Introducing VESPA-22: a groundbased microwave spectrometer for
measuring middle atmospheric water
vapour at polar latitudes
27 April 2012
EGU General Assembly 2012
Pietro Paolo Bertagnolio, Giovanni Muscari, Irene Fiorucci
and Massimo Mari
Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
Department of Earth Sciences, University of Siena
Distributed under Creative Commons Attribution 3.0
Our goal
To observe changes in the water vapour
concentration profile in the stratosphere and
mesosphere in the polar regions
Long-term (decadal trends)
Short-term (diurnal cycle)
With a new ground-based microwave
spectrometer to measure the 22.235 GHz
transition of water vapour as part of the
NDACC network
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it
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Outline
• Stratospheric H2O and its
•
•
•
•
•
impact on PSCs
The observational
challenges of the 22-GHz
H2O line
How does the technique
work?
Our instrumental setup
First measured and
calibrated spectra
Conclusions and future
work
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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2/14
Decadal change in stratospheric H2O as yet
not well understood
• Steady rise since 1980
• 10% decrease in 2000
• Influence on surface warming 30%
of GHG
from “Contributions of stratospheric
water vapor to decadal changes in the
rate of global warming.” S. Solomon et
al. – Science - 2010
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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3/14
Impact of H2O increase on Arctic PSC formation
+ 1 ppmv
H2O
from “Quantifying Denitrification and Its Effect on Ozone Recovery”,
Tabazadeh at al. – Science - 2000
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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4/14
The observational challenge
thermo
meso
strato
tropo
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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Balanced Beam-Switching Measurement Technique
𝑻∗𝒛 𝑨𝑹
𝑇𝑡𝑟𝑜𝑝 𝜖𝑡𝑟𝑜𝑝,𝑅
Stratosphere
𝑻∗𝒛 𝑨𝑺
𝑇𝑡𝑟𝑜𝑝 𝜖𝑡𝑟𝑜𝑝,𝑆
𝑻𝒄 𝝐𝒄
Receiver 𝑻𝒓𝒆𝒄
Troposphere
Compensating
Sheet
• 𝑇𝑆 = 𝑇𝑧∗ 𝐴𝑆 𝛼𝑡𝑟𝑜𝑝,𝑆 + 𝑇𝑡𝑟𝑜𝑝 𝜖𝑡𝑟𝑜𝑝,𝑆 + 𝑇𝑟𝑒𝑐
• 𝑇𝑅 = 𝑇𝑧∗ 𝐴𝑅 𝛼𝑡𝑟𝑜𝑝,𝑅 𝛼𝑐 + 𝑇𝑡𝑟𝑜𝑝 𝜖𝑡𝑟𝑜𝑝,𝑅 𝛼𝑐 + 𝑇𝑐 𝜖𝑐 + 𝑇𝑟𝑒𝑐
• 𝑇𝑆 − 𝑇𝑅 = 𝑇𝑧∗ 𝐴𝑆 𝛼𝑡𝑟𝑜𝑝,𝑆 − 𝐴𝑅 𝛼𝑡𝑟𝑜𝑝,𝑅 𝛼𝑐
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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VESPA-22 (water
Vapor Emission
Spectrometer for Polar
Atmospheres at 22
GHz)
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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Parabolic antenna
Half-Power Beam Width (HPBW) = 3.5°
Sidelobes < -40 dB below main lobe
Cross-polarization < -24 dB below main polarization
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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Noise diode calibration
𝑇𝑠 − 𝑇𝑅 = 𝐺 𝑉𝑠 − 𝑉𝑅 =
𝑇𝑁𝐷
𝑉 − 𝑉𝑅
𝑉𝑁𝐷+𝑅 − 𝑉𝑅 𝑠
Cold body (LN2)
Hot body
Calibration sources
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Noise diode calibration
8
x 10
"Raw" Calibration Spectra
Cold Target (77 K)
Hot Target (295 K)
Noise Diode 1 (84 K)
Noise Diode 2 (131 K)
FFT Counts
2
1.5
1
0.5
0
5000
10000
FFT Channels
15000
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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Trec =
312 K
10/14
Brightness Temperature [mK]
Calibrated Spectrum
18-04-2012 13:20-17:20
Effective Integration Time 80'
3000
2000
1000
0
-1000
-2000
22.15
22.2
22.25
Frequency [GHz]
22.3
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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11/14
Water Vapour Emission Line @ 22.235 GHz
18-19/04/2012
Effective Integration Time 3h40'
Brightness Temperature [mK]
300
200
100
0
-100
22.225
22.23
22.235
Frequency [GHz]
22.24
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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22.245
12/14
Conclusions
• Long-term monitoring of polar stratospheric water vapour
•
•
is needed
We designed and built a new 22-GHz spectrometer for
polar observations
We measured the first atmospheric spectra (“first light”)
Future work (now the fun starts…)
• Improve baseline flatness:
– λ/4 wobbler instead of fixed shift
– Delrin compensating sheet
– Front-end optimization
• Improve sensitivity and Trec
– Test single-sideband mixer
• Test with longer integration times from an high-altitude observatory
(Gran Sasso)
• Set up inversion algorithm
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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13/14
References
• Bertagnolio, P. P., Muscari, G., & Baskaradas, J. (2012). Development of a 22
•
•
GHz ground-based spectrometer for middle atmospheric water vapour monitoring.
European Journal of Remote Sensing, 51-61. doi:10.5721/EuJRS20124506
Solomon, S., Rosenlof, K. H., Portmann, R. W., Daniel, J. S., Davis, S. M.,
Sanford, T. J., & Plattner, G.-K. (2010). Contributions of stratospheric water vapor
to decadal changes in the rate of global warming. Science (New York, N.Y.),
327(5970), 1219-23. doi:10.1126/science.1182488
Tabazadeh, A., Santee, M. L., Danilin, M. Y., Pumphrey, H. C., Newman, P. A.,
Hamill, P. J., & Mergenthaler, J. L. (2000). Quantifying Denitrification and Its
Effect on Ozone Recovery. Science, 288(5470), 1407-1411.
doi:10.1126/science.288.5470.1407
Thank you for your attention!
EGU GENERAL ASSEMBLY 2012 – 27/04/2012
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14/14