Satellite Measurements of the Earths Radiation Budget

The arrival of the satellite era provided the first global direct measurements of clouds and ERB, starting with the basic instruments mounted on Explorer 7 in 1959 and TIROS 1 in 1960. The Nimbus series of experimental satellites, culminating in the important scanning radiometer observations from Nimbus 7, laid the foundation for the comprehensive Earth Radiation Budget Experiment (ERBE) from 1985-1990. ERBE placed broadband scanning instruments on two operational weather satellites and on the dedicated satellite, ERBS, which flew in a precessing low-inclination orbit to sample all times of day through its 72-day orbital repeat cycle. After ERBE, further scanning radiometers flew on the ScaRaB missions from 1994-1995 and 1998-1999. The CERES instruments on the TRMM, Terra, and Aqua satellites of the NASA Earth Observing System set the highest standards for accuracy (Wielicki et al. 1996).

Many of the satellites mentioned above were, or are, in sun-synchronous orbits with limited diurnal sampling. CERES overcomes this problem by incorporating additional data from the geostationary satellites used also by ISCCP. The ERBS and TRMM satellites, as well as the Russian satellites that carried the ScaRaB instruments, are in precessing orbits, providing coverage of all local times over the orbital repeat period; however, at any one location, the temporal sampling is still limited. To date, the only broadband radiometers in geostationary orbit are the GERB instruments on Meteosat-8 and Meteosat-9, which provide 15-minute data for the whole visible disk, albeit over a limited, although climatically important, geographical region (Harries et al. 2005). The broadband data from all of these instruments provide a wealth of information on the geographical distributions and the diurnal, seasonal, and interan-nual variability of the ERB, including the effect of clouds. This is possible because the signals are large (typically tens of W m-2 or more, as is clear from Figure 2.1) and thus are well observed relative to the instrument noise and absolute calibration.

Whereas multiyear changes in ERB due to El Niño/La Niña and the 1991 Mt. Pinatubo volcanic eruption are clearly apparent in the satellite tropical mean time series of outgoing LW radiation (Figure 2.3), it is much more challenging to study decadal and longer period variability and to search for trends associated with climate change. The latter topics place much more stringent demands on the measurements, which were seldom designed for such work. Some evidence has been put forward for the existence of decadal variations in the tropical radiation budget, particularly from the ERBS wide-field-of-view (WFOV) instrument; however, detection of such changes is hampered by various observational artifacts. Several of these were discovered after the original publication by Wielicki et al. (2002), including the existence of a spurious semiannual cycle due to aliasing of the diurnal cycle by the precessing satellite orbit. The first correction to the ERBS data adjusted for a spurious change in radiation flux that resulted from a downward trend in satellite altitude; the second correction adjusted for a previously unidentified degradation of solar transmission through the instrument dome. These two small corrections to the ERBS data reduced the change in LW flux by a factor of four and reversed the sign of the change in the net radiation, compared with the original study (Wong et al. 2006). The most recent results indicate a slight trend towards more LW emission (consistent with a weakening of LWCRF) but a larger trend towards less SW reflection so as to produce a net gain of energy to the Earth. Measurements of ocean heat content are consistent with a net gain of energy during the past two decades (Wong et al. 2006). It is interesting to note io

CT C

• CERES/TRMM SC a ScaRaB/Meteor SC

• CERES/TRMM SC a ScaRaB/Meteor SC

El Chichon eruption

1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 Time (year)

El Chichon eruption

Pinatubo recovery 88/90 La Niña Pinatubo eruption

1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 Time (year)

Figure 2.3 Time series of longwave radiation anomalies averaged over 20°N to 20°S based on the new ERBS Nonscanner WFOV Edition3_Rev1 (solid black line), Nimbus 7 Nonscanner (gray dashed line), ERBS Scanner (solid gray line), CERES/ Terra FM1 Scanner ES4 Edition2_Rev1 (dotted line), CERES/TRMM Scanner Edition2 (gray circle), ScaRaB/Meteor Scanner (triangle), and ScaRaB/Resurs Scanner (black circle) dataset. Anomalies are defined with respect to the 1985-1989 period (from Wong et al. 2006).

o that the GCMs, rightly or wrongly, do not reproduce the magnitude of decadal change in SW and net radiation flux between the 1985-1990 and 1994-1999 periods that are reported by ERBS (Wong et al. 2006). Investigation of changes in ERB over longer periods of time than that covered by ERBS WFOV must concatenate multiple satellite records (Figure 2.3).

Further evidence of the acute observational challenges presented by climate change is illustrated by the accuracy requirements for climate datasets listed by Ohring et al. (2005) and the error analysis of the current CERES sensors performed by Loeb et al. (2007). The stability requirement for measurements of the net SW flux at the top of the atmosphere necessary to identify the feedbacks from low clouds during climate change is listed by Ohring et al. (2005) as 0.3 Wm-2 per decade. Loeb et al. (2007) show that from the current CERES instrumentation, 10-15 years of data would be needed before a trend of this magnitude could be detected against the background of natural variability. Clearly, this is only achievable if such well-characterized sensors continue to be flown to provide an unbroken time series from which the cloud feedback could be inferred. The main problem is identifying such a small trend in the face of natural interannual and interdecadal variability, which Figure 2.3 shows can be much larger in magnitude than 0.3 Wm-2, at least regionally. Greater natural variability extends the time period needed to detect a trend of a given magnitude.

The strength of ERB measurements (i.e., that they provide information on the net effect of all the radiatively active constituents on the heating and cooling of the planet) can also be viewed as a weakness. Without additional information, it is difficult to unravel these effects. Retrievals of cloud properties have been combined with ERBE and later data to study the radiative effects of different cloud types and how they contribute to the changes in ERB that take place (e.g., during ENSO events). It is clearly important that complementary information should be available for aerosols.

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