In order to estimate the effect of horizontal advection on the change in direction of wind vectors with time, wind data from 71 observational stations in three areas: Oita City, California and Washington States, where the horizontal scale differs considerably from each other, are analyzed following the equation for the local rate of change of wind direction. The equation of rotation rate of wind vectors derived by Neumann (1977) is modified to a suitable form to discuss the direction of rotation of wind hodographs, because the shape of the wind hodograph is generally complicated and the origin of coordinate is outside the wind hodograph in most cases. Then, a daily weighted mean of the square of deviation from the daily mean wind vector is selected as a typical quantity of the direction of rotation of the wind hodograph. We can illustrate at once with this quantity the shape and the direction of rotation of wind hodographs. Furthermore, the daily mean of the rotation rate of wind vectors could be written in terms of the Coriolis parameter and the vertical vorticity which results from the daily mean wind field and the other terms. As the result of data analysis for three areas, it is shown that the effect of horizontal advection in the modified equation of rotation rate of wind vectors is comparable to that of the Coriolis force, but those places where this condition is satisfied are limited to the local area.
Using seven-year (1979-1985) rawinsonde data in the equatorial region, the 30-60 day variation in the geopotential height (H) and the temperature (T) fields and their relations with fluctuations in the tonal wind (U) field are studied. After examining in detail the 30-60 day variation in the time series, the lag-correlation method is applied to the seven intervals of four month duration which show active variation. The main results are as follows: (i) In the H field of all the troposphere and the T field in the upper troposphere, the zonal standing-wave-like structures with a little phase differences are found in the wide region from India to Central America through the date line. (ii) In the lower troposphere, the 30-60 day variations of H and U are in phase near the date line, but the signal of U leads that of H by about 120° over the Maritime Continent. In the upper troposphere, the signal of U leads that of H by about 120° near the date line. (iii) Though the vertical structure has a large year-to-year variability, the upper tropospheric H leads the lower one by 70°-120° in the phase over the western and mid Pacific. The properties of the H field obtained in the present work are considerably different from well-known ones in the U field. These results suggest that the simple Kelvin mode which has an out-of-phase structure between the upper and lower troposphere is not dominant in the 30-60 day variation.
In this paper we present results of several preliminary numerical experiments to study the mechanism of the observed variabilities in wind and convection associated with super cloud clusters(SCC), westerly wind bursts and 30-60 day oscillations in the western Pacific region. Using the Lau and Peng(1987) model, we show that the generation of eastward propagating precipitation pattern associated with the 30-60 day osillaition can be identified as a SCC. During its development stage, the SCC is accompanied by convective clusters moving in opposite direction(westward) to the SCC. Our results suggest that these westward propagating cloud clusters are associated with Rossby waves produced at the development stage of the 30-60 day disturbance due to mutual adjustment of the large scale flow and heating. If the boundary forcing is zonally symmetric and in the absence of other external forcings, the SCC eventually settles down to an organized eastward propagating precipitation pattern accompanied by a planetary east-west circulation cell in the equatorial zonal plane. It is found that intraseasonal oscillaition in the tropical atmosphere is a multi-scale process. Three basic spatial scales are identified, i.e., synoptic scale motions associated with a cloud cluster(1000-2000km), size of the SCC complex(-2000-4000km) and planetary circulation scale(wavenumber 1 and 2). The first is associated with westerly wind burst, double cyclone formation and high-frequency fluctuations of order of several days. The second and third are associated with slow eastward propagation associated with the 30-60 day oscillaition. The relationship between SCC and westerly wind burst is studied by examining the detailed spatial and temporal variation of the model SCCs as they propagate over tonally varying sea surface temperature. It is found that wave-CISK heating in the lower troposphere as well as surface heating are crucial in leading to the strongly asymmetric low level westerly wind bursts similar to those observed in the real atmosphere. The implication of the present results in terms of the development of a unified theory of low frequency oscillation is also discussed.
The characteristics of radar echoes of the precipitating snow clouds over the Ishikari Bay, Hokkaido, Japan were investigated statistically during four months, February and March of 1983 and 1984. In this analysis, the precipitating snow clouds were divided into three types: typical monsoon, final period of monsoon type and low pressure type based on the daily weather maps. Examined elements in this analysis were as follows; Size distributions of radar echo population, percentage occupied by the strong echo areas as against total precipitating echo areas, time changes of echo areas, locations of the center of individual large echo areas, life time of the echoes, and so on. Highly interesting characteristics were obtained; the log-normal distribution was obtained for the size ranges of radar echoes of all snowfall types; the percentage occupied by the echoes stronger than level 2 accounted for less than 40% of all echo areas; the total echo area showed sharp fluctuations; the locations at which the radar echo areas when the snow clouds invading the plain reached their maximal sizes were along the coast line; the average echo area having a life time of 1 hour was approximately 25km2 etc.
A statistical study is made of the relationship between the Northern Hemisphere atmospheric circulation and tropical convective activity during summer on intraseasonal time scales. An emphasis is placed on differences in the tropical-midlatitude interaction over East Asia between two 50-day periods: May 26-July 14 and July 15-September 2. Each period corresponds to the East Asian rainy season or the East Asian mid-summer season, respectively. Barotropic instability of climatological monthly mean 300mb stream functions is also investigated to interpret the observational results. It is found from correlative analysis that the tropical convection is not strongly related to the midlatitude flow over East Asia during the rainy season, and there are no definite indications that the tropical convection forces the midlatitude flow. On the other hand, during the mid-summer season convective activity around the Philippines and the Indo-China Peninsula has a large impact on the East Asian atmospheric circulation. In particular, geopotential height over the Yellow Sea is most strongly affected by the tropical convection. Active convection in the western tropical Pacific is accompanied by a wave train with zonal wavenumber 6 over the North Pacific, which originates from the western tropical Pacific. The location of the wave train does not depend strongly on the location of the convective activity. One of the major barotropic unstable modes for the climatological 300mb flow for August resembles the wave train observed over the North Pacific during the mid-summer season. It behaves like a standing wave, and has a large amplitude around the Yellow Sea. The e-folding time of this mode is 11 days when dissipation is ignored. Barotropic unstable modes for June do not have large amplitudes over East Asia. These results are consistent with the above observational results and suggest that barotropic instability plays an important role in the tropical-midlatitude interaction over East Asia during the mid-summer season.
The seasonal transition of the characteristics of the lower-level circulation systems around the Baiu front in China from spring to summer and their relation to the Northern Summer Monsoon are examined by using the observational data of 1979. The Baiu front in May (stage (A)) is the system within the baroclinic zone in middle latitudes. The surface high pressure belt with cooler air and large divergence is located just to the north of it and the subtropical high at the surface level to the south of the Baiu front is not so clearly found. The subtropical high with a strong lower-level southerly wind toward the Baiuu front around China was enhanced abruptly in the middle of June, nearly simultaneously with the onset of the Northern Summer Monsoon (the beginning of stage (C)). In stage (C) the low OLR area (intense heat source area) expands widely in South Asia (EQ-25°N/60°E-105°E). The pressure in the lower layer decreases there, which results in the geostrophic enhancement of the southerly wind in the subtropical high area around 120°E by producing the eastward pressure gradient. At the same time the belt-shaped low OLR area extending from India to Western Pacific region (-10°N/60°E-140°E) is formed, which intensifies the local Hadley cell with strong lower-level divergence around 20°N as its northern branch. It is concluded that the change in the lower-level wind structure of the subtropical high to the south of the Baiu front in the middle of June is due to the superposed effect of the two characteristics of the heat source distribution associated with the Northern Summer Monsoon. The additional analysis by using the pentad mean data from 1979 to 1983 supports this conclusion.
Optical thicknesses and volume spectra of aerosols were retrieved from spectral extinction and aureole measurements for the yellow sand events observed during 4-8 May 1982 at Nagasaki, Japan. The results showed a dominance of large particles of several micron radius and a reduction of submicron particles. The mode radius of the column volume spectrum obtained with the aureolemeter was larger than two microns during the yellow sand events. The aerosol optical thickness at a wavelength of 0.5μm and the Angstrom's exponent α took values of 0.5-1.0 and 0.1-0.5 in contrast with their normal values of 0.2-0.8 and 0.8-1.5, respectively. Trajectory analyses from the synoptic charts as well as the elemental composition, the number density and the scattering cross section of aerosols supported consistently the prevalence of the continental air mass associated with the frontal activity during the yellow sand events.
Phase functions of yellow sand particles were measured by a polar nephelometer during 24 April-11 May, 1982 at Nagasaki, Japan. They suggested a strong nonsphericity of the particles, which can be reconstructed by the semi-empirical theory of Pollack and Cuzzi or by Mie particles with large fictitious absorption. Spectral extinction cross section, single scattering albedo, asymmetry factor and backscattering phase function were estimated using the volume spectra retrieved from data of several instruments including the polar nephelometer data. Volume loading of yellow sand particles at the observation site was estimated as 666l/km2 at the maximum stage of the yellow sand event of 4-6 May and 183l/km2 for the secondary maximum of 8 May. Using two estimates of the absorption index (0.01 and a model variable with wavelengths), these values show that solar radiative heating from 0.08 to 0.4°C/day can be expected in the atmosphere over considerably wide area in one yellow sand event.
A new formulation is proposed to diagnose wave-mean flow interactions and Lagrangian-mean meridional circulations within a finite amplitude nongeostrophic system. In this scheme, the vertical coordinate is defined by the zonally averaged pressure along isentropic surfaces and all variables representing zonal mean fields are averaged with normalized weights proportional to the mass of air between two isentropic surfaces. Although the zonally averaged momentum, continuity and thermodynamic equations are formulated in a way similar to the transformed Eulerian mean (p-TEM) equations by Andrews and McIntyre, the thermodynamic equation of the new scheme does not have eddy terms. This makes it possible to correct for 'Stokes drift' in meridional circulations for finite amplitude disturbances. Using a perpetual January run of the NCAR community climate model, a preliminary diagnostic study demonstrates a practical advantage of the new scheme. Comparisons of the new scheme with the p-TEM show considerable differences in meridional cross sections of the mass streamfunctions and the Eliassen-Palm flux divergence. In particular, the mass streamfunction obtained with the new scheme forms a hemispheric single cell circulation in the lower stratosphere (the so-called Brewer-Dobson circulation). As a further application of the new coordinate system, a two dimensional axisymmetric tracer transport equation is formulated, where diffusion due to adiabatic motions is expressed by a symmetric tensor involving only one independent variable.