The freeze-drying of ensembles of air parcels in determining stratospheric water Department of Environmental Sciences Institute of Environmental and Natural Sciences Lancaster University Chuansen Ren & Robert MacKenzie

Outline  Dehydration near the tropical tropopause and the development of parameterization  Comparison with satellite measurements  Dehydration cases in winter and summer  Summary and perspective  Acknowledgements

flight measurements Water vapour Saturation water vapour Particle backscatter

Comparison of modelled size distribution (blue line) with APE-THESEO in- situ measurements on February 24, 1999 (black lines, Thomas et al. (2002)) near the tropopause. The model was tried to match the observations spatially and temporally.

Number density of ice particles as a function of the vertical velocity for 3 freezing temperatures. The mean mass diameters, used as the monodisperse dry aerosol sizes, are indicated by legends. The wet aerosol sizes at the freezing threshold 235.8K/216.0K/196.4K are magnified by a factor of 7.2/2.4/ K Synoptic condition range in the tropical tropopause layer

a. Detailed b1. Simplified—with time-step of 20 minutes } c. Gettelman et al Dehydration behaviors of different schemes for a single trajectory Total water (ppmv) b2. Simplified—with time-step of 6 hours as c timeobs.ab1b2cinst. -50hr hr~

The presence frequencies of optically thin clouds during the (a) 6–8 Dec 2000 and (b) 6–8 Jun 2001 time periods comparing with MODIS satellite results retrieved by Dessler and Yang (2003). winter summer

380K 370K 360K Means 2.1ppmv 5.4ppmv 2.5ppmv Averaged total water distributions (of 12 sets of domain-filling runs) on three potential temperature levels. Winter case (6–8 Dec 2000).

360K to 370K 370K to 380K Means 0.4ppmv 3.3ppmv Dehydration. Winter case (6–8 Dec 2000). Winter case 6—8 Dec 2000 Summer case 6—8 Jun 2001 Mean valueDehydrationLevelDehydrationMean value 2.1ppmv380K2.8ppmv 0.6ppmv 0.4ppmv 2.5ppmv370K3.3ppmv 3.6ppmv 5.4ppmv360K6.4ppmv

380K 370K 360K Means 2.8ppmv 6.4ppmv 3.3ppmv Averaged total water distributions (of 12 sets of domain-filling runs) on three potential temperature levels. Summer case (6–8 Jun 2001 ).

0.6ppmv 370K to 380K Means 3.6ppmv 360K to 370K Dehydration. Summer case (6–8 Jun 2001 ). Winter case 6—8 Dec 2000 Summer case 6—8 Jun 2001 Mean valueDehydrationLevelDehydrationMean value 2.1ppmv380K2.8ppmv 0.6ppmv 0.4ppmv 2.5ppmv370K3.3ppmv 3.6ppmv 5.4ppmv360K6.4ppmv

Summary  A Lagrangian, partial-column, microphysical model has been established which can capture some features of APE-THESEO observations, such as the number and size of ice crystals;  A parameterisation, deduced from the detailed model, maintains more of the essential cloud physics than current parameterisations, without significantly increasing calculation-time, showing similar dehydration behaviour to the detailed model.  Ensemble runs of space-filling trajectory sets are carried out. The results, bearing similar patterns of cloud-presence frequencies to MODIS satellite observations, show different dehydration behaviors in winter and summer cases.  Ensemble runs to estimate the annual cycle like ‘tape recorder’ and the stratospheric water content trend are going to be carried out in the next step.

Acknowledgements Funded by NERC CWVC, EC TroCCiNOx THANKS TO: P. Haynes, and V. M. Bonazzola, Cambridge University J. Methven, University of Reading A. E. Dessler, and P. Yang (2003), J. Climate, 16, A. Thomas, S. Borrmann, et al. (2002), J. Geophys. Res., 107, 10.29/2001JD T. Koop, and B. Luo, Swiss Federal Institute of Technology This presentation is soon available online at