We have performed initialization and forecast experiments using the ECMWF FGGE Level IIIb analysis data and the NCAR Community Climate Model. The performances of several initialization schemes, including nonlinear normal mode initialization (NNMI) with diabatic heating effects, were examined. Global diabatic heating distributiohns were evaluated from 1) a residual of the thermodynamic energy balance using FGGE analyses (referred to as "observed" heating) and 2) a time integration of the prediction model (referred to as model generated heating). Many 24-hour forecasts were carried out with initial condistions from each initialization procedure. While the performance of NNMI with the "observed" diabatic heating was superior, the initial large-scale divergent circulations were not retained during the forecast period unless the prediction model was forced to generate a diabatic heating sufficiently similar to the "observed" heating. We conclude that not only is a realistic heating distribution necessary to produce proper initial conditions, but also physical parameterization in the prediction model must be able to generate an appropriate heating distribution. This is necessary to correctly predict the quasi-stationary, large-scale tropical circulations.
Effects of large-scale orography on January atmospheric circulations are studied with the use of a general circulation model (the MRI⋅GCM). The model is a 5-layer tropospheric model with its top at 100mb. The horizontal resolution of the model is 5°×4°in the longitudinal and the latitudinal directions, respectively. Four cases of experiment are performed under the perpetual January condition to isolate the orographic effects of the Tibetan Plateau and those of the Rockies and Greenland.
The aim of the present paper is to investigate the formation mechanism of a convergent cloud band which appears east of the Korea Peninsula over the western Japan Sea under northwesterly winter monsoon. Numerical experiments with different surface conditions reveal that the land-sea contrast of thermal property between the Peninsula and the Japan Sea plays the leading role in the formation of the convergent cloud band. Less transformation over the cold land causes a mesoscale high pressure in the lower troposphere with its center at the southeast end of the Peninsula and a convergence zone forms over sea at the eastern margin of it. The convergence zone makes cumulus convections active and organized into a band. In addition, blocking effect of the mountains north of Korea acts to intensify the convergence zone.
Harada (1981a) found a cyclonic mesoscale vortex which often appears in the Kanto Plain onclear and calm nights. Numerical simulation of this vortex is carried out using a three-dimensionallocal wind model (Kimura and Arakawa, 1983), which is constructed from the Boussinesq equationswith the hydrostatic approximation. The characteristics of the simulated vortex agree wellwith Harada's results. The formation mechanism of the vortex is investigated by numerical experiments using a simplifiedtopography, which is a Guassian mountain with a crater. From the results, the mechanism isclarified as follows : (1) Positive vorticity is accumulated in the daytime in a 'heat low' by the anabaticwind developed over the mountain. The historical effects of the heat low are very important for thevortex formation. (2) At night, the katabatic wind prevails on the slope of the mountain. The vorticitybecomes low in the center region of the mountain because of strong divergence. As a result, thehighest vorticity region appears around the foot of the mountain. (3) If there is a crater at the sideof the mountain, further vorticity convergence occurs in the crater and an isolated vortex forms there.Although the effect of the crater is not completely clear, an estimation using the vorticity equationshows the convergence term is the most important for the vortex formation in the crater. This wind convergence is generated by katabatic winds on the sloping surface around the crater. Mechanicalinstability of the high vorticity zone at the foot of the mountain does not seem to be strong enough togenerate a vortex, alone. Finally, numerical simulation is carried out also for the northern part of Japan and shows thatthree similar vortices form around Hokkaido. One of them, near the coast of Tokachi Plain, is oftenclearly recognized from observed wind data at night in clear and weak synoptic-forcing days, thoughits southern half can not be observed becaue it extends over the sea.
In order to make clear the difference between the earth’s radiation budget over land and over sea, three components of the budget measured by the wide field of view sensors aboard NIMBUS 7 were analyzed for the period during November 1978 to October 1979. In each of the absorbed solar radiation and the emitted terrestrial radiation, it is confirmed that the zonal average over sea is about 40 W/m2 larger than over land covering the latitudinal range of 40 degrees around the solar declination. It is interpreted because the clear sky albedo over sea is about half of that over land and also the cloud amount over sea is about 13% less than that over land in the equatorial zone. In the absorbed solar radiation, it is found that in summer the zonal average over sea is about 15 W/m2 smaller than that over land covering the latitudinal range of 30 degrees centered 60N or 60S. It is estimated because in the latitude of summer the cloud amount over sea is so larger than over land as to cancel the clear sky albedo effect though clear sky albedo over sea is about half of that over land. In the emitted terrestrial radiation, it is also found that the minimum zone of it associated with ITCZ is lower over land than over sea, and the maximum zones of it associated with the subtropical high pressure zones are larger over land than over sea. This strangely means that the direct circulation over land looks like more active than over sea in the equatorial zone.
The synoptic climatology of monthly mean surface air temperature anomaly (MMATA) at 115 surface stations in Eurasia is assessed by two types of maps. The first consists of simple linear correlation coefficients between the MMATA at a station and the montly mean 700mb height anomaly (MMHA) at 134 grid points in the Northern Hemisphere. In winter, these fields generally contain two equally important centers: a center of maximum positive correlation approximately 1300km from the reference station and a negative center about 2800km to its northwest. In summer, the positive center is located an average of 800km from the reference station and strongly dominates the MMATA, while the negative center is no longer as important as in winter. The second kind of map is a special type of anomaly composite, constructed by multiplying the correlation coefficient at each grid point by the standard deviation of MMHA at the same point. This map shows the optimum distribution of height anomalies for abnormally warm (or cold) weather for winter and summer in each of several climatologically similar regions into which the Eurasian area is subdivided.
Long-term variations of large-scale circulations in the western Pacific regions are investigated by using cloud wind data derived from GMS during 6 years from 1978 to 1984. Comparison between monthly mean winds obtained by the satellite and those by the upper-air sonde observations indicates that upper cloud winds generally correspond to 200-mb winds and lower cloud winds correspond to about 850-mb winds. 6-year averaged monthly mean winds reveal that upper and lower winds in the western Pacific are largely affected by monsoon circulation systems. In the upper troposphere northeasterlies dominate in the equatorial western Pacific to the southeast of the Tibetan High during northern summer, while southeasterlies prevail during winter affected by an anticyclonic circulation generated over the Australian monsoon region. The first mode of winds obtained by an Empirical Orthogonal Function (EOF) analysis corresponds to wind anomalies associated with the 1982-83 El Nino event. Upper winds and lower winds of this eigenvector suggest that there exists a large-scale east-west circulation with rising motion in the eastern Pacific and sinking motion in the western Pacific. The first eigenvector also possesses an upper cyclonic anomaly circulation in the western Pacific subtropics. Wind anomalies corresponding to the western Pacific-Japan oscillation of the heat source found by Nitta (1986) are obtained by both EOF and composite analyses. When convective activities are intense in the western Pacific subtropics near 20°N, an upper cyclonic cell is found to the northwest of the heat source center and an anticyclonic cell appears north of the cyclonic cell. It is suggested that these anomaly circulations might have strong influence on large-scale summer-time circulations in the East Asia.
A distinct polygonal eye was observed for 15 hrs in the PPI image of typhoon 8019 WYNNE. The shape of radar eye of approximately 30km diameter was variably square, pentagonal and hexagonal, and rotated counterclockwise around its center. The period of rotation was 41-43 minutes for the pentagon and hexagon compared with 47-50 minutes for the square. The period decreases (tangential velocity increases) as number of corner of polygon becomes larger. The pentagonal shape occurs most frequently and persists for 111 minutes, contrary to 12 minutes for square shape. There are no observations of a triangular or heptagonal shapes. The deformation of the polygon is restricted within the narrow, limited portion with several kilometers width inside of inner eyewall of concentric double eye. The phenomenon coincides with the largest shear of tangential velocity. As the result of tracking small convective cells in the eye, the cells rotate in a Rankine vortex field; moreover, polygonal features have a higher rotational velocity than the cells inside the eye. The straight-sided portion of polygon becomes wavy as if instability exists at the inside boundary of eyewall. The phenomenon took place in the concentric double eye of well-developed tropical cyclones with approximately 920-950mb central pressure and 30-50km of eye diameter.
Two ground-based Doppler radars have been used to examine the wind fields and the internal structure of the rainband of Typhoon 8305. The rainband is located 300km to the northeast of the storm center and is embedded in a broad stratiform precipitation region. The air flow around the rainband is nearly two-dimensional along the rainband. Composite crosssections in the radial direction from the storm center reveal the secondary circulation associated with the rainband. A convergence zone with a large outward tilt exists from the inner edge of the rainband (the edge near the storm center) at lower levels to the outer edge at middle levels. Frictional inflow air at lower levels rises at the inner edge of the rainband and a mesoscale updraft of 2m s-1 forms. A mesoscale downdraft less than 1m s-1 exists in the maximum reflectivity zone outside the updraft zone. The downdraft is thought to be produced by the drag forces and evaporation of raindrops. The convergence between the relatively cold air associated with the downdraft and the low-level warm inflow relative to the storm center produces the updraft. This cloud dynamic mechanism is thought to play the main role in maintaining the rainband. Taking into account the band-relative air flow, the air with large θe value near the cloud base approaches from the outer region of the typhoon toward the storm center, and enters the rainband through the inner edge due to its large crossing angle of 25°. The warm air reaches the convergence zone and rises without passing through the intense rainfall area at the center of the rainband. Detailed analysis of air flow shows that convective-scale updrafts exist in the rainband at an interval of about 5km despite the relatively uniform reflectivity pattern. One of the interesting features is the circulation center embedded within the maximum reflectivity core in the rainband. A relative vorticity within this circulation exceeds 3×10-3s-1. The internal structure of the rainband has been compared with other observations and numerical modeling results. The propagation speed of the rainband and the surface pressure change associated with the passage of the rainband do not suggest a relationship between internal gravity waves and the rainband.
The three-dimensional structure of meso, β-scale organized convective system was observed using a X-band RHI radar. This system which was composed of several long-lasting convective clouds had a band-shaped configuration and it developed into the band of 50km in length and 20km in width. Main constituents of the band were meso γ-scale organized multicellular convective clouds in which new cellular echoes were successively formed about 5km apart from a pre-existing cellular echo on its right flank every 20min. The band also had a multicellular structure as a whole if constituent convective clouds are regarded to be large "cells". The successive formation of new convective echoes was observed about 15km apart from a pre-existing convective echo at the rear portion of the band. This double-organized structure was closely associated with the maintenance of the band as well as its movement.
Measurements of scattering coefficient of atmospheric aerosols as a function of relative humidity and electron-microscopic examination of individual particles of 0.1 to 0.5μm radius were carried out during the period from November 1983 to August 1984 in Nagoya. When scattering coefficient ratio is defined to be the ratio of scattering coefficient at ambient relative humidity to that at relative humidity less than 30%, its dependence on relative humidity (humidogram) varied greatly, but most of humidograms are classified into two types. Type A humidogram, which shows the rapid increase in scattering coefficient ratio at relative humidity of about 75%, appeared in the situation of strong wind and ambient relative humidity lower than 80%. In this case, sea-salt particles were often dominant among particles of 0.1 to 0.5μm radius. Type B humidogram, which shows the smooth increase in scattering coefficient ratio with relative humidity, was mainly observed in the situation of relatively weak wind and it was present in a wide range of ambient humidity. Sulfate-containing particles were predominant when type B humidogram was observed. In Nagoya, scattering coefficient of atmospheric aerosols at relative humidity lower than 80% is almost determined by the dry amount of aerosol particles. At relative humidity below 75%, the scattering coefficient ratio accompanied by type B humidogram tends to show higher value than that accompanied by type A. In the situation of relative humidity higher than 80%, only type B humidogram was observed. It can be concluded that aerosols of type B humidogram are important in causing a high scattering coefficient in the humid atmosphere of Nagoya.
A method of estimating the effective absorption property of atmospheric aerosols is proposed. This method is based on the dependence of reflectance of solar radiation at some altitudes on aerosol refractive index. Sensitivity studies of the monochromatic and entire solar spectrum reflectance to various radiative parameters show that this method has enough sensitivity to estimate the absorption property. This effective value may be different from the real value for an individual particle. To assess the validity of the effective value in radiative transfer calculation, modeling studies are performed, that is, some uncertainties due to complicated features, such as refractive index depending on radius and wavelength, non-Lambertian surface and wavelength dependence of the surface reflectivity are examined with a numerical model. The results indicate that these are small effects on the estimation and the effect of aerosols on earth-atmosphere radiation field can adequately be described by the effective values.
Effective absorption properties of atmospheric aerosols were estimated from five aircraft measurements of reflectance using a method described in our previous paper. Results indicate that the imaginary terms of the effective refractive indices vary within the range from 0 to 0.013 showing a tendency to increase in lower layer corresponding to an increase in industrial aerosols. The method is based on Mie theory and this may introduce additional uncertainties since shapes of real atmospheric particles are nat spherical in general. Determination of this effect on reflectance is vey difficult. But observed and calculated radiative flux divergences in the atmosphere are compared for rough investigation of above uncertainties. The results of the comparisons indicate that they are in good agreement.
A new type of sunshine-duration meter, 1) providing an optical system that consists of a mirror and a photodetector, was recently developed. The photodetector looks sky through a mirror rotating around an axis parallel to that of the earth's. A uniform sensitivity, independent of the incidence angle of the sun, is performed by an optical mirror with two diffusible surfaces. Effect of diffuse skylight has been remarkably well eliminated by an adoption of a pyroelectric detector.