Fujio Kimura

Diurnal Cycle of Convective Instability around the Central Mountains in Japan during the Warm Season

Abstract Convective rainfall often shows a clear diurnal cycle. The nighttime peak of convective activity prevails in various regions near the worlds 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 an...

Water sources in semiarid northeast Asia as revealed by field observations and isotope transport model

[1]xa0Origin of water vapor, which falls as precipitation over arid/semiarid area in northeast Asia, is investigated by modeling and observational approaches. The regional climate model evaluates the spatiotemporal variations of precipitation and water vapor budget, which are used to drive the single-layer isotope circulation model. Intraseasonal variations of δ18O in precipitation are well simulated during June-July-August of 2003. The δ18O in water vapor experiences rapid decreases according to the passage of synoptic-scale disturbances. The rapid decreases of δ18O are attributed to two processes. (1) The δ18O in air mass locally decreases over the western mountains associated with the mountain precipitation; the light vapor is advected to eastern Mongolia by the prevailing westerly wind. (2) Convective systems pass by in the vicinity of the observation site, which persistently keep the lower δ18O in the air mass. Origin of the water vapor during June-July-August in 2003 is evaluated by the colored moisture analysis in which the tracers are assigned depending on the region where the water vapor finally evaporated. Seasonal mean result indicates that the regions contributing to precipitation in Mongolia are not low-latitude regions but central Asia and western Siberia located to the northwest of Mongolia. Observed multilevel isotopic composition supports the model estimation. The moisture transport along the southwesterly wind of the Asian summer monsoon has difficulty reaching Mongolia as a monthly/seasonal mean perspective. However, eastern Mongolia and northeast China are situated on the border area between westerly wind moisture transport (by midlatitude synoptic cyclones) and southerly wind moisture transport (by Asian summer monsoon).

Impact of extensive irrigation on the formation of cumulus clouds

[1]xa0Irrigated farmland in an arid region creates a prominent heat contrast along its boundary. A statistical analysis of satellite images indicated that the frequency distribution of clouds has a clear contrast between the Hetao Irrigation District in China and the surrounding dry area in the summer. In this paper, we use a mesoscale model to clarify the impact of the extensive irrigation on the cloud formation. The numerical experiment assuming simplified irrigation indicates that the colder surface of the irrigated area induces a surface wind toward the dry area, where the surface temperature is higher. This wind is a part of a land-use-induced circulation. The circulation matures in the early afternoon and generates clouds near the border of the dry area and irrigated area.

Extensive dust outbreaks following the morning inversion breakup in the Taklimakan Desert

[1]xa0Extensive dust outbreaks occurred on the late morning of 26 March 2004 in the Taklimakan Desert. An image by the satellite AQUA shows the dust outbreaks as an enormous dust storm extending as far as 1000 km in the direction of east and west across the desert. A ground-based lidar was successful in observing the dust storm. Values of the backscattering ratio from near the ground surface to an altitude of 6 km above sea level rapidly increased as soon as the dust storm covered the lidar observation site. Numerical experiments including a diffusion experiment revealed that strong westerly winds appeared at the surface level following the morning inversion breakup and then induced the dust outbreaks. The nocturnal inversion layer prevented the strong westerly winds from spreading to the surface level until the breakup. Notable nocturnal inversion is considered to be easily formed in the Taklimakan Desert because of its deep basin topography which can accumulate cold air masses. These results show the stability of the planetary boundary layer to be an extremely important factor in dust outbreaks especially in basin deserts, in addition to the intensity of surface winds. This implies that the occurrence frequency of dust storms in the desert has a diurnal cycle.

Vegetation and topographic control of cloud activity over arid/semiarid Asia

[1]xa0Cloud patterns over arid/semi-arid area in the Northeast Asia are investigated by statistical analysis of the satellite observations in relation to the roles of topography and vegetation contrasts. In the morning, cloud frequencies are high over the mountainous areas in the northern Mongolia. In the afternoon and early evening, the frequencies increase in the entire analyzed domain. The frequencies are prominently enhanced over the slopes and mountains. The upslope wind prevails during the daytime where vegetation contrasts are not sharp or the slope is steep, even though vegetation covers the mountain which tends to cool the mountain slope. The land-cover-induced cloud formations are distinctly observed where the slope is gentle and vegetation contrast is sharp enough to form a prominent surface temperature contrast. Around the Yellow River, the cloud frequency rapidly increases in the afternoon due to the vegetation contrast, whereas it remains very low over the river. We hypothesized that the smaller sensible heat flux from the vegetated surface suppresses the development of the daytime mixed layer. Thus the cumulus clouds near top of the mixed layer are difficult to form. It is inferred from the statistical analysis that the afforestation of the desertified lands may cause the decrease of cloud frequency above it.

Numerical experiments on cloud streets in the lee of island arcs during cold‐air outbreaks

[1]xa0Numerous clouds streets often appear in the lee of island arcs during cold-air outbreak. These clouds are quite similar each other in shape and appearance. We found that two different kinds of cloud streets are coexistent in them, one is formed by a mountain and another is formed by the temperature contrast between islands and the sea. Since difference in the characteristics between them is small, it is difficult to simply discriminate by satellite images. However, two kinds of the cloud streets can be observationally classified by the statistics of the satellite images with the help of numerical experiments.

Climatic features of the water vapor transport around east Asia and rainfall over Japan in June and September

[1]xa0The climatic features of the water vapor transport (WVT), which is closely related to rainfall in the middle latitudes around east Asia, are investigated using reanalysis data in June and September. A large amount of water vapor is intermittently transported in September in association with the movement of typhoons, but continuously transported in June. We defined a typhoon term as that when a typhoon or tropical storm is in Region A (20N–40N, 120E–150E) to estimate the effect on the poleward WVT. In September, the poleward WVT through Segment B (25N, 130E–150E) in the typhoon term (38.5% of the total term) was about 84% of that in the 20-year-accumulated total term, indicating that the climatic poleward WVT is greatly affected by typhoons and tropical storms. It is concluded that the contribution of typhoons and tropical storms is indispensable for the formation of a climatic atmospheric field around east Asia in September.