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Bending Angle and Temperature Climatologies from Global Positioning System Radio Occultations

DOI: 10.7167/2013/795749

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The Global Positioning System (GPS) Radio Occultation (OR) technique provides estimates of atmospheric density, temperature, and water vapour content with high vertical resolution, global coverage, and high accuracy. We have used data acquired using this technique in the period 1995–2009 to create a reference climatology of radio occultation bending angle and atmospheric temperature which are used for meteorological studies. The bending angle is interesting because it is a direct measurement and independent of models. It is given with one-degree spatial resolution and 50-meter vertical sampling. In addition, we give the temperature climatology with one-degree spatial resolution and 100-meter vertical sampling. This dataset can be used for several applications including weather forecast, physics of atmosphere, and climate changes. Since the GPS signal is not affected by clouds and the acquisitions are evenly distributed in the globe, the dataset is well suited for studying extreme events (such as convective systems and tropical cyclones) and remote areas. 1. Introduction The Global Positioning System (GPS) Radio Occultation (RO) technique [1] enables measurements of the global atmospheric density structure under any meteorological condition [2]. As illustrated in Figure 1, the RO technique involves a GPS satellite transmitting the signal and a Low Earth Orbit (LEO) satellite carrying a receiver. The signal transmitted by the GPS satellite is refracted in the atmosphere, and the associated propagation delay, refractive index, and bending angle are measured on the LEO satellite. From the measurements, it is possible to estimate profiles of atmospheric parameters such as temperature, water vapour, and pressure [3]. These parameters are secondary products, derived from the refractivity together with the European Centre for Medium-Range Weather Forecasts (ECMWF) model, and are given with high vertical resolution. The highest accuracy on the refractivity is achieved between 5 and 25?km altitude with average errors estimated in the range 0.3%–0.5% [4]. The RO technique has improved the weather forecast in regions of the globe that is poorly covered by standard measurements, such as the southern hemisphere which is dominated by oceans [5]. For instance, forecasting the track of tropical cyclones (energized over the oceans) has greatly improved [6]. Also the upper atmosphere is better forecasted as in the case of the ECMWF Upper Troposphere Lower Stratosphere (UTLS) model [7]. Figure 1: The GPS RO technique scheme (the proportions are not respected) [ 1]. Dashed

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