SEVIRI - Cloud Top Height Factsheet

Transcription

1 Doc.No. : EUM/OPS/DOC/11/4199 Issue : v2a Date : 28 December 2011 WBS : EUMETSAT Eumetsat-Allee 1, D Darmstadt, Germany Tel: Fax: EUMETSAT The copyright of this document is the property of EUMETSAT.

2 Document Change Record Issue / Revision Date DCN. No Summary of Changes v1 06/10/2010 v2 01/06/2011 Contents totally replaced by CTH section in Central Facility MSG Meteorological Product Manual. v2a 28/12/2011 Add SEVIRI to document name. Page 2 of 5

3 General Description The Cloud Top Height (CTH) product is an image-based GRIB Edition 2 encoded product which indicates the height of the highest cloud. Using pixel-based cloud analysis retrieval, the CTH product is derived for 3x3 pixel areas, in which the highest cloud top is assigned to a 320 m thick cloud height band. If a specified minimum number of cloudy pixels are available within the CTH 3x3 pixel processing area, then the pressure level of the highest cloud is extracted for determining the aforementioned cloud height band. The pressure level is taken from a cloud analysis that uses forecast data for the determination of the cloud height in the geophysical unit of pressure. Currently, the product may contain up to 51 bands up to about 15 km. For the CTH height band determination, the International Civil Aviation Organization (ICAO) standard atmosphere (pressure and height) is used to derive the heights corresponding to the pressure levels determined in the cloud analysis product. This is done by interpolating the observed cloud top height between the pressure and ICAO height levels with values that are greater and less (respectively (p n, Z n ) and (p n+1, Z n+1 )) than the observed cloud top height. The interpolation relationship to convert the cloud top height (in hpa) to the geometrical height is as follows: Z CTH = Z n + ((ln(cloud_top_height) - ln(p n )) * (Z n+1 - Z n )) / (ln(p n+1 ) - ln (p n )) The encoded CTH product, disseminated to the user community, supplies the cloud top height in steps of 320 m (i.e. cloud tops at 320 m, 640 m, 960 m, etc. up to about 16,000 m). Within the product generation, the quality flag from the intermediate cloud analysis product for the cloudy pixels identified within the CTH processing segment is used, which indicates the quality of the cloud top height for these pixels. If more than a specified maximum percentage of the cloudy pixels within the CTH processing segment indicate poor quality for the cloud top height, then the CTH_quality_flag for the segment is set to height poor quality. The CTH product is generated for every repeat cycle for the 0 Operational Service and disseminated as a GRIB encoded product. An important user community for the CTH products concerns aviation. CTH products are typically used to provide aircraft crew with information on the presence of high-level clouds. The latest change increasing the frequency of the product generation and distribution improves the service for the aviation user community previously, with only an hourly dissemination, a single missed product immediately resulted in a gap of two hours between consecutive products. Settings: The minimum number of available cloudy pixels is set to 4 pixels. The maximum percentage allowed for pixels flagged in the intermediate cloud analysis product is set to 33%. Page 3 of 5

4 Product Examples An example of a CTH product from 28 April :00 UTC is given in Figure 1. Figure 2 shows the same CTH product within a limited area over the South American coast. CTH Bands Figure 1: An example of the CTH product (left) from 28 April :00 UTC. The colour scale presents only 16 values for 50 bands. A frequency histogram for the band occurrences is presented on the right. CTH Bands Figure 2: As Figure 1 but for a smaller area over the South American coast. (Note that the colour scale now covers 20 bands.) Page 4 of 5

5 Future Product Improvements The product depends on the cloud detection scheme, hence further product improvements depend wholly on improvements in the cloud detection scheme. Identified Product Limitations There are no known product limitations. Operations Limitations There are no known operations limitations. Product Change Record Date Change 8 June 2010 Increase EUMETCast dissemination frequency from every 3 hours to hourly. 15 June 2011 Increase CTH product generation and EUMETCast dissemination frequency from hourly to every repeat cycle. Relevant Documentation For more information the user is referred to the following documentation which is available on the EUMETSAT website: Cloud Detection for MSG - Algorithm Theoretical Basis Document (EUM/MET/REP/07/0132) MSG Meteorological Products Extraction Facility (MPEF) Algorithm Specification Document (EUM/MSG/SPE/022) Page 5 of 5