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.

Comparison of GPS with radiosondes

very good agreementfor wave structures

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

Cold point tropopause temperatures

deepconvection

NH winter climatology

Variability of tropopause temperature

Vertical structure at equator (NH winter)

IndonesiaAfrica South America

‘top’ ofconvectionnote eastward tilt

with height,characteristic of

Kelvin wave

TTL

high, coldtropopause over

South AsianMonsoon

NH summer climatology

deepconvection

Seasonal variation from GPS data

Equator 18 km

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

Equatorial sampling of CHAMP and SAC-C

Simple gridding procedure

Kelvin waves near the tropopause

eastward traveling

Kelvinwaves

Vertical structure

tropopause

eastward phasetilt with heightcharacteristic of Kelvin waves

How are Kelvin waves linked to deep convection?

Variations in tropical

convectionfrom OLR

measurements

Nov

Dec

Jan

Feb

Mar

Correlation of waves with convection (OLR)

wave variance at 16.5 km

OLR near Indonesia

Global-scale Kelvin wave forced by convection

note cold anomaliesabove convection,

as part of large-scalewave structure

modulation ofcold point

Residual temperature variance (small scales)

Sample of GPS tropical temperature profiles

note enhancedvariabilityabove ~15 km

Gravity waves observed by GPS/MET

Tsuda et al., JGR, 2000

maximum intropics

(see Alexanderet al.,JAS,2002)

Residual (small-scale) wave variance

maximumnear

tropopause

Residual (small-scale) wave variance

maximumjust belowu=0 line

QBO winds

Gravity waves interacting with a critical level

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)

Future: COSMIC + EQUARS Soundings in a Day

COSMIC EQUARS Radiosondes

high, coldtropopause over

South AsianMonsoon

NH summer climatology

deepconvection

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)

Persistent cirrus clouds over monsoon region

(HIRDLS measurements)cold

tropopause

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

Model simulation of gravity waves forced by deep convection

Alexander and Holton, 2000

Gravity waves interacting with a critical level

criticallevel

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

Understanding the Tropical Tropopause Layer (TTL)

Gettelman and Forster, 2002

Using GPS data to study the tropical tropopause

Bill Randel, NCAR

Vertical section through anticyclone (60-120 E)

warmtroposphere

cold lowerstratosphere

tropopause

deep convection

zonal winds

Background: stratospheric QBO

temperatures

CHAMP + SACC

Seasonal variation from GPS/MET data