Upper Tropospheric Humidity and Cirrus Clouds: Relationships Inferred by Combined AIRS and CALIPSO Data,

Evaluation of ECMWF Forecasts and Influence of Vertical Resolution on Ice Supersaturation

N. Lamquin (1), C. J. Stubenrauch (1), K. Gierens (2), J. Pelon (3) (1) C.N.R.S./ I.P.S.L. Laboratoire de Météorologie Dynamique, Ecole Polytechnique, France

(2) Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany(3) C.N.R.S./I.P.S.L. Service d’Aéronomie, Paris, France

AIRS (Atmospheric Infrared Sounder)L2, version 5, NASATemperature and water vapor vertical profiles, quality flags…(Susskind et al. 2003, 2006; Tobin et al. 2006) RHice calculation in 50 to 100 hPa – thick pressure layers

CALIPSO (Cloud-Aerosol Lidar and Infrared

Pathfinder Satellite Observation) Data from NASA Langley Research Center Atmospheric Science Data Center

archived at ICARE and Climserv (IPSL)High cloud top and base pressure: sub-visible cirrus, thin and thick cirrus

number of cloud layers, tropopause…

Estimation of highest cloud optical depth, colocations with retrieved AIRS cloud properties

Stubenrauch et al., JGR 2008

Relationships inferred from AIRS-CALIPSO synergyColocation: August 2006 – July 2007 Night, +60°/-60° latitude coverage

Lamquin, Stubenrauch and Pelon., JGR 2008

Contact: nicolas.lamquin@lmd.polytechnique.fr

Evaluation of ECMWF (European Center for Medium-range Weather Forecasts)

forecasts with new ice supersaturation schemeusing AIRS and CALIPSO

Lamquin, Gierens, Stubenrauch, and Chatterjee, ACPD 2008

Midlatitudes250-300 hPa

Midlatitudes300-400 hPa

Tropics150-200 hPa

Tropics200-250 hPa

Mean RHice remains < 100% even for clouds extending over the whole pressure layer (influence of vertical and horizontal humidity variability, supersaturation occuring over smaller vertical extents than cloud appear ?)

If significant, higher mean RHice for lowest optical depths at comparable geometrical thickness (more depletion of water vapor for largest optical depths)

Greater influence of geometrical thickness on mean RHice than optical depth (15 - 35 % vs. 5 – 10 %)

Difficulties for detection of ice supersaturation Multiple-scattering contribution increasing with optical depth

Distance of topof highest cloud

to tropopause

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

qs integrated by steps of 1 hPa:

q determined by means of the mixingratio in the AIRS layer:q = w / (1+w) RHice = q/qsat

Influence of vertical resolution on ice supersaturation occurrence

Confronting two resolutions in the ECMWF forecasts,Comparison with radiosoundings

Ice supersaturation in the upper troposphere is an explicit feature in the Integrated Forecast System, operational since Sept 13, 2006, was introduced by Tompkins et al. (2007), it adopts the supersaturated relative humidity (RH) threshold using empirical approximation of the results of Koop et al. (2000) given by Kärcher and Lohmann (2002), f(T).It has produced some changes in the statistics of UTH and cloud fraction in the IFS (decrease in high-level cloud cover).

Comparisons between ECMWF forecasted UTH and AIRS UTH show a good agreement for clear (or mostly clear) scenes.Cloudy scenes take into account a variability of RHi values found in AIRS subject to biases due to the vertical resolution, this has to be taken into account to evaluate the forecast of the nucleation of high clouds.

Small adjustmentsfor vertical griddings

←Colocation:

Europe windowOne year

Two resolutions ofECMWFforecasts

→

Spin up phase for the model to reach

constant ice supersaturation

occurrence→

Good agreement betweenECMWF UTH and

CALIPSO high clouds

High cloud nucleation in the model resets RHi to 100% while AIRS sees a variability of situations.Distributions consistentwith previous results (first part of poster).

Comparison AIRS-CALIPSO, LMD vs. L2 retrievals, high clouds.From study in Stubenrauch et al. (JGR 2008)

Separation into two classes of optical depth