Werner, F; Deneke, H (2020). Increasing the spatial resolution of cloud property retrievals from Meteosat SEVIRI by use of its high-resolution visible channel: evaluation of candidate approaches with MODIS observations. ATMOSPHERIC MEASUREMENT TECHNIQUES, 13(3), 1089-1111.

This study presents and evaluates several candidate approaches for downscaling observations from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) in order to increase the horizontal resolution of subsequent cloud optical thickness (tau) and effective droplet radius (r(eff)) retrievals from the native approximate to 3km x 3km spatial resolution of the narrowband channels to approximate to 1km x 1km. These methods make use of SEVIRI's coincident broadband high-resolution visible (HRV) channel. For four example cloud fields, the reliability of each downscaling algorithm is evaluated by means of collocated 1km x 1km MODIS radiances, which are reprojected to the horizontal grid of the HRV channel and serve as reference for the evaluation. By using these radiances, smoothed with the modulation transfer function of the native SEVIRI channels, as retrieval input, the accuracy at the SEVIRI standard resolution can be evaluated and an objective comparison of the accuracy of the different downscaling algorithms can be made. For the example scenes considered in this study, it is shown that neglecting high-frequency variations below the SEVIRI standard resolution results in significant random absolute deviations of the retrieved tau and reff of up to approximate to 14 and approximate to 6 mu m, respectively, as well as biases. By error propagation, this also negatively impacts the reliability of the subsequent calculation of liquid water path (W-L) and cloud droplet number concentration (N-D), which exhibit deviations of up to approximate to 89 gm(-2) and approximate to 177cm(-3), respectively. For tau, these deviations can be almost completely mitigated by the use of the HRV channel as a physical constraint and by applying most of the presented downscaling schemes. Uncertainties in retrieved r(eff) at the native SEVIRI resolution are smaller, and the improvements from downscaling the observations are less obvious than for tau. Nonetheless, the right choice of downscaling scheme yields noticeable improvements in the retrieved r(eff). Furthermore, the improved reliability in retrieved cloud products results in significantly reduced uncertainties in derived W-L and N-D. In particular, one downscaling approach provides clear improvements for all cloud products compared to those obtained from SEVIRI's standard resolution and is recommended for future downscaling endeavors. This work advances efforts to mitigate impacts of scale mismatches among channels of multiresolution instruments on cloud retrievals.