A Numerical Study of Precipitation Enhancement Potential in Frontal Cloud System
锋面云系降水中的增雨潜力数值研究

The macro and micro structures and the precipitation mechanism of a cold-frontal cloud with extensive and stable precipitation in Henan Province during 45 April 2002 are studied using the PSU/NCAR nonhydrostatic mesoscale model(MM5) and some observational data.Some basic variables such as atmospheric water budget and quasi-precipitation efficiency in association with exploitation and utilization of atmospheric water resources are calculated to evaluate the potential of precipitation enhancement in order to scientifically develop a conceptual model for the typical precipitating cloud system in the investigated region.The results show that the simulated clouds show different(precipitation) formation mechanisms in the different areas of the cold frontal system.The clouds within the frontal system usually display multi-layer and obvious non-uniform structure.It shows that ice crystals are at the upper(level,) the mixed ice and liquid water at the middle level,and the liquid water only at the low level.The cold- and warm-cloud processes coexist in the area.The plentiful cloud water is initially produced by warm-cloud process at both the low and middle levels.The ice crystals generated at the upper level are converted into snowflakes and graupel gradually by interacting with abundant cloud water at middle levels when falling and then precipitating particles melt to form rain after passing the melting level.It shows a distinct seederfeeder precipitation formation mechanism within the simulated cold-frontal clouds.Moreover,the simulation results also show that most of the precipitation is yielded along the cold-frontal line.The quasi-precipitation efficiency simply defined in the paper within the pre-frontal clouds and the post-frontal clouds is between 10% and 30%,while it reaches more than 30% near the inner area of the front.The quasi-precipitation efficiency even reaches more than 70% in the region with convective-stratiform mixed clouds.Only 23.1% of the total water vapor in the simulated area is converted into surface precipitation,which means most water vapor entering the studied region is not effectively converted into the surface precipitation.Thus,the cold-frontal clouds imply a very high seeding potential for precipitation enhancement.On the other hand,the simulation also displays a different kind of precipitation formation mechanism within the simulated clouds that intersect with the aforesaid part of the cold front.The analyses show that the precipitation from that part of clouds is chiefly yielded by ice phase process.After being generated at the upper level,ice crystals are most converted into snowflakes and less graupel.Then the melting of these ice particles,especially mass snowflakes,contributes to most of surface precipitation.Though the calculated quasi-precipitation efficiency within that part of clouds is as small as that within the pre-frontal and post-frontal clouds,the clouds still reveal a low seeding potential for precipitation