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 SwinbankMet 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

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.

COST workshop, 12th March 2004 3

Ozone

COST workshop, 12th March 2004 4

Radiation Dynamics, temperatures

Ozone assimilation in a GCM

TransportChemistry

Observationincrements

Ozone

Ozone: MIPAS, HIRS9

)(

)()(

LP

TTT

LPxx

x

LPLP

dt

dx

Cariolle scheme, no PSCs

Transport

Temperature

UM tracer (TVD/ Roe/ Superbee)

Eulerian dynamics

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Ozone differences (with and without Cariolle scheme)

2

1

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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

1.3 UM ozone with DA and TVD tracer advection scheme

0.9 UM TVD tracer advection scheme

0.2 HALOE estimate of WV tape recorder

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2. Difficulties with Cariolle scheme in vortex

Total ozone at 12Z on 24th September

Analysis with Cariolle scheme

Analysis without Cariolle scheme

TOMS

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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

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

COST workshop, 12th March 2004 10

Water vapour

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

sat

water

e

eRH

bsat

waterpseudo Te

eRH

bnorm RH

RHRH

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:

yh b )(xy

2okk

lklk B

yBx

2oR

))(( bba h xyKxx 1)( RHBHBHK TT

22ok

lkkllk

yPx

001000H

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Stratospheric WV and the B matrix

B: Correlations

New Dynamics50 levels

NMC Method

l

k

Plot: D. Jackson

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Stratospheric WV and the B matrix

B: STD RH (%)

New Dynamics50 levels

NMC Method l

k

Plot: D. Jackson

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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.

Plk = -0.01}

0.03ppmvRH%04.022

ok

lkkllk

yPx

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Single observation test case

temperature

background q

analysis q

observation

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.