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Bill Asher

Senior Principal Oceanographer





Research Interests

Air-Sea Exchange, Bubbles, Remote Sensing


Dr. Asher's research experience includes modeling the formation of secondary organic aerosols, studying the physics and chemistry of air-water transfer, determining the physicochemical properties of the marine surface microlayer, and measuring the concentration of trace organic compounds in natural aquatic systems.
His current research projects include developing thermodynamic models for predicting the formation of secondary organic aerosols, modeling the cycling and fate of volatile organic compounds in lakes and rivers, using infrared imaging to determine the relation of microscale wave breaking with air-water exchange processes, measuring the microwave emissivity of a foam-covered ocean surface, and characterizing spray droplets over the ocean surface at high wind speeds.


B.A. Chemistry, Reed College, 1980

Ph.D. Environmental Science and Engineering, Oregon Graduate Institute of Science and Technology, 1987


Modeling the Cycle and Source Apportionment of Volatile Organic Compounds in Lakes and Rivers

A set of models to predict how changes in sources and environmental conditions will affect surface water concentrations of volatile organic compounds are being developed to aid regulatory decision makers.

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Two models have been developed under this project. The first is LakeVOC, which predicts the change in concentration of a volatile organic compound (VOC) in both the epilimnion and hypolimnion of lakes and reservoirs in response to changes in source input and environmental parameters. The second model is StreamVOC, which calculates source apportionment for a particular VOC in a river or stream with multiple discrete and distributed source regions.

Our objective is to develop a set of models for predicting how changes in sources and environmental conditions will affect surface water concentrations of volatile organic compounds. These models are designed to allow regulators to easily study the effects of policy, planning, mitigation, and operational strategies on achieving national water quality requirements.

Fluxes, Air-Sea Interaction, and Remote Sensing (FAIRS) Experiment

The transfer of momentum, heat, and gas across the air-sea boundary is characterized and quantified by measuring the underlying physical mechanisms with remote sensing instruments.



2000-present and while at APL-UW

Laboratory heat flux estimates of seawater foam for low wind speeds

Chickadel, C.C., R. Branch, W.E. Asher, and A.T. Jessup, "Laboratory heat flux estimates of seawater foam for low wind speeds," Remote Sens., 14, doi:10.3390/rs14081925, 2022.

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15 Apr 2022

Laboratory experiments were conducted to measure the heat flux from seafoam continuously generated in natural seawater. Using a control volume technique, heat flux was calculated from foam and foam-free surfaces as a function of ambient humidity (ranged from 40% to 78%), air–water temperature difference (ranged from –9°C to 0°C), and wind speed (variable up to 3 m s-1). Water-surface skin temperature was imaged with a calibrated thermal infrared camera, and near-surface temperature profiles in the air, water, and foam were recorded. Net heat flux from foam surfaces increased with increasing wind speed and was shown to be up to four times greater than a foam-free surface. The fraction of the total heat flux due to the latent heat flux was observed for foam to be 0.75, with this value being relatively constant with wind speed. In contrast, for a foam-free surface the fraction of the total heat flux due to the latent heat flux decreased at higher wind speeds. Temperature profiles through foam are linear and have larger gradients, which increased with wind speed, while foam free surfaces show the expected logarithmic profile and show no variation with temperature. The radiometric surface temperatures show that foam is cooler and more variable than a foam-free surface, and bubble-resolving thermal images show that radiometrically transparent bubble caps and burst bubbles reveal warm foam below the cool surface layer, contributing to the enhanced variability.

Intense and small freshwater pools from rainfall investigated during SPURS-2 on 9 November 2017 in the eastern tropical Pacific

Reverdin, G., A. Supply, K. Drushka, E.J. Thompson, W.E. Asher, and A. Lourenço, "Intense and small freshwater pools from rainfall investigated during SPURS-2 on 9 November 2017 in the eastern tropical Pacific," J. Geophys. Res., 125, doi:10.1029/2019JC015558, 2020.

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1 Feb 2020

During the second Salinity Processes in the Upper Ocean Regional Study (SPURS‐2) 2017 tropical Pacific cruise, two drifters were deployed on 9 November. The drifters measured temperature and salinity in the top 36 cm, wave spectra, and the noise of rain drops. During a short nearly circular survey with a 1.8‐km radius around the drifters, the R/V Revelle measured air–sea fluxes, as well as temperature and salinity stratification in the top 1 m from a towed surface salinity profiler (SSP). A C‐band weather radar measuring rain rate within 1‐ to 100‐km range of the ship observed discrete rain cells organized in a system moving from the southeast to the northwest. Some of the intense rain cells were small scale (1 km in diameter or less) with short lifetimes yet dropped more than 5 cm of water in half an hour near the drifters, whereas the ship measured short rain episodes totaling 1.3 cm of rainfall mostly accompanied by very low wind. The data indicate a large spatial heterogeneity in temperature and salinity, with near‐surface freshening of up to 9 psu measured at different times by the two drifters (separated by less than 500 m) and by the SSP. The drifters indicate deepening of the fresh and cool surface layer during the rain, which then thinned during the following 40 min with very low wind speed (<2 m/s). Patchy surface‐trapped cold and fresh layers were also observed by the SSP east of the drifters. The high spatial and temporal variability of rainfall and surface‐trapped fresh pools is discussed.

Salinity Rain Impact Model (RIM) for SMAP

Jacob, M.M., W.L. Jones, A. Santos-Garcia, K. Drushka, W.E. Asher, and C.M. Scavuzzo, "Salinity Rain Impact Model (RIM) for SMAP," IEEE J. Sel. Top. Appl. Earth Obs. Remote Sens., 12, 16-79-1687, doi:10.1109/JSTARS.2019.2907275, 2019.

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15 Jul 2019

When oceanic rainfall occurs, it creates a vertical salinity profile that is fresher at the surface. This freshwater lens is mixed downward by turbulent diffusion, dissipating over a few hours until the upper layer (1–5 m depth) becomes well mixed. Thus, there will be a transient bias between the in situ bulk salinity and the satellite-measured sea surface salinity (SSS) (representative of the first centimeter of the ocean depth). Based on measurements of Aquarius (AQ) SSS under rainy conditions, a model called rain impact model (RIM) was developed to assess the SSS variations due to the accumulation of rainfall prior to the time of the AQ observation. RIM uses ocean surface salinities from hybrid coordinate ocean model and the NOAA global precipitation product, climate prediction center morphing, to estimate changes in the near-surface salinity profile. Also, the RIM analysis has been applied to soil moisture and ocean salinity with similar results observed. The Soil Moisture Active Passive (SMAP) satellite carries an L-band radiometer, which measures SSS over a swath of 1000 km at 40-km resolution. SMAP can extend AQ salinity data record with improved temporal/spatial sampling. This paper describes RIM that simulates the effects of rain accumulation on SMAP SSS, showing good correlation between the model and the observed SSS values. Given the better resolution of SMAP, the goal of this paper is to continue the previous analysis of AQ to better understand the effects of the instantaneous and accumulated rain on the salinity measurements.

More Publications

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center