A High-Resolution Prediction Study of Two Typhoons at Landfall

Abstract

The landfall on the China coast of two typhoons has been studied using both observational diagnostics and a 30-km, 19-level prediction system. The typhoons vary in size and in the amount of rainfall they produced after landfall. Use is made of a synthetic typhoon insertion scheme and the relatively dense observational network around China to provide objective analyses. The authors illustrate that, in the forecast model, as the typhoon?s outer circulation impinges on the China coast, an outer cloud band develops offshore, parallel to the coast, extending cyclonically north and east. During this time, the inner circulation weakens. As the outer cloud band strengthens, the center of the circulation approaches it from the east and there is a short-term intensification of both the inner region vertical motion and vorticity. Following landfall rapid dissipation of the core occurs. Using detailed diagnostics from the model integration, the changing thermal structure of the forecast vortex is illustrated. It is argued that, in the model, over the sea with near-zero sensible heat flux and a diagnosed low-level cold temperature anomaly inside the radius of maximum winds, downward mixing and horizontal, downgradient inflow cooling are the dominant near-surface processes. Over land, similar processes are operating, but diurnally varying sensible heat flux somewhat modifies the balance. Within the limitations of (a) the quality of the observational network, (b) the ability of the numerical model to represent real atmospheric processes, and (c) the limited number of events analyzed, the following conclusions are drawn. 1) The forecasts of the events, including the rainfall patterns, are quite realistic and provide encouragement for improvements in operational prediction. 2) The time evolution of typhoon structure in the analyses and forecasts shows (a) a weakening of the circulations as they approach land, (b) a slowdown in the weakening (or sometimes strengthening) near landfall, and (c) rapid decay after landfall. 3) As suggested previously, the strengthening seems due to enhanced inflow as the circulations approach the land from the sea. However, for these cases this occurs as the typhoon?s circulation impinges on the coastline and an offshore outer cloud band intensifies and interacts with the approaching vortex center. 4) Weakening after landfall seems related to (a) diurnally modulated cooling of the low levels due to enhanced downgradient inflow, (b) reduced downward mixing of warm air in the lighter wind region near the vortex center, (c) the finite heat capacity of the land and the limit placed on the maximum low-level temperatures by the surrounding sea temperatures, and (d) stabilization following enhanced convection at landfall. These processes act to increase the central pressure and inhibit the development of further deep convection by limiting the heat and moisture supply from the surface and reducing the moist convective instability necessary to grow deep convective clouds. Upper-air soundings areused to validate the evolving structures. 5) Lagrangian trajectory diagnostics suggest that the amount of rainfall at landfall and the subsequent circulation characteristics may in part be determined by the depth and extent of moist inflow from over the sea.