Introducing VESPA-22: a ground-based microwave spectrometer for

measuring middle atmospheric water vapour at polar latitudes

27 April 2012EGU General Assembly 2012

Pietro Paolo Bertagnolio, Giovanni Muscari, Irene Fiorucci and Massimo Mari

Istituto Nazionale di Geofisica e Vulcanologia, Rome, ItalyDepartment of Earth Sciences, University of Siena

Distributed under Creative Commons Attribution 3.0

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it2/14

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 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it3/14

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 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it4/14

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 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it5/14

Impact of H2O increase on Arctic PSC formation

from “Quantifying Denitrification and Its Effect on Ozone Recovery”, Tabazadeh at al. – Science - 2000

+ 1 ppmvH2O

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it6/14

The observational challenge

tropo

strato

meso

thermo

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it7/14

Balanced Beam-Switching Measurement Technique

Troposphere

CompensatingSheet

Receiver

Stratosphere

Signal beam

𝑻 𝒛∗𝑨𝑺

𝑇 𝑡𝑟𝑜𝑝𝜖𝑡𝑟𝑜𝑝,𝑆

Referencebeam

𝑻 𝒛∗𝑨𝑹

𝑇 𝑡𝑟𝑜𝑝𝜖𝑡𝑟𝑜𝑝 ,𝑅

𝑻 𝒄𝝐𝒄

𝑇 𝑆−𝑇 𝑅=𝑇 𝑧∗ (𝐴𝑆𝛼𝑡𝑟𝑜𝑝 ,𝑆−𝐴𝑅𝛼 𝑡𝑟𝑜𝑝 ,𝑅𝛼𝑐 )

•

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it8/14

VESPA-22 (water Vapor Emission Spectrometer for Polar Atmospheres at 22 GHz)

Parabolic mirror

Choppermirror

Quarter-wavelength shift

Receiver

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it9/14

Parabolic antenna

Half-Power Beam Width (HPBW) = 3.5°Sidelobes < -40 dB below main lobeCross-polarization < -24 dB below main polarization

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it10/14

Noise diode calibration

Cold body (LN2) Calibration sources

Hot body𝑇 𝑠−𝑇 𝑅=𝐺 (𝑉 𝑠−𝑉 𝑅 )=𝑇𝑁𝐷

𝑉 𝑁𝐷+𝑅−𝑉 𝑅(𝑉 𝑠−𝑉 𝑅)

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it11/14

Noise diode calibration

0 5000 10000 150000.5

1

1.5

2

x 108

FFT Channels

FFT

Cou

nts

"Raw" Calibration Spectra

Cold Target (77 K)Hot Target (295 K)Noise Diode 1 (84 K)Noise Diode 2 (131 K)

Trec = 312 K

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it12/14

22.15 22.2 22.25 22.3-2000

-1000

0

1000

2000

3000

Frequency [GHz]

Brig

htne

ss T

empe

ratu

re [m

K]

Calibrated Spectrum18-04-2012 13:20-17:20

Effective Integration Time 80'

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it13/14

22.225 22.23 22.235 22.24 22.245-100

0

100

200

300

Frequency [GHz]

Brig

htne

ss T

empe

ratu

re [m

K]

Water Vapour Emission Line @ 22.235 GHz18-19/04/2012

Effective Integration Time 3h40'

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it14/14

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

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”)

EGU GENERAL ASSEMBLY 2012 – 27/04/2012PIETRO PAOLO BERTAGNOLIO – pietropaolo.bertagnolio@ingv.it15/14

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!