Abstract
Unlike most altimetric missions, CryoSat-2 is not equipped with an onboard microwave radiometer (MWR) to provide wet tropospheric correction (WTC) to radar altimeter measurements, thus, relying on a model-based one provided by the European Center for Medium-range Weather Forecasts (ECMWF). In the ambit of ESA funded project CP4O, an improved WTC for CryoSat-2 data over ocean is under development, based on a data combination algorithm (DComb) through objective analysis of WTC values derived from all existing global-scale data types. The scope of this study is the analysis and inter-calibration of the large dataset of total column water vapor (TCWV) products from scanning MWR aboard Remote Sensing (RS) missions for use in the WTC computation for CryoSat-2. The main issues regarding the computation of the WTC from all TCWV products are discussed. The analysis of the orbital parameters of CryoSat-2 and all other considered RS missions, their sensor characteristics and inter-calibration is presented, providing an insight into the expected impact of these datasets on the WTC estimation. The most suitable approach for calculating the WTC from TCWV is investigated. For this type of application, after calibration with respect to an appropriate reference, two approaches were found to give very similar results, with root mean square differences of 2 mm.
Highlights
With an absolute value up to 50 cm and highly variable in space and time, the path delay due to the presence of water vapor in the atmosphere, or wet tropospheric correction (WTC), is still one of the major error sources in satellite radar altimetry
The fact that the obtained calibration parameters for these satellites are not exactly 1.0 for the scale factor and 0.0 for the offset can be attributed to the following different aspects of the presented methodology: (1) Different datasets for Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and F15-Sensor Microwave Imager/Sounder (SSM/IS) were used; (2) Possible latitudinal biases were not taken into account; and (3) no restrictions were imposed to, or effect corrected for, the viewing geometry of the SI-microwave radiometers (MWR) observations used, what may help explaining the slight overestimation/underestimation of the SI-MWR-derived WTC using the
This study presented an analysis of the water vapor dataset of SI-MWR sensors available for the computation of the WTC for CryoSat-2, launched in April 2010
Summary
With an absolute value up to 50 cm and highly variable in space and time, the path delay due to the presence of water vapor in the atmosphere, or wet tropospheric correction (WTC), is still one of the major error sources in satellite radar altimetry. Due to its high variability, the most accurate way to model this effect over open ocean is through the measurements of microwave radiometers (MWR) on board the altimetric missions. An accurate enough modeling of this effect can only be achieved through actual measurements of the atmospheric water vapor content at the time and location of the altimetric measurements For this purpose, dedicated microwave radiometers have been incorporated in the most recent altimetric missions. All of them have one band in the water vapor absorption line between 21 and 23.8 GHz plus one or two in “atmospheric window”
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