Integrated water vapour content retrievals from ship-borne GNSS receivers during EUREC4A

Abstract

<p>In the framework of the EUREC4A campaign, integrated water vapour (IWV) contents were retrieved over the open Tropical Atlantic Ocean using Global Navigation Satellite System (GNSS) data acquired from three research vessels : R/V Atalante, R/V Maria S. Merian, and R/V Meteor. This study describes the GNSS processing method and compares the GNSS IWV retrievals with IWV estimates from the ECMWF fifth ReAnalysis (ERA5), from the MODIS infra-red products, and from terrestrial GNSS stations located along the tracks of the ships. The ship-borne GNSS IWVs retrievals from R/V Atalante and R/V Meteor compare well with ERA5, with small biases (-1.62 kg/m2 for R/V Atalante and +0.65 kg/m2 for R/V Meteor) and a RMS difference about ~2.3 kg/m2. The results for the R/V Maria S. Merian are found  to be of poorer quality, with RMS difference of about 6 kg/m2 which are very likely due to the location of the GNSS antenna on this R/V prone to multipath effects. The comparisons with ground-based GNSS data confirm these results. The comparisons of all three R/V IWV retrievals with MODIS infra-red product show large RMS differences of 5-7 kg/m2, reflecting the enhanced uncertainties of this satellite product in the tropics. These ship-borne IWV retrievals are intended to be used for the description and understanding of meteorological phenomena that occurred during the campaign, east of Barbados, Guyana and northern Brazil.</p>

EUREC<sup>4</sup>A's <i>Maria S. Merian</i> ship-based cloud and micro rain radar observations of clouds and precipitation

“…The LWP retrieval is a neural network retrieval provided by the radar manufacturer, while the IWV is derived from a single-channel quadratic regression between the 89 GHz brightness temperatures obtained in clear sky and the IWV measured by the radiosoundings launched at the exact times. We assessed the IWV retrieval by comparing it to the IWV estimations obtained by GNSS (Bosser et al, 2021). We found a bias of 3.4 kg m −2 , in agreement with what was reported in Bosser et al (2021).…”

“…We assessed the IWV retrieval by comparing it to the IWV estimations obtained by GNSS (Bosser et al, 2021). We found a bias of 3.4 kg m −2 , in agreement with what was reported in Bosser et al (2021).…”

“…The bias between the mean value of the IWV distribution from W-band and the IWV distribution from GNSS is 3.4 kg m −2 , which is consistent with the bias estimated with ground-based GNSS stations reported in Fig. 9 of Bosser et al (2021). The spread between the GNSS and the radar-derived values of IWV can be due to the strongly varying bias component that affects the GNSS IWV estimations from MS Merian (Bosser et al, 2021), as well as to limitations in the IWV single-channel retrieval.…”

“…We compared the obtained IWV values with the IWV retrieved from GNSS by Bosser et al (2021). The mean of the IWV retrieved from W-band radar single-channel retrieval is 31.7 kg m −2 , the median is 32.3 kg m −2 , and the standard deviation of the distribution is 5.15 kg m −2 .…”

“…9 of Bosser et al (2021). The spread between the GNSS and the radar-derived values of IWV can be due to the strongly varying bias component that affects the GNSS IWV estimations from MS Merian (Bosser et al, 2021), as well as to limitations in the IWV single-channel retrieval.…”