MODELING EAST ASIAN SUMMER MONSOON IN 1998 WITH A COUPLED REGIONAL AIR-SEA MODEL. Xuejuan Ren

Transcription

1 INYS 2008 MODELING EAST ASIAN SUMMER MONSOON IN 1998 WITH A COUPLED REGIONAL AIR-SEA MODEL Xuejuan Ren Yaocun Zhang, Yongfu Qian, Huijuan Lin Department of Atmospheric Sciences, Nanjing University, Nanjing, P.R. China renxuej@nju.edu.cn

2 Outline 1. Introduction (motivation; background; objective) 2. Coupled air-sea model description (P( σ -RCM;; POM) 3. Simulation results: East Asian summer monsoon in Investigation of the differences between the coupled and uncoupled experiments 5. Conclusions

3 1. Introduction air-sea interaction processes Atmosphere Heat flux Surface stress Freshwater flux SST,sea-ice Surface stress Wave-induced Roughness length Ocean current Wave-induced stress Wave The air-sea interaction influences not only weather scale process, like storm, but also seasonal and longer time scale climate, like monsoon, Atmospheric oscillation and decadal variability.

4 1. Introduction AOGCM

5 1. Introduction Typhoon Sinlaku 09/06/2002 cyclone-ocean system hurricane-ocean model SST difference AVHRR SST composite image for hurricane Gert Sep. 1998

6 1. Introduction Necessity to develop the coupled regional air-sea model Climate variability in China is notably related with coastal ocean condition Besides the large-scale sea-land contrast of sea level pressure, the local air-sea interaction and sea-land thermal contrast over the SCS and the Indochina Peninsula regions may have effects on the summer monsoon. The CGCMs have certain ability to simulate the regional climate over East Asia and China, and there are markedly differences of summer precipitation between the observation and CGCM results The coarse resolution of OGCMs in CGCMs makes the simulations over the coastal sea quite roughly, and can not well depict the air-sea interaction over East Asian coastal sea. It is necessary to develop a coupled regional air-sea model for investigating the regional climate, especially East Asian monsoon.

7 Our plan: Regional Atmospheric Model (P σ -RCM,Regcm3) Wave. Model (WAM; WAVEWATCH-III) COUPLER Regional Ocean Model (POM) Land Process

8 2. Coupled Model description (P( σ -RCM; POM) The regional climate model: P-σRCM first developed by Kuo and Qian (1981; 1982) and improved by Qian(1985,1993), Zhang and Qian (1999a) and Liu and Qian (1999), is used to simulate atmospheric component in the coupled system. Numerical simulations (Liu and Qian, 1999; Wang and Qian, 2001) have shown that the P-σRCM has fair ability to simulate large-scale East Asian summer monsoon circulation. horizontal resolution of E-145 E, 5 S-45 N 5 vertical levels

9 2. Coupled Model description (P( σ -RCM; POM) The Princeton University s regional ocean model (POM) is used to simulate oceanic component in the coupled system (Blumberg and Mellor 1987). The version used in this study is improved by Chu and Chang (1997), Qian (1998,2000), Zhang and Qian, 1999b; Ren and Qian, 2000a; b. and tuned more suitable for the East Asian coastal sea region. horizontal resolution of E-145 E, 5 S-45 N 9 vertical levels σ vertical coordinate

10 2. Coupled Model description (P( σ -RCM; POM) atmosphere SST WIND STRESS HEAT FLUX FRESH WATER FLUX OCEAN Coupling technique

11 Experiment design uncoupled P-σRCM NCEP NCAR 1998 daily data and weekly SST fields as initial and boundary conditions. Three experiments from May to August uncoupled POM ocean temperature and salinity: Levitus 1982 data NCEP NCAR fluxes data Coupled P-σRCM and POM system

12 3. Simulation results the southwest monsoon flow illustrated by the 850 hpa wind obs cou Latitude-time sections of southwest wind (u>0, v>0) averaged over 105 E-120 E at 850 hpa for (a) NCEP NCAR and (b) coupled model simulation. Contours greater than 3 m s-1 are shaded (contour interval: 1.5 m s-1)

13 3. Simulation results and the WPSH represented by the 500 hpa geopotential height field. obs cou Latitude-time sections of the WPSH averaged over 120 E-130 E at 500 hpa (unit: gpdm for (a) NCEP NCAR and (b) coupled model simulation. Contours greater than 588 gpdm are shaded

14 3. Simulation results Obs. May June July Aug Observed monthly precipitation (1x100 mm month-1) for 1998: (a) May; (b) June; (c) July; (d) August. Contours greater than 2x100 mm month-1 are shaded

15 3. Simulation results coupled. May June July Aug The same as the previous slide, but for the coupled model simulation

16 Obs. SST SST Cou. SST before May during May after May 31-June 6 NCEP NCAR (left column) SST and coupled model simulated (left column) before, during and after the SCS summer monsoon onset in Contours greater than 30ºC are shaded

17 3. Simulation results Comparison between the coupled and the uncoupled model simulations (a) (b) (c) (a) Coupled model simulated low-level atmosphere wind field in August 1998 (b)corresponding difference between coupled and uncoupled P-σRCM simulation; (c) SST difference between coupled model and uncoupled POM simulation, averaged from June to August.

18 4. Investigation of the differences between the coupled and uncoupled experiments the cold drift of SST F=αF obs +(1- α)f cou (1) F represents the passed information, (wind stress or net heat flux) α is the weighting coefficient with the value ranging between 0 and 1. (α= 0.25,0.5,0.75) α= 0: full coupling of F α= 1: no coupling of F

19 F: wind stress ºC SST: 0.25 cm s -1 cm s -1 u v Time series for the absolute value of differences in SST (a) and surface current component u (b), v (c) between coupled model and uncoupled POM. Three lines are corresponding to three α values (0.25, 0.50, 0.75) for wind stress in formula (1)

20 F: net heat flux Time series for the absolute value of differences in SST (a) and surface current component u (b), v (c) between coupled model and uncoupled POM. Three lines are corresponding to three α values (0.25, 0.50, 0.75) for net heat flux in formula (1

21 5. Conclusions It is shown that the coupled model system simulates well the onset of the SCS monsoon, the time evolution of the low-level southwest flow and the WPSH from May to August in The model has certain ability to represent the northward migration of the monsoon rain belt, however, has a southward bias in August. The northward movement of rain belt from July to August is not correctly simulated enough in coupled system compared with the observation. The coupled simulation results show that the SST becomes warmer in the SCS and cooler in the BOB during the SCS onset period. the cold drift of the simulated SST in the coupled model. The reason for that is possibly the disagreement of the heat fluxes produced by the atmosphere model with ones that drive POM.

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