Regional and Mesoscale Meteorology Branch

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:

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:

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:

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]