Using GPS data to study the global tropopause

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Using GPS data to studythe global tropopause
Bill Randel National Center for Atmospheric
Research Boulder, Colorado
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Overview

• What is the tropopause, and why is it
  interesting?
• Structure and circulation of UTLS
• (Upper Troposphere / Lower Stratosphere)
• GPS observations of the tropical tropopause

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Vertical temperature profile
tropopause (tropics)
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Vertical temperature profile
tropical tropopause
midlatitude tropopause
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Latitudinal structure of the tropopause
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Latitudinal structure of the tropopause
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The tropopause is interesting because

• boundary between stratosphere and troposphere
• (air masses with different chemical and
• dynamical characteristics)
• tropical tropopause sets boundary condition for
  air entering the stratosphere (key for water
  vapor)
• relevant for understanding climate variability
  and change (a sensitive indicator of climate
  change?)

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Ozone
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• Air enters the stratosphere in the tropics

January
ozone photochemical source region (tropics)
Brewer-Dobson circulation
maximum ozone due to transport
tropopause
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Seasonal variation of column ozone
winter-spring maximum in both hemispheres
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QJRMS, 1949
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water vapor
very dry stratosphere
moist troposphere
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Stratosphere-troposphere exchange (STE)in
extratropics
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aircraft ozone measurements
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What processes maintain the tropical tropopause?
radiative control
influence of deep convection
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Understanding the Tropical Tropopause Layer (TTL)
Gettelman and Forster, 2002
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Key questions re tropical tropopause layer

• What processes control dehydration and cirrus
  formation? How important are large vs. small
  scale processes?
• What is the importance of convection for the
  thermal and chemical structure of the TTL?
• What is the detailed radiative balance, and how
  might this evolve in a changing climate?

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Comparison of GPS with meteorological analyses
High resolution GPS measurements more
accurately resolve the tropical tropopause
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Sample of GPS tropical temperature profiles
note enhanced variability above 15 km
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Comparison of GPS with radiosondes
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Tropical temp variability studied with GPS data

• Seasonal climatology and annual cycle
• Quasi-biennial oscillation
• Planetary-scale Kelvin waves
• Small-scale waves (inertia-gravity waves)

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tropical data from GPS (10 N 10 S)
obs per month
CHAMP SAC-C
GPS/MET
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Tropical tropopause temperatures
NH winter climatology
deep convection
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Variability of tropopause temperature
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Vertical structure at equator
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high, cold tropopause over South Asian Monsoon
NH summer climatology
deep convection
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Seasonal variation from GPS/MET data
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Amplitude of annual cycle in temperature
strong maximum just above the tropopause (amplitud
e 8 K) Why?
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Response of stratospheric water vapor to annual
cycle in tropopause temperature (tape recorder)
from Mark Schoeberl
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Quasi-biennial oscillation (QBO)
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Comparisons with Singapore radiosonde data
GPS/MET data
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Latitudinal structure of QBO
anomalies centered over equator
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Space-time variability on daily time
scales using CHAMP SAC-C data

• Kelvin waves
• identification
• coupling with zonal winds (QBO)
• forcing by tropical deep convection
• Small scales (gravity waves)

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Equatorial sampling of CHAMP and SAC-C
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Background stratospheric QBO
temperatures
zonal winds
CHAMP SACC
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Simple gridding procedure
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Planetary wave variability (gridded data)
19 km
17 km
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What are Kelvin waves?
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Kelvin waves in the ocean
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Vertical structure
tropopause
eastward phase tilt with height characteristic
of Kelvin waves
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Individual GPS profiles
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A second example
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Variations in wave amplitude
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Daily variations in wave amplitude
u0
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Residual temperature variance (small scales)
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Sample of GPS tropical temperature profiles
note enhanced variability above 15 km
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Gravity waves observed by GPS/MET
maximum in tropics (see Alexander et al., JAS,
2002)
Tsuda et al., JGR, 2000
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Residual (small-scale) wave variance
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Residual variance (small scales)
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Model simulation of gravity waves forced by deep
convection
Alexander and Holton, 2000
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Gravity waves interacting with a critical level
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Gravity waves interacting with a critical level
critical level
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How are tropopause temperatures linked to deep
convection?
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Variations in tropical convectionfrom
OLRmeasurements
Nov
Dec
Jan
Feb
Mar
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Correlation of waves with convection (OLR)
wave variance at 17 km
OLR near Indonesia
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Correlation of GPS temps and OLR near Indonesia
easterly winds in lower stratosphere
convection varies over this region
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Tropopause variations linked to convection
17 km
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January 28, 2002
convection directly over equator
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Correlation of GPS temps and OLR near Indonesia
westerly winds in lower stratosphere
convection varies over this region
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Vertical structure at equator
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Tropical tropopause temperatures
NH winter climatology
deep convection
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Key points

• GPS data allow high resolution view of ubiquitous
  wave variability near tropical tropopause.
• Kelvin waves (and smaller scales) strongly linked
  to stratospheric zonal winds
• Direct evidence for wave forcing from tropical
  deep convection
• Future work
• better understanding of smaller scales
• detailed variability of extratropical tropopause

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Future COSMIC EQUARS Soundings in a Day
COSMIC EQUARS Radiosondes
GPS radio occultation Data will be available in
2006
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annual variation at 17 km
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Stratospheric methane
photochemical loss of CH4
Brewer-Dobson circulation
high CH4 values from troposphere
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Contributions of Anthropogenic and Natural
Forcing to Recent Tropopause Height
ChangesSanter et al.Science, July 2003
observations (NCEP data)
climate model with changing CO2, ozone, aerosols,
...
decrease in tropopause pressure (increase
in altitude)