Wave-critical layer interactionsobserved using GPS data

Bill Randel, NCAR

Overview

• GPS radio occultation temperature measurements

• GPS observations of tropical temperatures

– large and small-scale waves

– coupling to convection and background flow

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! ~200 m

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

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

Randel and Wu, JRG, 2005

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

location and altitudeof convection

deduced from OLR data

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

Model simulation of gravity waves forced by deep convection

Alexander and Holton, 2000

no critical level

criticallevel(U>0)

Residual (small-scale) wave variance

maximumjust belowu=0 line

QBO winds

extension toearly 2006

using CHAMP

variance maximaonly for descending westerlies

QBO winds

Key points:

• Over the equator, small-scale waves have variance maxima

– near the tropopause (why?)– in lower stratosphere, just below U=0 line

(but only for descending westerlies)

Future: COSMIC + EQUARS Soundings in a Day

COSMIC EQUARS Radiosondes

Model simulation of gravity waves forced by deep convection

Alexander and Holton, 2000

Gravity waves interacting with a critical level

criticallevel

Quasi-biennial oscillation (QBO) in temperature

contours:+/- 0.5, 1.5, ...

cold point

result: QBO influence of ~ 0.5 K on tropical tropopause