## 3 Historical Review to State-of-the-Art
### Ceres
#### 3.3.3. CERES-Terra/Aqua  
The most recent CERES ADMs have been developed using an extended record of 5 years of RAPS data from the Terra satellite combined with 1 year and 8 months of RAPS data from the Aqua satellite, henceforth CERES-Terra/Aqua [24; 46]. The Aqua satellite launched in 2002 and, similar to Terra, hosts two CERES instruments that remain in orbit at the time of writing this paper. Aqua also flies in a 705 km sun-synchronous orbit, but with an ascending equator crossing at 13:30 local time (although the orbit is due to start drifting in local time in 2022). Separate ADMs are developed for Terra and Aqua over all scenes except permanent snow and sea ice. The approach applied is similar to CERES-Terra with a few key updates. For cloud-free scenes over ocean, the scene types are further segregated by aerosol optical depth in glint regions, and both aerosol optical depth and two aerosol types (rural and urban [47]) in non-glint regions. For cloud-free over land and fresh snow, the Ross-Li BRDF model [48; 49] is used that contains terms to account for an isotropic contribution, randomly distributed and oriented protrusions, and radiative transfer within a vegetation canopy that includes the hotspot effect. For permanent snow, separate shortwave ADMs are generated for Antarctica and Greenland due to the effects of elongated dune-like features in the snow, known as sastrugi, that are only found on Antarctica [50]. For sea ice, there is additional segregation by the brightness of the sea ice (Table 5), which accounts for variations in anisotropy due to features such as melt ponds (Figure 10). The presence of melt ponds reduces the overall albedo of the surface, but the liquid water has stronger anisotropy than the underlying ice surface (c.f., Figure 4). Improved cloud algorithms are employed [51] that include the addition of the 1.24 \(\upmu\)m channel for cloud optical depth retrieval over snow, a CO2-slicing method to retrieve high cloud over low cloud [52], and a rough ice crystal model [53]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Cloud Fraction (\%)** & **Sea Ice Fraction (\%)** & **Surface Brightness** & **Cloud Optical Depth** & **Cloud Phase** \\ \hline \multirow{8}{*}{0–1 (8)} & 0.0–1 & All & - & - \\  & 1–25 & All & - & - \\  & 25–50 & All & - & - \\  & 50–75 & All & - & - \\  & 75–99 & All & - & - \\  & 99–100 & Dark, mid, bright \({}^{\star}\) & - & - \\ \hline \multirow{8}{*}{1–25 (8), 25–50 (8), 50–75 (8)} & 0.0–1 & All & All & All \\  & 1–25 & All & All & All \\
1–25 (8), 25–50 (8), 50–75 (8) & 50–75 & All & All & All \\  & 75–99 & All & All & All \\  & 99–100 & Dark, mid, bright \({}^{\star}\) & & All \\ \hline \multirow{8}{*}{75–99 (16)} & 0.0–1 & All & \(\ln\tau<1\), \(\ln\tau\geq 1\) & All \\  & 1–25 & All & \(\ln\tau<1\), \(\ln\tau\geq 1\) & All \\  & 25–50 & All & \(\ln\tau<1\), \(\ln\tau\geq 1\) & All \\  & 50–75 & All & \(\ln\tau<1\), \(\ln\tau\geq 1\) & All \\  & 75–99 & All & \(\ln\tau<1\), \(\ln\tau\geq 1\) & All \\  & 99–100 & Dark, mid, bright \({}^{\star}\) & \(\ln\tau<1\), \(\ln\tau\geq 1\) & All \\ \hline \multirow{8}{*}{99–100 (N/A)} & All & 0–0.6 & Continuous in \(\ln\tau\) & Liquid, ice \({}^{\star}\) \\  & All & 0.6–0.7 & Continuous in \(\ln\tau\) & Liquid, ice \({}^{\star}\) \\ \cline{1-1}  & All & 0.7–0.8 & Continuous in \(\ln\tau\) & Liquid, ice \({}^{\star}\) \\ \cline{1-1}  & All & 0.8–0.9 & Continuous in \(\ln\tau\) & Liquid, ice \({}^{\star}\) \\ \cline{1-1}  & All & 0.9–1.0 & Continuous in \(\ln\tau\) & Liquid, ice \({}^{\star}\) \\ \hline \hline \end{tabular}  
* Surface brightness and cloud phase are determined based on thresholds applied to a sea ice brightness index and effective cloud phase index, respectively [24].  
\end{table} 
Table 5. CERES-Terra/Aqua shortwave ADM scene types for sea ice. Reproduced and adapted from [PERSON] et al. [24] © American Meteorological Society. Used with permission. The numbers in parentheses in the first column indicate the number of scene types.