Horizontal Vorticity in the Storm Environment

Question 1

Updraft Jet

Answer 1

Caused if the magnitude of the positive horizontal vorticity associated with the ambient shear is the same as the magnitude of the negative horizontal vorticity produced along the leading edge of cold air, then the air approaching the cold pool will tend to rise as a vertically-oriented jet.

Question 2

Where is the triggering of convective cells favored?

Answer 2

Along the downshear portion of a spreading cold pool.

Question 3

When will the strongest, most long-lived convective systems be produced?

Answer 3

When there is strong low-level vertical wind shear to balance the cold-pool generated circulation.

Question 4

What is one way we can evaluate environmental wind shear?

Answer 4

By plotting a hodograph.

Question 5

Hodograph

Answer 5

• depicts the vertical distribution of the horizontal wind.
• points are plotted as a function of wind direction and speed
• each point represents the horizontal wind at a specified elevation
• wind vectors for each level are produced by connecting the hodograph’s central origin point with the plotted point
• layer sear vectors are produced by connecting two sequential points.

Question 6

Gust front

Answer 6

the leading edge of the rain-cooled air in the planetary boundary layer.

Question 7

Gust front formation

Answer 7

The gust front forms along leading edges of mesoscale domes of rain-cooled air (i.e. surface-layer cold pools) that result from the amalgamation of evaporatively-cooled downdrafts from individual thunderstorms cells.

Question 8

Meso-highs

Answer 8

Thunderstorm cold pools are associated with meso-highs (i.e. bubble highs) and may actually be crudely analyzed on a synoptic sfc chart.

Question 9

The gust front is a density discontinuity

Answer 9

• cooler, denser surface air flows outward from the meso-high.
• high ThetaE air is uplifted above the outward-flowing cold pool
• New convection may form in associated with this uplift if the lifted parcels can reach their LFC.

Question 10

Gust front passage

Answer 10

Passage is characterized by:

• wind shift and abrupt increase in wind speed.
• abrupt temperature drop (most of the time)
• sharp pressure rise
• arc cloud (if boundary layer is moist)
• strong vertical wind shear

Question 11

Physical Mechanisms Controlling Storm Structure

Answer 11

• Buoyancy Processes
• Gust Front Processes
• Dynamic Processes

Question 12

Physical Mechanisms Controlling Storm Structure: Buoyancy Processes

Answer 12

• Lapse Rate (CAPE)

- Moisture Stratification

Question 13

Physical Mechanisms Controlling Storm Structure: Gust Front Processes

Answer 13

• Strength of cold pool

- strength of low-level vertical wind shear

Question 14

Physical Mechanisms Controlling Storm Structure: Dynamic Processes

Answer 14

• strength of the 4-6 km AGL vertical wind shear
• development of rotational (helical) updrafts and associated favorable vertical pressure gradients on the updraft flank.

Question 15

How does a thunderstorm develop a rotating updraft?

Answer 15

• vertical wind shear produces horizontal vorticity tubes
• these tubes are incorporated into a thunderstorm updraft and tilted vertically
• the updraft begins to rotate in the same sense as the vertical vorticity (cyclonic or anticyclonic)

Question 16

Storm splitting

Answer 16

• updraft intensity is maximized within rotating cores
• precipitation loading in updraft weakness between rotating cores produces a downdraft
• thus, a downdraft separates the cyclonic updraft from the anticyclonic updraft
• storm splitting occurs as each updraft sustains a separate storm cell

Question 17

Cyclonically-rotating updraft

Answer 17

• Bernoulli effects produce low (high) pressure on the right (left) side of the cyclonic updraft in the mid-troposphere
• cyclonic storm moves to the right of the mean vector wind
• SR supercells are favored within veering vertical wind profiles

Question 18

Anticyclonically-rotating updraft

Answer 18

• Bernoulli effects produce low (high) pressure on the left (right) side of the anticyclonic updraft in the mid-troposphere.
• Anticyclonic storm moves to the left of mean vector wind (SL supercell)

Question 19

No-shear ordinary cell

Answer 19

• gust front rapidly outruns the storm

- storm is left totally entrenched over boundary layer cold pool

Question 20

Moderate-shear multicell

Answer 20

-storm will move downshear at roughly the same speed as the MVW between the surface and 6 km altitude AGL.
-New cell growth is enhanced along the downshear portion of the gust front.
-> increases relative flow into the newly-developing
cells
-> increases the length of time cells stay in the vicinity
of strong low-level convergence zone and
associated lifting near gust front.

Question 21

Strong-Shear Supercell

Answer 21

• In strongly-sheared environments, the interaction between the updraft with the sheared environmental flow becomes an important contributor to the organization and sustenance of the convection.
• rotation develops on the flank of the updraft due to vertical tilting of pre-existing horizontal vorticity within the sheared flow.
• if the vertical wind shear extends through the middle-levels of the storm (sfc-6 km AGL), the rotation dynamically induces a negative (low) pressure anomaly (NPA) in the middle troposphere.

Question 22

Uni-directional shear

Answer 22

• favors storm splitting as NPA (low pressure) forms at mid-levels within rotating updrafts on both right (cyclonic) and left (anticyclonic) side of original storm.
• split cells are nearly identical, but rotate in opposite directions

Question 23

Curved Shear

Answer 23

• Veering shear vectors winds are climatologically favored in the environment.
• strongly veering winds favor strongly right-moving cells with cyclonic rotation. Left-moving anticyclonic cells are not favored.