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Weather Observation and Analysis
John Nielsen-Gammon
Course Notes
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Chapter 13. FRONTS AND
FRONTOGENESIS
13.1 Fronts as Temperature Gradients Fronts were first discovered during World War I, and the name was adopted by analogy to the fronts of battle during the war. Until data from a weather network covering a significant hunk of territory was regularly transmitted and plotted at a central location, it was difficult to recognize the patterns behind the sudden weather changes at different stations.
Today, fronts, along with highs and lows, are the most common features of weather maps, and even children are able to recognize the symbols. Nonetheless the working definition of a front remains somewhat elusive, and the decision about where a front lies is a judgment call that experienced weather analysts can disagree about. The basic definition of a front is a narrow, elongated zone with a locally strong temperature gradient. But how narrow is narrow, how elongated is elongated, and how strong is strong?
Some have argued that, because of this ambiguity, we should dispense with the concepts of fronts entirely and simply let the analyses of temperature, wind, pressure, etc. speak for themselves. Such an approach is attractive in its intellectual purity, but in practice, people expect to see fronts, and fronts are intimately related to weather patterns. The non-front folks would argue that the relationship between fronts and weather patterns is anything but simple, and that the mere presence of a frontal symbol without a depiction of the underlying weather elements is likely to be misleading.
Even the basic definition of a front includes many non-fronts. For example, imagine a coastline separating a warm land surface from a cold ocean. The air above the coastline would meet the criterion of a narrow, elongated zone of locally strong temperature gradient. Yet nobody would consider it to be a true front.
While this argument rages, we will attempt to construct a working definition of fronts that will serve us well enough for the time being. A synoptic-scale front is an air mass boundary that extends up into the troposphere. It includes at least a locally enhanced temperature gradient and a vector wind shift.
A vector wind shift means that the horizontal wind vectors on one side of the front are different from the horizontal wind vectors on the other side of the front. At ground level, the wind shift is either purely convergent or both convergent and cyclonic. Above ground level, the wind shift is only cyclonic.
though they extend up above the planetary boundary layer, because the strongest wind variations and temperature gradients tend to be at the surface. Surface fronts typically weaken with height, and while it is possible for surface fronts to connect with upper-level fronts and thereby extend through the entire depth of the troposphere, most surface fronts peter out near the 600 mb to 800 mb level.
Surface fronts have specific structural characteristics that are important for understanding the weather associated with them. The following discussion applies specifically to variations of weather elements observed at ground level.
First, the zone of strong temperature gradient is generally much wider than the zone over which the wind shift occurs. In other words, while the temperature gradient zone is rather narrow, the wind shift is very narrow.
Second, the wind shift, which is collocated with (or at worst within a few miles of) the pressure trough, is at the warm edge of the temperature gradient.
Third, if there’s a dewpoint change across the front, the dewpoint change tends to be even more rapid than the temperature change.
Fourth, while there’s often cloudiness and precipitation associated with cold fronts, they can occur on either side of the front, well ahead of the front, or not at all. Warm fronts are a bit better behaved: if there’s to be rain or snow associated with a warm front, it will usually be found ahead of it, on the cold side. The weather associated with stationary fronts and occluded fronts is similar to that associated with warm fronts.
While they tend to have certain distinguishing characteristics, technically the only absolute difference between warm fronts and cold fronts are the nature of the change of temperature as the front passes. Warm fronts bring warmer temperatures, while cold fronts bring colder temperatures. Even in this absolute sense, exceptions can occur if, for example, a cold front passes on a calm night or low clouds are present ahead of a cold front during daytime; both can cause temperatures to temporarily rise when a cold front passes.
The surface wind and pressure usually tell which way a front is moving: if the average of the winds on both sides of the front would carry the front in a particular direction, that’s usually the way the front will move. Using isobars you don’t even have to estimate an average: if isobars pass through a front, the direction of the geostrophic wind tells you the direction of frontal motion.
A stationary front is a front that is not moving much. No fronts are perfectly stationary, so how much can a stationary front move and still be stationary? A basic rule of thumb is about five miles per hour, or about two degrees of latitude per day.
At an occluded front, the air ahead of a warm front meets the air behind a cold front. Usually this situation is found near surface low pressure systems, where an occluded front can extend from the vicinity of a low pressure center to an intersection point between the warm and cold fronts. In most cases, the cold front is actually riding up over the top of the warm front, so the occluded front should be drawn as an extension of the warm front. Since both the warm front and cold fronts have temperature gradients, there are two temperature gradients associated with an occluded front: one ahead of the front and one behind. The front itself
Upper-level fronts are most often found in the vicinity of strong jet streams. They are typically oriented nearly parallel to the upper-level winds, and tilt toward the cold air.
That cold litany of facts makes upper-level fronts seem rather boring. A bit more interesting is the fact that all upper-level fronts are really narrow tongues of stratospheric air being dragged down into the troposphere. In a cross section, the high stratification within an upper- level front connects directly with the high stratification within the stratosphere, and aircraft observations have confirmed the presence of stratospheric ozone levels within upper-level fronts.
Upper-level fronts by themselves are not associated with much surface weather, but they are quite significant for aviation forecasting. The zone of an upper-level front often features strong turbulence excited by strong vertical wind shear, so commercial aircraft generally try to avoid them.
A so-called split front is actually two fronts: a surface cold front and an upper-level front that extends ahead of it. The best way to tell if a split front is present is from inspection of both surface and upper-level maps, but if only surface maps are available, the evidence for a split front is a band of moderate precipitation well ahead of the surface cold front, with low clouds and little precipitation associated with the cold front itself. The back edge of the moderate precipitation generally corresponds to the upper-level front. If you are expecting the precipitation to always be along the surface front, a split front can really wreck your forecast.
13.4 Fronts and Wind Shear According to thermal wind balance, the strong temperature gradients associated with synoptic-scale fronts should be associated with strong vertical wind shear. As with all geostrophic vertical shear, the shear vector (the vector difference between higher-level winds and lower- level winds) should be oriented along the isotherms with warmer temperatures to the right. Since fronts are oriented nearly parallel to isotherms, the vertical wind shear will be almost parallel to the temperature gradient too.
The above statements are consistent with what was earlier noted about upper-level fronts: that they tend to be associated with strong jet streams and be oriented parallel to them. Indeed, upper-level fronts will typically start on the left side of the jet (facing downwind) and slope under the jet, extending out the right side in the middle troposphere. This puts the strongest temperature gradient beneath the jet, where it must be if the wind speeds are to increase rapidly with height up to jet stream level.
With surface fronts, the shear is not so noticeable, not because it is much weaker but because the temperature gradients and temperature advection seem to attract the most attention.
As an example, consider a cold front oriented northeast-southwest, with the warmer air to the southeast. Suppose the surface winds behind the cold front are blowing from the northwest. Behind the surface frontal position, there must be a strong temperature gradient, so the vertical wind
the severe weather is not confined to the fronts but instead occurs in a broad swath within the warm sector. And it is the very shear that keeps the air masses in balance that allows thunderstorms to develop into rotating supercells and occasionally produce tornadoes. The severe weather is not produced by the two air masses clashing, it is produced by the two air masses getting along!
13.5 Gravity Currents At the surface, with friction, there will be a tendency for the wind to blow partly from high pressure to low pressure. Sometimes this can help cause the leading edge of the front to collapse into a near discontinuity, with several degrees of temperature difference across a kilometer or less. Such sharp discontinuities are often found with smaller fronts as well, such as gust fronts, squall lines, and sea breezes.
Once things get so small and air passes through them so quickly, there’s no time for the air to come into geostrophic balance. Instead, a phenomenon called a gravity current develops. Gravity currents, also known as density currents, have a sharp leading edge between 100 m and 1500 m deep, sloping back toward the cold air with a slope of about 1:1. Along the interface between the warmer and colder air, instabilities develop because of the strong vertical shear there.
Gravity currents are strange beasts because, quite independent of the sort of dynamical reasoning we have conducted so far, they have a very predictable velocity. The speed C of a gravity current is given by:
C U g h
where U is the component in the warm air ahead of the gravity current normal to the gravity current (positive if going away from the gravity current), h is the depth of the cold air several miles behind the gravity current, q is the potential temperature within the density current, and q’ is the difference between the potential temperature outside the density current and that within. Plugging in some sample numbers, say a normal wind of zero, a temperature difference of 6 K and a cold air depth of 500 m, the gravity current would be advancing at a speed of about 10 m/s into the warm air.
Notice that in this example, if the warm-air wind had been blowing at 10 m/s toward the cold air, the gravity current would be stationary, with no net velocity. If instead the warm air was blowing away front the gravity current at 10 m/s, the gravity current itself would be trucking along at 20 m/s. In effect, the last term in the gravity current equation gives the speed of the gravity current relative to the warm air.
When the environment of a gravity current is humid, distinctive clouds often form along the gravity current. A roll cloud is a cloud that forms due to the upward motion at the head of the gravity current. The cloud can be composed of air from the warm or cold side of the gravity current. When such a cloud appears in a satellite image it is called a rope cloud , a term descriptive of how it looks in a satellite image. A shelf cloud is a cloud located above the gravity current and represents a layer of humid air aloft that ascends as it passes over the gravity current.
Sometimes a squall line will form along a cold front, and the gravity current speed of the squall line will generally be faster than the speed of the original cold front. As a result, the squall line will propagate out ahead of the original cold front. Since the squall line now marks the leading edge of the strong temperature gradient, the cold front is often drawn coincident with the squall line. But sooner or later, if it doesn’t dissipate, the squall line moves so far out ahead of the front that the front
a low there. The name for this feature is a col or saddle point. The former name refers to a topographic feature, and the latter name refers to the similar appearance of lines of constant height on a saddle,
As for deformation, imagine an object placed in the middle of this vector field. The winds will act to stretch the object in the east-west direction and contract the object in the north-south direction. We could say that the object is being deformed, hence the term deformation.
Notice that we need not place the object at the origin for this to happen; deformation is present everywhere. No matter where the object is placed in this vector field, its left and right sides will be moving away
from each other and its north and south sides will be moving toward each other.
A useful quantity in connection with deformation is the axis of dilatation. This is the axis along which the imaginary object would be stretched (or dilated) most strongly. In the example above, the axis of dilatation is oriented east-west.
Now take the example above and rotate it counterclockwise by 45 degrees. Presumably we have not altered the magnitude of the deformation, since objects would still be stretched or shrunk by the same amount as before. The axis of dilatation will have rotated too, and now would be oriented northeast-southwest.
With the rotated wind pattern, consider the mathematical terms in the definition of deformation. After the rotation, each of the derivatives in the first term in parentheses is suddenly zero! (Don’t take my word for it, work it out for yourself.) But not to worry, the second term is now nonzero. Since v increases in the positive x direction, the first derivative is positive. Since u increases in the positive y direction, the other derivative is also positive. The lesson here is that there’s really nothing special about which of the terms contributes to deformation.
orientation (in the example case, east-west), deformation will cause the gradient to intensify. In fact, any angle between them less than 45 degrees will lead to intensification. If the angle is larger, all the way up to a maximum of 90 degrees, deformation will lead to a weakening temperature gradient. Right at 45 degrees, deformation will have no effect.
The process by which the temperature gradient intensifies (and, in mathematical terms, the time rate of change of the magnitude of the temperature gradient) is called frontogenesis. Frontogenesis is one of the most humorous-sounding words in all of meteorology, just behind “isodrosotherm”. The converse process, in which the temperature gradient weakens, is called frontolysis.
Deformation is one of the ways frontogenesis can take place, as long as the isotherms are oriented properly relative to the axis of dilatation. Another frontogenetic process is convergence. Both work well, but for large-scale motions the wind is approximately in geostrophic balance and the convergence of the geostrophic wind is almost zero, so big synoptic-scale fronts are almost always produced primarily by deformation.
While it is natural to think of frontogenesis as something that happens to a front, strictly speaking frontogenesis is something that
happens to an air parcel that may or may not be embedded within a front. It is actually possible for air to pass through a front, if it experiences frontogenesis on the way in and frontolysis on the way out. This most often happens with upper-level fronts: air comes in one end of the front and goes out the other.
13.8 Frontogenesis and Vertical Motion So far we’ve seen that deformation can happen as a result of large- scale wind patterns and it can produce frontogenesis. Now, at the scale of a synoptic-scale front, the wind will be in thermal wind balance, so if the wind pattern is such as to cause frontogenesis, somehow the vertical shear must be increasing at the same time. For reasons too convoluted to get into here, the same horizontal wind pattern that causes frontogenesis would by itself cause the vertical shear to weaken. So large-scale frontogenesis tries to throw the atmosphere completely out of balance. So what else happens that keeps the atmosphere (nearly) in thermal wind balance?
If the temperature gradient is changing, the pressure gradient must be changing too. In particular, a stronger temperature gradient means a stronger gradient at low levels between high pressure on the cold side and low pressure on the warm side. While the winds may initially have been in geostrophic balance, the stronger low-level pressure gradient will cause an additional acceleration from the cold side toward the warm side at low levels. This, in turn, implies the development of low-level convergence on the warm side and divergence on the cold side. Which itself means upward motion on the warm side and downward motion on the cold side. Mass conservation tells us that aloft there must be divergence on the warm side and convergence on the cold side, so the air would be blowing from warm to cold aloft. We’ve actually described a complete vertical circulation cell, with low-level air moving from cold toward warm, rising, moving aloft back toward cold, and sinking.
Now stay with me. The horizontal winds just described imply a Coriolis force directed toward the right of the winds. Since the new winds at the surface and aloft are in opposite directions, so too are the associated Coriolis forces. And the acceleration in response to the new Coriolis forces leads to an increase of shear, just the sort of thing we need to keep the atmosphere close to thermal wind balance.
Meanwhile the vertical motions just described involve the air on the warm side of the front rising, so the air over there would be getting cooler. On the cold side, the subsidence leads to warming. On the whole, the temperature gradient would be weakening because of this vertical circulation.
warmer than the sinking air, is a direct circulation. Conversely, it turns out that frontolysis is associated with upward motion on the cold side and downward motion on the warm side, that being an indirect circulation.
Notice that it is not the mere presence of the front that is causing the upward motion. Instead, it is the tendency of the front to strengthen or weaken that leads to an adjustment process that involves vertical motion.
True, there are other aspects of a front that produce upward motion even if the front is staying constant. One such situation occurs if a gravity current forms on the leading edge of the front. The gravity current will flow toward the warm air, and the warm air will be displaced upward. The other such situation is the convergence that surface friction induces along any trough, including a front. The convergence implies upward motion aloft. But both of these effects are strongest at ground level and would barely be felt at heights of 3 km or more. In contrast, the vertical motion induced by frontogenesis works even for upper-level fronts.
13.9 Fronts and Jet Streaks Why is a jet streak like a front? Because a jet streak, being an area of very strong winds, will typically have very strong vertical wind shear beneath it, and therefore a very strong horizontal temperature gradient. As air beneath the jet streak flows toward the jet streak, we would expect that, since the height gradient is intensifying, the height contours would be becoming closer together. This confluence would be associated with deformation and therefore (if the isotherms are close to parallel with the wind) frontogenesis.
So as an air parcel begins to move underneath a jet streak, we should expect the same direct vertical circulation as with the earlier frontogenesis example. In particular, there would be upward motion to the right (the warm side) and downward motion to the left (the cold side). Aloft, near the level of strongest winds, there would be a component of motion directed from right to left, across the height contours toward lower heights.
