1. ITSC-1227 February-5 March 2002 Use of advanced infrared sounders in cloudy conditions Nadia Fourrié and Florence Rabier Météo France Acknowledgement G. Hello and T. Labrot

2. ITSC-1227 February-5 March 2002 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.

3. ITSC-1227 February-5 March 2002 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

4. ITSC-1227 February-5 March 2002 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.

5. ITSC-1227 February-5 March 2002 Sensitive area 0 UTC 97/2/18 Adjoint model -36 hours Score function: forecast of the mean sea level pressure over the region of interest on 19 february 1997 at 12 UTC Sensitive area determined with temperature fields of the gradient with 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 12 UTC 97/2/19

6. ITSC-1227 February-5 March 2002 Synoptic situation for IOP 17 (12UTC 18/2/97) 50-200 200-300300-400400-500700-800500-600600-700800-900 900-1000 Clear sky Equivalent potential temperature at 850 hPa Wind velocity at 300 hPa Sensitive area

7. ITSC-1227 February-5 March 2002 12 UTC 18/2/97 (IOP 17) a pessimistic case <200 hPa 200-300 hPa 300-400 hPa 400-500hPa600-700hPa500-600hPa 700-800hPa 800-900hPa 900-1000hpa Clear sky <200 hPa 700-800 hPa200-300 hPa300-400 hPa600-700 hPa 500-600 hPa400-500 hPa 900-1000 hPa Clear sky 800-900 hPa 0% 10% 20% 30% 40%

8. ITSC-1227 February-5 March 2002 Overview over 10 FASTEX cases: Distribution of cloud top heights in sensitives areas and in the whole data set. Top pressure (hPa)

9. ITSC-1227 February-5 March 2002 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?

10. ITSC-1227 February-5 March 2002 Channel selection in cloudy conditions Experimental 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.

11. ITSC-1227 February-5 March 2002 Channel selection in cloudy conditions Results 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 Clouds at 800 hPa Clouds at 500 hPa Clouds at 250 hPa Clear sky257 channels 187 channels 97 channels Only 97 channels out of 300 always selected in the 4 channel sets.

12. ITSC-1227 February-5 March 2002 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