Effects of Soil Moisture of the Asian Continent upon the Baiu Front

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1 Present and Future of Modeling Global Environmental Change: Toward Integrated Modeling, Eds., T. Matsuno and H. Kida, pp by TERRAPUB, Effects of Soil Moisture of the Asian Continent upon the Baiu Front Fujio KIMURA* and Takao YOSHIKANE Institute of Geoscience, University of Tsukuba, Ibaraki , Japan INTRODUCTION Although the Baiu front has been studied by many scientists from various viewpoints for a long term, the formation process of the Baiu front has not been clearly explained yet. One of the reasons is that it is still difficult to simulate the Baiu front by GCMs. Yoshikane et al. (2001) investigated the Baiu front genesis using a regional model adapted to the eastern part of the Asian continent. Their simulations showed that the thermal contrast between the continent and the ocean is quite important for the formation of the Baiu front. This not only suggests that the sensible and latent heat flux from the land surface contributes to the Baiu front genesis but implies that the surface process on the continent may affect the activity of the Baiu front. The present paper will present a brief review of their study and discuss the effects of soil moisture of the Asian continent on the Baiu front. Figure 1 shows a schematic diagram of the large-scale circulation around the Baiu front referred from Ninomiya and Akiyama (1992). In early June, the Baiu front exists along the LLJ(Low Level Jet), which is located in a convergence zone between the southwesterly flow from the periphery of the Pacific high and the northwesterly flow from the west side of the Baiu trough. The meridional gradient of water vapor and temperature is large in the eastern part of the Baiu front, i.e., east of eastern Japan. On the other hand, the gradient of the temperature is small in the western part of the Baiu frontal zone, namely the part from China to western Japan. The figure also shows the Upper-Level Jet running parallel with the LLJ east of Japan. Matsumoto (1973) and Ninomiya and Akiyama (1974) suggested that the LLJ is formed by the downward transport of horizontal momentum from the upper layer by the cumulus convections in the Baiu front. However, Chen (1982) suggested that the LLJ is caused by thermal wind adjustment as a result of the condensation heating by convection, rather than by the vertical momentum mixing. Chen et al. (1998) showed that the intensification of the LLJ is quite sensitive to the presence of the deep convection according to their numerical experiments. *Present affiliation: Frontier Research System of Global Change. 101

2 102 F. KIMURA and T. YOSHIKANE Fig. 1. Schematic figure of the planetary-scale circulation systems related to the Baiu front, referenced by Ninomiya and Akiyama (1992). MODEL Model feature The Regional Atmospheric Modeling System (CSU-RAMS) developed at Colorado State University (Pielke et al., 1992) is adopted in this study. The radiation scheme in the original RAMS-3b is replaced by a band scheme by Nakajima et al. (2000) because it has higher accuracy than that of the original version. The Arakawa-Schubert type cumulus convective parameterization scheme (Arakawa and Schubert, 1974; Numaguti, 1995) is also utilized in addition to the original RAMS parameterization, which is a modified Kuo type parameterization scheme (Kuo, 1974; Tremback et al., 1990). The vegetation and soil type are assumed to be uniform in the model domain for simplicity. The sea surface temperature (SST) is estimated from the monthly mean SST data in June 1998 (Reynolds and Smith, 1994). Assumed conditions The initial and the boundary conditions are estimated from ten-day averaged global zonal mean ECMWF data in early and late June Because, all of the meteorological variables are stationary and are functions of only the height and the latitude in the global zonal mean data, there are no atmospheric disturbances of more than the zonal wave number zero. Although these simulations are timeintegrated over 20 days, the first ten days will be defined as a spin-up duration. The second ten-day will be evaluated as the simulation result. Although the solar zenith angle in the radiation scheme is estimated according to the solar calendar,

3 Effects of Soil Moisture of the Asian Continent upon the Baiu Front 103 Fig. 2. Ten-day averaged global zonal mean data form the ECMWF objective analysis of the early (a) and the late June (b). The dark, the semi dark, and the light-shaded areas represent the zonal mean wind of more than 20 m/s, 10 m/s and 5 m/s, respectively. The contour indicates the potential temperature at 5 K intervals. variation in the solar zenith angle will be small during these periods. Atmospheric disturbances simulated in the zonal mean runs are expected to be generated by the interaction between the zonal mean field and the surface process in the domain. The vertical cross sections of the zonal mean potential temperature and zonal component of wind velocity in early and late June are shown in Fig. 2. Some differences can be seen in the zonal mean data between early and late June. In early June, the westerly flow is stronger and the Upper-Level Jet is located more southward than those in the late June. The soil layer up to 1.8 m under the ground is divided into eleven layers, and initial soil wetness is assumed to be a uniform value of 0.2, which means 20% saturated soil moisture and quite dry conditions. Soil wetness will increase if precipitation occurs, but it will become drier in the case of little precipitation over several days. In the case of the wet soil simulation, the initial soil moisture is assumed to be a linear function with 0.4 at the soil surface and 1.0 at the 1.8 m level.

4 104 F. K IMURA and T. YOSHIKANE ZONAL MEAN SIMULATIONS Features of the simulated Baiu front by the zonal mean simulation Figure 3 shows the ten-day averaged rainfall and water vapor transport of the zonal mean runs using the global zonal mean field during early June (a) and late June (b), respectively (ZM-Standard). A rainfall zone is located from the southern part of China to Japan with prominent water vapor transport in early June (Fig. 3(a)). In late June, this zone shifts northward accompanied by the intensified anticyclonic circulation over the Pacific Ocean as shown in Fig. 3(b), although the Pacific high is not completely simulated. The location of the prominent water vapor transport approximately corresponds to that of the LLJ, which is sketched in Fig. 4. Fig. 3. Ten-day averaged precipitation (shadow) and the water vapor transport (arrow) by simulation of early (a) and late June (b). The dark and light-shaded areas represent the precipitation rate of more than 20 mm/day and 5 m/day, respectively. The arrow unit indicates 1000 kg/m 2/s.

5 Effects of Soil Moisture of the Asian Continent upon the Baiu Front 105 The simulated LLJ is defined as a strong wind belt in the lower layer below 700 hpa. The definition of the LLJ is referenced by a review in the paper, which is mentioned by Nagata and Ogura (1991). The horizontal wind at the level of 200 hpa of the ZM-Standard run is shown in Fig. 4(a) (in early June) and Fig. 4(b) (in late June) with a sketch of the LLJ at 850 hpa (thick arrow). In the early June, the Upper-Level Jet is meandering, namely heading south at the eastern part of the Tibetan Plateau and then going north again further east. As a result, a trough is formed around the East China Sea and westward of Japan. As shown in Fig. 3, a rain band, which resembles the observed Baiu front, appears around Japan in early and late June. This rain band has some features in common with those of the observed Baiu front: (1) The LLJ exists below a height of 700 hpa along the rain band. (2) The potential temperature gradient across the Fig. 4. The ten-day averaged horizontal wind at 200 hpa and the position of the Low Level Jet (LLJ) at 850 hpa in the simulation for early (a) and late (b) June.

6 106 F. KIMURA and T. YOSHIKANE front in the lower layer is larger in the eastern part of the Baiu front than in the western part. (3) The Upper-Level Jet is heading southward in the eastern part of the Tibetan Plateau and going north slowly around the area of the East China Sea and Japan. (4) The precipitation zone accompanied by the LLJ is aligned parallel with the Upper-Level Jet in the eastern part of the Baiu front which is located from the East China Sea and western Japan. (5) The water vapor of the LLJ is supplied both from the tropical zone in the South China Sea and from the Indian monsoon area of the Bay of Bengal. (6) The anti-cyclonic circulation appears in the northwestern Pacific Ocean. The mountain effects on the Baiu front Numerical experiments similar to the ZM-Standard except for no mountains (ZM-No Mountain) were conducted to investigate the mountain effects of the Fig. 5. The same as Figs. 3(a) and (b), except for the simulation without Mountains.

7 Effects of Soil Moisture of the Asian Continent upon the Baiu Front 107 Tibetan Plateau on the Baiu front. The precipitation and the water vapor transport in early and late June are shown in Figs. 5(a) and (b), respectively. In the early June, a strong water vapor transport zone still appears south of Japan. In late June, the Baiu frontal zone shifts northward increasing the amount of precipitation and water vapor transport. In comparison with the ZM-Standard, the position of the Baiu front shifts to the south, and the amount of the precipitation and water vapor transport are smaller in late June. Soil moisture RUN Figures 6(a) and (b) show the ten-day mean rainfall and water vapor transport in the case of a wet soil surface. The simulated Baiu front and LLJ are shifted southwest. These results show that the soil moisture in the Asian continent affects the position of the Baiu front and the intensity of precipitation. A drier soil Fig. 6. The same as Figs. 3(a) and (b), except for the simulation with wet soil surface.

8 108 F. KIMURA and T. YOSHIKANE surface of the continent tends to shift the Baiu front to the north. The effects of the soil surface, however, do not seem to be a dominant factor in the formation mechanism and positioning of the Baiu front. The soil moisture effects are much weaker than not only that of the zonal flow but also the orographic effects. Variability in large scale circulations would make it difficult to detect the soil moisture effects in a real situation. In early June, the simulated Baiu front is shifted remarkably around Taiwan, although shifting is prominent in the other part of the Baiu front in late June. This difference is caused by the difference in the zonal flow. In early June, because the westerly flow is still strong in the northern part of the domain, the position of the Baiu front is controlled by the westerly flow. In late June, the westerly flow becomes mild over almost the entire domain, and the thermal contrast affected by the soil moisture becomes more important to the positioning of the front. CONCLUSIONS Numerical experiments were conducted to investigate the mechanism of the Baiu front using a regional model. The Baiu front accompanied by the LLJ is formed by the experiments using the zonal mean field in early and late June 1998 as the initial and boundary conditions. The simulated Baiu front has a structures similar to that of the real Baiu front, i.e., the LLJ is located parallel to the precipitation zone and to the Upper-Level Jet in the eastern part of the domain. The numerical experiments fundamentally show that the Baiu front can be formed by the deformation of the zonal mean field due to the Land/Sea contrast and the topography. Although the amount of the rainfall in the Baiu front depends on the cumulus convective parameterizations, the fundamental structure of the Baiu front does not depend on them. A comparison between the zonal mean simulations for early and late June indicates that the Baiu front is formed at a higher latitude in late June, when the westerly flow is weak and the Upper-Level Jet is located northward. The location of the Baiu front is speculated to be quite sensitive to the zonal mean flow. The Baiu front accompanied by the LLJ is also represented by numerical experiments without topography, which suggests that the Baiu front could be reproduced by two factors alone, the zonal mean field and the Land/Sea contrast. The orography including the Tibetan plateau intensifies the precipitation of the Baiu front, although their effects are significant when the westerly flow is stronger and the Upper-Level Jet is located southward. Soil moisture of the Asian Continent is also important for the strength and the position of the Baiu front, especially in the case of a miled zonal wind. The soil moisture effects, however, are weaker than orographic effects. Acknowledgments We are grateful to Prof. T. Yasunari, Dr. H. Tanaka and other members of the University of Tsukuba, Prof. T. Koike of the University of Tokyo, and the other members of the GAME Tibet project for discussions during the study. We would also like to express our thanks to Prof. T. Nakajima of the University of Tokyo, Dr. A. Numaguti of the University of Hokkaido, and Dr. S. H. Lee, Japan Atomic Energy

9 Effects of Soil Moisture of the Asian Continent upon the Baiu Front 109 Research Institute, for providing technical comments. This work was partially supported by the Global Environment Research Fund (GERF) of the Environmental Agency of Japan. The data used are provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). REFERENCES Arakawa, A. and W. H. Schubert, 1974: Interaction of a cumulus cloud ensemble with the large-scale environment. J. Atmos. Sci., 31, Chen, C., W.-K. Tao, P.-L. Lin, G. S. Lai, S.-F. Tseng and T.-C. C. Wang, 1998: The intensification of the low-level jet during the development of mesoscale convective systems on the Mei-Yu front. Mon. Wea. Rev., 126, Chen, Q., 1982: The instability of the gravity-inertial wave and its relation to low-level jet and heavy rainfall. J. Meteor. Soc. Japan, 60, Kuo, H. L., 1974: Further studies of the parameterization of the influence of cumulus convection on large-scale flow. J. Atmos. Sci., 31, Matsumoto, S., 1973: Lower tropspheric wind speed and precipitation activity. J. Meteor. Soc. Japan, 51, Nagata, M. and Y. Ogura, 1991: A modeling case study of interaction between heavy precipitation and a low-level jet over Japan in the season. Mon. Wea. Rev., 119, Nakajima, T., M. Tsukamoto, Y. Tsushima, A. Numaguti and T. Kimura, 2000: Modelling of the radiative process in an AGCM. Appl. Opt., 39, Ninomiya, K. and T. Akiyama, 1974: Band structure of mesoscale clusters associated with low-level jet stream. J. Meteor. Soc. Japan, 52, Ninomiya, K. and T. Akiyama, 1992: Multi-scale features of Baiu. The summer monsoon over Japan and the East Asia. J. Meteor. Soc. Japan, 70, Numaguti, A., M. Takahashi, T. Nakajima and A. Sumi, 1995: Development of an atmospheric general circulation model, in Climate System Dynamics and Modelling, Reports of a New Program for Creative Basic Research Studies, edited by T. Matsuno, Vol. I-3, pp Pielke, R. A., W. R. Cotton, R. L. Walko, C. J. Tremback, W. A. Lyons, L. D. Grasso, M. E. Nicholls, M. D. Moran, D. A. Wesley, T. J. Lee and J. H. Copeland, 1992: A comprehensive meteorological modeling system RAMS. Meteorol. Atmos. Phys., 49, Reynolds, R. W. and T. M. Smith, 1994: Improved global sea surface temperature analyses using optimum interpolation. J. Climate, 7, Tremback, C. J., 1990: Numerical simulation of a mesoscale convective complex: model development and Numerical results. Ph.D. Dissertation, Atmos. Sci. Paper No. 465, Department of Atmospheric Science, Colorado State University, CO 80523, 247 pp. Yoshikane, T., F. Kimura and S. Emori, 2001: Numerical study on the Baiu front genesis by heating contrast between land and ocean. J. Meteor. Soc. Japan, 79, F. Kimura ( fkimura@baro.geo.tsukuba.ac.jp) and T. Yoshikane