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Fujio Kimura and Tuneo Kuwagata

Abstract

The thermally induced local circulation over a periodic valley is simulated by a two-dimensional numerical model that does not include condensational processes. During the daytime of a clear, calm day, heat is transported from the mountainous region to the valley area by anabatic wind and its return flow. The specific humidity is, however, transported in an inverse manner. The horizontal exchange rate of sensible heat has a horizontal scale similarity, as long as the horizontal scale is less than a critical width of about 100 km.

The sensible heat accumulated in an atmospheric column over an arbitrary point can be estimated by a simple model termed the uniform mixed-layer model (UML). The model assumes that the potential temperature is both vertically and horizontally uniform in the mixed layer, even over the complex terrain. The UML model is valid only when the horizontal scale of the topography is less than the critical width and the maximum difference in the elevation of the topography is less than about 1500 m.

Latent heat is accumulated over the mountainous region while the atmosphere becomes dry over the valley area. When the horizontal scale is close to the critical width, the largest amount of humidity is accumulated during the late afternoon over the mountainous region.

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Fujio Kimura and Tsuneo Kuwagata

Abstract

A new concept of a thermally induced local circulation is presented by numerical and observational studies. This wind system transports a low-level air mass from a plain to a basin, passing over a mountain ridge. The characteristics of the wind system are investigated using two- and three-dimensional numerical models.

Upslope winds develop over the mountain slopes surrounding the basin until late afternoon. These winds are composed of separate individual circulations both inside and outside the basin. The atmospheric temperature in the boundary layer within the basin becomes higher than that outside, so that the surface pressure becomes lower at the bottom of the basin than that outside.

At dusk, the thermal forcing due to the surface heat flux decreases, weakening the upslope winds, and then a plain-to-basin wind develops over the mountain ridges due to the pressure difference formed in the daytime. The plain-to-basin circulation is generated when the altitude of the mountain range is almost equal to or less than the maximum mixing height developed over the plain. Higher mountain ranges act as potential barriers of the circulation.

The plain-to-basin winds are most remarkable when the horizontal scale of the basin is less than approximately 100 km and the height of the mountain range is approximately equal to the maximum mixing height. For larger horizontal scales, the velocity of the plain-to-basin wind is weaker.

Two observational examples of the plain-to-basin wind are presented. The first example is known as a part of the system that carries pollutants from the Tokyo area to the Saku Basin and develops over a mountain pass in the evening. The other wind system develops during the afternoon over a valley that connects a basin to a plain. This wind system is observed for the first time in the present work.

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Fujio Kimura and Yugo Shimizu

Abstract

The authors present a linearized model of the heat transfer between the soil layer and the atmosphere. Using this model, the moisture availability at the surface can be estimated from the diurnal variations of the soil surface temperature and downward radiation, when the bulk exchange coefficient is given. The model can also estimate the diurnal variations of the sensible and latent heat fluxes. If the moisture availability is known, the bulk exchange coefficient can be estimated in a similar manner.

Two different methods are presented. The first method estimates the moisture availability from the difference between the daily mean air temperature and soil surface temperature. The second method can estimate the moisture availability from the daily variation of the soil surface temperature alone, without use of the air temperature. The accuracy of the heat fluxes by the second method is still quite high, even if they are estimated from the soil surface temperatures observed only twice daily. These methods are available only on bare-soil surfaces.

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Tomonori Sato and Fujio Kimura

Abstract

The roles of the Tibetan Plateau (TP) upon the transition of precipitation in the south Asian summer monsoon are investigated using a simplified regional climate model. Before the onset of the south Asian monsoon, descending flow in the midtroposphere, which can be considered as a suppressor against precipitation, prevails over northern India as revealed by the NCEP–NCAR reanalysis data. The descending motion gradually weakens and retreats from this region before July, consistent with the northwestward migration of the monsoon rainfall. To examine a hypothesis that the dynamical and thermal effects of TP cause the midtropospheric subsidence and its seasonal variation, a series of numerical experiments are conducted using a simplified regional climate model. The mechanical effect of the TP generates robust descending flow over northern India during winter and spring when the zonal westerly flow is relatively strong, but the effect becomes weaker after April as the westerly flow tends to be weaker. The thermal effect of the TP, contrastingly, enhances the descending flow over north India in the premonsoonal season. The descending flow enhanced by the thermal effect of the TP has a seasonal cycle because the global-scale upper-level westerly changes the energy propagation of the thermal forcing response. The subsidence formed by the mechanical and thermal effects of the TP disappears over northern India after the subtropical westerly shifts north of the plateau, the seasonal change of which is in good agreement with that in the reanalysis data. The retreat of the descending flow can be regarded as the withdrawal of the premonsoon season and the commencement of the south Asian monsoon. After that, the deep convection, indicating the onset of the Indian summer monsoon, is able to develop over north India in relation to the ocean–atmosphere and land–atmosphere interaction processes. Northwest India is known to be the latest region of summer monsoon onset in south Asia. Thus, the thermal and mechanical forcing of the TP has great impact on the transition of the Indian monsoon rainfall by changing the midtropospheric circulation.

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Takao Yoshikane and Fujio Kimura

Abstract

The formation mechanisms of both the South Pacific convergence zone (SPCZ) and the baiu front are investigated using a regional climate model. Some idealistic numerical experiments are carried out assuming zonally uniform and temporally constant atmospheric fields obtained from ECMWF analysis data as initial and lateral boundary conditions. A rainfall zone similar to the SPCZ is reproduced using a zonal mean atmospheric field of the Southern Hemisphere (SH) summer. The simulated SPCZ in the idealized model framework is highly sensitive to the variation of SST during 1997–98 in a manner similar to observation. The SPCZ is extremely weak in an experiment under the zonal mean field of the Northern Hemisphere (NH) early summer. Experiments with a different intensity of zonal wind speed and baroclinicity suggest that a mild zonal wind (weak baroclinicity) weakens the precipitation of the SPCZ and even occasionally suppresses precipitation when it is too weak. The heat contrast between the Australian continent and the South Pacific Ocean contributes to form another rainfall zone when the zonal flow is very weak. Under these conditions, the SPCZ becomes unclear, and the rainfall zone appears from the southeastern part of Australia to the east of New Zealand. The latter rainfall zone will be intensified if the orography in the Australian continent is magnified. This rainfall zone is formed by the heat contrast between land and ocean and is somewhat similar to the baiu front. A continent as large as Eurasia creates a better-defined rainfall zone, even under stronger zonal flow. The baiu front seems to be a rainfall zone caused by the heat contrast between land and ocean that differs from the SPCZ in its formation mechanism.

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Tomonori Sato and Fujio Kimura

Abstract

Convective rainfall often shows a clear diurnal cycle. The nighttime peak of convective activity prevails in various regions near the world's mountains. The influence of the water vapor and convective instability upon nocturnal precipitation is investigated using a numerical model and observed data. Recent developments in GPS meteorology allow the estimation of precipitable water vapor (PWV) with a high temporal resolution. A dense network has been established in Japan. The GPS analysis in August 2000 provides the following results: In the early evening, a high-GPS-PWV region forms over mountainous areas because of the convergence of low-level moisture, which gradually propagates toward the adjacent plain before midnight. A region of convection propagates simultaneously eastward into the plain. The precipitating frequency correlates fairly well with the GPS-PWV and attains a maximum value at night over the plain. The model also provides similar characteristics in the diurnal cycles of rainfall and high PWV. Abundant moisture accumulates over the mountainous areas in the afternoon and then advects continuously toward the plain by the ambient wind. The specific humidity greatly increases at about the 800-hPa level over the plain at night, and the PWV reaches its nocturnal maximum. The increase in the specific humidity causes an increase of equivalent potential temperature at about the 800-hPa level; as a result, the convective instability index becomes more unstable over the plain at night. These findings are consistent with the diurnal cycle of the observed precipitating frequency.

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Tsuneo Kuwagata and Fujio Kimura

Abstract

The thermally induced circulation in a deep valley during fair weather and weak synoptic wind conditions is simulated by a two-dimensional numerical model, in order to investigate the daytime planetary boundary layer evolution observed in the Ina Valley, a deep, two-dimensional valley in Japan. The numerical model can simulate the observed structure of the PBL fairly well, along with the daytime variations of the observed valley surface air temperature and surface pressure.

The numerical simulations suggest that the thermally induced cross-valley circulation creates a two-layer PBL structure. That is, a turbulent mixed layer develops due to sensible heating from the surface, reaching to heights of about 500–1000 m above the valley floor, while a quasi–mixed layer is formed above the turbulent mixed layer by the heat transport of the cross-valley circulation. The quasi–mixed layer is a new feature of the PBL. The upper limit of the quasi–mixed layer corresponds to the top of the cross-valley circulation, being somewhat higher than both sides of the mountains. The quasi–mixed layer can be clearly distinguished during the daytime in a deep valley having a depth of greater than about 1500 m. Since the quasi–mixed layer has a slightly stable stratification, the magnitude of the coefficient of vertical turbulence in this layer is much less than that in the turbulent mixed layer.

The results of the simulations reveal that the thermally induced cross-valley circulation transports heat from the mountainous regions to the central part of the valley, while water vapor is transported in the opposite manner. The potential temperature becomes horizontally uniform during the afternoon, except in the shallow layer of the upslope flow along the side slopes. On the other hand, the daytime distribution of specific humidity in the valley is rather complex, being affected not only by the cross-valley circulation, but also by the ambient wind along the direction of the cross valley. Water vapor tends to be accumulated over the mountainous regions during the daytime, resulting in the formation of cumulus clouds. Visible images observed by the NOAA satellite confirm the development of cumulus clouds over the mountainous regions in the Ina Valley during the afternoon.

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Tomonori Sato and Fujio Kimura

Abstract

Simple numerical experiments using a two-dimensional model were conducted to investigate the diurnal variation of water vapor in the lee of a mountain range, which strongly affects nighttime precipitation. In the daytime, moisture increases around the convergence zone in the lee of the mountain were formed by thermally induced local circulations in moderate ambient wind. Then, a region of enhanced precipitable water vapor (PWV) was formed. The region of enhanced PWV was transported to the lee side of the mountain by the ambient wind and finally reached the plain during the night. The convective instability in the lower troposphere and the possibility of deep convection were controlled by the advection of water vapor in the lee of the mountain, especially by the position of the enhanced PWV region. As a result, the diurnal cycle of the convective instability showed a phase difference between the mountainous region and the plain. The profile of equivalent potential temperature tended to be unstable in the lee of the mountain at night and contributed to the production of nighttime precipitation. The propagation speed of the enhanced PWV region was roughly determined by the ambient wind speed.

Some sensitivity experiments using bulk microphysics have suggested that the propagation speed of the enhanced PWV region is insensitive to the occurrence of a deep convection. The enhanced PWV region generated over a wider mountain range tends to travel farther, retaining its magnitude even until night. The water vapor transport process presented in this study agrees well with the actual diurnal variations of rainfall observed in many regions over land.

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Tsuneo Kuwagata and Fujio Kimura

Abstract

The development process of the daytime boundary layer under fair weather and weak synoptic wind conditions was observed in the Ina Valley, a deep two-dimensional valley in Japan. The daytime boundary layer over the bottom of the valley consisted of two sublayers. The lower sublayer is a turbulent mixed layer that reached to heights of 500–1000 m above the surface. The upper sublayer is formed by the local subsidence, which is part of the thermally induced cross-valley circulation, remaining a slightly stable stratification during the daytime. The specific humidity did not become vertically uniform in the upper sublayer due to the weakness of the turbulent mixing.

The heating rate of the boundary layer was larger over the valley bottom while smaller over the mountainous areas. The observed results suggest that the thermally induced cross-valley circulation (e.g., upslope flow along the side slopes) plays a role in the heat transport from the mountainous regions to the central part of the valley. The structures of the boundary layer obtained during these observations were also consistent with previous results observed in other basins and valleys.

The cross-valley circulation prevailed until the early afternoon. However, an up-valley wind along the valley developed during the late afternoon and continued to flow until midnight. The intensity of the up-valley wind increased with increasing the thermal contrast between the coastal and inland regions in central Japan.

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Hiroyuki Kusaka and Fujio Kimura

Abstract

A single-layer urban canopy model is incorporated into a simple two-dimensional atmospheric model in order to examine the individual impacts of anthropogenic heating, a large heat capacity, and a small sky-view factor on mesoscale heat island formation. It is confirmed that a nocturnal heat island on a clear, calm summer day results from the difference in atmospheric stability between a city and its surroundings. The difference is caused by anthropogenic heating and the following two effects of urban canyon structure: (i) a larger heat capacity due to the walls and (ii) a smaller sky-view factor. Sensitivity experiments show that the anthropogenic heating increases the surface air temperature though the day. (This factor strongly affects the nocturnal temperature, and the maximum increase of 0.67°C occurs at 0500 LST.) The larger heat capacity due to the walls decreases the daytime temperature and increases the nocturnal temperature. (The maximum increase of 0.39°C occurs at 0600 LST.) The smaller sky-view factor increases the temperature though the day, particularly during the first several hours after sunset. (The maximum increase of 0.52°C occurs at midnight.) In urban areas, this factor results in uniform cooling that occurs at a constant rate. The impact of the canyon structure is shown to be as significant as anthropogenic heating.

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