Numerical Simulation and Diagnosis Analysis on Heavy Rain in East Hebei by Typhoon Matsa

A heavy rain occurs in east Hebei during August 8—10, 2005. It is caused by a tropical storm which is resulted from the weakened typhoon Matsa. Mesoscale numerical model MM5 V3 is used to simulate the case. The precipitation time and intention as well as the synoptic situations are compared with the observation. Similarities are found. Based on mesoscale numerical model products, the physical mechanisms of the heavy rain are analyzed.The results show that the water vapor on low-level is quasi-saturated and the humidity layer is deep and stationary during the heavy rain. There are upper-level atmosphere divergence and the low-level atmosphere convergence over the rainstorm area, and they result in strong ascending motion which is the dynamical mechanism of the initiation and maintaining of the heavy rain.The meso-and low-level air circulation and the θ_e field are analyzed. Results indicate that the thermal structure of typhoon is asymmetry. Its eastern region is warm and the western is cold. The strong easterly low-level jet sustains in north of the typhoon. Energy and warm vapour are transported toward the rain-storm area.In meso-and low-level troposphere the values of helicity over the rainfall center are always positive. The precipitation center is corresponding with the maximum helicity center, and its movement follows the maximum helicity center. The maximum helicity center has certain designation function for the center of the heavy rain in the future. The values of local helicity are large probably because of the large horizontal wind velocity or the horizontal vorticity or of the both. Abnormity of air is lead to, and so is the rainstorm. The results also indicate that the rainstorm occurs in the maximum SRH (storm relative helicity) center or in its southeast isoline denseness area. At the same time, the values of SRH in the rainstorm area always exceed 120 m²/s² before or during the heavy rain. It means that the rotary environment wind field is advantageous to strengthen ascending motion. Enough flotage energy is provided for the convection.