On The Study Of The Mechanism Of Tropical Cyclone Remote Precipitation

Tropical cyclones may produce precipitation or torrential rainfall in an area in front of and away from the cyclone. We call this phenomenon as the Tropical-cyclone Remote Precipitation (TRP). The statistics in this paper show that TRP is a small probability event. It is difficult to predict this phenomenon in operational circumstance. The TRP can also be found in Japan and the United States. American scientists call this as Predecessor Rain Events ahead of tropical cyclone (PREs).The statistical analysis shows that 14.7% tropical cyclones could produce TRP events in 1971-2006 years. Majority of the events would last more than two days. A number of TRPs have the property of wide distribution and high rainfall rate. High frequency of TRPs occurs in July and August. Statistical analysis also shows that there are two areas with the high frequency of the TRPs occurring, i.e, encircling Bo Hai Sea ( regionⅠ) and adjacent area between Sichan and Shanci provinces ( regionⅡ).Diagnostic study with inter-comparison between the two group data of TRPs and Non-TRPs indicates that the prominent differences in lower level either in regionⅠorⅡare typhoon interact with a westerly trough in TRP data group and without a westerly trough in Non-TRP data group but with north-west dry flow instead. On the other hand, the inter-comparison shows tropical cyclones with TRP have strong south-east wet flow channel connected and Non-TRP cyclones without such channel. Prominent differences are also appeared in the upper level. In regionⅠ, jet stream is located to the north of the remote rainfall region, it incurs the divergence and ascending motion for TRP. On the other hand, the upper level jet stream covers the region and induces the descending motion in the same region for Non-TRP. Similar results are showed in the regionⅡ. The composite data analysis of moisture fluxes budget in regionⅠandⅡshows that tropical cyclones with TRP can transport moisture to reach the regionⅠandⅡand moisture transport for the cyclones without TRP is restricted only in a local area. The moisture increases remarkably for the area of TRP. On the contrary, moisture decreases obviously in the same area without TRP. Moisture transport of a tropical cyclone plays an important role for TRP.A numerical experiment with WRF/ARW model 3.1 version is conducted. Double nested domain is adopted with horizontal resolutions of 30km in outer region and 10km in inner region respectively. Vertical resolution is 27 levels. The initial and boundary condition are provided by the NCEP reanalysis data of"global final"with 1°×1°grid space. Typhoon Matsa (0509) producing typical TRP is selected for numerical experiment. Control experiment is conducted. The results fit with the observation well. Then three sensitive experiments are conducted and the results are as follows: (1) TRP will remarkably decrease if tropical cyclone is removed or its intensity is decreased. (2) Westerly trough is an important factor to TRP. The simulation shows that the strength of westerly trough has a direct proportion to the rain rate of TRP. (3) the simulation shows that topographic effect also plays a significant role in TRP. The remote rainfall would decrease 20% precipitation if the topography is removed.The study shows that the mechanism of the TRP is related to the interaction between the tropical cyclone and the mid-latitude trough, the ground topography would play an important role to increase the rainfall. The moisture transport of an offshore tropical cyclone is the most primary condition for the TRPs.