Development of Drifting Buoys to Measure Dynamic Ocean Topography and Precipitable Water Vapor

Lead PI: James Morison, University of Washington
Start Year: 2018 | Duration: 3 years
Partners: NOAA Pacific Marine Environmental Laboratory, Jet Propulsion Laboratory, NASA

We propose to build drifting buoys measuring sea surface height (SSH) to yield dynamic ocean topography (DOT= sea surface height-geoid) and atmospheric precipitable water vapor (PWV) content. SSH is one of ten Global Ocean Observing System’s (GOOS) Essential Ocean Variables important as the measure of long-term sea level rise, tides, and storm surges. DOT derived from SSH constitutes the surface pressure gradient that drives geostrophic surface velocity, Vgeo, another GOOS Essential Ocean Variable. DOT observations combined with density profiles such as measured by oceanographic buoys and profiling floats to infer velocity shear, make it possible to measure absolute water velocity versus depth. In the Arctic Ocean, the average sea ice drift, Vice, largely follows Vgeo, while the difference between Vice and Vgeo has a critical role in stabilizing the doming of the Beaufort Sea Gyre. Cross-shelf gradients in DOT drive shelf-basin exchanges important in maintaining the Arctic Ocean halocline.
Observations of PWV are needed to understand changes in atmospheric conditions globally and in the Arctic in particular. PWV in clouds reflects incoming solar radiation and traps long­ wave radiation near the surface, making soundings of moisture content critical to understanding the role of clouds in the surface heat budget, the water cycle, atmospheric dynamics, and their effect on sea ice. These effects are critical in operational forecasts of weather and radio propagation.
In spite of the importance of DOT and PWV, wholly autonomous insitu measurements of these variables have not been made. Satellite altimeters greatly expand the areal coverage of DOT observations, but in situ DOT and PWV observations are critical to provide ground truth for the satellites and fill high-frequency temporal gaps.
The proposed buoys, hereafter called DOT Buoys, will be suitable for surface or air deployment in sea ice or open water. The proposed DOT Buoys will provide heretofore unavailable in situ information to the ONR SODA and SIZRS programs in 2019-2020 and critical ground truth data for NASA’s launches of ICESat-2 (2018) and SWOT (2020).
The DOT Buoys will use precision dual-frequency OPS and Precise Point Positioning (PPP) processing of OPS data to determine DOT to 1-cm accuracy and PWV to I -mm accuracy. PPP and the dual frequency capability of the receiver address the key sources of OPS errors.PPP processing relies on a worldwide array of stationary GPS receivers to determine the errors in OPS satellite orbits and clocks. Corrections for these errors will be applied to raw code and phase information from the DOT Buoy OPS receptions to derive positions good to 1-cm accuracy. However, this requires that full code and phase information must be telemetered from the drifting buoy for post processing with I -week latency. The basic hardware and PPP scheme are well proven. Our challenge is to design an Iridium telemetry system and sampling strategy that measure DOT and WPV at appropriate spatial and temporal scales and buffers the data for transmission through an Iridium data link.
The Applied Physics Lab (APL) will build six DOT buoys combining the Iridium data telemetry, power systems, and ice-capable buoy hull of a proven APL drifting buoy with a dual­ frequency GPS receiver like that in a proven moored internally-recording GPS buoy built by our partners at the Pacific Marine Environmental Laboratory (PMEL). Past data from the PMEL buoy will be used to design the optimum sampling strategy for the.DOT Buoy. Our partners at the Jet Propulsion Laboratory (JPL) will perform the PPP processing .APL and JPL will evaluate the DOT Buoy performance and facilitate application of the buoys to planned ONR (SODA, SIZRS), NASA (ICESat-2, SWOT), and NOAA (IABP) programs in 2019-2021.