What causes storms to strengthen?

In light of Wednesday’s meteorological bomb of a storm that was just offshore, we have decided to write an article that explains why storms strengthen to begin with, and how they can get to be as strong as this storm got. Wednesday’s storm went under what we call “bombogenesis” (yes, that is a real meteorological term), meaning that its pressures dropped more than 24 millibars in 24 hours. At once point, surface analysis showed the storm being as strong as 955mb, which is equivalent to a category 3 hurricane!

As most of you probably know, a lower pressure means a stronger storm, and a higher pressure means a weaker storm — or if the pressure is high enough, an area of tranquil weather. Now the question becomes, what causes pressures to fall in a certain area, and why do they sometimes fall so rapidly?

The most important meteorological aspect for pressure falls is an area of upward vertical motion. If air is being lifted vertically, then pressure within that column of air has to decrease, because air is escaping that column when it is moving vertically. Naturally, if less air exists within a column, the pressure in that column will be less.

The atmosphere always wants to maintain balance, so to accommodate for the air that is being lifted vertically, there is a need for air to converge at the surface to replace what is lost at the surface, and to generate the lift to fill the void in that column of air as well. This is one reason why air converges at the surface in areas of lower pressures; it is all part of the balancing act of the atmosphere. Air also flows from higher pressures to lower pressures, being that lower pressures are an area of least resistance; another aspect of this balancing act. All areas of relatively higher pressures essentially shove air away, and it all converges where the lowest pressure is. This surface convergence leads to upward vertical motion, which leads to storm development, precipitation, and an additional lowering of pressure.

To illustrate this further, think about the opposite scenario: wouldn’t it make sense for pressure at the surface to be higher if there were downward vertical motion, meaning that air is being pressed downward towards the ground?

A water vapor animation taken Wednesday afternoon, beautifully illustrates the size and strength of the storm system (wx.rutgers.edu). You may need to click to animate.

Let’s go over the factors that cause upward vertical motion:

1) Divergence aloft.

Divergence aloft in meteorology is when air is separating — it’s heading in opposite directions. The atmosphere will always have a tendency to balance itself out. Thus, when air is separating aloft, there is a need to replace that separating air. This is when upward vertical motion comes in — beneath the separating air aloft, air must be rising vertically to replace that separating air, and reinforce it. It cannot be reinforced by sinking air above it, because once we are very high in altitude, we approach the stratosphere, where atmospheric motions are much weaker, and any movement of air would not be nearly enough to keep the atmosphere in balance — we need the strong motions from our atmosphere (troposphere) to keep everything in balance when air is diverging aloft.

Once the upward vertical motion is finally able to sufficiently spread out aloft, that means that the amount of air in that column can really start to decrease, since that is when it is leaving that vertical column.

Now that we know that divergence aloft leads to upward vertical motions from below, and that upward vertical motion leads to lowering pressures, it is now prudent to identify what causes divergence aloft.

This is where it is very important to look at jet streaks in the atmosphere — or localized areas where the jet stream is the strongest. It has been deduced that areas located in the right entrance (right side of where a jet streak begins) and the left exit (left side of where a jet streak ends) lead to upward vertical motion. If we take a look at Wednesday’s jet stream pattern, it becomes very clear why there was so much upward vertical motion.

The jet stream pattern Wednesday afternoon shows that our storm was located in an area that had two left exit and one right entrance region all juxtaposed. This leads to lots of upward vertical motion and pressure falls (NCEP).

Our storm was located just offshore of Cape Cod during late Wednesday morning and early afternoon. At that time, there was a powerful curved jet streak to the east of the storm, and our storm was in the left exit region of that jet streak. Additionally, there was an additional jet streak on the downstream side of the ridge in the Plains, of which our storm also in the left exit region. To the north, there was another strong jet streak due to confluent flow to the north, of which our storm was in the right entrance region.

Why is it these locations of the jet streak that produce upward vertical motion? I’ll try not to get too technical, but one explanation has to do with vorticity, or counterclockwise spin in the atmosphere. Intuitively, stronger vorticity can often mean stronger storms, because areas of low pressure have a counterclockwise flow (in the Northern Hemisphere). When looking at a jet streak, a vorticity maximum is usually located directly on the cold side of the center of the jet streak (north in a west-to-east jet streak, and west in a south-to-north jet streak). This is because on the cold side of the jet streak, as the air tries to flow counterclockwise, it does not run into any resistance. On the cold side of the jet streak, the initial counterclockwise motion runs away from the jet streak, which means the winds are getting weaker — this allows for the counterclockwise motion to be completed all the way through. However, on the warm side of the jet streak, the initial counterclockwise motion runs right into the heart of the jet streak and it thus gets sheared out in the direction of the jet streak, as opposed to being able to rotate counterclockwise. This means that we have a vorticity maximum on the cold side of the jet streak, and a vorticity minimum on the warm side of the jet streak.

An idealized diagram illustrating vorticity advection and a jet streak in the upper levels of the atmosphere, and associated vertical motions in the atmosphere.

Thus, the strong jet streak will be advecting the area strongest vorticity on the left exit side. However, on the right exit side, the weakest vorticity is getting advected. This is why we have upward vertical motion on the left exit side, yet downward vertical motion on the right side. Of course, the explanation behind why positive vorticity advection leads to upward vertical motion is a bit more complicated than the initial intuition that vorticity leads to more counterclockwise spin in the atmosphere, which is associated with lower pressures. Positive vorticity advection causes divergence aloft, which was the whole start of this section to begin with.

When vorticity is being advected into a region where the jet streak is weaker, the atmospheric motions — including the ones that are turning counterclockwise to begin with — get a little weaker as well. Thus, the atmosphere’s vorticity surrounding the vorticity maximum is also a bit weaker. As we said before, the atmosphere always has a tendency to balance itself out, which means that in order to accommodate for surrounding weaker vorticity, the vorticity maximum needs to somehow lose some of its vorticity. It does this by spreading out, or diverging, which explains the upper level divergence. An analogy for this is a figure skater: if they spread their arms out, their spinning slows down — if air diverges, its counterclockwise spinning, or vorticity, slows down.

A 500mb and vorticity image taken on Wednesday afternoon (3 hours after the jet streak image). Notice how the strongest vorticity and core of the storm is now located where the left exit region of the curved jet was previously located (as well as the right entrance region of the jet to the north). This means the strongest vorticity got advected into the exact region it should be advected into if you want explosive storm development (NCEP).

Of course, as the jet streak propagates with the vorticity maximum, any weakening is halted as the stronger atmospheric motions catch up. However, it is the initial balancing act the atmosphere does when strong vorticity is advected into a weaker vorticity environment that causes the upper level divergence, and thus the upward vertical motion, to begin with. This causes pressure falls.

There is even more to the explanation than just this — it is actually differential vorticity advection, meaning vorticity increasing with height (which is generally true considering how atmospheric flow increases with height) that is associated with the upward and downward vertical motions with the atmosphere; but in order to prevent this from turning into a college course, we will move onto the next factor.

2) A strong temperature gradient.

As some of you know, storm systems tend to form along frontal boundaries, where warm air and cold air are clashing. Once again, this goes back to the atmosphere wanting to balance itself out. The Poles are quite cold, and the Equator is quite warm, which gives the atmosphere a tendency to want to push the warm air towards the Poles, and cold air towards the Equator. If an extremely cold airmass is located very close to an extremely warm airmass, then you have lots of warm air rushing poleward right next to a location where lots of cold air is rushing equatorward. Over time, this can create counterclockwise, or cyclonic flow in the atmosphere.

Initially, we might have a scenario that looks like this:

Idealized scenario where cold air and warm air are located very close to one-another. The atmosphere tries to balance this by pushing warm air towards the cold air, and cold air towards the warm air.

The isotherms are initially purely west-to-east, and the warm air is rushing northward, and the cold air is rushing southward. This means we have a northerly flow to the west and a southerly flow to the east. This will eventually change the orientation of the isotherms with time.

Over time, a ridge of warm air will form to the east, and a trough of colder air will form to its west. This creates a temperature gradient not only in the north-to-south direction, but also in the west-to-east direction. This helps create cyclonic motion in the atmosphere.

Eventually, when enough warm air is getting pushed northward, and enough cold air is getting pushed southward, ridges of warm air and troughs of cold air can develop — this is part of the process behind our standard ridges and troughs. This allows temperature gradients to also form in the west-to-east direction as opposed to just the north-to-south direction. Thus, air will also flow in these directions to balance out the atmosphere, contributing to counterclockwise motion.

It is often not just the strong temperature gradients that create this counterclockwise motion, but it contributes to it. The ideal scenario for a strengthening storm is an area of vorticity that has already formed — perhaps maybe in the form of a shortwave trough — moving into an area that has a strong temperature gradient. That way, the area of vorticity can strongly be enhanced, which can help contribute to enough vorticity advection for upward vertical motion. If this is juxtaposed in an area with a very strong jet streak, explosive storm development can occur.

Storms develop along frontal boundaries, so it also makes sense that the stronger the temperature gradients, the stronger the fronts, and the stronger the storms.

A diagram showing a Low Pressure system and its associated cold and warm fronts (NOAA).

Not too dissimilar from the previous diagram, we see that warm, moist air is out ahead of the low pressure and its associated cold front, cool air is located to the north of the warm front, and much colder, drier air is located behind the cold front. The stronger these gradients are, the stronger the tendency will be for the atmosphere to need to balance itself out by blowing warm air towards cold air, and vice versa — thus frontal boundaries and storms. As a storm deepens and matures, its winds strengthen, and thus the ability to advect warm and cold air increases — strong warm air advection is also another trigger for lift and precipitation.

Over time, however, the cold front catches up to the warm front, leading to an occluded front. This is because cold, dry air is very dense, so the cold front gets a large “push” from that dense airmass. Additionally, the warm moist air out ahead of it is not dense at all, so the cold front has little resistance. The warm front moves slowly for the opposite reasons — it has a weak “push” from a lighter, warm airmass, and it’s attempting to push into a more dense airmass, leading to resistance.

The occluded front causes weakening because now we have cold air overtaking the warm air, which reduces the temperature gradient. Right when the occluded front begins to form is when the storm is mature, since this is when the initial gradients and “pushes” were maximized. Afterward, the storm slowly begins to weaken.

Surface analysis taken during the storm’s maturity showed it having a pressure of 955mb! Not surprisingly, a cyclone this mature had a well-defined occluded front (NOAA).

These are two of the main aspects that lead to storm development. There are actually more aspects than this, but the key takeaway is that being located in the left exit and right entrance regions of a jet streak, as well as being in an environment with a strong temperature gradient can lead to explosive storm development. In particular, there needs to be a situation where strong warm air advection can occur on the warm side of the storm, and strong cold air advection can occur on the cold side of the storm, as this leads to strengthening fronts and strengthening storms. In a surface Low Pressure system, the winds converge, since air flows from high pressures to lower pressures. This convergence at the surface leads to upward vertical motion, and thus a reinforced lower pressure at the surface. This process can particularly be augmented by divergence aloft via being in the left exit and right entrance region of a jet streak.

We will now leave you with a series of beautiful images from the powerful ocean storm that occurred on Wednesday.

A beautiful satellite image taken of the ocean storm, showing an eye-like feature, and a very strong line of thunderstorms on the southeast side of the storm (associated cold front). (NOAA).

Enhanced water vapor imagery of the powerful ocean storm. You may need to click to animate (wx.rutgers.edu).

Another beautiful enhanced water vapor image taken of the ocean storm (NASA).

The fact that the east side (warm side) of the storm was right over the Gulf Stream and its powerful circulation wrapped in plenty of cold air from Canada led to quite the impressive temperature gradient, so it is no surprise that the storm became so strong.

One final note: some of you may be wondering about the very prominent dry colors being shown in the water vapor imagery. This is actually an area of lots of dry air, which seems odd for a storm’s development; but it’s actually another classic indicator of a mature cyclone. All of those warm, moist colors on the southeast side of the storm is a bunch of convection out ahead of its strong cold front, and south of its strong warm front. This is where plentiful lift is occurring — it’s the warm sector. But what comes up must also come back down — the extremely strong areas of lift and precipitation are being balanced out by lots of subsidence and dry air behind it, which is reflected beautifully in the water vapor imagery.

Enjoy your weekend, everyone.