Shifting the diurnal cycle of parameterized deep convection over land

C. Rio, F. Hourdin, J.-Y. Grandpeix, and J.-P. Lafore

Motivation• Problem? Error in producing precipitating diurnal cycle over land• Observed precipitation peak late afternoon• Modeled precipitation peak in phase with insolation (mid-day)• Approach?• Control the triggering and intensity by sub-cloud processes–

boundary layer thermals + cold pool (cold wake)• Change convection parameterization-especially the closure for

deep convection from CAPE (Emanuel 1991) or horizontal moisture convergence (Tiedtke 1989), or temperature and humidity profiles below clouds (Emanuel and Zivkovi-Rothman 1999)

Physical processes being parameterized within grid cell

• Dry convection• Shallow convection (moistening the boundary

layer)• Sufficient energy that overcomes the inhibition at

boundary layer top• The triggering and growth of deep convection• Evaporation of rainfall under deep convective

cloud generates cold pools (cold wake)– self-sustaining thunderstorm

Physical processes being parameterized within grid cell

Parameterization for dry and shallow moist convection

• “thermal plume model”: Boundary layer convection and shallow clouds

• Prognostic for turbulent kinetic energy with mass-flux scheme to represent the vertical transport of TKE by thermals

Transition from shallow to deep convection

• Triggering of deep convection-• KE of parcels inside thermal –Available Lifting

Energy (ALE)– greater than the Convective Inhibition (CIN)

• ALEth=w*2/2 > |CIN|

• w* is the maximal vertical velocity within thermal, typically at shallow cloud top (boundary layer top)

The deep convection generated by strong thermal

• Determine the intensity of deep convection by convective power above inhibition

• The flux of kinetic energy associated with thermal– Available Lifting Power (ALP, in W m-

2)€

12Mbwb

2

Power consumed by CIN

Power loss by dissipation

€

ALP th = kthρw'3/2

wb=1 m s-1

Subscript b indicates the level of free convection (LFC)

With cold pool (cold wake in the grid cell)

• A fraction of the grid cell is covered by cold pool

• The dynamical lifting at cold pool (cold wake) gust front contribute to the kinetic energy of the updraft

• Updraft= thermal + dynamic lifted • Important to both deep convection triggering

and the intensity of further growth

Cold pool (cold wake) spread rate C*

Grandpeix and Lafore 2010

Wake Available Potential Energy

Potential energy kinetic energy

Contribution of wake dynamic lifting to the deep convection triggering and intensity

€

ALE wk =WAPE ∝ C*2

€

max ALE th ,ALE wk( ) > CIN

€

ALPwk = kwhwLwρC*3 /2 ∝ C*

3

€

ALP = ALP th + ALPwk

The available kinetic energy due to cold pool lifting

The new condition of the triggering of deep convection

The available lifting power of updraft lifted by cold pool

The new ALP in the mass flux closure

Diurnal cycle of convection on the EUROCS case: rain initiation and peak shift by the new ALP closure

•Over land•Idealized EUROCS case built from observation--•Atmospheric Radiation Measurement site over Southern Great Plains (USA)•27-30 June 1997•Initial conditions, large-scale forcing, SHF 120 W m-

2 and LHF 400 W m-2

Deep convection preconditioning: the change due to thermal plume model

E with CAPE

AT ALP +thermal plume

ATWAT+ wake

Thermal plume model:Faster deepening boundary layer in the morning (before deep convection)Warmer drier near surfaceCooler and moister in inversion layerIncrease the inhibition at boundary layer top, but not enough to delay deep convection

The delay of deep convection by ALE condition

In ALE implemented models the precipitation and deep convection start to generate once CIN is overcome by ALEth

Deep convection continuation: the wake effect in maintaining deep convection

Precipitation weakens the thermalDynamical lifting associated with wake gust front reinforce convection

AT: ALP +thermal plumeWithout wake parameterization

ATW:AT+ wake

GCMs

Conclusions

• With new parameterization the single-column version of general circulation model LMDz can simulate a realistic diurnal cycle of convective rainfall

• Thermal drives shallow convection in the morning, play a key role in the preconditioning and triggering of deep convection

• Cold pool (cold wake) reinforce and maintain deep convection in the the afternoon

• The ALE and ALP concepts couple the wake and deep convection parameterizations

Modification to the ALP closure to adapt the parameterization for over ocean cases

wb=1 m s-1

Rio et al. 2013

Wbmax=6 m s-1 Δp=500 hPa

DHARMA: cloud-resolving simulation performed with the Distributed Hydrodynamic-Aerosol-Radiation Model Application (DHARMA, Stevens et al. 2002; Ackerman et al. 2000)SP: large-scale model in 1D mode with CAPE closureNP: large-scale model in 1D mode with ALP closure and varying wb ALPCV: large-scale model in 1D mode with ALP closure plus large scale convergence and wb=1 m s-1

WB1: large-scale model in 1D mode with ALP closure and wb=1 m s-1

WB05: large-scale model in 1D mode with ALP closure and wb=0.5 m s-1

Convection over tropical ocean

Active

Suppressed

TWP-ICE case

LFC mass flux produced by different models

Diurnal cycle of convection over semi-arid land

precipitation

trigger

end

3D experiments on parameterization sensitivity test: Diurnal cycle