The Evolution of Convective Systems over Africa and the Tropical Atlantic

1. The Evolution of Convective Systems over Africa and the Tropical Atlantic Joanna Futyan and Tony DelGenio GIST 25, Exeter, 24th October 2006

2. 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

3. 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

4. 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 Weak 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 height stratiform height convective Melting level ~ 5km Z Z • LIS - staring imager - counts lightning flashes

5. 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)

6. Results from the tracking algorithm • Systems form preferentially over elevated terrain • Small short lived systems dominate population, but longer lived large systems are known to dominate rainfall + cloud cover

7. 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 <220K 1 - warm developing, 2 - cold developing, 3 - mature, 4 - cold dissipating, 5 - warm dissipating

8. Observed Evolution from Geostationary Satellite Data • Well defined lifecycle - approach works! • Land systems are deeper + brighter • Ocean systems have larger precipitating fraction • Results are for JJAS 2005 • GERB-like and TRMM 3B42 data ‘Africa’ ‘Atlantic’ large deep systems

9. 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 convective stratiform Mature, large deep systems

10. TRMM PR – Evolution • See expected behavior over land • Over ocean - convective fraction is essentially constant • Also little variation in strength/ depth of convection over ocean • weakens in final stages • Most intense convection seen in mature stage over land

11. 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 x100 (right hand axis) Ocean scaled by x5 (right hand axis)

12. 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

13. Discussion… • Results suggest a fundamental difference in evolution between land and ocean land ocean? time time • 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 stratiformregions • 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

14. Didn’t the title mention GERB… • Examine radiative properties for 1 month (July 2006) of release quality GERB 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 July 2006 GERB data JJAS 2005 GERB-like data

15. Contribution of large deep systems to cloudiness • Large deep systems contribute almost all of the cold cloudiness over the course of the month

16. Contribution to cloud forcing… • Large-deep systems dominate cold cloud forcing

17. 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 forcing More details in Futyan and Del Genio, 2006, under revision for J. Climate