HALO 2020 – Cloud effects on solar actinic radiation: Evaluation of satellite-aided radiative transfer model calculations with HALO measurements

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In this project UV/VIS spectral actinic flux density measurements of six HALO missions will be used for an evaluation of radiative transfer model calculations based on operational satellite cloud retrievals. Five of the HALO missions have already been completed: TECHNO (2010), NARVAL-I (2014), OMO (2015), EMERGE (2017/2018), and CAFE-Africa (2018) with a total of around 75 research flights. In addition, data of CAFE-Brazil (2020) will be included. The main purpose of the spectral actinic flux density measurements is to derive photolysis frequencies that are important for photochemical research. The HALO measurements provide a rare opportunity for an evaluation of satellite derived photolysis frequencies because they provide high resolution spot samples that cover different altitudes and mission areas globally. Moreover, during TECHNO, NARVAL and OMO also nadir spectral radiance measurements were performed on HALO by a mission partner. These measurements cover the complete solar spectral range and provide independent, local information on cloud properties below the aircraft. That will help to interpret and correctly apply the satellite data. The main question addressed in this project is whether or not radiative transfer modelled and measured data can be brought into reasonable agreement by taking into account available satellite cloud information and if consistent results are obtained for different mission areas and altitude ranges. If this approach is successful, predictions of evaluated, user-defined 3D fields of photolysis frequencies and their error estimates can be produced based on satellite cloud information. These photolysis frequency fields can be used to test chemistry transport model predictions or they can serve directly as an input for such models in future studies. A corresponding case study is intended within this project. From these applications also future HALO missions and their scientific analyses could benefit.