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CONTENTS
1. Earth Radiation Budget Measurements from Satellites
2. Radiation Budget Sensors on MSG
1. Earth Radiation Budget Measurements from Satellites
All such measurements to date have been made from satellites in low earth orbit (LEO). The first data were from single sun-synchronous polar orbiters, such as Nimbus 6 and the highly successful Nimbus 7 satellite, launched in 1978. However, the data from single satellites must be used with caution, as they cannot provide proper temporal sampling. There are strong diurnal variations in the radiation budget, particularly over land, in response to the diurnal variation of solar heating.
The importance of the diurnal sampling problem was recognised by NASA when planning the Earth Radiation Budget Experiment (ERBE). Identical sensors were mounted both on the operational polar orbiters (NOAA-9 and NOAA-10) and on the dedicated Earth Radiation Budget Satellite (ERBS), launched by the space shuttle into a 57 degree inclination orbit in 1984. ERBE precesses with a repeat cycle of 36 days, so that improved diurnal coverage was provided about every month. ERBE provided excellent data over a 5-year period from 1985 until 1990, but for most of this period only one of the polar orbiters was available for merging with ERBS. Because of this, and the fact that ERBS only provided information on the mean diurnal cycle averaged over a month, it is generally accepted that even ERBE provided incomplete diurnal coverage.
The first ScaRaB instrument was launched on a Russian Meteor satellite in 1994 and operated for one year. The satellite was in a polar orbit, but not sun-synchronous and having a long precessional cycle (about 200 days), so that diurnal coverage was poor. ScaRaB failed in March 1995, so at present there are no broad-band measurements being made of the radiation budget, despite the acknowledged importance of such data, although further ScaRaB launches are planned.
Looking further ahead, NASA is developing a sensor known as CERES (Clouds and the Earth's Radiant Energy System), an improved version of the ERBE instrument. CERES radiometers will be mounted on the TRMM spacecraft, to be launched into a 35o LEO around 1998 and on the EOS polar orbiting platforms at the turn of the century.
All the satellites carrying radiation budget sensors planned for launch around the turn of the century use polar orbiting or other low earth orbits. Four such satellites could provide coverage of the diurnal cycle with a temporal resolution of 3 hours (each of which provides 2 measurements per day), if the equator crossing times are arranged at this separation. This number of satellites is indeed planned, but it is most unlikely that they will satisfy this requirement, given the various operational and other constraints acting within the different agencies and countries responsible. In any case, 3 hourly sampling still represents rudimentary temporal resolution. At least hourly measurements are needed to resolve the diurnal cycle of tropical convection properly, and no practicable system of polar orbiting or other LEO satellites can deliver this.
If similar sensors were mounted on the operational geostationary satellites launched by other agencies (GOES, GMS and INSAT) and the data were merged with those from the polar platforms, then a cost-effective and comprehensive observing system for the Earth's radiation budget could be achieved, without the need for additional platforms. This is consistent with the philosophy of the International Satellite Cloud Climatology Project (ISCCP) applied to the closely-related problem of generating a properly sampled, global cloud climatology. In ISCCP, the polar orbiters provide the means for inter-calibrating the geostationary sensors, which provide the required temporal resolution.
2. Radiation Budget Sensors on MSG
Spectral broad-band radiometers on a geostationary platform, providing measurements of ERB parameters within time frames of less than an hour, and global coverage, will provide the scientific community with an invaluable dataset that will help to solve the problems mentioned above and therefore enhance our understanding of climate.
Through its Earth Radiation Scientific Advisory Group (ERSAG), the European Space Agency considered, and has agreed to the proposal to mount a Geostationary Earth Radiation Budget instrument (GERB) on the Meteosat Second Generation (MSG) geostationary satellite, due for launch in 2000. ESA commissioned an industrial study and the contractors, British Aerospace Space Systems (BAe) produced a Phase A design which would meet the requirements laid down by ERSAG. Subsequently NERC placed a contract with BAe (now Matra Marconi Space) for a Phase B1 design.
At the EUMETSAT Council meeting on 30 November 1995 the UK supported by the Netherlands invited EUMETSAT to provide a financial contribution to GERB to a maximum of 3 MECU. Considerable interest was expressed and it was stated that a proposal by GERB should be considered at the next Council meeting. Since then support for the inclusion of the GERB instrument on MSG-1 has been provided and its development is on schedule. Funding is now requested for the provision of GERB instruments on the second flight of MSG in order to permit long-term monitoring of the Earth Radiation Budget. And the operation of the Ground Segment during the lifetime of this instrument. EUMETSAT is also offered an option to fund a third GERB, which would fly on the third MSG, plus provision of funds to extend the Ground Segment operations for the life of this third instrument. This option could be taken up once the first GERB has been successfully commissioned in orbit
The area of the globe seen by MSG is limited, and development and launch of GERB-2 and 3 will enable wider coverage through cross-calibration with GERB-1. (This implies that the MSG1 spacecraft is shifted in longitude once MSG2 is operational)
The following list covers the additional uses of GERB-2 and 3 data:
The importance of temporally-resolved ERB measurements should be emphasised. Measurements of the ERB with high temporal resolution have considerable potential for studying the interaction between radiation and other physical processes. The region covered by MSG is of particular interest in understanding such interactions, as it covers not only deep continental convection forced by strong surface heating over land, but also the maritime convection over the tropical Atlantic and the important area of stratocumulus convection over the South Atlantic. Observations from GERB could thus provide new information on the interactions between the radiation budget terms, tropical convection and dynamics, and new ways of validating these interactions in diabatic models. It is not possible to obtain rigorous tests of these processes represented in the models with only ERBE data from polar orbiting instruments, because of the serious sampling problems. The continuous temporal coverage would also provide ERB information for field studies, whether at the land surface or within the atmosphere. GERB will provide a unique new view of these processes, and a unique new set of quantitative data with which to study the most important climate-radiative processes.
The benefits of GERB-2 and 3 are :
A very high priority is given to the exploitation of GERB (and SEVIRI) data for environmental research. The excellent time resolution of GERB (a full Earth disk image every 15 minutes) will allow exacting new tests and validation of the physics of climate models. Such high time resolution is essential, but completely beyond the capabilities of polar orbiters in LEO. Information from GERB would enable tests of the accuracy of our understanding of the basic interactions between radiative heating, convection and surface processes in such models. The importance of the observations proposed from GERB have been widely recognised, and a further two instruments would allow a more comprehensive coverage of the globe and enhance the accuracy and reliability of the resulting data through cross-calibration of in-flight instruments. Furthermore, in the unlikely event of GERB-1 failing, the development effort will not have been wasted.
The exploitation of GERB data ranks as a high priority in each of the participating countries, as well as many others including the US (GERB Science Team participants), and recent EC support for GERB-1 exploitation and improvement of the ground segment confirms the continuing commitment at European level. The credibility of the proposing team in this area is strongly supported by their considerable experience in a number of similar or equivalent satellite missions, and the data exploitation from these. Thus, the proposing team has experience, inter alia, with the NASA ERBE project, the French/German ScaRaB instrument, instruments on NASA’s Upper Atmosphere Research Satellite (eg HALOE, ISAMS), and on the Nimbus series of satellites (Nimbus 4, 5, 6, 7). A great deal of experience resides in the team, promising a highly successful GERB exploitation. As we have noted, there is close synergy between GERB and the main MSG sensor, SEVIRI, and the GERB team has the expertise to extract the maximum benefit from this synergy. It is anticipated that over the early part of GERB development, the partners will generate agreements which will be designed to extend and strengthen co-operation across Europe, and indeed more widely, in the exploitation of the data.
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