cost workshop, 12th march 2004 1 assimilation of stratospheric ozone and water vapour at darc alan...
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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.
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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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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
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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
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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
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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
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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.