Shifting the diurnal cycle of parameterized deep convection over land

1. Shifting the diurnal cycle of parameterized deep convection over land C. Rio, F. Hourdin, J.-Y. Grandpeix, and J.-P. Lafore

2. 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)

3. 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

4. Physical processes being parameterized within grid cell

5. 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

6. 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)

7. The deep convection generated by strong thermal Subscript b indicates the level of free convection (LFC) Power consumed by CIN Power loss by dissipation wb=1 m s-1 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)

8. 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

9. Cold pool (cold wake) spread rate C* Wake Available Potential Energy Potential energy  kinetic energy Grandpeix and Lafore 2010

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

11. 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

12. Deep convection preconditioning: the change due to thermal plume model Thermal plume model: Faster deepening boundary layer in the morning (before deep convection) Warmer drier near surface Cooler and moister in inversion layer Increase the inhibition at boundary layer top, but not enough to delay deep convection AT ALP +thermal plume E with CAPE ATW AT+ wake

13. 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

14. Deep convection continuation: the wake effect in maintaining deep convection Precipitation weakens the thermal Dynamical lifting associated with wake gust front reinforce convection AT: ALP +thermal plume Without wake parameterization GCMs ATW: AT+ wake

15. 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

16. Modification to the ALP closure to adapt the parameterization for over ocean cases wb=1 m s-1 Wbmax=6 m s-1 Δp=500 hPa Rio et al. 2013

17. 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 closure NP: 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

18. Convection over tropical ocean TWP-ICE case Active Suppressed

19. LFC mass flux produced by different models

20. Diurnal cycle of convection over semi-arid land precipitation end trigger

21. 3D experiments on parameterization sensitivity test: Diurnal cycle