legrandangle-lgm

By Will Dunham

(Reuters) – A dangerous weather phenomenon called a bomb cyclone that occurs in mid-latitudes – between Earth’s tropics and the polar regions – could bring strong and damaging winds, torrential rain, heavy snow, flooding and frigid temperatures. It shows some characteristics of a hurricane, but it is not one.

Here is an explanation of bomb cyclones.

WHAT IS A BOMB CYCLONE?

A bomb cyclone, also called explosive cyclogenesis or bombogenesis, is a mid-latitude cyclone that has intensified rapidly. A cyclone is a low-pressure weather system – one in which the atmospheric pressure at the center is lower than in surrounding areas – with the wind turning inward. It circulates counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

A bomb cyclone’s winds can reach hurricane force — 74 miles (119 km) per hour — and stronger. These storms usually form in winter and can produce large amounts of precipitation. They have a lifespan of about a week, during which they reach maximum intensity in about four to five days and then disappear in the last two days, according to Jon Martin, professor of meteorology at the University of Wisconsin in Madison.

HOW DOES A BOMB CYCLONE FORM?

Bomb cyclones form when conditions at the surface and at the level of the jet stream are ideal for the storm to intensify. The jet stream is a narrow band of strong winds in the upper atmosphere. A variety of atmospheric processes combine to create these storms. Nearly all bomb cyclones have a precursor disturbance in the winds in the middle troposphere – the lowest region of Earth’s atmosphere – about 5 to 8 km above the planet’s surface, Martin said.

Another important feature that many, but not all, explosive cyclogenesis events have in common is a warm ocean surface. Many of the most intense bomb cyclones originate over the oceans. Precipitation can be miraculous. When water vapor turns into liquid and ice, as in these storms, enormous amounts of energy are released – called latent heat energy. Some of that energy strengthens the storm even further.

Because atmospheric pressure drops so low, pressure differences during the storm can become very large, creating strong winds that can have devastating consequences.

WHEN AND WHERE ARE THEY MOST LIKELY TO FORM?

Explosive cyclogenesis usually occurs over the oceans and usually during the cold season in both hemispheres – roughly from November to March for the Northern Hemisphere and roughly from May to August in the Southern Hemisphere, although these storms can occur earlier or later. Areas that are particularly sensitive are in so-called storm tracks along the east coasts of continents, because this is where the warmest ocean currents occur, such as the Kuroshio off Japan and the Gulf Stream off North America, Martin said.

According to John Knox, an atmospheric scientist and professor of geography at the University of Georgia, bomb cyclones can be very destructive and pose a particular danger to shipping because many of them occur over the oceans. Some bomb cyclones have occurred on the Great Lakes of North America and have caused shipwrecks there as well, Knox said.

WHAT HAPPENS TO THE ATMOSPHERIC PRESSURE?

The average atmospheric pressure at sea level at mid-latitudes is about 1012 millibars or mb. With cyclones this regularly drops to 980 mb. With bomb cyclones it can drop to 950 mb or less, and the rate at which they intensify is at least 24 mb in 24 hours.

WHY CAN’T WE CALL IT A HURRICANE?

Although bomb cyclones can unleash hurricane force winds and sometimes exhibit hurricane characteristics, they are not hurricanes. They form from different physical processes and do not have the symmetry of hurricanes, which are also low-pressure systems. Bomb cyclones occasionally develop “eyes” similar to those in the center of a hurricane, Knox said. But a bomb cyclone originates in the mid-latitudes and is associated with weather fronts — a boundary between two air masses with different characteristics such as temperature — and a strong jet stream, Knox said. A hurricane originates in the tropics and is not associated with weather fronts or a strong jet stream, Knox said.

ARE BOMB CYCLONES COMMON?

Global climate change is causing increasingly frequent and extreme weather events around the world, according to experts. But are bomb cyclones becoming more common or more intense? Martin said it’s not clear if that’s the case. The fact that the Earth is warming has implications for the cyclone dynamics that scientists are currently trying to figure out, Martin said. A warmer planet means more water vapor in the air and that would tend to make at least the latent heat part of these storms’ power stronger, Martin said. However, the warming is not uniform, Martin said. Because observations indicate more warming at high latitudes, Martin said, this could make bomb cyclones weaker overall.

(Reporting by Will Dunham in Washington, Editing by Rosalba O’Brien)