1. Diurnal and semi-diurnal cycles of convection in an aqua-planet GCM
Myong-In Lee, Max J. Suarez,
Kyu-Myong Kim, and Siegfried D. Schubert
Aqua-Planet Experiment Workshop April 2005, Reading, UK

2. Outline The diurnal cycle of NASA/GMAO AGCM
Sensitivity to treatment of convection
Results from aqua-planet experiments
Diurnal and semi-diurnal cycles of convection
Sensitivity of the atmospheric tides
Summary

3. NASA/GMAO Atmospheric Model
NASA’s Seasonal-to-Interannual Prediction Project (NSIPP) version 1, Bacmeister et al. (2001)
Operational prediction model and tested up to ½ degree resolution
Numerics
Finite Difference (Suarez and Takacs 1995)
Cumulus convection
Relaxed Arakawa-Schubert (Moorthi and Suarez 1992)
More detailed condensate budget in the updraft
Large-scale condensation
Diagnostic cloud scheme based on RH, similar to Slingo (1987)
PBL/vertical diffusion
Local diffusion by Louis et al. (1982)
Radiation
Chou and Suarez (1999) for SW and Chou and Suarez (1994) for LW
Others
Gravity wave drag (Zhou et al., 1996)
Mosaic LSM (Koster and Suarez, 1992,1996)

4. Diurnal Cycle of NASA/GMAO AGCM (compared with TRMM)
Six years (1998–2003) of TRMM Microwave Instrument (TMI) rain retrievals at 2.5°x2.5° grid box (Tom Bell 2004)
Model AMIP simulation for the same period at 2.5°x2° horizontal resolution
Seasonal statistics (e.g., JJA)
Rainfall (R) in each grid point is fit to sinusoidal harmonics

5. TRMM
MODEL

6. TRMM
MODEL

7. Diurnal Cycle Simulation in the NCAR CCSM2 (CAM2 AGCM)
From Dai and Trenberth (2004)

8. Sensitivity to Treatment of Convection
Definition of sub-cloud layer
Choice of convective relaxation time

9. Diurnal Cycle of PBL (Temperature, JJA, Great Plains, U.S.)
OBS
Model

10. Determine cloud base properties
1. Strapping Process (defining sub-cloud layers)
Levels up
ground
Low-Strapping
Case
strapping levels
High-Strapping
cloud base
averaged profiles
Determine cloud base properties
2. Relaxation Time Scale Tests
Rather than “quasi-equilibrium” achieved at each time step,
only the fraction of the cumulus mass flux relaxes the state toward equilibrium (RAS), assuming certain time-scale.
Control: 30 minutes for all cloud types
Low strapping
EXP: Relaxation time varying from 2 hrs (shallow) to 12 hrs (deep)
High-Strapping

11. Phase of Diurnal Cycle
(24-h)
TRMM
CTRL
EXP

12. Histogram for diurnal (24-h) component
LST
TRMM
CTRL
EXP
Histogram for diurnal (24-h) component
(40 S - 40 N)

13. Phase of Semi-Diurnal Cycle
TRMM
CTRL
EXP

14. Histogram for diurnal (12-h) component
LST
TRMM
CTRL
EXP
Histogram for diurnal (12-h) component
(40 S - 40 N)

15. Aqua-planet with Swamp Experiment (same forcing as APE)
1. Flat Surface with No Mountain
2. Solar Radiation at TOA – (as in APE)
- fixed equinoctial insolation, symmetric about the equator (eccentricity and obliquity set to zero)
- diurnal cycle retained
- solar constant as 1365 W/m2
3. Ozone – (as in APE)
- zonally uniform and symmetric about the equator
- obtained from annual mean climatology of AMIP II
4. Radiatively active gases and various geophysical constants
- Followed as in APE design

16. Lower-Boundary Condition
1. Model generated surface temperature – swamp
- interactive ocean with finite heat capacity (~ 10 meter depth of swamp)
- surface temperature is changing by the net heat flux onto the surface
2. Surface characteristics
surface albedo- globally uniform as 10%
no sea ice
- roughness – 3.21 x 10-5 m

17. Zonal Mean States
Surface Temperature (C)
Precipitation Rates (mm/day)

18. Zonal Mean States
Temperature (C)
U (m/s)

19. Atmospheric Tides (SLP)
(Whiteman and Bian 1996,BAMS)
observation
swamp aqua-planet

20. Diurnal and Semi-diurnal Cycles in the Swamp Aqua-planet

21. Diurnal
(24-h)
Semi-
diurnal
(12-h)

22. Moist Static Energy (J/kg)
CpT+gZ+Lq Lq

23. Atmospheric Tide Sensitivity Experiments
Experiment Name
Description
CONTROL
Control swamp aqua-planet experiment
Exp-oz
No diurnal SW radiation by ozone
(zonal mean ozone contribution prescribed)
Exp-q
No diurnal SW radiation by water vapor
(zonal mean water vapor contribution prescribed)

24. Precipitation Rates (mm/day)
Zonal Mean States
Precipitation Rates (mm/day)
Surface Temperature (C)
CNTR
Exp-oz
Exp-q
CNTR
Exp-oz
Exp-q

25. Atmospheric Tides (SLP)
(24-hour)
(12-hour)
0.5 mb
1.0 mb
control
Exp-oz
Exp-q

26. control
Exp-oz
Exp-q

27. Moist Static Energy (J/kg)
CpT+gZ+Lq Lq
control
Exp-oz
Exp-q

28. Summary-1
The diurnal cycles of simulated rainfall in the AGCM is compared with the TRMM observations: Phases are earlier than observed--both over the land and ocean--by several hours.
The diurnal phase of land convection is substantially changed by modifying the relaxation time and sub-cloud layer definition in the RAS convection scheme. However, the phase of oceanic convection is not changed.

29. Summary-2
The swamp-APE model simulates fairly realistic phase and amplitude of the diurnal and semi-diurnal tides. Experiments without diurnal variation of ozone and moisture reduce the model solar tide by 30% and 60%, respectively.
The forcing of oceanic convection was investigated in swamp aqua-planet experiments: Nocturnal precipitation is driven by the destabilization associated with nocturnal radiative cooling by water vapor in the mid-troposphere.
Atmospheric tide reduction weakens amplitude of rainfall diurnal cycle, due to the increase of stability by reducing nocturnal cooling of mid-troposphere (but, no change in phase).