1D Radiative Transfer Model Calculations of Solar Actinic Flux Densities with Satellite Cloud Products – Comparison with Airborne Measurements on HALO

Clouds are highly variable in their structure as well as phase, resulting in a potentially large influence on actinic flux densities. Chemistry-transport models rely on accurate simulations of actinic flux densities to reproduce the essential impact of photolysis processes, thus the need for accurate radiative transfer calculations in the presence of clouds arises. Current studies show that under clear sky conditions simulated and measured UV/VIS actinic flux densities are typically within 10%, independent of wavelength. On the other hand, the impact of clouds on actinic radiation is more difficult to reproduce correctly when dependent on cloud structure, phase and position, flux densities can be significantly smaller or greater compared to clear sky conditions.

Following a similar approach by Ryu et al., 2016, UV/VIS spectral actinic flux densities were calculated utilizing cloud products from geostationary satellites (NASA SatCORPS). In this work, the latest version of the libRadtran model has been used, as well as aerosol properties (MODIS, MOD08_D3), surface albedos (MODIS) and total ozone columns (TEMIS, MSR-2) from polar-orbiting satellites as key input to simulate actinic flux densities in a range 280-650 nm. The evaluation of the model results is made by comparison with measured data from several campaigns with the research aircraft HALO (High Altitude and Long Range Research Aircraft) with a total of around 90 flights.

Using the NASA SatCORPS products (cloud phase, cloud optical depth, cloud top height, cloud liquid or ice water content and cloud particle size) 1-D radiative transfer calculations were conducted. Radiative properties of water clouds are reliably reproduced using look-up tables based on pre-conducted radiative transfer calculations using Mie theory. On the other hand, ice clouds and their correct parametrizations are challenging because of the wide range of possible ice crystal variations. Moreover, small-scale variations captured by the highly resolved aircraft measurements cannot be reproduced completely, due to the lower spatial and temporal resolution of satellite observations. The final intent of this study is to assess the quality of the radiative transfer modelled actinic flux densities and their potential to improve chemistry-transport models.