Characterization of tropical convective systems

Henri Laurent IRD/LTHE

Cooperation with Brazil

CTA (Centro Técnico Aeroespacial)

CPTEC (Centro de Previsião do Tempo e Estudos Climaticos)

Cooperation with Africa

AGHRYMET center, Niamey (Niger)

Motivations

Tropical rainfall is mostly of convective origin (e.g., 90% in West Africa)

Rainfall is the key parameter of climate in tropical regions • convection is essential for atmospheric circulation (vertical transport of water and energy)• impacts: food production, flooding, ressources, health,..

Why to study the convective systems ?

•vector of the tropical precipitations

• cloud clusters at mesoscale: link between general atmospheric circulation and hydrological scales

• atmospheric water cycle and energy balance

Monsoon

ocean/continent.

Chemistry, aerosols

Rainfall fields

Cloud microphysics, aerosols

Retroactions with the surface

Hurricane development

convective system studies

convective systems

meso-scale

~ 100 km

Local scale

~1-10 km

large scale

~ >1000 km

AMMA: African Monsoon Multidisciplinary Analysis

Global

Mesoscale

Regional

Sub-Meso

YearSeasonDayHour

Global Sea Surface Global Sea Surface Temperatures and Temperatures and TeleconnectionsTeleconnections

Monsoon System andMonsoon System and

Mesoscale Mesoscale Convective Convective systemssystems

ConvectiveConvective

CellsCells

Scale Interactions

Transport of Water Vapour,

Chemical Species, Aerosol etc

Major River BasinsMajor River Basins

Catchments,Catchments,

VegetationVegetation

Pools, Pools, VegetationVegetation

Easterly Waves Easterly Waves

104 km

103km

102km

101km

SOP EOP LOP

Tropical Atlantic Variability Tropical Atlantic Variability

Objective: understanding of monsoon improve climate and weather forecasts

Convective systems: Amazonia - Africa

LBA Large-Scale Biosphere AtmosphereAmazon Mesoscale Campaigns :WET-AMC jan-fev 1999DRYtoWET-AMC sep-oct 2002

(Hapex-Sahel 1992, EPSAT-Niger 1989-2000)AMMA African Monsoon Multydisciplinary Analysis Long Observing Period 2001-2010 Extended Observing Period 2005-2007 4 Special Observing Periods 2006

Warm Cold

IR10.8 Top Temperature MSG-1, 14 July 2003, 02:00-08:45 UTC, IR10.8

Tracking of convective systems

infrared satellite threshold identification image and tracking

14 h 00

14 h 30

15 h 00

15 h 30

infrared satellite threshold identification database image

and tracking

Importance of well organized systems (large/long lived) for the total convective cloud cover

Mathon and Laurent., QJRMS., 2001

Distributions of convective systems

Importance of a few major convective systems

% cloud cover

Lifetime > 24 h

zonal velocity

Over West Africa, 50 % cloud cover ~ systems > 24 h

Over Amazonia: shorter lifetime of convective systems

50 % cloud cover ~ systems > 12 h

Laurent et al., JGR, 2002

different distributions of convective systems

WETAMC/LBA jan-fev 1999 (Silva Dias et al. JGR 2002)

Short-lived CS, explosive convection Convection at minimum of total cloud cover (energy)

Machado et al., JGR., 2002

Diurnal cycle: Amazonian wet season

understanding of monsoons

need to improve convection in GCMs

comparison of CS: African monsoon – Amazonian monsoon

Niamey mesoscale network (since 1989) • Classification of rainfall events

(e.g., Amani et al. Water res. Res., 1996)

• 90 to 95 % of rainfall from mesoscale events

• Up to ~ 80 % of rainfall from major events with coherent propagation

Convective systems and precipitations Sahel

rain events (surface) / cloud events (satellite)

233 K213 K

V > 10 m/s

lifetime > 3h• OCS = 12 % of MCS

• OCS = 78 % of cloud cover (233 K)

Mathon et al., J. Applied Meteo., 2002Lebel et al., JGR, 2003

Organized convective systems (OCSs)

Satellite events = 83% of total rainfallAmong them:

- 94% OCS- 6% others

30

28

26

24

22

20

18

16

14

12

10

nom

bre

550500450400350300250200

pluie totale (mm)

90

91

9293

94

96

97

98

99 evt sol

année = evt satellite

annual rainfall number of events

Atmospheric Circulation convective systems wind regimes during WETAMC/LBA

Westerly regime (monsoon) : large systems, moderate rainfall, weak propagation

Easterly regime (break): explosive convection, shorter and more intense rainfall, steady westerly propagation

Silva Dias et al., Rickenbach et al., Laurent et al., Machado et al.,… JGR 2002

MCS propagation

wind 700 hPa

Rondônia state

Tracking of convective systems (satellite) and rain cells (radar)

Laurent et al., JGR, 2002

Relationship between high-level divergence and CS expansion

vt

A

AAE

.1

?

Divergence

Convergence

Divergence at top of convective systems

t

A

AAE

1

vt

A

AAE

.1

?

tl

Q

lQt

A

AAE v

11 .

the condensation term is dominant, particularly during the early stage of CS

divergence is important in the CS mature stage

Machado and Laurent, Monthly Weather Rev., 2004

Divergence at top of convective systems

Relationship between high-level divergence and CS expansion

Application to nowcasting

Relationship between initial expansion and lifetime

Operational tracking of convective systems

Outcome of a cooperative project Brazil-France

Operational version of the CS tracking methodology:

FORTRACC (Forecasting and Tracking Convective Cloud Clusters) running at CPTEC (Brazilian center for weather forecasting and climate studies)

www.cptec.inpe.br

www.cpetc.inpe.br

Mesoscale convective systems from satellite data

- characterization of CS life cycle

- rainfall associated with CS

- interactions with atmospheric dynamics

- retroactions CS-surface

Operational tracking of CS

- Nowcasting

- climatology

Summary