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BURST POINT SERIES:
10 Mechanisms

METEOROLOGIST JEFF HABY

This 10 point series examines processes that can create an initial burst point for thunderstorm convection. The topics include:

1. Temperature Convergence
2. Moisture Convergence
3. Outflow Convergence
4. Frictional Convergence
5. Solar Heating Max
6. Natural Conv. Boundaries
7. UVV Max
8. Man-made Boundaries
9. Old Boundaries
10. Instability Max

This series of Haby Hints looks at 10 mechanisms that can create a burst point for convection. The burst point is the location where a thunderstorm first forms and climbs vertically throughout the troposphere. Convection is air rising due to positive buoyancy.

1. Temperature Convergence

Temperature is important since it influences the density of the air. Warmer air is less dense and it is easier for less dense air to lift in the vertical. This is a reason why storms often form first in the afternoon. In the afternoon, solar heating tends to be at a maximum. Also, different regions of air with different temperatures have trouble mixing. When warmer air moves toward colder air, instead of completely mixing, much of the warmer air will deflect over the colder air. Since colder air is denser and warmer air is less dense, it is the warmer air that will tend to rise over the colder air. Thus when air streams interact that have different temperatures it can help initiate vertical motions. Convergence is air piling into a region. For temperature convergence to increase instability what is needed is a piling of warm air. This can create a relatively larger bubble of warm air that when interacting with surrounding air will tend to help it rise. An analogy is to think of a developing bubble of air in a pot of water that is warming toward boiling. Bubbles will collect and build and then break away to rise to the top of the water surface. In the case of temperature convergence, warm air is being pooled together by the surrounding wind flow. This can create a location that is the burst point for thunderstorm development.

2. Moisture Convergence

Moisture is one of the critical ingredients for thunderstorm development. A region with more moisture will be less dense than an adjacent region that has less moisture when both have the same temperature. This is important since the region that is less dense will be more inclined to rise. This is one ingredient that can lead to the initial burst point for convection. Moisture content influences the density of air and when these air parcels approach each other the less dense parcel will tend to ride over the denser parcel instead of thoroughly mixing with it. Wind direction has an important influence on the moisture content of the air. Air flowing from an oceanic source will often have a higher moisture content while air blowing from the continent interior will tend to be drier. Moisture content can vary locally by other factors such as the influence from wet vs. drier soils, lakes, urban vs. nonurban, vegetation type and irrigation practices.

When air streams converge, the moisture within those air streams will converge also and this leads to dewpoints increasing. Boundaries such as fronts, outflow boundaries and drylines are locations convergence can occur. Moisture convergence can increase the dewpoint of the air and thus increase the instability of the air. Lifting, instability increase and moisture increase can all occur along a boundary. This makes a boundary a prime candidate for a burst point somewhere along the line (often where best convergence takes place).

The previous Haby Hint focused on temperature convergence. Both temperature and moisture convergence will contribute to increasing the Theta-E of the air. The intersections of a higher temperature axis with a higher dewpoint axis will have higher Theta-E values. Increasing Theta-E contributes to increasing instability since it results from a combination of temperature and dewpoint increasing. This location called a Theta-E ridge can be a burst point for convection.

3. Outflow Convergence

Outflow is a boundary that results from a thunderstorm or group of thunderstorms. This gust front has different temperature and moisture characteristics from the surrounding environmental air. Thus, it has a different density than the surrounding air. This is important since the outflow can act as a new lifting mechanism to develop more thunderstorms or to sustain a thunderstorm that has already developed. An outflow boundary could develop new convection right away or the boundary could sit around and even help develop new convection a day later. The convergence between two different outflow boundaries can be a significant lifting mechanism since at their intersection there will be a near doubling of the convergence. This can be a prime focal point for a new burst point to occur.

Before a storm hits, often a gusty, more humid and cooler wind will be felt. This is the gust front. The coolness of the air is due to a combination of evaporative cooling and the air that was brought from aloft ending up being cooler than the environmental air. The density difference helps accelerate the air toward the surface thus the wind can be very gusty and at times can produce damaging straight line wind. This gust front continues to move and mixes with environmental air. The mixing is gradual and the time it takes for the gust front to thoroughly mix with the environmental air is variable. Air with different densities has trouble mixing thus the influences of the outflow can be sustained for many hours.

Outflow can be as significant of a lifting mechanism as a front, upper level divergence and low level warm air advection. Thus, the development of one storm can signal the development of several storms or a grouping of storms since the lifting from the outflow combines with preexisting lifting mechanisms already in place. These outflows are difficult to predict since it is difficult to pin point exactly where the first storm will form. Once a storm does develop, then monitoring outflow is a major player in mesoscale forecasting since it leads to more burst points.

4. Frictional Convergence

Low level convergence will lead to rising air. This is because the air piles together and has no place to go except up. The most dramatic examples of low level convergence occur along fronts and low pressure systems but more subtle convergence can occur within an air mass that can also lead to a burst point for convection.

Subtle convergence can occur when air has to flow into topographic barriers, over land with different friction characteristics, and at different velocities. When air flows into a barrier such as a large hill, some of the air will deflect around the barrier while some will go over it. This will lead to convergence as the air moves into the topographic barrier. Air just above the surface will flow faster since there is less friction but this air will interact and slow down as it interacts with higher elevations. Also, air will flow at different velocities over different surfaces. For example, air will flow faster over a water surface than over a land surface with trees. Air will flow faster over a treeless field than over a forest. When friction is weaker such as over water or a treeless field, the wind will flow faster. When this faster moving wind flows into a rougher surface it will cause convergence to occur. Convergence caused by topography and land cover can ignite a thunderstorm if only subtle convergence is needed to get a storm going.

5. Solar Heating Max

One contribution to instability is how warm the boundary layer can warm during the day. The amount of warming at any one place is going to depend on a variety of factors. The warm air tells part of the story but the other part of the story is the moisture content in the air. A location that is slightly cooler can be more unstable than a warmer location if it has a significantly higher moisture content. This writing will focus on factors at the surface that determine the heating and moisture content of the air near the surface.

One factor that influences the amount of heating is clouds. Clear regions will generally warm more than cloudy regions. The sun coming out and staying out can be a critical ingredient for thunderstorm development. Another factor is the heat capacity of the surface that is being warmed by the sun. Land will warm faster than water. Dry soil will warm faster than wet soil. Bare soil will warm faster than densely vegetated soil. There is a trade off in that increasing moisture also helps make the air more unstable. Thus, regions undergoing a combination of both significant warming while retaining high dewpoints will be favored regions for the burst point for convection. Wet soils can have an overall positive impact on instability even though it may prevent the temperature from warming as high as it could have been.

Solar heating depends on several factors such as the amount of unobstructed sunlight, time of year, air mass in place, cloud cover, particulate matter in the air and ground cover. Solar heating is cumulative during the day and tends to reach of maximum in the afternoon. This is a common time of the day storms will first form. During the late spring and early summer the amount of daylight is at a maximum. This produces more cumulative heating throughout the day and makes it more likely that storms will form when solar heating is needed to break a capping inversion. Severe thunderstorms are most common this time of the year also, from April to June. A solar heating max that initiates vertical motions can be the location of the burst point that starts a thunderstorm or severe storm outbreak.

6. Natural Conv. Boundaries

Two ways of categorizing convergence boundaries are natural and man-made convergence boundaries. This writing will focus on natural convergence boundaries. Natural convergence boundaries help create convergence when the wind is from a particular direction.

One of the most common convergence boundaries are ones created by elevation changes. When the land elevation changes, wind flow is forced to interact with this boundary. Higher above the surface the wind is less influenced by friction, but when it moves into a higher elevation land region the wind will be forced to interact with the land. This will cause the air to slow down. This slowing causes convergence to occur. This convergence occurs most significantly on the windward facing slope of the elevated region. The windward side in any particular weather situation is going to depend on the wind direction. This location can be a burst point for convection to occur due to the extra lifting provided by the windward slope. The slope does not have to be a mountain. Gently rising elevation changes over large areas and big hills can also aid in burst point convergence.

Other natural boundaries are more subtle but can also act as the focal point for first convection. One example is a water and land boundary. Wind flows faster over water thus when it interacts with land it will slow down and convergence. This is especially true over large lakes and ocean-land boundaries. Another example is plains and forest boundaries. The plains region tends to be flatter with less trees and this causes the wind to be stronger. When this air flows into a forest region it can lead to convergence. A final example is a natural boundary produced by river valleys. The land sloping on each side of the valley and vegetation changes away from the river over distance can lead to convergence taking place. When no storms have formed yet but are about to, a convergence boundary is a popular place for first convection due to the extra lift provided.

7. UVV Max

UVV stands for Upward Vertical Velocity. There are many lifting mechanisms that contribute to rising air. Sinking air mechanisms, call DVV (Downward Vertical Velocity), are also significant since they can reduce or cancel lifting mechanisms. The net impact of UVV and DVV over a region determines whether the air will rise or sink at that location. UVV and DVV motion is the result of dynamic lifting or sinking and does not result from instability. The motion of UVV and DVV is much slower than instability release or downdraft motions in a storm but with persistence and enough lift the air can rise or sink a significant amount over hours of time.

A forecaster will examine all the potential UVV and DVV mechanisms over the forecast region. The region where the mechanisms coming together result in the greatest lift is a potential burst point for convection. The three critical ingredients for thunderstorms are moisture, lift and instability. A high UVV value indicates lifting will be a positive aid in potential thunderstorm development. The 700-mb UVV panel shows the net result of UVV or DVV for each location over an area. This can be used as a quick guide to see where the greatest UVV is coming together. This will often be a circular or linear region of UVV on the model panel. The location that has the highest UVV can be the burst point for convective storms when adequate moisture and instability are in place also. The link below goes over various UVV mechanisms and how they influence a precipitation forecast. Other factors that go into a precipitation forecast are explained also:

http://www.theweatherprediction.com/precipfx/

8. Man-made Boundaries

Two ways of categorizing convergence boundaries are natural and man-made convergence boundaries. This writing will focus on man-made convergence boundaries. Man-made boundaries help create convergence that can influence where convection first forms.

Examples of man-made boundaries include farming-vegetation boundaries, urban and rural boundaries and land use boundaries. Humans have dramatically changed the vegetation type is certain areas. Some of this is due to agriculture, farming and ranching. The introduction of different vegetations will change the character of the surface. Some crops give off more moisture than others while some vegetation absorbs solar radiation more than others. These moisture and thermal differences that change over distance due to differing vegetation type can create a subtle convergence boundary.

Urban and rural areas vary in the amount of thermal absorption and moisture emission. During the day, urban areas are often warmer and less humid while rural areas are cooler and more humid. The temperature difference can be especially remarkable. When winds are not too strong, mesoscale air masses can develop over the urban and rural areas. The differing thermal and moisture characteristics between these mesoscale air masses can cause convergence to occur which can be the focal point for thunderstorms. The extra heat over an urban area can set off a burst point of convection that first forms near the urban area and then moves downstream with the wind.

Land use boundaries are created by the division of farm land from forest land. The differing moisture and thermal characteristics between trees and agricultural crops can create a subtle convergence boundary. It needs to be stressed that these boundaries will tend to be subtle (not more obvious like cold fronts). These subtle boundaries can make all the difference in a borderline situation between storms forming or not forming and where the storms first develop.

9. Old Boundaries

An old boundary refers to a surface and low level boundary that was set down the previous day that has not completely dissolved and mixed out with the environmental air yet. There can be a variety of reasons why a boundary remains in place after many hours and continuing into the next day. Boundaries that slowly move or stall over several hours can be caused by old fronts (stationary front, dissolving cold front or warm front), an old outflow boundary set up from a complex of thunderstorms the day before that is gradually dissolving away, and any other boundary that remains present due to weak winds not mixing out a boundary that was established the previous day.

An old boundary can have subtle moisture, wind and temperature differences on each side of it that can aid in convergence as the day progresses. If the environment has adequate instability and moisture but increased lift is needed, an old boundary can be just enough to provide that extra low level convergence that is needed to be the burst point for convection. Forecasters look for these boundaries and other mesoscale boundaries when doing a meso-analysis. Overlays of surface and low level temperature, dewpoint, wind direction, wind speed, etc. from a network of closely spaced weather stations can be used to find these subtle and non-subtle boundaries that can aid in the formation of thunderstorms. The website location that many forecasters use is the Storm Prediction Centerís Mesoscale Analysis Pages at the following link. A future Haby Hints series in the spring severe weather season will cover these mesoscale analysis updates and the weather information they provide along with example case studies:

http://www.spc.noaa.gov/exper/mesoanalysis/

10. Instability Max

Instability can be increased by increasing low level temperatures, increasing low level moisture or cooling in the middle levels of the troposphere. Warming temperatures and increasing dewpoints have been covered in previous topics. The cooling temperatures aloft is the main focus of this writing.

Cooling temperatures aloft can be described as a cold pool aloft, cold air advection aloft or a lifting mechanism that is cooling the air aloft such as dynamic lifting or evaporative cooling. These processes help weaken the cap aloft and enhance instability. Where the cap weakens the most and where the greatest cooling occurs aloft will be a location that a rising thermal from the surface will have the easiest time breaking the cap. This can be a burst point for initial thunderstorm development.

The three ingredients for thunderstorms are lifting, adequate moisture and instability. Cooling aloft will increase instability and make it easier for a rising parcel of air to break the cap and develop into a thunderstorm. Thus, not only are surface conditions important but what is occurring aloft can be very significant also in determining where convection will develop first. Lifting aloft and cooling aloft are important to examine right along with surface temperatures and dewpoints.