A numerical study of warm could interactions



A two-dimensional, slab-symmetric cloud model with detailed microphysics is presented. Drops were classified into 37 size classes representing a droplet spectrum from 1 µm to 4 mm radius. Each class of droplet was subjected to condensation and collection processes. New droplets were formed by the nucleation process. Results show that the model is capable of simulating the life history of a warm cloud from the initiation to the dissipation stages. A warm, moist perturbation or impulse, 1 km wide and 1 km deep with base at 800 m, is used to trigger convection in the model. Comparison of our study on isolated clouds with actual observations based on Day 261 of GATE reveal some realistic features of the model. Numerical experiments on cloud interactions show that merging occurs when a calm environment is initially used. No merging occurs when a sheared flow is incorporated in the model. When the two impulses are introduced simultaneously, the upshear cell develops more strongly compared with an isolated cell or its downshear counterpart. The growth of the downshear cell seems suppressed. When the second impulse is introduced 15 minutes later, the newer cell develops more vigorously while the older cell dissipates. The lifetime is longer and the rainfall heavier when the new cell is located upshear of the older cell. The formation of rainfall from the simulation is quite rapid; the rainfall lasted for about 10 to 15 minutes. This feature is attributed to the use of a top-hat profile of the temperature and humidity for triggering initially the cloud growth and to the use of detailed microphysics in the model.


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