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| Titel |
High time resolution observations of the polar stratosphere and mesosphere using a ground-based 230-250 GHz microwave radiometer |
| VerfasserIn |
D. A. Newnham, P. J. Espy, M. A. Clilverd, D. J. Maxfield, P. Hartogh, K. Holmén, S. Blindheim, R. B. Horne |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063451
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| Zusammenfassung |
| Microwave radiometry is used to measure thermal emission by the Doppler- and
pressure-broadened molecular rotational lines of atmospheric gases, from which vertical
abundance profiles can be determined. Since solar radiation is not required for the
measurement, the technique has the advantage that continuous observations are possible
including throughout the polar winter.
We describe the development of a passive microwave radiometer [Espy, P. J., P. Hartogh,
and K. Holmen (2006), Proc. SPIE, 6362, 63620P, doi:10.1117/12.688953] for ground-based
remote sensing of the polar middle atmosphere. The instrument measures nitric oxide (NO),
ozone (O3), and carbon monoxide (CO) vertical profiles over the altitude range 35-90 km
with time resolution as high as 15 minutes, allowing the diurnal variability of trace chemical
species to be investigated. Heterodyne detection of atmospheric emission at 230 GHz and 250
GHz (wavelength ~1.25 mm) with a receiver noise temperature of 300 K is achieved using a
superconductor-insulator-superconductor (SIS) mixer cooled to 4 K. The down-converted
signals at 1.35 GHz and 2.10 GHz are analysed using both a moderate-resolution (28
kHz, 220 MHz bandwidth) and a high-resolution (14 kHz, 40 MHz bandwidth)
chirp-transform spectrometer (CTS). The instrument was operated semi-autonomously
at Troll station (72Ë 01’S 02Ë 32’E, 1270 m above sea level), Antarctica during
2008-10 and at the Arctic LIDAR Observatory for Middle Atmosphere Research
(ALOMAR, 69Ë 16’N, 16Ë 00’E, 380 m above sea level), northern Norway during
2011-12.
NO volume mixing ratio (VMR) profiles have been inverted from calibrated
brightness temperature spectra of the NO line centred at 250.796 GHz, observed
above Troll station, using the Microwave Observation Line Estimation and Retrieval
(MOLIERE) version 5 code. A priori pressure, temperature, ozone, water vapour, and NO
profiles above 30 km were calculated using the Sodankylä Ion and Neutral Chemistry
(SIC, version 6.8) model under geomagnetically quiet conditions. For altitudes
up to 30 km MIPAS/Envisat (Michelson Interferometer for Passive Atmospheric
Sounding) data were combined with 10-year (1999–2008) averages of ozonesonde
data from Neumayer station (70Ë 39’S, 08Ë 15’W). Reference data for radiative
transfer calculations are from HITRAN 2008. The retrieval is constrained with
measured CTS channel response curves. Data inversion was performed from the
ground to 120 km with NO and water vapour profiles adjusted in the forward model
calculations to provide the best fit to the observed brightness temperatures. The area of
the normalized averaging kernels is -¥0.5 for atmospheric layers between 35–83
km, indicating good information retrieval over this altitude range, although the
measurements contribute to the retrieved NO VMR profiles up to at least 90 km. The
vertical resolution is estimated from the width of the averaging kernels to be 8 km. |
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