GCSS workshop9/18/06
CONVECTIVE INTENSITIES AND LIFECYCLES – SOME QUESTIONS FOR GCSS
Tony Del Genio, GISSwith help from Yonghua Chen, Joanna Futyan
GCSS workshop9/18/06
IPCC AR4 coupled AOGCMs produce a wide variety of stratiform contributions to total tropical precipitation…
…and it matters
Dai (2006)
GCSS workshop9/18/06
Two different ideas about how moist convection affects climate sensitivity
“thermostat”
SW
“adaptive iris”
↑LW
(Zipser, 1977)
GCSS workshop9/18/06
Need to know 3 things: 1. Drop size distribution 2. Size-fallspeed relationships 3. Cumulus updraft speed
Then: 1. Liquid mostly rains out, short-circuits much of SST dependence of anvil 2. Primary SST dependence is thicker anvil at higher SST due to higher level of neutral buoyancy 3. Clausius-Clapeyron ensures IWP asymptote to max at high SST
Result: ~neutral cloud feedback – more ice at top offsets less ice + liquid at bottom – but depends on updraft speed, mesoscale dynamics (Del Genio et al. 2005)
GCSS workshop9/18/06
“Shape of the CAPE”: Rising parcels more buoyant, but over smaller depth, over land than ocean →
more vigorous updrafts over landBut other ideas (deeper PBL, more aerosol loading)
ocean
land
(Lucas et al. 1996; Zipser and Lutz 1994)
| | |lightningthreshold?
ocean
land
GCSS workshop9/18/06
Diagnosis of growth of cumulus updraft kinetic energy with height from large-scale thermodynamic
structure (Gregory, 2001)
buoyancydue to parcelT, q excess
dragdue tocondensedwaterloading
downwardcumuluspressuregradientforce
dilutionbyentrainment
Let’s take it for a spin…
GCSS workshop9/18/06
Land-ocean difference in updraft speed diagnosed
Weak increase in updraft speed with warming over land, even less over ocean -
primarily due to upward shift in freezing level
0°C current
0°C 2xCO2
GCSS workshop9/18/06
Budget of vertical velocity variance for midlatitude deep convective cases – would be nice to see for all
CRMs and different environments
(Khairoutdinov and Randall 2002)
GCSS workshop9/18/06
SCMs underpredict downdraft mass fluxes in a summer midlatitude case study (i.e., those that even
have downdrafts) – what can CRMs tell us about how to do better?
(Xie et al., 2002)
CRMsSCMs
GCSS workshop9/18/06
GCM ENSO anomalies weaker, more zonal than observedTRMM TMI precip. GCM precip.
(Chen et al. 2006)
GCSS workshop9/18/06
GCM stratiform rain fraction much smaller than observed, responds in opposite way to El Niño
MEAN ENSO
(Chen et al. 2006)
GCSS workshop9/18/06
Convective lifecycle composite study – using cloud top T and size to
define developing, mature, dissipating stages of systems
(Futyan and Del Genio 2006)
GCSS workshop9/18/06
TRMM PR convective/stratiform partitioning and radar reflectivity profiles develop differently over
land and ocean – do CRMs do this?
(Futyan and Del Genio 2006)
GCSS workshop9/18/06
Almost all GCMs now detrain condensate but have no mesoscale anvil dynamics – how is this related
to what we know about the parent convection?land
ocean
(Houze 1989)(Futyan and Del Genio 2006)
GCSS workshop9/18/06
Why not create an archive of GCSS case study results for analysis by the community, analogous to
what GCM groups have done for IPCC AR4?
GCSS workshop9/18/06
Summary
• Basic microphysics and thermodynamics suggests convective cloud feedback nearly neutral, but depends on cumulus updraft speed and mesoscale dynamics
• Large-scale predictors for updraft speed capture land-ocean and even some regional differences in GCM – what about PBL depth and aerosols?
• Slightly “stronger” thunderstorms in warming climate due to freezing level shift – is this consistent with CRMs?
• How can CRMs guide GCM determinations of occurrence, characteristics of cumulus-scale downdrafts?
• What can our knowledge of parent convection and grid-scale parameters tell us about the formation and evolution of mesoscale anvils?
• How can we “entrain” a larger community into the analysis of CRMs and thereby gain more from all these case studies?
GCSS workshop9/18/06
ADDITIONAL SLIDES
GCSS workshop9/18/06
TRMM data
(Del Genio et al. 2005)