The results of atmospheric and oceanic CO2 observations conducted by the Japan Meteorological Agency in the western North Pacific are presented for the period 1990-1993, covering an El Niño/Southern Oscillation (ENSO) event (onset: spring, 1991; withdrawal: summer, 1992). The atmospheric CO2 concentration over the region south of 30°N increased drastically between 1990 and 1991 during the winter (4.0ppmv) and summer (4.5ppmv), although the values are not seasonally adjusted. Over the other two years of observations, the growth rates were smaller or at times negative. Oceanic CO2, which is expressed in units of the mole fraction (ppmv) in dry air equilibrated with seawater, showed a significant increase, especially in low latitudes during both seasons of the years 1991-1993, compared to the 1990 values. Oceanic CO2, normalized at a constant temperature, also revealed a significant increase, with larger and smaller magnitudes for winter and summer, respectively. This implies that the increased summer oceanic CO2 results mostly from changes in surface seawater temperature, while for that in winter, other factors, unknown at present, are more related to the increased oceanic CO2. Calculated values of ΔpCO2 and CO2 flux at the air-sea interface reveal that in winter, the region north of 10°N acts as a sink of CO2, with a maximum net flux of --10.0mmol·m-2·d-1. The region south of 10°N, however, at times becomes a weak source of CO2, with a maximum net flux of 2.4mmol·m-2·d-1. In summer, the western North Pacific becomes a weak source or is almost in equilibrium with atmospheric CO2. The increase in winter oceanic CO2 or ΔpCO2, as related to the 1991/92 ENSO event, is not as distinct as in the 1982/83 ENSO event, although lower temperatures and higher salinity in the surface seawater were commonly found during both the events. The response of the CO2 flux in the tropical western North Pacific to the 1991/92 ENSO event was rather small compared to the magnitude of the rate changes obtained in previous results for the central or eastern tropical Pacific.
An observational study has been conducted of the southern summer rainfall variability over the semiarid region of southern Africa (SASA) since 1958, as related to the tropical African rainbelt, tropical atmospheric circulation, and sea surface temperature (SST) patterns. A rotated empirical orthogonal function (R-EOF) analysis was applied to the global monthly SST anomalies. On a year-to-year basis, the first R-EOF mode, related to the El Niño/Southern Oscillation (ENSO) phenomenon, was most strongly correlated with the southern African rainfall and two rainbelt indices; the latitude of its center of gravity and the total rainfall that occurs over the entire rainbelt zone. The anomalous warming of the central-eastern equatorial Pacific, identified as the first R-EOF mode, was associated with increased 700hPa heights throughout the tropics except for the eastern Pacific, especially marked over southern Africa and the eastern Indian Ocean-central Pacific region. Simultaneous intensification of the 700hPa subtropical high over southern Africa was found to cause a northward displacement in the rainbelt's center of gravity and a decrease in total rainfall. These changes in the rainbelt parameters resulted in decreased rainfall for the SASA region, located south of the main rainbelt. On the other hand, the anomalous cooling of the eastern Pacific was related to a weakening of the southern African sub tropical high. This leads to a simultaneous southward advance and enhanced convection of the rainbelt, which ultimately result in increased SASA rainfall. On the decadal time scale, the third R-EOF mode, characterized by a dominant signal over the South Atlantic, is most strongly correlated with trends in the SASA rainfall and rainbelt parameters. The warming of the South Atlantic and simultaneous increases in the 700hPa heights over southern Africa result in decreased rainfall; cooling and decreased 700hPa heights lead to increased rainfall.
A new method, utilizing a 22.235GHz-water vapor emission line, to retrieve precipitable water content (PWC) is proposed. To obtain PWC, the method includes estimating the portion of the water vapor contribution to the brightness temperature, integrated over the microwave K-band (18-26.5GHz) range. It is appropriate to use the K-band range in order to retrieve PWC because the emission (or absorption) in this frequency range is weak and the line strength is little dependent on atmospheric temperature and pressure. By using several atmospheric models with typical cloud types, the following approximate relation to retrieve PWC is introduced: PWC=aTb.H2O+b(g/cm2) where a=0.2135±0.0021, b=-0.0420±0.0251, Tb.H2O=1/Δf∫f2f1Tb.obs(f)df-[Tb.obs(f2)+Tb.obs(f1)]/2+ΔTb. Tb.obs (f) is a brightness temperature observed at a frequency of f GHz, and f1 and f2 are lower and upper boundary frequencies for integration, which are 18 and 26GHz, respectively, and Δf=f2-f1. ΔTb is a correction factor for the term correcting cloud and water vapor continuum emission. It is best for ΔTb to be set at 0.2K empirically. By using actual radiosonde data along with clouds introduced depending on atmospheric conditions, an error is determined from the above approximate method for retrieval of PWC. The error for PWC is less than 5% under the average atmospheric condition (PWC>1g/cm2 and liquid water path (LWP)<0.1g/m2), and is about 6% for rather cool atmospheric conditions (PWC<1g/cm2 and LWP<0.01g/m2).
The East Asian monsoon circulation during the El Niño episodes of '86/87 and '91/92 is studied diagnostically. It is found that a southerly wind anomaly appeared in the lower troposphere along the coast of the East Asia during the mature phases of these two El Niño events. In the case of '86/87, the mature phase covered the boreal summer and the East Asian summer monsoon was intensified. On the other hand, in the case of '91/92, the northern winter was within the mature phase and the East Asian winter monsoon was weakened. Examination of the circulation features suggests that the effects of the El Niño events on the East Asian monsoon are felt through the variation of convective activities over the western equatorial Pacific. Convections in this area are strongly influenced by the evolution of sea surface temperature anomalies in the equatorial Pacific and are strongly suppressed during the mature phase, which exerts significant influences on the direct monsoonal circulation over the western tropical Pacific and the East Asia. The relationship between the East Asian monsoon and El Niño in its mature phase is also confirmed by a historical 850hPa wind dataset that covers 6 events prior to the '86/87 event. The dataset also suggests that an inverse relationship does not hold during the La Niña periods.
Two suboptimal data assimilation schemes for stable and unstable dynamics are introduced. The first scheme, the partial singular value decomposition filter, is based on the most dominant singular modes of the tangent linear propagator. The second scheme, the partial eigendecomposition filter, is based on the most dominant eigenmodes of the propagated analysis error covariance matrix. Both schemes rely on iterative procedures like the Lanczos algorithm to compute the relevant modes. The performance of these schemes is evaluated for a shallow-water model linearized about an unstable Bickley jet. The results are contrasted against those of a reduced resolution filter, in which the gains used to update the state vector are calculated from a lower-dimensional dynamics than the dynamics that evolve the state itself. The results are also contrasted against the exact results given by the Kalman filter. These schemes are validated for the case of stable dynamics as well. The two new approximate assimilation schemes are shown to perform well with relatively few modes computed. Adaptive tuning of a modeled trailing error covariance for all three of these low-rank approximate schemes enhances performance and compensates for the approximation employed.
In order to evaluate the impact of assimilating precipitable water content (PWC) and rain flag data derived from the Special Sensor Microwave Imager (SSM/I), an assimilation method is developed which modifies the relative humidity (RH) and divergence of background data provided by the Japan Spectral Model (JSM). The assimilation method consists of the following procedures: (1) JSM is integrated to produce background data for the assimilation of SSM/I data. (2) If the model does not predict precipitation in areas where precipitation is observed by SSM/I, RH is raised to a critical RH so that the model precipitation schemes produces precipitation. (3) In areas where precipitation is not observed by SSM/I, RH increments are calculated on each levels based on difference in PWC between SSM/I and the background data, and they are added to the background RH. In this calculation, the correlation coefficients between JSM forecast error of PWC and that of RH on each level are utilized. (4) After the modification of RH, non-linear normal mode initialization with the precipitation process is performed to adjust divergence of the background data. The assimilation method adopted in the present study reduced the positional error of precipitation forecasts by JSM over 12 hours for the case of 21 UTC 12 July 1988. The results of the forecast experiments for this case suggest that assimilation of PWC is effective in eliminating model precipitation in the observed rain-free areas, and that assimilating rain flag data is effective in generating model precipitation in the observed precipitation areas. Assimilation of PWC also improved RH forecast in the lower-troposphere. This was due to large statistical correlation between the JSM forecast error of PWC and that of RH in this layer.
On the night of 9 August 1993, Typhoon 9307 passed near the southern part of Japan and caused heavy rainfall and many land-slide disasters across the Ohsumi Peninsula. A notable feature of this event is that the rainfall amount was very large on the lee side of the mountains. The 9-hour rainfall exceeded 150mm at the greater part of the stations on the lee side, while it was only 80mm at the station on the windward side. All the land-slide disasters occurred on the lee side. The heavy rainfall was mainly produced between the eyewall and the rainband of the typhoon. Radar echoes showed strong reflectivity over the mountains during the rainfall event. The rainfall increasing process was studied by using a two-dimensional numerical model. The results suggested that the seeder-feeder mechanism contributed to the rainfall enhancement on the lee side. Namely, the precipitating particles from the upper cloud grew over the mountains by accreting cloud water produced by the orographic lifting, and the grown particles were carried to the lee side by the strong wind of the typhoon. In addition, it was also suggested that the downdraft of the mountain wave made an additional contribution to the rainfall enhancement by transporting many ice particles to the lower layer.
By applying a seasonally modulated condensation heating on tropical Kelvin waves with a certain form of damping, the parametric instability and super (sub) harmonic response of the Kelvin waves are investigated. It is suggested that the excitation of intraseasonal oscillation and tropospheric QBO may be related to the superharmonic and subharmonic resonances with the seasonal cycle, respectively. The main features of the spatial structures and propagation of these resonant modes were shown and compared with those of intraseasonal oscillation and tropospheric QBO to examine this point of view.
In the afternoon of September 22, 1988, a supercell storm developed over the Ishikari Plain, in Hokkaido, Japan. A single-funnel F1 tornado was spawned in Chitose City, near Sapporo. The characteristics of the structure and the life cycle of the Chitose tornado were revealed by radar observations, ground survey, photographs, videotape, and meteorological data analyses. The Chitose tornado had a lifetime of 20 minutes and funnel width of 150m at cloud base. The life cycle of the tornado was divided into four stages according to the detailed structure of the funnel and damage characteristics: the formation stage was characterized by a descending funnel, the mature stage characterized by touchdown of a visible funnel and a formation of strong dust cloud, the shrinking stage characterized by tilting and meandering funnel, and decaying stage that lead to the dissipation of the funnel. The mesocyclone of 7km in diameter developed at 2.5km AGL in the parent cloud and was accompanied by a misocyclone of 1km in diameter. The angular momentum of the tornado circulation at the height of 400m AGL was nearly equal to that of the misocyclone at 2.5km AGL. The time and space relationship between surface damage, the tornado funnel and the mesocyclone was revealed. The width of the surface damage area was nearly equal to the width of the dust cloud (200m) and was 10 times larger than the surface funnel width. Although the surface funnel initially formed right below the misocyclone, the funnel top near the cloud base and the funnel bottom near the surface moved in quite different ways, since the parent cloud moved at a speed of 5ms-1 and the surface funnel at a speed of 1.2ms-1.
The effect of the earth's rotation on down-slope windstorms induced by a mountain range, which is uniform in the north-south direction, in an eastward uniform environmental flow with a constant buoyancy frequency is analytically investigated. The velocity field is assumed to be uniform in the north-south direction. After an appropriate non-dimensionalization, the non-dimensional Coriolis parameter f becomes small. To the O(f) approximation, the wind speed component in the east-west direction is not altered by the rotation, and the same down-slope windstorms occur as in the non-rotational case. On the other hand, the wind speed component in the north-south direction, which is everywhere uniform in the non-rotational case, is altered by the rotation. At the leeward foot of an east-west symmetric mountain profile, the southward wind component is increased by the rotation.
The vertical flux of horizontal momentum by three-dimensional mountain waves is examined using a linear theory. Analytic solutions for two different wind directions are obtained using numerical Fourier analysis. Although the mountain examined in this note is a fairly two-dimensional one, the results show that, at the tropopause level, the momentum flux greatly decreases to 1/4-1/3 of its near-surface value even over the center area of the mountain when the mean wind crosses the mountain at 45 degrees. It is also suggested that the decrease of the momentum flux observed in IOP-3 of PYREX is caused by both the finite length of the Pyrenees mountains and the slant attack angle of the mean wind.