Validation of 7 Years in-Flight HY-2A Calibration Microwave Radiometer Products Using Numerical Weather Model and Radiosondes

Zhilu Wu,Jungang Wang,Yanxiong Liu,Yang Liu,Wenxue Xu,Xiufeng He

doi:10.3390/rs11131616

Abstract

Haiyang-2A (HY-2A) has been working in-flight for over seven years, and the accuracy of HY-2A calibration microwave radiometer (CMR) data is extremely important for the wet troposphere delay correction (WTC) in sea surface height (SSH) determination. We present a comprehensive evaluation of the HY-2A CMR observation using the numerical weather model (NWM) for all the data available period from October 2011 to February 2018, including the WTC and the precipitable water vapor (PWV). The ERA(ECMWF Re-Analysis)-Interim products from European Centre for Medium-Range Weather Forecasts (ECMWF) are used for the validation of HY-2A WTC and PWV products. In general, a global agreement of root-mean-square (RMS) of 2.3 cm in WTC and 3.6 mm in PWV are demonstrated between HY-2A observation and ERA-Interim products. Systematic biases are revealed where before 2014 there was a positive WTC/PWV bias and after that, a negative one. Spatially, HY-2A CMR products show a larger bias in polar regions compared with mid-latitude regions and tropical regions and agree better in the Antarctic than in the Arctic with NWM. Moreover, HY-2A CMR products have larger biases in the coastal area, which are all caused by the brightness temperature (TB) contamination from land or sea ice. Temporally, the WTC/PWV biases increase from October 2011 to March 2014 with a systematic bias over 1 cm in WTC and 2 mm in PWV, and the maximum RMS values of 4.62 cm in WTC and 7.61 mm in PWV occur in August 2013, which is because of the unsuitable retrieval coefficients and systematic TB measurements biases from 37 GHz band. After April 2014, the TB bias is corrected, HY-2A CMR products agree very well with NWM from April 2014 to May 2017 with the average RMS of 1.68 cm in WTC and 2.65 mm in PWV. However, since June 2017, TB measurements from the 18.7 GHz band become unstable, which led to the huge differences between HY-2A CMR products and the NWM with an average RMS of 2.62 cm in WTC and 4.33 mm in PWV. HY-2A CMR shows high accuracy when three bands work normally and further calibration for HY-2A CMR is in urgent need. Furtherly, 137 global coastal radiosonde stations were used to validate HY-2A CMR. The validation based on radiosonde data shows the same variation trend in time of HY-2A CMR compared to the results from ECMWF, which verifies the results from ECMWF.

Highlights

Haiyang-2A (HY-2A) is the first marine dynamic environmental satellite of China, which was launched in August 2011
The results showed the accuracy of HY-2A was ~1 cm compared with Jason-1 microwave radiometer (JMR) and calibration microwave radiometer (CMR) and the accuracy variation from different retrieval models was 1–2 mm [5,11]
Results we present the validation results of the HY-2A wet tropospheric correction (WTC) and precipitable water vapor (PWV) product using the numerical weather model (NWM) dInattah.isTsheectgiolno,bwaledpirsetrsiebnut ttihoenvoafliWdaTtiConarnedsuPltWs oVf tahreeHfiYrs-t2AprWesTenCteadndbPriWeflVy,parsodWuTctCusainndg tPhWe V usuNaWllyMsudffatear. cTohnesgidloebraalbdleisntroibisuetiaonndofshWoTuCldabned ePxWclVudaeredfwirshtepnrecsoemntpeadrberdietfolyN, WasMWTdCataan. dThPeWdVata cleuansuinagllymseutfhfoerdcaonndsirdeesrualbtsleanreoiaslesoanpdressheonuteldd.bTeheexcdleutdaeildedwahgerneecmomenptasrebdetwtoeNeWn tMhedWataT.CT/PheWdVataand NWclMeanairneginmveetshtoigdataendd. results are presented

Summary

Introduction

Haiyang-2A (HY-2A) is the first marine dynamic environmental satellite of China, which was launched in August 2011 It is equipped with an altimeter, a calibration microwave radiometer (CMR), a microwave scatterometer, and a scanning microwave radiometer, which could measure ocean water vapor, sea surface wind field, sea surface height, and sea surface temperature, etc. The algorithm to calculate WTC and PWV from TB is usually developed before satellite launching and regular validation and calibration should be performed during the in-flight satellite period. VMF3 data on a global 1◦ × 1◦ grid derived from ERA-Interim NWM data in an epoch-wise format (6 h), i.e., the VMF3-EI product was used in this paper, including the tropospheric wet delay. As the successor of VMF1, the VMF3-EI with higher accuracy and better spatial resolution is suitable for the validation of HY-2A WTC in this study

Methods

Results

Conclusion