There’s Something in the Water

In the fast paced world of weather, Hurricane Irma is old news. There’s already a Wikipedia page on it. But, people that were in Irma’s path are still cleaning up (at least at the time I’m writing this). In case you’ve already forgotten, or were living in a Faraday cage underground, here’s a quick recap. Among the factoids: Irma was the strongest hurricane ever recorded in the Atlantic basin and it was a Category 5 (the highest the scale goes) for the longest period of time of any Atlantic hurricane. The island of Barbuda took a direct hit from Irma and is now desolate and decimated. Jacksonville, which did not take a direct hit, received record flooding due to winds blowing the St. Johns River inland, while heavy rains inland were trying to flow out to sea. And, the hearing impaired mocked Manatee County, Florida for using a sign language interpreter that didn’t know sign language. Just in the U.S. alone, 26 people died.

Satellite imagers with higher resolution than VIIRS captured the damage. First, Landsat (~30 m spatial resolution) showed how vegetation was stripped from the soil in Antigua, Barbuda and the Virgin Islands. And, Worldview-4 (~30 cm resolution!) captured images of damaged structures in the Florida Keys and other islands in the Caribbean for Digital Globe (not a paid advertisement or endorsement). Our newest satellite, GOES-16, monitored Irma all the way from birth to death. (Shout out to my collegues at CIRA who provided the imagery used in that article!) And, of course, the VIIRS Day/Night Band showed the extent of power outages in Florida, which I won’t talk about further because I’ve already been beaten to it.

But, VIIRS works during the day, too. And it captured an aspect of Irma’s impact not mentioned above. We’ll start by taking a look at a VIIRS True Color image from 31 August 2017:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (1840 UTC 31 August 2017)

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (1840 UTC 31 August 2017)

Remember, you can click on an image to bring up the full resolution version. Let’s compare this “before” image with one taken after Irma hit:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (1813 UTC 12 September 2017)

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (1813 UTC 12 September 2017)

Notice anything different between the two images?

Apart from all the clouds (which are always different between two images), it shouldn’t take long to notice a change in the water surrounding Florida and, to a lesser extent, the Bahamas. You see, hurricanes bring with them heavy rains, high winds and waves and storm surge. The winds and waves churn up sediment at the bottom of the ocean – like this guy, only more, at least in shallow areas like the Florida Keys and the Bahamas. The storm surge causes beach erosion and flooding along the coasts while the heavy rains cause inland flooding (of both the “flash” and “river” variety). And, when was the last time you saw crystal clear floodwater? Floodwater is filled with dirt from the soils it eroded. Plus, there’s often garbage, raw sewage and toxic chemicals that may make it as hazardous as the hurricane itself. And, let’s not mention floating fire ant colonies because no one want to think about those – except I just did.

If you look closely, you may even see this sediment and pollution beginning to be entrained in currents in the Gulf of Mexico as well as on the Atlantic side of Florida. And, remember that the Atlantic side of Florida is home to the Gulf Stream (the current, not the aircraft).

Of course, we don’t have to just compare two days. We can monitor this sediment and pollution for as long as it’s there. Here’s a video showing both the before image (31 August 2017) and 6 days after (12-17 September 2017):


 
You can view it in full screen by clicking on the icon in the lower right corner of the video. After watching it several times, you should see two things: sediment around the Florida Keys does get pulled into the Gulf Stream, with visible eddies where the polluted water meets the clean water; and the polluted water generally gets darker with time. The latter is due to the fact that more of the dirt and sand and garbage settle out with time, slowly restoring the ocean to its pre-Irma appearance.

You might also notice the ocean around the Bahamas is always lighter in color. This is true even in the “before” image. This is because the water is very shallow in the Bahama Banks, and you can see all the way to the bottom. But, offshore on the west side of the largest island (Andros) the water becomes nearly white after Irma’s passage:

Comparison of VIIRS True Color images before and after Hurricane Irma (2017)

Comparison of VIIRS True Color images before and after Hurricane Irma (2017)

Go back to the video and see that it barely darkens over time. It is possible that, just like flood-induced erosion changes the landscape on the ground, the storm-induced waves and surge may have altered the underwater topography (“bathymetry”) of the Grand Bahama Bank and made the water even shallower. We’ll just have to wait and see how dark it gets.

Postscript: our VIIRS-like geostationary imager, the Advanced Baseline Imager (ABI) on GOES-16 also saw this sediment in the waters off the coast of Florida: click here. Remember, ABI doesn’t have a green wavelength visible band, but that’s no problem for CIRA’s Synthetic True Color imagery! [/end shameless plug]

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Hurricane Isaac: Before, During and After

While Hurricane Isaac (then a tropical storm) did not destroy Tampa, Florida as many people feared, it certainly left its mark on the Gulf Coast. With many locations from Florida to Louisiana receiving more than 12″ of rain, and levees unable to keep out the storm surge, flooding was (and still is) a major problem. Look at these aerial photos of Isaac’s aftermath in Louisiana. The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard Suomi NPP saw that flooding, also.

But first, let’s look at the high resolution infrared (IR) window channel (I-05, 11.45 µm) which, at ~375 m resolution, is the highest-resolution IR window channel on a public weather satellite in space today. This image was taken when Isaac was still a tropical storm in the middle of the Gulf of Mexico:

VIIRS I-05 image of Tropical Storm Isaac, taken 18:50 UTC 27 August 2012

VIIRS I-05 image of Tropical Storm Isaac, taken 18:50 UTC 27 August 2012

This image uses a new (to this blog, anyway) color scale, developed by our colleagues at CIMSS, that really highlights the structure of the clouds at the top of Isaac. The color scale is included in the image. For comparison, here’s the GOES Imager IR window channel (channel 4, 10.7 µm) image from roughly the same time:

GOES-13 Imager channel 4 image of Tropical Storm Isaac, taken 18:45 UTC 27 August 2012

GOES-13 Imager channel 4 image of Tropical Storm Isaac, taken 18:45 UTC 27 August 2012

GOES has ~4 km resolution in its IR channels. VIIRS provides amazing details of the structure of tropical cyclones that you just can’t get with current geostationary satellites.

The real story from Isaac, however, is the flooding. It’s hard to capture flooding from a visible and infrared imaging instrument, since flooding usually occurs when it’s cloudy. Clouds block the view of the surface when looking at visible and infrared wavelengths. But, large quantities of water that fail to evaporate or drain into the local rivers after a period of several days can be seen after the skies clear. That’s what happened with Isaac.

Here are before-Isaac and after-Isaac images of the southern tip of the Florida Peninsula. These are false color (“pseudo-true color”) composites of VIIRS channels I-01, I-02 and I-03. These images were taken on the afternoon overpasses of 23 August and 29 August 2012. Many cities on the east coast of Florida got 10-16 inches of rain (250-400 mm for those of you outside the U.S.). See if you can pick out the flooding.

False color RGB composite of VIIRS channels I-01, I-02 and I-03 taken before and after Tropical Storm Isaac (2012)

False color RGB composite of VIIRS channels I-01, I-02 and I-03 taken before and after Tropical Storm Isaac (2012)

If you have been following this blog, you know that, in the “pseudo-true color” RGB composite, water shows up very dark – in most cases, almost black. That’s not always true, of course. You can see sun glint (particularly in the “before” image) that makes water a lighter color and shallow water (where visible radiation [i.e. channel I-01] is able to penetrate to the bottom) shows up as a vivid blue.

Now, notice the Everglades. Many areas of the Everglades, particularly on the east side, appear darker in the “after” image, because those swampy areas have a lot more water in them. Water has a lower reflectivity than vegetation or bare ground at these wavelengths.

The effect of water on the land surface shows up even better in the moderate resolution channel M-06 (0.75 µm). M-06 is a channel not shown before because it is perhaps the worst channel for producing interesting images. M-06 was designed to aid in ocean color retrievals and/or other uses that require atmospheric correction. The M-06 detectors saturate at a low radiance, so any radiation at 0.75 µm that reflects off of clouds, aerosols or the land surface easily show up. About the only things that have low reflectivity in M-06 are atmospheric gases and water surfaces without sun glint. Ocean color retrievals need a very clean atmosphere with no aerosols or clouds and no sun glint to work correctly. You also need to be able to identify what is or is not water, which is what makes M-06 useful for identifying flooding.

Here are the similar before-Isaac and after-Isaac images of Florida from M-06:

VIIRS channel M-06 images of southern Florida taken before and after Tropical Storm Isaac (2012)

VIIRS channel M-06 images of southern Florida taken before and after Tropical Storm Isaac (2012)

Both the land and optically thick clouds saturate M-06, so this channel is useless at identifying clouds over land (except you can see some cloud shadows). Sun glint is saturating the pixels over the Gulf of Mexico in the “before” image, while it is mostly to the east of Florida in the Atlantic Ocean in the “after” image. In the “after image”, reflective cirrus clouds over the Gulf of Mexico show up that are not as easily visible in the RGB composite. Of primary importance here, however, is the dark appearance of the Everglades in the “after” image. All that flood water reduced the reflectivity of the land surface, making it appear darker. That means, if you know where the clouds (and, hence, the cloud shadows) are, it may be possible to use M-06 to identify large flooded areas.

Louisiana and the coast of Mississippi were the hardest hit by Isaac, and the flooding is easily visible here, too. In fact, the massive flooding is easier to see in the RGB composite in this region. Compare the “before” and “after” images, taken on 26 August 2012 and 1 September 2012:

False color RGB composites of VIIRS channels I-01, I-02 and I-03 of southeast Louisiana

False color RGB composites of VIIRS channels I-01, I-02 and I-03 of southeast Louisiana

To make it easier to see, here’s a quick animation of the before and after images. Watch the highlighted areas.

Animated GIF of false color RGB composites taken from VIIRS before and after Hurricane Isaac

Animated GIF of false color RGB composites taken from VIIRS before and after Hurricane Isaac

After the passage of Hurricane Isaac, Lake Maurepas and Lake Pontchartrain almost appear to merge into one big lake! Other flooding is visible near Slidell, Bay St. Louis, Pascagoula Bay, and the heavily hit parishes of Plaquemines, St. Bernard, Lafourche and Terrebonne.

Thin cirrus clouds are visible in the “after” image, which limit the ability of M-06 to detect some of the flooding, but M-06 is still able to see the large areas of flooding highlighted in the animation above. M-06 also detects reflection off of the Twin Spans as well as the Lake Pontchartrain Causeway. And this is at ~750 m resolution!

VIIRS channel M-06 images of southeastern Louisiana taken before and after Hurricane Isaac (2012)

VIIRS channel M-06 images of southeastern Louisiana taken before and after Hurricane Isaac (2012)

So, don’t try to do ocean color retrievals in pixels obscured by big bridges.