1. COST workshop, 12th March 2004 1 Assimilation of stratospheric ozone and water vapour at DARC Alan Geer, Carole Peubey, Ross Bannister, Roger Brugge, William Lahoz, Stefano Migliorini, Alan O’Neill Data Assimilation Research Centre, University of Reading, UK David Jackson, Hazel Thornton, Richard Swinbank Met Office, UK Funded by: The ASSET project, which is a shared-cost project (contract EVK2-CT-2002-00137) co-funded by the Research DG of the European Commission within the RTD activities of the Environment and Sustainable Development sub-programme (5th Framework Programme). http://darc.nerc.ac.uk/asset All ESA data ©2002

2. COST workshop, 12th March 2004 2 Introduction - context University of Reading (with help from UK Met Office) is assimilating Envisat data to produce stratospheric analyses of temperature, water vapour and ozone, as part of the EU FP5 ASSET project. Unified Model configuration:  3.75 by 2.5 resolution  40 levels in vertical  approx 1km vertical resolution in the stratosphere Assimilated data:  All operational data - in stratosphere and upper troposphere this includes ATOVS, sonde and aircraft observations  Retrieved temperature and ozone from MIPAS  Radiance assimilation of ozone from HIRS9 (ATOVS)  MIPAS water vapour to follow.

3. COST workshop, 12th March 2004 3 Ozone

4. COST workshop, 12th March 2004 4 Radiation Dynamics, temperatures Ozone assimilation in a GCM TransportChemistry Observation increments Ozone Ozone: MIPAS, HIRS9 Cariolle scheme, no PSCs Transport Temperature UM tracer (TVD/ Roe/ Superbee) Eulerian dynamics

5. COST workshop, 12th March 2004 5 Ozone differences (with and without Cariolle scheme) 2 1

6. COST workshop, 12th March 2004 6 1. No chemistry and no observations reveals that vertical transport in the tropical pipe is too fast No MIPAS With MIPAS observations, ozone distributions recover slowly below 40km UM tracer advection scheme? Gregory and West 2002 Effect of assimilation of temperature observations on model dynamics? e.g. Douglass et al 2003 Estimate of tropical pipe speed at 23km /mm s -1 1.3UM ozone with DA and TVD tracer advection scheme 0.9UM TVD tracer advection scheme 0.2HALOE estimate of WV tape recorder

7. COST workshop, 12th March 2004 7 2. Difficulties with Cariolle scheme in vortex Total ozone at 12Z on 24th September Analysis with Cariolle scheme Analysis without Cariolle scheme TOMS

8. COST workshop, 12th March 2004 8 2. Difficulties with Cariolle scheme in vortex Instantaneous ozone tendency for each term in the Cariolle scheme (estimated for 18Z 22nd September) DU Total column column ozone above level 26, 57hPa Analysis, 18Z 22nd September

9. COST workshop, 12th March 2004 9 MIPAS ozone O-B at the lowest levels Accepted Rejected by quality control MIPAS ozone: ESA Level 2 NRT retrievals between 31st Dec 2003 and 2nd Jan 2004 Figure from CNR IFAC: www.ifac.cnr.it/retrieval/products.html

10. COST workshop, 12th March 2004 10 Water vapour

11. COST workshop, 12th March 2004 11 Control variable Relative humidity  Currently the control variable in the Met Office system  Preserves cloud distributions when temperature is varied  Background errors are not Gaussian Pseudo relative humidity (Dee and Da Silva 2003)  Temperature increments do not affect specific humidity and vice-versa. Normalised relative humidity (Holm et al 2002, ECMWF)  Gaussian background errors  Supersaturation is suppressed

12. COST workshop, 12th March 2004 12 Does the B matrix link the stratosphere to the troposphere? A single observation, colocated with model grid point k : The approximate analytical solution is: Analysis increment at model grid point l, due to the observation at point k :

13. COST workshop, 12th March 2004 13 Stratospheric WV and the B matrix B: Correlations New Dynamics 50 levels NMC Method l k Plot: D. Jackson

14. COST workshop, 12th March 2004 14 Stratospheric WV and the B matrix B: STD RH (%) New Dynamics 50 levels NMC Method l k Plot: D. Jackson

15. COST workshop, 12th March 2004 15 Does the B matrix link the stratosphere to the troposphere? Point k : 100hPa,  k = 20%RH Point l : 50hPa,  l = 2%RH Residual  y = 50%RH, observation error  o = 10%RH A typical value of specific humidity at point l might be 3ppmv. Is this small increment a problem? In the absence of observations, water vapour is long lived. A series of increments over several months can have a cumulative effect. With observations of stratospheric water vapour, such an increment would soon be corrected. P lk = -0.01 }

16. COST workshop, 12th March 2004 16 Single observation test case temperature background q analysis q observation

17. COST workshop, 12th March 2004 17 Conclusions Ozone In the UM, the Brewer-Dobson circulation (as indicated by ozone) is too fast in the absence of Cariolle scheme ozone chemistry and ozone observations:  Assimilation of dynamical observations increases tropical pipe speed  UM tracer transport scheme is known to produce vertical transport that is too fast in the tropical pipe. With Cariolle scheme and assimilation of ozone observations, generally realistic ozone distributions are achieved. Cariolle scheme causes difficulties in polar vortex. Water vapour Ongoing experiments with control variables: relative humidity or pseudo relative humidity? Spurious correlations between troposphere and stratosphere may not be a big problem except around the tropopause.