infrasound technology ws – bermuda, november 6 th, 2008 1 microbarom signals recorded in...

Infrasound Technology WS – Bermuda, November 6 th, 2008 1

Microbarom signals recordedin Antarctica - a measure for

sudden stratospheric warming?

L. Ceranna, A. Le Pichon, E. Blanc & L. Evers

BGR / B3.11, Hannover, Germany

KNMI, DeBilt, The Netherlands

CEA / DASE, Bruyères-le-Châtel, France


Content

The Antarctic Infrasound Array I27DE

mb-Signal Detections at I27DE

Causes and Effects

Conclusions

• siting and design

• microbarom signals recorded from Feb-2003 to Oct-2008

• background noise level and prevailing winds• variability of the stratospheric wave duct• sudden stratospheric warming (SSW) and its signature


Siting and Design of the Infrasound Station I27DE

5°W10°W

71°S

72°S

satellite image of the Ekström Ice Shelf

at the German Neumayer Research Base, Antarctica

• nine element infrasound array• 2 km aperture• optimum detection conditions for mb-signals (Tdom=5 s)• noise reduction by snow coverage• continuous operation since Feb-2003


Track of the Ocean Swell around Antarctica

• averaged 990 hPa isobar from Jun-1979 to Feb-2000 [Simmonds et al. 2003]• considered as path of the clockwise moving ocean swell around Antartica• predicted direction of continuous mb-signals throughout the years

HWM-93 radial stratospheric wind speed from source to I27DE [Hedin et al. 1996]

Mar

- Nov

Dec - Feb


Detections of mb-Signals at I27DE, Feb-2003 – Oct-2008

Tdur > 100 sΣ130,500~63 / day

• station azimuth follows to first order well the variation predicted by HWM-93• trace velocities (VT) of detections are in accordance with typical range for Is phases• Austral winter 2006 is an exception - strong decrease in number of detections - strong increase in VT


Causes and Effects for anomalous mb-Signal Detection

• although no comparable observations at I55US (Windless Bight, Antarctica) not an artifact, since array configuration remained unchanged - anomaly is only limited to Austral winter 2006

• absence of a stratospheric duct, i.e. It instead of the usually prevailing Is phases - It shows higher trace velocities compared to Is - It is subjected to strong attenuation at 5 s period

• increased background noise level due to increased prevailing wind speeds because wind speeds above 10 m/s strongly affect the detection capability - wind speed < 10 m/s (72 %); wind speed < 15 m/s (84 %)

• sudden stratospheric warming (SSW) events are known as a potential source for reduced detection capability of infrasound stations [Evers & Haak, 2005] - more frequently in northern hemisphere (minor & major events) - SSW are associated with a slowing down of the polar vortex leading to a decrease and sometimes to a reversal in zonal wind speed [Hoffmann et al., 2007] - in 2002 a major SSW event observed in Antarctica [Varotsos, 2003]


mb-Signal Detection in the Periods from 01-Mar to 31-Oct

Σ 20,000> 10 m/s (29 %)no wind: ~28,000

Σ 6,000> 10 m/s (38 %)no wind: ~10,000

Σ 18,000> 10 m/s (32 %)no wind: ~26,000

high wind speed (increased background noise level)explains gaps in signal detection, however, it is NOT

causing the reduced number of detections duringAustral winter 2006


ECMWF Temperature and Wind Speed Profiles

ECMWF profiles (61/91 levels) ~ [0 70] km

• between 75°S and 55°S every 1°• between 80°W and 55°E every 1°• from 01-Mar to 31-Oct every 24 h• path length: 2700 → 1000 → 1800 km

sound speed radial wind speed

source

I27DE


Effective Sound Speed Ratios: ECMWF, HWM-93

ECMWFaveraged along path

HWM-93averaged along path

Veff=sound speed + radial wind speed Veff-ratio=max{Veffstrat}/Veff

surf

• stratospheric duct always exists, even in 2006: excluding It observations

• note: high Veff ratio is essential precondition for measuring high VT

duct

no duct

duct

no duct


Effective Sound Speed Ratios: ECMWF, 61/91 levels

ECMWFat the source

ECMWFat the station

Veff=sound speed + radial wind speed Veff-ratio=max{Veffstrat}/max{Veff

tropo}


Effective Sound Speed Profiles, averaged along PathM

SIS

E/H

WM

-93

smooth gradients

?

?

?

?

? ?

?

• station azimuth > 0° during Austral winter are in contradiction to HWM-93 prediction, therefore, no stratospheric duct• SSW occur along with reversal in stratospheric wind direction• observed in 2005, 2006, and 2007, however, most pronounced in 2006


Effective Sound Speed Profiles, averaged along Path (cont.)E

CM

WF

• clear events showing increase of Veff in upper tropopause / lower stratosphere• high Veff values in the upper stratosphere, however, in 2006 lower than in 2005 and 2007 being in accordance with stratospheric cooling at that time• during Austral winter 2005 to 2007 SSW events occurred and and can be correlated to observations• Veff ratio between troposphere and stratosphere instead of ground to stratosphere


Examples of Wave Propagation in 2006

I27DE

11-Jul-2006β=-54.5°VT=370 m/s

11-Jul-2006β=-38.8°VT=361 m/s

15-Sep-2006β=-86.1°VT=366 m/s

Is

Is

IsIwIs


Examples of Wave Propagation in 2006 (cont.)

06-Mar-2006 β= -68.7°VT=NaN

08-Mar-2006β= -65.5°VT=NaN

- no rays -

Veff in troposphere ≈ Veff in stratosphere

Veff in troposphere {source} > Veff in stratosphere {receiver}

I27DE


I27DE’s siting and design are well suited for measuring mb-signals. Moreover, I27DE has been operated for more than 5 years without any major problems.

Generally, detection capability is reduced due to increased background noise level caused by prevailing wind speeds above 10 m/s.

During Austral winter 2006 an anomaly was observed for mb-signals, the number of detections was strongly reduced while trace velocities shows unusual (for Is) high values.

- The high trace velocities VT might be caused by high effective sound speeds Veff in the troposphere along with relatively low values in the stratosphere (in 2006 stratospheric cooling was observed).

- The reduced number might be explained by SSW events. Although such events were also observed in 2005 and 2007 (β > 0°), they were most prominent in 2006.

Detection of mb-signals is a measure for SSW events.

Conclusions

infrasound technology ws – bermuda, november 6 th, 2008 1 microbarom signals recorded in...

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