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Ruth Doherty et alRuth Doherty et al: Influence of convective transport on tropospheric ozone and its precursors in a chemistry-climate model, 2005

Mark Lawrence et alMark Lawrence et al: The balance of effects of deep convective mixing on tropospheric ozone, 2003

Matus Martini Jan 4, 2008

Happy new year!Happy new year!

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Outline

• climate change motivation

• convection scheme

• comparison Lawrence v. Doherty

• discussion

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IntroFuture emissions projections indicate strong growth of O3 precursor emis

in the tropics – how these are processed by convection will be an important determinant of future O3 conc and the oxidizing capacity of the atmosphere.

Convection is important in the context of understanding how climate variability and future climate change may affect tropospheric chemistry, via changes in both vertical mixing and the distribution of LNOx.

O3 and also O3 precursors• sfc precursor sources, esp. short-lived precursors such as NOx,

isoprene, and other natural and anthro hydrocarbons are uplifted to MT+UT at the expense of LT

• most LNOx added to the UT• isoprene and its degradation products

- major source of tropical UT HOx

- peroxy acetyl radical, which affect NOx by promoting PAN formation (over land where isoprene and LNOx emis co-located)

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Balance between transport and variety of chem effects

Dickerson et al. 1987; Pickering et al. 1990 showed that the upward transport of O3 and precursors can result in substantial local increases in the tropo O3 column over polluted regions, since the lifetimes of these gases and O3 production efficiency (per NOx molecule) increase with altitude.

Lelieveld and Crutzen, 1994 downward mixing of O3-rich air from UT towards the surface can reduce the column mean lifetime of O3; using a simple tropo CTM: this effect outweigh the effect of mixing above polluted regions, resulting in a net 20% decrease in tropo O3. Number of studies following... the overall effect of conv overturning over clean regions (such as the equatorial Pacific) globally reduces tropo O3.

Lawrence et al. – effect of conv changes in precursor emis on tropo O3 burden is more important than the convective redistribution of O3

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Task of the convection schemeTask of the convection scheme

To calculate the collective effects of an ensemble of convective clouds in a model column as a

function of grid-scale variables.

The parameterization needs to describe

condensation/evaporation and (vertical) transport

Most models treat shallow (non-precipitating) convection as a separate process

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Determine vertical distribution of heating, moistening and momentum changes

Cloud model (single cloud, cloud spectrum)

Determine the overall amount of the energy conversion at the grid scale

Closure (equilibrium, time scale of

energy release)

Determine occurrence/location of convection

Trigger (CAPE, moisture convergence)

Structure of a convective parameterization

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Lawrence et al.

3D global offline Model of Atmospheric Transport and Chemistry - Max Planck Institut für Chemie ver 3.1 (MATCH-MPIC): meteorology driven by the NCEP reanalysis data at T21 (64x32 grid cells) 28L up to ~2hPa and a 30-min time step

combination of two conv parameterizations: [Zhang and McFarlane, 1995; Hack 1994], photochemical scheme includes a parameterization of NMHC up to C5 molecules (esp. isoprene)

test runs - conv transport of O3/trace gases completely shut off (except water vapor and potential temperature)

modeled meteorology remains the same in all runs

LNOx source not modified

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Doherty et al.

STOCHEM-HadAM3 coupled CTM-GCM

HadAM3 (Pope et al., 2000) is the atmospheric component of the HadCM3 atmosphere-ocean GCM. HadAM3 was driven by prescribed monthly SSTs for 1980–2002 and was run at 3.75 x 2.5 and 19L

vertical levels are concentrated at the sfc and extend to 10 hPa

metfields are passed to the CTM every 3 h

STOCHEM is a Lagrangian tropo CTM: atmosphere divided into 50 000 equal mass air parcels that are advected by HadAM3 winds

air parcels mapped onto 5 x 5 with 9L up to 100 hPa

chem fields from STOCHEM are not fed back into the radiation scheme in HadAM3

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Doherty et al.

The conv scheme in HadAM3 is a bulk mass-flux scheme with an instability closure (Gregory et al., 1997) and is based on that of Tiedtke (1989).

STOCHEM uses the Collins et al. (2002) Lagrangian convective mixing scheme.

This mixing scheme uses the 3-D convective updraft mass fluxes generated from HadAM3 and represents:

– rapid convective updrafts

– entrainment and detrainment of air parcels

– slower mass balance subsidence of larger regions of surrounding air

Although downdrafts are calculated in the GCM conv scheme, these are not implemented in the mixing scheme.

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Chemistry v. Meteorologymixing of O3 precursors v. O3 vertical transport

Lelieveld and Crutzen (1994) 20% decrease in the global O3 burden due to convective mixing, using a global model with no NMHC chemistry.

the effects of mixing O3 over-rode any chemical effects due to mixing of O3 precursors.

Lawrence et al. (2003), detailed NMHC chemistry (similar to that used Doherty et al.), found a global O3 burden increase of 12% from convective mixing

effects of changes in precursor chemistry due to mixing (in particular lifting of sfc NOx emis) outweigh the effect of vertical mixing of O3 itself.

Doherty et al. (2005), O3 decreases by 13% due to convective mixing, and the effect of O3 vertical transport outweighs the effect of precursor mixing changes. Differences in the patterns of TC O3 change are large with decreases almost everywhere in our study, compared to increases everywhere in Lawrence et al. (2003).

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O3 chem prod: NO+HO2, NO+CH3O2, NO+RO2

O3 chem destr: O1D+H2O, O3+OH, O3+HO2, O3+hydrocarbons (C2H4, C3H6(2 routes), C5H8, MVK) and other minor net O3 losses

OH burden 8% increaseNOx lifetime 21.2% decrease mainly due to downward transport of LNOxPAN increase by similar amount

Convection impact on tropo O3

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STOCHEM-HadAM3 control – convection off

convected isoprene is a significant source of PAN (reservoir for NOx)in the cold UT PAN is relatively stable ~1monthsimpact of conv on NOx: vertical transport + isoprene-PAN-NOx chem

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STOCHEM-HadAM3 control – convection off

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Discussion

Similarities:

MT O3 increases due to NOx emis mixing

conv reduces tropo O3 lifetime

(Stevenson et al 2005)

STOCHEM-HadAM3 had the shortest lifetime 17 days

MATCH-MPIC had the longest 28 days

even if convective mass fluxes quite ‘similar’, response to convection is significantly different

uncertainty range?

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• WV distribution and LNOx emis were kept const (in both studies)• conv updraft fluxes are a factor of 2-3 stronger everywhere in

HadAM3 compared to MATCH-MPIC• emission source strengths:

NO sfc, aircraft and ship similar

LNOx 7 Tg N/yr compared to 2 Tg N/yr in Lawrence et al.

higher anthro NMHC emis by 5-20 Tg/yr

higher isoprene emis by ~225 Tg/yr (~65%)

-> more PAN formed, larger UT NOx reduction

-> higher isoprene -> higher UT HOx• chemical schemes: isoprene degradation scheme

(isoprene oxidation generates peroxy radicals HO2 and RO2 required for O3 production)

Discussion - differences

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• ligtning in the tropics is a major NOx source - directly associated with convection

• LNOx and WV kept const

• 3.75x2.5 some LS convection can be resolved

the conv is not completely turned off

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• deep conv mixing: O3 and precursors• over tropical land: conv lofts surface sources of NOx and NMHCs

(MT+UT increase at the expense of LT), downward transport of UT LNOx

• UT NOx -> PAN (by the introduction of peroxy acetyl radicals, an oxidation product of isoprene)

• overall effect is a reduction in UT NOx and increase in UT HOx• in UT : despite an overall increase in chem production, O3 decreases

(vertical transport of O3 itself is more important than changes in chem)

not only in tropics!

in MT: strong NOx increase (10-50ppt) in the north mid-lat at the expense of the sfc (transported to the remote tropo, remote oceans –vertical mixing more dominant)

Conclusion

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STOCHEM-HadAM3 control – convection off

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• Mid-latitudinal conv also reduces UT O3 13% decrease in the global O3 burden (despite a 4% increase in chem production)

• apparent contradiction: the mean tropo O3 lifetime reduces substantially (16%) – less O3 resides in the UT where its lifetime is longest

• substantial diff in conv mass fluxes: strength and height of deep conv• diff in isoprene chem schemes• diff in LNOx and isoprene emis magnitudes

• Two similar models (resolution and complexity) -> divergent results• importance of deep conv mixing – interannual and longer timescales• further studies with other models needed to constrain this range of

uncertainty

Conclusion

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Convection impact on tropo O3

O3 prod: NO+HO2, NO+CH3O2, NO+RO2

O3 destr: O1D+H2O, O3+OH, O3+HO2

O3+hydrocarbons (C2H4, C3H6(2 routes), C5H8, MVK) and other minor net O3 losses

OH burden 8% increaseNOx lifetime 21.2% decrease mainly due to downward transport of LNOxPAN increase by similar amount