Jeannette van den Bosch, Air Force Research Laboratory; Earl Spillar, AFRL/RDS
Keywords: SDA, modeling skylight radiance, MODTRAN, Starfire Optical Range, Izana
Abstract:
Daytime skylight has long been known to be impacted by the scattering of particles of various origins (e.g., organic matter, dust, soot, urban pollutants, biomass burning, water vapor, sea salt) in the visible and near infrared (roughly 0.4 to 2.5 microns). Previous work described at the AMOS Conference has shown that skylight radiance is, in turn, the limiting factor for observations of satellites during the daytime. However, actual observations of daytime skylight radiance climatologies at sites are rare in the literature. To fill the gap, MODerate resolution atmospheric TRANsmission (MODTRAN) modeling using default parameters have been used to estimate performance. In this paper, in situ data is used to constrain MODTRAN modeling at two sites with the results showing the default parameters most likely do not reflect the actual skylight radiances at these sites.
MODTRAN calculations are used to create model skylight radiance climatologies for Starfire Optical Range (SOR), Kirtland AFB, NM and Izana (Tenerife Island), Spain. For both sites a monthly aerosol climatology is created based on data from Aerosol Robotic Network (AERONET) CIMEL sun photometers located at each site which can then be used to constrain MODTRAN. These inputs are the monthly averaged aerosol optical depth at 550 nm (AOD550) and columnar water vapor (CWV). Other MODTRAN input parameters include the current value of CO2 (400 ppmV) and site-specific columnar O3 in DU. For simplicity, the line-of-sight (LOS) is a 45 degrees zenith angle from the target site towards space. A geographical-seasonal model atmosphere is chosen for each site. The chosen model is run with all input parameters set to default and then compared with the CIMEL-constrained model runs.
A naive use of MODTRAN can lead to 100-300% overprediction of skylight radiances (corresponding to a performance penalty of 0.38 to 0.6 magnitudes) when compared to aerosol-constrained modeling through the seasons.
Date of Conference: September 15-18, 2020
Track: Atmospherics/Space Weather