A Numerical Study Of Cloud Condensation Nuclei Effects On Precipitation And Intensity Of The Typhoon Morakot

Aerosols effects acting as cloud condensation nuclei (CCN) on the precipitation and structure of the landfalling typhoon Morakot (the eighth typhoon in2009) was investigated with Weather Research and Forecasting Model (WRF), two numerical experiments were carried out:one with the background concentration of CCN set to2000cm-3which named CCN experiment and the other with300cm-3which named Ctrl experiment. The results indicate that, a large number of cloud condensation nuclei lead to increasing of the precipitation in typhoon’s main part, the most growths locate at the peripheral rainband which are distributed nubbly, resulting of aerosols invigorate the meso-and small-scale convections. The development of rainband tends to block the surface flow inflow, which makes the decreasing rainfall in the eyewall; by contrast, the large value of precipitation locates in eyewall in Ctrl experiment.The concentration of background cloud condensation nuclei is higher in CCN experiment than Ctrl experiment. Intensity is weaker when Morakot approaches to land in CCN experiment, its max radar reflectivity shows that typhoon eye zone is larger and the structure of rainband is looser. The amount of low levels (1-1.5km) inflow moisture from the spiral rainband on the northern typhoon decreases, which means the’fuel’for maintaining eyewall’s development; at the meaning time, cold and dry air is involved in typhoon’s eastern part. The speed of low levels in CCN experiment is lower than Ctrl.The amounts of microphysical variables (such as the rain, snow, and graupel mix ratios) increase with the concentration of cloud condensation nuclei increasing, such as snow and graupel. The vertical section of typhoon shows that convection is developed on the northern typhoon, the droplets of snow can rise to high levels; typhoon eye zone get larger with eyewall outward movement. LPI is high in the peripheral rainband and offshore, which means continental aerosols is taken to sea by typhoon. The probability of lightning getting larger indicates that its intensity being weaker.The increasing concentration of cloud condensation nuclei leads to change in the typhoon’s main part while the zones have little differences which’s far from the typhoon centre. The precipitable water and absolute vorticity are getting higher, those are lower in eyewall. There is a divergent zone in typhoon’s eastern, the downdraft caused by divergence leads to cool pool which is against to the intensity of typhoon.