## 3 Historical Review to State-of-the-Art
### Ceres
#### 3.3.2 CERES-Terra  
The next generation of CERES shortwave ADMs were generated using observations from the Terra satellite, henceforth CERES-Terra [23; 43]. Terra was launched in 1999 into a 705 km sun-synchronous orbit with a descending equator crossing at 10:30 local time (although the orbit is now starting to drift in local time). At the time of writing this paper, two CERES instruments are still operational on the Terra satellite. One of the instruments was often dedicated to RAPS mode in the initial years of flight. Two years of data from CERES-Terra were used to generate the CERES-Terra ADMs, providing at least an order of magnitude increase in angular sampling at any given solar zenith angle compared to CERES-TRMM. This allowed for a substantial increase in angular bin resolution (e.g., 2\({}^{\circ}\) in solar zenith, viewing zenith, and relative azimuth angles for cloud-free ocean scenes) and a further segregation of scene types. Cloud properties were inferred in a consistent manner to CERES-TRMM, but using the Moderate Resolution Imaging Spectroradiometer (MODIS) that was available on the Terra satellite instead of VIRS (Table 4). For CERES-Terra, separate ADMs were defined over cloud-free land and desert for each 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) region, for each month, and for normalized difference vegetation index (NDVI) differing by 0.1. Instead of the traditional sorting-by-angular-bins method, an eight parameter fit was applied to model the bidirectional reflectance distribution function for these scenes [45]. An additional 60 scenes were defined over s \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Surface Type** & **Cloud Fraction** & **Surface** & **Snow/Sea Ice** & **Cloud Optical** \\  & **(\%)** & **Brightness** & **Fraction (\%)** & **Depth** \\ \hline \multirow{6}{*}{Permanent snow (10)} & 0.0–0.1 & Bright, dark * ** & - & - \\  & 0.1–25 & All & - & All \\  & 25–50 & All & - & All \\  & 50–75 & All & - & All \\  & 75–99.9 & All & - & All \\  & 99.9–100 & Bright, dark *  & - & Thin (\(\tau\leq 10\)), \\  & 0.0–0.1 & All & 0.0–0.1 & - \\  & 0.0–0.1 & All & 0.1–25 & - \\  & 0.0–0.1 & All & 25–50 & - \\  & 0.0–0.1 & All & 50–75 & - \\  & 0.0–0.1 & All & 75–99.9 & - \\  & 0.0–0.1 & Bright, dark *  & 99.9–100 & - \\  & 0.1–25 & All & 0.0–0.1 & All \\  & 0.1–25 & All & 0.1–25 & All \\ Fresh snow (25), & 0.1–25 & All & 50–75 & All \\ sea ice (25) & 25–50 & All & 0.0–0.1 & All \\  & 25–50 & All & 0.1–25 & All \\  & 25–50 & All & 25–50 & All \\  & 25–50 & All & 50–75 & All \\  & 50–75 & All & 0.0–0.1 & All \\  & 50–75 & All & 0.1–25 & All \\  & 75–99.9 & All & 0.0–0.1 & All \\  & 75–99.9 & All & 0.0–0.1 & All \\  & 75–99.9 & All & 0.0–0.1 & All \\  & 75–99.9 & All & 0.1–25 & All \\  & 99.9–100 & Bright, dark *  & \multicolumn{2}{c}{All} & Thin (\(\tau\leq 10\)), \\  & & & & thick (\(\tau>10\)) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: CERES-Terra shortwave ADM scene types for permanent snow, fresh snow, and sea ice. Reproduced and adapted from [PERSON] et al. [23] © American Meteorological Society. Used with permission. The numbers in parentheses in the first column indicate the number of scene types. using angular bins of \(2^{\circ}\) or \(5^{\circ}\) in solar zenith angle, and \(5^{\circ}\) in viewing zenith and relative azimuth angles. For cloudy scenes, CERES-Terra ADMs took a distinctly different approach. Over ocean, sigmoidal functions were fit between \(\ln(f\tau)\) and the shortwave radiance in \(2^{\circ}\) angular bins for liquid, ice, and mixed phase cloud, where \(f\) is the cloud fraction and \(\tau\) is the cloud optical depth (Figure 8). A similar approach was taken over land, but coarser \(5^{\circ}\) angular bins and an additional correction to account for the surface reflection was applied. Under this definition, the cloudy scenes essentially become continuous, and the number of scene types is therefore more difficult to quantify from this point forward.