Applications of a Regional Climate Model to Study Climate Change

over Southern China

Keith K. C. Chow

Hang-Wai Tong

Johnny C. L. ChanCityU-IAP Laboratory for Atmospheric Sciences,

Department of Physics & Materials Science,

City University of Hong Kong

Outline of the Presentation

1. Introduction to the Regional Climate Model (RCM)

2. Two applications of the RCM in studying the Asian summer monsoon:

• Effects of land surface heating over the Indochina Peninsula on the climate over Southern China.

• Time-lagged effects of soil moisture over the Tibetan Plateau on the climate over Southern China.

Introduction to the Regional Climate Model

• The regional climate model (RCM) was developed at the China National Climate Center and the City University of Hong Kong, based on the NCAR RegCM2.

• Objectives of the RCM development:

1. Simulating the onset and evolution of the Asian Summer Monsoon (ASM), particularly the onset of the South China Sea summer monsoon in May.

2. To study the physics of the ASM.

One ultimate goal is to have a reliable seasonal prediction of the ASM rainfall over China.

Basic Configuration of the RCM

• 20 vertical levels, 60-km horizontal resolution and a domain size of 135 165 grid points.

• Initial and lateral boundary conditions: ECMWF ERA40 reanalysis data. Lateral boundary conditions are fed in every 6 hr within a 15-grid buffer zone.

• Sea-surface temperature data are taken from the NOAA Optimum Interpolation SST V2 weekly mean data, with a 1 spatial resolution.

Model Domain without buffer zone

Physical Parameterization Schemes

• Land surface processes: BATS (Dickson et al. 1993).

• Planetary layer: Holtslag (Holtslag et al. 1990).

• Cumulus: Anthes-Kuo

• Large scale precipitation: Pal et al. 2000.

• Atmospheric radiation: CCM3

Total Precipitation (mm) in 1998Left: simulations Right: observations (TRMM)

Low-level Geopotential Height (m) and Vector Wind at 850 hPa in May 1998

Left: simulations Right: reanalysis data (ECMWF)

Low-level Geopotential Height (m) and Vector Wind at 850 hPa in June 1998

Left: simulations Right: reanalysis data (ECMWF)

Upper-tropospheric Temperature (K) averaged from 200 to 500 hPa in 1998

Left: simulations Right: reanalysis data

(ECMWF)

General Performance of the Model

• Successful in reproducing the pattern and quantity of precipitation over the China Mainland.

• Precipitation over the equatorial region of Indian Ocean in May is poorly simulated.

• Surface pressure over ocean is generally higher.

• A better cumulus parameterization scheme may help solve the above problems.

Effects of Land Surface Heating on the Summer

Monsoon over South China

• To study the effects of surface heating over two upstream regions of South China:

(1) Indochina peninsula

(2) Indian peninsula

• Surface heating = sensible heat + net longwave radiation emitted at the surface.

Focus

Maximum incident Solar Radiation in May is over the latitudes of Indochina and India

Experiments• Three experiments are performed for April to

June of 1998:

1. Control experiment (CTRL) with an optimal setting for simulating the summer monsoon.

2. Experiment (EXPIC) with surface heating removed in the Indochina landmass (10 - 20N ,94 - 109E).

3. Experiment (EXPIND) with surface heating removed in the Indian landmass (10 - 20N,75 - 90E).

EXPICDifferences (CTRL-EXPIC) in total Precipitation (mm)

EXPICDifferences (CTRL-EXPIC) in

850-hPa Geopotential Heights (m)

EXPICDifferences(CTRL-EXPIC) in 850-hPa Wind Field (m/s)

EXPICDifferences(CTRL-EXPIC) in vertical velocity (0.01 Pa s-1)

averaged from 10 to 20N

EXPINDDifferences (CTRL-EXPIC) in total Precipitation (mm)

EXPINDDifferences (CTRL-EXPIC) in

850-hPa Geopotential Heights (m)

EXPINDDifferences(CTRL-EXPIC) in 850-hPa Wind Field (m/s)

EXPINDDifferences(CTRL-EXPIC) in vertical velocity (0.01 Pa s-1)

averaged from 10 to 20N

Summary

• The surface heating of Indochina has significant effect on the precipitation over local region as well as remote regions of southern China and Western Pacific.

• The heating effect of the Indian peninsula on precipitation is relatively local and mainly affects the Bay of Bengal region.

• Effect of surface heating over Indochina is deep into the upper troposphere.

• Surface heating over Indochina may help increase the upper-tropospheric zonal wind over the subtropical region.

• In the lower troposphere, the surface heating of Indochina apparently has significant effect on:

1. the strength of the subtropical high.

2. the onset of the South China Sea summer monsoon.

Time-lagged Effects of Spring Tibetan Plateau Soil Moisture on the Monsoon

over China in Early Summer

Introduction

• It has been shown in numerous studies that a clear relationship exists between the amount of winter/spring snow over the Tibetan Plateau and the monsoon rainfall over the Yangtze River region and south China in the following summer.

• The snow amount is directly proportional to the rainfall over the Yangtze River region while it is inversely proportional to that over southern China.

• Soil moisture due to snow-melt is a possible factor since its memory is relatively long.

• Soil moisture can change the local surface evaporation and the soil thermal properties and so the surface heating effect.

• To test the hypothesis that soil moisture is a key factor for the effect of spring Tibetan snow cover on the summer monsoon rainfall over China.

• To investigate the effects of initial soil moisture over the Tibetan Plateau in spring on the local and regional climate in early summer.

Objectives of this Study

Experiments• Four experiments run from April to June for the

year 1998:

1. A control experiment (CTRL) with an optimal setting for simulating the summer monsoon. The initial soil moisture over the Tibetan Plateau (>4500km) is 25.6%.

2. Five sensitivity experiments as same as CTRL except the initial soil moisture over the Tibetan Plateau is increased to 32%, 38.4% and 44.8% (EXP1, EXP2, EXP3).

• Only the results in June will be discussed.

Local Effect of Initial Soil Moisture:

soil moisture over central Tibetan Plateau (28-36N,80-90E)

500-hPa temperature (K) and vertical velocity (hPa/hr)

over central Tibetan Plateau (28-36N,80-90E)

Surface heat fluxes over

central Tibetan Plateau (28-36N,80-90E)

Total precipitation (mm) over Yangtze River region (28N-33N, 115E-120E) and southern China (20N-27N,105E-120E)

Differences (EXP3-CTRL) in total precipitation (mm)

Differences (EXP3-CTRL) in 500-hPa velocity field (m/s)

Differences (EXP3-CTRL) in 500-hPa velocity field (m/s)

Differences (EXP3-CTRL) in temperature (K) averaged for Lat. 28-33N

Differences (EXP3-CTRL) in meridional wind (m/s) averaged for Lat. 28-33N

Summary

• The initial soil moisture over the Tibetan Plateau (ISMTP) can have significant effects on the early-summer climate over the local region as well as the remote regions.

• In the local region, the increase in ISMTP could result in the decreases in upper-tropospheric temperature, vertical velocity, the surface heating, and evaporative fluxes in early summer.

• In the remote regions, the increase in ISMTP could result in:

1. increase of precipitation over the Yangtze river region,

2. decrease of precipitation over southern China,

3. weakening of the Tibetan High.

• The patterns of precipitation differences due to the changes in ISMTP are consistent with those due to anomalous snow cover over the Tibetan Plateau in winter or spring.

Some Implications from these Studies

• The results from the RCM indicate that the climate of Southern China could be easily influenced by environmental changes in the neighboring regions, such as the Indochina Peninsula and the Tibetan Plateau.

• Important environmental changes over these regions include the changes in land-use, e.g. deforestation, urbanization, and desertification. These may result in the changes in surface heating, soil moisture, and surface evaporation over these regions.

Acknowledgement

We would like to thank the contributions of two previous group members Xueli Shi and Yiming Liu of the China National Climate Center.

Thank you for your attention!