A modified definition of precipitation efficiency (PE) is proposed based on either cloud microphysics precipitation efficiency (CMPE) or water cycling processes including water vapor and hydrometeor species [large-scale precipitation efficiency (LSPE)]. These PEs are examined based on a two-dimensional cloud-resolving simulation. The model is integrated for 21 days with the imposed large-scale vertical velocity, zonal wind, and horizontal advections obtained from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). It is found that the properly defined PEs include all moisture and hydrometeor sources associated with surface rainfall processes so that they range from 0% to 100%. Furthermore, the modified LSPE and CMPE are highly correlated. Their linear correlation coefficient and root-mean-squared difference are insensitive to the spatial scales of averaged data and are moderately sensitive to the time period of averaged data.

Although there have been many observational and modeling studies of tropical cyclones, understanding of their intensity and structural changes after landfall is rather limited. In this study, several 84-h cloudresolving simulations of Typhoon Nari (2001), a typhoon that produced torrential rainfall of more than 1400 mm over Taiwan, are carried out using a quadruply nested-grid mesoscale model whose finest grid size was 2 km. It is shown that the model reproduces reasonably well Nari's kinematic and precipitation features as well as structural changes, as verified against radar and rain gauge observations. These include the storm track, the contraction and sizes of the eye and eyewall, the spiral rainbands, the rapid pressure rise (ϳ1.67 hPa h Ϫ1 ) during landfall, and the nearly constant intensity after landfall. In addition, the model captures the horizontal rainfall distribution and some local rainfall maxima associated with Taiwan's orography.A series of sensitivity experiments are performed in which Taiwan's topography is reduced to examine the topographic effects on Nari's track, intensity, rainfall distribution, and amount. Results show that the impact of island terrain on Nari's intensity is nearly linear, with stronger storm intensity but less rainfall in lower-terrain runs. In contrast, changing the terrain heights produces nonlinear tracks with circular shapes and variable movements associated with different degrees of blocking effects. Parameter and diagnostic analyses reveal that the nonlinear track dependence on terrain heights results from the complex interactions between the environmental steering flow, Nari's intensity, and Taiwan's topography, whereas the terraininduced damping effects balance the intensifying effects of latent heat release associated with the torrential rainfall in maintaining the near-constant storm intensity after landfall.

Precipitation efficiency is estimated based on vertically integrated budgets of water vapor and clouds using hourly data from both two-dimensional (2D) and three-dimensional (3D) cloud-resolving simulations. The 2D cloud-resolving model is forced by the vertical velocity derived from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The 3D cloud-resolving modeling is based on the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) simulation of Typhoon Nari (in 2001). The analysis of the hourly moisture and cloud budgets of the 2D simulation shows that the total moisture source (surface evaporation and vertically integrated moisture convergence) is converted into hydrometeors through vapor condensation and deposition rates regardless of the area size where the average is taken. This leads to the conclusion that the large-scale and cloud-microphysics precipitation efficiencies are statistically equivalent. Results further show that convergence (divergence) of hydrometeors would make precipitation efficiency larger (smaller). The precipitation efficiency tends to be larger (even >100%) in light rain conditions as a result of hydrometeor convergence from the neighboring atmospheric columns. Analysis of the hourly moisture and cloud budgets of the 3D results from the simulation of a typhoon system with heavy rainfall generally supports that of 2D results from the simulation of the tropical convective system with moderate rainfall intensity.

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