Joanna Futyan and Tony DelGenio

GIST 25, Exeter, 24th October 2006

The Evolution of Convective Systems over Africa and the

Tropical Atlantic

Outline

• Background & Motivation

• Identifying and tracking convective systems

• Compositing by lifecycle stage– Definition of lifecycle stages– Results from geostationary satellite data– Matched TRMM PR and LIS data

• Differences in evolution for land and ocean systems

• Radiative forcing of large-deep systems

• Summary + conclusions

Motivation

• Deep convective systems likely to play important role in determining cloud feedback– but nature of changes open to debate– and they are poorly represented in GCMs

• African/ Atlantic region provides interesting contrast between some of the strongest convection found anywhere and more weakly buoyant oceanic convection– + availability of GERB data and data from ongoing

AMMA field campaign

Background – TRMM Precipitation Radar and Lightning Imager

• TRMM samples all local times over a 46 day period• PR - measures back-scatter from large water

droplets or ice particles– Shape of the reflectivity profile determines the rain type, and is

used to estimate the surface rain rate

stratiform convective

Melting level ~ 5km

Z

Z

height heightWeak upward motion, particles drift downward + grow by vapor diffusion. Corresponds to steady, horizontally uniform rain areas

Strong upward motion, particles grow by coalescence or rimming and fall out in heavy showers

• LIS - staring imager - counts lightning flashes

Identifying and Tracking Convective Systems…• Want to track though entire lifecycle and include as

much of the cirrus anvil as possible– Use multiple threshold detect and spread approach (Boer +

Ramanathan, 1997)

Thresholds at 165,200,235 Wm-2 (~232,244,254K)

• Track via maximum area overlap at successive thresholds

• Track all systems present for > 3hrs, min size 4 GERB footprints (Req>50km)

Results from the tracking algorithm

• Small short lived systems dominate population, but longer lived large systems are known to dominate rainfall + cloud cover

• Systems form preferentially over elevated terrain

Defining Lifecycle stage

• Define life cycle stages based on evolution of system size and IR Tb

– Decreasing Tb developing– Increasing R mature,

detraining– Decreasing R, increasing Tb

dissipating

• Example is typical large, deep land system– Large deep - R>300km,

Tmin <220K1 - warm developing, 2 - cold developing, 3 - mature, 4 - cold dissipating, 5 - warm dissipating

Observed Evolution from Geostationary Satellite Data

• Well defined lifecycle - approach works!

• Land systems are deeper + brighter

• Ocean systems have larger precipitating fraction

large deep systems

‘Africa’

‘Atlantic’

• Results are for JJAS 2005 • GERB-like and TRMM 3B42 data

Matched TRMM PR data - land ocean differences

• Convection is deeper and more intense over land

• Ocean systems have more stratiform signature (bright band)

• Evidence of re-evaporation of rainfall in land systems– Contributes to lower

mean stratiform rain rate

Mature, large deep systems

convective

stratiform

• Also little variation in strength/ depth of convection over ocean– weakens in final stages

• Most intense convection seen in mature stage over land

• See expected behavior over land

• Over ocean - convective fraction is essentially constant

TRMM PR – Evolution

And LIS data…

• Peak lightning occurrence and highest flash rates occur late in lifecycle over ocean– Consistent with continued convective activity after

peak size is reached

Ocean scaled by x5(right hand axis)

Ocean scaled by x100(right hand axis)

Role of propagating systems?• Do systems which propagate out over the African coast

and dissipate over ocean play a role?

• Result remains if exclude these systems• In fact, systems do not appear to have memory

Discussion…• Results suggest a fundamental difference in evolution

between land and ocean

• Over land, systems become more stratiform with time

• Over ocean, convective fraction remains essentially constant– High sustainability - new cells generated continuously– Lifecycle controlled by balance between rate at which new cells are generated and dissipation of decaying stratiform regions

– Diurnal cycle acts as cut-off for new convection?– Even long-lived large deep systems reach maturity before/ around

midnight• form earlier and take longer to dissipate

land ocean?

time time

Didn’t the title mention GERB…

• Examine radiative properties for 1 month (July 2006) of release quality GERB data

July 2006 GERB data

JJAS 2005 GERB-like data

• Albedo and OLR slightly higher for GERB data

• Land ocean differences remain

• Differences may be due to different time period as well as difference in datasets

Poor sampling

• Large deep systems contribute almost all of the cold cloudiness over the course of the month

Contribution of large deep systems to cloudiness

Contribution to cloud forcing…

• Large-deep systems dominate cold cloud forcing

Summary and Conclusions

• Classification by lifecycle stage based on IR geostationary data provides a valuable framework for analysis of less well sampled data

• African systems have colder cloud tops, higher albedo, deeper convection and more frequent lightning than Atlantic systems, but Atlantic systems have higher precipitating fraction

• Storms which propagate from land to ocean behave like local oceanic systems

• Land and ocean systems evolve differently - Atlantic systems do not show the expected evolution to more stratiform, less active behavior

• Large deep systems dominate cloudiness and cloud forcingMore details in Futyan and Del Genio, 2006, under revision for J. Climate