Sensitivity of MJO to the CAPE lapse time in the NCAR CAM3.1
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Transcript of Sensitivity of MJO to the CAPE lapse time in the NCAR CAM3.1
Sensitivity of MJO to the CAPE lapse time in the NCAR CAM3.1
Ping Liu, Bin Wang
International Pacific Research CenterUniversity of Hawaii
Sponsored by SciDAC project, computations partly finished at SDSCThanks to: Jerry Meehl
2007 CCSM AMWG meeting at NCAR
In CAM3.1 T42L26, MJO is weak in amplitude and irregular in propagation as in CCM3 (Maloney 2001) and CAM2 (Liu et al 2005)
Variance of 20-80-day filtered U850 inextended winter season (NDJFMA) during 1979-2001
Variance of 20-80-day filtered precipitation inextended winter season (NDJFMA) during 1979-2001
Variance of 20-80-day filtered OLR inextended winter season (NDJFMA) during 1979-2001
Power spectra(10N-10S)1979-2001Winter Nov-Apr850hPa u
Power spectra(10N-10S)1979-2001Winter Nov-AprOLR
IrregularRegression of U850
Onto 155E inExtended winter
During 1979-2001With filtered data
Why?• Observational studies indicate a close coupling
exists between large-scale disturbances and convection associated with MJO (Wang 1988, …)
• A precondition of moisture (or buildup) by boundary layer convergence and/or shallow convection before deep convection associated with MJO bursts (Hendon, Salby, Maloney, Sperber…)
Why?
• Experiments with CCM3 (Maloney 2001, Zhang 2005) and CAM2 (Liu 2005) disclose that either model with alternative convective schemes or a revised closure can simulate much improved MJO although deficiencies remain
• Consequently the convective schemes probably have flaws in 1) deep convection configuration; 2) partition of deep/shallow convection
Where?Basic theories in the Zhang and McFarelane
(1995) scheme for deep convection
(1) A mass flux scheme based on Quasi-Equilibrium
theory (Arakawa and Schubert 1974)
(2) Uniform mass flux at cloud base for updraft
(3) Convection is triggered wherever there is net
positive CAPE (including CIN). Or CAPE
threshold is positive (70 J/kg in code).
(4) Scheme closed on CAPE consumed exponentially
at a specified time scale (2 hours in paper, 1 hour
in code: tau=3600.).
Hypothesis“Convection frequently occurs pre-maturely in the CCSM2”
(Dai 2004). Add a RH threshold for triggering deep convection can enhance the precipitation variability (Zhang and Mu 2005) but not for the RAS (Maloney 2001) in CCM3. A too frequent deep convection might prevent a reasonabe partition of shallow/deep convection then moisture buildup does not occur. So
(3) Is the CAPE threshold low?
The QE theory requires
(4) the specified time for CAPE lapse too short?
s
sADJLS
5LS
43ADJ
10~ lfor typica AS74) in 154 (EQ
hours 10 ~ )1010(~ ,
More evidence“Tropical atmosphere have a thermal-dynamical
background of CAPE at 1000 J/kg” – Heat engine theory by Renno (1996; reversible)
Zhang and McFarelane (1995) table
LTM (1979-2001) DJFM Pseudo-Adiabatic CAPE, J/kg interval 500, thick 1500
Experiments
CAPE threshold: 3 and 10 times
CAPE lapse time: 1, 2, 4, 6, 8, 10 hours
Run: AMIP 1978.9 ~ 2002.8
Model: CAM3.1 T42L26
Results
CAPE threshold: 3 and 10 times
210, 700 J/kg
LTM (1979-2001) DJFM P-A (left) and RV (right) CAPE, J/kg
LTM (1979-2001) DJFM precipitation, mm/dayInterval 3, thick 9
Regression of U850Onto 155E in
Extended winterDuring 1979-2001With filtered data
Indications from CAPE threshold experiments
Lifting the CAPE threshold to 10 times as large as that in control can help the mean state and MJO to some extent, but obviously cannot significantly improve the structure of MJO.
Results
CAPE lapse time: 4, 6, 8, 10 hours
LTM (1979-2001) DJFM Reversible CAPE, J/kg interval 200, thick 800
LTM (1979-2001) DJFM Pseudo-Adiabatic CAPE, J/kg interval 500, thick 1500
LTM (1979-2001) DJFM precipitation, mm/dayInterval 3, thick 9
LTM (1979-2001) DJFM Reversible CAPE, J/kg interval 200, thick 800
Power spectra(10N-10S)1979-2001Winter Nov-Mar850hPa u
Power spectra(10N-10S)1979-2001Winter Nov-MarOLR
MJO based on ZM8HR
Variance of 20-80-day filtered U850 inextended winter season (NDJFMA) during 1979-2001
Variance of 20-80-day filtered precipitation inextended winter season (NDJFMA) during 1979-2001
Variance of 20-80-day filtered precipitation inextended winter season (NDJFMA) during 1979-2001
Regression of U850Onto 155E in
Extended winterDuring 1979-2001With filtered data
Frictional convergence in a composite MJO life cycle
• Maloney (1998; 2001)
• Liu (2005)
NOAA
ZM8HRFirst two EOFs of10N~10S mean filtered OLR
Power spectra(10N-10S)1979-2001Winter Nov-MarOLR
Power spectra(10N-10S)1979-2001Winter Nov-MarOLR
Partitioning of shallow and deep convection
LTM (1979-2001) DJFM ratio of shallow convective to total precipitationInterval 10%, shaded >= 50%
LTM (1979-2001) DJFM ratio of shallow (left) and deep (right) convective to total precipitation
Interval 10%, shaded >= 50%
Summary
. Lifting the CAPE threshold does not significantly improve MJO
. Lengthening the CAPE lapse time enhances MJO variability and improves its structure.
. The CAPE lapse time is optimal at 8 hours to simulate the MJO in both variability and structure. Frictional convergence mechanism functions from the Indian Ocean to western Pacific, which is close to observational facts. A 4:5 partition of shallow and deep convection is a key feature in this case.
. ZM8HR is an ideal starting point for further work.
Future work
Local CAPE lapse time
Standard plots for ZM8HR based on AMWG packages 1 and 2, please see
http://www.soest.hawaii.edu/pliu/zm8hr/1/sets.htm
http://www.soest.hawaii.edu/pliu/zm8hr/2/variab.html