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Using GPS data to study the tropical tropopause Bill Randel National Center for Atmospheric Research...
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Transcript of Using GPS data to study the tropical tropopause Bill Randel National Center for Atmospheric Research...
Using GPS data to studythe tropical tropopause
Bill RandelNational Center for Atmospheric Research
Boulder, Colorado
“You can observe a lot by just watching” (Yogi Berra)
Overview
• GPS radio occultation temperature measurements
• GPS observations of the tropical tropopause:
– low frequency variability (seasonal cycle, QBO)
– large and small-scale waves
GPS Radio Occultation
Occulting LEO
Occulting GPS
20 msecdata
Ionosphere
Neutral atmosphere
Earth
(LINK 1)
Basic measurement principle:
Deduce atmospheric properties based on precise measurement of phase delay and amplitude.
* high vertical resolution! ~100 m
Availability of GPS data:
• GPS/MET (1995-1997)
• CHAMP (2001-present)
• SAC-C (2001-2002)
• COSMIC (launched April 2006) (6 satellites)
number of tropical profilesper month (20 N – S)
each LEO satellite ~ 100-200 occultations/day
Sample of GPS tropical temperature profiles
Temperature profiles are characterized by high variability (planetary waves, gravity waves), closely linked to convection. GPS data offer a new tool to understand this variability.
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)
references: Randel et al., JGR, 2004 Randel and Wu, JGR, 2005
Vertical structure at equator (NH winter)
IndonesiaAfrica South America
‘top’ ofconvectionnote eastward tilt
with height,characteristic of
Kelvin wave
TTL
Amplitude of annual cycle in temperature
strong maximumjust above the
tropopause(~8 K )
Why?
cold point
Amplitude of annual cycle in temperature
strong maximumjust above the
tropopause(~8 K )
Why?
thermodynamic balance
smalllong radiative time scale
in lower stratosphere
hence, amplified T response
Quasi-biennial oscillation (QBO) in temperature
contours:+/- 0.5, 1.5, ...
cold point
result: QBO influence of ~ 0.5 K on tropical tropopause
Recent cooling of tropical tropopause echoed instratospheric water vapor decreases
stratosphericwater vaporfrom HALOE
satellite
tropicaltropopause
temperatures
r=.72
Randel et al, JGR, 2006
Space-time variability on daily time scalesusing CHAMP + SAC-C data
• Kelvin waves
– identification– forcing by tropical deep convection
• Small scales (gravity waves)
– coupling with background winds
Global-scale Kelvin wave forced by convection
note cold anomaliesabove convection,
as part of large-scalewave structure
modulation ofcold point
Gravity waves observed by GPS/MET
Tsuda et al., JGR, 2000
maximum intropics
(see Alexanderet al.,JAS,2002)
Key points:
• GPS data allow high resolution view of ubiquitous wave variability near tropical tropopause.
• Kelvin waves (and smaller scales) strongly linked to tropical deep convection. Global-scale dynamical
response in TTL, with cooling near tropopause overconvection.
• Maximum wave variance near tropopause (why?). Waves are coupled to background winds (QBO)
• Future: COSMIC (6 more satellites) EQUARS (equatorial orbit)
Circulation of the South Asian summer monsoon
deep convection
monsooncirculationnear 15 km
- - - cross sectioncold lower stratospherehigh, cold tropopause
warm troposphere
winds
tropopause
16 km cloudsfrom HIRDLS
100 hPa relativehumidity from MLS
Persistent high clouds over monsoon region
observed100 hPa
circulation
(zonal mean
removed)
response tolow frequency
tropical heating(Gill, 1980)
Correlation of GPS temps and OLR near Indonesia
convection variesover this region
easterly winds in lower stratospheretropopause
TTL
convection variesover this region
westerly winds in lower
stratosphere
(waves do notpropagatevertically)
Correlation of GPS temps and OLR near Indonesia
tropopause TTL
Comparison of near-coincident CHAMP and SAC-C retrievals
Hajj et al., JGR, 2004
mean bias
std. dev. ~ uncertainty of single measurement
from Pan et al., JGR, 2004
zonal wind(from analysis)
tropopausefrom aircraft
profilermeasurements
potential vorticity
(from analysis)
double tropopause associated with break near subtropical jet
poleequator
Vertical section through anticyclone (60-120 E)
warmtroposphere
cold lowerstratosphere
tropopause
deep convection