The diagnostic study of the biennial oscillation (BO) of the ENSO/monsoon system is performed by using the sea surface temperature (SST), sea surface wind, sea level pressure, and outgoing longwave radiation data. The BO is a prominent feature of the ENSO/monsoon system that results in the modulation of convective activity aroud the maritime continent. The BO tends to change its anomalous states during the Northern Hemisphere spring. The SST variation around the South China Sea also has a BO with a phase transition that tends to occur about a half year earlier, that is, during the Northern Hemisphere autumn. The intensification of the northeast winter monsoon (NEWM) probably modifies the anomalous state of the SST which persists until the northern summer. The SST anomalies around the South China Sea are favorable for modifying the convective activity around the maritime continent during the northern winter to summer. The strength of the convective activity further regulates the SST anomalies in the tropical central-eastern Pacific through the Walker circulation during the northern summer to winter. This anomalous SST state is probably responsible for the strength of the NEWM by controlling the extratropical circulation pattern. The NEWM changes the anomalous state of the SST around the South China Sea. Thus the cycle of the BO of the ENSO/monsoon system is completed. It is, thus, strongly suggested that the tropical-extratropical interaction plays an essential role in the BO of the ENSO/monsoon system.
The beam filling error is a significant problem in rain rate retrieval from passive microwave radiometer measurements from space. Rain will not be uniformly distributed throughout footprints of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). This inhomogeneous rainfall distribution within the field of view (FOV) and non-linear relationship of rain rates and brightness temperature (R-T) produces errors. This error can be reduced if we have some information on rain/no-rain at a higher spatial resolution. The Visible IR Scanner (VIRS) on the TRMM observatory will provide cloud coverage, type and cloud top temperature at high resolution (2 km at nadir). In this study, we have examined how good the rain/no-rain criteria should be from VIRS in order to improve TMI rain retrievals using a simple passive microwave rain retrieval algorithm. It is found that the improvement in rain retrieval using VIRS rain/no-rain information depends on the intensity of rain rates. Generally, it is better to overestimate raining fractions of moderate and heavy rain within a footprint than underestimate them. On the other hand, it is better to underestimate raining fractions of light rain than overestimate them. We can have improvements of rain retrievals by the TMI in heavy rainfall (r > 20 mm hr-1) with the aid of VIRS measurements when scattering effects are considered.
This paper is concernd with interannual and interdecadal variabilities of summer rainfall and temperature patterns in China and their association with 500 hPa height in the Northern Hemisphere (NH), tropical convective activities and global sea surface temperature anomaly (SSTA). The temporal evolutions and spatial structures of interannual variation of summer (JJA) rainfall and temperature from 1951 to 1994 over China are revealed through EOF analysis. The spatial pattern of EOF1 for rainfall (EOF1.R) is dominated by a maximum over the middle-lower reaches of the Yangtze River, and a large negative value region in the middle reach of the Yellow River is also obvious. The spatial pattern of EOF1 for temperature (EOF1.T) reflects coherent variations over most regions of China, and it is dominated by a maximum over the middle-lower reaches of the Yangtze River. Linear increase and decrease trends are found in the time coefficients of EOF1.R and EOF1.T, respectively. The quasi-biennial oscillation (QBO) signal is also strong after the middle of the 1970's in repect of their time coefficients. The coupled patterns of rainfall and temperature are picked up through the singular value decomposition (SVD) analysis. The spatial patterns and their temporal evolutions of SVD1 for rainfall (SVD1.R) and SVD1 for temperature (SVD1.T) are quite similar to those of EOF1.R and EOF1.T. There is an abrupt change in the middle 1970's in the time coefficients of SVD2.R and SVD2.T. The variations of summer rainfall and temperature coupled patterns in China are closely connected with the 500 hPa height anomaly over the Northern Hemisphere (NH). The Pacific-Japan (PJ) and Eurasia (EU) teleconnection patterns play a very important role in the spatial patterns of SVD1.R and SVD1.T, especially in the East Asia monsoon region along the middle-lower reaches of the Yangtze River. The abrupt change of China summer climate in the middle 1970's is related with the intensification and southerly location of the western Pacific subtropical high and also the geopotential height changes over Eurasia and in the regions to the north of the Japan Sea in 1977 or 1978. Correlations between the summer rainfall and temperature coupled patterns and monthly-averaged out-going longwave radiation (OLR) and high-cloud amount (HCA) data are significant with the PJ teleconnection pattern. There exist positive correlations between the coupled patterns and sea surface temperature anomaly (SSTA) in the North Pacific and the tropical western Paciftc. A comparison study shows that there are coherent variations between summer rainfall in the middle-lower reaches of the Yangtze River and in the western part of Japan. It is also demonstrated that there are close correlations between the summer temperature variations in China and in Japan.
Baroclinic flows in an open cylinder are simulated using a three-dimensional global-grid numerical model. Five types of flow observed in our laboratory experiments are obtained numerically: axisymmetric flow, steady wave flow, wavenumber vacillation, amplitude vacillation and irregular flow. Their regimes are presented as a function of the basic angular speed, and the characteristics of each flow are described.
A method is proposed for calibrating radar estimates over the entire detection area using more than one conventional radar and raingauge datum. In this algorithm, two parameters are used to correct two major causes of errors in radar estimating, namely, (1) errors arising from the instability of the sensitivity of radar hardware, and (2) errors based upon the vertical profile of precipitation. By use of these parameters and the field of the height of the radar beam, a correction factor, which is an estimate of the ratio of the actual precipitation to the radar estimate on each grid of the precipitation field, is described with a simple function. The correction factors are determined hourly with the least-squares method by comparing radar estimates not only with raingauge measurements, but also with radar estimates from different radars. The method works well for the various types of precipitation, which change according to the nature of disturbances or to the season. The correction method is found to be accurate enough for operational use. By correcting radar estimates with this algorithm, the discontinuity found around the borders of domains for respective radars in the composite map is improved. This method is useful not only over land areas, but also over a wide range of the sea, where there is no other proper method for calibrating, especially in operational use. This algorithm is used as a part of the process for producing radar-AMeDAS precipitation charts, which the Japan Meteorological Agency provides operationally as fields of hourly precipitation amounts.
We outline the correlations between the height of the constant pressure levels in the lower stratosphere in the Japanese region and the 11-year solar cycle. We point out that the position of the highest correlations varies substantially from one period to another over Asia and the western Pacific Ocean, whereas over most of the rest of the hemisphere it is comparatively stable. This variability of the position of the highest correlations, in addition to the larger variability in the size of the correlations over the subtropical parts of Central Asia than elsewhere in the same latitudes, gives rise to a large zonal asymmetry in the long-term correlation pattern. And it probably also creates a large variability in the position of the largest solar correlations with total ozone in the same area, because the axis of highest correlation between ozone and the sun is associated with the position of the highest correlations between the sunspot cycle and the heights in the lower stratosphere.
The abrupt northward shift of the active convective region from 10°N to 25°N around 150°E on July 25-29 (Pentad 42) seen in the seasonal cycle of the summer monsoon over the western North Pacific (convection jump) and other associated phenomena preceding this convection jump are investigated using data of equivalent black body temperature derived from GMS and sea surface temperature (SST) for the period of 1980-1994 together with circulation field data objectively analyzed by the ECMWF. The mechanism of the maturing process of the summer monsoon between middle June and late July are examined by comparing 8-year remarkable convection jump years (typical cases) and other years (atypical cases). Tongue-shaped warm-SST areas (warmer than 29°C) are observed in early July around 20°N, 150-160°E, preceding the typical convection jump. This warming of SST is closely related to the appearance of a weak-wind region (weakening of easterlies) around 25°N, 140°E-160°E in late June. These weak easterlies are likely to be associated with the propagation of a Rossby wave induced by the occurrence of the active convection near the Philippine Islands in middle to late June. Most of the atypical cases appear in the El-Nino years.
The interannual variability of the seasonal evolution of the zonal mean wind and wave activity in the southern hemisphere troposphere is investigated, using a global analysis data-set for 1979-1993 provided by the National Meteorological Center (NMC), to understand the dynamical coupling between the troposphere and the stratosphere. The interannual variance of the zonal mean wind at 300 hPa calculated for each calendar day has maxima at 60°S and 40°S almost all year round. We found that a zonal wind index defined by the difference of the zonal wind speed between 60°S and 40°S is a sensitive indicator of two typical latitudinal profiles of the zonal mean wind in the southern hemisphere troposphere: the single-jet and the double-jet profiles. The seasonal evolution of zonal wind indices is separated into two groups, i.e., DS (double-single) years and SD (single-double) years. DS years are characterized by a double-jet structure in early winter and a single-jet in late winter, while in SD years the zonal mean wind has a single-jet structure in early winter and changes its profile into a double-jet in late winter. The appearance of DS and SD years is similar to that of the two categories of the westerly jet profile in the southern hemisphere upper stratosphere in Shiotani et al. (1993). Wave activity is diagnosed by the Eliassen-Palm (E-P) flux. Clear differences are seen in May-June between DS and SD years in the horizontal component of E-P flux. Variation of the horizontal component precedes change in the zonal mean wind, indicating its importance in transitions between the single- and double-jet. The zonal wind structure has influence on planetary wave 1 activity. The vertical component of wave 1 E-P flux is small (large) at 60°S and 500 hPa when the zonal wind has a double-jet (single-jet) structure during winter. The weak flux at 500 hPa in the years with a double-jet structure yields weak wave 1 activity in April-June but gives strong wave 1 activity through more efficient propagation in August-November at 100 hPa relative to the years with a single-jet structure.
In this study, normal-mode energetics diagnosis was carried out for the 9-year period of ECMWF (European Center for Medium-Range Weather Forecasts) analysis during winters (DJF) of 1986 to 1994. The diagnosis was performed also for operational global analyses by JMA (Japan Meteorological Agency) and NMC (National Meteorological Center) during the winter of 1988/89 for comparison. Special attention is focussed on the total energy spectra for Rossby and gravity modes over the 3-D scale index. The result was compared with our former studies for other datasets, including original and re-analyses of the First GARP Global Experiment (FGGE). The result for the 9-year average of ECMWF shows that the slopes of the Rossby-mode energy spectra in the eigenfrequency domain are estimated as 2.2, 2.1, 3.5, 5.0, 5.5 for vertical modes m=0 to 4, respectively, whereas the corresponding slopes are -1.8, -1.7, -1.6, -1.3, -1.0, respectively, for the gravity mode. We confirm that the energy spectra of Rossby and gravity modes are connected with each other in the high-frequency range, at least for the vertical modes m=1 to 4. However, the mechanism to create the spectral peaks within the Rossby modes is not known. The intercomparison of the normal-mode energetics for JMA, NMC, and ECMWF shows that the Rossby-mode energy levels are virtually the same, having a minor discrepancy less than 8%. We find that the total energy level pertaining to the gravity modes is about 45 (1O3 Jm-2) and shows a minor discrepancy of only about 8% between the analyses. According to our former studies for the FGGE reanalyses, the corresponding energy levels for the gravity modes are 90 and 41 (103 Jm-2) for GFDL and ECMWF, respectively. Hence, we can conclude that the total energy levels of the gravity modes in the recent operational analyses are converging to the value of ECMWF FGGE data. Since the amount of the total energy of the atmosphere is about 6800 (103 Jm-2), the energy level of the gravity mode is found to be only 0.7% of the total energy.
A comprehensive observation during the late period of the Baiu season was performed in July, 1988, in the project "A Study of Mechanism and Prediction on Heavy Rainfalls during Rainy Season in Japan" (Asai, 1990). Spatially and temporally dense (-100 km, 3-6 h) sounding data and Doppler radar data were collected during this special observation. In this study, in order to investigate the mesoscale and convective scale features of the heavy rainfall events in the late period of the Baiu season, the low-level wind field and evolution processes of radar echoes were analyzed by using the Hokkaido University Doppler radar, which was located in Saikai town, Nagasaki Prefecture. Two successive mesoscale rainfall events associated with the Baiu front (Case 1 and Case 2) which were observed on July 17 and 18, 1988, formed the main units of analysis. Both rainfall events occurred in association with a cloud cluster. Heavy rainfall amounts exceeding 100 mm were experienced in limited areas during Cases 1 and 2 (for example, rainfall amounts at Isahaya during Case 1 and Case 2 were 165 mm/3h and 104 mm/3h, respectively). The Doppler radar analyses revealed important mechanisms of the heavy rainfall events, which are described briefly as follows. In Case 1, enhancement of the Baiu frontal convergence by gust fronts from convective clouds, and the merging between the Baiu frontal rainband and isolated echoes generated south of the Baiu front played an important role in the organization of a well developed rainband. The rainfall event of Case 2 occurred about 4 hours after Case 1 and in almost the same region. In Case 2, an organized arc-shaped echo caused the heavy rainfall, and gust fronts contributed to the organization of this echo. The advection of warm and moist air from the undisturbed region of the Case 1 rainfall also played an important role in its development. From these case studies, it can be concluded that mesoscale and convective scale features influencing the formation and maintenance of Baiu frontal rainbands are (1) enhancement of low level convergence by convective outflow (gust front), (2) merging effect of convective clouds, and (3) mesoscale contrast of temperature and rainfall produced by mesoscale rainfall.
We formulate the quasi-geostrophic 3-dimensional E-P flux of stationary waves by means of a quasi-geostrophic potential vorticity equation on a sphere with a divergent component introduced by Matsuno (1970, 1971). It is found that the expression for the E-P flux has the same form as that of Plumb (1985), even when the wave is represented by a geostrophic wind with a divergent component.
The atmospheric boundary layer over the central area of Tokyo was studied by a Mie-scattering polarization lidar. Negative correlation of the depolarization ratio with relative humidity was observed in the lower atmosphere throughout the year. Contributions of hygroscopic or water-soluble and deliquescent aerosol particles, especially sea-salt particles to the depolarization ratio are discussed in the light of the chemical compositions of the surface aerosol in Tokyo.
We investigated the interannual variation of the sea ice over the Sea of Okhotsk by using radar observation data and gridded ice-coverage data for the years 1969-1994. It became evident that the abrupt decrease of sea ice over the southern part of the Sea of Okhotsk occurred in 1989, and that this reduced-ice condition has continued since then. In addition, the winter time Aleutian low has abruptly weakened since 1989. According to the lag-correlation analysis, this weakening has brought about the reduction in extent of the ice-coverage.