High-spatial-resolution mapping of precipitable water vapour using SAR interferograms, GPS observations and ERA-Interim reanalysis

Abstract

Abstract. A high spatial and temporal resolution of the precipitable water vapour (PWV) in the atmosphere is a key requirement for the short-scale weather forecasting and climate research. The aim of this work is to derive temporally differenced maps of the spatial distribution of PWV by analysing the tropospheric delay "noise" in interferometric synthetic aperture radar (InSAR). Time series maps of differential PWV were obtained by processing a set of EN-VISAT ASAR (Advanced Synthetic Aperture Radar) images covering the area of southern California, USA from 6 October 2007 to 29 November 2008. To get a more accurate PWV, the component of hydrostatic delay was calculated and subtracted by using ERA-Interim reanalysis products. In addition, the ERA-Interim was used to compute the conversion factors required to convert the zenith wet delay to water vapour. The InSAR-derived differential PWV maps were calibrated by means of the GPS PWV measurements over the study area. We validated our results against the measurements of PWV derived from the Medium Resolution Imaging Spectrometer (MERIS) which was located together with the ASAR sensor on board the ENVISAT satellite. Our comparative results show strong spatial correlations between the two data sets. The difference maps have Gaussian distributions with mean values close to zero and standard deviations below 2 mm. The advantage of the InSAR technique is that it provides water vapour distribution with a spatial resolution as fine as 20 m and an accuracy of ∼ 2 mm. Such highspatial-resolution maps of PWV could lead to much greater accuracy in meteorological understanding and quantitative precipitation forecasts. With the launch of Sentinel-1A and Sentinel-1B satellites, every few days (6 days) new SAR images can be acquired with a wide swath up to 250 km, enabling a unique operational service for InSAR-based water vapour maps with unprecedented spatial and temporal resolution.

Error Evaluation of L-Band InSAR Precipitable Water Vapor Measurements by Comparison with GNSS Observations in Japan

“…After calculating 𝜎 _ using the ZHD error (𝜎 ) through Equation ( 23), we could finally obtain the absolute error of InSAR PWV (𝜎 _ ) as 2.96 mm through Equation (24). The estimated 𝜎 _ was slightly larger than the PWV errors of other observation techniques evaluated in previous studies [12,20,[39][40][41] (Figures 5 and 6).…”

“…Regarding the neutral atmospheric delay, InSAR observes the relative difference in atmospheric delays between two acquisition times. In this study, we estimated the "InSAR zenith total delay difference" (dZTD InSAR ), whose offset was calibrated using the "GNSS zenith total delay difference" (dZTD GNSS ) according to the formulae provided in previous studies [19,20]. Hereinafter, "InSAR initial value" (IV) denotes the phase delay in the zenith direction converted from the satellite line-of-sight direction using the incidence angle θ and a simple trigonometric function.…”

“…The absolute dZTD error of InSAR, which is the relative error against the radiosonde dZTD, is equal to the root of the square sum of (1) the relative error of the GNSS dZTD (σ ZTD_GNSS ), ( 2) the processing error of the GNSS ZTD (σ Sys_GNSS ), and (3) the relative error of dZTD InSAR from dZTD GNSS (σ R ). Using Equation (20), σ dZTD_InSAR can be expressed as follows:…”

“…This is due to the fact that noise characteristics largely differ depending on the microwave center frequency; however, in previous studies, only the observation accuracy of C-band SAR was evaluated. (e.g., [19,20]). Currently, three frequency bands are used for SAR satellites: X-band (e.g., 9.65 GHz), C-band (e.g., 5.40 GHz), and L-band (e.g., 1.25 GHz).…”

A Novel Procedure for Generation of Sar-Derived ZTD Maps for Weather Prediction: Application to South Africa Use Case

“…Satellite observations can undoubtedly give a valuable and complementary contribution in this sense, as they can provide additional water vapor observations over broad areas. Of particular interest is the Synthetic Aperture Radar (SAR), which can exploit the SAR Interferometry (InSAR) technique to obtain water vapor distribution products characterized by high spatial resolution and high accuracy, about 2 mm (Tang et al, 2016). Experiments on the ingestion in NWPMs of water vapor data derived by ENVISAT-ASAR satellite are reported by Pichelli et al (2015) and Mateus et al (2016).…”