More and more violent tornadoes. However, it remains an atmospheric mystery. why?

On the night of December 10-11, a series of powerful tornadoes left a 1,000-mile trail of destruction through parts of Arkansas, Mississippi, Missouri, Kentucky, Tennessee, and Illinois. Dozens of people died, billions in damages. Hurricanes are among the deadliest and destructive weather phenomena on Earth. However, in 2021, they remain a mystery – and it is difficult to predict.

On December 10, a powerful storm system approached the central United States from the west. While that storm brought heavy snow and sleet to the cold western and northwestern part of the Midwest, the South enjoyed near-record warmth, thanks to warm, moist air blowing north from the Gulf of Mexico. When warm and cold air masses collide, the less dense warm air rises to cooler levels in the atmosphere. As this warm air cools, the moisture it contains condenses into clouds and storms form.

When this instability is combined with significant wind shear – winds that change direction and speed at different altitudes in the atmosphere – they can create ideal conditions for powerful rotating storms – that is, hurricanes -.

Since 1925

There would have been at least 28 on the night of December 10-11. One of them reportedly traveled nearly 400 miles across four states. This is very unusual. Most hurricanes stay on Earth for short periods of time and travel short distances – 5 to 6 kilometers on average. Long and very long tornadoes — tornadoes that travel at least 40 and 160 kilometers, respectively — are relatively uncommon. They account for less than 1 percent of all hurricanes in the United States. The previous record of a long-track hurricane dates back to 1925, when Tree State Tornado 350 kilometers across Missouri, Illinois, and Indiana. the Quad-state tornadoAs it is called last weekend’s hurricane, it is expected to break this record.

Hurricanes are relatively common in the central United States, averaging about a thousand years. The area that does it Hurricane Alley mentioned, from central Texas to eastern Nebraska and Iowa was the hardest hit, especially during the months of April, May and June.

Hurricanes also sometimes occur in our country. However, there is one big difference: they are less powerful here. But in the United States, tornadoes are a serious problem that is getting worse. Global warming is causing more and more deadly tornadoes, and they are increasingly occurring over a wider area than in the past.

Direct link to global warming

in a recent study James Elsner of Florida State University describes how hurricanes become more powerful. And the increase in hurricane strength is significant: 5.4 percent annually from 1994 to 2016. His findings are backed by official storm reports from US weather agencies, all of which show an upward trend in hurricane strength and also indicate that hurricanes take longer and have wider paths.

The study found a direct link between global warming and the number and strength of hurricanes: a combination of increased heat and humidity, Available Convective Potential Energy (CAPE) and increased wind shear. Simply put, when heat, humidity, and erratic winds converge at the highest levels, you get stronger tornadoes.

Conditions that cause hurricanes will be facilitated by a variable jet stream. Due to the rapid warming of the Arctic, the difference in temperature between the north and south has narrowed. This reduces pressure differences between the arctic and mid-latitudes, and softens the winds of the jet stream.

Just as slow-flowing rivers usually follow a meandering path, the slow-flowing jet stream also tends to meander more. Large north-south ripples in the jet stream generate energy in the atmosphere. Incidentally, it’s the same process that was also responsible for the polar vortex that hit the United States earlier this year.

More and more intense, but no progress has been made in forecasting in the past ten years

In recent decades, tornadoes have changed, too – they are relatively less common in Oklahoma, Texas and Kansas, and are the traditional Alley tornado, but are more common in Illinois and other states along the Mississippi River and east.

More and more severe, however, and this is an additional problem, and therefore not better to predict. exactly the contrary. In 2011, the average time for people to receive tornado warnings was about 13 minutes. But now that’s down to 8.4 minutes. Think about it: If you have less than nine minutes to prepare, what else can you do?

It’s not that we declined to predict – there are more hurricanes. But no real progress has been made in the past ten years. This lack of progress with hurricane warnings is frustrating when you consider how adept meteorologists are at predicting other severe weather, including tornadoes. In 2019, the National Hurricane Center’s forecast three days before the storm were more accurate than the 1990 forecast one day before.

Why is it so hard to predict?

Why is it difficult to predict when a hurricane will occur? Scientists know that hurricanes consist mainly of massive and violent thunderstorms, especially violent storms that spin like small tornadoes. These storms are especially common in the central and southeastern parts of the United States, where warm moist air from the Gulf of Mexico meets dry air from the western and southwestern mountains. They will form mainly in the spring and early summer.

The problem is that meteorologists can look at two giant thunderstorms that look identical, and only one of them will cause a tornado. This is also the reason why the false alarm rate of tornado warnings is so high: forecasters can’t easily tell when a storm that looks like it might trigger a hurricane will actually do it.

Scientists understand the ingredients needed to create the kind of supercelled storms that produce the most powerful tornadoes. There are four. You need a lot of moisture in the atmosphere and a lot of wind shear, or changes in wind speed and direction (this creates a storm). You also need atmospheric instability, which can cause air currents, and upward or upward movement of air which rotates around the storm along a vertical axis.

But what causes a hurricane occurs on a much smaller scale – perhaps at the level of individual particles in the atmosphere – and is strongly influenced by the peculiarities of local geography. Even trees can disrupt surface circulation, unlike grasslands, and this can affect hurricane formation. Weather conditions that lead to a tornado in Oklahoma will not necessarily cause a tornado in Alabama.

We don’t even know how that happens

Many storms even produce rotating winds without creating an actual hurricane. The question meteorologists are trying to solve is, how, when and why these rotating winds move to a point where you have a very narrow, intense vortex that we call a hurricane.

It is possible (but not counterintuitive) that tornadoes form from the bottom up. They will begin as a turbulence on the ground and then connect upwards with a thunderstorm. Being a small vortex on the surface comes into contact, for whatever reason, with the rising current of the thunderstorm. And then you get an effect like a snowboarder spinning: she pulls her arms in, stretches and spins faster and faster.

But it’s also possible for tornadoes to descend from a storm cloud – or a combination of both. The tornado seems to form all the way from top to bottom all at once, all at once.

And now, scientists don’t know. why? Currently, weather radar cannot get a good glimpse of rapid conditions at a relatively low altitude leading to tornadoes. The processes that determine whether tornadoes form or not appear to occur on a time scale of a minute or less, only tens of meters from the surface, an area that is difficult to scan with radar.

The solution still sounds like science fiction for now

The only way to improve hurricane forecasts, researchers say, is to take it head-on. Hurricane forecasts have become very good in recent decades because scientists have been able to study their every move intensely. It helps that they move slower than hurricanes and last for days. We can fly planes in and out of a hurricane’s eye wall and collect all kinds of data.

On the other hand, tornadoes are smaller in size and short-lived. The scientists who study don’t have that wealth of data to factor in their predictions. To make matters worse, tornadoes easily damage scientific equipment. Any kind of sensor is often destroyed before they can sample everything you hope to collect. So it is very difficult to get a complete data set for the phenomenon.

What is needed is another breakthrough. One that makes it possible to model and simulate every molecule of the atmosphere. Such molecular modeling may sound like science fiction now, but it may be possible in the future. There are specialized branches of physics where they do it literally, ie modeling any molecule, but usually in very small sizes, down to a cubic centimeter at most. It is a matter of increasing this capacity over time.

(EVB)