Evaluation of Radiative Transfer Model Calculations of Solar Actinic Flux Densities with HALO Aircraft Measurements

Solar actinic radiation is driving atmospheric photochemistry. Consequently, chemistry-transport models rely on accurate model predictions of actinic flux densities to correctly reproduce the essential impact of photolysis processes. Cloud effects are most challenging in this context because of their potentially large influence and their variability. In this study, the effects of clouds, aerosols and ground albedos on solar actinic radiation are investigated using 1D satellite-aided radiative transfer calculations and in-situ aircraft measurements.

Spectral actinic flux densities in the range 280-650 nm are calculated with the latest version of the libRadtran model utilizing cloud products from geostationary satellites (NASA SatCORPS) as well as aerosol properties (MODIS, MOD08_D3), surface albedos (MODIS, MCD43A3) and total assimilated ozone columns (TEMIS, MSR-2) from polar orbiting satellites as key input parameters. The evaluation of the performance of the model output is made by comparison with data from several campaigns with the research aircraft HALO (High Altitude and Long Range Research Aircraft) where spectral actinic flux densities were measured during a total of around 90 research flights.

As a prerequisite to study cloud influence, clear-sky cases were investigated in detail to quantify the impact of the aerosol optical thickness and surface albedo on spectral actinic flux densities. Over land, radiative transfer calculations show good agreement with the measured data independent of wavelength and altitude within about 10% under clear sky conditions. Over the ocean the situation is complicated, because ocean surface albedos (OSA) are not available from satellite observations. Available OSA parametrizations, that depend on atmospheric conditions, tend to lead to a slight overestimation of upward-directed actinic flux densities in particular in the visible range, but the agreement for total actinic flux densities is still comparable with that over land. With sufficient agreement of modelled and observed actinic flux densities under clear sky conditions, flight paths with clouds will be included comprising above-cloud, in-cloud and below-cloud conditions. In the model, liquid cloud effects can be parametrized using Mie theory, but ice clouds pose a more complex problem, due to the wide range of possible structures of ice crystals. Finally, the intent of this study is to asses the quality of the radiative transfer modelled actinic flux densities based upon the satellite-derived cloud information.