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