Towards an operational Ice Cloud Imager (ICI) retrieval product

<p>The second generation of the EUMETSAT Polar System (EPS-SG) will include the Ice Cloud Imager (ICI), the first operational sensor covering sub-millimetre wavelengths. Three copies of ICI will be launched that together will give a measurement time series exceeding 20 years. Due to the novelty of ICI, preparing the data processing is especially important and challenging. This paper focuses on activities related to the operational product planned, but also presents basic technical characteristics of the instrument. A retrieval algorithm based on Bayesian Monte Carlo integration has been developed. The main retrieval quantities are ice water path (IWP), mean mass height (<span class="inline-formula"><i>Z</i>_m</span>) and mean mass diameter (<span class="inline-formula"><i>D</i>_m</span>). A novel part of the algorithm is that it fully presents the inversion as a description of the posterior probability distribution. This is preferred for ICI as its retrieval errors do not always follow Gaussian statistics. A state-of-the-art retrieval database is used to test the algorithm and to give an updated estimate of the retrieval performance. The degrees of freedom in measured radiances, and consequently the retrieval precision, vary with cloud situation. According to present simulations, IWP, <span class="inline-formula"><i>Z</i>_m</span> and <span class="inline-formula"><i>D</i>_m</span> can be determined with 90&thinsp;% confidence at best inside 50&thinsp;%, 700&thinsp;<span class="inline-formula">m</span> and 50&thinsp;<span class="inline-formula">µm</span>, respectively. The retrieval requires that the data from the 13 channels of ICI are remapped to a common footprint. First estimates of the errors introduced by this remapping are also presented.</p>