1. Influence of ice supersaturation, temperature and dynamics on cirrus occurrence near the tropopause N. Lamquin (1), C.J. Stubenrauch (1), P.-H. Wang (2) Vienna, European Geophysical Union 16 April 2007 (1) CNRS/IPSL Laboratoire de Météorologie Dynamique, Ecole Polytechnique, Palaiseau, France (2) Science and Technology Corporation, Hampton, VA 23666, USA

2. Cirrus clouds require high supersaturation to form, RHi > RHi critical RHi critical depends on the type of nucleation, temperature, dynamics Homogeneous nucleation: -freezing of aqueous solution droplets at T < -40°C Heterogeneous nucleation: -requires lower supersaturation and involves aerosol particles -produces thinner cirrus Is cirrus formation thermodynamically controlled ?

3. SAGE II, June 1987 – May 1991 (prior Pinatubo) Source: http://oea.larc.nasa.gov -Limb occultation at satellite sunrise / sunset at 7 wavelengths between 0.4 & 1 μm -pathlength: 200km x 2.5 km -vertical resolution: 1 km -vertical profile ends at ‘opaque’ cloud with extinction(1.02μm) > 2.10-2 km-1 Wang et al., Atm. Res. 1994, JGR 1996, Atm. Res. 1998, JGR 2001 SAGE cloud data provided by Pi-Huan Wang 00.030.33.0 Optical Depth Sub-Visible Cirrus Thin CirrusCirrus Cirrostratus SAGE IITOVS Source: Lynch, D.K., K. Sassen, D.O’C. Starr and G. Stephens. Cirrus. Oxford University Press, 2002

4. TOVS Path-B climatology: 1979, 1987- 1995, … Scott et al., BAMS 1999; Stubenrauch et al. J. Climate 2006 (5 layers,  100hPa) - atmospheric temperature (9 layers,  10hPa), water vapor (5 layers,  100hPa) - effective cloud amount (ECA), cloud top pressure (Stubenrauch et al. 1999) MSU+HIRS MSU+HIRS R m ( i,  ) along H 2 O, CO 2 absorption bands, good spectral resolution 3I Inversion (Chédin, Scott 1985) - D e, IWP of cirrus (CIRAMOSA, Rädel et al. 2003, Stubenrauch et al. 2004) - upper tropospheric relative humidity (Stubenrauch & Schumann 2005) -determined for clear sky and cloud scenes with ECA < 0.6 - RHi in two 200 hPa-thick layers: 100-300 hPa, 300-500 hPa

5. Tropopause SAGE 300-500 hPa TOVS layer for RHi 100-300 hPa TOVS layer for RHi Tropics Midlat South Midlat North Tropopause and region of study RHi taken in the layers situated under the tropopause for each region

6. RHi distributions -INCA measurements (Ovarlez et al. 2002): peak of cirrus RHi distribution at 100 %, we find 60 % because of layer thickness → we define supersaturation by RHi > 60 %. -Microwave Limb Sounder measurements (Spichtinger et al. 2003): 5.98 % supersaturated clear events in the Tropics at 215 hPa while we find 6.5 % in a 200 hPa- thick layer centered around 200 hPa. Tropics, 100-300 hPaMidlat North, 300-500 hPa 60 % Clear: 6.5 % super- saturated events RHi clear < RHi SVC < RHi cirrus, 60 % works for all regions

7. SVC occurrence as function of ISS occurence Positive correlation: -SVC formation is thermodynamically controlled -correlation is stronger in the tropics -extending results of Gierens, JGR 2000 (MOZAIC NH midlat)

8. SVC and Cirrus occurrence (4 years) -Seasonal occurrences of SVC and Cirrus at (latitude,longitude) versus seasonal occurrence of ISS -« All seasons » = data taken at all seasons -Cirrus occurrence follows SVC occurrence in the tropics -Cirrus occurrence is constant in midlatitudes : 215 K : 230 K - 250 K

9. Midlatitudes, two T domains (8 years) ECMWF ERA-40 wind fields, « Strong updraft » = strong ↑ and weak ↔ different behaviours in NH and SH midlatitudes strong large-scale updrafts increase strongly Ci occurrence in NH, not in SH warm T (het. nucleation): Ci formation thermodynamically controlled cold T: on average constant Ci occurrence « Warm » = T > 240 K « Cold » = T < 240 K

10. Tropics, influence of dynamics (8 years) « Weak » = all winds are weak « Strong » = one is strong, the other is weak Strong large-scale updraft increases already Ci occurrence at low ISS occurrence In situations with strong horizontal winds (may diffuse moisture): less Ci

11. Midlatitudes North, two T domains, influence of dynamics (8 years) cold T: horizontal wind as important as updraft front dynamics at meso-scale

12. Conclusions ● SVC: stronger thermodynamic control in the tropics ● Tropics: cold T, Ci formation thermodynamically controlled, stronger updrafts increase Ci formation already at low ISS occurrence ● Midlatitudes: warm T: Ci formation thermodynamically controlled, heterogeneous nucleation cold T: probably meso-scale processes dominate Outlook: ● AIRS: RHi on thinner layers ● Calipso: thin cirrus with more precise data ● link to models

14. Coherence of datasets (1) SAGE thin cirrus and cirrus SAGE no high clouds SAGE SVC SAGE \ TOVS no hghhigh no hgh ci 30%8% high ci 28%34% → ~28% of SAGE cirrus too thin to be detected by TOVS

15. Coherence of datasets (2) Cloudy SAGE Clear SAGE Cloudy TOVS Clear TOVS Cloudy Clear Sum ( Clear/Clear + Cloudy/Cloudy ) = 63.5 % but… « Cloudy » = Cloudy of high clouds

16. Winds Horizontal (√u 2 +v 2 ), vertical (w) winds averaged on the 200 hPa-thick pressure levels « Weak » and « strong » winds defined by regional and seasonal distributions using edges at 20 % Strong updraft Weak vertical Strong horizontal Weak horizontal Supersaturation occurrence is calculated seasonally, regionally and for each « wind case »

17. Determination of RHi Precipitable water column: 300-100 / 500-300 hPa, W = → RHi(Δp) = gρ W /  q s_ice (p)dp 3I retrieved atmospheric T profile (30 levels) → p s calculated by Sonntag’s formulae (Sonntag, 1990): q s determined by integration, steps of 1 hPa: