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Use of advanced infrared sounders in cloudy conditions

Use of advanced infrared sounders in cloudy conditions. Nadia Fourrié and Florence Rabier Météo France Acknowledgement G. Hello and T. Labrot. Context.

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Use of advanced infrared sounders in cloudy conditions

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  1. Use of advanced infrared sounders in cloudy conditions Nadia Fourrié and Florence Rabier Météo France Acknowledgement G. Hello and T. Labrot 27 February-5 March 2002

  2. Context • Rabier et al (2002) have shown that it was possible to reduce the number of channels to assimilate in a NWP system and that a constant set gave promising result for clear sky conditions. This study has to be extended to the other observation conditions. • IASI can resolve some of the small structures identified by sensitivity studies (Prunet et al. 1998). • McNally (ITSC-11) showed with ECMWF model clouds that sensitive areas are widely covered by high-level clouds. 27 February-5 March 2002

  3. Outline • Introduction • Clouds and sensitive areas • Tools: MAIA and sensitive areas • 10 FASTEX cases • Channel selection : preliminary results • Channel selection for clear sky and clouds at some pressure levels • Conclusions and Future work 27 February-5 March 2002

  4. Clouds and sensitive areas: Tools • FASTEX (Fronts and Atlantic Storm-Track Experiment, Joly et al 1999): 10 cases corresponding to the zonal weather regime which is favourable to the development of cyclonic system over Europe. • MAIA (Mask AVHRR for Inversion ATOVS) method used for the retrieval of cloud parameters in each IASI pixel. • From the cloud top temperature, a cloud top pressure is deduced, using an analysis profile. 27 February-5 March 2002

  5. Sensitive area Score function: forecast of the mean sea level pressure over the region of interest on 19 february 1997 at 12 UTC Adjoint model -36 hours 12 UTC 97/2/19 0 UTC 97/2/18 Sensitive area determined with temperature fields of the gradientwith respect to initial conditions of the score function.They are regions where small errors in the initial conditions can increase and lead to major forecast errors 27 February-5 March 2002

  6. Synoptic situation for IOP 17 (12UTC 18/2/97) 50-200 200-300 300-400 400-500 500-600 600-700 700-800 800-900 900-1000 Clear sky Equivalent potential temperature at 850 hPa Wind velocity at 300 hPa Sensitive area 27 February-5 March 2002

  7. 12 UTC 18/2/97 (IOP 17) a pessimistic case 900-1000hpa 200-300 hPa 300-400 hPa 400-500hPa 500-600hPa 600-700hPa 800-900hPa Clear sky 700-800hPa <200 hPa 40% 30% 20% 10% 0% <200 hPa Clear sky 500-600 hPa 400-500 hPa 800-900 hPa 700-800 hPa 600-700 hPa 300-400 hPa 200-300 hPa 900-1000 hPa 27 February-5 March 2002

  8. Overview over 10 FASTEX cases:Distribution of cloud top heights in sensitives areas and in the whole data set. Top pressure (hPa) 27 February-5 March 2002

  9. Summary • Sensitive areas are mainly covered by clouds. Variability of the cloud top level with respect to the meteorological situation. • For 8 cases out of 10, cloud top mainly located above 700 hPa. For 7 cases out of 10, sensitivity maximum located below 700 hPa. • McNally (2001) already showed that sensitive areas are widely covered by high clouds with ECMWF model clouds. • What consequence for the channel selection? 27 February-5 March 2002

  10. Channel selection in cloudy conditionsExperimental framework • 24 profiles of a mid-latitude station. • 4 types of situations considered: clear sky, clouds at 800, 500 and 250 hPa (low-level, mid-level and high-level). • 300 channels selected with the so-called Rodgers iterative method for an analysis in temperature and humidity. • A constant channel set deduced from the average of the 24 selections. 27 February-5 March 2002

  11. Channel selection in cloudy conditionsResults • Distribution of the channels with respect to the cloud level Surface-800hPa: 43; 800-500hPa:70 ; 500-250hPa: 90; 250hPa-top of the atmosphere: 97 27 February-5 March 2002

  12. Conclusion • Sensitive areas widely covered by clouds • Preliminary work on channel selection in cloudy conditions.About a third of channels sounding in the stratosphere. Future work • Extension of this study with profiles from the sensitive areas. • Adress in more details the channel selection for cloudy conditions • Cloud detection 27 February-5 March 2002

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