## 3 Historical Review to State-of-the-Art
### Ceres
#### 3.3.1 CERES-TRMM  
The first CERES shortwave ADMs were generated using observations from the CERES instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite, henceforth CERES-TRMM [22,40]. The TRMM satellite was in a 350 km circular orbit with an inclination angle of 35\({}^{\circ}\), permitting observations from approximately 38\({}^{\circ}\) S-38\({}^{\circ}\) N after taking into account the CERES scan extent. The precessing orbit facilitated sampling of scenes within this latitude range at all solar viewing geometries, addressing some of the most serious issues of missing data experienced in the ERBE ADMs. Every third day the instrument was operated in rotating azimuth plane scan (RAPS) mode, where the instrument rotates in azimuth as it scans in elevation, to more effectively sample the angular space. Nine months of available observations from CERES-TRMM (January-August 1998 and March 2000) were used to generate ADMs. A subtle but important update in the CERES-TRMM ADMs relative to the ERBE ADMs was that solar-viewing geometry was defined at a field-of-view reference level of 100 km above the Earth's surface (rather than at the Earth's surface) to account for transmission of shortwave radiation through the atmosphere beyond Earth's horizon, and the top-of-atmosphere reference level for the irradiance was defined to be 20 km (rather than 30 km) [41]. Angular binning was similar to that used by ERBE, with 2 additional bins each in viewing zenith angle and relative azimuth angle (Figure 6b). However, by utilizing the Visible and Infrared Scanner (VIRS) instrument also flying on the TRMM satellite, a far greater number of scene types were defined (Table 3). Notably, cloud-free ocean scenes were additionally segregated by surface wind speed, and cloudy scenes were additionally segregated by cloud optical depth and cloud phase. This was achieved by applying thresholds to VIRS pixels within the CERES footprint for cloud detection and applying an iterative inversion for the retrieval of cloud optical depth and cloud phase [42]. An example of the additional value gained by segregating scenes with varying cloud optical depth and cloud phase is shown in Figure 7. A scene becomes more isotropic with increasing \(\tau\) as a greater fraction of the photons reaching the sensor go through the multiple scattering process. While the differences in anisotropy due to cloud optical depth and phase are not as large as those of different solar zenith angle and surface type as indicated in ERBE ADMs (compare variations within Figure 7 to those within Figure 4), there is a distinct and sometimes large change in anisotropy with cloud properties. Accounting for these additional parameters therefore leads to improved regional irradiance prediction,  
Figure 6: (**a**) ERBE and (**b**) CERES-TRMM shortwave ADM viewing zenith and relative azimuth bins. All ADMs are assumed to be azimuthally symmetric, so only 0-180\({}^{\circ}\) is shown. with an estimated monthly mean uncertainty of less than 0.5 W m\({}^{-2}\) and an instantaneous error of less than 10 W m\({}^{-2}\) at the 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) scale [40]. In addition, the CERES-TRMM ADMs show improved self-consistency, as defined and discussed further in the next section. \begin{table}
\begin{tabular}{c c c c} \hline
**Surface Type** & **Cloud Thermodynamic Phase** & **Cloud Fraction (\%)** & **Cloud Optical Depth** \\ \hline \multirow{4}{*}{Ocean (336)} & \multirow{4}{*}{Liquid, ice} & 0.1–10, 10–20, 20–30, 30–40, & 0.01–1.0, 1.0–2.5, 2.5–5.0, 5.0–7.5, 40–50, 50–60, 60–70, 70–80, & 7.5–10, 10–12.5, 12.5–15, 15–17.5, 17.5–20, 20–25, 25–30, 30–40, \\  & & 99.9–100 & 40–50, >50 \\ \hline Moderate–high tree/shrub & \multirow{4}{*}{Liquid, ice} & 0.1–25, 25–50, 50–75, 75–99.9, 99.9–100 & 0.01–2.5, 2.5–6, 6–10, 10–18, \\  & & 99.9–100 & 18–40, >40 \\ \hline \end{tabular}
\end{table} 
Table 3: CERES-TRMM shortwave ADM scene types that contain cloud. Reproduced and adapted from [PERSON] et al. [22] (c) American Meteorological Society. Used with permission.