With the very high horizontal resolution (T63 and T159), an aqua-planet version of the JMA operational global model was integrated. For both resolutions, the Madden Julian (M-J) oscillation with the period of 40 days has appeared. The structures with the scale of zonal wave number one in two different resolution models are nearly identical, while the amplitude of the M-J oscillation in the T159 model is two times larger than that in the T63 model. To the east of the equatorial convergence in the lower troposphere, moist convective systems are mainly found in the equatorial region, while to the west of the convergence, convective systems are found in the off-equatorial tropics. Moisture budget analysis indicates the importance of frictional convergence near the surface for the maintenance of the M-J structure. In the T159 model, the ensemble of cloud clusters (a super cloud cluster) behaves similarly to that in the nature analyzed by Nakazawa (1988). However, the scale separation between the M-J oscillation and the super cloud cluster is not obtained in our model. We examine the influence of the M-J oscillation upon the generation of tropical cyclones. A large number of tropical cyclones are generated near or near-west of the equatorial lower convergence associated with the oscillation. These cyclones contribute to the formation of a symmetric Rossby wave structure elongated in the zonal direction.
A two-dimensional, non hydrostatic, compressible model is used to simulate the airflow over the Pyrenees in two lee wave events (IOP-3 and IOP-9) during the PYREX program. For each case, the mean flow is initialized by a single upstream sounding. The bottom topography used in the model is taken from the real mountain profile after smoothing out the wavelength shorter than 10km. In all cases, the model simulates well several characteristics of the lee waves: the wavelength, amplitude, and position. This is a strong indication that the lee waves observed in these events were excited by non linear wave-wave interactions of long mountain waves, because the topographic forcing in the model has negligible spectral amplitude at the wavelength of the lee waves (∼10km). Sensitivity experiments show that the phases of the lee waves are very sensitive to a small change of the mean wind and that their amplitudes are increased by the addition of smaller-scale topography. In both cases, the simulated downward momentum fluxes agree well with the observed fluxes around 4km height: -15×104N/m in IOP-3 and +10×104N/m in the third day of IOP-9. The simulated fluxes are, however, almost constant through the troposphere, while the observations show rapid decrease of absolute values above 4km height. This overestimation of the simulated momentum fluxes in the upper half of the troposphere is caused by the overestimation of the amplitude of long mountain waves at these heights; these long mountain waves control the vertical profile of the momentum flux there. It is suggested that the time evolution of the mean wind and the lateral momentum flux divergence found in the real atmosphere give rise to this overestimation.
On the afternoon of 27 June 1991, the Okayama area in the western part of Japan was hit by strong gusty winds and a heavy precipitation of rain and hail associated with severe thunderstorms. The synoptic condition was that the area was located to the south of the Baiu-front with the northern edge of the Pacific Subtropical Anticyclone (PSA) prevailing. At one location in the area, a gusty wind blew down 18 utility poles that were built to withstand winds of 51m/s. The present work was derived from this incident. Virtually all available data from various sources were collected and analyzed. These sources include weather surveillance radars, meteorological observatories, densely deployed surface anemometers for air pollution monitoring, commercial aircraft, videotaped imageries and a damage survey. The result of the analysis reveals that at least four wet downbursts (either microbursts or macrobursts) occurred in the area and that one of them, which was associated with hail precipitation, blew down the 18 utility poles. Furthermore, the environmental conditions are found to be similar to those identified as favorable for the development of wet microbursts in northern Alabama by Atkins and Wakimoto (1991). The potential risk of downburst on the northern edge of the PSA is also suggested.
A new algorithm to estimate precipitable water over land using the split-window data from the NOAA satellite is proposed. The estimation is based on the ratio of the split window channel brightness temperature difference and the ratio of the split window channel brightness temperature variance. The algorithm involves two main steps. The first is to extract high-confidence pixels which do not contain a large influence due to unresolved clouds, and to calculate the first-guess of precipitable water using the split-window variance ratio technique proposed by Jedlovec (1990). The second step corrects the first-guess of precipitable water using the difference between the mean-air temperature of the model atmosphere and the mean-air temperature calculated from the satellite data. By using this algorithm, the precipitable water over an area of about 33km×33km can be estimated, while the influences due to unresolved clouds and unknown temperature profiles decrease. Compared with four radiosonde observations during August of 1992, the satellite-derived precipitable water data are in good agreement with the radiosonde data.
A three-dimensional numerical simulation of a convective-cloud ensemble was performed in order to study the formation process of long-lasting precipitating convective cells. As the initial condition, random small thermals were given in the horizontally homogeneous atmosphere. The convective cells were classified into three types: The first type (S-type) was short-lived and had a downshear-tilting updraft. The second type (F-type) was forced by the other cells and persisted for a long time, although the airflow structure was similar to S-type. The third type (L-type) was long-lasting and had an upshear-tilting updraft. In the formation process of an L-type cell, the updraft root must move in a downshear direction faster than the rising air parcels in order to build an upshear-tilting updraft. The fast movement of the updraft root is initially forced by the neighboring cells. The results showed there are three patterns in this process. In Pattern I the updraft root moves with outflow diverging from one short-lived cell. In Pattern II it moves with outflow boundaries of several short-lived cells which form successively. In Pattern III it moves together with outflow diverging from a pre-existing L-type cell. Once an upshear-tilting updraft is built, the cell produces a strong cold air pool near the surface and is self-maintained at its edge.
The seasonal cycle and the onset and retreat dates in the summer monsoon in Indonesia, Australia and New Guinea are analyzed using 15-year (April 1978-December 1992)-averaged 5-day mean 1-degree latitude-longitude gridded GMS high-cloud-amount data. An analysis of these data shows seasonal cycles of the convective zone (ITCZ) cloud defined by regions with more than 30 percent of the mean high-cloud amount. The high clouds associated with the ITCZ increase in Java and northern New Guinea during November and subsequently spread to the eastern Indonesian and Australian regions during December and January. Comparision of the dates of the onset of the summer monsoon winds defined as simultaneous observations of westerly wind at 850hPa and easterly wind at 200hPa derived from the 9-year (1980-88) mean of the 5-day mean 2.5-degree latitude-longitude ECMWF 850hPa and 200hPa wind data showed that convective clouds are enhanced over the land areas over northern Australia, New Guinea, Sulawesi and southern Borneo, where a tendency for early onset and late retreat of the summer monsoon clouds is observed. The onset and retreat dates as derived from this study fill a major gap in these dates over Indonesia, Australia and New Guinea, where determination of onset (retreat) dates has been hindered by poor rainfall data coverage.
By using the stationary wave activity flux of Plumb (1985), the present study carried out a diagnostic analysis of the phenomenon in which a Rossby wave propagating from high to low latitudes affects the Baiu front around Japan in early summer. According to one composite and one case (during 20-29 June of 1982) analysis, there is a large horizontal southeastward component of the flux occasionally stretching from the Okhotsk Sea, via east of Japan (or Japan) to the sub-tropical region (about 25-27.5°N/155°E), when the blocking anticyclone around the Okhotsk Sea (the Okhotsk blocking anticyclone) establishes itself. On the other hand, a wide and strong ascending flux covering the area from East Siberia to the Okhotsk Sea (exactly where there is a wide and large divergence of the flux) was found during 20-29 June, 1982. These suggest that this area is one of the important forcing-source-areas for the quasistationary wave propagation, although a complete forcing mechanism of the propagation has not been found yet.
The seasonal change of the water budget in the Congo river basin is investigated by using hydrometeorological data averaged over long-term periods. Vapor flux convergence is calculated using the global objective analysis data of the ECMWF from 1985 to 1988. Precipitation and river discharge data mainly cover the periods 1920-1960 and 1932-1959, respectively. Evapotranspiration is estimated as precipitation minus vapor flux convergence on the monthly basis. The atmospheric water balance terms are related to the Normalized Difference Vegetation Index (NDVI) derived from the NOAA/AVHRR averaged from 1985 to 1987. On the monthly basis, the NDVI and evapotranspiration are in phase with the seasonal change of precipitation in the evergreen forest region, which mainly covers the northern part of the basin. In contrast, the NDVI and evapotranspiration lag precipitation by one month in the southern deciduous forest region covering the southern part of the basin. As for the entire basin, the lag-relationship between the NDVI/evapotranspiration and precipitation is similar to that for the southern deciduous forest region. In the dry season of the southern deciduous forest region, evapotranspiration exceeds precipitation in the entire basin, causing a decrease of the basin storage to its minimum value. In addition, from the viewpoint of the seasonal change of precipitation and evapotranspiration, it is concluded that the feature of the seasonal change of the water budget in the entire basin mainly reflects the characteristics of the southern deciduous forest region.
A three-dimensional non-hydrostatic anelastic model using a radiative-nesting lateral boundary condition is presented, and is applied to the simulation of the Yamaji-kaze on 27 September 1991. A 2.5km-resolution version of the model, which is double-nested with the operational hydrostatic model (Japan Spectral Model; JSM) of the Japan Meteorological Agency (JMA), successfully simulates the downslope wind and the associated internal hydraulic jump in the lee of the Shikoku Mountains. The non-hydrostatic anelastic model, which was used in a previous study (Saito, 1993), is modified so that it can express the time-changing environmental information through the lateral boundary condition. Variational calculus is used in order to obtain a three-dimensional mass-consistent initial wind field which satisfies the anelastic continuity equation and the upper and lower boundary conditions. The Orlanski-type radiation condition is employed for the lateral boundary condition of the inner model, using interpolated grid point values of the outer model as the external reference value. The validity of the nesting procedure is shown by a comparison with the non-hydrostatic linear analytic solution over a three-dimensional mountain. The Yamaji-kaze of the 27 September 1991 windstorm caused by Typhoon 9119 is presented, and realistic simulations are conducted using the 10km and 2.5km resolution non-hydrostatic models which are nested with JSM. The 10km-resolution model reproduces well the time change of the synoptic field by the approach of T9119, but does not simulate a conspicuous downslope wind. On the other hand, the 2.5km-resolution model reproduces well the downslope wind and the associated Yamaji-kaze front (internal hydraulic jump in the lee of the Shikoku Mountains). The characteristics of the model-reproduced wind field and the response of the jump to the magnitude of the environmental wind are generally consistent with the conceptual models of the Yamaji-kaze which have been proposed in previous studies (Saito and Ikawa, 1991a; Saito, 1993). Sensitivity experiments show that both the ground temperature and ground roughness affect the intensity of the downslope wind. In case of the Yamaji-kaze, not only the orographic characteristics of the Shikoku Mountains, which were emphasized in previous studies, but also the presence of the Hiuchi-nada (southern part of the Inland Sea in the lee of the Shikoku Mountains) contributes to intensifying the surface wind in the coastal plain through the difference in roughness between sea and land.
The influence of horizontal grid resolution (Δx) and horizontal diffusion on the maximum velocity of the sea breeze circulation is discussed using the results of a two-dimensional numerical model. The computed maximum updraft (WMAX) of the sea breeze decreases as Δx increases when Δx is larder than a certain value. The maximum velocity, however, approaches a constant as Δx is decreased to values less than the value. Increasing the grid interval is similar to smoothing the peak values of the velocity. The peak values, therefore, are decreased as the grid size is increased. Further, it was found that the WMAX is significantly weakened by horizontal diffusion. The magnitude of WMAX at a given point is not very meaningful, since the value can be altered by changing the grid size and the smoothing method. The area-weighted WMAX appears to be more physically significant in numerically simulated results. Therefore, use should be made of an appropriately fine grid for studying a given phenomena.
In three cities (Ahwaz, Bushehr and Shiraz) of the Persian Gulf area in Iran, atmospheric concentrations of chemical components of aerosols less than 2μm in diameter and sulfur dioxide were measured on June 3-7 in 1991. The concentrations of elemental carbon and particulate organic carbon were 2.5-6.4 and 5.1-14.3μgCm-3, respectively. Sulfate, nitrate and chloride concentrations were 2.0-6.4, 0.6-12 and 0.3-0.9μgm-3, respectively, and sulfur dioxide concentration ranged from 1.1 to 3.7 ppbv. The atmospheric concentrations of elemental carbon, particulate organic carbon, sulfate and sulfur dioxide in the cities were as high as those in the urban area of Japan.