IWV retrieval from ship-borne GNSS receiver in the framework of the MAP-IO project

<p>In the framework of the research project &#8220;Marion Dufresne Atmospheric Program &#8211; Indian Ocean&#8221; (MAP-IO), which is aiming at collecting long-term atmospheric and marine biology observations in the under-instrumented Indian and Austral Oceans, a Global Navigation Satellite System (GNSS) receiver was installed on the research vessel (RV) Marion Dufresne in October 2020 to describe, and monitor, global moisture changes in these areas. GNSS raw data are recorded continuously and used to retrieve integrated water vapor contents (IWV) along the RV route.</p><p>After a data quality check that confirmed that a wise choice of location of the GNSS antenna on the RV is crucial to avoid mask, signal reflection and interference from other instruments that may degrade IWV retrieval, a first assessment of the GNSS analysis performances was carried out by comparing the vertical component of the estimated positions to sea surface height model. The differences are on the order of 20 to 30 cm; they are consistent with both the error budget for sea surface height determination using GNSS and the sea surface height model formal errors.</p><p>An evaluation of GNSS-derived IWV was conducted using IWV estimates from the ECMWF fifth ReAnalysis (ERA5) and ground-based GNSS reference stations located nearby the tracks of RV Marion Dufresne. Preliminary analyses show encouraging results with a mean root mean square error of ~2-3&#160;kg&#160;m<sup>-</sup><sup>2</sup> between ERA5 and GNSS-derived IWV. The use of ultra-rapid GNSS orbit and clock product was also investigated to assess the performance of near real-time GNSS-derived IWV estimation for numerical weather prediction purposes.</p>


GNSS raw data quality check
GNSS raw data are processed in PPP mode with Gipsy-Oasis II 6.4 over the period Oct. 2020 -Jun.2021 following 3 strategies: ZWD is estimated as random-walk process with process parameter of 5 mm h −1/2 .
See also [Bos+21] for a more complete description of the processing strategies and IWV retrieval.Interruptions in PTU acquisitions when the ship is docked prevent to compute IWV over the whole period for ultra and rapid.
Differences are quasi equally due to differences in ZTD retrieval and conversion from ZTD to IWV.
Good agreement between rapid and repro.
Agreement between ultra and repro is also quite conclusive and highlight the potential use of ship-borne GNSS IWV for NWP purposes.

Context
Studies highlight the performance of IWV retrieval from ship-borne GNSS antenna, with RMS with respect to classical techniques in the range 1-3 kg m −2 [Wan+19; Bos+21; M än+21] The challenge lies in the simultaneous estimation of kinematic position and ZTD.Marion Dufresne Atmospheric Program -Indian Ocean (MAP-IO): collection of long-term marine biology and atmospheric observations in Indian & Austral Oceans Installation of a GNSS receiver on the RV Marion Dufresne in October 2020 to describe and monitor global moisture in the atmosphere.GNSS raw data are recorded continuously and used to retrieve integrated water vapor contents (IWV) along the RV route.

First
step: raw data quality check with TEQC [Est+99] Nsat : Number of satellites by 24h sessions MP: Multipath on L1 and L2 carriers Interference induced by antenna environment.(Lowest value expected) % obs : Percentage of used observations Percentage of complete to possible observations O/S: Observations / Slips Ratio between complete observations and the number of slips from GNSS raw data.(Highest value expected) A change in GNSS antenna location occurs in Mar.2021 (bold vertical line) in order to improve MP and O/S values.¯This change is shown to improve both these indicators.
P. Bosser et al.MAP-IO -Ship-borne GNSS IWV 6 / 11 First assessment: GNSS height estimates from repro solution Geoid height values from raw and smoothed GNSS ellipsoid height estimates.Modeled geoid height derived from mean sea surface model CNES CLS 2015 [Puj+18] and oceanographic tide FES2014b [Lya+16] Significant height of combined wind waves and swell product from ERA5 Decrease of variability of differences in height after antenna location change.Large variability explained by wind waves, swell Low variability at the end of the period when docked.Systematic positive offset in Kerguelen area: Overall agreement between all the techniques; radiosondes from La R éunion tend to overestimate IWV.Different characteristics of IWV values: Southern areas: ∼ 15 kg m −2 [5 → 30 kg m −2 ] Northern areas: ∼ 30 kg m −2 [20 → 65 kg m −2 ] Sensing of severe Weather events as Danilo (2021-12) and Iman (2021-65) tropical cyclones.RMS with ground GNSS range from 2.2 to 3.1 kg m −2 Causes for large differences: High differences in height between antennas Lower quality of ship-borne IWV before the change of location of the GNSS antenna.Large deviations with radiosondes (also observed as comparing to ground GNSS); the kind of radiosonde (M10) may be in cause [Boc+13; Dup+20; Lee+20].Differences with ERA5 extraction are consistent with recent studies [Bos+21; M än+21] Differences are shown to be reduced after the change of the location of the antenna ( * ).