## 3 Method
### Part 3: Processing of Instantaneous Albedo to Daily Mean RSF (Level-3)
#### 3.3.1. Daylight Conditions (\(\theta_{0}<84^{\circ}\)): Modeling Albedo Diurnal Cycle  
A consecutive time period of 5 min bins with daylight conditions (\(\theta_{0}<84^{\circ}\)) is called a \"daylight block (DLB)\" (Figure 7a). Depending on the grid box's location and season, a single UTC day may contain zero, one, or two DLBs. Each DLB was processed separately. All the remapped Level-2b observations (output from Section 3.2) of previous, current, and next UTC day were collected, and only those were selected from which the observation's time stamp falls within the temporal range of the daylight block. Each observation was assigned to the temporally nearest 5 min bin. In cases where more than one observation fell within the same 5 min bin, priority was given to the observation with the lowest temporal difference with the 5 min bin center. The temporal interpolation method used here, called the \"constant meteorology method\", has been documented extensively by [PERSON] et al. (2016) and used subsequently in the CERES processing, where it is also called the \"CERES-only (CO) method\" ([PERSON] et al., 2017). An illustrative example for a hypothetical DLB is shown in Figure 8, showing a DLB with two observations, in this case from NOAA-17 and NOAA-16. Each observation has an associated scene type, that is, a combination of surface type and cloud properties (cover, phase, optical thickness). Figure 7. Solar zenith angle (SZA; \(\theta_{0}\)) for all temporal “5 min bins” during a single UTC day in a single 0.25\({}^{\circ}\) grid box, with (**a**) only one relevant daylight block (DLB), and (**b**) two relevant DLBs. An iteration was started over all observations in the DLB. The scene type of each observation was used to select its corresponding albedo model, which describes the variation of TOA albedo depending on solar zenith angle \(\theta_{0}\) (Section 2.1). Each 5 min bin of the DLB has its own \(\theta_{0}\) (Figure 7a), and hence can be assigned an albedo value based on the selected albedo model, resulting in a diurnal cycle. In Figure 8 this is shown for the first observation (NOAA-17; overcast) by the light orange curve. The diurnal cycle's albedo should not be too far off the observed albedo, but there will be a difference in most cases because the albedo model provides an average value and not an instantaneous observation. However, rather than the absolute magnitude, it is mostly the shape of this curve which is important. Therefore, the diurnal cycle curve (from the albedo model) was scaled to match the observation, as shown by the dark orange curve in Figure 8. This was done by calculating the ratio between observed and modeled albedo, and applying it to scale the entire diurnal cycle (Equation (4) in [42]). Some situations require additional corrections. For instance when an observation around solar noon has a relatively high observed albedo but due to errors in the auxiliary input data, the diurnal cycle is much lower and more curved. The scaling could then lead to erroneous albedo values, exceeding 100% around sunrise and sunset. In these cases, iterative modifications were applied to the scene type, in steps of +25% cloud cover, and then, if needed, steps of +15 optical thickness, which 'flatten' the albedo model's diurnal cycle and hence decrease the risk of excessive scaling (which would lift albedo's over 100%). As soon as the (iteratively modified) diurnal cycle does not exceed 100%, it is accepted. The iteration then proceeded to the next observation, in this example NOAA-16 with a clear-sky scene type. The same steps were applied to this observation, that is, determination of the albedo model's diurnal cycle (light pink curve in Figure 8) and scaling of the albedo model's diurnal cycle (dark magenta curve in Figure 8). The final step consists of interpolating between the scaled diurnal cycles. This was done for each 5 min bin as a linear mean of the cycles corresponding to the closest preceding and closest following observation.