"Norlun" instability/inverted troughs
Posted 23 January 2007 - 03:30 PM
Climatology of the NORLUN Instability Trough: Initial Development and Use in Improving New England Snowstorm Forecasts
Gregory A. Zielinski, Daniel K. Cobb and Hendricus J. Lulofs (National Weather Service, Caribou, Maine)
Almost every winter in New England, there is at least one occurrence of a snowstorm that was not forecasted or an “under forecasted” snowstorm that produced snowfall totals above 10 inches when the original forecast was only for 1-2 inches. In such cases, NWS warnings are often issued in the middle of the event, when heavy snow is already occurring, which limits the effectiveness of the warning. The most common cause for these problematic forecasts, particularly from the perspective of the general public, is the presence of a NORLUN instability trough (NORLUN), a trough that has the ability to focus and lift moisture to form intense and persistent snow squalls (Lundstedt, 1993). Identification of the trough as the driving force in such scenarios occurred in the early 1990s following two separate snowstorms that hit the Portland, ME, area (21 March 1992) and Cape Cod (19 February 1993) with 3-4 inch snowfall rates and total accumulations of 20-30 inches. Unfortunately, numerical models do not appear to predict consistently development of the NORLUN (QPF) nor do they appear to predict reliably snowfall amounts (QPE). Consequently, we are in the process of developing a detailed climatology of the NORLUN as a means to identify the processes involved in NORLUN formation and the parameters essential for recognizing, and thus predicting the presence and impact of the trough. An ultimate goal is the construction of a conceptual model, which would facilitate consistent prediction and provide adequate warning lead times for such events.
In addition to a re-evaluation of the storms originally assessed by Lundstedt, we initially evaluated the characteristics of two NORLUN events during the 2001-2002 winter (21 December 2001 and 16 January 2002). From these analyses we suggest that there are at least two NORLUN types. A Type I NORLUN appears to occur most frequently as a 25 to 50 mile wide band of heavy snowfall amounts with the axis of heaviest snowfall oriented NW-SE extending from the coast into the hills of interior New England. Synoptically, this type appears to be characterized by a weakening surface low moving into the St Lawrence River valley with a developing ocean storm too far to the east to affect New England directly. This situation is often associated with onshore flow of potentially buoyant air from 500 to 1500 meters above the surface ahead of a trough axis that connects the two low centers. A Type II NORLUN produces a snowfall axis that tends to mirror coastal topography in areas south of Mid-Coast Maine. In this case, there is only one low (i.e., an ocean storm passing too far to the east to directly affect New England) with a significant inverted trough extending to the northwest of the developing low center. The inverted trough appears to be the result of an advancing shortwave aloft with heavy snow along and ahead of the trough axis. However, the orientation of the snow bands tends to be parallel to the coastline and not necessarily parallel to the trough indicating that a secondary mesoscale response, such as coastal convergence induced by frictional differences between sea and land, may be more directly responsible for the maintenance of the 2 to 4 inch per hour snow rates. Consequently, geography of the coastline may be critical to development of a Type II event. We expect that these characteristics, as well as others discovered in the evaluation of additional NORLUN events, will be components of our conceptual model for the NORLUN instability trough.
WHAT IS AN INVERTED TROUGH?
METEOROLOGIST JEFF HABY
In the mid-latitudes of the Northern Hemisphere a trough is usually seen as a southerly bulge in the height contours. The lowest heights are generally located to the north of the trough. In an inverted trough situation, the height contours bulge to the north. This is more common in the tropical regions where regions of low pressure ride south of a mid-latitude high pressure but can happen in the mid-latitudes when low pressure is south of high pressure. An inverted trough bulges to the north. At first it may look like a ridge, but on further inspection it is a trough. Both a trough and an inverted trough have a cyclonic (counterclockwise) flow pattern. A trough will tend to have more westerly winds associated with it while an inverted trough will tend to have more easterly winds associated with it. If an inverted trough is actually a ridge then the winds will be flowing with the height contours in the opposite direction (anti-cyclonic direction). The direction of windflow through the feature is how a ridge is discerned from an inverted trough.
Tropical waves will show up as inverted troughs because they are generally south of mid-latitude high pressure and have an easterly wind associated with them. The image below shows how a typical trough in the mid-latitudes looks and that of the inverted trough.
Posted 23 January 2007 - 03:43 PM
You can clearly see the mesolow.
Posted 23 January 2007 - 03:51 PM
From Wikipedia, the free encyclopedia
A polar low is a small-scale, short-lived atmospheric low pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. The systems usually have a horizontal length scale of less than 1,000 km and exist for less than a couple of days. They are part of the larger class of mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms.
Polar lows have been referred to by many other terms, such as comma cloud, mesocyclone, polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more-vigorous systems that have near-surface winds of at least gale force (17 m/s).
Polar lows were first identified on the meteorological satellite imagery that became available in the 1960s, which revealed many small-scale cloud vortices at high latitudes. The most active polar lows are found over certain ice-free maritime areas in or near the Arctic during the winter, such as the Norwegian Sea, Barents Sea, Sea of Japan, and Gulf of Alaska. Polar lows dissipate rapidly when they make landfall. Antarctic systems tend to be weaker than their northern counterparts since the air-sea temperature differences around the continent are generally smaller. However, vigorous polar lows can be found over the Southern Ocean.
Polar lows can have a wide range of cloud signatures in satellite imagery, but two broad categories of cloud forms have been identified. The first is the "spiraliform" signature consisting of a number of cloud bands wrapped around the centre of the low. Some polar lows have the appearance in satellite imagery of tropical cyclones, with deep thunderstorm clouds surrounding a cloud-free eye, which has given rise to the use of the term "Arctic hurricane" to describe some of the more active lows. These systems are more common deep within the polar air. The second is a "comma-shaped" signature that is found more frequently with systems closer to the polar front.
Polar lows form for a number of different reasons, and a spectrum of systems is observed on satellite imagery. A number of lows develop on horizontal temperature gradients through baroclinic instability, and these can have the appearance of small frontal depressions. At the other extreme are the polar lows with extensive cumulonimbus clouds, which are often associated with cold pools in the mid- to upper-troposphere.
Polar lows are very difficult to forecast and a nowcasting approach is often used, with the systems being advected with the mid-tropospheric flow. Numerical weather prediction models are only just getting the horizontal and vertical resolution to represent these systems.
Here is an image of a polar low taken back in 1987. As you can see it is almost an exact replica of the mesolow that formed off the New England coast last night. It is common for these lows to form north of an arctic boundary.
CAN UPPER LEVEL DIFFLUENCE CAUSE RISING AIR?
METEOROLOGIST JEFF HABY
Divergence occurs when a stronger wind moves away from a weaker wind or when air streams move in opposite directions. When divergence occurs in the upper levels of the atmosphere it leads to rising air. The rate the air rises depends on the magnitude of the divergence and other lifting or sinking mechanisms in the atmosphere. The diagram below shows two examples of divergence.
Diffluence is the spreading of wind vectors. In a diffluent pattern the height contours become further spaced from each other over distance. Does this spreading out of the wind vectors and height contours cause the air to rise? The diagram below is an example of 300-mb diffluence.
In a diffluent pattern, two distinct phenomena occur at the same time. First, strong wind is moving into weaker wind. Where the height contours are closer spaced, the wind velocity is higher. As you know, a strong wind moving into a weak wind is convergence. Second, as height contours spread apart, a divergence of air occurs. The convergence due to stronger wind moving into weaker wind replenishes the mass lost due to the divergence in the diffluent flow. In the diagram on the next page, notice in the diffluent pattern that strong wind is moving into weaker wind and the air streams are diverging over distance also.
The effect of convergence and divergence occurring at the same time is no vertical motion. The air is merely being deformed into a new shape. The air is spreading out, but it is not rising or sinking. It is upper level divergence that causes rising air. The two best examples of upper level divergence are PVA and divergence associated with the right rear and left front quadrants of a jet streak. Upper level diffluence by itself does not cause rising air.
Haby sez: An upper level diffluence pattern by itself does not cause rising air. It is upper level divergence that causes rising air.
Posted 23 January 2007 - 06:42 PM
Those things sure do looks like hurricanes.
Posted 23 January 2007 - 09:21 PM
Posted 23 January 2007 - 09:28 PM
What is a Norlun trough?
Near the surface and in the lower few thousand feet of the atmosphere, a trough is a weak disturbance featuring a wind shift and convergence of air - that is, air coming together. Occasionally, with cold air aloft, conditions can become quite favorable for clouds and precipitation ("instability") and it's this combination of converging air and instability that makes a Norlun trough. These events have, in the past, deposited surprise snowfalls of over a foot from the Eastern Massachusetts coastline through the coast of Maine. In this instance, it appears as though the heavier band of snow associated with the Norlun trough will set up mostly offshore, but close enough to New England that it's worth noting the potential for enhanced accumulations on outer portions of Cape Ann and Outer Cape Cod, and it's not impossible that this band could extend into the Central coast of Maine - in fact, that's looking more and more likely given a band of snow that's already established around the York County coast! While the bulk of this heavier snow band will remain just offshore, over the Gulf of Maine and waters east of Massachusetts, a few inches are possible along the immediate coast from Northern York County, ME, to the Knox County coastline between Monday midday and overnight Monday night. Keep in mind that these bands of snow are very difficult to predict, but those along the immediate coastline should be prepared for enhanced accumulation of a few inches in the areas mentioned.
This Norlun trough will be slow to completely disappear from the weather map, as a series of upper level disturbances moving overhead will enhance it for the next day or two. Nonetheless, Tuesday is likely to bring early lingering snow showers to Eastern New England, and otherwise lots of clouds, some sunny breaks and scattered snow showers to most of New England as weak disturbances continue to ride overhead in the jet stream winds aloft. All the while, a series of weak storm centers will be pulling northeast from the Great Lakes into Southern Canada, and the counter-clockwise flow of air around each of these low pressure waves will gradually feed milder air northeast into New England. The result will be steadily moderating temperatures into Wednesday.
Posted 23 January 2007 - 10:03 PM
The forecast was for 1-3 inches of snow for Cape Cod and they got 30 inches.
Posted 23 January 2007 - 10:14 PM
Back in '93 the National Weather Service had one of the worst busted forecasts ever thanks to one of these Norlun troughs.
The forecast was for 1-3 inches of snow for Cape Cod and they got 30 inches.
Posted 02 March 2007 - 06:09 AM
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