The collection of aerosol particles using an Andersen high-volume air sampler was carried out during the austral summer, between 15 November 1986 and 13 March 1987, on board the icebreaker Shirase over the Indian and Antarctic oceans. Data sampling was also made during the cruise of the Hakuho Maru from 3 June to 1 August 1986, over the northern Pacific Ocean. The collected samples were analyzed by ion-chromatography to examine the mass concentrations of methanesulfonic acid (CH3SO3H or MSA) and non-sea-salt sulfate (nss-SO2-4) within the aerosols. Methanesulfonate (CH3SO-3 ), anion component of MSA, was detected in the fine aerosol particles less than 1.1μm in diameter. In general, the CH3SO-3 concentrations increased with decreasing air temperature, whilenss-SO2-4 concentrations showed an opposite tendency. The highest concentrations of CH3SO-3 and nss-SO2-4 were found to be 0.067μgm-3 and 0.77μgm-3, respectively, near 40°S where wind speeds were relatively strong. The results demonstrate first that, at higher latitudes, low air temperatures accelerate MSA production while the lower concentration of oxidants, such as H2O2 and OH, retards nss-SO2-4 production from MSA and SO2. Second, strong wind contributes to the increase of the flux of dimethylsulfide ((CH3)2S or DMS) from the ocean to the atmosphere.
The characteristics of snow crystal growth by vapor diffusion at water saturation and in free fall were quantitatively investigated in a vertical supercooled cloud tunnel for periods up to 30 min at temperatures from -3 to -23°C. The results obtained are as follows: 1) the basic growth habits were plates (>-4.0°C), columns (-4.0--8.1°C), plates (-8.1--22.4°C) and columns (<-22.4°C), respectively. At about -5.5, -12, -14.5, -16.5 and -18°C, crystal shapes were enhanced with time; 2) for an isometric crystal, the slope of a log-log plot between the crystal mass and the growth time showed the Maxwellian value of 1.5. The mass growth rate of a shape-enhanced crystal was larger than that of the isometric crystal, indicating more effective vapor transfer on the former; 3) in the case of shape-enhanced planar crystals grown at around -12, -14.5 and -16.5°C, ventilation effects became recognizable, whereas the effect was not evident for needle crystals grown at about -5.5°C. This suggests that the characteristic length of the flow field even around a needle crystal is along the a-axis. The ventilation effect became significant when the Reynolds number exceeded about 2 (sector) and 5 (dendrite); 4) linear relationships between the drag coefficient and the Reynolds number were found in log-log plots.
In order to study the Yamaji-kaze-a typical downslope wind found in Japan, the two-dimensional flow over an asymmetric mountain is simulated by use of a non-hydrostatic model. The Yamajikaze front and the reversed wind behind the front-characteristic features of the Yamaji-kaze-are explained in terms of the internal hydraulic jump and its associated circulation. Numerical experiments for a homogeneous atmosphere show that the behavior of the internal hydraulic jump is significantly affected by the inverse Froude number and the shape of the mountain. When the inverse Froude number is large, a quasi-steady state solution such that the hydraulic jump remains on the lee side of mountain is obtained, with the associated reversed flow being generated just behind the hydraulic jump. In the case of the Yamaji-kaze, the asymmetry of the Shikoku Mountains and the blocking effect of the Chugoku Mountains impede the propagation of the Yamaji-kaze front and allow the reversed wind to occur more readily. For the case of the Yamaji-kaze observed on 21 April 1987, a notable inversion layer was found at a level near the mountaintop. It is confirmed, by numerical experiments of a heterogeneous atmosphere with the observed thermal stratification, that the surface wind strengthens in the presence of the inversion when compared to that without the inversion. Development and propagation of an internal hydraulic jump are qualitatively simulated under the observed thermal stratification and timechanging wind profile. On the basis of the experimental results a conceptual model of the Yamaji-kaze is proposed.
Measurements of direct solar irradiance and sky radiance were carried out in Sendai, Japan for the period from September 1981 to May 1985 using a scanning spectral radiometer (aureolemeter). Size distributions of columnar total aerosols were retrieved by inverting both spectral optical thickness and solar aureole radiance data. The size distribution of aerosols due to the El Chichon eruption in 1982 was estimated as the difference between columnar volume spectra before and after the eruption. The results indicate that the El Chichon aerosol had a monomodal volume spectrum with a mode radius about 0.5 μm; their contribution to the total aerosol volume reached the maximum in the winter of 1983, i. e. December 1982 to February 1983, then gradually decaying to the normal level prior to the eruption by the spring of 1985. A seasonal model of tropospheric aerosols over Sendai was constructed by subtracting the volume spectrum of the El Chichon aerosol model from that of the columnar total aerosols, and successfully represented by a bimodal log-normal function. The aerosols in spring and summer seasons have different features in respect of volume spectra. The coarse particle mode aerosols with radii around 3μm are predominant in spring while the accumulation mode aerosols with radii around 0.2μm are predominant in summer.
Two different types of aerosols were observed during Polar Stratospheric Clouds (PSCs) events at Syowa Station (69°00'S, 39°35'E) using a lidar. Thermodynamical discussion on particle forms in the Antarctic winter stratosphere suggests that these two types were possibly nitric acid trihydrated (NAT) crystal and ice crystal, and in the early stage of a PSCs event most of the particles are NAT, and in the fully developed stage the major component is ice crystals. Early stage PSCs sometimes contained an ice crystal particle layer in their lower layer, possibly due to water vapor inlet from the troposphere; the ice particles appear without nucleation on NAT particles. The region where PSCs particles can actively form expanded to near the tropopause in the fully developed stage of PSCs. Particle descent from the stratosphere to the troposphere is certainly active under such conditions, and thereby this motion is an important sink of stratospheric particulate matter and relating gases. Denitrification of the stratosphere due to this will be an important process for disturbing Antarctic ozone.
In order to measure the liquid water content of a melting snowflake, a new automatic instrument based on a filter paper technique has been developed and used for field observation of sleet. Analysis of the observational data indicated that the relationship between the liquid water content w and the mass of the snowflake m (mg) could be expressed as W=βm-a.α and β range from 0.27 to 0.95 (0.53 in an average) and 0.07 to 0.51 (0.25 in an average), respectively. A simple model for the melting of snowflakes showed that α was a parameter depending on the fall velocity of the snowflake alone and parameter β depends on the quantity of heat transferred from ambient air to the snowflake and the density of the ice skeleton in the melting snowflake as well as the fall velocity.
The daytime thermal low and nocturnal thermal high over central Japan are analyzed for fair weather and weak synoptic wind conditions during the spring. The thermal low and high were usually analyzed making use of the sea level pressure, calculated by extrapolating temperature profiles to sea level with a lapse rate of Γ=0.0065 K m-1. However, such analysis overestimates the strength of the low/high by a factor of 2 to 3. In this study, overestimation was avoided by estimating the meso-scale pressure field for given constant levels with the application of a new method. During the daytime, pressure in the atmospheric planetary boundary layer (PBL) decreases with time due to PBL heating, with the daytime thermal low developing in the late afternoon. The strength and depth of the low are about -2 mb and 2000 m ASL, respectively. The thermal low is caused by the horizontal difference in the PBL heating rate over a complex terrain, this difference being attributed to a combination of three effects: the basin heating effect, the plain-plateau effect, and the sea breeze cooling effect. The first two effects are especially important. The nocturnal thermal high develops around sunrise. The strength and depth of the high are about 1.6 mb and 2000 m ASL, respectively. The thermal high is caused by the horizontal difference in the PBL cooling rate over a complex terrain, and this difference is attributed to a combination of the basin cooling effect and the plain-plateau effect, which have the opposite effects of those in the daytime.
The structural features of the eastward-propagating super cloud clusters and their westwardpropagating internal structures observed over the Pacific Ocean are described based on the GMS IR fundamental histogram data and the Global Objective Analysis (GANAL) Data. The analysis period is 1∼20 June 1986 during which four organized groups of cloud clusters (super cloud clusters) with a longitudinal scale of about 3000 km successively propagated eastward over the equatorial western Pacific Ocean. The main results are as follows. (1) The hierarchical structure of the super cloud clusters observed in 1∼20 June 1986 was associated with the equatorial eastward-propagating Kelvin wave-type mode and several degrees off equatorial westward-propagating "easterly waves" . (2) The equatorial eastwardpropagating mode was dominant between 10°N and 10°S, mainly in the zonal wind component. The phase relation between the wind and the height anomaly showed this mode had a Kelvin wave-type structure, although the propagation speed was as slow as 10°∼12°/day. The vertical structure indicated the downward energy propagation. The conversion from the convectively constructed potential energy to kinetic energy was not distinct, while the energy flux input from the southern hemisphere was indicated in the upper troposphere. (3) The structure of the westward mode associated with the westward-traveling cloud clusters corresponded to the "easterly waves" . To the rear of the lower convergence of the eastward mode (in SCC region), the upper level structure of the "easterly waves" was more firmly formed with the enhanced cloud activity producing the energy conversion from the available potential to the kinetic energy of the "easterly waves". Its amplitude in the lower troposphere was not significantly influenced by the eastward mode.
A three-dimensional mesoscale numerical model is presented, designed with the capability of simulating the airflow and orographically-induced rain in the presence of steep irregular terrain. The model utilizes a fourth-order accurate version of Arakawa's potential enstrophy and total energy conserving scheme to improve the simulation of nonlinear aspects of the airflow over steep topography, along with an adiabatic reference atmosphere to reduce the effects of orographic truncation errors. The precipitation processes are represented by large-scale condensation. The model is applied to the island of Hawaii. The results of a 24-h simulation indicate the model is generally successful in reproducing leeward eddies depicted in the composite surface airflow patterns. A nondimensional analysis of vortex parameters suggests that the simulated vortex pattern is an atmospheric analog of the Karman vortex street and compares favorably with an observed case of Hawaii vortices. Spatial distribution of simulated rainfall is also generally in good agreement with observations. However, the lack of precipitation in the lowland along the windward as well as leeward coast is apparent. The excess rainfall over the southern peninsula suggests the need for an finer grid mesh to improve representation of the effects of subgrid-scale mountain forcing. An application of the model to regional climate modeling through the use of large-scale objective analyses is also discussed.
In order to assess the practical feasibility of a dynamical one-month forecast, we performed several sets of ensemble forecasts using the lagged average forecast method. The results obtained by verifying the eight wintertime cases are summarized as follows: (1) The eight-case average anomaly correlation coefficient of the ensemble forecast for the 1-30 day mean 500 hPa geopotential height in the Northern Hemisphere is 0.54, and its average root mean square error is 55 m. The skill of the ensemble forecast is better than that of both the ordinary dynamical forecast starting from the latest intitial state and the climate forecast. So it seems that the one-month forecast using the lagged average method is effective for practical use. (2) For the 10-day mean 500 hPa geopotential field, the merit of ensemble averaging is apparent after the second 10-day mean (11-20). But in the third 10-day mean (21-30), the average root mean square error is slightly larger than that of the climate forecast. So its skill is marginal for practical use.