In order to make clear the problem of the four-day circulation, we construct a simple axisymmetric model. This model contains the mechanism first proposed by Gierasch (1975), i.e., upward transports of angular momentum by a meridional circulation with the aid of very large horizontal viscosity which dissipates differential rotation. Further, a suppressing effect on this mechanism due to finiteness of the horizontal eddy viscosity is also involved. The velocity and the temperature field are represented by a few fundamental modes. Terms expressing the nonlinear interactions among the modes are explicitly written in the mode equations. Stationary solutions of this system are obtained mainly by a two-layer model, for both an infinite and a finite horizontal eddy viscosity. First, we determine magnitude of the mean zonal flow (U) as a function of the meridional circulation (V) from angular momentum balance. In the case of an infinite horizontal viscosity, U is simply proportional to V. Its ratio (U/V) is given by the inverse ratio of period of planetary rotation (τΩ) to the time constant of vertical diffusion (τν) (i.e., U/V/≈τΩ.) In the case of a finite horizontal viscosity, U has a maximum value for a certain value of V. Its maximum value is determined by the ratio of horizontal viscosity to vertical one as well as τΩ. Next, associating this U-V relation with the vorticity equation in the zonal direction, we classify types of solutions according to the effect which dominates and balances solenoidal term in the vorticity equation. The types of solution are as follows. Thermal wind balance of the Venus type: The vertical gradient of centrifugal force due to atmospheric rotation dominates. Thermal wind balance of the earth type: The vertical gradient of centrifugal force due to atmospheric rotation coupled with planetary rotation dominates. Direct cell balance: The frictional force associated with the meridional circulation dominates. Kinds of balance are determined on a two-dimensional parameter space of τΩ/τν and the latitudinal differential heating denoted by Gγ. In an infinite horizontal viscosity case, thermal wind balance of the Venus type appears in the whole range of large Gγ. In the case of finite viscosity, solutions of this balance can exist only in a more restricted domain in the τΩ/τυ-Gγ diagram. Gγ of this domain has an upper limit depending τΩ/τυ, and only a direct cell balance can correspond to a Gγ value beyond the upper limit. In a portion of the domain where thermal wind balance of the Venus type is realized, solution of direct cell balance is also obtained as a stable solution. Thus for this parameter range, two utterly different states, a fast zonal motion accompanied by a slow meridional circulation and a strong meridional circulation associated with a slight zonal motion are possible as stable stationary states for the same differential heating. The former corresponds to the four-day circulation, while the latter means a direct cell between day side and night side in actual situation. Results of the numerical experiments by Young & Pollack (1977) are discussed in the light of the present results.
Numerical experiments of tropical cyclones are performed using a moving variable grid scheme proposed by Kitade (1979). The formulation for parameterization of cumulus convection is based on the concept of penetrative convection. The physical processes, such as the surface friction, latent and sensible heat supplies from sea surface, eddy dissipation and diffusion, dry convective adjustment and large-scale condensation, are also contained with conventional ways in present model. The initial vortex develops into a mature disturbance in about 2 days. Its structure is similar to that of the tropical cyclone in the real atmosphere and that in numerical experiments performed by many researchers. The simulated tropical cyclone has some asymmetric features due to the variable Coriolis parameter in spite of the adoption of an initial nearly symmetric vortex. Furthermore, the developed tropical cyclone has dynamically unstable regions in the upper layer, which may also contribute to the asymmetry of the simulated vortex. The simulated vortex moves north-north-westward, which is due to the effect of variable Coriolis parameter pointed out by Rossby (1948). The moderate tropical cyclone moves at a speed of about 3km hour-1 for westward and at about 7km hour-1 for northward components. The meandering about the mean path occurs with a period of about 25 hours, which seems to be due to the Magnus effect suggested by Yeh (1950). When the sea surface temperature is changed, the path of the vortex center is deviated a little from that in the basic experiment. The deviation is partly explained for the following reason. The strength of vortex is strongly influenced by sea surface temperature. According to Rossby (1948), on the other hand, the northward acceleration due to variable Coriolis parameter increases with the increase of strength of vortex. Therefore the sea surface temperature affects the path of vortex. The variation of speed of vortex movement through its life cycle is also understandable from such point of view. It is confirmed by an experiment with non-uniform sea surface temperature that the asymmetric heating of cumulus convection affects the path of the vortex center.
A two-dimensional nonlinear numerical model of cellular convection has been developed in order to investigate the effects of anisotropy of eddy diffusivity, large-scale subsidence, and latent heat release upon the degree of cell flatness. Each of these physical factors has been shown to increase the cellular aspect ratio from that typical of small-scale cumulus to values considerably greater than unity, more comparable to that of mesoscale cellular convection (MCC). The dry convective form of this model agrees well with previous laboratory and numerical findings for classical Boussinesq convection. The model also shows that the preferred aspect ratio more closely simulates a square root relationship to the degree of anisotropic diffusivity (as postulated by Priestley) than that predicted by linear theory.
Modification of rainfall system by orographic effects was studied on the basis of the echo structure observed quantitatively by the use of a RHI radar which was set in Owase in the southeastern side of Kii peninsula during the period of June to July in 1977. Four cases of rainfalls were analyzed (cases A, B, C and D). In cases A, B and D continuous rain originated from middle-level clouds occurred over the broad area in Kii peninsula, and in case C convective rain was found out only in the limited area along the sea shore. In case C easterly wind was prevalent at low levels and a precipitating convective cloud system, which had the size of about 4km in height and about 15km in width of E to W, was formed around the sea shore. It existed for more than two hours over the nearly same region. This long-lasting system was maintained as the group of small precipitating convective clouds which were formed successively in the same area over the sea and which travelled to the mountain side showing the same evolution. It can be inferred that the horizontal convergence caused by orographic effect in the windward side of mountains and by a downdraft in a previously developed cloud would have played an important role in the successive formation of new convective clouds. In case A easterly wind was prevalent in the lower layer and a low-level cloud system which seemed to be the same type as case C was formed around the sea shore below middlelevel precipitating clouds. Precipitation originated from middle-level clouds increased remarkably in low-level clouds. However, in case B in which westerly wind was prevalent in the lower layer, a low-level cloud system was not detected and precipitation from middlelevel clouds was not observed to increase at low-levels. In case D the “seeding effect” of precipitation particles of middle-level clouds on low-level clouds was suggested.
Electrical properties of raindrops originated from middle-level precipitating clouds were studied on the basis of the simultaneous observations of the charge on raindrops and the structure of RHI radar echoes. Main results are as follows: Raindrops with large positive charge were observed only when the intensive echo-regions in the streaks from generating cells passed over the site of observation. Except for the raindrops with large positive charge, large and small raindrops from middle-level clouds tended to be charged negatively and positively, respectively. The amount of charge on raindrops originated from warm rain clouds was smaller than that from middle-level clouds. It was inferred that the amount of charge on precipitation particles from middle-level clouds was not changed when the particles fell through low-level convective clouds.
A statistical method for retrieving the clear radiance from several adjacent fields of view of HIRS/2 which are partially contaminated by clouds is developed using the digital picture data of AVHRR on TIROS-N satellites. The accuracy of this method is compared with that of McMillin's method by simulation study. The error of the present method is about 2/5 that of McMillin's method. The two methods are also applied to the actual data of radiances observed by satellite and the conclusions obtained in the simulation study are again confirmed.
A helicopter-borne radiosonde system is developed to determine thermal structures of the lower atmospheric layer. It consists of a radiosonde instrument, a winch, a receiver with a receiving antenna and a recorder. An on-board winch hangs down the radiosonde instruments from a hovering helicopter and winds it, up with a nylon cord tied at the top of the instrument, to the helicopter again after measurements. Radiosonde sensors attached to or packed in a metalic watertight cylindrical vessel can measure dry- and wet-bulb temperatures and pressure, and detect also water temperature when it alights on the water. Measurement signals are transmitted from the radiosonde instrument to an on-board receiver. Easy and rapid movement of a helicopter from place to place makes it possible to measure (1) vertical profiles of the dry-and wet-bulb temperatures above any target site by making a helicopter to hover at given height and (2) horizontal distributions of dry- and wet-bulb temperatures along a prescribed path of a flying helicopter suspending the radiosonde instrument at a given height. Correction of surface temperatures remotely-sensed by means of a helicopter-borne infrared thermographical instrument at the height of about 300m for absorbing and emitting effects of atmospheric water vapor is carried out as an application of the radiosonde system to indicate that reduction of temperatures due to effects of water vapor is several degrees Centigrade. Another example is also shown.
The applicability of the piecewise linear function in place of a similar smoothly-varying current profile is examined in the baroclinic context. Within the framework of smallperturbation linearization, the behavior of the vertical velocity and the horizontal divergence is analyzed at the discontinuity of the current shear. In the conventional geostrophic-type instability regime, the discontinuity in the horizontal divergence at the shear discontinuity is suppressed, and, therefore, the piecewise linear profile leads to a useful approximation to the true solution. In the symmetric-type instability regime, however, due to the magnified discontinuity in the horizontal divergence at the shear discontinuity, the solution thus c btained will show a major distortion, rendering the piecewise linear profile inadequate for modeling the smoothly-varying current profile. Using exemplary current profiles, numerical results are presented to demonstrate the behavior of the horizontal divergence near the discontinuity of current shear.