Showing papers in "Papers in Meteorology and Geophysics in 1992"

The Dome Emission Effect on the Performance of Pyrgeometers with Silicon Domes.

Abstract: 赤外放射フラックスの測定に広く用いられるEppley社のシリコン製ドーム付赤外放射計 (PIR) について、ドームの射出効果を定量的に調べた。温度を室温から-25°Cまで制御できる黒体錐とドーム温度を変化させるための装置を有する検定用黒体槽を開発し、PIRの測器定数およびドーム係数を求めた。ドームの射出効果をドーム温度を用いて補正することにより、温度上昇・下降時に見られたPIR出力のヒステリシスが消失し、入射放射量対PIR出力の検定回帰直線が精度良く得られることが確かめられた。PIRを用いた下向き赤外放射フラックスの野外測定では、ドーム効果によって、それを無視した場合には晴天大気下の地上観測では20W/m2以上の過大評価、航空機観測では10～20W/m2の過小評価がなされることがわかった。ドーム温度を測定しドーム効果を適正に補正することにより、10W/m2前後の誤差で赤外放射フラックスが測定されることを示す。

Climatology of Mesoscale Warm and Cold Fronts in the Kanto Plain.

Abstract: A statistical analysis is made of mesoscale warm fronts (MWFs) and mesoscale cold fronts (MCFs) in the Kanto plain by using hourly surface data covering eleven years. A series of objective criteria are used to select 190 MWFs and 177 MCFs, and composite analyses are made of their local structure, seasonal and diurnal variabilities, and the larger-scale environment. MWFs are associated with southeast-southwest winds or northwest winds blowing against a shallow cold-air pool having a depth of a few hundred meters. The MWF associated with southeast-southwest winds corresponds to the warm-air intrusion during the passage of a cyclone, while the MWF associated with northwest winds corresponds to a strong northwesterly surge. The frequency of MWFs is highest between 06 and 12 JST. The MCF generally occurs during a cold-air surge. From late autumn to winter, the MCF is mainly associated with northwest winds accompanied by shallow cold air in its leading edge, and is more frequent in the nighttime than in the daytime. From spring to summer, MCFs associated with northeast or east winds are more frequent than those associated with northwest winds, and have a strong tendency to occur in the afternoon.

A study of tropical cyclone motion with a nested-grid model including rainwater prediction

Abstract: Numerical experiments of tropical cyclone motion are performed with the use of a triply-nested grid model which has been developed primarily for the study of tropical cyclones. One of the features of the model is that the mixing ratios of cloud water and rainwater are taken to be predicted variables because evaporation of rainwater and convective downdraft are considered to be important. Cumulus parameterization and other aspects of the model are basically the same as those described in Yamasaki (1986, 1987) except for the use of the longitude-latitude coordinates, inclusion of topography, finer vertical resolution (ten-layer model) and so on. The grid sizes of the coarse, intermediate and fine grid areas are taken to be 15/4, 5/4 and 5/12 degrees, respectively. Several typhoons in August and September 1990 (SPECTRUM period) are chosen for the numerical experiments. The initial condition is taken from the objectively analyzed global data of JMA. An axially symmetric vortex is superimposed on this data. The objective of this study is to see to what degree the model can predict typhoon motion and what are the primary causes of prediction errors. It is found that while the model appears to show somewhat good performance for the prediction of typhoon motion, the primary causes of prediction errors are poor prediction of the behavior of the subtropical high and the initial fields of wind and conditional instability.

Forecasting Aftershock Activities.

Abstract: 日本列島とその周辺に発生する地震の余震活動の予報可能性について検討した。最初に、大森の公式と Gutenberg and Richter の関係式とから余震発生の危険度関数を導いた。この危険度関数には4つのパラメータが含まれる。それらのパラメータの値を、1969-1990年の期間に発生した47個の地震の余震系列についてそれぞれ評価した。次に、余震系列は非定常 Poisson 過程であると仮定し、パラメータについては47個の地震の余震系列の平均値を用いて、日本列島とその周辺の地震に関する平均的な余震発生確率を計算した。いくつかの地震について、実際に観測された余震活動と予測値を比較したところ、両者の一致はかなり良いことがわかった。予報精度を更に高めるためには、余震発生の危険度関数に含まれる4つのパラメータについて、地震のタイプや地域による違いを考慮に入れる必要があろう。

Stepwise Variations of Strain Observed by Two Strainmeters Installed in Nearby Boreholes in a Fracture Zone.

Abstract: Stepwise variations of strain (hereafter called the strain step) were observed by a couple of borehole strainmeters installed 30 meters apart in a fracture zone of the southern Fossa Magna. Strain changes for a year after the installation carried at December, 1986—January, 1987 were apparent expansion indicating the cooling of grout and the relaxation of stress concentrated around the strainmeter. For two years after the installation the number of the strain steps decreased gradually. No clear relationships are seen between the occurrence of strain steps and the environmental conditions such as the rainfall, the atmospheric pressure, the groundwater temperature, and the groundwater level. The strain steps that occurred in the two boreholes are independent, which means that the spatial extent of the phenomenon is less than the distance between the strainmeters. The amplitude-frequency relationship for the strain steps is analogous to those of fracture processes such as earthquakes and acoustic emission in rocks. The decrease in the number of strain steps with time is represented by a formula similar to Ohmori's for the aftershock sequences of earthquakes. A laboratory experiment showed that the frequency of strain step occurrence depends on the stress levels in the surrounding media. Consequently it is inferred that the strain steps observed by our strainmeters are caused by local fracturing processes around the boreholes, the number of which indicates relaxation of the initial stress induced by the installation of the strainmeters.

A Relation between Verkley's 1987 Modon and 1990 Modon.

Abstract: Verkley (1987, 1990) constructed modons as a model of the atmospheric blocking phenomenon. They are exact solutions of the barotropic potential vorticity equation on a sphere. In this note, it is shown that the 1987 modon changes continuously into the 1990 modon as the value of -dQ/dΨ within the modon goes to zero, where Q and Ψ are potential vorticity and streamfunction of the 1987 modon, respectively. Thereby it is revealed that the stability properties of the 1990 modon can be studied by means of Arnol'd's invariant of the 1987 modon though Arnol'd's invariant can not be defined for the 1990 modon.

An Application of the Magnetotelluric Method to the Analysis of the Geoelectric Field Observed by Using Telegraphic Facilities

Abstract: Impedance tensors for periods of 5.3 to 1920 minutes were estimated by making use of the stable data of the geoelectric field over a long range of time and the geomagnetic variations due to a large-scale magnetic storm. The wavelength of geomagnetic variations used as the inducing field in applying the magnetotelluric method is very long. Multiple coherencies between each component of the geoelectric field and both N-S and E-W components of the geomagnetic field are high; coherencies between the two horizontal components of the geomagnetic field are low. Therefore, estimated impedance tensors are reliable. Since skewnesses are not so high, the subterranean resistivity structure beneath the observation area can be regarded to be two-dimensional and coordinate transformation was performed referring to the principal axes calculated from impedance tensors. Apparent resistivities obtained from transformed data for both H- and E-polarizations are different by a factor of about 100.

A Note on Hopf Bifurcations in Topographically Forced Flows.

Abstract: In this note, we investigate analytically how the bifurcation diagram of Pedlosky (1981)'s barotropic model is modified by introducing meridional structure into the forced basic zonal flow. If the basic zonal flow deviates from a uniform one to some degree, then two Hopf bifurcation points emerge (i.e., periodic solutions are generated) on the steady solution curve corresponding to blocking flows and steady solutions between them lose their stability. Moreover, the diagram may be qualitatively altered as the basic zonal flow further deviates from a uniform one.

A Note on Elliptical Modons.

Abstract: For any stationary solution of the potential vorticity equation, the streamfunction Ψ and the potential vorticity Q are functionally related, i.e., Q=F (Ψ). For analytical modons which are considered as one of the models of atmospheric blocking, the functional form of Q=F (Ψ) is assumed to be linear and therefore the modon boundary is assumed to be circular. Modons with boundaries other than a circle have been numerically investigated by several authors. Boyd and Ma (1990) calculated numerically the functional form of Q=F (Ψ) for elliptical modons elongated in the basic flow direction on an ƒ plane. In this note, the functional form of Q=F (Ψ) is calculated analytically for modons with an elliptical boundary slightly different from a circle. For modons elongated in the basic flow direction, the result agrees qualitatively with the above-mentioned numerical one. On the other hand, for modons elongated in the direction perpendicular to the basic flow, the functional form of Q=F (Ψ) is shown to be qualitatively different from that for modons elongated in the basic flow.