Efficient Methods to Account for Cloud-Top Inclination and Cloud Overlap in Synthetic Visible Satellite Images

Autor(en)

Leonhard Scheck, Martin Weissmann, Bernhard Mayer

Abstrakt

Visible satellite images contain high-resolution information about clouds that would be well suited for convective-scale data assimilation. This application requires a forward operator to generate synthetic images from the output of numerical weather prediction models. Only recently have 1D radiative transfer (RT) solvers become sufficiently fast for this purpose. Here computationally efficient methods are proposed to increase the accuracy and consistency of an operator based on the Method for Fast Satellite Image Synthesis (MFASIS) 1D RT. Two important problems are addressed: the 3D RT effects related to inclined cloud tops and the overlap of subgrid clouds. It is demonstrated that in a rotated frame of reference, an approximate solution for the 3D RT problem can be obtained by solving a computationally much cheaper 1D RT problem. Several deterministic and stochastic schemes that take the overlap of subgrid clouds into account are discussed. The impact of the inclination correction and the overlap schemes is evaluated for synthetic 0.6-μm SEVIRI images computed from operational forecasts of the German-focused COSMO (COSMO-DE) Model for a test period in May–June 2016. The cloud-top inclination correction increases the information content of the synthetic images considerably and reduces systematic errors, in particular for larger solar zenith angles. Taking subgrid cloud overlap into account is essential to avoid large systematic errors. The results obtained using several different 2D cloud overlap schemes are very similar, whereas small but significant differences are found for the most consistent 3D method, which accounts for the fact that the RT problem is solved for columns tilted toward the satellite.

Organisation(en)

Institut für Meteorologie und Geophysik

Externe Organisation(en)

Ludwig-Maximilians-Universität München, Hans-Ertel-Zentrum für Wetterforschung, Hans Ertel Centre for Weather Research

Journal

Journal of Atmospheric and Oceanic Technology

Band

35

Seiten

665–685

Anzahl der Seiten

20

ISSN

0739-0572

DOI

https://doi.org/10.1175/JTECH-D-17-0057.1

Publikationsdatum

2018

Peer-reviewed

Ja

ÖFOS 2012

105206 Meteorologie

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/efficient-methods-to-account-for-cloudtop-inclination-and-cloud-overlap-in-synthetic-visible-satellite-images(a8f7fc6c-bae6-42cb-9112-778cacc395ce).html