Using a new analysis technique, the heat budget of the daytime atmospheric planetary boundary layer (PBL) over a complex terrain was obtained for 55 meteorological stations in central Japan under fair weather and weak synoptic wind conditions. Under such conditions, the daytime wind and temperature fields in the PBL are dominated by thermally induced local circulations. The PBL heating rate Qs at each station was estimated from routine observational data. The topographical features of the meteorological stations were divided into five categories: cape, coastal plain (within 20 km of the coastline), inland plain, basin bottom, and mountainous area. The basin bottom and inland plain exhibit large values of Qs, due to local subsidence heating acting as a compensating current of the upslope flow. The mountainous area, on the other hand, has a small value of Qs since the upslope flow removes heated air from this region. In the cape and a large part of the coastal plain, cold air advection due to the sea breeze depresses Qs. The daytime PBL heating causes a surface pressure depression proportional to Qs, resulting in a thermal low over the inland area.
The daytime PBL heating process and the air-land heat budget are analyzed for central Japan under fair weather and weak synoptic wind conditions, making use of routine observational data. The convective boundary layer (CBL) depth increases as the PBL heating rate Qs increases. According to the horizontal distribution of Qs, the depth of the CBL is larger over the inland basin and inland plain areas, while it is smaller over the coastal and mountainous areas. The mountainous area has a large CBL height (ASL) in spite of small values of Qs due to the higher ground level. The CBL height increases monotonically from the coastal region toward inland areas. The air-land heat budget is estimated for 11 sub-regions. Overall, there is no significant difference in the heat budget among the sub-regions, except that the Kanto Plain has a somewhat larger sensible heat flux and smaller latent heat flux, due to having the smallest percentage of vegetated area and the largest percentage of urban area. The Bowen ratio is estimated as 1.03 when averaged over central Japan. Surface moisture availability β, the ratio of the bulk latent heat transfer coefficient to the sensible heat transfer coefficient, is estimated as 0.29 for the mean value during the spring season over central Japan. Regional variation of β ranges from 0.16 to 0.46 over all the sub-regions.
A frictional boundary layer is coupled to a two-level model of the free atmosphere to investigate the wave-CISK (conditional instability of the second kind) in the tropics. To relax several assumptions made in a previous analytical study (Wang, 1988), the full set of linear model equations is solved numerically. In the stable parameter regime for the inviscid wave-CISK, the frictional convergence in the boundary layer generates a slowly eastward-propagating, unstable mode which has finite growth rate even for the short wave. In contrast to Wang (1988), the unstable mode does not have a maximum of the growth rate at any finite wavenumber, and is not equatorially trapped but has a wavy meridional structure at the longwave limit.
The 30-60 day tropical intraseasonal oscillation (ISO) has a quite complicated structure. For example, different behavior is observed between the wavenumber-one zonal wind and the wavenumberzero temperature (or geopotential). An attempt is made to understand such a structure from a unified view, using a linear response model based on the primitive equations. The thermal forcing, which is externally prescribed, has a 40 day period with variable amplitudes and moves eastward over a distance of 120° of longitude. This model simulates well the observed structure. The key to understanding the structure of the ISO lies in the contrast between the slow moving speed of the forcing and the fast group velocity of Kelvin modes. Regions with upward motion move slowly with the forcing, while areas with downward motion promptly extend to the entire longitude owing to the Kelvin modes. Thus, the vertical velocity and divergence fields have the same wavenumber-one structure as the heating. The zonal wind is the integral of the divergence, so that it must exhibit a wavenumber-one structure. On the other hand, the temperature must have a wavenumber-zero structure. The reason is that high temperatures result from diabatic heating in the forced region and adiabatic heating due to downward motion in the unforced region, and the maximum of diabatic heating and that of downward motion in the unforced region occur almost simultaneously. The geopotential shows similar behavior via the hydrostatic equation. Thus, the wavenumber-one structure of the zonal wind and the wavenumber-zero structure of the temperature (geopotential) necessarily correspond to each other. Marked structural changes occur in one cycle of the ISO. The slowly moving heat source excites a Matsuno-Gill (MG) pattern. When the heating amplitudes become small in the eastern portion of the forced region, a free Kelvin mode separates and moves eastward. Thus, a MG pattern having a slow speed is observed in the forced region, while a fast Kelvin mode is formed in the unforced region. Further, it can be shown that the vertical structure of the geopotential should be different from that of the zonal wind. The effects of the values of the imposed parameter on the structure of the model ISO are also examined. Moreover, five time scales which characterize the ISO are proposed, and the relationships which they must satisfy in order to reproduce the observed structure are discussed.
We studied the structure of snow trails from convective snow cloud developed along the west coast of Hokkaido Island, Japan in winter monsoon season. Photographic observations on the snow trails were carried out in company with radar and surface meso-network observations from 20 January to 20 February 1987. Snow trails were categorized into "burst type" and "virga type" from their morrphology. The former type was a peculiar case for snow showers. It had sharp edges and accompanied by a strong downdraft, which was also recognized by surface wind gusts and temperature drops. The downward velocity of downdraft was estimated about 10 m/s from time-series photographs. On the other hand, the latter type, which is a typical case of ordinary snowfall, showed little change in surface wind and temperature; and it caused no significant downdraft. Burst-type snow trails occurred where the surrounding sub-cloud layer was conditionally unstable and below 0°C temperature. The sub-cloud condition, high humidity and low temperature was quite different from that of thunderstorm or squall line downbursts. The downdraft from precipitating snow cloud in the convective mixing layer was not recognized as a dry microburst nor wet microburst. Therefore, we named this burst-type snow trail "snowburst".
Results are presented from a numerical simulation of the stratospheric circulation using a spectral general circulation model of the Meteorological Research Institute. The model has 23 layers in the vertical extending from the surface to approximately 70km in the middle mesosphere and uses the spectral transform method with a rhomboidal 13 truncation. The model includes all major physical processes as well as realistic radiative transfer. The model also includes Rayleigh damping as a parameterization of gravity wave breaking. The model has been run for two years, and the results are compared with observations. The model generally simulates the overall stratospheric fields of temperature and winds reasonably well. Seasonal variations of the stratospheric fields are also successfully reproduced. Successful aspects of the current simulation include: clear separation of the subtropical and polar night jet streams; stratospheric summer easterlies of the proper speed and extent; interhemispheric differences in the stratospheric circulation; major and minor sudden warming events; and an equatorial semi-annual oscillation. Obviously deficient results include: excessively cold polar night vortices; and much less westerly winds in the equatorial semi-annual oscillation. A simulated final warming in the Northern Hemisphere and a minor warming in the Southern Hemisphere are briefly described, along with the evolution of easterlies from late-winter to spring.
Part I of this study examined the large-scale situations and cloud variations of the Baiu front during July 1982, and showed that the large-scale increases of baroclinicity around the front and the low-level southerly wind toward the front contributed to the increase of cloud amounts in the frontal zone (the active phase). The regional difference in the cloud variations of mesoscale periods was also described. In Part II, the vertical structure of the Baiu front, features of disturbances in/around the Baiu frontal zone were studied for the six 5-day subperiods of July 1982, based on the relative vorticity fields. The results showed that during the active phases the Baiu frontal mesoscale disturbances tend to develop at -120°E, under the influence of the disturbances in the upper westerly jet. In Part III, the space-time scale and structure of the disturbances in⁄around the Baiu front are examined by spectral analysis mainly on the relative vorticity fields. The interaction between the disturbances in the Baiu front and in the upper westerly jet is also examined. The results of Part III are summarized as follows. From spectral analysis, the disturbances of a synoptic-scale (-6-day) and three mesoscale (-3.5-, -2.5-, 1-day) periods are found in/around the Baiu front. These disturbances developed through interaction between them, and showed a complicated evolution process. Many shallow-weak vortices were generated around the Quinghai-Xizang Plateau with 1-day period. Some of them in the Baiu front (30°N-40°N) propagated east along the low-level Baiu jet (B-jet) and began to develop into the long-lived mesoscale disturbances around the east coast (-120°E) of the continent. In the period when a synoptic-scale disturbance in the upper westerly jet (W-jet; in 40°N-50°N) developed around 120°E (the active-shallow Baiu front), the aforementioned weak vortices developed into the Baiu frontal shallow mesoscale (-2.5-day period) disturbances, making pairs with the deep mesoscale (-2.5-day period) disturbances propagated from the west in the W-jet. Some of the shallow and the deep mesoscale disturbances developed as subsystems of one synoptic-scale disturbance. In the period when the Baiu front east of -120°E showed a baroclinic structure (the active-deep Baiu front), the aforementioned shallow-weak vortices in the B-jet developed into the Baiu frontal deep mesoscale disturbances with a period of -3.5 days, coupling with the upper-level mesoscale (-3.5-day period) disturbances propagated from Central Asia in the W-jet. Although the period, vertical structure and evolution process of these mesoscale frontal disturbances differed from each other in accordance with the large-scale and/or synoptic-scale frontal situations, they caused the cloud variations of mesoscale periods in the frontal zone.
Simultaneous measurements of aerosols by use of an airborne optical particle counter (OPC) and a ground-based lidar were performed during January, 1986, over the Tsukuba area. The measurements were taken over the middle and lower troposphere in order to obtain both spatial and size distributions, to infer an extinction-to-backscatter ratio (S1) and a complex index of refraction (m). Bimodal distributions were commonly found in the aerosol volume size distributions from the OPC measurements over the flight range (310-4270 m). Although the spectral shape of the accumulation mode exhibited a slight difference between altitudes, it was reasonable to use a constant S1 estimated from the aerosol size distribution near the surface in analyzing the lidar signal. This was true except for the cases when a heavy dust layer was found in the higher troposphere, which may have originated from a Kosa (Asian dust) event. Assuming spatial homogeneity of aerosol optical properties, the mean value of S1 and its range of variation was inferred from the lidar signal along with additional information on optical thickness. Moreover, the tropospheric aerosol refractive index was estimated from the relationship between S1 and m based on the OPC data. In this experiment, two heavy-dust layers were found, one aloft near a height of 5000 m and the other just above the ground surface. In order to determine the ranges of optical parameters for the aerosols in both layers, it was necessary to treat each level separately. Since no in situ data was available for the upper dust layer, S1 for the upper layer was assumed while the data was analyzed for the lower layer to estimate the range of S1 there. The mean extinction-to-backscatter ratio and the imaginary part of the complex refractive index for the total air column of the lower dust layer (less than 4300 m) were estimated respectively as ranging from 32 to 66, and less than 0.04, assuming the real part as 1.55±0.03.
Relationships between the seasonal sea surface temperature values of four oceanic regions, namely the western Arabian Sea, the eastern Arabian Sea, the Bay of Bengal and the sea off Somalia coast with the Southern Oscillation Index, position of the 500 mb ridge along 75°E in April and the summer monsoon rainfall for the period 1948 to 1972 are studied. There is positive and significant relationship between the preceding winter sea surface temperatures in the Bay of Bengal and the 500 mb ridge position along 75°E in April. The study further shows that sea surface temperature anomalies in the Arabian Sea and the north Indian Ocean during the summer monsoon are caused by the monsoon circulation/monsoon activity over India rather than vice versa. It was also found that the premonsoon sea surface temperature values in three of the above study areas are useful for predicting the monsoon rainfall.