Forecasting Mesoscale Processes Final Exam Study Guide

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

Front

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

Back

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

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.

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

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

Front

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

Back

Front

Describe the basic (e.g., horizontal radar) structure of a progressive and serial derecho.

Back

Progressive Derecho:

Characterized by a short curved squall line oriented nearly perpendicular to the mean wind direction with a bulge in the general direction of the mean flow.

Serial Derecho:

Consists of an extensive squall line which is oriented such that the angle between the mean wind direction and the squall line axis is small. A series of LEWPs and bow echoes move along the line.

Front

Identify and describe the key synoptic elements of the weather setup supportive of the 3 synoptically-evident severe weather outbreaks types according to the Johns (1993) paper, including supercell outbreaks, progressive derechos and serial derechos. Be sure to compare and contrast the three types in your response.

Back

Supercell Outbreaks:

Spring season

Deep, strong surface low

Strong trough, negative tilt

High CAPE

Very strong shear

Discrete supercells

Tornadoes, hail

Progressive Derecho:

Late spring-summer season

Weak to moderate surface low

Zonal to weak trough

Very high CAPE

Moderate to strong shear

Bow echoes

Damaging winds

Serial Derecho:

Fall-early spring season

Surface low: Strong, extratropical cyclone

Broad trough, negative tilt

Low to moderate CAPE

Very strong shear

Squall lines

Damaging winds, embedded tornadoes

Front

Identify and describe some key environmental factors (e.g., shear, instability like CAPE/MUCAPE, mid-level (4-6 km) system relative winds [SRW], low-level system-relative inflow (0-2 km), 0-6 km mean wind and DCAPE) associated with bow echoes that create intense winds and derechos (including both weak forcing [WF] and strong forcing [SF] types).

Back

WF Bow Echo/Derecho:

High CAPE

MUCAPE -- Important in elevated environments

Moderate to strong 0-6 km shear

Moderate to strong mid-level shear (4-6 km SRW), aiding RIJ

0-2 km SR Inflow -- Moist inflow

High DCAPE

0-6 km Mean Wind -- Zonal/NW flow; supports fast forward motion

SF Bow Echo/Derecho:

Moderate CAPE

MUCAPE -- Often less critical

Strong to very strong 0-6 km shear

Moderate to strong mid-level shear (4-6 km SRW), aiding RIJ

0-2 km SR Inflow -- Strong inflow with frontal lifting

Moderate DCAPE; downward momentum transport from aloft

0-6 km Mean Wind -- SW flow; synoptic-scale organization