The Earth Story

Hurricane Harvey fed off record warm Gulf of Mexico waters

This satellite shot captures Hurricane Harvey at the time it was a category 4 storm, just before it made landfall in Texas. The storm started off as a disturbance crossing the Yucatan Peninsula, but when it made it into the warm waters of the Gulf of Mexico it rapidly strengthened. Harvey became the second costliest disaster ever to hit the United States after it doused the city of Houston with record rainfall totals. A new analysis points squarely at the Gulf of Mexico as the main driving force behind the power of this storm.

Hurricanes feed off of warm temperatures in ocean waters. In the process, they remove heat from the ocean and leave the water behind it cooled off. However, other things can happen that affect ocean temperatures; for example, the winds blowing over the water could mix the shallow layer with deeper water, cooling the surface water by pushing the warm water deeper. As a consequence, it’s not easy for scientists to match up the energy of the storm with the energy of the ocean.

Harvey though represented a unique case. For much of the surrounding month it was the only major weather event in the Gulf of Mexico, and it traveled over areas that are well instrumented so that scientists could see how much heat it removed from the water. On top of that, scientists also have available the Global Precipitation Measurement Satellite system, which enabled estimates of Harvey’s rainfall over wide areas.

A team led by a scientist at the National Center for Atmospheric Research took these two measurements and converted them into total energy. The ocean cooled by a certain amount over a certain volume – that’s an energy measurement. Harvey produced a certain amount of rainfall over a certain area – that’s also an energy measurement. When they compared the number of joules pulled out of the Gulf to the energy released over land by Harvey – they were virtually identical, within 1% of each other. The energy that drove Harvey was the energy in the Gulf; basically every Joule of energy it pulled out of the Gulf, it dumped on Texas.

Harvey became such a disaster because it had an ample supply of energy in the shallow waters of the Gulf of Mexico. Prior to the storm, the waters of the Gulf were at their hottest temperature ever recorded, more than 1.5°C above the long term average. Those temperatures extended downwards, making the Gulf heat content also a record. When Harvey passed over these waters, it cooled them by 2°C. That extra 1.5°C in the Gulf of Mexico in 2017 was enough to almost entirely fuel the storm; had the Gulf not been at record temperatures, Harvey would not have had the energy to produce that rainfall.

Global ocean heat content has been rising steadily since the 1980s as the ocean takes up much of the extra heat kept in the atmosphere by greenhouse gases. The close match between energy taken out of the ocean and energy dumped by the storm verifies that the extra energy trapped in the atmosphere is feeding storms like Harvey. The extra heat in the Gulf of Mexico directly triggered flooding in Houston, and as ocean temperatures continue to increase, it will be able to continue powering devastating storms.

-JBB

Image credit: https://commons.wikimedia.org/wiki/File:Harvey_2017-08-26_0055Z.jpg

Original paper: https://doi.org/10.1029/2018EF000825

How many Harveys?

Houston, Texas is a flood-prone city. The city sits on a flat coastal plain where one river, known as the Buffalo Bayou, spreads out into an estuary to reach Galveston Bay. The city has grown into the 4th largest in the United States, and now encompasses several other large streams. The combination of a coastal plain, with little slope to drive water downhill, and exposure to the Gulf of Mexico, makes it a natural place for flooding. The city has seen 3 major floods in the past 3 years alone, but Hurricane Harvey topped them all.

Harvey started its voyage over unusually warm waters in the Gulf of Mexico. In fact, because of the heat in the Gulf this year Houston and Galveston are both expected to have their hottest years ever recorded. The storm sucked up this energy and moisture from the Gulf, but then it ran into a weather system that locked it in place for several days. It dumped the largest rainfall totals ever recorded in the United States on some portions of the Houston area.

The “largest rainfall totals ever recorded” seems like a true extreme. Local officials called the storm a “Biblical” rainfall; that would mean that Harvey was something we couldn’t have prepared for and might never have to prepare for again. However, new research just published by scientists from MIT suggests that Harvey is just the beginning.

Dr. Kerry Emanuel has previously developed a model that simulates the effects of hurricanes. Hurricanes are complicated, turbulent beasts and simulating processes in them requires a huge amount of computer time, but his model instead focuses on the results. Using input parameters such as water temperature, wind conditions, and atmospheric parameters, the simulation produces low-pressure systems that sometimes intensify into hurricanes. This simulation setup allows them to produce a large number of storms and then compare their actual statistical distribution to the ones observed in the real world.

Their hurricane model is able to accurately match the observed rate of occurrence hurricanes that struck Texas in the period 1900-2000. Given those historical statistics, over the period 1980-2000a rainfall event the size of Harvey has a 1 in 100 chance of occurring somewhere in the state of Texas. Taking into account the odds of it hitting a specific location such as Houston, the city of Houston could expect to see a comparable storm once every 2000 years.

However, the weather is no longer what it once was and the statistics for 1900 aren’t even valid today. The Gulf of Mexico is warmer than it was 100 years ago; that is showing up in Houston’s daily temperatures and it showed up in Harvey’s rainfall totals. The scientists took estimates for CO2 contents in the atmosphere for the period 2080-2100 from one of the scenarios in the IPCC report where there is little to no limitation of CO2 release and asked what that would do to storms like Harvey.

Under those conditions, the planet is warmer, the Gulf is warmer, and there is more water to feed strong storms. Under those conditions, a storm dumping as much rain as Harvey dumped on Houston will hit Texas once every 5.5 years, and will hit Houston directly once every 16 years. Not only will Harvey-level storms become more likely, but also storms several times larger than Harvey, storms that are basically impossible today, will be the new “100 year storms” by the turn of the 22nd century.

Right now, humans already don’t have the climate of the year 1980. We’ve added a lot more heat to this planet. The scientists also gave a general estimate of where we are right now and said that a Harvey level of storm has probably gone from about a 1 in 100 year event in the state of Texas as a whole to a 1 in 20 year event due to CO2 increases since 1980.

In other words, as of right now, Harvey level storms are possible in Texas in the next decade. In the latter half of the 20th century, unless we change our current emissions path, Harvey level storms will be happening every decade in the state of Texas. Hurricane Harvey did an estimated $200 billion or so worth of damage to the Houston area. Under those conditions, downtown Houston basically becomes uninhabitable – by the time recovery is finished from one storm, the next storm is a year or two away.

-JBB

Image source: CNN http://cnn.it/2wG4HaD

Original paper: http://bit.ly/2hrtgPe

Press release: http://bit.ly/2zJqzBd_ _

Waugh Bridge Bats

After Hurricane Harvey dumped record-breaking amounts of rain on the city of Houston, Texas, there was a huge amount of standing water in the city and surrounding area. A handful of residents have come down with diseases due to exposure to the water, including one known death due to a flesh-eating bacterial infection.

However, despite standing water throughout the city, there weren’t widespread reports of people coming down with diseases due to mosquito bites – at least not a huge surge compared to normal years. For that, you can possibly thank these critters – the Waugh Bridge Bats.

Bats inhabit several large caves in Texas, and since humans began constructing sheltered sites in cities bat colonies have moved into housing beneath bridges in several major cities including Houston and Austin. This photo from the Houston Chronicle captures the bat colony leaving the Waugh Bridge for the night. Bats can reportedly consume up to 2/3 their body weight in insects, including mosquitoes, during an evening out, and therefore they definitely help as mosquito control when there is a major flooding like that hurricane. In fact, this bridge even flooded during the storm, sending the bat colony out for safety – it’s back now.

Unfortunately, the bats of Texas have one new worry – White Nose Syndrome. White Nose Syndrome is a disease that starts showing up with a white fungus growing on the nose of bats. It has killed millions of bats in the United States since it was first discovered in 2007. It has reached 30 states, and in some cases the mortality rate has been more than 90%. Texas is home to some of the largest bat colonies in the United States, and the disease was just detected in that state for the first time.

-JBB

Image credit: Houston Chronicle/Kathy Adams Clark (Used for non-profit/outreach) http://bit.ly/2z2XlN0

References: http://bit.ly/2iPCwk4 http://www.houstontx.gov/parks/bats.html

500-year storms

As I’m writing this, the “500 year floodplains” of several rivers near Houston, Texas are occupied by water for the 3rd time in just over a year. That’s an interesting statement since records of weather in Texas don’t go back 500 years. How exactly do scientists know what the weather was like 500 years before there were records? These flood events have filled the “500 year floodplain”, but calling something a 500-year flood is a statistical statement.

Rather than requiring records going back a thousand years, a 500-year rainfall event or a 500-year flood is defined based on probability. This plot shows measured peak discharge rates in the Red River plotted against how often that event has happened. When a few fractions are calculated, the flow data in these rivers tends to produce a line in this space. Given enough measurements of flood stage on a river, enough to fill the lower edge of this plot, a few decades of data is enough to establish the probability of flood events of a certain size.

Even if a thousand years of records don’t exist, using a plot like this and projecting, weather forecasters can assess how often an event of a certain size should occur on average. In the case of Texas, flood events have exceeded the “500 year floodplain” volume for these rivers 3 times in the past 3 years.

Worth remembering though, just because a storm is a “thousand year storm” doesn’t mean it will be a thousand years before it happens again. An area could go hundreds of years with no “hundred year storms” and then suddenly have several of them within a short period and the statistical estimate of recurrence interval would still be accurate. That’s a statistical note – kind of like a baseball player hitting 4 home runs in a game or a basketball player making 20 shots in a row. The event is rare, but given enough chances, rare events do happen.

The odds of 3 “500 year events” occurring within the space of 3 years is roughly 1 divided by (500) cubed – 1 in 125 million. The odds of these 3 events happening this often is not zero, but exceptionally small. That simple calculation would mean that you’d get this type of flooding hitting Houston 3 times this rapidly once since the Jurassic period, again on average (maybe 3 or 4 clusters like this since the development of hard shelled life!)

However, that quick calculation makes other assumptions: a representative climate and an unaltered floodplain. If the climate of an area changes substantially, then the data built up regarding recurrence intervals won’t be relevant any more. Since it could take decades to truly recognize that the previous data is no longer relevant, major climate change is a potential problem in this type of statistical estimate. As the atmosphere today is very different than the atmosphere when this type of data started being collected, it is very likely that our previous estimates of “500 year floods” and “500 year storms” are no longer accurate in many areas around the world.

Furthermore, one major issue in Houston Texas is urban development. Covering surfaces with concrete to build buildings and parking lots removes any ability for water to seep into those soils and instead dumps all the rain that falls on those surfaces into nearby channels. Many grasses that fill floodplains also have the ability to absorb floodwaters and slow river currents, but as Houston has developed these floodplains have been paved over for development, making this flooding worse. The rainfall that used to occupy the 500 year floodplain might well be plausible every few years.

-JBB

Image credit: http://serc.carleton.edu/details/images/3984.html http://serc.carleton.edu/quantskills/methods/quantlit/floods.html

Small footnote: This article was originally written when a storm in South Carolina was described as the “worst in 1000 years”. It has been repurposed as the main statistical argument is the same today, as are the caveats.

Hurricane Harvey

With winds reaching upwards of 130-140 mph (209-225 kph), Hurricane Harvey made landfall along the Texas coast between Port Aransas and Port O'Conner late on Friday night. The storm has stalled over the state, causing devastating flooding, especially in the southeastern areas, which fall on the "dirty side" of the storm's rotation. Residents who safely sheltered in place during the worst of the storm itself are now evacuating their homes as river levees threaten to give way and water is released from dams to relieve pressure.

Houston, my former hometown, is experiencing never-before-seen devastation, with flood waters in some cases reaching the signs above the freeways. Austin and the Hill Country are under flash flood advisories.

Rain is supposed to continue the rest of the week. Current reports from NASA show the storm moving back into the Gulf of Mexico. Although it no longer has an organized center, the storm may well strengthen over the warm waters of the Gulf, with possible winds of 45 mph (72 kph).

As I wait to hear that my sister and her husband have safely reached a shelter away from the threat of the levee on the Brazos river being breached, I ask you to please keep Texas in your thoughts.

CW

Image:

http://bit.ly/2vtElol

Sources:

https://go.nasa.gov/2vyeJa0

http://bit.ly/2vtElol

I’m upstream of this, so for tonight’s video I’m just going to share a news report fro Hurricane Harvey hitting Texas. Well have more to say about this soon.

“Brown Ocean” fuels some hurricanes

If you only look at this radar pattern from Tropical Storm Erin in 2007, you might not think there’s anything unusual about it. It’s clearly a tropical storm, there’s even a well-developed eye.

But if you take a look at the labeled cities on the map, you might note that there’s something odd here indeed. This storm still has a well-developed eye and it is sitting in northern Oklahoma. That’s about 800 kilometers (500 miles) from where the storm made landfall and somehow there is still a developed eye and circulation pattern. That’s at the least odd, and also impressive.

I’m currently hiding indoors from the rain bands of Harvey (landfall south of me, only a flooding threat for me), so here’s an animation of a hurricane viewed from space. I love how they threw in the ISS.