sensitivity of numerical simulations of easm* to different convection schemes

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Sensitivity of Numerical Simulations of EASM* to

Different Convection Schemes

Haoming Chen, Tianjun Zhou, Rucong Yu, LASG, Institute of Atmospheric Physics, Chinese Academy of SciencesLASG, Institute of Atmospheric Physics, Chinese Academy of Sciences

Richard Neale, Jim Hack National Center for Atmospheric ResearchNational Center for Atmospheric Research

University Allied Workshop University Allied Workshop (1-3 July, (1-3 July, 2008)2008)

* EASM : East Asian Summer Monsoon

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Outline

Introduction Introduction

Model and Data DescriptionModel and Data Description

Results Results

Climatological mean pattern

Seasonal Variation

Summary and Discussion Summary and Discussion

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Introduction

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

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Introduction

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) .

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Questions

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?

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CAM3.5 and Experiments

ExperimentConvective

Scheme Integration Time References

CNTLZhang and McFarlane

1977.01 - 2003.12Zhang and McFarlane

(1995)

NR Neale and Richter 1977.01 - 2002.12

Wu Wu and Zhang 1977.01 - 2000.12 Wu et al.(2007)

Zhang Revised Zhang 1978.01 - 1999.12 Zhang et al. (2002)

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.

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Climatological mean pattern

RainfallRainfall

Meridional Meridional Monsoon Monsoon

CellCell

EASM ClimatologyEASM Climatology

Horizontal Horizontal CirculationCirculation

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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

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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. (90-140Total Precip. (90-140°°E, 0-45E, 0-45°°N)N)

Tropical Precip (90-140Tropical Precip (90-140°°E, 5-15E, 5-15°°N)N)

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

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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

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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

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Water Vapor Transport

Three main branches of water vapor transport:

southwesterly from ISMsouthwesterly from ISM southeasterly from southeasterly from

western Pacific western Pacific cross-equator flow cross-equator flow

straddling 105straddling 105°°E-150E-150°°EE

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

Vertically integration from 1000 – 100 hPa

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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

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Seasonal Variation

Abrupt Jump

•WPSH ridge•Rainfall belt

Zonal averaged rainfall

Seasonal March

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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

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North Jump of WPSH Ridge and Rainfall BeltNorth Jump of WPSH Ridge and Rainfall BeltW

PS

H R

idg

eW

PS

H R

idg

eR

ain

fall

Bel

tR

ain

fall

Bel

t

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DiscussionDiscussion

Strong temperature gradient in both the meridional and zonal directions;

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

Temp. averaged between 200 - 500 hPaTemp. averaged between 200 - 500 hPa

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Summary

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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