Mesoscale Convective Systems: Recent Observational and Diagnostic Studies

 Robert Houze

Department of Atmospheric SciencesUniversity of Washington

DEFINITION

Mesoscale Convective System (MCS)

A cumulonimbus cloud system that produces a contiguous precipitation area ~100 km or more in at least one direction

Questions

Why do tropical and midlatitude MCSs look different?

Does layer lifting occur in a mature MCS?

Is rear inflow really from the rear?

What controls the size of MCSs?

What controls the movement of MCSs?

Why do tropical and midlatitude MCSs look different?

Houze et al. 1989, 1990

Tropical & midlatitudes“Symmetric”

Midlatitudes(later stages)“Asymmetric”

Radarreflectivity

Conv.

Strat.

Skamarock et al. 94

Symmetric

MCV

(Tropics & midlatitudes)

No Coriolis Coriolis

Asymmetric(Midlatitudes)

Parcel vs. layer lifting in an MCS

Parcel viewpointZipser 1977

CrossoverZone

Layer viewpoint: Bryan and Fritsch 2000

“Slab” or Layer Overturning

MAUL

TOGA COARE Airborne Doppler Observations of MCSs

Convective region flights

0.5-4.5 km

Note!

Layer viewpoint: Kingsmill & Houze 1999

0ze

Layer viewpoint: Mechem, Houze, & Chen 2002

TOGA COARE23 Dec 92

200X (km)

150 250200150 250X (km)

Z (k

m)

Y (k

m)

0

2

4

6

8

10

12

14

50

100

150

1000 km

1000

km

Moncrieff & Klinker 1997

plan view

cross section

A B

A B

Is rear inflow really from the rear?

Diversity of stratiform structure: Parker & Johnson 2000PATTERNS OFEVOLUTION OF STRATIFORM PRECIPITATION IN MIDLATITUDESQUALL LINES

Kingsmill & Houze 1999 Documented airflow

shown by airborne Doppler inTOGA COARE MCSs

Stratiform region flights

0°C

0 192Horizontal Distance (km)

11

0

Hei

ght (

km)

192

11

0

Hei

ght (

km)

0

90 km

Horizontal Distance (km) 100 km

Refl.

Radial Velocity3.5 km level

JASMINE: Ship radar, Bay of Bengal, 22 May 1999

RadialVelocity

Reflectivity1.5 km level

0 192Horizontal Distance (km)

12

0

Hei

ght (

km)

192Horizontal Distance (km)

12

0

Hei

ght (

km)

0Horizontal Distance (km) 100 km

Radial Velocity3.5 km level

JASMINE: Ship radar, Bay of Bengal, 22 May 1999

Reflectivity1.5 km level

Horizontal Distance (km) 100 km

Refl.

Radial Velocity3.5 km level

JASMINE: Ship radar, Bay of Bengal, 22 May 1999

RadialVelocity

Reflectivity1.5 km level

90 km

Factors determining the size of MCSICAPE, sustainability, diurnal cycle

Chen et al. 1996

Sizes of MCSs observed in TOGA COARE

“Super Convective Systems”(SCS)

Kingsmill & Houze 1999

Examplesof TOGACOAREMCSs

Satellite IR overlaid with A/C

radar

100 km

Yuter & Houze 1998CS map Convective echo

Stratiform echo Satellite IR

% o

f grid

% o

f grid

Mea

n IR

tem

p (K

)

x (km)

y(km)

Statistics for all TOGA COARE satellite/radar comparisons

Percent of 24 km square grid covered by A/C radar echo in all the MCS

Yuter & Houze 1998

Portion of 240 km scale grid covered by convective radar echo

Statistics for all TOGA COARE satellite/radar comparisons

Yuter & Houze 1998

Schumacher & Houze 2003

TRMM Precipitation radar:% of 2.5 deg grid covered by stratiform radar echo

Annual Average

Factors determining the movement of MCS:Waves in the environment, cold pool dynamics

Nakazawa 1988

INTRASEASONALENSEMBLE VARIATION

SUB-ENSEMBLE

MESOSCALE CONVECTIVE SYSTEM

Chen, Houze,& Mapes 1996

AnalyzedIR data

3°N-10°S208°K threshold

IN TOGA COARE

MCSs moved individually

with wave much of the

time 12

13

15

14

Longitude

Tim

e (d

ay)

A/Cflights

on 12-14Dec

Serra & Houze 2002

TEPPS—East Pacific ITCZ

Ship radardata

Easterly wave and cold pool propagation hard to distinguish

equator

40NJASMINE: May 1999

60E 100E

NOAA Ship R.H. Brown

JASMINE IR sequence

(courtesy P. Zuidema)

Webster et al. 2002 IR over Bay of Bengal during JASMINE

Ship track

5 10 25 302015May 1999

Mapes et al. (2002)

West Coast of

South Am.

GravityWave

hypothesis

JASMINEMCS

JASMINEMCS

Carbone et al. 2002

WSR88-Dradar dataover U.S.in time/

longitudeformat

ConclusionsCoriolis effect explains why midlatitude MCSs exhibit asymmetry and develop MCVs as they evolve—and why tropical MCSs don’t have asymmetryParcel lifting gives way to layer lifting in mature MCSs when potentially unstable inflow air becomes moistened—circulations become mesoscale!Midlevel inflow enters stratiform regions from various directions—controlled by environment shearMax size of MCSs related to sustainability of low-level moist inflow—get biggest systems over oceans and with LLJsMCSs motion may be determined by waves propagating through the environment—gravity waves, inertio-gravity waves,…