The level of neutral buoyancy (LNB) around the Japan Islands during the Baiu season in 2001-2005, estimated by lifting a low-level air with the maximum equivalent potential temperature in a vertical direction, is statistically examined from objective analysis data with the horizontal resolution of 20 km, in order to clarify the surrounding atmospheric conditions under which cumulonimbi can form and develop. Three and two peaks are found in the vertical profiles of the appearance rate of the LNB over the sea and on the land, respectively. The high-level (∼ 150 hPa) peak is very weak in June, and the low-level (∼ 900 hPa) peak does not appear on the land. The other peak is found at the middle level (N 700 hPa). The theoretical study shows that the LNB during the Baiu season strongly depends on the lapse rate between the low and middle levels, when the variation of the low-level temperature is within a few degrees. This is because the LNB during the Baiu season rapidly becomes higher as the lapse rate increases. The statistical results show that the lapse rate around the Japan Islands strongly depends on convective activities over upstream regions, especially the southern part of China. Therefore, strong convective activities over the Baiu frontal zone in June produce the middle-level LNB around the Japan Islands to suppress the development of cumulonimbi. In July, the northward shift of the area with strong convective activities considerably releases such suppression, associated with that of the Baiu frontal zone. The middle-level LNB also frequently appears over the other areas around the Baiu frontal zone, although it scarcely appears over the western part of the North Pacific Ocean. Therefore, the middle-level LNB that suggests the existence of cumulonimbi with a cloud top lower than several kilometers is one of the characteristic features around the Baiu frontal zone.
A single-Doppler radar velocity retrieval method, based on an optimally determined moving frame of reference, is developed. This method enables the recovery of the complete three-dimensional wind field using reflectivity, and radial wind observations, detected by a single Doppler radar over multiple volume scans. This method first computes a set of optimal moving speeds (U, V, W), which can be functions of the height. It is considered optimal in the sense that when a reference frame is traveling at this speed, as much of the radar reflectivity, interpolated onto this frame can be conserved as possible. The perturbation velocities (u′, v′, w′) are then solved simultaneously, with respect to the moving speeds, by minimizing a cost function through the three-dimensional variational (3D-Var) approach. This cost function consists of a set of weak constraints, mainly the equation for radar reflectivity conservation, the continuity equation, the geometric relationship between the radar-observed radial wind and the Cartesian winds, and a smoothness filter. Finally, by adding the optimal moving speed to the perturbation velocities, a complete three-dimensional wind field is obtained. The performance of this method is tested under different scenarios, including cases with deep convections, embedded in an environment without mean wind, with constant mean wind, with vertical wind shear of the horizontal mean wind, and with differences in the radar observational time at each grid point within one volume scan are considered. This method is demonstrated to be feasible by applying it to recover the three-dimensional wind structure of a subtropical squall line from Doppler radar data, collected during the 1987 Taiwan Area Mesoscale Experiment (TAMEX).
More than 400 upper-air soundings at different geographical locations over the Japanese Islands for various seasons reveal that vertical fine structures of temperature and water vapor, having a venical scale of several hundreds of meters, commonly exist in the troposphere between 1 and 8 km MSL. The temperature and specific humidity fluctuations associated with the fine structures range between −0.4 and 0.4 K, and between −0.6 and 0.6 g kg−1, respectively. The specific humidity fluctuations are negatively correlated with the temperature fluctuations, and the positive (negative) peaks of the former are located right below the local maxima (minima) of buoyancy frequency. The existence of such fine structures of temperature and water vapor was also confirmed by an observation using simultaneously a radiosonde and a Raman lidar, which measures water vapor independently.
Extreme precipitation events are the major causes of severe floods in China. In this study, four time series of daily, 2-day, 5-day, and 10-day annual maximum precipitation from 1951 to 2000 at 651 weather stations in China were analyzed. The generalized extreme value (GEV) distribution was used, to model the annual extreme precipitation events at each station. The GEV distribution was also modified to explore the linear temporal trends in the extreme events. The results showed that more than 12% of the stations have significant (p-value < 0.10) linear trends. Decreasing trends are mainly observed in northern China, and increasing trends are observed in the Yangtze River basin and northwestern China. The return periods of extreme precipitation have changed for stations with significant temporal trends. The 50-year event observed in parts of the Yangtze River basin, and northwestern China during 1951-60, has become a more frequent 25-year event in the 1990s. The spatial distribution of the return levels of the 651 stations are closely related to the climatic mean precipitation, and are influenced by the East Asian summer monsoon system (return levels are measures of extremity—for example, a 10 year return level is the value that can be expected to be exceeded on average once in every 10 years). The return period of extreme precipitation, that caused the 1998 severe floods in the Yangtze River basin, was also evaluated from a probabilistic perspective.
The divergent component of water vapor transports was constructed using evaporation, precipitation, and total precipitable water estimated from the Special Sensor Microwave Imager (SSM/I). The SSM/I moisture budget parameters were then compared with those from the National Centers for Environmental Prediction (NCEP), the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA40), and the Japanese 25-year Reanalysis Project (JRA25) data over the Asian monsoon region for the May to September (MJJAS) period from 1988 to 2000. The climatology of SSM/I water vapor transports clearly indicates three major water vapor sources for the Asian monsoon, i.e., the subtropical Indian Ocean and Pacific Ocean in the Southern Hemisphere, and the North Pacific Ocean. In contrast, sinks are located in the Asian summer monsoon trough, the equatorial convective zones over the Indian and western Pacific Oceans, and over the East Asian monsoon region from the northern tip of Philippine Sea to the Kuroshio extension region. These sources and sinks are linked to well-known large-scale rotational circulation features, i.e., the cross-equatorial flow associated with monsoon circulation over the Indian Ocean, the anticyclonic circulation along the western periphery of the western North Pacific High, and the cross-equatorial flow north of New Guinea. In conjunction with the fluctuation of these sources and sinks, the northward propagating climatological intraseasonal oscillations in water vapor flux convergence are evident in the South and East Asian monsoon regions in the SSM/I data. From the comparison of water budget parameters of NCEP, ERA40, and JRA25 reanalysis with SSM/I-derived features, we found that the general features of all three reanalyses are in good agreement with those from SSM/I; however, the magnitudes of water vapor transports are comparatively weaker in all three reanalyses than what SSM/I measurements suggested. In addition, much weaker water vapor transports in three reanalyses are found in the intraseasonal oscillation signals with less distinct patterns, compared to what are inferred from the SSM/I measurements.
A mid-tropospheric mesoscale cold core passed over the Sea of Japan on 14 January 2001 during a cold air outbreak. In association with the passage of the cold core, intensification and northward and southward shifts of the Japan-Sea Polar-Airmass Convergence Zone (JPCZ) were observed. In order to clarify a response of the JPCZ to a change of upper-level environment, the shift and intensification processes associated with the passage of the cold core were studied mainly using a non-hydrostatic cloudresolving model. When the cold core approached the JPCZ, the JPCZ was intensified and shifted northward. The intensification of the JPCZ is caused by a change in a decrease of convective stability associated with the passage of the cold core. As a ridge ofpotential temperature is formed along the intensified updraft of the JPCZ, a leeward region of the JPCZ is locally heated by the horizontal advection in a westerly wind, which is predominant in the upper levels. This hydrostatically reduces the low-level pressure in the north of the JPCZ, and increases the wind speed blowing into the JPCZ from the south. The divergence equation indicates that the abrupt change in the pressure as well as the adjusted wind fields cause the northward shift of the JPCZ. In contrast, the JPCZ decayed and shifted southward when the cold core moved away from the JPCZ. The pressure decrease, which occurred in the north of the JPCZ, is not maintained during the southward shift, and the pressure gradient across the JPCZ becomes relatively uniform. This results in the decrease of the wind speed blowing into the JPCZ from the south, and the JPCZ shifted southward due to the advection.
Numerical simulations of a cloud cluster over a Meiyu-Baiu front were conducted using a cloud-resolving non-hydrostatic model. The purpose was to demonstrate thermodynamic impact of the diurnal heating of the mainland China upon the nocturnal evolution of the cluster. Simulations were conducted within a domain covering eastern and southern China to reproduce land-surface heating on the southern side of the front. The case simulated was a rainband associated with the cluster, which formed on the late afternoon of 22 June 2003. The environment is characterized by a synoptic-scale southerly inflow of warm and moist air in the lower troposphere. A control simulation reproduced the rainband successfu11y, and showed that the evolution resulted from the strong latent instability over the frontal convergence zone, due to the southerly inflow of the warm and moist air. The origin of this air was the surface heat flux over a cloud-free area to the south of the front. The importance of the heating was proved by a sensitivity simulation without insolation, which failed to reproduce deep convective updrafts. The water budget analysis demonstrated that the evaporation from the ground, rather than the moisture convergence, contributed to the increase in the precipitable water before the rainband evolution. This study, thus, indicates that the continental surface, heated by insolation, has a significant impact upon the nocturnal evolution of a cloud cluster over a Meiyu-Baiu front.
A series of numerical simulations on a heavy rain episode over a Meiyu-Baiu frontal zone was conducted, to demonstrate the role of the heated landmass over mainland China on the evolution and duration of the rainfall. The case studied here was a long-lived convective rainband, which formed over the Yangtze River basin, and brought heavy rainfall reaching 380 mm in 20 hours. The synoptic-scale situation was characterized by the indistinct meander of the upper-level flow, and the significant low-level southerly inflow from a subtropical high. Simulations using a cloud-resolving non-hydrostatic model were performed within a domain covering central and southern China, to reproduce land-surface heating to the south of the front. Sensitivity experiments without shortwave radiation, without terrain, and with modified land-surface conditions, were conducted to examine the factors effective in the evolution and duration of the rainband. The control simulation reproduced the rainband, and its reproducibility was good when compared to the observational evidence. The evolution resulted from the latent instability, due to the inflow of the warm and moist air from the fine-weather area to the south of the front. The rainband was not reproduced when the surface heating effect was excluded. The duration became short as the heated area was reduced, and was represented as a function of the meridional width of the heated area, and meridional velocity component in the lower troposphere. These results indicate that land-surface heating, over fine-weather areas to the south of the front, is a crucial factor for the rainband evolution and its duration.