Forecasting Mesoscale Processes Final Exam Study Guide

Forecasting Mesoscale Processes Final Exam Study Guide

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Science

University

Hard

Created by

Josie Nelson

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46 questions

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1.

FLASHCARD QUESTION

Front

Given the linear perturbation term for supercells, sketch and describe (physically and mathematically) the linear dynamical effects of a strong updraft at mid-levels in a convective cell interacting with moderate-to-strong environmental shear that is either a) unidirectional, b) curved/veering or c) curved/backing. In your sketch, be sure to label perturbation low- and high-pressure centers and the sense of the linear perturbation pressure terms. Diagnose any induced perturbation pressure gradient force and associated vertical accelerations. Discuss implications for supercell storm development and propagation (e.g., right vs. left moving storms).

Back

a) Unidirectional

(slide 57)

Result: You get a low (high) perturbation pressure induced downshear (upshear) of the updraft

--> These pressures induce upward (downward) accelerations downshear (upshear) of updraft

Implication: Provided shear + buoyancy is strong enough, PGF will be strong enough to lift parcels to LFC downshear of current updraft

--> New cell growth occurs on downshear side and is suppressed on upshear side associated with dynamical force

--> Cell propagates downshear

b) Curved/Veering

(slide 58)

Result: With veering hodograph, vertical perturbation PGF is upward directed on south side or right flank of supercell

--> Veering (CW) curvature --> preferred growth on right flank --> hence, right, deviate motion relative to typical eastward cell motion (0-6 km flow)

c) Curved/Backing

A backing (CCW) hodograph results in upward PGF on north side or left flank

Preferred growth on left flank --> hence, left deviate motion relative to typical eastward cell motion

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2.

FLASHCARD QUESTION

Front

Identify Doppler radar (reflectivity and radial velocity) signatures associated with downbursts and tornadoes within bow echo storms.

Back

Downbursts:

Look for a prominent "rear inflow notch (RIN) (region of lower reflectivity behind the main convective line, often co-located with the RIJ)

Tornadoes:

A "hook echo" is a signature of mesocyclonic rotation (hook-shaped extension of high reflectivity, usually on the right-ear flank of the storm)

Look for the "tornado vortex signature" (TVVS) (a tight area of rotating winds on the velocity product). You'll see a small area with red and green velocities wrapping together, indicating strong rotation

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3.

FLASHCARD QUESTION

Front

Identify, describe and explain the underlying dynamical processes of the three (3) primary mesovortex genesis hypotheses. Be sure to compare and contrast them. For example, identify the primary take-aways from each hypothesis and identify how these are similar and different for each.

Back

#1 -- Trapp and Weisman (2003):

(slides 56-58)

Two stages

1) "Early QLCS stage"

*Tilting of environmental vorticity by convective updrafts

Dynamical Focus: Local convective updrafts

Main Process: Tilting of ambient horizontal vorticity

Result: Weak pockets of vertical vorticity

2) "Mature QLCS stage"

*Stretching and amplification of vertical vorticity by system-scale convergence zones

Dynamical Focus: System-scale lifting/convergence

Main Process: Stretching of vertical vorticity

Result: Strong, organized mesovortices

#2 -- Atkins and St. Laurent (2009b)

(First Mechanism)

Two source regions for air parcels entering the mesovortex

1) Cold pool, either via RIJ or convective downdrafts

2) Storm-relative inflow

Step 1:

Physical Process: Horizontal vorticity exists due to vertical wind shear

Source: Environmental

Step 2:

Physical Process: Tilt that vorticity downward via updrafts and downdrafts

Source: Gust front dynamics

Step 3:

Physical Process: Vertical vorticity is generated at low levels

Source: From shear via tilting

Step 4:

Physical Process: Stretching intensifies vertical vorticity

Source: From convergence/updraft zone

#3 -- Atkins and St. Laurent (2009b)

(Second Mechanism)

Similar to Trapp and Weisman (2003)

Step 1: Baroclinic generation -- Gust front produces horizontal vorticity due to density and pressure gradients

Step 2: Upward tilting -- Convective updrafts tilt this vorticity into the vertical

Step 3: Stretching -- Convergence along gust front intensifies the vertical spin into mesovortices

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4.

FLASHCARD QUESTION

Front

Explain why we cannot necessarily reject the role of shearing instability in the formation of mesovortices.

Back

Wheatley and Trapp (2008) do cite horizontal shearing instability as a potential source of meso-vortex generation in their cool-season QLCS case!

This highlights that in certain environments, especially where baroclinic or shear-driven processes dominate, instability in the low-level shear profile may play a significant role in generating vertical vorticity.

5.

FLASHCARD QUESTION

Front

Describe the individual and potentially combined role of the descending rear in-flow jet (RIJ) & mesovortices in the production of damaging wind in bow echoes within a QLCS. What is a key feature in the strongest damaging winds associated with bow echoes and mesovortices?

Back

Important:

Bow echoes are formed through the intensification of a RIJ, which is driven by a pressure gradient induced by buoyancy due to the upshear-tilted updraft.

Meso-vortices create intense horizontal pressure gradients due to (ζ’)2 relationship (the non-linear p' term!)

Result:

Meso-vortices without a RIJ tended to be much weaker or non-damaging than those associated with a RIJ

*The most damaging winds with bow echoes/meso-vortices tend to occur where the effects of the descending RIJ and the meso-vortex overlap

6.

FLASHCARD QUESTION

Front

List a general (simple) definition of a derecho and explain the key elements of one formal definition of a derecho (note: it's not necessary to list all of the details but emphasize the key ideas).

Back

A derecho is a widespread and long-lived, violent convectively induced windstorm that is associated with a fast-moving band of severe thunderstorms usually taking the form of a bow echo

Key Elements:

Widespread wind damage

Long-lived event

Violent wind speeds

Convectively induced

Fast-moving band of storms

Bow echo structure

7.

FLASHCARD QUESTION

Front

Describe in words and graphically the warm and cool season climatology of derecho frequency in the CONUS.

Back

(starts on slide 71)

Warm Season:

Most common type of derecho in the CONUS

High instability

Moderate shear

Progressive, localized structure

Triggered by surface heating, cold pools

Cool Season:

Low to moderate instability

Strong shear

Serial structure, large-scale squall lines

Triggered by synoptic systems (fronts, jet streaks)

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