Ben Holt, Research Scientist, Ocean Circulation and Air-Sea Interaction Group, NASA’s Jet Propulsion Laboratory (JPL)
Research interests: Ben Holt uses remote sensing data to study the geophysical state of polar sea ice and snow, investigate circulation along coasts, and detect marine pollutants. He also uses these data to develop new instruments and techniques for the microwave measurement of sea ice thickness.
Current research: Studying polar sea ice from the ground has never been easy. Winter Antarctic temperatures average about -50˚C (-58˚F), while summer Arctic high temperatures struggle to reach 0˚C (32˚F). Along with frigid temperatures, the vast distances over which data must be collected can create logistical nightmares. Polar orbiting Earth observing satellites, such as NASA’s Terra and Aqua, and the use of synthetic aperture radar (SAR), which does not need illumination from the sun and can penetrate through clouds, ushered in a new era of polar data collection. Even with the benefits of satellites, though, researchers still need to conduct field campaigns at the poles to validate these remotely-sensed data.
For Ben Holt, field campaigns are an important aspect of his research into the impact of ocean waves on sea ice in the marginal ice zone in the Arctic Ocean, a project funded by the U.S. Office of Naval Research. Holt uses satellite data along with data collected by aircraft and during field campaigns to derive sea ice morphology, ice type, and floe size distribution over thousands of miles of Arctic ocean. A field campaign was conducted in fall 2015 in the Beaufort and Chukchi Seas to measure and observe waves and their impact on sea ice. Along with the use of multi-sensor SAR data collected by satellites (such as Canada’s RADARSAT-2 and the European Space Agency’s [ESA] Sentinel-1A), Holt also relies on data collected by instruments on aircraft (through campaigns such as NASA’s Operation IceBridge and as flown by the uninhabited aerial vehicle SAR [UAVSAR]) and other fine resolution imagery of sea ice, such as from the U.S. Geological Survey’s (USGS) Landsat 8 satellite. In addition, as a member of the science definition team for the upcoming joint NASA-Indian Space Research Organisation SAR (NISAR) mission, Holt is helping to define science and mission requirements for deriving SAR-based polar sea ice motion products.
Holt also is conducting research at lower latitudes. In the Great Lakes, he is part of a team studying seasonal patterns of thermal bar overturning and relationships to the lake circulation and the generation of small eddies. Thermal bars form in the Great Lakes in the spring when the water begins to warm. As this water warms above freezing, it becomes denser. This warmer, denser surface water sinks, and a curtain of water running parallel to shore that is 4˚C (39˚F)—the temperature at which water has its highest density—develops. With continued summer warming, the thermal bar gradually moves farther offshore into deeper, cooler water. The difference in water density within the thermal bar restricts mixing between coastal and offshore waters. Thermal bars are areas of not only warmer water temperatures, but also elevated nutrient concentrations, which can make them areas in which fish congregate. On the other hand, thermal bars also can help retain pollutants in coastal waters. For this research, Holt and his colleagues are using a lake surface temperature product from the Advanced Along-Track Scanning Radiometer (AATSR) aboard the ESA Envisat satellite as well as SAR imagery from multiple sensors.
Finally, Holt uses remote sensing data to detect and track marine pollutants. In one study, he is using an analysis of NASA UAVSAR polarimetric SAR imagery to estimate the thickness and transport properties of oil spills. This work is currently using data collected off Norway in June 2015 during a controlled spill experiment and is based on earlier work using UAVSAR collections from the Gulf of Mexico as a result of the Deepwater Horizon oil spill in 2010. Holt also uses sea surface temperature, optical properties and chlorophyll, and surface roughness data from multiple satellite sensors to detect coastal pollution in Southern California caused by stormwater runoff and wastewater plumes. The goal of this project is to identify sources of pollution impacting local water quality and the movement of this pollution.
Data products used:
- SAR data sets available through the Alaska Satellite Facility Distributed Active Archive Center (ASF DAAC), including:
- Operation IceBridge data available through the National Snow and Ice Data Center DAAC (NSIDC DAAC)
- Moderate Resolution Imaging Spectroradiometer (MODIS) sea surface temperature (SST) data from the Terra and Aqua Earth observing satellites and ocean color data from Terra and Aqua available through the Ocean Biology DAAC (OB.DAAC)
- ALOS-2 data available through the Japan Aerospace Exploration Agency
- Envisat data available through ESA
- Landsat 8 data available through USGS
Research findings: In his research into the effect of waves on Arctic sea ice, Holt and his colleagues found that an increase in open water during the summer period in the Arctic Ocean has led to more ocean waves due to the increased fetch, or distance across the open ocean, that wind can blow. This increase in ocean waves impacts summer ice melt as well as the formation of young sea ice during the fall period of sea ice growth and advance.
While studying thermal bars in several locations in Lake Superior, Holt and his colleagues noted a thermal gradient between warmer nearshore waters and colder offshore waters that enhances cyclonic coastal currents. The research team observed small eddies, and identified and mapped basic eddy characteristics including diameter, location, and rotation. The eddies were located within the region of sharp thermal gradients (3-5˚C [37.4-41˚F] per 3 km [1.86 mi]). While the spatial and temporal coverage of the eddies were uneven, more eddies were seen in SAR images taken in late summer along the southern and eastern shores as well as areas where the boundary current interacts with topographic features such as islands and promontories.
Finally, in his pollution research, Holt and his colleagues were able to derive a method of identifying thicker emulsified oil from thinner oil using UAVSAR data. They are continuing to evaluate this method under varying ocean conditions and with varying oil properties.
Read about the research:
Wang, Y., Holt, B., Rogers, W.E., Thomson, J. & Shen, H.H. (2016). Wind and wave influences on sea ice floe size and leads in the Beaufort and Chukchi Seas during the summer-fall transition 2014. Journal of Geophysical Research: Oceans, 121(2). doi:10.1002/2015JC011349
Holt, B., Johnson, M.P., Perkovic-Martin, D. & Panzer, B. (2015). Snow depth on Arctic sea ice derived from radar: In situ comparisons and time series analysis. Journal of Geophysical Research: Oceans, 120(6). doi:10.1002/2015JC010815
McKinney, P., Holt, B. & Matsumoto, K. (2012). Small eddies observed in Lake Superior using SAR and sea surface temperature imagery. Journal of Great Lakes Research, 38(4). doi:10.1016/j.jglr.2012.09.023
Minchew, B., Jones, C. & Holt, B. (2012). Polarimetric analysis of backscatter from the Deepwater Horizon oil spill using L-band synthetic aperture radar. IEEE Transactions on Geoscience and Remote Sensing, 50(10). doi:10.1109/TGRS.2012.2185804
For more information
- Operation IceBridge
- Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR)
- NASA/ISRO SAR mission
- Moderate Resolution Imaging Spectroradiometer (MODIS)