An experimental method for determining the crossover function is studied for those days when a light mist is observed.
An artificial seeding experiment was carried out over the Genkai Sea, Japan, using liquid carbon dioxide. The seeded cloud was followed by an aircraft and radar at Kyushu University. A radar-echo intensity of 19 dBZ was formed in the seeded cloud where no radar echo was observed previously. The results of the experiment are analyzed using a numerical model [Weather Research and Forecasting Model (WRF), version 3.1] to evaluate the effects of the artificial seeding. The numerically simulated radar echo has a slightly stronger intensity and wider area than that observed. The results of the experiments were similar to those produced by the numerical model, however. Evaporation of cloud liquid water; growth of snow particles that is due to deposition, autoconversion, aggregation, and collection; and convection caused by released latent heat are shown in the numerical results as effects of the seeding experiment. The falling snow particles cause downward flow, which is compensated for by upward flow that causes the formation of new cloud liquid water as a secondary effect of seeding, although this secondary effect is confirmed only in the numerical results.
Abstract:In order to bring large amounts of precipitation, the new seeding method using liquid carbon dioxide (LC) was suggested by Fukuta (1996a). The method was applied to the supercooled convective cloud in a post-frontal weather condition in northern Kyushu, Japan, on October 27 1999. In the seeding experiment, LC seeding and the subsequent observation by aircraft were carried out and the features of a seeded echo were observed by radar. Consequently, the aircraft observation confirmed the further development of the seeded cumulus together with a fuzzy aspect of the cloud surface, which indicates the feature consisting of ice particles. Furthermore, the observed cloud top was quite consistent with the cloud top estimated by the thermodynamic analysis following parcel theory. Therefore, the observed results indicate the artificial effects by LC seeding. On the other hand, the radar observation confirmed an artificially induced echo, which showed spreading of the echo area and took a unique mushroom shape in the RHI pictures. The maximum width of the echo reached 24 km and the total amount of estimated radar precipitation of the seeded cumulus was approximately 2.4 million ton, traversing a distance of 60 km in 1 h 40 min. The observed and estimated results are consistent with the hypothesis of the new seeding method, which induces the dynamic and microphysical processes consisting of two fundamental processes. In addition, it was found that dynamical interaction between the seeded and the adjacent natural cumuli was an important factor in the formation of the secondary cumulus. The observational fact will give new viewpoint into the future seeding study.
An elliptical eye was observed in Typhoon Herb during its passage around the Sakishima Islands located in the southwest of the Okinawa Islands in Japan on 30–31 July 1996. Later analysis showed that this elliptical eye had a structure with wavenumber 2 in the radar echo distribution, rainfall field, and pressure field. It is argued here that this structure is likely caused by a dynamic instability arising from radial shear in the tangential flow. To examine this possibility, a linear eigenvalue problem was set up using the asymmetric balance model in which the basic flow was based on the observed surface wind in Typhoon Herb, and the stability of the basic flow for both two- and three-dimensional perturbations was examined. Although for large Rossby numbers, the asymmetric balance model is not formally valid for higher azimuthal wavenumber perturbations, results certainly show that an unstable mode appears with the perturbation with wavenumber 2. In particular, the largest growth rate of the perturbation with wavenumber 2 is shown to occur for the three-dimensional perturbation. The eigenmode structure and rotation period of this three-dimensional perturbation with wavenumber 2 are comparable with the observations. These results suggest that barotropic instability due to the radial shear of tangential flow is relevant to the formation of the elliptical eye of Typhoon Herb.
It has been well-known that extraordinary deep local depressions are frequently observed at Urakawa on the west side of the Hidaka mountain range in Hokkaido (Arakawa, 1960), when the easterly prevails near the surface. Any easterly near the surface has a critical level, because the westerly prevails over Japan usually. The present numerical investigations show that the local depressions are responsible for a shear flow including a critical level. The results are as follows :1) In the case of a uniform flow, the surface pressure differences between the both feet of the mountain range increase with the wind speed when it is small, and are maintained with small constant values when it is large.2) The local depressions on the lee side are detected on the mid-slope, not on the foot, of the mountain range and the upper atmosphere is affected significantly by the mountain range.3) In the case of a shear flow including a critical level, the surface pressure differences increase with increasing wind speed.4) The local depressions appear on the foot of the mountain range and the atmosphere above the critical level is not affected seriously by the mountain range.
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