Monthly Archives: April 2012

Remote Islands, part I: Easter Island

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With the I-bands having ~375 m resolution at nadir, VIIRS is a powerful instrument. We have already seen the detailed imagery it produces of severe thunderstorms and tropical cyclones. But, you might ask (particularly if you’re thinking you need a vacation), what remote islands is it able to see?

Well, it can see Easter Island. Yes, the one with all the big-headed statues (moai).

False color RGB composite (I1-I2-I3) image of Easter Island, 20:44 UTC 25 April 2012

False color RGB composite (I1-I2-I3) image of Easter Island, 20:44 UTC 25 April 2012

At approximately 24.6 km x 12.3 km, VIIRS has no problem identifying the triangular island, as this false color (I1-I2-I3) RGB composite shows. In this image, taken at 20:44 UTC on 25 April 2012, the 163 km2 island appears to be dwarfed by a thunderstorm just to its north.  If you zoom in, you can see several small cumulus clouds over the island along with their shadows. Unfortunately, it is not quite the resolution needed to see the individual moai.

As Easter Island is in the southern hemisphere, it is autumn there now. The average high temperature is down to 76 °F (from a summertime peak of 79 °F in February). April and May are listed as the wettest months, so an image of Easter Island not obscured by clouds this time of year may be a rare occurrence.

The Last Line of Storms from the 14 April 2012 Tornado Outbreak

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The second major tornado outbreak of the year took place on 14 April 2012 (after the 2 March outbreak that slammed Indiana and Kentucky). At last count, 115 tornadoes were reported from Oklahoma to Iowa. Credit must be given to the Storm Prediction Center, National Weather Service offices, and local TV and other media outlets for accurately predicting the severe weather event and keeping people informed as it happened, and the people of the area for paying attention to the weather. It must be counted as a success on many levels that 115 tornadoes over 4 states only resulted in 6 deaths (and those deaths occurred in the toughest situation to warn people – a rain-wrapped tornado in the middle of the night where the tornado sirens were disabled due to a lightning strike earlier in the day).

The last bout of severe weather occurred with a squall line that formed in the late evening (~02:30 UTC 15 April 2012) along the dry line in western Texas and quickly expanded into Oklahoma and Kansas. This line produced the deadly tornado in Woodward, OK, along with many reports of 1-2″ diameter hail. Suomi-NPP passed over this line of storms between 07:45 and 07:50 UTC (15 April). The high resolution infrared window band, I-5 (11.45 µm), shows the immense scale of this storm system stretching from Wisconsin and Minnesota to Texas, in great detail. Be sure to click on the image, then on the “1497×1953” link below the banner to see it in full resolution. (The full resolution image is ~2MB in size.)

View of a squall line over the Central Plains from VIIRS channel I-5, 7:45 UTC 15 April 2012

View of the squall line over the Central Plains from VIIRS channel I-5, 7:45 UTC 15 April 2012

The color scale here is the same one used for the 2 March 2012 tornado outbreak image and the 25 January squall line over southeast Texas. The darkest blue pixels visible amongst the white overshooting tops (more easily visible on the southern end of the squall line) have a brightness temperature below -77 C, indicative of very strong convection.

VIIRS view of Invest 97S at night

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On 5 April 2012, the Joint Typhoon Warning Center was watching an area of the Mozambique Channel for possible development of a tropical cyclone. This area was named Invest 97S. As 6 April 2012 was a full moon, this is a good case to test the capabilities of low-light visible imagery channels for detection of tropical cyclone development at night.

The Operational Linescan System (OLS) aboard the Defense Meteorological Satellite Program (DMSP) satellite F-18 has a low-light visible channel (that inspired the development of the Day-Night Band (DNB) for VIIRS). The image below is from this channel on F-18, taken at 17:22 UTC, 5 April 2012 (courtesy the Naval Research Laboratory).

DMSP OLS low-light visible image of Invest 97S, taken at 17:22 UTC, 5 April 2012

DMSP OLS low-light visible image of Invest 97S, taken at 17:22 UTC, 5 April 2012. Image courtesy Naval Research Laboratory.

The landmass on the right of the image is Madagascar with Mozambique on the left side of the image. A low-level circulation is visible in the clouds just off the coast of Madagascar in the center of the image.

Suomi-NPP passed over the area at 23:02 UTC. The images below are taken from the VIIRS DNB, which is a low-light visible channel (centered at 0.7 µm) with higher radiometric resolution, a higher signal-to-noise ratio and higher spatial resolution. The second image is a zoomed-in version of the first.

VIIRS DNB image of Invest 97S taken at 23:02 UTC, 5 April 2012

VIIRS DNB image of Invest 97S taken at 23:02 UTC, 5 April 2012. Image courtesy Dan Lindsey and Steve Miller.

Zoomed-in image of Invest 97S from the VIIRS DNB taken at 23:02 UTC, 5 April 2012

Zoomed-in image of Invest 97S from the VIIRS DNB taken at 23:02 UTC, 5 April 2012. Image courtesy Dan Lindsey and Steve Miller.

In the nearly six hours that elapsed between the DMSP OLS image and the VIIRS DNB image, you can see that the line of deeper convection to the southwest of the circulation center has moved further south away from the center of the circulation and outflow from these storms has cleared out the low level clouds from where the storms used to be.

Compare these images with the high-resolution infrared window channel (11.45 µm), I-5, from VIIRS, seen below.

VIIRS channel I-5 image of Invest 97S, taken at 23:02 UTC, 5 April 2012

VIIRS channel I-5 image of Invest 97S, taken at 23:02 UTC, 5 April 2012.

The low level circulation is difficult to distinguish, given that there is no significant temperature contrast between the low level clouds and the background (ocean) surface. The deeper convective clouds are easy to spot in I-5, however.

The information provided by the VIIRS DNB near full moon events would be a great help to tropical cyclone forecasting in cases such as this where, typically, only IR data is available at night. Assuming latency issues with VIIRS can be solved, of course.

In the end, Invest 97S failed to develop into a tropical cyclone, which spared Madagascar and Mozambique – both of which had been affected by the cyclones Giovanna and Funso earlier this year.

Time-lapse of the Lower North Fork Fire

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On 26 March 2012, strong winds, high temperatures and low humidities re-ignited embers from a controlled burn that took place the previous week near Conifer, CO. The Lower North Fork fire quickly spread in the high winds, eventually burning more than 4000 acres and damaging or destroying 27 homes. Three people were killed, presumably because they were unable to evacuate before their homes were engulfed in flame. One family’s daring escape from the fire was caught on a cell phone camera and made national news (CAUTION: strong language has not been edited out). Many interesting pictures of the fire may be found here, here, and here.

Channel I-4 of VIIRS (centered at 3.74 µm) captured the hot spot from the Lower North Fork fire on each of Suomi-NPP’s afternoon (ascending) overpasses last week. These images make up the loop shown below.

5-day loop of I-4 images of the Lower North Fork fire

5-day loop of afternoon I-4 images of the Lower North Fork fire

In this image loop, the color scale represents observed brightness temperature such that warmer pixels appear darker and cooler pixels appear lighter. Pixels warmer than 330 K appear black, and pixels colder than 250 K appear white. The time between each image in the loop is approximately 24 hours.

The first image in the loop, taken at 20:24 UTC on the 26th, captured the hot spot shortly after the fire was first reported. The hot spot as seen by I-4 expanded significantly during the first 24 hours, before lighter winds and firefighting efforts greatly limited the growth of the fire. Over the last three frames, the hot spot can be seen to cool and shrink slightly.

Low (liquid) clouds can be seen as dark splotches on the images from the 28th and 29th of March, which should not be confused with fires. This is due to the fact that liquid clouds are highly reflective at 3.7 µm, and the reflection of solar radiation during the day increases the observed brightness temperature, so they appear darker. The persistently bright sideways “C” shape to the northeast of the fire is Chatfield Reservoir, which has a low brightness temperature due to the low water temperature in the reservoir and the relatively low emissivity of liquid water at this wavelength. Cherry Creek Reservoir (to the northeast of Chatfield Reservoir) and Marston Lake (to the north of Chatfield Reservoir) can also be seen.

With clear skies, the burn area shows up quite clearly in the I-band false color RGB composite of I-1, I-2 and I-3, taken at 20:06 UTC 27 March 2012 – the same time as the second frame of the loop above.

RGB composite of VIIRS channels I-1, I-2 and I-3 of the Lower North Fork fire, 20:06 UTC 27 March 2012

RGB composite of VIIRS channels I-1, I-2 and I-3 of the Lower North Fork fire, 20:06 UTC 27 March 2012

The burn area shows up as a sizeable dark brown spot in the forests (which show up as green) southwest of Denver.

After the driest and warmest March on record in Denver, hopefully this is not the start of a long, devastating fire season (link goes to PDF file).