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| Titel |
Influence of changes in humidity on dry temperature in GPS RO climatologies |
| VerfasserIn |
J. Danzer, U. Foelsche, B. Scherllin-Pirscher, M. Schwarz |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 7, no. 9 ; Nr. 7, no. 9 (2014-09-09), S.2883-2896 |
| Datensatznummer |
250115893
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| Publikation (Nr.) |
 copernicus.org/amt-7-2883-2014.pdf |
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| Zusammenfassung |
Radio occultation (RO) data are increasingly used in climate
research. Accurate phase (change) measurements of Global Positioning
System (GPS) signals are the basis for the retrieval of near-vertical profiles of bending angle, microwave refractivity, density,
pressure, and temperature. If temperature is calculated from
observed refractivity with the assumption that water vapor is zero,
the product is called "dry temperature", which is commonly used to
study earth's atmosphere, e.g., when analyzing temperature
trends due to global warming. Dry temperature is a useful quantity,
since it does not need additional background information in its
retrieval. However, it can only be safely used as proxy for physical
temperature, where moisture is negligible. The altitude region above
which water vapor does not play a dominant role anymore, depends
primarily on latitude and season.
In this study we first investigated the influence of water vapor on
dry temperature RO profiles. Hence, we analyzed the maximum altitude
down to which monthly mean dry temperature profiles can be regarded
as being equivalent to physical temperature. This was done by
examining dry temperature to physical temperature differences of
monthly mean analysis fields from the European Centre for
Medium-Range Weather Forecasts (ECMWF), studied from 2006 until
2010. We introduced cutoff criteria, where maximum temperature
differences of −0.1, −0.05, and
−0.02 K were allowed (dry temperature is always lower than
physical temperature), and computed the corresponding altitudes. As
an example, a temperature difference of −0.05 K in the
tropics was found at an altitude of about 14 km, while at
higher northern latitudes in winter it was found at an altitude of
about 9–10 km, in summer at about
11 km.
Furthermore, regarding climate change, we expect an increase of
absolute humidity in the atmosphere. This possible trend in water
vapor could yield a wrongly interpreted dry temperature trend. As
a consequence, we performed a model study, investigating the
increase in height of the transition region between moist and dry
air. We used data from the fifth phase of the Coupled Model
Intercomparison Project (CMIP5), analyzing again monthly mean dry
temperature to physical temperature differences, now from the years
2006 to 2050. We used the highest emission scenario RCP8.5
(representative concentration pathway), studying all available
models of the CMIP5 project, analyzing one internal run per model,
with the goal to identify the altitude region where trends in dry
temperature can be safely regarded as reflecting trends in physical
temperature. From all models we therefore choose a selection of
models ("max 8" CMIP5 models), which showed the largest trend
differences. As a result, our trend study suggests that the lower
boundary of the region where dry temperature is essentially equal to
physical temperature rises about 150 m decade−1. |
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