Diagnostic and Numerical Study on Physical Process of Strong Rainfall Associated with Rammasun (1409)

The development evolution and landfall process of typhoon Rammasun (1409) was investigated using high-resolution simulation produced by the WRF (weather research and forecasting) model. The diagnostic and numerical study on the physical process of strong rainfall focused on the period of Rammasun featuring the rapid enhancement of land surface friction and the landfall in the South China Sea, using three-dimensional surface precipitation equations and the definition of precipitation efficiency. The results showed that a strong average precipitation intensity (P_S) was maintained in the main circulation area of Rammasun, and the relative contributions of P_S over the land and ocean were opposite. The enhancement of land surface friction was conducive to more water vapor convergence to the land (Q_WVA represented vertically integrated three-dimensional flux convergence/divergence rate of moisture, and Q_WVA was positive), resulting in local atmospheric humidification over the land within a short period before the landfall (Q_WVL represented vertically integrated negative local change rate of water vapor , and Q_WVL was negative during the duration). The water vapor was rapidly transformed into liquid-phase and ice-phase hydrometeors by cloud-related microphysical processes (Q_CLL and Q_CIL represented vertically integrated negative local change rate of liquid-phase and ice-phase hydrometeors respectively, and they were negative during the duration), which promoted the rapid development of clouds and the intensification of precipitation intensity over the land. When the center of the circulation was located in the Beibu Gulf, the relative contribution of Q_WVA over the ocean was significantly enhanced. The change in the underlying surface during the landfall period led to remarkable changes in the moisture-related microphysical processes, causing significant changes in the cloud system and the strong precipitation center. The surface evaporation over the ocean caused more effect and more significant changes, compared with the one over the land. The average Q_CLL and Q_CIL in the main circulation were basically “positive–negative–positive” within the period of Rammasun movement to the coastal region of South China. The content of hydrometeors was mainly increased (decreased) when the circulation center was located in the Beibu Gulf (during the landfall). A high precipitation efficiency was maintained in the main circulation area of Rammasun. From the contact of the main circulation to the land, the precipitation efficiency over the land rapidly increased, while the precipitation efficiency over the ocean maintained a high value during most of the integration period but reduced only after the second and third landfalls.