UTLS WorkshopBoulder, Colorado

October 19 - 22, 2009

Characterizing the Seasonal Variation in Position and Depth of the Mixing Layer in the UTLS Based on Observations from the ACE-

FTS and TES Satellite Instruments

Dave MacKenzie1, D. B. A. Jones1, M. Hegglin1, J. Worden2, C. D. Boone3, K. A. Walker1,3, P. F. Bernath3,4, C. Carouge5, L. Murray5

davem@atmosp.physics.utoronto.ca

1University of Toronto, 2Jet Propulsion Laboratory, 3University of Waterloo, 4University of York, 5Harvard University

Motivation: TES Observations over Central Asia in Summer TES shows enhanced O3 in the

middle and upper troposphere over central Asia

This feature is robust and is observed each summer (see poster by John Worden)

Enhanced O3 over central Asia may reflect the influence of mixing of stratospheric ozone into the troposphere

TES O3 at 464 hPa, July 2007

0 50 100 150Longitude

[Worden et al. 2009]

TES O3 at 464 hPa, July 2007

Description of Data and Model

Tropospheric Emission Spectrometer (TES)Instrument on AURA satelliteMeasurements made in nadir (5.3km x 8.3km footprint)Sun-synchronous orbit with 1:43pm ascending nodeVertical resolution is ~6-7km

Atmospheric Chemistry Experiment (ACE)Fourier Transform Spectrometer (FTS) instrument on SCISAT-I satelliteSolar occultation during sunrise/sunsetCovers 85S - 85N and 10-100km altitudeEffective vertical resolution in UTLS ~1km

GEOS-Chem Model (GEOS5 v8-01-04)Chemistry Transport Model (CTM)Driven by assimilated GMAO meteorological fields4° latitude by 5° longitude, 47 vertical levels (top level ~0.01hPa)Vertical resolution in UTLS ~1.1kmLinearized ozone (LINOZ) chemistry in stratosphere

Using CO/O3 Correlations to Characterize Mixing

High CO (flat) indicates tropospheric air; high O3 (steep) is stratospheric

Points in between indicate a region of mixing

Distribution of mixing data with altitude relative to tropopause gives mixing layer width (fwhm) and peak (mode)

Mixing widths are sensitive to the method used to define stratospheric and tropospheric end members

Altitude - TP Height [km]

CO [ppbv]

O3 [

ppbv

]N

umbe

r of D

ata

Lat=[30N,50N], Lon=[0,175E], July 2007

Lat=[30N,50N], Lon=[0,175E], July 2007

Stratospheric air (CO < 30 ppb)Tropospheric air (O3 < 100 ppb)

Mixed layer

Tropopause

GEOS-Chem - ACE Mixing Layer Width Comparison

Zonally averaged, 10° latitude bins, seasonal quasi-climatology

Tropical data sparse for satellite Model mixing width similar to satellite

with largest difference in northern hemisphere subtropics

Smallest (largest) width in northern mid-latitudes in fall (spring) in GEOS-Chem; mixing widths are also large is summer in ACE-FTS data

M

ixin

g W

idth

[km

]

Latitude [degrees north] Latitude [degrees north]

GEOS-Chem Zonal Mixing Layer Width ACE Zonal Mixing Layer Width

M

ixin

g W

idth

[km

]

Zonal Mixing Layer Widths

GEOS-Chem - ACE Mixing Layer Peak Comparison

ACE-FTS mixing layer found slightly above tropopause; GEOS-Chem mixing layer is below (above) tropopause in subtropics (extratropics)

Mixing layer consistently lower in summer months

M

ixin

g P

eak

[km

from

trop

]

Latitude [degrees north] Latitude [degrees north]

GEOS-Chem Zonal Mixing Layer Peak ACE Zonal Mixing Layer Peak

M

ixin

g P

eak

[km

from

trop

]

Zonal Mixing Layer Peaks

Tropopause

Regional Mixing Layer in GEOS-Chem July 2007 Enhanced mixing over Eastern

Europe and central Asia Most mixing occurs in extratropics,

just poleward of subtropical jet The mixing widths will depend on

the definition of the stratospheric and tropospheric end members, but the spatial distribution and the relative mixing widths will be preserved

Mixing layer is lowest over the eastern Mediterranean

Low mixing layer over eastern Atlantic and central Asia may indicate downward transport from the stratosphere

Mixing Layer Width [km], July 2007

Mixing Layer Peak [km from trop], July 2007

0 7

-3 3

Evidence of Stratospheric Influence Low CO and high O3 located equatorward of the regions with enhanced mixing

widths and in the vicinity of the region with the lowest mixing peak

0 70 100

40 140 -4 2

Meteorological Conditions for July 2007

0 2

-0.15 0.15

Pressure tendencies indicate strong descent over the eastern Mediterranean and southeast of the Caspian Sea where the mixing peak is low These regions of descent are linked to the heating in the Asian monsoon region [Rodwell and Hoskins, 1996]

GEOS-Chem (GEOS-5) potential vorticity (PV) for July 2007 indicates stratospheric PV values (PV > 2 PVU) near these regions of descent (1 PVU = 1.0x10-6 K m2 kg-1 s-1)

Contribution of the stratosphere to total ozone

0 100

0 50

GEOS-Chem total ozone [ppb] in the upper troposphere

GEOS-Chem stratospheric ozone tracer [ppb] in the upper troposphere Ozone from the stratosphere contributes as much as 45% to the total ozone over Turkey and central Asia

Summary There is good agreement between mixing layer widths and peaks estimated

from GEOS-Chem and ACE-FTS data

GEOS-Chem has a smaller mixing layer in the subtropics, but otherwise there is small difference between the two

Larger differences are seen when comparing the location of the peak of the mixing layer with the greatest differences seen at high latitudes

The mixing layer in GEOS-Chem appears smallest (largest) in fall (spring) and lowest with respect to the tropopause in summer

Enhanced region of mixing found near the Mediterranean and central Asia, poleward of the subtropical jet in summer

Omega and PV fields suggest downward and equatorward transport from the stratosphere in this region

TES observes high O3 in the middle and upper troposphere in over central Asia in summer and GEOS-Chem suggests that as much as 45% of this O3 could be from the stratosphere