Abstract: Tianchi is a central composite volcano with potential danger of eruption.Tephra of Tianchi eruption in 1215(±15)has diffused to the Japanese sea and southern Japan.However,the study about simulation of the tephra of Tianchi is little.This paper tries to recur to the ash distribution of this eruption by simulating the tephra dispersion of Tianchi volcano eruption.This paper adopts the mathematic model for tephra dispersion proposed by Suzuki(1983).Based on Suzuki's formula,it simulates the tephra dispersion of Tianchi eruption of 1215(±15).There are different densities of the grains from this eruption because of the difference of content of air bubbles.Wind speed and air parameters relate with the altitude.According to the wind speed,we adopt three models:30m/s,MW1 and MW2.The conclusions are as follows:1.The value of the dispersal parameter β of Tianchi eruption in 1215(±15)is 0.45,which indicates most of tephra congregating at the top of the volcano eruption column;2.The differences of settling rates for different diameter particles are very big.For the particles of diameter in 2～1cm,the settling rate achieved 8.95m/s,but for the particles of diameter in 0.005～0.001cm,the settling rate only 0.03m/s;3.There is a turning point in the relations chart of particle diameters and in the settling rate.It is speculated that this turning point possibly relates with transformation of the eruption column from the convective region to the diffusion region;4.The probability densities of diffusion are different for different diameter particles.Big particles start to diffuse at the bottom of the eruption column,but small particles only start to diffuse near the peak of eruption column.And the probability densities of diffusion of small particles are much bigger than that of big ones;5.In the three models the diffusion distance simulated by the first model is farther than that of MW1 model,but the width is smaller.MW2 model is not suitable for this region;6.There is an exponential relationship between the ash deposit thickness and the distance from the volcanic crater in the downwind direction 20km away from the crater.It is nearly 800 years after the eruption.We can only estimate that the downwind is 120°,so in this model it is regardless of the change of the wind direction along with altitude.In addition,there are also some problems about the numerical simulation which need to be solved,for examples,the shape of the volcanic ash particles,the error near the crater,and the second big deposit thickness and etc.

Key words: Tianchi volcano, tephra fallout, numerical simulation, diffusion model

摘要： 文中以Suzuki火山灰扩散数学模型为基础,考虑了空气参数随海拔高度的变化和不同大小的颗粒由于内含气泡数量的不同而造成的密度不同,计算了不同尺寸颗粒的最终沉降速度和沉降时间。并对喷发柱扩散概率浓度的计算公式进行了修正,对长白山天池火山千年大喷发空降碎屑物的空间分布进行了数值模拟。模拟时根据风速随高度的变化应用3个模型:1)固定风速30m/s;2)风速从地球表面线性增加到对流层顶部,在平流层的速度为对流层顶部风速的0.75倍(又称MW1模型);3)风速在对流层与MW1相同,但是从对流层顶部到20km高处风速线性减小,20km高度以上的风速为对流层顶部的10%(又称MW2模型)。通过与前人的结果进行比较,说明了模型的合理性,最后分析了模拟结果与前人结果之间存在差异的原因。

关键词: 天池火山, 空降碎屑物, 数值模拟, 扩散模型