Numerical Experiment of Orographic Heavy Rainfall due to a Stratif orm Cloud

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Abstract
Our study of the rainfall around the Suzuka mountains during the past ten years has shown that the heavy rainfalls with a maximum daily precipitation of 200mm or more usually occur in connection with typhoons. The heavy rainfall areas lie on the lee side along the mountains and near the ridge. During those storms, strong south-easterly winds prevailed across the mountains in the lower troposphere, the middle and lower troposphere were almost saturated, and the stability was convectively unstable or nearly neutral. In some cases, the stability was nearly neutral, the time change of the rainfall rate was relatively small, and the convective rainband was not found by radar observation. Assuming these cases are due to a stratiform cloud, numerical experiments are carried out in order to make a model of such orographic heavy rainfall. In the first model, steady airflow over simplified mountains with the scale similar to that of the Suzuka mountains is computed, based on a two-dimensional and linearized equation, assuming that the atmosphere is saturated and has a pseudo-adiabatic lapse rate in which the undisturbed basic flow across the mountains has its maximum speed in the lower troposphere. The generation of precipitation is assumed to be due to the process of warm rain. A portion of the condensed liquid water in the ascending current is converted into cloud droplets with a certain size distribution, and another portion is spent to grow drops. In this cloud, drops grow by the collision and coalescence process, and evaporation occurs in the descending current. Some drops fall to the ground as rain. The steady state of these processes is computed. The computed rainfall rate with the model is very small having a maximum of only about 1mm/hr around the ridge, and the distribution is different from that observed around the Suzuka mountains. In the second model, in addition to the first, uniform light rain with the function of seeding falls from the upper cloud. The results using the second model are similar to that observed. It is found that the function of the rain from the upper cloud with the rainfall rate of a few millimeters per hour is important for such orographic heavy rainfall. In fact, the smaller the rate of decrease of the wind speed above the low level strong wind crossing the mountains, the more the heavy rainfall area shifts leeward with the rainfall over the lee side of the mountains becoming heavy. A similar relationship is obtained in this experiment. Furthermore, it is found from the experiment that the precipitation distribution around the ridge changes considerably due to the size distribution of drops of uniform rain falling from the upper cloud and the number density of cloud drops. The smaller the drops of rain falling from the upper cloud, the larger the rainfall rate around the ridge. On the other hand, the larger the generated cloud droplets or the smaller the number density of cloud drops, the larger the rainfall rate. The rainfall rate around the ridge decreases considerably if the number density of cloud drops exceeds about 100/cm3.

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