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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

Extension of the prognostic model of sea surface temperature to rain-induced cool and fresh lenses

Bellenger, H., K. Drushka, W. Asher, G. Reverdin, M. Katsumata, and M. Watanabe, "Extension of the prognostic model of sea surface temperature to rain-induced cool and fresh lenses," J. Geophys. Res., 122, 484-507, doi:10.1002/2016JC012429, 2017.

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1 Jan 2017

The Zeng and Beljaars (2005) sea surface temperature prognostic scheme, developed to represent diurnal warming, is extended to represent rain-induced freshening and cooling. Effects of rain on salinity and temperature in the molecular skin layer (first few hundred micrometers) and the near-surface turbulent layer (first few meters) are separately parameterized by taking into account rain-induced fluxes of sensible heat and freshwater, surface stress, and mixing induced by droplets penetrating the water surface. Numerical results from this scheme are compared to observational data from two field studies of near-surface ocean stratifications caused by rain, to surface drifter observations and to previous computations with an idealized ocean mixed layer model, demonstrating that the scheme produces temperature variations consistent with in situ observations and model results. It reproduces the dependency of salinity on wind and rainfall rate and the lifetime of fresh lenses. In addition, the scheme reproduces the observed lag between temperature and salinity minimum at low wind speed and is sensitive to the peak rain rate for a given amount of rain. Finally, a first assessment of the impact of these fresh lenses on ocean surface variability is given for the near-equatorial western Pacific. In particular, the variability due to the mean rain-induced cooling is comparable to the variability due to the diurnal warming so that they both impact large-scale horizontal surface temperature gradients. The present parameterization can be used in a variety of models to study the impact of rain-induced fresh and cool lenses at different spatial and temporal scales.

Rainfall measurements in the North Atlantic Ocean using underwater ambient sound

Yang, J., W.E. Asher, and S.C. Riser, "Rainfall measurements in the North Atlantic Ocean using underwater ambient sound," Proc., IEEE/OES China Ocean Acoustics Symposium, 9-11 January, Harbin, China (IEEE/OES, 2016).

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8 Aug 2016

Quantification of rainfall over the ocean is critical in understanding the global hydrological cycle. However, oceanic rain has proven difficult to measure due to problems associated with platform motion and flow distortion combined with the spatial and temporal variability of rainfall itself. Passive acoustic rain gauges avoid these issues by using the underwater sound generated by raindrops on the ocean surface to detect and quantify rainfall. In this paper, the operating principles for and data from the Passive Aquatic Listener (PAL), which uses underwater ambient sound to measure rainfall rate and wind speed, are presented. PAL was incorporated onto thirteen Argo profilers that were deployed in September, 2012 as part of the US National Aeronautics and Space Administration-sponsored Salinity Processes in the Upper ocean Regional Studies (NASA SPURS) field experiment in the North Atlantic Ocean. PAL-Argo was initially deployed within a 200 km x 200 km box, PAL-Argos now cover a 1600-km x 600-km region, and continue to telemeter rain rate and wind speed data. Comparisons of these PAL data with in situ and satellite measurements show good agreement for both rain rate and wind speed. Seasonal and inter-annual variability of wind and rain fields in the region are also presented.

Satellite and in situ salinity: Understanding near-surface stratification and sub footprint variability

Boutin, J., and 21 others, including W.E. Asher and K. Drushka, "Satellite and in situ salinity: Understanding near-surface stratification and sub footprint variability," Bull. Am. Meteor. Soc., 97, 1391-1407, doi:10.1175/BAMS-D-15-00032.1, 2016.

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1 Aug 2016

Remote sensing of salinity using satellite-mounted microwave radiometers provides new perspectives for studying ocean dynamics and the global hydrological cycle. Calibration and validation of these measurements is challenging because satellite and in situ methods measure salinity differently. Microwave radiometers measure the salinity in the top few centimeters of the ocean, whereas most in situ observations are reported below a depth of a few meters. Additionally, satellites measure salinity as a spatial average over an area of about 100 x 100 km2. In contrast, in situ sensors provide pointwise measurements at the location of the sensor. Thus, the presence of vertical gradients in, and horizontal variability of, sea surface salinity complicates comparison of satellite and in situ measurements. This paper synthesizes present knowledge of the magnitude and the processes that contribute to the formation and evolution of vertical and horizontal variability in near-surface salinity. Rainfall, freshwater plumes, and evaporation can generate vertical gradients of salinity, and in some cases these gradients can be large enough to affect validation of satellite measurements. Similarly, mesoscale to submesoscale processes can lead to horizontal variability that can also affect comparisons of satellite data to in situ data. Comparisons between satellite and in situ salinity measurements must take into account both vertical stratification and horizontal variability.

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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