The response of flow in a barotropic, β-plane, channel model atmosphere with the external forcing due to realistic topography is examined by making use of spectral expansion of physical variables in wavenumber space. In the model planetary waves are excited by zonal flow over the topographic perturbations while the fluctuation of zonal flow is influenced by the planetary waves through the form-drag of topography. In the case where nonlinear interactions between planetary waves are absent stationary solution can be obtained analytically and its dependency on parameters such as meridional channel width, dissipation rate for waves and zonal flow is studied. It is revealed that for plausible range of parameters describing real atmosphere a multiplicity of stationary solution cannot be readily realized. In the case including nonlinearity of wave-wave interactions the deviation of response of flow from the stationary solutions determined in the former case is examined by numerical experiments as a nonstationary problem. A meandering solution corresponding to blocking situation which is expected to exist as one of multiple stationary solutions for rather small dissipation rate is found to be feeble to nonlinear interactions between waves and to change into another stationary or quasi-periodic state.
The validity of surface pressure tendencies included in the SYNOP and the SHIP obser- vations is investigated. The variational technique under the strong constraint of the continuity equation is proposed to analyze the mass-adjusted three dimensional wind fields in the σ-coordinate system from the observational data of surface pressure tendency and wind. Dividing the wind field into the vertically averaged mean and the deviation field, two differential equations to be solved for Lagrange multipliers are introduced. One case is treated as the numerical example. On that occasion, in place of surface pressure tendencies, 12 hours surface pressure changes are used. It is demonstrated that the original fields of horizontal wind and 12 hours surface pressure change are modified largely so as to satisfy the continuity equation in the vicinity of large scale orographies such as Tibetan Plateau, Rocky Mountains and Greenland, and of strong cyclones. And it is also shown that the patterns of induced vertical velocity are in good correspondence with the surface pressure distribution and large scale orographies. On the other hand, the magnitudes of it in the high and the middle latitude are much smaller but in the low latitude they are considerably larger than those obtained by the conventional initialization scheme. By making use of these mass-adjusted three-dimensional wind fields as the initial ones, the 48 hours prediction with the 5-level primitive equation model is performed, and the results are compared with those predicted with the same model but starting from the initial fields derived by the conventional initialization scheme. It is concluded that noises created by gravity waves become remarkably small if we use these adjusted fields as the initial ones. The problem to select the most suitable value for the weight relating to the error variance of the observation data is also discussed.
This is a continuation of a recent study on medium range numerical weather prediction utilizing a global spectral model and FGGE/MONEX observations. In the present study the impact of diabatic initialization and steep orography over the monsoon region are examined in a number of medium range numerical prediction experiments. The steep orography is the so-called 'Envelope Orography' which is almost a kilometer higher than the conventional orography over most major global mountain chains. The physical initialization, proposed here, contains a reconstruction of the humidity field such that a close balance between the advective and the radiative forcing is achieved over most of the rain-free areas. Over the tropical rain areas the humidity analysis is structured to a cumulus parameterization scheme of the global spectral model and the observed rain. The initial observations of the rain come from a mix of satellite and surface based observations. The proposed physical initialization recovers a substantial part of the observed rain. Two interesting prediction experiments on tropical cyclogenesis-one on the formation of the onset vortex and the other on the formation of a monsoon depression are described in the context of the physical initialization and steep orography. In both cases realistic forecasts on the medium range time frame are obtained. A realistic track (or motion) of the Arabian Sea onset vortex was obtained with the steeper orography. The storm was noted to move too far inland when a smoothed orography was used. The experiments on the Bay of Bengal depression show a major improvement with the physical initialization. The formation and track up to 6 days is in close agreement with observations. The major result of this study is on the prediction of the time averaged motion field or the stationary components. Results of four medium range prediction experiments from the winter and summer FGGE periods are illustrated. These 7 to 10 day averaged motion fields contain details on many spatial scales, most of these details are reasonably simulated by the prediction. This success in the prediction of the stationary component of the flow field is attributable to the improved physical parameterization in the model.
The possibility for two-wavelength radar to detect the microphysical processes in the melting layer is discussed based on model calculations. We calculated the characteristics of microwave scattering by precipitation particles based on the non-coalescence and non-breakup model and monodispersed coalescence or breakup model in the melting layer and obtained the vertical profiles of radar reflectivity factor for both 3.21cm wavelength (X-band) and 5.6cm wavelength (C-band). The results of the calculations show a possibility to detect the shift of the size distribution associated with coalescence and/or breakup of melting snowflakes in the upper-half region of the melting layer. The reason can be well understood by considering that the radar sensitivity of the microwave scattering to the change in particle size deviates between the two wavelengths, and that the sensitivity to the change in the other parameters such as fall velocity of particles is equal between the two wavelengths. Therefore the two-wavelength radar measurement is useful to detect the shift of the particle size distribution in the melting layer.
Microphysical processes in the melting layer were studied using two-wavelength radar observation. Dependence of microwave scattering by precipitation particles on radar wavelength was utilized applying the model calculations shown in Part I of Yokoyama and Tanaka (1984). The observation was performed from the end of June to the begining of July, 1979 (rainy season in Japan) using C-band (5.6cm) and X-band (3.2cm) radars. It is clearly found from the results of the observation that coalescence processes are predominant in the upperhalf region of the melting layer as was already expected by the model calculation in Part-I. How predominant the coalescence processes in the melting layer are depends on precipitation types, that is, the coalescence processes in the precipitation associated with stratiform and stationary clouds are more predominant than with cellular clouds. The effects of evaporation was also found when the atmosphere was dry around the melting layer. In the lowerhalf region of the melting layer, it was difficult to detect the microphysical processes when we use C- and X-hand radars.
Large amounts of aerosols were injected into the lower stratosphere by the volcanic eruptions of El Chichon (17.3°N, 93.2°W) in late March and early April 1982. A ruby lidar at Meteorological Research Institute (MRI), Tsukuba (36.1°N, 140.1°E) in Japan detected the first increase of aerosols around the altitude of 15km on April 25, 1982. A dominant layer with the maximum value of the scattering ratio, Rmax=12.3 was observed at an altitude of 22.5km on May 5. On May 23, it grew to a layer with a value of Rmax=45.2 at 23.5km. After large fluctuations of Rmax occurred in the first 5 months, Rmax began to increase again in September, and Rmax reached 21.3 at 23.5km in October. During October 1982 through September 1983, Rmax-1 decreased gradually with a time constant of about 6 months. The data of the maximum value of the optical mixing ratio of aerosols, γ694(=Rmax-1) at a wavelength of 694.3nm observed at Tsukuba were compared with γ340 obtained by UV lidar at 340nm at Fukuoka (33.6°N, 130.3°E) during October 1982 through May 1983. The altitudes of Rmax were almost the same at both observatories. The ratio of γ694 to γ340 was estimated to be about 10 from these observations. The aerosol backscattering coefficient integrated over an altitude range of 16.5-30.5km, B, exibited enhancements in May and December 1982. During December 1982 through Septembr 1983, the value of B decreased with a time constant of about 7 months at Tsukuba. Latitudinal and time variations of B were briefly discussed comparing with recent model simulations of transport processes of air parcels.
The concentration of atmospheric CO2 was measured on the summit of Mt. Fuji, Japan by using a flask sampling method in October 1980 and by operating a system for continuous measurement in July to October 1981. The diurnal CO2 variation was scarcely found, but irregular variations with periods of 3 to 5 days usually appeared especially in July and August, reflecting the difference of air masses arrived at the observation site in association with synoptic-scale weather systems. The CO2 concentrations at Mt. Fuji were found to be very close to those in the middle troposphere over Japan by comparing with the results of aircraft measurements. A trend of secular increase in the atmospheric CO2 concentration was suggested even from our present results for the limited observational period in two successive years.