In this two-part study, we made several rudimental considerations of the Jovian atmospheres as a step to achieve a comprehensive understanding of the Jovian fluid systems. In Part I, we address the two basic problems: (1) The depth of mean zonal flows alternating between eastward and westward directions, and of the vortex-spot motions such as the Great Red Spot, whether they are extending into the deep interior of the molecular fluid layer or are confined to a thin atmospheric layer. (2) The relative importance of the two energy sources, the solar energy and the internal heat energy, as driving forces of the atmospheric motions. The following results are obtained: (1) The only possible configuration of the steady flow in the barotropic molecular fluid interior is the axisymmetric zonal flow, as long as a baroclinic upper part of the molecular fluid layer is thin enough compared to the radius of the planet. Because both the mean zonal flows and the vortex-spot motions are almost steady flows, the vortex-spot motions must be confiend to a baroclinic atmospheric layer, while the mean zonal flows may extend deep into the interior. (2) By extending the formulation of the available potential energy to the convectively unstable condition, it is shown that the main driving force of the Jovian fluid systems is the internal heat energy.
The primary purpose of Part II of this two-part study is to offer a rationale to study the dynamics of the Jovian atmospheric layer (an upper part of the molecular fluid layer of the major planets, where the fluid can be treated as an ideal gas)based on a scale analysis. The results obtained are as follows: (1) The dynamics of the Jovian atmospheric layer can be described as almost two-dimensional flow under the geostrophic balance without strong vertical motions such as accompanying cumulus convection.Horizontal motions are purely determined by the conservation of potential temperature that does not include the vertical heat transport term.Consequently, the dynamics of the Jovian atmospheric layer is not appropriately described by the previously known geophysical fluid dynamic regimes (quasi-geostrophic, planetary-geostrophic, intermediate-geostrophic).We designate this dynamical regime as "thermo-geostrophic". (2) The cloud patterns in the Jovian atmospheres are not the resultants of cumulus convection, but rather they are mostly of long-enduring stratus-type clouds purely advected by horizontal motions. This is because the observed vertical heat flux from the interior is sufficiently explained by the slow vertical motions of large-scale nearly-geostrophic dynamics, in contrast to the Earth's atmosphere, where the cumulus convection activities are indispensable in this respect.
The tropical intraseasonal oscillation is studied by use of a simple model based on a CISK mechanism.In this model, the CISK parameter is chosen to be large over areas of high sea surface temperature (SST) distribution in order to describe the high convective activity that occurs in those regions. The unstable disturbance generated in the regions of high convective activity (high SST) grows as it moves eastward.The disturbance has a baroclinic structure and its horizontal pattern consists of a combined Kelvin and Rossby type response.As the unstable disturbance propagates to the less convectively active regions (low SST), it decays with time, generating free waves by dispersion.These free waves propagate further eastward to eventually become one of the sources of the next unstable disturbance generated in the convectively active region. It is shown that the tropical intraseasonal oscillation can be interpreted as a pulsation induced by the repetition of this process.
The Nimbus-7 Earth Radiation Budget (ERB) Experiment data set is used to identify and describe the planetary-scale, quasi-stationary irrotational flow anomalies which develop in the tropospheric wind field during the major El Niño/Southern Oscillation (ENSO) event of 1982-1983. A statistical correspondence was developed (Krishnamurti and Low-Nam, 1986) between the broad-band Outgoing Longwave Radiation (OLR) field and the divergent wind at 200mb during the FGGE year. The resultant regression relationship is here applied to a sequence of OLR fields that cover the period of the El Nibo episode to estimate the associated upper-level divergence. Further analysis then yields estimates of the velocity potential, divergent flow, and vertical motion. Vigorous thermally-direct Hadley and Walker circulation anomaly modes are demonstrated to exist in the tropics. The drought regions over Indonesia, northern Australia, Africa, and northern Brazil are found to lie in regions of subsidence resulting from the anomalous east-west circulations that form during the ENSO event. The drought regions over the northern Pacific Ocean from the Philippines to Hawaii and in the southern Pacific Ocean are demonstrated to lie in descending branches of the north-south Hadley circulation anomalies. Teleconnections into the mid-latitudes are clearly discernible in the fields. A wave train, with ascent regions over New Zealand, the equatorial Pacific Ocean, and the southern United States, and forced descent regions in the subtropical Pacific Ocean north and south of the equator, is evident as a dominant feature. At the height of the El Niño, the monthly-averaged mean vertical motion anomaly over the primary sea surface temperature maximum in the equatorial Pacific Ocean reaches 1cm/sec while the resultant divergent, east-west 200mb flow anomalies reach 4m/sec. The vertical motion anomalies display a high degree of correspondence to independently-derived estimates of the global cloud-cover distribution.
Interannual and intraseasonal variations of convective activities in the tropical western Pacific during summer and their impact on the Northern Hemisphere circulation are investigated by using satellite cloud amount, sea surface temperature (SST) and geopotential data for 7 years (1978-1984). During summers when SST in the tropical western Pacific is about 1.0°C warmer than normal, active convection regions consisting of a number of typhoons and tropical depressions are shifted northeastward from the normal position near Philippines to the subtropical western Pacific around 20°N and cloud amounts both in the middle latitudes and in the equatorial regions are greatly suppressed. A high pressure anomaly with little vertical tilt predominates in middle latitudes extending from East China, through Japan Islands to North Pacific during these summers. Analyses of 5-day mean cloud amount reveal that the convective activity is largely modulated by the intraseasonal variations (ISV). The amplitude of ISV of convective activity in the Philippine Sea around 15°N-20°N is more intensified in warm SST summers than in cold SST summers resulting in stronger season mean convective activities in the former than in the latter. Correlation computations between 5-day mean tropical cloud amount and 500mb geopotential height show that there exist wave trains of geopotential height emanating from the heat source region near Philippines to North America. Daily analyses of geopotential height indicate that these wave trains appear to be generated when convective activities in the Philippine Sea become intense and that the amplification occurs downstream from the western Pacific to the west coast of North America taking about 5 days. It is concluded that Rossby waves are generated by the tropical heat source associated with ISV, and high pressure anomalies over East Asia and Northwest Pacific during warm SST summers can be understood as the results of frequent occurrence of Rossby wave generation.
Distributions of composited convective heating and drying vary substantially during the life cycle of six cloud clusters identified by Frank (1978) in Phase III of GATE. Convective heating (Q1-QR) and drying (Q2) begin in the lower troposphere. As cloud clusters mature, intensification of upper level convective heating increases; while below 600mb drying accompanies decreases in convective heating. The upper level convective heating reaches its peak, when the upper level cloud cover is largest; whereas the Q2 maximum at 850mb coincides with the peak radar echo intensity. The depletion of moist static energy (Q1-Q2-QR) in the lower trposphere begins in the growing stage; whereas the upper level heating due to vertical eddy fluxes is most significant in later stages, when deep clouds are prominent.
The existence of the medium-scale waves (wavenumbers 4-6), especially wavenumber 5, is a "ubiquitous" feature of the Southern-Hemisphere general circulation during the summer season. The data generated by the FGGE III-b analyses of the European Center for Medium Range Weather Forecasts were used to explore the role played by this wave regime on three dynamic aspects of the Southern-Hemisphere general circulation: eddy transport property, vacillation of atmospheric energetics, and downstream development. The medium-scale waves contribute about a half of the total eddy transport of sensible heat and momentum in the Southern-Hemisphere summer general circulation. It is inferred from the contrast between the sensible heat transport by this wave regime in the upper and lower levels along the Pacific storm track that the sea surface temperature may affect the development of medium-scale waves. The momentum transport by this wave regime maintains the jet stream. The standard deviations of eddy energy variables are largely attributed to the medium-scale waves. If the atmospheric energetics of the summer Southern Hemisphere exhibit a vacillation, then the medium-scale wave should play the most important role, compared to other scales of atmospheric motions. Time series analysis of various energy variables prove this conjecture. The medium-scale waves showed several cases of downstream development during the 1978/79 summer. One case was used to illustrate physical processes involved over various phases of the entire life cycle of downstream development. It was shown that the downstream development of medium-scale waves was amplified by the baroclinic process during the developing stage and maintained by the barotropic process during the decaying stage.
Atmospheric tides induced in a global atmospheric general circulation model in January are analyzed. The model produces detailed structure in the thermal forcings of diurnal and semi-diurnal modes within the planetary boundary layer (PBL) in the subtropics and within the mid-troposphere in the tropics. Compared with the forcings by Forbes (1982a, b), the model gives larger ones in the lower part of the troposphere for both modes. The thermal forcings of diurnal and semi-diurnal tides within the stratosphere roughly agree with the forcings by Forbes (1982a, b). The model produces tidal forcings other than the migrating tides, such as (ƒ=-1, m=5), (ƒ=1, m=3), (ƒ=-1, m=9), (ƒ=1, m=1), ..., where _??_ is the frequency in day-1 and m is the zonal wavenumber. Such non-migrating modes* have large thermal forcings within the PBL and the mid-troposphere in the tropics. Strong diurnal heating over land in low latitudes essentially forces them.
Tatsumi (1985) first applied a spectral method to a limited area forecast model using modified Fourier basis functions together with a transform method and the prescribed spectral coefficient of the additional basis function. Another limited area spectral method having the spectral coefficient of the additional basis function not prescribed but predicted, is formulated based on the Galerkin procedure, and compared with Tatsumi's method. Some merits of Tatsumi's method are discussed. It is shown that the use of a transform method in combination with a modified Fourier basis function leads to erroneous interaction coefficients and false energy generation. In addition, the Gibbs phenomenon for higher order derivatives and the effects of a boundary relaxation term in limited area spectral methods are demonstrated.
A case study is made of the orographically distorted fields of surface meteorological elements in typhoons by using one pair of northern-crossing typhoons and another pair of southern-crossing ones over the Island of Taiwan. The orographic effects on the surface pressure, temperature and humidity are evaluated in terms of their anomalies from the corresponding five-year averaged monthly means. The important results of the present investigation are as follows: (1) Formation of a pressure trough occurs on the lee side, i.e., on the west (east) side for northern (southern)-crossing typhoons, of the Central Mountain Range (CMR), and a pressure ridge forms on its windward side. The ridge-trough system becomes more intense as the wind component across the mountain range increases. (2) Generally heavy rainfall occurs over the windward side of the CMR and a Foehn, i.e., the warm and dry air, is observed on its lee side. The intensity of the Foehn is closely related to the strength of the wind across the mountain range and the rainfall intensity on the windward side. (3) Formation of secondary vortices occurs on the west side of the CMR and over the southern tip of the island in the case of a smaller northern-crossing typhoon. In the case of the southern-crossing typhoon, a secondary vortex forms on the southwest side of CMR in advance of its crossing over the mountain range when the airstream approaches the north-eastern region of the island with a large angle of incidence to the CMR axis.
A feasibility study of delineation of convective rainfall area is presented using the split window (11μm and 12μm) data measured by the Advanced Very High Resolution Radiometer (AVHRR) on board NOAA-7. Non-precipitating cirrus clouds and low-level cumulus clouds, which cause a erroneous rainfall area delineation, and cumulonimbus clouds are objectively distinguished by the split window data. The satellite rainfall areas, in this study, are defined as those of cumulus and cumulonimbus clouds whose tops are over 700mb level. The satellite rain-clouds are compared with concurrent radar data for eight cases. The study was performed over the area of 33°-39°N and 135°-141°E during the summer season in 1984 by taking into account the availability of digital radar data and the atmospheric condition for convective clouds. Statistics show that this method is better than the conventional single infrared method (utilizing -20°C of brightness temperature as a threshold for rain clouds) by 12% for the probability of detection and 13% for the false alarm ratio. The false alarm ratio by our method is generally better than the conventional single infrared method for any cases studied. The improvement of the false alarm ratio by our method over the single infrared method is comparable to that shown by other authors who use the method combining visible and infrared data. Significant improvement in rain area delineation by our method in comparison with the single infrared method can be obtained in the situation where single layer cirrus clouds or low-level rain-clouds dominate over the study area.
Analyzing the aerological data at Syowa Station and the surface data at Syowa and Mizuho Stations Antarctica in 1979, we discuss the mechanism of cloud formation during high-pressure conditions. It was frequently (over 30%) overcast during high-pressure conditions at Syowa Station except in December, and a similar tendency can be found at Mizuho Station. Moreover, snowfalls were observed at Syowa Station even in these conditions and the frequency of such days was rather larger in winter than in summer. In many cases of high-pressure, a stable layer or an inversion layer was found; some stable layers were around 800mb level and others were near the ground surface. It was considered that the formation and development of clouds was connected directly with the stable layers. The main mechanisms for forming clouds would be the weak convection in the layer below stable layer or inversion layer, the mixing of cold air with moist air because of wind shear and surface radiative cooling, and for developing clouds would be radiative cooling at the cloud top.
This note demonstrates the observational evidence of abrupt enhancement of convective activities and simultaneous low-level westerly bursts which occurred in the equatorial western Pacific in November of 1986. These abrupt changes were followed by the eastward movement of the convective systems and low-level westerlies to the east of the dateline resulting in the development of the 1986-87 El Niño.
Measurements of aerosol particles and light scattering coefficient were carried out at Nagoya in Japan during the Asian dust-storm events in April in 1979 and 1980. Asian dust-storm particles collected with an impactor were examined through a transmission electron microscope and most of them were found to be mainly composed of water-insoluble material showing irregular shape on the collecting surface. The mean ratio of maximum length to width is 1.4 for these particles. It is also found that most of these particles were surrounded by water-soluble material.