* CRONOLOGÍA DE LA ÉPOCA PRIMITIVA

The radiosonde technique is able to deliver profiles of meteorological parameters with a high accuracy and vertical resolution using a sensor attached to an automatic radio-sounding balloon. Data collected by the sensor is transmitted via radio link back to a ground receiving station. Accurate observations of pressure, temperature and relative humidity can be detected at increasing altitudes from the ground up to approximately 30 km. Because of the high vertical resolution of this direct measurement method, radiosonde is a very important observational tool that can be assimilated with forecast models and also used for evaluation studies of WV retrieval using ground and remote sensing techniques. Radiosonde WV measurements are also used in the context of climate research, including studies of trends in upper tropospheric WV, and stratospheric dehydration and tropospheric- stratospheric exchange processes (Miloshevich et al., 2006 and Zhang et al., 2011).

Figure 3.4 presents the current Australian radiosonde network as well as the sites located in the greater Victorian region. The limitation of radiosonde is its restricted coverage and low spatial and temporal resolution of data due to the high maintenance and cost to keep a site operational. Many

regions including polar, mountains, desert and unpopulated areas around the globe experience similar problems with ground-based observation networks (Kuo et al., 2004 and Fu, 2011). Figure 3.4 also depicts the sparse distribution of radiosonde stations nationally, especially in the centre of Australia where the region is dominated by desert. A bias exists for stations located towards the coastline where most of the population reside. Only 3 radiosonde stations were operational in the greater Victorian region – located in Melbourne (MELB), Wagga Wagga (WAWA) and Mt Gambier (MTGA). These sites were used in this research as truth data for a validation study of three co-located GPS stations. A poor temporal resolution is also a major limitation, with two radiosonde observations per day, typically occurring at 11:00 and 23:00 (UTC).

Figure 3.4: Distribution of the Australian radiosonde network (left) and, radiosonde sites in the greater Victorian region (right).

Table 3.1 presents the associated accuracies at increasing altitudes of each meteorological parameter detected using radiosonde. The radiosonde technique is highly accurate for observations of the troposphere with a 1 – 2 hPa accuracy for pressure, 0.5°C accuracy for temperature and 5% accuracy for relative humidity.

Table 3.1: Radiosonde measurement accuracies (National Oceanic and Atmospheric Administration, 1997)

Variable Range Accuracy

Pressure ~1000 hPa (surface) – 100 hPa 1 – 2 hPa

100 – 10 hPa 2 %

Temperature ~1000 hPa (surface) – 100 hPa 0.5 °C

100 – 10 hPa 1 °C

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As mentioned previously, radiosonde profiles from stations within the greater Victorian region were used in this thesis as truth data for evaluation purposes due to high accuracy and vertical resolution. This profile data will also be tested as an additional point observation input for the observation model of the GPS tomography processing (see Chapter 7).

3.3.2. Ground-based synoptic weather observation network

The synoptic weather observation station network within Victoria consisted of 69 operational stations (Figure 3.5). Data from these automatic weather stations are provided by the Australian Bureau of Meteorology, with a temporal resolution of 3 hours. The synoptic network provides pressure (hPa), temperature (K) and partial WV pressure (hPa) parameters.

Figure 3.5: Distribution of the Victorian synoptic weather station network.

These observations, when co-located with the GPS receiver, can provide sufficient ground meteorological information to eliminate the hydrostatic delay from the GPS path delay to extract the wet delay component relating to WV. In theory, it would be preferable to have every CORS GPS station equipped with a synoptic meteorological sensor, however, in Australia and more relevantly Victoria, this is far from the case. During the Jan 2011 campaign (see Section 3.5) only 20% of CORS GPS receivers had a co-located synoptic station within a 20 km radius. For the PWV estimates at each station and the tomographic modelling procedures (Chapter 6 and 7) accurate meteorological values of temperature, pressure and relative humidity were needed at the location of every GPSnet station in order to estimate SWD. Therefore, an interpolation method is adopted using grid data from the Australian NWP model ACCESS-R (see Section 4.3).

The synoptic data however, was used for the Saastamoinen modelling of hydrostatic delay (Eq. (2.28)) in the estimation of GPS-derived PWV for the ground-based validation (Section 3.5).

Figures 3.6, 3.7 and 3.8 present the synoptic observations of pressure (hPa), temperature (K) and WV partial pressure (hPa) from the Melbourne observatory (MOBS) over a 60-day period from 1 Dec 2010 to 31 January 2011. These measurements were limited to ground observations and present the relative trend for the Melbourne region at surface level. It should be noted that unstable atmospheric conditions occurred from 8 – 15 Jan 2011 due to a severe weather phenomena.

Figure 3.6: Pressure profile from the Melbourne Observatory (MOBS) synoptic station from 1 Dec 2010 to 31 January 2011. Data is produced with a temporal resolution of 3 hours.

Figure 3.7: Temperature profile from the Melbourne Observatory (MOBS) synoptic station from 1 Dec 2010 to 31 January 2011. Data is produced with a temporal resolution of 3 hours.

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Figure 3.8: Water vapour profile from the Melbourne Observatory (MOBS) synoptic station from 1 Dec 2010 to 31 January 2011. Data is produced with a temporal resolution of 3 hours.

These meteorological measures can be used to estimate the dry and wet effects within the troposphere represented as the sum of total refractivity. Figures 3.9, 3.10 and 3.11 present the application of these meteorological observations in the segregated form of total, dry and wet refractivities, respectively. The trend of dry refractivity is quite stable and is not affected by varying densities of WV as opposed to wet refractivity, which was dominated by these densities.

Figure 3.9: Refractivity estimates using synoptic measurements with the Saastamoinen model. Synoptic data is from the Melbourne Observatory (MOBS) station from 1 Dec 2010 to 31 January 2011. Data is produced with a temporal resolution of 3 hours.

Figure 3.10: Dry refractivity estimates using synoptic measurements with the Saastamoinen model. Synoptic data is from the Melbourne Observatory (MOBS) station from 1 Dec 2010 to 31 January 2011. Data is produced with a temporal resolution of 3 hours.

Figure 3.11: Wet refractivity estimates using synoptic measurements with the Saastamoinen model. Synoptic data is from the Melbourne Observatory (MOBS) station from 1 Dec 2010 to 31 January 2011. Data is produced with a temporal resolution of 3 hours.