Bias induced by the AOD representation time scale in long-term solar radiation calculations. Part 1: Sensitivity of the AOD distribution to the representation time scale

Abstract

The temporal resolution of aerosol optical depth (AOD) affects the prediction of surface irradiance and hence impacts solar resource assessments. Most current global AOD databases used for solar radiation estimation span from monthly to yearly time scales, which are much coarser than the typical hourly or sub-hourly time step used for irradiance calculations. The relative variation of the AOD’s mean and standard deviation (SD) calculated from data at monthly and daily temporal resolutions is assumed here as a proxy for the expected differences in the calculated long-term average of solar irradiance when using monthly and daily AOD data. This is specifically analyzed in Part 2 of this study. In this Part 1, the changes in the mean and SD of AOD caused by an alteration of the time scale from daily to monthly time resolution are investigated first. A database from 214 AERONET sites with long historical record series is used, as well as data from worldwide multi-year evaluations of AOD from a numerical weather model coupled with an aerosol transport model. The AERONET AOD data is fitted to the log-normal distribution and the results are separately analyzed for multiple AOD representation time scales from daily to monthly, and for sites with prevalence of coarse, mixed or fine aerosols. When the averaging period is increased from one to 30days, three effects are noticed regardless of the aerosol regime: (i) the range of possible AOD values narrows; (ii) the probability around the modal AOD increases; and (iii) the AOD distribution’s mode moves toward higher values. On average, the mean AOD at sites dominated by coarse or mixed aerosols does not significantly vary (it does by only less than 1%) when the averaging time increases from one day to 30days, although with significant dispersion from site to site (≈±5%). In contrast, again on average, it declines ≈4% at the fine aerosol sites. Conversely, the SD decays exponentially fast at all sites—faster at the fine aerosol sites—on average. For an averaging period of 30days, the SD of AOD over Europe and inland areas of Asia can be as low as 40% of the daily SD, while for most other land areas it stays at ≈60% of the daily SD. The impact that this change in the AOD SD has on the prediction of surface irradiance is investigated in this study’s Part 2.