Nonconvective partial condensation in a coarse grid box of low-resolution climate models has been parameterized in terms of a heavy-tailed probability density function of “liquid water” relative humidity. In our diagnostic scheme, temperatures of the cloud and clear-air parts in a model grid box are assumed to be equal, so that a strong buoyant force does not act on the clouds. When the present diagnostic scheme is adopted, nonconvective stratiform clouds begin to form at grid-averaged relative humidifies lower than 70%. Numerical experiments on the baroclinic wave growth in a midlatitude β-plane channel have shown that our scheme brings earlier occurrence and larger amount of precipitation than the “all-or-nothing” scheme or the Le Treut-Li (1991) scheme. Moreover, it has been indicated that our scheme produces nonconvective precipitation not only along the warm and cold frontal regions, but also in the warm sector of an occluding extratropical cyclone.
The formation mechanism of a systematic vertical-velocity reversal over East Asia was studied during the cold season by using MU radar data and global analysis data. Although active gravity waves may modify the vertical wind pattern in the stratosphere, clear reversal patterns are also observed in the periods when gravity waves are quiet. The frontogenesis function is positive in all the reversal periods. These facts suggest that the reversal is a part of the frontogenetically forced secondary circulations associated with an upper tropospheric jet streak. Calculation of the geostrophically forced secondary circulation following frontogenesis theory supports this conclusion.
Numerical simulations were used to investigate the mechanisms of the physical and biological interactions between the terrestrial ecosystems and climate on a regional scale. Also, a study was conducted on how the regional interactions influence seasonal and interannual variations of regional atmospheric carbon dioxide concentrations. The numerical simulation was performed using a physical regional climate model, including sophisticated biological land surface processes. The experimental area is the Japanese Islands and surrounding area. The regional climate model can estimate the nonlinear physical and biological interactions between terrestrial ecosystems and the atmosphere using a short time step (a few minutes), and a fine grid scale (about 30km). Experimental time integrations were performed for six years and five months, with the results of the last six years being examined. The model reproduced the seasonal variations of meteorological elements, and also reasonably well reproduced the heat and water budgets for the land surface for each year. The interannual variations of atmospheric carbon dioxide concentrations calculated by the model exhibited a characteristic phenomenon of stepwise increase in the lower troposphere similar to that found in data observed over Japan. The model results suggested that the characteristic phenomenon in the lower tropospheric carbon dioxide concentrations over Japan was related to the interannual variations of vegetation activity on and around the Japanese Islands. The vegetation activity was influenced by the interannual variations of climate around the Japanese Islands. From examinations of the principal elements that influence the vegetation activity, it was suggested that interannual variations of downward short-wave radiation over the land surface during the experimental period were mainly responsible for the interannual variations of vegetation activity. Interannual variations of vegetation activity influenced the interannual variations of the net carbon dioxide flux between the land surface and the atmosphere. In turn, these variations influenced the interannual variations of carbon dioxide concentrations in the lower troposphere over Japan.
During the Baiu period in 1996 the enhancement of rainfall in a slow southward-moving band-shaped convective cloud system was observed in the downwind side of Yaku-shima Island by the PPI-radar of Tanega-shima Meteorological Observatory and in some cases the dual-Doppler radar system of Nagoya University. Common atmospheric conditions in these cases were strong low-level west-southwesterly winds and stability which led to a Froude number between 0.3 and 0.7. Mainly on the basis of a case study on the radar-echo band observed on June 22 and numerical simulation on airflows modified by the orographic effect of Yaku-shima Island, the orographic enhancement of rainfall in the downwind side of the island is considered to have been caused by the combination of the following processes. (1) Convective clouds in the slow southward-moving system developed in the zone of low-level horizontal convergence intensified due to winds modified by the orographic effect of the island and they produced intensive rainfall in the downwind side of the island. (2) New convective clouds formed and developed in the zone of low-level horizontal convergence over the sea north of Yaku-shima Island, which was caused by the confluence of ambient winds around the island and winds modified by its orographic effect. The line of convective clouds which formed to the southeastern side of the slow southward-moving system mentioned in (1) produced rainfall in its downwind side. (3) In the slow southward-moving system, precipitation particles in the new cloud line are provided to convective clouds by ambient winds which turned around the southern end of the island. This process was observed as the merging of the two bandshaped radar-echoes and would have caused the efficient formation of precipitation in the downwind side of the island.
During the Baiu season (17 June-8 July 1991) we carried out simultaneous tropospheric observation by using the MU (Middle and Upper atmosphere) radar (VHF band, Kyoto University) and meteorological radars (C band of Osaka Meteorological Observatory, X band of Hokkaido University and C/Ku band of Kyoto University). Vertical distributions of three components of wind field and precipitation particles were observed by the MU and C/Ku-band radars, respectively. The C- and X-band radars were used to investigate horizontal distributions of precipitating clouds in the meso-α and -β scales, respectively. Several meso-β and -γ-scale cloud systems were observed around a meso-α-scale cyclone center during 4-5 July when rainfall was the strongest in the whole observational period. They were divided into two groups of convective clouds i) near a surface warm front and ii) near a surface cold front, and iii) one group of stratiform clouds on the north-western side of the surface cold front. In i), a remarkable updraft inside a precipitating cloud extending up to an altitude of 14km was produced by a convergence (inflows coming from the front and rear of the precipitating cloud) at an altitude of 4-5km and by strong southerly wind in the middle troposphere. In ii), a narrow rainband with gust front was seen at the leading edge of the surface cold front.Two meso-γ-scale rotor circulations were found in front of and inside the rainband, respectively. In iii), south-easterly (north-westerly) ascent (descent) flows were observed above (inside/under) the cold frontal surface extending up to an altitude of about 9km. Below the cold frontal surface, there was a dry region without precipitation, and a part of the descending westerly flow returned to the back of the precipitating cloud. In this study, vertical structures of meso-β and -γ-scale cloud systems with characteristic wind flows as mentioned above were revealed by the detailed three components of wind field in both clear and precipitating atmosphere. They were presented as smaller cloud systems in the hierarchical structure of cloud clusters near the meso-α-scale cyclone in the central region of the Japan Islands.
It is found that the appearance of the western Pacific (WP) pattern and the Pacific/North American (PNA) pattern in the northern winter is statistically related to a biennially oscillating sea surface temperature (SST) in the South China Sea (SCS) and associated precipitation variability over the tropical western Pacific in the following way; (1) When precipitation is strongly suppressed (enhanced) east of the Philippines at the warm (cold) phase of NINO4 SST, the WP (the inverse WP) pattern tends to appear. This is qualitatively consistent with a local Hadley circulation theory. (2) When negative (positive) precipitation anomalies are small east of the Philippines or when those are shifted westward at the warm (cold) phase of NINO4 SST, the PNA (the inverse PNA) pattern tends to appear rather than the WP (the inverse WP) pattern. (3) The above variability of the precipitation in the tropical western Pacific corresponds to that responsible for the biennial oscillation in the SCS SST and the neighboring SST.