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 Union16 April 2007

(1) CNRS/IPSL Laboratoire de Météorologie Dynamique, Ecole Polytechnique, Palaiseau, France

(2) Science and Technology Corporation, Hampton, VA 23666, USA

• Cirrus clouds require high supersaturation to form, RHi > RHicritical

• RHicritical 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 ?

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

0 0.03 0.3 3.0 Optical Depth

Sub-Visible Cirrus

Thin Cirrus Cirrus Cirrostratus

SAGE II TOVS

Source: Lynch, D.K., K. Sassen, D.O’C. Starr and G. Stephens. Cirrus. Oxford University Press, 2002

TOVS Path-B climatology:1979, 1987- 1995, …Scott et al., BAMS 1999; Stubenrauch et al. J. Climate 2006

- atmospheric temperature (9 layers, 10hPa), water vapor (5 layers, (5 layers, 100hPa)100hPa)

- effective cloud amount (ECA), cloud top pressure (Stubenrauch et al. 1999)

MSU+HIRS MSU+HIRS Rm(i,) along H2O, CO2 absorption bands, good spectral resolution

3I Inversion3I Inversion (Chédin, Scott 1985)

- De, 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

TropopauseSAGE

300-500 hPaTOVS layer

for RHi

100-300 hPaTOVS layer

for RHi

Tropics

MidlatSouth

MidlatNorth

Tropopause and region of study

RHi taken in the layers situated under the tropopause for each region

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 %60 %

Clear:6.5 %super-saturatedevents

RHi clear < RHi SVC < RHi cirrus, 60 % works for all regions

SVC occurrence as function of ISS occurence

Positive correlation:-SVC formation is thermodynamically controlled -correlation is stronger in the tropics-extending results ofGierens, JGR 2000 (MOZAIC NH midlat)

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

< T >: 215 K< T >: 230 K- 250 K

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

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

Midlatitudes North, two T domains, influence of dynamics (8 years)

• cold T: horizontal wind as important as updraft• front dynamics at meso-scale

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

Coherence of datasets (1)

SAGEthin cirrusandcirrus

SAGEnohighclouds

SAGE SVC

SAGE\TOVS no hgh high

no hgh ci 30% 8%high ci 28% 34%

→ ~28% of SAGE cirrus too thinto be detected by TOVS

Coherence of datasets (2)

CloudySAGE

ClearSAGE

CloudyTOVS

ClearTOVS

Cloudy

Clear

Sum ( Clear/Clear + Cloudy/Cloudy ) = 63.5 %but…

« Cloudy » =

Cloudy of high clouds

Winds• Horizontal (√u2+v2), 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 %

Strongupdraft

Weakvertical

Stronghorizontal

Weakhorizontal

Supersaturation occurrence is calculated seasonally,regionally and for each « wind case »

Determination of RHi

• Precipitable water column: 300-100 / 500-300 hPa, W =

→ RHi(Δp) = gρ W / qs_ice(p)dp

• 3I retrieved atmospheric T profile (30 levels)

→ ps calculated by Sonntag’s formulae (Sonntag, 1990):

• qs determined by integration, steps of 1 hPa:

gdp

qs

p

p0