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

Wei Li,Weimin Yu,Mingsheng Liao,Lu Zhang,Wei Tang

doi:10.5194/amt-9-4487-2016

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 ENVISAT 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 high-spatial-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.

Highlights

The performance of interferometric synthetic aperture radar (InSAR) data when deriving digital elevation models (DEMs) or precisely measuring surface deformation of the Earth is limited by the tropospheric delay mainly caused by the water vapour content in the lower part (≤ 1.5 km) of the troposphere (Beauducel et al, 2000; Liao et al, 2013; Zebker et al, 1997)
We presented the results of the temporal evolution of the precipitable water vapour (PWV) over southern California, USA using SAR interferograms during the period from 6 October 2007 to 29 November 2008
We used the outputs from the ERA-Interim model to produce maps of the conversion factor for mapping zenith wet delay onto PWV at each pixel in the radar scene

Summary

Introduction

The performance of interferometric synthetic aperture radar (InSAR) data when deriving digital elevation models (DEMs) or precisely measuring surface deformation of the Earth is limited by the tropospheric delay mainly caused by the water vapour content in the lower part (≤ 1.5 km) of the troposphere (Beauducel et al, 2000; Liao et al, 2013; Zebker et al, 1997). The water vapour contributes to only about 10 % of total atmospheric delay, this source of error is not eliminated due to its high spatial and temporal variability. Our aim in this paper is to investigate the tropospheric delay “noise” of InSAR as a meteorological signal to measure the water vapour content in the atmosphere. We will present a new approach for accurate water vapour estimation with a high spatial resolution by combing InSAR observations, GPS data and a global atmospheric model (ERA-Interim), and we will assess its performance.

Objectives

Results

Conclusion