown that the
GPS-based radio occultation technique also
has the potential to provide, in the middle
to lower troposphere, high resolution
profiles of atmospheric refractivity,
combining the effects of temperature and
water vapour in this region of the
atmosphere.
In response to the GCOS IP, CEOS undertook
to ensure continuity of GPS radio
occultation measurements with, at a minimum,
the spatial and temporal coverage
established by COSMIC. CEOS will also
continue efforts to exploit the
complementary aspects of radiometric and
geometric determinations of temperature and
moisture in the upper air.
Essential Climate Variables: Upper Air Wind
Speed and Direction
Measurements of atmospheric winds are of
primary importance to weather forecasting,
and as a variable in the study of global
climate change. Upper air wind speed and
direction is a basic element of the climate
system that influences many other
variables.
Horizontal wind may be inferred by motion
vectors or by humidity and ozone tracers in
geostationary imagery. Substantial
information can be derived by these methods
but quality control is difficult and
vertical resolution is poor. Planned
instruments for geostationary satellites
promise improved information, but the
limited vertical resolution and the problems
of accurate height assignment of winds will
remain areas to be improved.
For global NWP models, wind profile
information – mostly over land – is
available mainly from radiosondes. Satellite
Doppler wind lidar technology is being
developed to provide line-of-sight wind
profiles of acceptable coverage and vertical
resolution, but thick cloud is a limitation.
Geostationary imagers offer wind profile
information by cloud tracking, or through
tracking of highly-resolved features in the
water vapour channels in cloud-free areas.
Coverage may be supplemented in future by
tracking ozone features in satellite
imagery. Regional NWP models also rely
heavily on radiosondes (over land) and
aircraft (over ocean and over the poles) for
atmospheric wind profile measurements, but
they would benefit from improved satellite
data.
At present, geostationary multi-channel
visible and infrared imagers, such as
INSAT/Kalpana, SEVIRI and VISSR, are used to
measure cloud and water vapour motion
vectors from which tropospheric wind
estimates may be derived. Atmospheric motion
vectors generated from the global ring of
geostationary imagers provide improved data
in terms of coverage, spatial and temporal
resolution, and accuracy of both wind
vectors and height assignment.
Though valuable, because they offer wind
information in areas of the world where
otherwise there would be none, atmospheric
wind vectors are single level observations
which are only available where there are
suitable image features to be tracked.
Geostationary satellite measurements have
been supplemented by the addition of water
vapour wind motions from polar orbiters
(MODIS). Plans need to be made to continue
the polar-orbit wind measurements.
In the longer term, laser instruments such
as Doppler lidars offer the promise of
directly measuring clear air winds and winds
within optically thin aerosol and cloud
layers. Although such active instruments
will provide a global coverage of vertically
resolved, highly accurate measurements, the
coverage offered by polar missions, such as
that planned for the research-oriented
ALADIN on the ESA ADM-Aeolus mission, is
limited to measurements twice a day along
the satellite line of sight.
Hyperspectral observations are needed to
improve the vertical resolution of
atmospheric motion vectors derived from
geostationary satellite observations,
especially in clear areas. The first
opportunity for these observations may be
the IRS payload on EUMETSAT’s MTG-S-1
mission.
CEOS identified two actions in response to
the GCOS requirements:
— To commit to reprocessing the
geostationary satellite data for use in
reanalysis projects before the end of the
decade;
— To identify options for continuing
improvements to wind determinations
demonstrated by MODIS and to be demonstrated
with ALADIN on the ADM-Aeolus mission.
The study of clouds, their location and
characteristics plays a key role in the
understanding of climate change. Low, thick
clouds primarily reflect solar radiation and
cool the surface of Earth. High, thin clouds
primarily transmit incoming solar radiation,
but at the same time they trap some of the
outgoing infrared radiation emitted by Earth
and radiate it back downward, thereby
warming the surface. Earth’s climate system
constantly adjusts in a way that tends
toward maintaining a balance between the
energy that reaches Earth from the Sun and
the energy that is reradiated from Earth
into space. This process is known as Earth’s
‘radiation budget’. The components of the
Earth System that are important to the
radiation budget are the planet’s surface,
atmosphere and clouds.
The IPCC points out that even the most
advanced climate models cannot yet simulate
all aspects of climate, and that there are
particular uncertainties associated with
clouds and their interaction with radiation
and aerosols.
Weather forecasters are able to draw on a
range of satellite data on clouds in
improving models and in making forecasts.
For both global and regional NWP models,
satellite instruments offer detailed
information on cloud coverage, type, growth
and motion. The coverage is global from
polar-orbiting satellites and (with the
exception of high latitudes) geostationary
satellites. Infrared imagers and sounders
can provide information on cloud cover and
cloud-top height with good horizontal and
temporal resolution. Hyperspectral
observations in the 14 mm band are ideal to
derive accurate cloud-top height
information. For example, observations in
the oxygen A band by SCIAMACHY, MERIS
(Envisat) and GOME-2 (MetOp) have been used
to derive cloud-top pressure in an
independent way. By using observations in
the NIR part of the spectrum, for example
from AVHRR observations, bulk cloud
properties such as liquid water content can
be derived.
Passive microwave imagers and sounders
(SSM/I, AMSU/B, MHS) give information on
cloud liquid water, cloud ice and
precipitation. Microwave information is
valuable for regional mesoscale models which
have sophisticated parameterisation of cloud
physics. In the context of nowcasting and
very short-range forecasting, meteorological
satellite data are well suited to monitoring
the rapid development of
precipitation-generating systems in space
and time.
In the field of climate research, the MODIS
and MISR spectroradiometers on the Terra
mission have enabled viewing of cloud
features at higher resolutions than were
previously available. MODIS measurements
allow more precise determination of the
contribution that clouds make to the
greenhouse warming of Earth. MISR is
observing angles at which sunlight is
reflected from clouds. These observations
are critical in support of new research on
the radiative properties of clouds. Also on
the Terra mission, the ASTER radiometer,
which measures visible and infrared
wavelengths, complements the other
instruments by providing high-resolution
views of specific targets of interest.
For weather forecasting, satellite
instruments will continue to offer a wealth
of useful information on clouds. On
polar-orbiting missions, HIRS, AMSU-A, MHS
and IASI offer improved information on
clouds. Geostationary imagers and sounders
(on MSG, GOES, Elektro-L, INSAT,
Himawari/MTSAT and FY-3 series) contribute
to retrieval of information about cloud
cover, cloud-top temperature, cloud-top
pressure and cloud type, and are close to
meeting regional NWP modelling needs for
these variables.
Retrievals not only comprise the temperature
and moisture profiles, but also fractional
cloud cover, cloud-top height, cloud-top
pressure, surface temperature and surface
emissivity from both infrared and microwave
soundings.
The increased use of imagery data to
determine cloud amount helps improve the
performance and the number of retrieved
profiles. In general, IASI has increased
sounding performance to a level very
significant for global and regional NWP. On
Suomi NPP and the forthcoming JPSS series of
satellites, parameters that may be derived
from VIIRS include cloud cover.
The WCRP International Satellite Cloud
Climatology Project (ISCCP) has developed a
continuous data record of infrared and
visible radiances since 1983, utilising both
geostationary and low-Earth orbiting
meteorological satellite data. A range of
products has been derived, but unfortunately
the record suffers from inhomogeneities.
Reprocessing the data to account for orbital
drift and other issues has helped to reduce
uncertainties in the observations.
The active satellite instruments on CloudSat
and Calipso are crucial for the validation
of cloud parameters observed by passive
instruments, in particular cloud top height
and type. EarthCARE will provide new
insights by observing with lidar, radar,
multi-spectral imager and a broadband
radiometer in synergy.
A key to predicting climate change is to
observe and understand the global
distribution of clouds, their physical
properties – such as thickness and droplet
size – and their relationship to regional
and global climate. Whether a particular
cloud will heat or cool Earth’s surface
depends on the cloud’s radiating temperature
– and thus its height – and on its albedo
for both visible and infrared radiation,
which depends on the number and details of
the cloud properties. As clouds interact
with radiation at all wavelengths, a
multitude of observations can be used to
infer cloud properties.
Because clouds change rapidly over short
time and space intervals, they are difficult
to quantify from low-Earth orbits. High
temporal sampling provided by geostationary
satellites is better suited to monitor
rapidly changing conditions, albeit on a
regional scale. Full 3D observations of
cloud structure is a capability that has now
been provided by NASA–CSA’s CloudSat and
NASA–CNES’s Calipso since 2006 and will
eventually be offered by ESA–JAXA’s
EarthCARE mission. Together, these missions
are capable of measuring the vertical
structure of a large fraction of clouds and
precipitation, from very thin cirrus clouds
to thunderstorms producing heavy
precipitation. However, the Calipso lidar is
unable to penetrate thick clouds and the
radar on CloudSat cannot penetrate heavy
rain.
Traditionally, basic macro- and
micro-physical information on the structure
of clouds (determination of whether water or
ice particles are present) is being obtained
from VIS and IR multi-spectral imagery, such
as that provided by MODIS and MISR on Terra
in LEO, and GOES and SEVIRI in GEO. These
measurements are important for climate
purposes as the structure of clouds
(particle size and phase) greatly affects
their optical properties, and hence their
albedo. This has been demonstrated by the
WCRP International Satellite Cloud
Climatology Project which, since 1983, has
provided a record of cloud properties
derived from multi-spectral VIS/IR imagery
observations that were initially collected
for operational meteorological
applications.
Together with cloud-top temperatures,
information on the 3D structure of clouds
can be used as a basic tool for the realtime
surveillance of features such as
thunderstorms. Microwave observations
provided by instruments such as SSM/I on
DMSP and AMSU-A and MHS on NOAA and EUMETSAT
polar platforms, have enhanced capabilities
over the VIS and IR multi-spectral
observations through their ability to probe
the entire cloud and not only the cloud top.
However, one limitation of these sensors is
their coarse spatial resolution.
Additional phase and cloud particle
information is available from polarimetric
radiometers such as POLDER on Parasol
(ending late 2013) and from GOME-2 on MetOp.
As these instruments observe the UV–VIS–NIR
part of the spectrum at moderate spectral
resolution, very accurate information on
macro-physical cloud properties can be
obtained. However, for detailed process
studies, the users’ requirements for cloud
data are unlikely to be met until data from
instruments such as ATLID or the cloud
profiling radar on EarthCARE become
available.
A good example of international cooperation
is the multiple satellite constellation
comprising CloudSat, Aqua, Aura, Calipso and
Parasol (the A-train), which has flown in
orbital formation since April 2006. Its
objectives are to gather data needed to
evaluate and improve the way clouds are
represented in global models, and to develop
a more complete knowledge of their poorly
understood role in climate change and the
cloud–climate feedback. CloudSat maintains a
tight formation with Calipso, with a goal of
overlapping measurement footprints at least
50% of the time. Calipso carries the
dual-wavelength, polarisation-sensitive
lidar CALIOP that provides high-resolution
vertical profiles of aerosols and clouds.
CloudSat and Calipso maintain a somewhat
looser formation behind Aqua, which carries
a variety of passive microwave, infrared,
and optical instruments. Since late 2011,
Parasol (initially planned for two years) is
placed 9.5 km under the A-train and
continues its nominal mission observing
clouds and aerosols (but due to end late
2013).
EarthCARE (to launch late in 2016) will fly
a cloud/aerosol lidar, cloud radar,
multi-channel imager and broadband
radiometer for measuring clouds and aerosols
simultaneously with top-of-atmosphere
radiances.
In responding to the GCOS IP, CEOS
recognised that accurate measurement of
cloud properties has proved to be
exceedingly difficult. CEOS agreed to
support investigations of cloud properties
and cloud trends from combined satellite
imager and sounder measurements (with
horizontal as well as vertical information)
using Cloudsat/Calipso for validation.
Water forms one of the most important
constituents of Earth’s atmosphere and is
essential for human existence. The global
water cycle is at the heart of Earth’s
climate system, and better predictions of
its behaviour are needed for monitoring
climate variability and change, weather
forecasting and sustainable development of
the world’s water resources. A better
understanding of the current distribution of
precipitation, and of how it might be
affected by climate change, is vital in
support of accurate predictions of regional
drought or flooding.
Information on liquid water and
precipitation rate is used for initialising
NWP models. A variety of satellites provide
complete global coverage, but they present
two major challenges. Firstly, the satellite
sensors (such as visible/IR imagers on
geostationary weather satellites) typically
observe quantities (such as cloud height and
cloud-top temperature) related to
precipitation, so algorithms must be
developed to get the best estimates from
each particular sensor. Secondly, the mix of
available data is constantly changing in
space and time.
The new generation of geostationary imagers,
available since the start of EUMETSAT’s
Meteosat Second Generation, also allows for
the observation of cloud liquid water path
and particle size at high temporal
resolution (15 min).
Microwave imagers and sounders offer
information on precipitation of marginal
horizontal and temporal resolution,
acceptable to marginal accuracy (though
validation is difficult). Satellite-borne
rain radars (such as those on TRMM and
CloudSat), together with plans for
constellations of microwave imagers, offer
most potential for improved observations and
form the core of the proposed Global
Precipitation Measurement Mission. For
regional NWP, no satisfactory precipitation
estimates are available from satellites at
present, although they are the only
potential source of information over the
oceans. Geostationary satellites do provide
vital information on the location of
tropical cyclones.
Increasing amounts of useful microwave data
– such as those from the TRMM mission – are
becoming available. TRMM was dedicated to
studying tropical and subtropical rainfall
and carried the first spaceborne
precipitation radar, JAXA’s PR instrument,
and NASA’s TMI microwave imager. Data from
PR and TMI have provided new insights into
the internal composition of tropical
thunderstorms associated with hurricanes.
NASA, JAXA and partner agencies plan to
continue this collaboration in future to
develop the GPM constellation of satellites
that will be launched from 2014 onwards. The
GPM series will provide global observations
of precipitation every three hours to help
develop the understanding of the global
structure of rainfall and its impact on
climate. The CNES–ISRO Megha-Tropiques
mission (launched October 2011) is providing
measurements of water vapour, condensed
water and radiative fluxes, from which
information on the water cycle and tropical
rainfall will be derived; MADRAS, a passive
multi-frequency radiometer, will collect
data on rain over the oceans.
The 94 GHz cloud radars on CloudSat and
(from 2016) EarthCARE provide complementary
information on light precipitation.
EarthCARE’s Doppler capability will provide
additional detail on sedimentation
velocities.
Future coordination of these satellite
programmes, as well as the efforts of the in
situ measurement community, was addressed by
the Integrated Global Water Cycle
Observations Theme of the IGOS Partnership.
The first element of IGWCO is a ‘Coordinated
Enhanced Observing Period (CEOP)’ which is
taking the opportunity of the simultaneous
operation of key satellites of Europe, Japan
and USA to generate new data sets of the
water cycle.
The IGWCO Theme report is available from
www.earthobservations.org/wa_igwco.shtml.
This document represents a comprehensive
overview of the state-of-the-art in water
cycle observations and formulates
recommendations for an international work
programme to better understand, monitor and
predict water processes.
To meet GCOS IP needs, CEOS agencies have
committed to ensure continued improvements
to precipitation determinations demonstrated
by TRMM and planned by GPM from 2014. JAXA
and NASA are leading a CEOS study team to
establish the basis for a Global
Precipitation Constellation – building on
GPM to incorporate measurements from more
countries over an extended period.
Ozone (O3) is a relatively
unstable molecule, and although it
represents only a tiny fraction of the
atmosphere, it is crucial for life on Earth.
Depending on its location, ozone can protect
or harm life on Earth. Most ozone resides in
the stratosphere, where it acts as a shield
to protect the surface from the Sun’s
harmful ultraviolet radiation. In the
troposphere, ozone is a harmful pollutant
which causes damage to lung tissue and
plants. Man-made chemicals and very low
stratospheric temperatures over Antarctica
combine to deplete stratospheric ozone
concentrations during the southern
hemisphere’s winter.
The total amount of O3 in the
troposphere is estimated to have increased
by 36% since 1750, due primarily to
anthropogenic emissions of several
O3-forming gases.
Satellite instruments have for many years
provided data measuring interactions within
the atmosphere that affect ozone, and more
advanced sensors will soon be in orbit to
collect more detailed measurements,
increasing knowledge of how human activities
are affecting the protective ozone layer.
Total column measurements of ozone have been
provided over long periods by NASA’s TOMS
and NOAA’s SBUV instruments. Stratospheric
ozone profiles have also been measured by
instruments such as HALOE and MLS (UARS
mission), GOME (ERS-2), and SAGE III (part
of the International Space Station
payload).
From 2002 to 2012, GOMOS, MIPAS and
SCIAMACHY on ESA’s Envisat mission provided
improved observations of the concentration
of ozone and trace gases in the
stratosphere. Operation of GOME-2 on
EUMETSAT’s MetOp satellites guarantees the
continuity of these observations for another
decade.
A wide range of instruments dedicated to,
or capable of, ozone measurements are
planned for the next decade. On the Aura
mission (launched 2004), HIRDLS, OMI and MLS
study and monitor atmospheric processes that
govern stratospheric and mesopheric ozone,
and continue the TOMS record of total ozone
measurements. TES on Aura is used to create
three-dimensional maps of ozone
concentrations in the troposphere. AIRS on
Aqua and CrIS on Suomi NPP/JPSS also supply
an ozone product that has some application
in the lower stratosphere and also can be
used to identify regions of
stratospheric/tropospheric mixing. OMPS on
Suomi NPP is already providing ozone profile
measurements including an experimental limb
profiler that measures the distribution of
ozone at higher vertical resolution by
looking through the atmosphere at an
angle.
IASI and GOME-2 on the MetOp series have
provided information since early 2007 on
both total column ozone and vertical
profile. The SBUS Ozone Profiler on China’s
FY-3 series has contributed further data
since its launch in 2008.
Though the infrared imagers on the GOES and
Meteosat geostationary platforms have
limited capabilities to provide vertical
information on ozone, they provide total
stratospheric ozone amount with a high
temporal resolution. This information can be
used to depict stratospheric dynamical
processes, relevant for NWP applications.
The IGOS theme on Atmospheric Chemistry
Observations has developed a strategy for
the integrated provision of chemistry
observations (and associated meteorological
parameters) required to realise the theme’s
objectives, including the monitoring of
atmospheric composition parameters related
to climate change.
Essential Climate Variables: Earth
Radiation Budget (Including Solar
Irradiance)
Earth’s radiation budget is the balance
within the climate system between the energy
that reaches Earth from the Sun and the
energy that returns from Earth to space.
Satellite measurements offer a unique means
of assessing Earth’s radiation budget. The
goal of such measurements is to determine
the amount of energy emitted and reflected
by Earth at the top of the atmosphere. This
is necessary to understand the processes by
which the atmosphere, land and oceans
transfer energy to achieve global radiative
equilibrium, which in turn is necessary to
simulate and predict climate.
Systematic observations of the Earth System
energy balance components are noted by the
IPCC as being of key importance in narrowing
the uncertainties associated with the
climate system. In addition to these
continuous global measurements of the
radiation budget, which are necessary both
to estimate any long-term climatic trends
and shorter-term variations overlying these
trends, measurements on a regional scale are
useful to understand better the dynamics of
certain events or phenomena and to assess
the effect of climate change, for example on
agriculture and urban areas.
In general, three types of measurements are
currently possible:
— The shortwave and longwave radiation
budget at the top of the atmosphere;
— The shortwave radiation budget at Earth’s
surface;
— The total incoming broadband radiation
flux.
Since the mid-1960s, NASA has been measuring
the net radiation with the ERBE, ACRIM, and
CERES sensors. The MISR spectroradiometer
(also on Terra with CERES) provides data on
the top of the atmosphere, cloud and surface
hemispheric albedos, and aerosol opacity.
Continuity of Total Solar Irradiance (TSI)
measurements was assured by the launch of
the SORCE mission at the beginning of 2003,
carrying four instruments (TIM, SOLSTICE,
SIM, XPS) that operate over the 1–2000 nm
waveband and measure over 95% of the
spectral contribution to TSI. ESA’s
EarthCARE will embark a broadband radiometer
(BBR) together with instruments providing
profile information (ATLID, CPR).
The French–Indian mission Megha-Tropiques
(launched October 2011) carries the
broadband ScaRaB radiometer, similar to the
instrument flown in the mid-1990s on the
Russian Meteor and Resurs satellites, for
ERB measurements over the tropical and
equatorial regions.
An increasing number of radiation budget
measurements are featuring on operational
meteorology missions. These include: GERB
(operating since September 2002 on Meteosat
and measuring shortwave and longwave
radiation every 15 minutes from a
geostationary orbit); CERES and TSIS on
JPSS; and continued narrowband information
from the HIRS, AVHRR, SEVIRI
(top-of-atmosphere and surface radiative
fluxes) and VIIRS instruments.
Second-generation Chinese meteorological
satellites FY-3 include a radiation budget
capability (ERM) and a solar irradiance
monitor (SIM).
An important component of the Earth
Radiation Budget is the Outgoing Longwave
Radiation. This is calculated from
multi-spectral infrared imager observations,
such as those from AVHRR or imagers on
geostationary platforms.
The past multi-satellite record of
measurements suffers from an absence of
absolute calibration. It is recognised that
development of absolute, spectrally resolved
measurements is needed to provide
information on variations in climate
forcings and responses, and to calibrate the
operational meteorological satellite
sensors.
In support of the GCOS IP, CEOS aims to make
absolute, spectrally resolved measurements
of radiance emitted and reflected to space
by Earth for information on variations in
both climate forcings and responses.
Essential Climate Variables: Carbon
Dioxide, Methane and Other Long-Lived
Greenhouse Gases
Trace gases other than ozone may be divided
into three categories:
— Greenhouse gases affecting climate
change;
— Chemically aggressive gases affecting the
environment (including the biosphere);
— Gases and radicals affecting the ozone
cycle, thereby affecting both climate and
environment.
The presence of trace gases in the
atmosphere can have a significant effect on
global change as well as potentially harmful
local effects through increased levels of
pollution. The chemical composition of the
troposphere, in particular, is changing at
an unprecedented rate. Meanwhile, the rate
at which pollutants from human activities
are being emitted into the troposphere is
now thought to exceed that from natural
sources (such as volcanic eruptions).
The IPCC noted in 2007 that:
— Changes in atmospheric concentrations of
greenhouse gases and aerosols, land cover
and solar radiation alter the energy balance
of the climate system;
— Global greenhouse gas emissions due to
human activities have grown since
pre-industrial times, with an increase of
70% between 1970 and 2004;
— Carbon dioxide (CO2) is the
most important anthropogenic greenhouse gas.
Its annual emissions grew by about 80%
between 1970 and 2004.
The IPCC concluded that “most of the
observed increase in globally averaged
temperatures since the mid-20th century is
very likely (over 90% probability) due to
the observed increase in anthropogenic
(man-made) greenhouse gas concentrations”.
They consider that reductions in greenhouse
gas emissions and the gases that control
their concentration would be necessary to
stabilise radiative forcing.
Measurements from satellite sensors have
already made an important contribution to
the recognition that human activities are
modifying the chemical composition of both
the stratosphere and the troposphere, even
in remote regions.
A variety of instruments provide
measurements on the concentration of trace
gases. In general, high spectral resolution
is required to detect absorption, emission
and scattering from individual species. Some
instruments offer measurements of column
totals, i.e. integrated column measurements,
whilst others provide profiles of gas
concentration through the atmosphere
(usually limited to the upper troposphere
and stratosphere, using limb
measurements).
To date, the instruments on UARS (operated
1991–2005) have provided the most
significant source of data on trace gases
and have been vital for studies of
stratospheric chlorine chemistry,
stratospheric tracer-tracer correlation,
tropospheric water vapour, the chemistry of
the Arctic lower stratosphere in winter, and
tropospheric aircraft exhaust studies.
The last decade has seen the arrival of new
and significant capabilities, with advanced
instruments on Terra (MOPITT, providing
global measurements of carbon monoxide and
methane in the troposphere), and Envisat
(GOMOS, MIPAS and SCIAMACHY, providing
profiles of trace gases through the
stratosphere and troposphere). AIRS (on
Aqua) and IASI (on MetOp) also contributed
to such information and their sounder
products help quantify atmospheric mixing
and help determine sources and sinks.
On NASA’s Aura mission (launched 2004),
HiRDLS, an infrared limb-scanning
radiometer, carries out soundings of the
upper troposphere, stratosphere and
mesosphere to determine concentrations of
trace gases, with horizontal and vertical
resolutions superior to those previously
obtained. On the same mission, MLS measures
concentrations of trace gases for their
effects on ozone depletion, TES provides a
primary input to a database of 3D
distribution on global, regional and local
scales of gases important to tropospheric
chemistry, and OMI continues the TOMS record
for atmospheric parameters related to ozone
chemistry and climate. JAXA’s GOSAT mission
(launched 2009, follow-on planned from 2018)
and NASA’s OCO-2 mission (from 2014) are
expected to make significant contributions
to observations of trace gases, particularly
carbon dioxide and methane. The
DLR–CNES Merlin mission (to be launched in
2017) is expected to monitor methane
concentration in the atmosphere.
The IGOS IGACO Theme for observations of
atmospheric chemistry has considered all
relevant chemical species to interpret
properly the observations and intends to
monitor the research required to improve
understanding of Earth processes so that air
quality evolution can be predicted. ESA is
considering atmospheric composition missions
(such as PREMIER and Carbonsat) to meet
these needs.
The CEOS Response to the GCOS IP cautions
that demonstrations of potential future
operational measurements are neither
complemented by plans for operational
implementation nor any R&D
follow-on.
