High-Resolution Rain Maps from an X-band Marine Radar and Their Use in Understanding Ocean Freshening

Ship-based X-band radar observations of rain were collected with high spatial resolution during the 2016 and 2017 Salinity Processes in the Upperocean Regional Study 2 (SPURS-2) field experiments in the eastern tropical Pacific Ocean.These observations were collected with a repurposed marine radar that is not typically used for weather monitoring.The radar images captured during SPURS-2 show the spatial extent and variable intensity of rain at a horizontal resolution of 180 m within 30 km of the ship.When analyzed alongside collocated measurements of oceanic and atmospheric properties collected during SPURS-2, the radar-derived rain maps enable a clearer understanding of the impact of spatially and temporally varying freshwater fluxes on ocean salinity.Ocean surface freshening, measured by ship gauges, is found to be affected by local rain accumulation, and also by prior rain accumulation in surrounding locations that was measured by radar.In one example, the X-band marine radar measured rain directly ahead of the ship's path.The ship then sampled a near-surface freshening signature within the time period expected based on the ship speed, ship heading, and rain area measured by the radar. Oceanography| Vol.32, No.2 58 created by rain are referred to here as rain layers.Significant progress has been made in understanding the vertical one-dimensional processes that contribute to rain layer formation and evolution.Using in situ measurements of salinity and temperature and one-dimensional models, Asher et al. (2014), Anderson and Riser (2014), Drushka et al. (2016), and Thompson et al. (2019) demonstrated that the formation of near-surface vertical salinity gradients depends on local rain rate, net heat flux, wind speed, and ocean turbulence.However, according to observations made by You (1995), Thompson et al. (2019), and Drushka et al. (2019), surface freshening and vertical salinity stratification are also often detected when rain is not present locally, but where it rained recently near the location of salinity measurements.