The global characteristic features of the lower tropospheric disturbances in the tropics are examined using the FGGE level III-b wind data during the northern summer of 1979. The activity of the disturbance with periods shorter than 10 days is greater in the northern equatorial region than in the southern equatorial region. Two spectral peaks at periods of 3-4 days and about 6 days both corresponding to westward moving disturbances with the zonal wavenumbers of 10-12 are pronounced in the zonal means of the time spectra in the northern equatorial region. No dominant spectral peak exists in the southern equatoriai region. The results of the power spectral analysis at individual grid points reveal large regional difference of the disturbance characteristics showing that the lower tropospheric disturbances. in the tropics are largely affected by the regional conditions. The 3-4 day period disturbances which correspond to the African wave predominate in a wide area from the central Africa to the central Atlantic and from the equator to about 25°N. The African wave disturbances have maximum amplitudes near the west coast of Africa and decrease their amplitudes in the eastern Atlantic. In the western Atlantic and the Caribbean Sea disturbances with a longer period (-6 days) become dominant. In the tropical Pacific the largest amplitudes of lower tropospheric disturbances are found from the western Pacific to the East China Sea. The dominant spectral peaks in a period range of 6-10 days exist in a region extending northwestward from the equatorial western Pacific to the East China Sea and this region generally corresponds to typhoon tracks. It is interesting to note that the period of the spectral peak gradually shifts from about 6 days in the equatorial western Pacific to about 10 days in the East China Sea along these tracks. A pronounced spectral peak of about 6 days is found in the eastern Pacific off the coast of Central America. Although the disturbance activity is quite low in the central Pacific as compared with those in the eastern and western Pacific, there exist weak spectral peaks in a 3-4 day period range along the ITCZ in the central Pacific. Disturbances with large power spectra of the 6-7 day period are found in the Southeast Asia from Indochina to North India and their intensity becomes maximum over the northern Bay of Bengal. These disturbances may correspond to the monsoon disturbances propagating along the monsoon trough region and intensified over the northern Bay of Bengal. Variations with longer periods (>13.3 days) predominate in the Asian monsoon region extending from the Indian Ocean through Southeast Asia to western Pacific. These variations are mostly due to the 30-50day period oscillations of the Asian monsoon activity which have been recently discovered. Similar long period variations, but with a 13.3 day period, are found in the North America-Atlantic monsoon region. The results of the spectral analysis of winds are compared with these of the outgoing longwave radiation (OLR) data. The spatial distributions of the time variations of OLR are quite similar to those of winds indicating that the convective activity is strongly coupled with the large-scale lower tropospheric disturbances in the tropics.
The evolution of the Baiu frontal zone over the China Continent during the early summer of 1979 is examined based on the observation data. The Baiu frontal zone is a quasi-stationary cloud zone with a large horizontal gradient of specific humidity. In late May when the Baiu frontal zone still stays in the South China Plain the air temperature increases rapidly in the North China Plain and the horizontal temperature gradient in the lower layer in the Baiu frontal zone vanishes. A comparison is made between the characteristics of the Baiu frontal zone over the China Continent for the two periods (I) (1-20 May) and (II) (10-29 June). In the period (I) the stratification is stable and the frontal surface with a relatively large temperature gradient tilts northward with height in the Baiu frontal zone. The widely spread stratiform clouds as well as convective clouds cover the frontal zone. On the other hand, the stratification is convectively unstable and the Baiu frontal zone consists of active deep cumulus clouds in the period (II). It is suggested that the vanishment of the horizontal temperature gradient in late May greatly affects the difference between the stabilities for moist convection in the two periods. The ground surface temperature attains the nearly maximum value in late May and the amount of sensible heat supplied from the ground to the atmosphere is large in the North China Plain. The mixed layer with a thickness of 2000-3000m develops there through dry convection. The sensible heat supplied from the ground cancels out the cold air advection due to the transient eddies and contributes to the maintenance of the high air temperature there. It is also suggested that the air temperature in the lower layer increases rapidly in a few days after the passage of a synoptic-scale disturbance by the sensible heat from the ground there. Thus it is concluded that the heating from the ground in the North China Plain contributes greatly to the change in the structure of the Baiu frontal zone over the China Continent.
An extremely sharp pressure dip accompanied with a mesoscale disturbance was observed in the northwestern quadrant of Typhoon 8124 (Gay) passing off the coast of the Boso peninsula. The amount of pressure drop was so large that the minimum pressure value associated with the disturbance (967.4mb) was almost comparable to the central pressure of this typhoon (965mb). The steering velocity and the lifetime of the disturbance was estimated to be a little faster than the typhoon movement and more than five hours respectively. The structure was analyzed based on the data from dense network of surface meteorological observations and a doppler radar installed in MRI, and it was shown that the surface wind field and the cloudiness distribution had internal gravity wave characteristics. The disturbance was accompanied with a rainband system, and the surface pressure dropjump phenomena seems to be closely related to the finer structure of systematic convective activity with the east-west wavelength of 90km embedded in the rainband.
The diurnal variation of wind in the atmospheric boundary layer over the Kanto plain is studied in order to find the contribution of each of the land-sea breeze and the diurnal variation of turbulence as a function of the magnitude of the larger-scale pressure gradient. The land-sea breeze is found to be the main source of the diurnal variation of wind when the pressure gradient is less than 5ms-1 in geostrophic wind speed. As the pressure gradient intensifies the land-sea breeze weakens and is replaced by a turbulence-induced diurnal variation, which dominates over the land when the pressure gradient exceeds 10ms-1.
The integrated surface sensible heat flux and the maximum mixing depth are obtained by routine aerological data and daily maximum and minimum temperature data measured at the Tateno Aerological Observatory located 60km east of Tokyo. Annual variations of the components of the surface heat balance are also estimated from the integrated surface heat flux, net radiation and the downward flux of short-wave radiation. Annual variations of the convective velocity scale, the standard deviation of the vertical velocity fluctuations, and the eddy diffusivity are estimated by the surface heat flux and mixed layer height. The potential temperature gradient of the stable layer capping the mixed layer is 0.003°C/m during the winter season, 0.005°C/m in midsummer, and 0.003°C/m under the travelling anticyclones. The integrated surface sensible heat flux from sunrise to a time when the daily maximum temperature occurs is largest in early spring (170ly), smallest in winter (50ly). The maximum mixing depth is highest in March (1, 500m) and lowest in December (700m). The surface sensible heat flux is largest in spring because of low temperatures, aridity, and a small amount of evapotranspiration. The Bowen ratio is unity in May and November. The lowest value of the Bowen ratio is 0.4 in July. Agreement of the abovementioned values with more accurate observations, is reasonably high. So this simple method for estimating parameters of the mixed layer structure is ascertained to be useful.
Infrared greybody emissivity of high altitude semitransparent cloud at anvil-head accompanying large scale convective cloud system and the relationship to the visible albedo was estimated from GMS-1 radiation measurements. Generally we can not estimate the greybody emissivity of semitransparent cloud like cirrus from the only satellite radiation measurement because satellite observations do not directly give the cloud top temperature which is needed for estimate of the greybody emissivity. However, the greybody emissivity of semitransparent cloud can be easily estimated under the condition that the temperature of the anvil may be approximately equal to that of the core of the system. Seventeen examples satisfying nearly the condition were picked out from imageries of GMS-1 (22 Sept. 1979) in the tropical region. The author could estimate the greybody emissivity of many semitransparent clouds from the only radiation measurement of the satellite. Obtained relationships between cloud greybody emissivity and the visible albedo showed radiative character between the theoretical relationships for the cloud of long ice cylinders and that of spherical droplets. In addition, it showed that the greybody emissivity of semitransparent cloud increases more quickly than the albedo with increasing of the optical thickness.
By inspection of the image constructed from the brightness temperature difference between the 11μm and 12μm satellite data, semi-transparent cirrus clouds where brightness temperature in 11μm and visible reflectivity are relatively low are identified clearly as having a bigger brightness temperature difference area than that of cloud free areas. Effective emissivity is determined for semi-transparent cirrus clouds using the simple cloud model where scattering is neglected and only absorption is considered. Cirrus clouds often have 'black' parts in this wavelength. The radiance of the 'black' part of the cirrus clouds whose temperature is assumed to be equal to that of semi-transparent part and the clear radiance just off the cirrus clouds which is assumed to be equal to the radiance at the bottom of the cloud are used for effective emissivity calculation. A simple relationship between the effective emissivity for 11μm and 12μm has been determined empirically from 860 satellite measurements for eight cirrus clouds cases which have 'black' parts. The bi-spectral method has been developed to retrieve cloud temperature and effective emissivity for semi-transparent cirrus clouds using the effective emissivity relationship between 11μm and 12μm. We have compared the retrieved cloud temperature and effective emissivity by our method with those estimated from the 'black' part of the cirrus cloud. It shows reasonable agreement for the cirrus cloud whose effective emissivity is larger than 0.4.
In order to investigate microphysical processes in the stratiform precipitations including melting layers, size distributions of the precipitation particles were measured by "filterpapers method" at three observational stations with different altitude on Mt. Fuji. The following conclusions on the microphysical processes in the stratiform precipitations were obtained inspecting the time variations of the size distribution at each station: i) In the upper part of the melting layer, small sized snowflakes (melted diameter<0.1cm) were predominant in volume and number. ii) In the middle part of the melting layer, melting and growth due to coalescence of the melting snowflakes seem to have occurred simultaneously, and the large sized particles(>0.2cm) were found frequently. iii) In the lower part and just below the melting layer, precipitation particles had finished melting. The large sized particles decreased in volume and number perhaps due to breakup and therefore the medium sized particles (0.1-0.2cm) were predominant. iv) The vertical change of the size distribution of raindrops was insignificant far below the melting layer.
A method of measurement of atmospheric sulfuric acid particles was developed by modifying a method proposed by Penzhorn and Filby (1976). This method used an assembly for the generation of diazomethane and methylation of sulfuric acid. The minimum detection of H2SO4.t was 3ng. The amount of sulfuric acid particles generated in a test chamber determined by this method agreed well with those by barium chloranilate and titration methods. On field sampling gaseous NH3; which converted H2SO4 to (NH4)2SO4 was eliminated by using an NH3 denuder. In Tokyo, in August of 1981 the concentrations of sulfuric acid particles were 0.3-4.7μg m-3. In Sapporo they were 0.2-1.3μg m-3; between May and July in 1981, and 0.1-1.4μg m-3 between November in 1981 and January in 1982 respectively. The cumulative fraction by weight of sulfuric acid particles less than 2.1μm in diameter was 90%. The ratio of H2SO4/SO2-4 increased with decreasing particle diameter.
The collection efficiency of uncharged natural snow crystals for the aerosol particles of 0.1 to 6μm in diameter was experimentally examined. Each scavenging test was performed under the condition where the external electric field and phoretic forces were negligibly small. The collection efficiency decreased with the decreasing aerosol size from 6 to 0.2μm in diameter. However, the collection efficiency attained its minimum value at around 0.2μm, and increased with the decreasing aerosol size from 0.2 to 0.1μm. As for the aerosol particles of 0.2 to 6μm in diameter, the interception was considered to be the main scavenging mechanism. The experimental results also showed that the difference between the collection efficiencies of rimed and unrimed snow crystals was small. The collection efficiency determined in the present experiments showed a good agreement with the observational values obtained by Murakami et al. (1981, 1983).
The attachment rate of 0.1μm dia. particles to stationary uncharged snow crystals was experimentally obtained using an aerosol chamber. The experiment was performed under conditions where the external electric field and phoretic forces were negligibly small. The measured attachment rates were considerably greater than the values calculated from the stationary Brownian diffusion theory. This discrepancy might result from the increase of the measured attachment rate due to additional factors (image force, turbulent diffusion, etc.) in the experiment. The attachment rate of stationary snow crystals was extrapolated to the collection efficiency of falling snow crystals using the ventilation coefficient. The comparison between the measured and extrapolated collection efficiencies showed that the convective Brownian diffusion was the main scavenging mechanism although the additional factors (image force and turbulent diffusion) might enhance the collection efficiency.