Mesoscale Convective Systems: Recent Observational and Diagnostic Studies Robert Houze

Mesoscale Convective Systems: Recent Observational and Diagnostic Studies

 Robert Houze

Department of Atmospheric SciencesUniversity of Washington

10th Conf. on Mesoscale Meteorology, Portland, OR, June 23-27 2003

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?

Houze et al. 1989, 1990

Tropical & midlatitudes“Symmetric”

Midlatitudes(later stages)“Asymmetric”

Radarreflectivity

Conv.

Strat.

Skamarock et al. 1994

Symmetric(Tropics & midlatitudes)

No Coriolis Coriolis

Asymmetric(Midlatitudes)

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?

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

1000 km

1000

km

Moncrieff & Klinker 1997

plan view

cross section

A B

A B

TOGA COARE convection in

a GCMwith ~80 kmresolution

Mean heatingin convective

line

Horizontalwind

Pandya & Durran 1996

gravity wave response to heating

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?

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

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?

Chen et al. 1996

Sizes of MCSs observed in TOGA COARE

“Super Convective Systems”(SCS)

Yuter & Houze 1998Percent of 24 km square grid covered by A/C radar echo in all the MCS

All TOGA COARE satellite/radar comparisons

Precipitation Convective Stratiform

% % %

Yuter & Houze 1998Percent of 240 km square covered by A/C radar echo in all the MCS

All TOGA COARE satellite/radar comparisons

Hypothesis:

The size of the MCS is determined by the environment’s

ability to sustain an ensemble of convection over time.

Question:

What factors control and limit sustainability?

Kingsmill & Houze 1999: TOGA COARE a/c soundings

Hei

ght (

m)

Schumacher & Houze 2003

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

Annual Average

Inference: Sustainability promoted by moist boundary layer that is

not interrupted by the diurnal cycle

Stratiform Rain Fraction

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?

Traditional view:

Cold pool dynamics

Recent studies:

Waves in environment

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

equator

40NJASMINE: May 1999

60E 100E

NOAA Ship R.H. Brown

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

ConclusionsCoriolis effect explains why midlatitude MCSs exhibit late-stage asymmetry not observed in the tropics.Layer lifting occurs in mature MCSs, possibly as a gravity wave response to the net heating in the convective region.Midlevel inflow enters stratiform regions from various directions—controlled by environment wind.Max size of MCSs related to sustainability of low-level moist inflow—get biggest systems over oceans and with LLJsMovement of an individual MCS may be in part determined by waves propagating through the environment—gravity waves, inertio-gravity waves,…

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

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)

Nakazawa 1988

INTRASEASONALENSEMBLE VARIATION

SUB-ENSEMBLE

MESOSCALE CONVECTIVE SYSTEM

JASMINE IR sequence

(courtesy P. Zuidema)

Serra & Houze 2002

TEPPS—East Pacific ITCZ

Ship radardata

Easterly wave and cold pool propagation hard to distinguish

Carbone et al. 2002

WSR88-Dradar dataover U.S.in time/

longitudeformat

Examplesof TOGACOAREMCSs

Satellite IR overlaid with A/C

radar

240 km