slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Understanding the Tropical Biases in GCMs: Double-ITCZ, ENSO, MJO and Convectively Coupled Equatorial Waves PowerPoint Presentation
Download Presentation
Understanding the Tropical Biases in GCMs: Double-ITCZ, ENSO, MJO and Convectively Coupled Equatorial Waves

Loading in 2 Seconds...

play fullscreen
1 / 20

Understanding the Tropical Biases in GCMs: Double-ITCZ, ENSO, MJO and Convectively Coupled Equatorial Waves - PowerPoint PPT Presentation


  • 167 Views
  • Uploaded on

Understanding the Tropical Biases in GCMs: Double-ITCZ, ENSO, MJO and Convectively Coupled Equatorial Waves. The tropical biases: One of the main bottlenecks for climate modeling. The major difficulties for understanding and alleviating these tropical biases.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Understanding the Tropical Biases in GCMs: Double-ITCZ, ENSO, MJO and Convectively Coupled Equatorial Waves' - zedekiah


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Understanding the Tropical Biases in GCMs:Double-ITCZ, ENSO, MJO and Convectively Coupled Equatorial Waves

the major difficulties for understanding and alleviating these tropical biases
The major difficulties for understanding and alleviating these tropical biases
  • They all involve some forms of feedback, such as the ocean-atmosphere feedback and the wave-heating feedback, making it difficult to determine the real cause of the bias;
  • The biases need to be traced back to specific model characteristics, such as certain aspect of the physical parameterizations, in order to provide useful guidance on how to improve the model simulations.
slide4

How to attack the problem?

Difficult to understand the success of some schemes/ parameters

Difficult to try all combinations of schemes/parameters

Possible missing physics in all existing schemes

Simulations and Predictions

Model Improvement

(Treatments)

Structure Analysis (Symptoms)

Feedback and Physical Relationship Analysis (Mechanisms)

gcms analyzed 27 models including almost all the major gcms used for predictions and projections
GCMs analyzed: 27 models including almost all the major GCMs used for predictions and projections
  • 22 IPCC AR4 coupled GCMs (IPCC Fourth Assessment Report to be released in 2007; from PCMDI data archive)
  • NCEP operational GFS and CFS (in collaboration with Wanqiu Wang of NCEP)
  • ECMWF model (from DEMETER archive)
  • NASA GMAO GEOS5 GCM currently under development (in collaboration with Siegfried Schubert, Max Suarez, Julio Bacmeister of NASA GMAO)
  • GFDL next generation GCM currently under development (in collaboration with Leo Donner of GFDL)
  • Seoul National University GCM (in collaboration with Myong-In Lee of NASA GMAO)
slide6

The double-ITCZ problem: Symptoms (1) Excessive (insufficient) precipitation over much of tropics (equatorial western Pacific); (2) Cold SST bias over much of tropics

Obs

NCAR

GFDL

Double-ITCZ

Shading: SST Contours: precipitation

From Lin (2006a)

the double itcz problem mechanisms
The double-ITCZ problem: Mechanisms

(1) Biases in AGCM’s climatology initiate the biases in the coupled runs; (2) Biases in ocean-atmosphere feedback parameters amplify or suppress the initial problems.

SST gradient - trade wind (Bjerknes) feedback (e.g. Bjerknes 1969, Neelin and Dijkstra 1995; Pierrehumbert 1995; Sun and Liu 1996; Jin 1996; Clement et al. 1996; Liu 1997; Cai 2003)

SST - LHF feedback (e.g. Wallace 1992; Liu et al 1994; Zhang et al. 1995)

SST - SWF feedback (e.g. Ramanathan and Collins 1991)

Neelin and Dijkstra (1995) showed that any excessive positive feedback (or insufficient negative feedback) tends to shift the whole system westward, leading to a double-ITCZ pattern. However, few previous studies have evaluated quantitatively the feedback parameters in GCMs.

From Lin (2006a)

slide8

The double-ITCZ problem: Mechanisms(1) Excessive tropical precipitation in AGCMs leads to enhanced Walker circulation and surface flux cooling

Precipitation

Latent heat flux

Excessive

Excessive

Surface zonal wind stress

Surface downward shortwave flux

Overly strong

Insufficient

Annual mean along the equator (5N-5S)

the double itcz problem mechanisms 2 overly positive ocean atmosphere feedback parameters
The double-ITCZ problem: Mechanisms(2) Overly positive ocean-atmosphere feedback parameters

Bjerknes x vs SST

Precip vs SST

Overly positive

SST-LHF LHF vs SST

Qair vs SST

Overly positive

SST-SWF SWF vs SST

Cld vs SST

Insufficiently negative

Linear regression for 5N-5S averaged monthly data

the enso problem symptoms 1 large scatter in enso variance 2 too short enso period in many models
The ENSO problem: Symptoms(1) Large scatter in ENSO variance (2) Too-short ENSO period in many models

Interannual variance of SST along the equator (5N-5S)

CCSM3

Normalized spectrum of Nino3 SST

CCSM3

From Lin (2006b)

existing enso theories
Existing ENSO theories

(6) Stochastic forcing theory (McWilliams and Gent 1978, Lau 1985, Penland and Sardeshmukh 1995, Blanke et al. 1997, Kleeman and Moore 1997, Eckert and Latif 1997)

(1) Slow coupled mode theory (Philander et al. 1984, Gill 1985, Hirst 1986, Neelin 1991, Jin and Neelin 1993, Wang and Weisberg 1996)

(2) Delayer oscillator theory (Suarez and Schopf 1988, Battisti and Hirst 1989)

(3) Advective-reflective oscillator theory (Picaut et al 1997)

(4) Western Pacific oscillator theory (Weisberg and Wang 1997)

Quasi-standing oscillation within Pacific basin triggered or forced by free oceanic waves

(5) Recharge oscillator theory (Jin 1997a,b)

From Lin (2006c)

a new observation based mechanism for enso the coupled wave oscillator lin 2006c d
A new observation-based mechanism for ENSO: The coupled wave oscillator (Lin 2006c,d)

ENSO amplitude and period are determined by circum-equatorial coupled equatorial waves, and their interactions with the off-equatorial Rossby waves

the enso problem mechanism incorrect representation of the coupled wave oscillator
The ENSO Problem: MechanismIncorrect representation of the coupled wave oscillator

Too-fast phase speed

Realistic phase speed

SSH

SSH

x

x

CCSM3 ENSO Period=2.5 yrs

MPI ENSO Period=4 yrs

slide14
The MJO and CCEW problems: SymptomsOnly half of the models have the waves, but usually with too weak variances and too fast phase speeds

Obs

GFDL

NCAR

slide15

The MJO and CCEW problems: SymptomsThe problem is especially severe for MJO, with very weak variance, no coherent eastward propagation, and no significant spectral peak

All season

Asian summer monsoon

CCSM3

Spectrum of precipitation at 0N85E

North American monsoon

West African monsoon

(Lin et al. 2006a,b,c, Lin 2007)

the mjo and ccew problems mechanisms
The MJO and CCEW problems: Mechanisms

Vertical heating profile

In collaboration w/ Leo Donner Stratiform heating

In collaboration w/ Myong-In Lee Moisture pre-conditioning

Column-integrated diabatic heating has six major components (Mean state and higher-frequency modes affect the MJO through the nonlinear terms)

In collaboration w/ Ping Liu Shallow/midtop convection

In collaboration w/ Myong-In Lee Radiation feedback

Model resolution

In collaboration w/ Wanqiu Wang

IPCC runs Air-sea coupling

slide17

The MJO and CCEW problems: Treatments Moisture trigger often significantly enhances the variances of CCEWs, and sometimes slows down the phase speeds

No convection

Strong trigger

Weak trigger

No trigger

Effect on MJO is not monotonic

Lin, Lee. Kim, Kang (2006d)

slide18
The MJO and CCEW problems: Treatments Moisture trigger significantly enhances the fraction of large-scale precipitation

No convection

Strong trigger

Weak trigger

No trigger

Lin, Lee, Kim, Kang (2006d)

slide19

Recommendation: A model development strategy for alleviating the tropical biases

Difficult to understand the success of some schemes/ parameters

Difficult to try all combinations of schemes/parameters

Possible missing physics in all existing schemes

Simulations and Predictions

Model Improvement

(Treatments)

Structure Analysis (Symptoms)

Feedback and Physical Relationship Analysis (Mechanisms)

Understand the reasons of past successes/failures

Save time and computer resources in testing parameters

Know the directions of future improvements