Tag Archives: moorings

Sea Ice from Satellite at 20-m Resolution

I am a self-taught amateur on remote sensing, but it tickled my pride when a friend at NASA asked me, if I could tell a friend of his at NOAA on how I got my hands on data to produce maps of radar backscatter to describe how the sea ice near Thule Air Base, Greenland changes in time and space.

Wolstenholme Fjord, Greenland Feb.-5, 2017 from Sentinel-1 radar. The data are at 20-m resolution

Wolstenholme Fjord, Greenland Feb.-5, 2017 from Sentinel-1 radar. The data are at 20-m resolution

In about 4 weeks from today I will be working along a line near the red dots A, B, and C which are tentative locations to place ocean sensors below the sea ice after drilling through it with ice fishing gear. The colored line is the bottom depth as it was measured by the USCG Healy in 2003 when I was in Thule for the first time. Faint bottom contours are shown in gray.

I discovered the 20-m Sentinel-1 SAR-C data only 3 weeks ago. They are accessible to me (after making an account) via

https://scihub.copernicus.eu/dhus/#/home

where I then search for a specific geographic area and time frame using the following “product”

Product Type: GRD
Sensor Mode: IW
Polarization: HH

Screenshot on how I search for the Sentinel-1 SAR-C DATA.

Screenshot on how I search for the Sentinel-1 SAR-C DATA.

The more technical detail can be found at

https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar

where one also finds wonderful instructional videos on how to work the software.

The data file(s) for a typical scene are usually ~800 MB, however, for processing I use the free SNAP software (provided by European Space Agency) via a sequence of steps that result in a geotiff file of about 7 MB.

Screenshot of SNAP software and processing with [1] input and [2] output of the Feb.-5, 2017 data from Wolstenholme Fjord.

Screenshot of SNAP software and processing with [1] input and [2] output of the Feb.-5, 2017 data from Wolstenholme Fjord.

This .tiff file I then read with Fortran codes to tailor my own (quantitative or analyses) purposes.

Start of Fortran code to covert the SNAP output geotiff file into an ascii file with latitude, longitude, and backscatter as columns. The code has 143 lines plus 80 lines of comment.

Start of Fortran code to covert the SNAP output geotiff file into an ascii file with latitude, longitude, and backscatter as columns. The code has 143lines plus 80 lines of comment.

The final mapping is done with GMT – General Mapping Tools which I use for almost all my scientific graphing, mapping, and publications.

Please note that I am neither a remote sensing nor a sea-ice expert, but consider myself an observational physical oceanographer who loves his Unix on a MacBook Pro.

Working the Night shift aboard CCGS Henry Larsen in the CTD van in Aug.-2012. [Photo Credit: Renske Gelderloos]

Working the Night shift aboard CCGS Henry Larsen in the CTD van in Aug.-2012. [Photo Credit: Renske Gelderloos]

If only my next problem, working in polar bear country with guns for protection, had as easy a solution.

Polar bear as seen in Kennedy Channel on Aug.-12, 2012. [Photo Credit: Kirk McNeil, Labrador from aboard the Canadian Coast Guard Ship Henry Larsen]

Polar bear as seen in Kennedy Channel on Aug.-12, 2012. [Photo Credit: Kirk McNeil, Labrador from aboard the Canadian Coast Guard Ship Henry Larsen]

Sea Ice, Oceanography, and Nature’s Way to Paint

I am going to sea next week boarding the R/V Sikuliaq in Nome, Alaska to sail for 3 days north into the Arctic Ocean. When we arrive in our study area after all this traveling, then we have perhaps 18 days to deploy 20 ocean moorings. I worry that storms and ice will make our lives at sea miserable. So what does a good data scientist do to prepare him or herself? S/he dives into data:

Map northern Chukchi Sea with mooring locations (red and blue symbols), contours of bottom topography, and radar backscatter from space.

Map northern Chukchi Sea with mooring locations (red and blue symbols), contours of bottom topography, and radar backscatter from space. Slightly darker shades especially in the bottom segment are interpreted as sea ice. The offset in grey scale between top and bottom is caused by me using different numbers for two different data segments to bring the data into a range that varies between 0 and 1.

The image above is my first attempt to determine, if our planned mooring deployment locations are free of sea ice or not. The darker tones of gray are sea ice with the white spots probably thicker or piled-up ridges of rougher sea ice. The speckled gray surface to the north is probably caused by surface waves and other “noise” that are pretty random. There is a data point ever 40 meters in this image. It also helps to compare these very high-resolution ice data with products that the US National Ice Center (NIC) and the National Weather Service provide:

Ice Chart of the Alaska office of the National Weather Service (link)

Ice Chart of the Alaska office of the National Weather Service

The above is a wonderful map for general orientation, but it is not good or detailed enough to navigate a ship through the ice. The two maps agree, however, my patch of ice to the south of the moorings are represented as the orange/green patch on the top right (north-east). The orange means that 70-80% of the area is covered by ice and this ice is thicker than 1.2 meters and thus too thick for our ship to break through, but there are always pathways through ice and those can be found with the 40-m resolution maps.

In summary, on Sept.-29, 2016 all our moorings are in open water, but this can change, if the wind moves this math northward. So we are also watching the winds and here I like the analyses of Government Canada

Surface weather analysis from Government Canada for Oct.-2, 2016.

Surface weather analysis from Government Canada for Oct.-2, 2016. The map of surface pressure is centered on the north pole with Alaska at the bottom, Europe on the top, Greenland on the right, and Siberia on the left.

It shows a very low pressure center over Siberia to the south-west and a high pressure center over Arctic Canada to our north-east. This implies a strong wind to the north in our study area. So the ice edge will move north into our study area. If the High moves westward, we would be golden, but the general circulation at these latitudes are from west to east, that is, the Low over Siberia will win and move eastward strengthening the northward flow. That’s the bad news for us, but we still have almost 2 weeks before we should be in the area to start placing our fancy ocean moorings carefully into the water below the ice.

While this “operational” stuff motivated me to dive into the satellite radar data that can “see” through clouds and fog, I am most excited about the discovery that the radar data from the European Space Agency are easy to use with a little clever ingenuity and a powerful laptop (2.5 MHz Mac PowerBook). For example, this hidden gems appeared in the Chukchi Sea a few days earlier:

Close-up of the ice edge in the northern Chukchi Sea on Sept.-23, 2016. The mushroom cloud traced by sea ice and associated eddies are about 10-20 km across.

Close-up of the ice edge in the northern Chukchi Sea on Sept.-23, 2016. The mushroom cloud traced by sea ice and associated eddies are about 10-20 km across.

It is a piece of art, nature’s way to paint the surface of the earth only to destroy this painting the next minute or hour or day to make it all anew. It reminds me of the sand-paintings of some Native American tribes in the South-West of the USA that are washed away the moment they are finished. Here the art is in the painting, just as the pudding is in the eating, and the science is the thinking.

Oceanography below Petermann Gletscher for 400 Days

Ocean data from 810 meters below sea level under one of Greenland’s last remaining ice shelves arrives every 3 hours at my laptop via a 3-conductor copper cable that passes through 100 meter thick ice to connect to a weather station that via a satellite phone system connects to the rest of the world. This Ocean-Weather station on the floating section of Petermann Gletscher has reported for 400 days today. I am still amazed, stunned, and in awe that this works.

The station started 20th August of 2015 as a small part of a larger joint US-Swedish expedition to North Greenland after friends at the British Antarctic Survey drilled holes through the Empire-State-Building thick ice shelf. It is powered by two 12 Volt car batteries that are recharged by two solar panels. When the sun is down, the car batteries run the station as in winter when temperatures reached -46 C. When the sun is up, the solar cells run the station and top off the batteries. The voltage during the last 400 days shows the “health” of the station:

Battery voltage at the Petermann Ocean-Weather Station from Aug.-20, 2015 through  Sept.-23, 2016. The polar night is indicated by slowly declining voltage near 12 V while during the polar day voltage is near 14 V with oscillations in spring and fall during the transition from 24 hours of darkness to 24 hours of sun light.

Battery voltage at the Petermann Ocean-Weather Station from Aug.-20, 2015 through Sept.-23, 2016. The polar night is indicated by slowly declining voltage near 12 V while during the polar day voltage is near 14 V with oscillations in spring and fall during the transition from 24 hours of darkness to 24 hours of sun light.

There is an unexplained outage without data from February 12-25 (Day 175-189) which happened a day after the first data logger shut down completely without ever recovering. Our station has 2 data loggers: A primary unit controls 2 ocean sensors, atmospheric sensors, and the Iridium satellite communication. The secondary unit controls 3 ocean sensors and the GPS that records the moving glacier. Remote access to the secondary logger is via the primary, however, each logger has its own processors, computer code, and back-up memory card.

Inside of University of Delaware command and control of five ocean sensors and surface weather station. Two data loggers are stacked above each other on the left.

Inside of University of Delaware command and control of five ocean sensors and surface weather station. Two data loggers are stacked above each other on the left.

The primary logger failed 11th February 2016 when we received our last data via Iridium satellites until Keith Nicholls and I visited the station 27th and 28th August 2016 via helicopter from Thule, Greenland. Since I could not figure out what went wrong sitting on the ice with the helicopter waiting, I spent a long night without sleep to swap the data logger with a new and tested unit. I rewired sensors to new data logger, switched the Iridium modem, transceiver, and antenna, changed the two car batteries, and now both data loggers with all five ocean sensors have since reported faithfully every 3 hours as scheduled as seen at

http://ows.udel.edu

Lets hope that the station will keep going like as it does now.

The major discovery we made with the ocean data are large and pronounced pulses of fresher and colder melt waters that swosh past our sensors about 5 and 25 meters under the glacier ice. These pulses arrive about every 14 days and this time period provides a clue on what may cause them – tides. A first descriptive report will appear in December in the peer-reviewed journal Oceanography. Our deeper sensors also record increasingly warmer waters, that is, we now see warm (and salty) waters under the glacier that in 2015 we saw more than 100 km to the west in Nares Strait. This suggests that the ocean under the glacier is strongly coupled to the ambient ocean outside the fjord and vice versa. More on this in a separate future posting.

Time series of salinity (top) and potential temperature (bottom) from four ocean sensors deployed under the ice shelf of Petermann Gletscher from 20th of August 2015 through 11th of February 2016. Temperature and salinity scales are inverted to emphasize the vertical arrangements of sensors deployed at 95m (black), 115 (red), 300 m, and 450 m (blue) below sea level. Note the large fortnightly oscillations under the ice shelf at 95 and 115 m depth in the first half of the record. [From Muenchow et al., 2016]

Time series of salinity (top) and potential temperature (bottom) from four ocean sensors deployed under the ice shelf of Petermann Gletscher from 20th of August 2015 through 11th of February 2016. Temperature and salinity scales are inverted in order to emphasize the vertical arrangements of sensors deployed at 95m (black), 115 (red), 300 m, and 450 m (blue) below sea level. Note the large fortnightly oscillations under the ice shelf at 95 and 115 m depth in the first half of the record. [From Muenchow et al., 2016]

P.S.: The installation and year-1 analyses were supported by a grants from NASA and the Jet Propulsion Laboratory, respectively, while the current work is supported by NSF for the next 3 years. Views and opinions are mine and do not reflect those of the funding agencies.

Petermann Gletscher Ocean Station Revisited

Standing on floating Petermann Gletscher last sunday, I called my PhD student Peter Washam out of bed at 5 am via our emergency Iridium phone to check the machine that Keith Nicholls and I had just repaired. We had prepared for this 4 months and quickly established that a computer in Delaware could “talk” to a computer in Greenland to receive data from the ocean 800 m below my feet on a slippery glacier. For comparison the Empire State Building is 480 m high. The closest bar was 5 hours away by helicopter at Thule Air Force Base from where Keith and I had come.

Cabled ocean observatory linked to a University of Delaware weather station on Petermann Gletscher, Greenland on 28 August 2016. View is to the north.

Refurbished ocean observatory linked via cables to a University of Delaware weather station on Petermann Gletscher, Greenland on 28 August 2016. View is to the north.

Remote Petermann Gletscher can be reached by helicopter only of one prepares at least two refueling stations along the way. Anticipating a potential future need, we had placed 1300 and 1600 liters of A1 jet fuel at two points from aboard the Swedish icebreaker Oden in 2015. The fuel was given to Greenland Air with an informal agreement that we could use the fuel for a 2016 or 2017 helicopter charter. Our first pit stop looked like this on the southern shores of Kane Basin

Refueling stop on north-eastern Inglefield Land on 27 August 2016. Air Greenland Bell-212 helicopter in the background, view is to the north.

Refueling stop on southern Washington Land on 27 August 2016. Air Greenland Bell-212 helicopter in the background, view is to the south towards Kane Basin.

Helicopter flight path on 27/28 August 2016 to reach Petermann Gletscher (PG) via southern (Fuel-S) and northern (Fuel-N) fuel stops in northern Inglefield and southern Washington Land, respectively. Background color is ocean bottom depth in meters.

Helicopter flight path on 27/28 August 2016 to reach Petermann Gletscher (PG) via southern (Fuel-S) and northern (Fuel-N) fuel stops in northern Inglefield and southern Washington Land, respectively. Background color is ocean bottom depth in meters.

Upon arrival at the first (northern-most) Peterman Gletscher (PG) station we quickly confirmed our earlier suspicion that vertical motion within the 100 m thick glacier ice had ruptured the cables connecting two ocean sensors below the ice to data loggers above. We quickly disassembled the station and moved on to our central station that failed to communicate with us since 11 February 2016. Keith predicted that here, too, internal glacier motions would have stretched the cables inside the ice to their breaking point, however, this was not to be the case.

My first impression of this station was one of driftwood strewn on the beach of an ocean of ice:

Looks can be deceiving, however, and we found no damage to any electrical components from the yellow-painted wooden battery box housing two 12 Volt fancy “car batteries” at the bottom to the wind sensor on the top. Backed-up data on a memory card from one of two data loggers (stripped down computers that control power distribution and data collections) indicated that everything was working. The ocean recording from more than 800 meters below our feet was taken only a few minutes prior. In disbelief Keith and I were looking over a full year-long record of ocean temperature, salinity, and pressure as well as glacier motions from a GPS. This made our choices on what to do next very simple: Repair the straggly looking ocean-glacier-weather station, support it with a metal pole drilled 3.5 m into the glacier ice, and refurbish the adjacent radar station. We went to work for a long day and longer night without sleep.

Selfie on Petermann Gletscher on sunday 28 August 2016 after 33 hours without sleep. Weather station and northern wall of Petermann in the clouds. It was raining, too.

Selfie on Petermann Gletscher on sunday 28 August 2016 after 33 hours without sleep. Weather station and northern wall of Petermann in the clouds. It was raining, too.

When all was done, University of Delaware graduate student Peter Washam did the last check at 5:30 am sunday morning. Since then our Greenland station accepts Iridium phone calls every three hours, sends its data home where I post it daily at

http://ows.udel.edu

The data from this station will become the center piece of Peter’s dissertation on glacier-ocean interactions. Peter was part of the British hot water drilling team who camped on the ice in 2015 for 3 weeks while I was on I/B Oden responsible for the work on the physical oceanography of the fjord and adjacent Nares Strait. Alan Mix of Oregon State University prepared and led the 2015 expedition giving us ship and helicopter time generously to support our work on the ice shelf of Petermann. Saskia Madlener documented the scope of the 2015 work in a wonderful set of three videos

Ocean & Ice – https://vimeo.com/178289799
Rocks & Shells – https://vimeo.com/178379027
Seafloor & Sediment – https://vimeo.com/169110567

A first peer-reviewed publication on this station and its data until 11 February 2016 will appear in the December 2016 issue of the open-access journal Oceanography with the title The Ice Shelf of Petermann Gletscher, North Greenland and its Connection to the Arctic and Atlantic Oceans.

Ghosts of Discovery Harbor: Digging for Data

Death by starvation, drowning, and execution was the fate of 19 members of the US Army’s Lady Franklin Bay Expedition that was charged in 1881 to explore the northern reaches of the American continent. Only six members returned alive, however, they carried papers of tidal observations that they had made at Discovery Harbor at almost 82 N latitude, less than 1000 miles from the North Pole. Air temperatures were a constant -40 (Fahrenheit or Celsius) in January and February. While I knew and wrote of this most deadly of all Arctic expeditions, only 2 days ago did I discover a brief 1887 report in Science that a year-long record of hourly tidal observations exist. How to find these long forgotten data?

My first step was to search for the author of the Science paper entitled “Tidal observations of the Greely Expedition.” Mr. Alex S. Christie was the Chief of the Tidal Division of the US Coast and Geodedic Survey. He received a copy of the data from Lt. Greely. His activity report dated June 30, 1887 confirms receipt and processing of the data, but he laments about “deficient computer power” and requests “two computers of standard ability preferable by young men of 16 to 20 years.” Times and language have changed: In 1887 a computers was a man hired to crunch numbers with pen and paper.

Data table of 15 days of hourly tidal sea level observations extracted from Greely (1888).

Data table of 15 days of hourly tidal sea level observations extracted from Greely (1888).

While somewhat interesting, I still had to find the real data shown above, but further google searches of the original data got me to the Explorer’s Club in New York City where in 2003 a professional archivist, Clare Flemming, arranged and described the “Collection of the Lady Franklin Bay Expedition 1881-1884.” This most instructive 46 page document lists the entire collection of materials including Series III “Official Research” that consists of 69 folders in 4 Boxes. Box-4 File-15 lists “Manuscript spreadsheet on Tides, paginated. Published in Greely (1888), 2:651-662” as well as 3 unpublished files on tides and tide gauges. With this reference, I did find the official 1888 “Report on the United States Expedition to Lady Franklin Bay” of the Government Printing Office as digitized from microfiche as

https://archive.org/details/cihm_29328

which on page 641 shows the above table. There are 19 more tables like it, but at the moment I have digitized only the first one. Unlike my colleagues at the US Coast and Geodedic Survey in 1887, I do have enough computer power to graph and process these 15 days of data in mere seconds, e.g.,

Hourly tidal observations at Discovery Harbor taken for 15 days by Greely in 1881 and Peary in 1909.

Hourly tidal observations at Discovery Harbor taken for 15 days by Greely in 1881 and Peary in 1909.

A more technical “harmonic” analyses reveals that Greely’s 1881 (or Peary’s 1909) measured tides at Discovery Harbor have amplitudes of about 0.52 m (0.59) for the dominant semi-diurnal and 0.07 m (0.12) for the dominant diurnal oscillation. My own estimates from a 9 year 2003 to 2012 record gives 0.59 and 0.07 m for semi-diurnal and diurnal components. This gives me confidence, that both the 1881 and 1909 data are good, just have a quick look at 1 of the 9 years of data I collected:

Tidal sea level data from a pressure sensor placed in Discovery Harbor in 2003. Each row is 2 month of data starting at the top (August 2003) and ending at the bottom (July 2004).

Tidal sea level data from a pressure sensor placed in Discovery Harbor in 2003. Each row is 2 month of data starting at the top (August 2003) and ending at the bottom (July 2004).

There is more to this story. For example, what happened to the complete and original data recordings? Recall that Greely left Discovery Harbor late in the fall of 1883 after supply ships failed to reach his northerly location two years in a row. This fateful southward retreat from a well supplied base at Fort Conger and Discovery Harbor killed 19 men. Unlike ghostly Cape Sabine where most of the men perished, Discovery Harbor had both local coal reserves and musk ox in the nearby hills that could have provided heat, energy, and food for many years.

It amazes me, that a 1-year copy of tidal data survived the death march of Greely’s party. It took another 18 years for the complete and original records to be recovered by Robert Peary who handed them to the Peary Arctic Club which in 1905 morphed into Explorer’s Club of New York City. I suspect (but do not know), that these archives contain another 2 years of data that nobody but Edward Israel in 1882/83 and the archivist in 2003 laid eyes on. Sergeant Edward Israel was the astronomer who collected the original tidal data. He perished at Cape Sabine on May 29, 1884, 25 years of age.

Edmund Israel, astronomer of the Lady Franklin Bay Expedition of 1881-1884.

Edmund Israel, astronomer of the Lady Franklin Bay Expedition of 1881-1884.

References:

Christie, A.S., 1887: Tidal Observations of the Greely Expedition, Science, 9 (214), 246-249.

Greely, A.W., 1888: Report on the Proceedings of the United States Expedition to Lady Franklin Bay, Grinnell Land, Government Printing Office, Washington, DC.

Guttridge, L., 2000: The ghosts of Cape Sabine, Penguin-Putnam, New York, NY, 354pp.