Solar Radiation Measurements on HALO

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New instruments for spectral measurements of solar radiation on HALO will be developed and applied in two demonstration missions (OMO, ML-CIRRUS). The target quantities are (i) spectral irradiance which determines the solar radiation budget of the atmosphere, and (ii) spectral actinic flux density which controls atmospheric photochemistry through photo-dissociation processes. Solar spectral radiance from the nadir direction will be measured simultaneously for remote sensing purposes. For the irradiance measurements it is of great importance to accurately distinguish between up-welling and down-welling flux density incident on cosine receivers on top and underneath the airplane. It is therefore planned to apply stabilizing platforms which track the movements of the airplane and correct the position of the receivers with sufficient time response. For the actinic flux density the main challenge is to establish measurements with both high time resolution and high accuracy in the UV-B range by using suitable combinations of spectral monochromators and detectors. This is crucial to quantify ozone photolysis besides other processes. Setup, tests and operations of the instruments will be made by two PhD students requested within this proposal. The measurements will provide (i) spectral cloud radiative forcings and (ii) photolysis frequencies which are essential objectives of the two demonstration missions ML-CIRRUS and OMO and possibly further missions within the application period.