Precipitation samples were collected in a large-scale region (more than 2,000 km from the north to the south of the Tibetan Plateau) and a middle scale region (with an area about 9,000 km2) in the Tibetan Plateau to study the impact of monsoon precipitation on stable isotopes. In the large-scale region, samples of precipitation and river water were collected at several stations from the north to the south along the Qinghai-Tibet Highway on the plateau. Results show that the Indian monsoon not only influences the spatial variation of stable isotope in precipitation, but also the relationship between δ18O and δD. Deuterium excees (d) in areas where precipitation is directly controlled by monsoon precipitation is lower, while in the northern Tibetan Plateau, d is in higher value due to the influence of continental air mass and strong local hydrological recycling. A detailed study on oxygen stable isotope in precipitation was carried out on the middle scale in the Nagqu Basin area. The temporal variation of δ18O in daily precipitation at 6 sites shows quite similar trends, indicating that δ18O in precipitation in the middle Tibetan Plateau is mainly controlled by large-scale synoptic condition, rather than the local meteorological factors. The spatial variation of δ18O in precipitation in the basin agrees with the Tibet-scale spatial variation trend and local hydrological recycling. Temporal variations of δ18O in precipitation also show a close relation to the movements of monsoon precipitation. The onset of monsoon in the earlier summer leads to low δ18O value. There is a negative relation between δ18O and SWI (south wind index). High SWI, which represents intensive transport of moisture from south with the monsoon movement, is coincided with lower δ18O values, while low SWI, which represents moisture from inland air mass or local evaporated moisture, is associated with higher δ18O values.
The oscillation of separation distance between centers of interacting binary vortices of typhoon-scale in a quiescent environment is investigated numerically with a vorticity equation. It is found that the separation distance between interacting vortices exhibits a tendency to oscillate with time regardless of whether they merge eventually or repel each other. The amplitude of oscillation depends on the initial separation distance as well as the structure of the vortex. The beta-effect has little influence on it during the first couple of days, but becomes important beyond that, due to the secondary circulaions. The maximum amplitude reaches more than one hundred kilometers for the case of strong vortices with a moderate size. The period of oscillation, typically one day or longer, in general, increases with the initial separation distance and shows an increasing tendency with time for foxed initial separation distance. With a simple analysis on the angular momentum, the oscillation of separation distance is described in terms of the eddy-mean interaction based on the plane polar coordinate system of which the origin is the center of binary vortices. In agreement with the previous studies, the analysis indicates that for paired vortices that rapidly merge together (repel each other) the anomaly or the deviation from the axisymmetric part, in average, has a phase line of inward-right tilt (inward-left tilt) near the origin. However, in a case where the vortices do not readily merge or repel in a short time range but maintain the temporal oscillation in the separation distance with a large amplitude, the phase configuration of the anomaly exhibits the alternation between the inward-right and inward-left phase tilt.
As a contribution to the GAME/HUBEX program, we used historical data and the Regional Climate System Model (RCSM) to analyze and simulate precipitation and streamflow in the Xixian basin. Historical data for the period 1982 to 1988 indicates that peak precipitation and streamflow occurs during the summer and early fall, when this region is affected by the East Asian summer monsoons and typhoons, respectively. In preparation for long-term coupled atmospheric and streamflow model simulations, we calibrated the RCSM’s semi-distributed hydrologic model (TOPMODEL) for the Xixian basin using observations from 1982 to 1984 and validated for the period 1985 to 1988 with good results. Long-term hydroclimate simulations generated for the period January 1979 to December 1983 using the RCSM captured important hydroclimate characteristics of the region. The simulated seasonal precipitation and streamflow variations agree well with observations during late fall to spring. Summertime precipitation and streamflow were overestimated in the hindcast. The over-estimated precipitation may be due to the input large-scale forcing and the difficulty in simulating convertive precipitation during the monsoon season using a mesoscale atmospheric model. Coupled modeling of regional climate and streamflow is a relatively new capability. Implementation of this technique to river basins in the East Asia region will result in an increased predictability of water resources for this region.
The performance of current mesoscale numerical models is evaluated in a case of model intercomparison project (COMPARE III). Explosive development of Typhoon Flo (9019) occurred in the case in September 1990 during the cooperative three field experiments, ESCAP/WMO-led SPECTRUM, US-led TCM-90,and former USSR-led TYPHOON-90 in the western North Pacific. Sensitivity to initial fields as well as impact of enhanced horizontal resolution are examined in the model intercomparison. Both track and intensity predictions are very sensitive to the choice of initial fields prepared with different data assimilation systems and the use of a particular synthetic tropical cyclone vortex. Horizontal resolution enhanced from 50 km through 20 km down to a 10 km grid has a large impact on intensity prediction. This is presumably due to a better presentation of inner structure with higher resolution. There is little impact on track prediction in this target period when the typhoon was in its before-recurvature stage. While most models show large biases in underestimating central pressure deepening, some of the participating models with a particular initial field succeed in reproducing qualitatively the time evolution of central pressure, including slow deepening in the first half and rapid deepening in the second half of the simulation period of 72 hours. However, differences leading to different intensity predictions among models have yet to be identified. Intercomparison of the simulation results shows that wind field has a close relationship with precipitation distribution. This suggests that better prediction of precipitation distribution is crucial for better prediction of wind field, and vice versa. Through the COMPARE III experiments, it has become clear that precise simulation of tropical cyclone structure, especially in the inner-core region, is very important for accurate intensity prediction. Consideration, therefore, should be given to this point, when improvements in resolution, initialization, and physics of numerical models for tropical cyclone intensity prediction are reviewed.
A new mesoscale-convection-resolving model (MCRM) for tropical cyclones (TCs) is proposed along a line of Yamasaki’s (1986) model in which mesoscale organized convection (MC) is resolved by the grid, but cumulus-scale convection (CC) is treated as the subgrid-scale. The most significant difference from Yamasaki’s model is that Kuo’s (1965) formulation is used to represent the CC-effects. Some important modifications from Kuo are made: (1) it is assumed that part of moisture converged in an air column is used for CC and that the ratio depends on low-level convergence and the degree of latent instability; (2) the ratio of moisture consumed for heating to moisture redistributed by CC is assumed to be significantly larger than that given by Kuo; (3) turbulent and dynamic entrainment and detrainment are taken into account so that the vertical profile of heating, which is crucial to MC, may be effectively controlled; (4) cloud water and rainwater of the subgrid-scale are treated with prognostic equations. Numerical experiments are performed with an axisymmetric model having a horizontal grid size of 10 km. The results are discussed compared with those from a 1 km grid model that can resolve CC. It is shown that the new model can simulate important features of MC and a TC, provided that the values of model parameters are specified properly. The CC-effects are made clear by comparison with a case that does not include them. It is shown that CC has two different effects, depending on the low-level relative humidity. One is to give realistic growth of MC through the upward transports of heat and moisture under a very humid condition, and the other is to enhance the formation and growth of MC when the air is not very humid. This study suggests that though Kuo’s parameterization was not proposed as that representing the CC-effects, a reasonable model can be developed when Kuo’s formulation is adopted in the framework of the MCRM. Important aspects of the model that contribute to realistic simulation of MC and a TC are discussed in comparison with those of Yamasaki’s model.
The spatial and temporal evolution of the moisture field over the subtropical northwest Pacific during the summer of 1995 is investigated using daily total precipitable water from combined SSM/I-TOVS data, and pentad upper tropospheric humidity (UTH) data, in conjunction with NCEP reanalysis data. From analysis of the combined water vapor field, the westward movement of a dry airmass is observed along the 20-30°N latitude zone from near the dateline to the south of Japan throughout the summer of 1995. Extended EOF analysis of total precipitable water reveals that the westward moving pattern takes place in association with an expanding North Pacific subtropical high, maintaining an oscillatory component exhibiting a period of some 15-25 days. a concomitant dipole-like oscillating anomalous circulation with approximately a 20-day period between the South China Sea and south of Japan appears to influence the westward expansion of the subtropical high or vice versa. The analysis also suggests that the fluctuations of the North Pacific high are in response to a local Hadley-type circulation which is induced by westward-moving anomalous convection episodes along 10-20°N.
The processes associated with the diurnal variation of precipitable water over the “semi-basin” were studied using GPS and radiosonde data. Precipitable water over the semi-basin exhibited a pronounced diurnal variation, with a maximum around 18-20 JST and a diurnal range of 10 to 25 mm. The variation of precipitable water agreed well with the variation of ESE component of the surface wind, due to a thermally induced local circulation. The diurnal variation of water vapor was recognized in two different layers of Layer L (surface to 1.5 km), and Layer H (1.5 km to 3.0 km). In the increasing period (09:00 to 20:00 JST), precipitable water increased simultaneously all over the south part of the mountains in the north Kanto District. The variation of precipitable water in the Layer L would be, in large part, controlled by convergence of surface wind. Furthermore, the amount of water vapor in Layer H over the semi-basin that is located on the lee side of mountains increased abruptly from evening to midnight. It was expected that the water vapor that had been transported into the mountains due to the thermally induced local circulation traveled into the semi-basin by ambient wind and/or return flow, and the moisture advection caused the increase of water vapor in Layer H over the semi-basin. In the decreasing period (21:00 to 08:00 JST), precipitable water decreased simultaneouly all over the GPS network using the present study. The decrease in water vapor coincided with increase in potential temperature in the Layer H. The large scale subsidence due to the Pacific High seemed to be necessary for the prominent diurnal variation of precipitable water over the semi-basin.
Temperature lapse rate, potential vorticity, Brunt-Väisälä frequency, along with VHF wind profiler observations were employed to examine the temporal variation of the tropopause height and stratospheric air intrusion during an event of tropopause folding. The present study suggests that the tropopause defined by 1.6 PVU is more adapted to analyzing stratospheric air intrusion, compared with the tropopause defined by temperature lapse rate or Brunt-Bäisälä frequency. However, the latter two tropopause definitions are more relevant to localizing stratospheric air intrusions into the troposphere than the potential vorticity when the air is intruded in a slantwise direction over the observation site. The intruded air in a slantwise direction is observed as the temperature lapse rate and Brunt-Väisälä frequency anomalies, corresponding to the temperature anomaly, which is predicted as the low level tube by Bithell et al. (1999). The present study suggests that the slantwise intrusion is a type of low level tube developed during tropopause folding.