Category Archives: Computing

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 and 2016 Arctic field work

The sea ice in the Arctic Ocean is quickly disappearing from coastal areas as we are entering the summer melt season. This year I follow this seasonal event with nervous anticipation, because in October and November we will be out at sea working north of northern Alaska. We plan to deploy a large number of ocean sensors to investigate how sound propagates from the deep Arctic Ocean on to the shallow Chukchi Sea. This figure shows our study area with the ice cover as it was reported yesterday from space:

Ice concentration for June 14, 2016 from SSM/I imagery. Insert show study area to the north of Alaska and planned mooring locations (red box).

Ice concentration for June 14, 2016 from SSM/I imagery. Insert show study area to the north of Alaska and planned mooring locations (red box).

Zooming in a little further, I show the coast of Alaska along with 100 and 1000 meter contour of bottom depth over a color map of ice concentrations:

Ice concentrations from SSM/I to the north of norther Alaska with planned mooring locations across the sloping bottom. The 100 and 1000 meter contours are shown in gray with blue and red symbols representing locations of ocean and acoustic sensors, respectively.

Ice concentrations from SSM/I to the north of norther Alaska with planned mooring locations across the sloping bottom. The 100 and 1000 meter contours are shown in gray with blue and red symbols representing locations of ocean and acoustic sensors, respectively.

My responsibilities in this US Navy-funded project are the seven densely packed blue triangles. They indicate locations where I hope to measure continuously for a year ocean temperature, salinity, and pressure from which to construct sections of speed of sound and how it varies in time and space. I will also measure ice draft as well ice and ocean currents from which to estimate the roughness of the sea ice over time. Sea ice and ocean properties both impact sound propagation from deep to shallow water and vice versa.

A first question: What will the ice be like when we get there? This is the question that has the 40 or so people all working on this project anxiously preparing for the worst, but how can we expect what challenges are to come our way?

Doing my homework, I downloaded from the National Snow and Ice Data Center all gridded maps of ice concentrations that microwave satellites measured almost daily since 1978. Then I crunch the numbers on my laptop with a set of kitchen-sink Unix tools and code snippets such as

set ftp = 'ftp://sidads.colorado.edu'
set dir = 'pub/DATASETS/nsidc0081_nrt_nasateam_seaice/north'
...
wget -r -nd -l1 --no-check-certificate $ftp/$dir/$year/$file

along with fancy and free Fortran and General Mapping Tools to make the maps shown above. With these tools and data I can then calculate how much sea ice covers any area at any time. The result for custom-made mooring area at almost daily resolution gives a quick visual that I use to prepare for our fall 2016 expedition. The dotted lines in the top panel indicate the dates we are in the area.

Time series of daily ice concentration in the study area for different decades from January-1 through Dec.-31 for each year from 1980 through 2015. Panels are sorted by decade. The red curve is for 2015 and is shown for comparison in all panels.

Time series of daily ice concentration in the study area for different decades from January-1 through Dec.-31 for each year from 1980 through 2015. Panels are sorted by decade. The red curve is for 2015 and is shown for comparison in all panels.

The story here is well-known to anyone interested in Arctic sea ice and climate change, but here it applies to a tiny spec of ocean between the 100 and 1000 meter isobath where we plan to deployed ocean sensors for a year in the fall of 2016. For the two decades of the last century, the ice cover looks like a crap shoot with 80% ice cover possible any month of the year and ice-free conditions unlikely but possible here or there for a week or two at most. The situation changed dramatically since about 2000. During the last six years our study area has always been free of ice from late August to early October, however, our 2016 expedition is during the transition from ice-free October to generally ice-covered early November, but, I feel, our saving grace is that the sea ice will be thin and mobile. I thus feel that we probably can work comfortable on account of ice for the entire period, but the winds and waves will blow us away …

Weather will be most uncomfortable, because fall is the Pacific storm season. And with little or only thin ice, there will be lots and lots of waves with the ship pitching and rolling and seeking shelter that will challenge us from getting all the work done even with 7 days for bad weather built into our schedule.

I worked in this area on larger ships in 1993, 2003, and in 2004. Here is a photo that Chris Linder of Woods Hole Oceanographic Institution took during a massive storm in the general vicinity in October of 2004. The storm halted all outside work on the 420 feet long USCGC Healy heading into the waves for 42 long and miserable hours:

Icebreaker taking on waves on the stern during a fall storm in the Beaufort Sea in October 2004. [Photo Credit: Chris Linder, Woods Hole Oceanographic Institution]

Icebreaker taking on waves on the bow during a fall storm in the Beaufort Sea in October 2004. [Photo Credit: Chris Linder, Woods Hole Oceanographic Institution]

Oh, I now also recall that during this four-week expedition we never saw land or the sun. It was always a drizzly gray ocean on a gray horizon. The Arctic Ocean in the fall is an often cruel and inhospitable place with driving freezing rain and fog.

How to Power Modern Economies: Read Your Meter

Read you meter at home. This fun-filled advice was given by Sir David MacKay in a wonderful TEDx talk about how we heat our homes, get to work, run our computers, and how it all scales across countries and continents. The idea is really about how we run our lives while also trying to pass on a livable planet to our grand-children without the politically correct “greenwash” and self-righteous “claptraps”. Read your meter, do some algebra, and embrace the adventure to explore your home, your life, and the energy it all takes. If you read this far, watch the movie

David MacKay taught physics and information theory at the University of Cambridge in England. I learnt of him via Ruth Mottram in one of her many tweets. Dr. Mottram studies climate impacts of Greenland glaciers and works at the Danish Meteorological Institute. The tweet made me buy the book “Information Theory, Inference, and Learning Algorithm’s” that David MacKay wrote a few years back. It arrived today.

What piqued my interest was the advanced math that goes into designing networks that send and transform information such telephone calls via wireless, computer networks, and how to deal with imperfect channels of communication. My marriage comes to mind, too, because what I say is not always what I mean which is not always what my wife hears and vice versa, but I digress. Imperfect communication channels are one challenge we will face in an experiment to explore acoustic underwater data transmissions that hopefully will take place next year out of Thule Air Force Base, Greenland. Water and ice are imperfect communication channels that we need to use wisely to make our whispers carry far. Try to talk to a person across a busy street in Manhattan with all its hustle and bustle; you need to find something smarter and more effective than just simple shouting.

David MacKay wrote a second book that is close to his TEDx talk and is called “Sustainable Energy without the Hot Air.” Experimenting at home like any good physicist does, he discovers that “… the more often I read my meter, the less gas I use!”

There is so much more to this man, his work, and ideas as a physicist with a keen interest in the big picture without skipping the details. Sadly, he died yesterday of cancer too early only 48 years of age.

Thule, Greenland in Sharp Focus

I want to fly like an eagle
To the sea
Fly like an eagle
Let my spirit carry me

Steve Miller Band, 1976

The eagle “sees” the ground, because the twinkling sensation of light tickles her nerves. Today’s cameras work without the twinkle and tickle. They store numbers (digits) that approximate the amount of light passing through the lens. Satellite sensors work the same way. The data they beam to earth give me the soaring feeling of flying like an eagle, but there is more to the bits and bytes and digits sent home from space to our iPhones, laptops, and the internet.

Aerial photo taken Oct.-13, 1860 of Boston, MA by J.W. Black.

Aerial photo taken Oct.-13, 1860 Boston, MA from a balloon by J.W. Black.

The Metropolitan Museum of Art in New York houses the earliest existing aerial photo that was taken from a balloon hovering 600 meters above Boston, Massachusetts. Within a year the American Civil War broke out and this new technology became an experimental tool of war. It advanced rapidly, when air craft replaced the balloon during the First World War. Sharp photos of bombed-out battle and killing fields along the entire Western Front in France were taken by both Allied and German soldiers every day. Placing these photos on a map for efficient analyses of how a land- sea- or ice-scape changes over time, however, was impossible, because photos do not record precise locations.

Modern satellite photos are different. We now have fancy radar beams, computers, and several Global Position Systems (GPS) with atomic clocks to instantly calculation satellite tracks every second. This is why we now can both take photos from space AND map every dot or pixel that is sensed by the satellite moving overhead at 17,000 miles an hour snapping pictures from 430 miles above. The camera is so good that it resolves the ground at about 45 feet (15 meters). This is what such a (LandSat) picture looks like

LandSat photo/map of Thule, Greenland Mar.-17, 2016. The airfield of Thule Air Force Base is seen near the bottom on the right. The island in ice-covered Westenholme Fjord is Saunders Island (bottom left) while the glacier top right is Chamberlin Gletscher.

LandSat photo/map of Thule, Greenland Mar.-17, 2016. The airfield of Thule Air Force Base is seen near the bottom on the right. The island in ice-covered Westenholme Fjord is Saunders Island (bottom left) while the glacier top right is Chamberlin Gletscher.

Everyone can download these photos from the United States Geological Survey which maintains a wonderful photo and data collection archive at

http://earthexplorer.usgs.gov

but the tricky part is to turn these images or photos into maps which I have done here. More specifically, I wrote a set of c-shell and nawk scripts along with Fortran programs on my laptop to attach to each number for the light sensed by the satellite (the photo) another two numbers (the map). These are latitude and longitude that uniquely fix a location on the earth’s surface. A “normal” photo today has a few “Mega-Pixels,” that is, a few million dots. Each scene of LandSat, however, has about 324 million dots. This is why you can discern both the runways of Thule Air Force Base at 68 degrees 45′ West longitude and 76 degrees 32′ North latitude. The pier into the ice-covered ocean is just a tad to the south of Dundas Mountain at 68:54′ W and 76:34′ N. A scale of 5 kilometers is shown at the top on the right. For spatial context, here is a photo of the pier with the mountain in the background, that is, the object shown in the photo such as mountain, ship, and Helen serves a rough, but imprecise reference:

Dr. Helen Johnson in August 2009 on the pier of Thule AFB with CCGS Henry Larsen and Dundas Mountain in the background. [Credit: Andreas Muenchow]

Dr. Helen Johnson in August 2009 on the pier of Thule AFB with CCGS Henry Larsen and Dundas Mountain in the background. [Credit: Andreas Muenchow]

This photo shows the airfield and Saunders Island

Thule AFB with its airport, pier, and ice-covered ocean in the summer. The island is Saunders Island. The ship is most likely the CCGS Henry Larsen in 2007. [Credit: Unknown]

Thule AFB with its airport, pier, and ice-covered ocean in the summer. The island is Saunders Island. The ship is most likely the CCGS Henry Larsen in 2007. [Credit: Unknown]

The satellite image of the ice-covered fjord with Thule, Saunders Island, and Chamberlin Gletschers shows a richly texture field of sea ice. The sea ice is stuck to land and not moving except in the west (top left) where it starts to break up as seen by the dark gray piece that shows ‘black’ water peeking from below a very thin layer of new ice. There is also a polynya at 69:15′ W and 76:39′ N just to the south of an island off a cape. A polynya is open water that shows as black of very dark patches. A similar albeit weaker feature also shows to the east of Saunders Island, but it is frozen over, but the ice there is not as thick as it is over the rest of Westenholme Fjord. I suspect that larger tidal currents over shallow water mix ocean heat up to the surface to keep these waters covered by water or dangerously thin ice. There are also many icebergs grounded in the fjord. They cast shadows and from the length of these shadows one could estimate their height. Here is another such photo from 2 days ago:

LandSat photo/map of Thule, Greenland Mar.-21, 2016. The airfield of Thule Air Force Base is seen near the bottom on the right. The island in ice-covered Westenholme Fjord is Saunders Island (bottom left) while the glacier top right is Chamberlin Gletscher.

LandSat photo/map of Thule, Greenland Mar.-21, 2016. The airfield of Thule Air Force Base is seen near the bottom on the right. The island in ice-covered Westenholme Fjord is Saunders Island (bottom left) while the glacier top right is Chamberlin Gletscher.

I am using the satellite data and maps here to plan an experiment on the sea ice of Westenholme Fjord. Next year in March/April I will lead a team of oceanographers, engineers, and acousticians to place and test an underwater network to send data from the bottom of the ocean under the sea ice near Saunders Island to the pier at Thule and from there on to the internet. We plan to whisper from one underwater listening post to another to communicate over long ranges (20-50 kilometers) via a network of relay stations each operating smartly at very low energy levels. We will deploy these stations through holes drilled through the landfast ice 1-2 meters thick. The work is very exploratory and is funded by the National Science Foundation. Wish us luck, as we can and will use it … along with aerial photography that we turn into maps.

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.