Comparing GPS radio occultation observations with ...

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International Global Navigation Satellite Systems Society IGNSS Symposium 2013 Outrigger Gold Coast, Qld Australia 16-18 July, 2013

Comparing GPS radio occultation observations with radiosonde measurements over Antarctica R. Norman1, J. Le Marshall1,2, B.A. Carter1, K. Zhang1, G. Kirchengast3, S. Alexander4 , C-S.Wang1 and Y. Li1 1

Satellite Positioning for Atmosphere, Climate and Environment (SPACE) Research Centre, RMIT University, Melbourne, Australia, Tel: 99256735, email: [email protected]

2

Centre for Australian Weather and Climate Research (CAWCR), Bureau of Meteorology, Melbourne, Australia

3

Wegener Center for Climate and Global Change (WEGC), University of Graz, Graz, Austria

4

Australian Antarctic Division, Hobart, Tasmania, Australia

ABSTRACT GPS Radio Occultation (RO) is a space-based technique for sounding the Earth’s atmosphere. This technique has been shown to significantly improve weather forecasting and climate monitoring over many regions of the Earth. The GPS RO technique uses specially-designed GPS L-band frequency receivers on-board Low Earth Orbit (LEO) satellites to receive signals from GPS satellites. The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) is a joint Taiwan and USA satellite program and was launched into orbit in April 2006. GPS RO data from this constellation of six FORMOSAT-3 (Formosa Satellite Mission #3), LEO (800 km altitude) micro-satellites provide an observational data type for operational meteorology and significant information on the thermodynamic state of the atmosphere. In the Antarctic region there are only 16 radiosonde (RS) weather stations mainly distributed along the coastal fringe. As such this RS network is far from ideal for studying the atmosphere, meteorology and climatology in the Antarctic region. It does however provide excellent reference stations to test and validate the GPS RO technique as a suitable meteorological data type in the Antarctic region. In this study the COSMIC GPS RO temperature and pressure profiles are compared to those measured using radiosondes in the Antarctic region. Annual area weighted average temperature profiles from the Antarctic region are presented. KEYWORDS:

COSMIC

Radio

Occultation,

Antarctica,

radiosonde,

satellites,

1. INTRODUCTION Of the worlds continents Antarctica is the world’s coldest, windiest and driest. Antarctica is the fifth largest continent approximately 1.3 times the size of Europe and nearly twice as large as Australia. Approximately 98% of Antarctica is covered in more than 1 mile (1.6 km) of ice. It is generally regarded as a desert having no permanent human residents and typically less than 5,000 human visitors at any one time. Monitoring of the Antarctic climate is difficult due to the almost uninhabitable conditions, sparse human population and lack of infrastructure, especially inland. Monitoring of the Antarctic climate is important for weather forecasting, developing climate models and for understanding the effects of climate change. There are currently only 16 radiosonde (RS) weather stations in Antarctica and they are predominantly distributed along the coastal fringe. Space-based remote sensing of the Antarctic atmosphere offers far greater spatial and temporal capability, collecting data from all regions of Antarctica and surrounding southern ocean. The RS network provides excellent reference stations to test and validate atmospheric retrieval techniques in the Antarctic region. GPS Radio Occultation (RO) is a robust space-based Earth observation technique, providing significant information for atmospheric profiling and meteorological applications. The GPS RO technique requires GPS receivers onboard Low Earth Orbit (LEO) satellites to measure the received radio signals from GPS satellites. The received GPS signal is refracted by the earth’s atmosphere and using a complex atmospheric retrieval process, atmospheric profiles such as temperature, pressure, water vapour and electron concentration in the ionosphere can be determined. This relatively new observational data type provides significant information on the thermodynamic state of the atmosphere, improving atmospheric analyses and prognoses. It is therefore important to know and understand how GPS RO measurements compare with more conventional atmospheric and meteorological sounding devices. In this study the GPS RO temperature and pressure profiles are compared to those measured from radiosondes, in the Antarctic region. The GPS RO technique use GPS navigation satellites and LEO satellites to create a robust space-based earth observation platform. The GPS consists of ~ 30 global positioning satellites orbiting the earth twice daily, travelling at an altitude of ~20,200 km and continuously transmitting L-band frequencies signals of 1.57542 GHz (L1) and 1.2276 GHz (L2). The refractive gradients in the atmosphere cause the paths of the GPS L-band electromagnetic signals to bend/refract according to Snell’s law. GPS RO atmospheric retrieval techniques enable atmospheric profiles of temperature, pressure and water vapour to be calculated (Hajj et al., 1994, Melbourne et al., 1994, Kursinski et al., 1996, Ware et al., 1996, Rocken et al., 1997, Healy and Eyre, 2000, Hernandez-Pajares et al., 2000). The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) is a joint Taiwan and USA satellite program and was launched into orbit in April 2006. GPS RO data from this constellation of six FORMOSAT-3 (Formosa Satellite Mission #3), Low Earth Orbit (LEO) (800 km altitude) micro-satellites provides a new observational data type providing significant information on the thermodynamic state of the atmosphere. The RO data from this constellation has been shown to improve atmospheric analyses and prognoses (Cucurull et al., 2007 and Le Marshall et al., 2010). A number of papers have been published on using GPS RO for climate applications Anthes et al., 2008, Anthes 2011 and Le Marshall et al., 2012. GPS RO data obtained from COSMIC for the period from day 194 in 2006 to 2010 will be used in this study.

2. RO DATA In this study post-processed GPS RO data from the COSMIC Data Analysis and Archival Center (CDAAC) at UCAR in Boulder (http://www.cosmic.ucar.edu/index.html) was used for RO comparisons with RS. The received COSMIC GPS RO data is routinely processed at UCAR and freely available from the UCAR website (www.cosmic.ucar.edu) in both BUFR and netCDF formats. The data available includes bending angle, refractivity, pressure, temperature and moisture profiles and data quality quantifiers. UCAR provides global coverage of around 1000-2500 COSMIC GPS RO events daily. Figure 1 shows the COSMIC GPS RO tangent point locations (1744 RO events) for Day 61, 2nd March, 2009.

Figure 1. The 1744 F3C RO locations for 2nd March 2009. (Image courtesy of UCAR).

Radiosondes are typically launched twice a day (up to an hour before 00:00 and 12:00 UTC) from more than 800 locations worldwide. The approximate locations of the radiosonde weather stations are shown in Figures 2 and 3. Figure 2 reveals the high concentration of RS stations in the northern hemisphere and the relatively low density of stations in Antarctica and ocean regions. The GPS RO events from Figure 1 are far more evenly distributed.

Figure 2. Location of radiosonde weather stations. (Image courtesy of World Meteorology Organization (WMO)).

Figure 3. Location of radiosonde weather stations in Antarctica.

It should be noted that RS suffer from some biases and errors (Luers and Eskridge, 1998; Wang et al., 2003) and thus the differences in RO and RS results should not be assumed to be solely due to RO retrieval errors. Co-location mismatches between RS and RO data have been found to introduce noise into the RO-RS comparisons (Sun et al., 2010). Radiosonde balloons drift with height and can take ~ 2 hours to ascend from the earth’s surface to the stratosphere and may drift more than 200 km horizontally (Sun et al., 2010). The RO tangent point location typically drifts more than 100 kilometres during a single RO event. The RO atmospheric retrieval technique also assumes a spherically stratified atmosphere, ignoring the horizontal ionospheric and atmospheric gradients along the signal path. 3. RO RESULTS The results from a typical single RO event are compared to collocated RS results in Figure 4. In this example the RS balloon was launched around 00:00 UT, from the Casey weather station located at 62.28°S, 110.52°E, day 38 in 2010. This comparison used the CDAAC UCAR RS file sonPrf_C002.2010.038.00.18.G29_2010.2640_nc and the corresponding RO file wetPrf_C002.2010.038.00.18.G29_2010.2640_nc. The RO event occurred at 00:18 UT with the occultation point located at 67.01°S, 111.09°E which is ~85.6 km from the weather station. The RO data is represented by the blue dots and the red dots represent the RS data in Figure 4. Figure 5 shows the temperature difference between the RO and RS data for altitudes above 2 km. The temperature profiles from the RO data typically spans altitudes much higher than the RS data, ranging to the upper stratosphere boundary.

Figure 4. Geometric height versus Temperature where the “blue dots” represent RO results and the “red diamonds” represent the RS data on day 38, 2010 above Casey, Antarctica.

Figure 5. RO-RS temperature bias for a RO event on day 38, 2010 above Casey, Antarctica.

Figure 6 shows annual RO-RS temperature bias and root mean square (rms) error using UCAR (CDAAC V2010.2640) and all 16 radiosondes in the Antarctic region from day 194 in 2006 to 2010. A spatial and temporal buffer of 200 km and 3 hrs was chosen as the constraint for the co-located RO and RS data. This spatial and temporal buffer provided sufficient comparison data to demonstrate the consistency in the RO data. It should be noted that the 2006 data set is less than the other years and begins on day 194. In the years 2007 to 2010 the

temperature bias is less than 0.5 °C. This is consistent with other studies, Norman et al., 2013 showed RO-RS comparisons over the Australian region and globally using identical buffer constraints. Comparisons were made at pressure levels 30, 50, 100, 150, 200, 250, 300, 400, 500, 700 and 850 hPa. For each of the years 2007 to 2010 approximately 500 RO-RS colocation data points were averaged at each pressure level. 0 100 200 2006 Bias 2007 Bias

Pressure [hPa]

300

2008 Bias 2009 Bias

400

2010 Bias 2006 RMS

500

2007 RMS 2008 RMS

600

2009 RMS 2010 RMS

700 800 900 -1

0

1

2

3

4

Temperature bias [C]

Figure 6. RO-RS temperature bias, Antarctica, 2006(194) to 2010.

The co-located RO and RS average pressure profiles for the Antarctic region for the year 2007 are shown in Figure 7. The profiles are almost indistinguishable from each other. 25

Geometric height [km]

20

15

RO RS 10

5

0 0

200

400

600

800

1000

Pressure [hPa]

Figure 7. RO and RS average pressure profiles, Antarctica, 2007.

Annual average temperatures over the Antarctic region were also determined using RO data and 10° x 10° lat/lon grids from 60°S to 90°S and 0° to 360°E. The area of each grid was taken in to consideration in the calculation of the average annual temperatures. The annual average RO grid point temperatures are shown in Figures 8 and 9. Figure 9 shows RO average annual temperatures starting at 300 hPa and clearly shows the annual temperature differences. The annual average temperature in 2007 at 50 hPa is -60.86 °C and almost 2 °C warmer than the corresponding 2008 average temperature. 0 100

Pressure [hPa]

200 300 2007

400

2008 2009

500

2010 600 700 800 900 -70

-60

-50

-40

-30

-20

-10

0

Temperature [C]

Figure 8. RO grid point average annual temperature profiles, Antarctica, 2007-2009. 0

Pressure [hPa]

50

100 2007 2008 2009

150

2010 200

250

300 -63

-62

-61

-60

-59

-58

-57

-56

-55

Temperature [C]

Figure 9. RO grid point average annual temperature profiles, Antarctica, 2007-2009, starting from a pressure level of 300 hPa.

4. CONCLUSIONS Post processed radio occultation data from CDAAC at UCAR and provided by the COSMIC/FORMOSAT-3 were compared to co-located radiosonde data from Antarctica for the period from day 194 in 2006 to 2010. Radio occultation provides high vertical resolution soundings of the atmosphere. The robustness and accuracy of the COSMIC GPS radio occultation were examined and compared to co-located radiosonde measurements with a spatial and temporal buffer of 200 km and 3 hrs. The results presented show good agreement between the radiosonde and radio occultation data with biases less than 0.5 °C. For the years 2007 to 2010 the biases between 500 and 100 hPa were less than 0.25 °C. The Antarctic region is sparsely populated with conventional atmospheric monitoring devices. In order to fully appreciate the atmospheric state over the Antarctic region remote sensing techniques such as GPS radio occultation will become very important. The radio occultation also has the advantage of being able to operate in almost all weather conditions which is extremely important in the Antarctic region. Annual average temperatures over the Antarctic region (60°S to 90°S and 0° to 360°E) were determined using radio occultation data and 10° x 10° lat/lon grids over this region. The area within each grid location was determined and the average temperature weighted accordingly. Average annual Antarctic temperatures were presented for the years 2007 to 2010. The temperature versus pressure profiles kept a similar shape where cold point temperatures were less than -60 °C. The results also revealed the difference in average annual temperature for 2007 and 2008 at 100 hPa to be as high as ~ 2 °C. The results presented clearly demonstrate that the GPS RO technique provides a very useful meteorological data type for the data sparse Antarctic region.

ACKNOWLEDGEMENTS This work is supported in part by the Australian Space Research Program project: “Platform Technologies for Space and by the Australian Antarctic Division AAS project no. 4159: “GPS Radio Occultation for studying the Antarctic Atmosphere and Climate Analysis.” The authors would like to thank all members of the FORMOSAT-3 / COSMIC science mission and CDAAC, UCAR for providing the radio occultation and radiosonde data. REFERENCES Anthes RA, Bernhardt PA, Chen Y, Cucurull L, Dymond KF, Ector D, Healy SB, Ho S-P, Hunt DC, Kuo Y-H, Liu H, Manning K, McCormick C, Meehan TK, Randel WJ, Rocken C, Schreiner WS, Sokolovskiy SV, Syndergaard S, Thompson DC, Trenberth KE, Wee T-K, Yen NL, and Zeng Z (2008) The COSMIC/FORMOSAT-3 Mission: Early Results, Bull. Amer. Meteor. Soc, 89, 313-333. Anthes RA, (2011) Exploring Earth’s Atmosphere with Radio Occultation: Contributions to Weather, Climate and Space Weather, Atmos. Meas. Tech., 4, 1077–1103. Cucurull L, Derber JC, Treadon R, Purser RJ (2007) Assimilation of Global Positioning System radio occultation observations into NCEP’s Global Data Assimilation System, Mon. Weather Rev.,

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