Flashcards in Severe Weather Deck (49)
List the types of severe weather soundings discussed in class? (Hint 4)
Loaded Gun, Inverted V, Wet Microburst, Elevated Convection.
Discuss the characteristics of the Inverted V sounding (Type IV). IE What does the sounding look like and how do atmospheric conditions change through the sounding?
· Named Inverted-V since the dewpoint depression decreases significantly with height
· Sounding has dry air (low RH) in lower troposphere with nearly saturated air (high RH) in middle troposphere
This sounding type is characterized by a relatively dry, well-mixed lower layer, with RH increasing with height, giving the appearance of an "inverted V."
What types of weather are associated with the Inverted V (Type IV) sounding and why?
· Convection tends to be high based since Convective Condensation Level is at a high elevation
· Most common severe weather: Strong winds > 58 mph; this is due to negative buoyancy of evaporationally cooled air aloft that causes it to accelerate toward the surface
· Gust fronts from inverted-V storms can have a large temperature gradient from one side to the other due to evaporative cooling
· Hail and tornadoes are not common due to the dry boundary layer, high cloud base and unorganized wind shear
It is typically associated with high based thunderstorms with vigorous, evaporatively driven downdrafts and microbursts.
Where is the Inverted V (Type IV) sounding most common?
· Most common in interior Western U.S. (especially interior Southwest U.S.)
It is commonly observed during the summer season over the High Plains and the mountain and plateau regions of the Western U.S.
Discuss the characteristics of the Loaded Gun (Type I) sounding. IE What does the sounding look like and how do atmospheric conditions change through the sounding?
· Severe weather sounding (large CAPE, very unstable LI)
· Large hydrolapse in mid-levels (mT air in boundary layer and capped by cT air)
· There must be an inversion above mT air
This sounding is characterized by a moist, fairly well-mixed layer of at least 100-150 hPa depth, separated from a dry layer above by a capping inversion. Lapse rates above the cap are typically nearly dry adiabatic.
Where is the Loaded Gun (Type I) sounding most common?
· Great Plains during the spring severe weather season.
What types of weather are associated with the Loaded Gun (Type I) sounding?
· Most common severe weather: Large hail, tornadoes, convective wind gusts of 58mph or greater
· If speed /directional wind shear and strong low-level jet are present on sounding, severe weather chances are enhanced
Discuss the characteristics of the Wet Microburst sounding. IE What does the sounding look like and how do atmospheric conditions change through the sounding?
Mid-level dry air
· Similar to goal post sounding but with more moisture (higher precipitable water in sounding)
Where is the Wet Microburst sounding most common?
· Sounding most commonly found east of Rockies
What types of weather are associated with the Wet Microburst sounding?
· The dry air aloft will entrain into the downdraft and cause evaporative cooling. This increases the negative buoyancy and can result in microbursts and macrobursts
· If supercells develop they are most likely to be high precipitation supercells
· Most common severe weather: winds > 58mph, small hail near the 3/4" threshold, tornadoes possible (depends on low level shear and CAPE)
Discuss the characteristics of the Type II sounding? IE What does the sounding look like and how do atmospheric conditions change through the sounding?
It is characterized by a deep moist, conditionally unstable layer with relative humidities of >60% from the surface up to 7 km AGL.
Where is the two II sounding most common?
This sounding type is common in the tropics, but observed at times over much of the U.S. east of the Rocky Mountains, especially in the summertime over the Gulf Coast and Southeast.
What types of weather are associated with a type II sounding and why?
There is no capping stable layer so that widespread convection is typically observed.
What is the type III sounding and where is most common?
This sounding type is similar to Type 2, except 10-15°C cooler. It is commonly observed near cold-core, upper-level troughs and cyclones.
What are hodographs used for?
The primary purpose of a hodograph is to reveal vertical wind shear.
Because the storm moves through its environment, the wind it experiences is often very different from the ground-relative winds. Storm-relative winds can also be calculated on a hodograph
What is plotted on a hodograph and how is it plotted?
The hodograph is based on wind vectors, rather than wind barbs. To create a hodograph, wind vectors are plotted on a polar coordinate chart. Then their endpoints are connected.
Vertical wind shear is a description of how the velocity of the horizontal wind changes with height. We determine the vertical wind shear by taking the vector difference between the horizontal wind at two levels.
The total magnitude of vertical wind shear over a particular depth is an important factor in anticipating possible storm structure and evolution. Estimating total vertical wind shear is done by combining the lengths of all the shear vectors over a particular depth (the net length of the hodograph)
You can determine the direction of the mean wind shear vector (but not the magnitude) by drawing a line from the point that plots the surface wind to the point plotting the 6-km (20-kft)wind
Calculating the mean wind shear vector is simply a matter of averaging the x and y components of each of the single layer wind shear vectors
What are the possible hodograph shapes and how do those shapes relate to storm morphology?
Strong straight shear tends to produce a pair of splitting mirror-image supercells.
Wind shear profiles with at least this much clockwise-curvature, common in the Great Plains, are responsible for producing dominant right-moving supercells.
Occasionally, the environmental shear creates a counterclockwise curving hodograph, which favor dominant, left-moving supercells.
Describe how lightning forms.
1. First, we need a charged thunderstorm. While there is still considerable debate as to how a thunderstorm becomes charged, a popular theory is that collisions between graupel and ice crystals results in a transfer of negative charges to the graupel and positive charges to the smaller ice crystal. This may have to do with the difference in temperature between the warmer graupel. The thunderstorm becomes charged because the updrafts carries the lighter positively charged ice crystals to the upper portions of the storm and the heavier negatively charged graupel particles to the lower portions of the storm - thus creating a dipole.
2. Second, the negatively charged lower portions of the storm induces a positive charge at the earth's surface. This creates an electrical potential between the bottom of the storm and the surface. When the electric fields exceed that of the breakdown voltage, a stream of electrons flow from the cloud base to the ground in steps of 50 to 100 m. These descending finger like paths are known as the stepped leaders.
The step leaders are very faint and are essentially invisible to the human eye. The stepped leader is important because it creates an ionized channel that will allow for the flow of charge during the remainder of the lightning stroke
3. When the stepped leader gets near the ground (~100 m or so) positive charges moves from the ground up toward the stepped leader --these are called Connecting Leaders. The connecting leaders may come from almost any pointed object on the ground: trees, antennas, grass, flag poles, telephone poles, people, really tall towers, etc.
4. Once the connecting leader touches the stepped leader, negative charged particles flow from the cloud to the ground and positive charges flow from the ground to the cloud. These charges flow along the ionized air channel formed by the stepped leader.
Describe lightning climatology.
In the US, lightning most frequently occurs in Florida and along the Gulf coast. Across the world, the cast majority of lightning occurs over land with a high concentration near the equator in north Africa, northern south America, central America and southeast Asia.
Describe Lightning Safety.
If outdoors, avoid water, high ground and open spaces. If indoors, avoid water, stay away from doors and windows, do not use a telephone. If lighting is in the area go indoors for at least 30 minutes after the last observed lightning or thunder.
Describe the characteristics and severity of MCC's
Describe the climatology of MCC's.
Average of 35 MCCs per year in the US.
Monthly Totals- 86 % occur during May , June, July, August
--The Great Plains experiences the most MCCs during the warm season.
--The Great Plains receives upwards of 18 percent of it's annual warm season precipitation from MCC's.
--The southeast experiences the most MCCs during the Spring season (primarily April ). On average, the Southeast receives 0 to 4 %.
--However, MCC activity is highly variable from year to year and warm-season precipitation totals are highly dependent upon MCC precipitation across much of the southeastern agricultural centers (e.g., Mississippi River floodplain)
What are the key features associated with Squall Lines and Bow Echoes? (Hint 10)
Cold Pool which forces lift along a linear axis (gust front).
Updraft and updraft towers which would be the CB's which form along the leading edge.
Downdraft which exists behind the gust front.
Stratiform Precipitation Area which is a large area of continuous rain.
Mid level low pressure which develops between the upflow and low level cold pool.
Rear Inflow Jet which forms in response to the mid level low. It is most common with trailing startiform systems. The strength depends on the strength of the mid level low which is regulated by positive and negative bouyancies between updraft and downdrafts generated by temperature differences between the updraft and cold pool. Some are forced to the surface by convergence with the updraft. Non-descending RIJ's tend to promote a more longlived MCS.
MCV is a region of weak cyclonic rotation generated by pressure falls from widespread latent heat release.
Wake low develops behind the cold pool and MCS do to latent heat release in the stratiform region overhead and the descending rear inflow.
Line end vortices and book end vortices from in the mature stage - this is where the MCV may develop.
Rear Inflow Notch
How do squall lines and bow echos develop?
In strongly sheared environments, the evolution of a squall line begins with an initially narrow line of strong convective cells, with light precipitation often extending downshear of the convective cores. Some of the cells may be supercells. As the system matures, the narrow line of strong cells persists, with bow-shaped segments of cells also beginning to develop. Lighter precipitation begins to extend somewhat rearward (upshear), but to a lesser extent than in weaker shears. In the dissipating stages, the leading cells weaken and become more scattered and the region of lighter precipitation extends even farther rearward (upshear). Squall lines tend to propagate in the direction of the 0-3 Km mean vertical wind shear vector at the speed of the cold pool.
As a squall line matures, it typically develops rotation at each end. The development of these line-end vortices is most apparent and significant for relatively short lines (less than 200 km, 110 n mi, in length). As we’ll discuss later, line-end vortices in close proximity to each other in bow echoes are given the special name bookend vortices. This development is schematically presented here for a 150 kilometer (80 n mi) long squall line evolving in an environment characterized by weak-to-moderate low-level shear.
Line-end vortices usually develop during the early to mature stages or between two to four hours into the lifetime of the convective system, just behind the zone of most active convection. When line-end vortices first develop, the cyclonic and anticyclonic vortices are often of nearly equal strength, promoting a symmetric, bowed shape in the precipitation field. However, if the vortices last for more than two to three hours (i.e., beyond four to seven hours into the lifetime of the system), the northern, cyclonic vortex tends to become stronger and larger than the southern, anticyclonic vortex. As this occurs, the convective system becomes asymmetric, with most of the stratiform precipitation region found behind the northern end of the system, and the strongest leading-line convective cells found near the southern end. In weak-to-moderate shear environments, the northern line-end vortex is typically observed to move rearward with time.
Bows develop as the rear inflow jet strengthens and often indicate very strong winds at the surface.
The optimal condition for the generation of new convective cells is when there is a balance between the horizontal vorticity produced by the cold pool and the opposite horizontal vorticity associated with the ambient low-level vertical wind shear on the downshear flank of the system.
Squall Line and Bow Echo Severity.
Name some Derecho types
- Produced by multiple bow echoes which are embedded in a squall line
- several hundred miles in length
- associated with a strong, migrating low pressure system
- found in the warm sector of a mid-level cyclone
- tend to occur with a well-defined 500mb trough upstream of a mid-latitude cyclone
- corresponds largely to cool season events (Sept - Apr)
- often found in the left exit region of a jet sreak
- 24% of events were serial
- These are strongly forced events.
-Characterized by a single bow-shaped system which propagates north of and parallel to an east-west boundary
-an upstream ridge is usually present for these events
-corresponds largely to warm season events (May-Aug)
-often found in or near the right entrance region of a jet streak
-76% were progressive however, this may have been too high
-These are weakly forced events.
combination of serial and progressive characteristics
may exhibit a zonal pattern or combinations of trough/ridge patterns
have no particular seasonal distribution
Explain Derecho Climatology
the pattern of derecho occurrence is a distinct one
the greatest incidence tends to follow the mean position of the jet stream which is the demarcation line between warm and cold air at the surface, ie a frontal boundary
How do derechos form?
Defined as a widespread and long-lived windstorm associated with a band of rapidly moving showers and thunderstorms
Squall lines - derechos evolve from squall lines (linear MCS)
Bow Echoes - the radar signature most commonly associated with the derecho is the bow echo, indicated by a bulge in the squall line, bulge indicated the presence of faster moving winds, produced by the descent of the RIJ to the surface, the apex of the bow represents the area of strongest winds, the region where the most significant wind damage will occur.
Tend to develop with westerly or northwesterly flow aloft
form where 850mb dew points are highest, this produces the elevated convection which can contribute to downbursts
as they move, they migrate toward areas with high CAPE (often south of the boundary)
they will continue as long as sufficient CAPE is available and the shear is strong
There is lower/mid-level dry area which fosters evaporative cooling and further downward motion
additionally, there may be strong mid-level winds in place which can augment the RIJ
Tend to develop in environments where the LI is less than -6
81% produced tornadoes, the majority (92%) were EF2 or less
Strongly forced derechos (Serial) had relatively cool and dry soundings with high shear and low CAPE (cold season)
Weakly forced derechos (Progressive) had warmer and moister soundings with low shear and high CAPE (warm season)
hybrids had mixed charachteristics
What does the radar signature Hook Echo indicate?
It is a signature produced by precipitation held aloft that wraps around the mid-level mesocyclone. Since the mesocyclone has counterclockwise winds, the reflectivity signature of a hook echo will have a cyclonically shaped hook.