Numerical experiments on the orographic snowfall over the mountainous region of northern Japan in winter are conducted using a 2-dimensional non-hydrostatic model with a cloud microphysical parameterization which predicts not only the mixing ratio of water species but also the number density of ice species. The orographic effect on the snowfall is investigated by sensitivity tests from cloud microphysical aspects.
Modification of air-mass and formation of snow clouds over the Sea of Japan, enhancement of snowfall over the windward slope of the mountainous area and disappearance of clouds on the lee side are simulated by an experiment where a simplified orography and sea-land distribution of northern Japan is given. The snow clouds gradually increase their top heights from sea, off the west coast, to inland, and the number density of cloud ice increases with the decrease of cloud-top temperature. The influence of the differences of roughness and temperature between sea and land on the snowfall distribution is examined as subsidiary factors. According to comparative experiments, the difference of temperature between sea and land contributes to the enhancement of snowfall over the windward side of land area, while the difference of roughness does not have a substantial influence.
Forced condensation is evaluated assuming that the air-mass is lifted up by the mountain, and precipitation efficiency is defined as the portion of the forced condensation which converts to precipitation over the land. In the sensitivity experiments on the mountain-top height, the precipitation amount over the land increases significantly when the mountain height exceeds the height of cloud base. The precipitation efficiency is about 40% when the mountain height is below 600m, while it reaches about 80% when the mountain height is greater than 1000m. On the other hand, in the warm-rain experiments, the precipitation amount as well as the precipitation efficiency are only about 1/3 those of the experiments with ice phase, because the greater part of the condensed cloud water goes to the lee of the mountain before it converts to rain. Sensitivity experiments where ice nucleation rates are changed over the land are conducted, and it is shown that the snowfall over the land decreases with the suppression of ice nucleation over the mountain, while it increases with the enhancement of ice nucleation over the land, even without mountain. These experimental results suggest that not only the forced condensation but also the natural seeding over the mountainous area-the ice nucleation by the decrease of the temperature in cloud due to the orographic ascent-plays an important rôle in the orographic enhancement of the snowfall over northern Japan.
A seeding experiment in which the ice nucleation rates are enhanced over a specified zone in the Sea of Japan demonstrates the possibility of artificial modification of the snowfall.