title goes here for lessonfebruary 2002 mesoscale convective systems book sources: markowski and...

Title goes here for lesson February 2002

Mesoscale Convective Systems

book sources:Markowski and Richardson (2009), chapter 10

Houze 1993: Cloud Dynamics, chapter 9

MCS comet modules:MCSs: squall lines and bow echoes (unnarrated, 1999)

Severe convection II: MCSs (narrated, 2002)Tropical MCSs (unnarrated, 2013)

http://www.meted.ucar.edu/convectn/mcs/

http://www.meted.ucar.edu/mesoprim/severe2/

http://www.meted.ucar.edu/tropical/synoptic/trop_MCS/index.htm


Overview

Introduction to MCSs Squall lines

definition types synoptic setting importance of shear conceptual model of SL with stratiform region pressure perturbations origin of rear inflow jet

Bow echoes Tropical squall lines Mesoscale Convective Complexes

300 km


Introduction


Definition Mesoscale convective systems (MCSs) refer

to all organized convective systems larger than ~50 km (long axis)

Some classic convective system types include: squall lines bow echoes line echo wave patterns

mesoscale convective complexes (MCCs) are large MCSs (Maddox 1980) definition is based on IR imagery: <241 K cloud shield at least 100,000 km2

<221 K cloud shield at least 50,000 km2

Eccentricity >0.7 Must last at least 6 hrs


Where and when

MCSs occur worldwide mainly in the warm season

MCC global distribution:

Laing and Fritsch (1993)

TRMM data


Compare this to the distribution of hail and tornadoes …

Conclusion: in mid-latitudes the distribution is similar. MCSs and MCCs are surprisingly common in the tropics.


US tornado tracks


MCS examples

warm ocean MCS Dryline Squall line in Texas


MCC examples

30 April 2004, 1:32 UTC


MCS morphology

linear symmetric asymmetric

amorphous


MCS morphology linear

TS LS PS

Fig. 9.11, adapted from Parker and Johnson (2000)

TS

PS


Upscale growth of convection towards a squall line is accelerated when the deep layer (surface to cloud top) mean shear is close to aligned to the boundary along which cells first initiate.

MCS morphology


MCS morphology

squall lines can be continuous or cellular


Synoptic patterns

Favorable conditions conducive to severe MCSs and MCCs often occur with identifiable synoptic patterns

Synoptic forcing may be quite weak.

Temperature (C, dashed) and dewpoint (C, solid)


Squall Lines


Squall line definition A squall line is any line of

convective cells. 100-1000 km long

no strict size definition Usually trailed by stratiform

precip (TS)

Squall line animation 1 Squall line animation 2 (mov file)

http://severewx.atmos.uiuc.edu/17/online.17.1.html

http://severewx.atmos.uiuc.edu/17/172-parallel-squall/squall_line.mov


Initial organization Squall lines may either be

triggered as a line along some boundary (e.g. dryline)

or organize into a line from an amorphous cluster of cells

Thus the majority of cases developed from pre-existing convergence lines

pre-existingconvergence line

pre-existingconvergence line


Life cycle

Little is known about the initial and dissipating stages.

Mature structure is well known. It can be steady-state (long-lived MCS) or a brief transition

(Leary and Houze 1979)


Environmental factors Both CIN and CAPE impact MCS structure and evolution

low-levelshear

deepshear


Importance of shear For a given CAPE, the strength and longevity of an MCS

increases with increasing depth and strength of the low-level shear S0-3(between 0-3 km AGL)

For midlatitude environments, |S0-3| : weak |S0-3| <10 m/s moderate |S0-3| 10-18 m/s strong |S0-3| >18 m/s

A balance is needed between baroclinically generated hor. vorticity and ambient low-level hor. vorticity (RKW theory) Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A Theory for

Strong, Long-Lived Squall Lines. J. Atmos. Sci., 45, 463–485.

To continue initiating convection, the cold pool needs to lift air above the LFC. The higher the LFC (drier air), the deeper the layer in which the RKW condition needs to be assessed deeper ambient shear deeper cold pool


Which low-level shear matters?

It is the component of low-levelvertical wind shear perpendicular to the line that is most critical for controlling squall line structure & evolution

Note that the initial line orientation is often due to convergence lines (fine lines) independent of LL shear


Impact of shear on longevity Short-lived squall lines tend to

form under weak LL shear

Long-lived squall lines have at least 10 m/s of line-normal wind shear in the lowest 3 km Severe squall lines have more

CAPE and/or shear than non-severe ones


Classic evolution with weak shear

The characteristic squall line life cycle is to evolve from a narrow band of intense convective cells to a broader, weaker system over time


Classic evolution with stronger shear

Stronger shear environments produce stronger long-lived lines composed of strong leading line convective cells and even bow echoes.


Later evolution in moderate-to-strong shear

Several bow echoes may formAsymmetry (if present) explained by the Coriolis force


Cold pool motion and shear

Squall lines often have their ‘own’, intrinsic propagation speed, which may be different from the deep-layer mean wind

This speed tends to be controlled by the speed of the system cold pool New cells are constantly triggered along its leading edge Speed is close to cold pool density current speed

In mid-latitudes an "average" gust front speed is ~15 m/s. Longevity of a squall line depends on its ability to “keep up” with

the gust front LL shear Du needs to match gust front speed c.

v

vghU


RKW theory:ambient shear and MCS longevity


low-level wind shear & updraft slope

note vertical aspect ratioheight is exaggerated by ~5


wind shear & updraft slope

heig

ht

heig

ht

Buoyancy tilting (by shear) produces a larger downward BPPGA, and thus a weaker net upward acceleration


RKW’s optimal state implies that LL shear Du = c

or Du = c for an erect updraft

Density current plowing into westerly shear (RKW’s optimal state)

𝑑𝜂𝑑𝑡

=−𝜕𝐵𝜕 𝑥

The integration of

along the control volume gives:

The vertical vorticity flux, integrated along the top of the control box, cancels only under RKW’s optimal state

𝑢𝑅 , 0❑

❑2 = (Δ𝑢)2=2∫

0

𝐻

𝐵𝐿𝑑𝑧=𝑐2

Judiciously choose control volume such that u=0 at z=d . Then

S


LL shear Du vs. c, and storm evolution

Squall lines may move thru the optimal state over time. c tends to increase as cold pool deepens / strengthens


The RKW theory for squall line maintenance is not without controversy …

Fig. 9.20: the cold pool strength usually exceeds the low level (0-3 km) shear.


deep shear may matter as well

cold


Squall line motion

squall lines sometimes re-orient normal to the low-level shear


Conceptual model of the mature stage of a symmetric squall line

(textbook Fig 9.7, adapted from Houze et al. 1989)



(textbook Fig 9.7, adapted from Houze et al. 1989)Fig. 9.8



LL conv

UL divFig. 9.3model output(Rotunno et al. 1988)


LL shear

deep shear

deep shearH Lupdraft wSpD

2'2


Trailing stratiform region

sustained by a mesoscale updraft above the freezing level this updraft probably is buoyancy-driven.

buoyancy is due to latent heating (condensation/freezing)


Surface pressure fields

+3 mb

-3 mb

interpret mesohigh and wake low hydrostatically

weaker shear

stronger shear


Vertical cross section of reflectivity and pressure perturbations

mature stage pressure perturbations

low under convective towers, near LFC, dominates

due to buoyancy source


Observed pressure perturbations

low near LFC due to buoyancy source

(base of CAPE layer) hydrostatic high below

plus stagnation point high ahead of gust front

S

E W

LeMone and Tarleton 1986, JtechLeMone et al 1987, Mon Wea Rev


The Rear-Inflow Jet (RIJ)

down-gradientsubsident, evaporatively-cooledmay join gust front feeder flow


Buoyancy distribution, the trailing stratiform region, and the rear-inflow jet

The convective line is positively buoyant from the LFC to the LNB

This B+ spreads into the TSR

Decreasing B+ towards the rear implies a decreasing B-induced low below the B+

This implies a horizontal PGF towards the convective line

This drives the rear inflow jet (RIJ)


The RIJ is stronger in more unstable environments

More CAPE implies more B in convective line stronger B-induced low at its base stronger PGF stronger RIJ possible straight-line wind damage & bow-echo formation

Updraft should be more erect!!

Fig. 9.22


Bow Echoes


Bow echo definition

Bow echoes are relatively small (20-120 km long), bow-shaped systems of convective cells noted for producing long swaths of damaging surface winds.

bow echoes


Bow echo environments

deep shear:

Bow echo and supercell

shallow shear:

bow echo only

red line: downburst winds


evolution of a bow echo

S

towards a balance between ambient, RIJ, and cold pool vorticities


Bow echo evolution


Reasons for bow echoes intensity

Rear-inflow jet connects to gust front feeder flow


Bow echoes and supercells Bow echoes represent

mesoscale organization, supercells are individual storms.

Both require strong shear.

Supercells within squall lines tend to become bow echoes, but cells at the ends of lines can remain super-cellular for long periods of time.


mature bow echo: RIJ

Fig. 9.25


mature bow echo: hook echo

Fig. 9.26 Fig. 9.26: simulated vortex lines around the RIJ

RIJ

3 km DD winds(NOAA + NRL P-3)

BAMEX – Bow Echo and MCV experiment


Rear-inflow notch

The reflectivity notch, if sustained or growing, is an indication of a strong RIJ and possible damaging straight-line surface winds.


The MARC signature

MARC: Mid-altitude radial convergence(applied to the radial that is normal to the squall line)

rear-inflownotch


MARC characteristics:

MARCs are locally enhanced convergent areas (velocity differentials), embedded within a larger region of convergence extending from 60 to 120 km in length

width: 2-6 km

height: mid-levels (~5 km AGL)

depth: 3-9 km

magnitude: ~25-30 m/s


Derechoes: definition

If the cumulative impact of the severe wind from one or more bow echoes covers a wide enough and long enough path, the event is referred to as a derecho.

To be classified as a derecho, a single convective system must produce wind damage or

gusts greater than 26 m/s within a concentrated area with a major axis length of at least 400 km.

The severe wind reports must exhibit a chronological progression and there must be at least 3 reports of F1 damage and/or convective wind gusts of 33 m/s (65 kts) or greater separated by at least 64 km.

Additionally, no more than 3 hours can elapse between successive wind damage or gust events.


Derechoes cont’d

A progressive derecho is a single bow-shaped system that typically propagates north of and parallel to a weak stationary boundary

Serial derechos are most commonly a series of bow-echoes along a squall line (usually located within the warm sector of a cyclone)


MCCs


MCC Definition

A mesoscale convective complex is defined via IR satellite imagery (Maddox 1980).

<241 K (-32°C) cloud shield at least 100,000 km2

<221 K (-52°C) cloud shield at least 50,000 km2

Eccentricity >0.7 Must last at least 6 hrs


Mesoscale convective vortices form in some MCSs


MCCs may form mid-to-UL cyclonic vortices

1. These vortices may be large relative to LR (Rossby radius of deformation)

2. Mechanism the same as that for tropical cyclogenesis

3. Can be explained in terms of PV creation due to latent heating

7 July 198211:30 UTC

7 July 198216:30 UTC

source: Fritsch et al 1994


1. Squall line bows out, stratiform region increases in size.

2. Mesoscale Cyclonic Vortex (MCV) may develop (due to f)

Odten long-lived squall lines in moderate-to-strong shear eventually become asymmetric, with an MCV


MCV creation due to diabatic PV generation define IPV

consider diabatic heating and cooling profiles in sustained deep convection

infer isentrope distortions and PV anomalies

p

fgP )(


MCV creation due to diabatic PV generation define IPV

consider diabatic heating and cooling profiles in sustained mesoscale deep convection

infer isentrope distortions and PV anomalies

source: Fritsch et al 1994

p

fgP )(

SW NE


Tropical squall lines


Tropical squall lines

Overall, squall lines in the tropics are structurally similar to midlatitude squall lines. Notable differences include:

develop in lower shear, lower LFC, less-CAPE environments taller convective cells system cold pools are generally weaker less of a tendency toward asymmetric evolution AND most tropical squall lines move from east to west


Tropical Squall Lines

example: Arizona monsoon


Typical evolution of tropical squall lines

Examine divergence, B and pB at three stages: Convective stage Intermediary stage Stratiform stage

Houze’s book

con div


Summary MCS structure and evolution depend on the characteristics of

the environmental buoyancy and shear, as well as the details of the initial forcing mechanism.

The strength and the degree of organization of most MCSs increases with increasing low-level vertical wind shear values.

MCS evolution and motion is heavily controlled by the cold pool. MCS longevity depends on the interaction between cold pool (c)

and low-level vertical wind shear (Du). Deeper shear may be important as well.

The Coriolis effect significantly impacts long-lived MCSs (> 4 hrs).


References

Fritsch, J. M., Murphy, J. D., Kain, J. S.. 1994: Warm Core Vortex Amplification over Land. J. Atmos. Sci., 51, 1780–1807.

Hilgendorf, E.R. and R.H. Johnson, 1998: A study of the evolution of mesoscale convective systems using WSR-88D data. Wea. Forecasting, 13, 437-452.

Houze, R.A., 1977: Structure and Dynamics of a Tropical Squall-Line System. Mon. Wea. Rev., 105, 1540-1567.

Johns, R.H., 1993: Meteorological conditions associated with bow echo development in convective storms. Wea. Forecasting, 8, 294-299.

Johnson, R.H., and P.J. Hamilton, 1988: The relationship of surface pressure features to the precipitation and airflow structure of an intense midlatitude squall line. Mon. Wea. Rev., 116, 1444-1472.

Maddox, R. A., 1983: Large-Scale Meteorological Conditions Associated with Midlatitude, Mesoscale Convective Complexes. Mon. Wea. Rev., 111, 1475-1493.

Przybylinski, R.W., 1995: The bow echo: Observations, numerical simulations, and severe weather detection methods. Wea. Forecasting, 10, 203-218.

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