In order to investigate the mesoscale structures and appropriate mechanisms for their maintenance in the Mei-yu front leading edge during the TAMEX (Taiwan Area Mesoscale EXperiment) IOP 9 (15 June 1987) off the east coast of Taiwan, we measured the fine-scale in-situ data by different sensors and sensed radar data by two airborne radars mounted on the NOAA P-3 research aircraft traversing the frontal system at six different altitudes. Based upon the sudden increase of the turbulence intensities, the position of the frontal leading edge at each flight level was identified exactly and, after deducting the propagating distance of the frontal system, the composited vertical cross sections of the system in kinematics, dynamics and thermodynamics was feasible. The significant findings are: the frontal edge was parallel to the isolines of horizontal component of wind and perturbed air density and the frontal edge demonstrated a well-defined density current, the variation of thermodynamic parameters was not obvious, but the pattern of potential temperature revealed a cold core in the lower level behind the frontal edge, which coincided with the position of the heaviest air density. The probable mechanisms for the propagation of the density current and the maintenance of the frontal system were proposed to be the intense horizontal pressure gradient force from rear to front in the cold core region and the moderate convective instability at the head of the system as well as the kinetic energy transport from the mean flow.
A method of quality control (QC) to reject erroneous observational data has been developed in the Numerical Weather Prediction System of the Japan Meteorological Agency (JMA). Each observed value is processed in the QC, by evaluating its deviation from the first guess(FG) value given by the forecasted field, comparing it with a threshold value defined by statistics and rejecting it if it falls outside the threshold range. In the previous method (Static QC), the threshold value was defined as a constant regardless of the weather conditions. In the new method (Dynamic QC, in this paper), the threshold value is defined as a variable linearly depending on the local horizontal gradient and three-hour tendency of the FG. With a statistical study, we found a relationship between the deviations, and both the horizontal gradient and the tendency of FG were approximately proportional. Then this relationship was applied to the Dynamic QC. In almost all regions, the Dynamic QC reduces erroneous judgements given in the Static QC where wrong observed values were accepted in less disturbed weather conditions and right values were rejected in more disturbed conditions. Forecast experiments using the Dynamic QC revealed that the forecast scores and the fit in root mean square error (RMSE) to rawinsonde observations were improved in the tropics and the Southern Hemisphere in winter. For some cases, the improvement of the scores in Anomaly Correlation Coefficient (ACC) was equivalent to more than one days extension of the forecast term, and the ratios of fit in RMSE were improved by more than 10%. The forecast results for the Northern Hemisphere were neutral. The Dynamic QC method was put into operations at JMA on March 17, 1997. After that, the scores of the operational global forecasts have significantly improved.
In this study, we demonstrate remarkable warming trends in northern winter and spring not only at the surface but also in the lower troposphere during a recent 30 year period (1964-93). In winter, major warming has happened over central Siberia and the northwestern Canada/Alaska region. The vertical structure of the warming tendency is somewhat different between these two regions. In spring, tropospheric warming is most noticeable over the northern part of North America. A three-dimensional rotated EOF analysis of the temperature anomaly field in these seasons confirmed the warming trends as the most dominant mode at the surface and in the lower troposphere. The rotated EOF analysis also shows that an interdecadal-scale fluctuation with abrupt changes in the periods of 1976/77 and 1988/89 is another dominant mode in the temperature field in winter. The spatial structure of this mode shows a wavenumber-three spatial pattern with positive factorloadings over North America, Northern Europe and the eastern part of Eurasia. In spring, a fluctuation of 10-13 year period is noticed in the second largest mode, which suggests a possible relation with the solar cyle of the same period. Possible forcing factors of these tropospheric temperature trends and fluctuations are briefly discussed.
Precipitation distribution around the Tibetan Plateau was examined by using satellite data of the Geostationary Meteorological Satellite Infrared channel (GMS/IR) and Special Sensor Microwave/Imager (SSM/I). These data were compared with observed precipitation and a product of Xie and Arkin (1997), in 1993 monsoon season. Three kinds of algorithms were used based on an index of fractional coverage of clouds, meso-scale convection centers, and scattering signal of the microwave. IR-based estimates showed two increased centers in the southern and southeastern plateau with a zonal gap behind the Himalayas, but the SSM/I-based estimate failed to yield representative distribution due to infrequent data sampling. IR-based estimates were combined, and the differences between daytime and nighttime precipitation and distribution variability were clarified, in relation to the onset of the rainy season. The differences between the daytime and nighttime precipitation depend on the larger-scale topography. The stepwise onset of the rainy season is known to be related to different circulation patterns around the plateau. In the middle of June, the first heavy precipitation was observed in the southeastern plateau, associated with a strong southwesterly moisture inflow from the meandering of westerlies at the 500hPa level south of the plateau. After the middle of July, a stable Tibetan anticyclone was established, and continuous precipitation prevails only in the southern plateau. The importance of studies on the regional activities of meso-scale convection and the transitional period ushering in the mature rainy season are discussed.
Size (radius: 0.15-1.8μm) and number concentration of polar stratospheric clouds (PSCs) were observed with a balloon-borne particle counter at Ny-Ålesund, Norway (79°N, 12°E) in winter of 1994/95 (December 18, 1994 and January 17, 1995). The measurements suggested that PSCs actively formed in the cold winter stratosphere. During the balloon-borne measurements, the depolarization ratio of PSCs was monitored with a lidar near the balloon observational site to see the phase (liquid or solid) of PSCs. Both measurements suggested that many sub-layers were contained in the PSCs. Particle size distribution of the early PSCs event (December 18, 1994) was different from that observed on January 17, 1995, after active PSCs events. This difference possibly suggests that particle formation processes were largely disturbed by redistribution of chemical constituents relating with PSCs formation and growth in the polar winter stratosphere. Lidar measurements suggested active formation of liquid phase PSCs in addition to solid phase PSCs. The number-size distribution pattern showed enhancement of the number of particles with their radius larger than 1.8μm in the solid state sub-layer of PSCs. On the other hand, not only large particles (r>1.8μm) but also smaller particles (r=0.25-0.6μm) were enhanced in the liquid state PSCs.
Fundamental dynamics of extra-tropical low-frequency variations were investigated by numerical experiments based on a simple spherical barotropic model with forcing, dissipation and topography. Nonperiodic solutions that show low-frequency variations were obtained for realistic parameter ranges. The time average of the solutions has a zonally varying jet structure under the influence of the topography, as the climatological mean state has in the upper troposphere. Some disturbances, which are defined as a deviation from the time average, grew rapidly in a favorable area of upstream and equatorward of the jet maximum. Linear characteristics of the time-averaged basic states, such as eigenmodes, optimal modes, and linear responses of isolated initial perturbations, were examined in detail and were compared with the rapidly growing disturbances in the nonlinear nonperiodic solutions; the rapid growth is explained well by the linear evolution of optimal modes rather than that of unstable eigenmode. The wave activity diagnostics reveal the importance of the propagation of Rossby waves in the zonally varying basic state to the growth of disturbances around the jet exit region.
By using the NCEP/NCAR Reanalysis dataset for 17 years (1979-1995), the structure and behaviour of quasi-stationary planetary waves (QSPWs) in the Southern Hemisphere winter were studied in relation to the tropospheric zonal wind variations shown by Aoki et al. (1996). An empirical orthogonal function (EOF) analysis was made of 300hPa height fields to detect the fluctuation of horizontal QSPW structure. The composite analysis based on the EOF analysis indicated that the QSPWs change their amplitude in the eastern hemisphere and phase in the western hemisphere between the single-jet and double-jet profiles. The wave activity flux diagnosis suggests poleward propagation from the subtropics during the period of single-jet and the equatorward propagation from polar latitudes during the double-jet period. It is shown that the horizontal EP flux divergence by total waves, especially due to high-frequency disturbances, plays an essential role in the maintenance of the difference between the two jet profiles. As regards the total (horizontal and vertical) divergences, the contribution of the QSPWs to forcing of the zonal-mean zonal wind anomalies is important in higher latitudes around 60°S.
An ensemble of three 40-year parallel simulations was performed using a T42 AGCM version of the Japan Meteorological Agency global model to answer the question why extraordinary cool and hot summers in East Asia, especially Japan and Korea, tend to occur very frequently inrecent years from the late 1970s to the early 1990s. Three independent long-term integrations from January 1955 to December 1994 were forced by the same SST boundary condition observed on the global scale. Our AGCM simulations employing prescribed observed SSTs were successful in reproducing extratropical circulation anomalies that bring about the decadal-scale amplitude modulation of interannual variations of summer mean temperatures in the vicinity of Japan. During the period from the beginning of 1980s to the early 1990s, the interannual variability of the east-west gradient of summertime SST anomalies between the South China Sea and the tropical western Pacific east of the Philippines became appreciably large, was accompanied by anomalous cumulus convection around the Philippines, and its phases coincided quite well with those of model-simulated lower-tropospheric geopotential height variations near Japan. The anomalous convective heating substantially affected summertime lower tropospheric circulation anomalies in East Asia through the dynamic process of the excitation of PJ teleconnection pattern (Nitta, 1987). The anomalous SST forcing from the tropics is crucially responsible for the frequent occurrence of extreme cool and hot summers in Japan and Korea from the late 1970s to the early 1990s. The presence of strong east-west gradient of SST anomalies across the Philippines is primarily attributed to the significant coupling of weak (strong) South Asian summer monsoon and the warm (cold) episode of ENSO. The warm episodes that occurred during the period from the late 1970s to the early 1990s are appreciably different from a typical model of El Niño event exemplified by Rasmusson and Carpenter (1982) in terms of seasonal evolution. It is anticipated that both unusually persistent ENSO signals from the preceding winter until summer and the associated South Asian summer monsoon activity strongly regulate the formation of the east-west SST gradient near the Philippines in boreal summer.
The slowly evolving annual cycle (signified as “L-mode”) was defined by the sum of the first three sinusoids in a series of climatological pentad mean data of infrared equivalent black body temperature, 850hPa wind, temperature, geopotential height and specific humidity, while the rapidly varying annual cycle (denoted by “S-mode”) was determined by adding up the remaining sinusoids. During early summer, the L-mode exhibited a blocking-type configuration with a ridge over the Kamchatka-Okhotsk region, whereas a trough stretched eastward from northern China (center of a continental heat low) to Japan and beyond. The establishment of a locally independent L-mode anticyclonic cell over the Sea of Okhotsk enhanced low-level easterly anomalies from the vicinity of the Aleutian islands through northern Japan. The L-mode easterly anomalies merged with southwesterly anomalies along the southeastern periphery of a continental-scale heat low, thus generating a pronounced cyclonic shear zone around Japan accompanied by low-level moisture convergence. This L-mode lower tropospheric trough in intrinsically association with the east-west temperature gradient between East Asia and the western North Pacific plays a vital role in the formation of Baiu system in early summer. The L-mode southwesterlies along the east coast of China, which are attributed to the prominence of the continental-scale heat low, served as a bridge that links the monsoon westerlies of tropical origin with the westerly jet of mid-latitude origin. This resulted in producing a low-level westerly duct extending from the South China Sea to the central North Pacific. At the Baiu onset phase (mid-June), the S-mode onset cyclone of convective origin developed over the South China Sea, and concurrently the S-mode onset anticyclone organized to the northeast of the onset cyclone. The consolidated effect of these onset vortexes amplified in the vicinity of the low-level westerly duct was to cause the northward advection of warm and moist air from the tropics to southern Japan. By mid-July, the Asian continental heat low reached its peak and the summer monsoon over Southeast Asia became fully established. At the Baiu withdrawal phase (late July), the continental heat low began to decay because of land surface cooling; nevertheless, the L-mode Pacific High still developed northward and was most intense at the beginning of August. As the east-west temperature gradient between the ocean and continent decreased, the L-mode lower tropospheric trough near Japan dissipated, whereas L-mode WNPM (summer monsoon over the western North Pacific) trough prevailed in the subtropics. The withdrawal of Baiu was also characterized by the dominance of S-mode disturbances as well as the Baiu onset. It is thus anticipated that the seasonal evolution of L-mode circulation field, associated with continent-ocean thermal contrast, strongly regulates the activities of S-mode perturbations in and around the low-level westerly duct. It seems likely that the westerly duct served as a wave guide for the barotropic Rossby wave dispersion of the S-mode disturbances. The presence of horizontally sheared L-mode flow may also contribute substantially to the development and persistence of S-mode vortexes via in situ barotropic interaction process between the two modes. At any rate, the combined contribution of the L-mode flow and S-mode disturbances is to make such local climatological events as the onset and withdrawal of Baiu so rapid and dramatic.
The local front observed over the Kanto plain on December 5, 1996 has been analyzed as an example of one of those that often appear when a synoptic-scale low advances in the Japan Sea in early winter. Winds in the lower layer, blowing from the northern and western mountains since the previous night, supplied cold air to the middle of the plain. Before dawn, synoptic-scale southerly winds intensified from the south of the plain, and the local front formed along the southern margin of the cold air mass. The southerly winds over the lifted frontal surface entrained the cold air from the lower layer and transported it northward in a “transition layer, ” characterized by relatively cold southerly winds. The depths of the transition and lower layers tended to be maintained above the inland areas north of the front. The front advanced northward, corresponding to the surplus of the northward flow in the transition layer over the cold air supply in the lower layer. As a result of the persistent cold air flow, even during the daytime, the front became stationary over the middle of the plain in the afternoon. Additionally, a very shallow (-50m) structure of the frontal head existed prior to the frontal passage through the central area of Tokyo.
We propose an effective method for treating singular value decomposition (SVD) analysis. This method is economical in the sense that the problem can be reduced to a matrix problem of the temporal size, not spatial size. Thus, this method is efficient when the spatial size is very large and the number of the ensemble is much smaller, as in a typical climatological problem. With this method, it is very simple to change the area of analysis by the use of weights, which are introduced to deal with the inhomogeneity of the data.
An idealized precipitation process in a 3-dimensional unsteady mesoscale flow with a Boussinesq approximation is considered. The process is assumed as follows: Immediately after precipitation particles are generated, they attain such an equilibrium that they fall with a constant terminal vertical velocity relative to the flow, and their horizontal velocity is equal to that of the flow. The ground is in general not flat, and other forcings (e. g., thermal) may be included in order to induce the vertical flow velocity. Based on these assumptions, a simple formula describing the concentration of trajectories of precipitation particles at the ground is derived. In particular, in the case of a steady flow, the concentration (scattering) of trajectories occurs when w0 is positive (negative). Here w0 is the vertical flow velocity at the generation point of particles. In the case of an unsteady flow, the unsteadiness may suppress or enhance the concentration (scattering) of trajectories.
The monthly evapotranspiration (ET), which was estimated from assimilated atmospheric data from European Centre from Medium-Range Weather Forecasts (ECMWF) and gridded global precipitation data introduced by Xie and Arkin, was examined in relation to the vegetation activity for 1987 and 1988 over Asia. The vegetation activity was represented by the Normalized Difference Vegetation Index (NDVI) that was calculated from satellite observation. Over Siberia, the annual marches of the ET and the NDVI were quite similar. Furthermore, bimodal annual variations of the NDVI and ET were observed in Punjab (around Pakistan and northern India) where bi-seasonal cultivation is seen. The ET-NDVI relationships were analyzed for seven vegetational cover types and revealed that slopes of ET-NDVI regression lines are distinguished depending on the vegetation types. The results presented in this paper demonstrate the possibility of investigating the continental-scale vegetation activity and the ET, which is derived from assimilated gridded global data.