Long time series of GPS-derived Integrated Precipitable Water ...

Long time series of GPS-derived Integrated Precipitable Water ...

GPS derived Integrated Water Vapour content and its relationship with 6 years
of surface radiation balance at MZS (Terra Nova Bay)
P. Sarti*, M. Negusini*, C. Lanconelli^, A. Lupi^, C. Tomasi^

*Istituto di Radioastronomia, Istituto Nazionale di Astrofisica, Bologna, Italy
^Istituto di Scienze dellAtmosfera e del Clima Consiglio Nazionale delle Ricerche, Bologna, italy

Abstract. A wide variety of monitoring networks are nowadays working in polar regions. GNSS-GPS networks are constantly increasing the number of observing sites and most of the times GPS systems are co-located with sensors used for investigating other
scientific fields. A full exploitation of GPS networks can therefore be achieved through interdisciplinary studies; a very rewarding exchange of information is realized when applying GPS derived IWV to atmospheric and climate investigation. In particular, at Mario
Zucchelli Station a permanent GPS system is observing since 1998 and it is co-located with two radiometers that started operations in 2000. The aim of this study is to investigate the relationship between the radiation fluxes and the GPS-derived atmospheric integrated
water vapour (PW). Surface downwelling longwave (maximum at 10 m) and shortwave (maximum at 0.5 m) radiation fluxes for clear sky conditions were isolated from a 6 years dataset, spanning December and January austral summer months. These measurements
were collected with a CNR1-Kipp&Zonen net radiometer. Additional information provided by GPS technique unable us to evaluate the role of PW on the downwelling longwave clear sky flux. Inter-annual variability of the IWV and its radiative contribution on the local
climate are investigated. The measured radiation is also compared to the output values obtained through a Radiative Transfer Code (SBDART, Ricchiazzi et al. 1998), forcing their runs with measured surface and atmospheric profiles of meteorological parameters (T, p,

2. GPS data analysis: The GPS data set spanning six years, from 2000 to 2005,
1. Introduction: Human-induced variations on aerosols and greenhouse gases


appear to have a significant impact on climate system. For a variety of different
scenarios, numerical models predict worrying climate change in the near future. With
little uncertainty, within the end of this century, temperature rise will deeply influence
the whole Earths ecosystem. One important role in climate change is played by water
vapour: it is the strongest greenhouse gas and its content variations in the low and
middle troposphere are expected to originate positive feedback process. It is important
to estimate its concentration as continuously and densely as possible. Water vapour
observations performed in the Antarctic are particularly important: they supply
information for depicting the global distribution and circulation of water vapour and its
relation with global warming. In the short history of Antarctic science, time series of
water vapour records have been obtained using radiosonde observations. These
latter are sparse and, often, difficult to perform on a regular basis. Furthermore, an
uneven distribution of observations as well as the use of different sensors remarkably
complicates the assessments of water vapour trends. Space geodetic techniques
using radio signals, in particular Very Long Baseline Interferometry (VLBI) and Global
Positioning System (GPS) have proved to be effective tools for sounding the
atmosphere and e.g. inferring about integrated water vapour content (Bevis et al.
1992, Niell et al. 2001). GPS observations alone do not allow monitoring the complex
impact of water vapour on climate system: its vertical distribution and its radiative
forcing effects cannot be determined. They should rather be regarded as an ancillary,
although precise, source of information in climatology, as well as in meteorology and

was processed with Bernese software version 5.0 (Dach et al. 2006) with the aim of
estimating a long time series of Zenith Total Delays (ZTD) and subsequently compute values
of Integrated Precipitable Water Vapour. TNB1 coordinates and velocities were estimated
using a geodetic network formed by a selection of Antarctic and peri-Antarctic GPS stations,
and they were fixed in the following step of the analysis in order to compute ZTD corrections
every hour. The Zenith Hydrostatic Delay (ZHD) has been computed using different models:
Hopfield (1969) and Saastamoinen (1972), both in its original and modified form.
Atmospheric parameters values were recorded at Eneide, pressure readings were corrected
by -5.2 mbar in order to take into account the height difference between the GPS and the
meteorological site. Zenith Wet Delay (ZWD) has been computed using both Hopfield and
Saastamoinen (modified) models. During more dry winter season, it sometimes happens that
ZHD values determined with both models are higher than the corresponding ZTD estimated
values, this leads to negative ZWD values and this behaviour must be further investigated.
The Precipitable Water (PW) has been computed from ZWD applying the mean
temperature formula Tm=0.78T0+46.15 derived by the radiosoundings performed at Terra
Nova Bay during Antarctic summers according to Bevis et al. (1992), were T0 represents the
surface temperature. GPS capability of retrieving atmospheric PW can be evaluated through
a comparison with PW content computed with radiosounding data. This comparison can be
done over the two months period spanning December 2002 and January 2003. The
agreement of the two series is very good, with small discrepancies only exceeding the 1 mm
level over a few points of the series. The hourly GPS-derived PW time series has a higher
resolution than the radiosounding series (12 vs. 1 value, respectively) and represents an
efficient method for evaluating the total content of water vapour in the atmosphere, its
variability as well as the presence of water vapour when no radiosoundings are performed
(this is always the case for the closing season of Mario Zucchelli Station).

3. Measuraments of Radiant Flux density:

Longwave and shortwave radiation flux measurements were regularly being performed during austral summer periods from 1999 to
2004, at the Clean Air Facility of Icaro Camp (MZS) to define the upwelling and downwelling terms of the radiation balance. These measurements aim at determining the surface radiative effects of aerosols and
clouds, these latter being dependent mainly on cloud coverage and cloud type characteristics. The four radiation balance terms were obtained by using a Kipp&Zonen CNR-1, located at 2.5 m above the ground,
in order to achieve an overall view of the sky with a 2 sr field-of-view. Observations performed with the four sensors of CNR-1 were separately sampled in steps of 3 s and their average, computed over 60 s,
stored along with their standard deviations. The long-wave downwelling radiant-flux density measurements taken from November 22, 2003 to February 5, 2004 are shown in Figure 5 together with the
simultaneous measurements of surface temperature. The time patterns of radiant-flux density turn out to range between 170 and more than 300 W m-2, being the lower values associated with clear sky
conditions, whereas the rest of such data were taken for cloudy sky conditions. In particular, the higher values are related to the presence of thicker and lower clouds having higher emission temperature. On the
other hand, the presence of thin cirrus clouds is expected to cause lower radiant-flux density values not too different from those measured for clear sky conditions. The comparison between the radiant-flux density
and the surface temperature time patterns gives evidence of the fact that the clear sky radiant-flux density varies in time in a similar manner to surface temperature. In order to relate the GPS-derived PW with the
measurements of performed at Terra Nova Bay, only the data associated with clear sky conditions were selected. This was achieved using the Long and Ackerman (2000) automatic method based on four tests
made examining the simultaneous measurements of global and diffuse components of incoming short-wave radiation at the ground. The selected clear sky data are presented in Figure 6b.

L a T0 4

Figure 2: The long time series of Zenith Total Delay; the series spans a
six-year period. The observations were being performed with the GPS
permanent system (TNB1) installed at Mario Zucchelli Station.

T0 surface temperature
Tm vertical average

Figure 3: Long time series of Precipitable Water values derived by GPS.
The annual water vapour content variation is evident: dryer conditions are
determined during the Antarctic winters.

The analysis of a six-year set of continuous GPS observations was performed aiming at
sensing the water vapour content and its variations with time in the area of Terra Nova Bay. GPS capabilities of
retrieving Precipitable Water vapour was confirmed, despite the very low humidity that characterize the Antarctic
troposphere, especially during the winter period. A validation of PW content was achieved through a comparison with
PW computed using 108 radiosonde profiles. The test was performed over a two-month period (December 2002
January 2003), paying a careful attention to both the processing of GPS and radiosonde data. The results fully confirm
that GPS observations of PW at Terra Nova Bay can be considered as a useful ancillary information for atmospheric
studies. To this respect, with the purpose of relating the PW measured with GPS to the radiative properties of the
atmosphere at Terra Nova Bay, more accurate studies are planned in analyzing more extended sets of GPS and
radiosounding observations through more advanced procedures taking into account the day-to-day variations in the
thermal and moisture conditions of the atmosphere that can be observed at Terra Nova Bay in different seasonal
periods. In addition, the analysis of the measurements of downwelling radiant flux density performed routinely at Campo
Icaro with a Kipp&Zonen CNR-1 net radiometer showed that an important influence is exerted by atmospheric water
vapour on the thermal energy balance of the atmosphere also at Antarctic sites, suggesting that a significant
contribution to the knowledge of the radiative transfer of thermal atmospheric radiation can be provided by relating these
longwave radiation measurements to simultaneous measurements of Precipitable Water.

1. This research was carried out in the framework of the Programma Nazionale di Ricerche
in Antartide and financially supported by PNRA S.C.r.l. The authors would like to thank
Dr. Andrea Pellegrini and all his colleagues for their fundamental work with atmospheric
weather stations and radiosoundings.

Figure 6: (a) Scatter plot of the incoming radiant flux density and the corresponding
surface temperature over the whole five-year period from 2000 to 2004; (b) Clear sky
data scatter plot obtained applying the Long and Ackerman (2000) method to the fiveyear data set shown in (a).

Figure 4: Comparison between the time patterns of
PW obtained from the GPS observations and those
derived from the Vaisala RS-80A radiosonde data.

4. Conclusions:


Figure 5: Time patterns of the downwelling radiant flux
density (upper part) and surface temperature (lower
part) measured at Icaro Camp during the period from
November 22, 2003 to February 5, 2004. The
downwelling radiant flux can be related to the surface
temperature introducing the concept of apparent
emittance a as shown in the box.

Figure 1: Overview of the area nearby Mario
Zucchelli Station. The location of the permanent GPS
station TNB1 is marked with a red triangle on the
upper left part of the picture. On the centre-right part
of the picture, the locations of the Automatic Weather
Station Eneide and the radiosoundings station are
identified by a blue circle and a black square,
respectively. The distances between the three
locations does not exceed a few hundreds meters;
the three observing sites and their sensors can
therefore be regarded as co-located.

Figure 7: Plot of the 15-minute average values of the clear sky Bibliography
1. Bevis M, Businger S, Herring TA, Rocken C, Anthes RA, Ware RH (1992) "GPS meteorology:
effctive emittance measured during five years (2000-2004) versus
remote sensing of atmospehric water vapour using the Global positioning system", JGR, 97, D14,
GPS-derived values of PW, compared with the values computed
2. Bevis M, Businger S, Chiswell S, Herring TA, Anthes RA, Rocken C, Ware RH (1994) "GPS
applying the SBDART code (Ricchiazzi et al. 1998) to 5 monthly
meteorology: Mapping Zenith Wet Delays onto Precipitable Water", Am Met Soc, 33, 379-386
mean atmospheric models (open circles), and the 5 variation of 3. Dach R., U. Hugentobler, P. Fridez and M. Meindl (2006), Bernese GPS Software Version 5.0,
Astronomical Institute of University of Berne, 612 pp.
December atmospheric models (solid circles).

4. Hopfield H.S., (1969) - Two-quadratic tropospheric refractivity profile for correcting satellite dataJournal of Geophysical Research, 74, pp. 4487-4499
5. Long C.N. and T. P. Ackerman (2000), Identification of clear skies from broadband pyranometer
measurements and calculation of downwelling shortwave cloud effects, J. Geophys. Res.,
105(D12), 15609-15626.
6. Niell AE, Coster AJ, Solheim FS, Mendes VB, Toor PC, Langley RB, Upham CA (2001)
"Comparison of measurements of atmospheric wet delay by radiosonde, water vapour radiometer,
GPS and VLBI" J Atm and Oceanic Tech, 18, 830-850
7. Ricchiazzi P, S. Yang, C. Gautier and D Sowle (1998), SBDART: A research and teaching
software tool for plane-parallel radiative transfer in the Earths atmosphere, Bull. Am. Meteor. Soc.,
79(10), 2101-2114.
8. Saastamoinen I.I. (1973) - Contribution to the theory of atmospheric refraction - Bulletin
Godsique, 107, pp. 13-34

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