CONVECTIVE INTENSITIES AND LIFECYCLES – SOME QUESTIONS FOR GCSS

GCSS workshop9/18/06

CONVECTIVE INTENSITIES AND LIFECYCLES – SOME QUESTIONS FOR GCSS

Tony Del Genio, GISSwith help from Yonghua Chen, Joanna Futyan


IPCC AR4 coupled AOGCMs produce a wide variety of stratiform contributions to total tropical precipitation…

…and it matters

Dai (2006)


Two different ideas about how moist convection affects climate sensitivity

“thermostat”

SW

“adaptive iris”

↑LW

(Zipser, 1977)


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)


“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


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…


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


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)


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


GCM ENSO anomalies weaker, more zonal than observedTRMM TMI precip. GCM precip.

(Chen et al. 2006)


GCM stratiform rain fraction much smaller than observed, responds in opposite way to El Niño

MEAN ENSO

(Chen et al. 2006)


Convective lifecycle composite study – using cloud top T and size to

define developing, mature, dissipating stages of systems

(Futyan and Del Genio 2006)


TRMM PR convective/stratiform partitioning and radar reflectivity profiles develop differently over

land and ocean – do CRMs do this?

(Futyan and Del Genio 2006)


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)


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?


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?


ADDITIONAL SLIDES


TRMM data

(Del Genio et al. 2005)

CONVECTIVE INTENSITIES AND LIFECYCLES – SOME QUESTIONS FOR GCSS

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