Sensitivity of numerical simulations of easm to different convection schemes
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University Allied Workshop (1-3 July, 2008). Sensitivity of Numerical Simulations of EASM* to Different Convection Schemes. Haoming Chen, Tianjun Zhou, Rucong Yu, LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences Richard Neale, Jim Hack

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Sensitivity of Numerical Simulations of EASM* to Different Convection Schemes

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University Allied Workshop (1-3 July, 2008)

Sensitivity of Numerical Simulations of EASM* to Different Convection Schemes

Haoming Chen, Tianjun Zhou, Rucong Yu,

LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences

Richard Neale, Jim Hack

National Center for Atmospheric Research

* EASM : East Asian Summer Monsoon


Model and Data Description


Summary and Discussion


Climatological mean pattern

Seasonal Variation


  • The huge Asian summer monsoon system can be divided into two subsystems: the Indian summer monsoon (ISM) and the East Asian summer monsoon (EASM) systems;

  • EASM is a hybrid type of tropical and subtropical monsoon


  • Simulation of the Asian summer monsoon and its variability has proved to be one of the most challenging issues for general circulation models(Kang et al., 2002; Wang et al., 2004; Meehl et al., 2006).

  • The CAM model has been widely used for the climate research, but its performance in EASM has not been fully evaluated.

  • The coupling of convective processes with the large-scale dynamics is crucial for modeling the distribution of precipitation(Zhang, 2005) . The simulation of EASM rainfall and circulations are sensitive to convection schemes (Huang et al., 2001) .


1) Can the new version NCAR CAM3.5 reasonably reproduce the climatic features of EASM?

2) What are the influences of different convection schemes on EASM simulations?

CAM3.5 and Experiments

  • CAM3.5 is the recently improved version AGCM in NCAR

  • Finite-Volume dynamical core;

  • Horizontal resolution is about 2.5°longitude by 1.9°latitude with 26 vertical levels;

  • Changes to convection and cloud processes, land model and chemistry modules compared to CAM3;

  • Three revised convection schemes are applied, and four AMIP-type experiments are performed.




EASM Climatology




Climatological mean pattern

JJA Mean Precipitation

  • Two major rainfall belts:

    ★ tropical monsoon trough

    ★ subtropical Meiyu /Baiyu/ Changma front

  • None of the schemes realistically reproduce the observed Meiyu rain band; the simulated tropical rainfall are relatively weak

  • The revised schemes improve the rainfall simulation, but the details depend on schemes

Taylor Diagram

  • The three revised schemes improve the precipitation simulation;

  • The NR scheme simulates more realistic rainfall in EASM and its tropical rainfall belt;

  • The subtropical rainfall simulated by Wu scheme are most reasonable.

Total Precip. (90-140°E, 0-45°N)

Tropical Precip (90-140°E, 5-15°N)

Subtropical Precip. (90-140°E, 25-35°N)

100 hPa Wind and Geopotential Height

  • Tibetan High

    ★ Subtropical Westerly

    ★ Tropical Easterly Jet

  • The simulated High shift westward; the westerly and easterly are stronger;

  • The circulation are generally similar in different schemes

500 hPa Wind and Geopotential Height

  • A strong anticyclone dominates the subtropical western Pacific

  • The simulated WPSH is stronger than the reanalysis and extends about 20° westward and its ridge shift more than 5° north

Water Vapor Transport

  • Three main branches of water vapor transport:

  • southwesterly from ISM

  • southeasterly from western Pacific

  • cross-equator flow straddling 105°E-150°E

  • The southwesterly transport is too weak in the model, whereas the southeasterly transport extends northward.

Vertically integration from 1000 – 100 hPa

Meridional Monsoon Cell

  • The normal Hadley cell is replaced by a meridional monsoon cell;

  • The model simulates weak monsoon cell, which is closely related to rainfall biases in the model

Averaged over 90 -140 °E



Zonal averaged


  • Abrupt Jump

  • WPSH ridge

  • Rainfall belt

Seasonal Variation

Zonal Averaged Rainfall

  • The major rainfall advances toward the north since March, Mei-Yu /Baiyu/ Changma begins along the Yangtze River valley in June; heavy rainfall withdraw southward in August;

  • The model can reproduce the poleward progress and southward withdraw; the subtropical rainfall shifts northward

Averaged over 110°E~125°E

North Jump of WPSH Ridge and Rainfall Belt

WPSH Ridge

Rainfall Belt


Temp. averaged between 200 - 500 hPa

  • Strong temperature gradient in both the meridional and zonal directions;

  • Zonal “+ - +” pattern in temperature difference fields greatly reduce the meridional thermal contrast.


  • The CAM3.5 model can realistically reproduce the main circulation of EASM, but the precipitation simulated is poor, especially in the subtropical Meiyu/Baiyu/Changma rainfall belt, which is closely related to the biases of the monsoon circulation in subtropical region;

  • The model can capture the southward advance and northward withdraw of the main rain belts, as well as the two north jump of WPSH, but failed to represent the north jump of rainfall belt;

  • The three revised schemes generally improve the model performance in EASM, and the simulation of EASM rainfall depends on convection schemes, but the circulation is less sensitive to different schemes

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