The near-real-time (NRT) model evaluation is an assessment of how the forecast behaves relative to observations that are NRT available, allowing the modelling group and the end users to have a quick overview of the quality of the forecast. Forecasts of dust optical depth (DOD) are compared with total aerosol optical depth (AOD) provided by the Aerosol Robotic Network (AERONET) for selected dust-prone stations located in Northern Africa, Middle East and Southern Europe (stations list).
The AERONET program is a federation of ground-based remote sensing aerosol networks established by NASA and PHOTONS (Univ. of Lille 1, CNES, and CNRS-INSU) and is greatly expanded by collaborators from national agencies, institutes, universities, individual scientists, and partners. The program provides a long-term, continuous and readily accessible public domain database of aerosol optical, microphysical and radiative properties for aerosol research and characterization, validation of satellite retrievals, and synergism with other databases. The network imposes standardization of instruments, calibration, processing and distribution. Version 2 Level 1.5 of AERONET products are used for the NRT evaluation. Level 1.5 data are automatically cloud screened but may not have final calibration applied. Since AERONET sun photometers do not yield AOD at 550 nm, this variable is calculated from AOD at 440, 675 and 870 nm and the Ångström exponent 440-870 using the Ångström law.
To minimize the sources of error, it is intended to restrict the comparison to situations in which mineral dust is the dominant aerosol type. Threshold discrimination is made by discarding observations with an Ångström exponent 440-870 higher than 0.6.
Rather than time-interpolated, AERONET observations are assigned to the nearest multiple-of-3 hour. In case more than one observation is assigned to the same hour, only the closest-in-time is considered.