Clouds
5. Lydie Lavanant
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At CMS, three-dimensional fields of temperature and humidity are produced in a near-real time way since the beginning of 2009 from the acquisition of the advanced IASI interferometer and microwave AMSU / MHS sounders of METOP by using a 1dVar method. The purpose of this chain is to provide the satellite retrieved atmospheric fields before the release of the analysis and especially if possible with more realistic estimations of humidity and instability of the air, parameters which are useful for Nowcasting applications.
A Météo-France internal Web server displays dynamically and in real time all necessary fields for helping French Nowcasters. Besides an ‘external clone’ (http://retmetop.meteo-spatiale.fr/ ) of the site is also available for everybody outside Météo-France with the products of the last ten days over the ‘Europe-Atlantic’ and CMS HRPT area.
The scheme works in clear and cloudy conditions. Two methods are combined for the cloud detection and the cloud characterization.
First, the AVHRR data at full resolution segmented in the IASI footprint (for HRPT acquisitions) or the AVHRR radiances analysis available in the IASI level1c files (for global acquisitions) are processed. The cloud detection algorithm is a succession of thresholds tests applied to every AVHRR situation to various combinations of the AVHRR channels. When the situation is flagged cloudy a further process is done to determine its cloud type. If the situation is defined as an opaque cloud, the cloud top height is estimated from the AVHRR and the forecast. The scheme allows to determine the heterogeneity of the IASI scene and to estimate up to 3 cloud layers.
For semi-transparent cloud layers a further process is applied based on the CO2 slicing technique as described by Smith and Frey (1990).
Finally, IASI using the previous cloud products to select a sub-set of cloud-affected channels useful in the assimilation of the cloudy radiances.
Example of retrieved cloud type in the IASI footprint.
References:
Lavanant L., P. Marguinaud : Operational cloud masking for the OSI SAF global METOP/AVHRR SST product. EUMETSAT Conference, 2007.
Dahoui M., L. Lavanant, F. Rabier, T. Auligné: Use of Modis imager to help dealing with AIRS cloudy radiances. Quart. J. R. Met. Soc., 131B, 610, pp 2559-2580.
Lavanant L.: Using AVHRR radiances analysis for retrieving atmospheric profiles with IASI in cloudy conditions. ITSC-16 proceedings. 2008.