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ABSTRACT
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 cloudresolving 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.
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1. Introduction
Precipitation is directly produced by cloud microphysics processes at convective temporal and spatial scales. But the occurrence of precipitation is associated with environmental dynamics and thermodynamics of weather and climate events. So precipitation can be simply assumed to be proportional to the condensation rate (microphysical view) or the moisture flux in the convective systems (large-scale view). However, due to reevaporation of rain and local atmospheric moistening, not all condensation or moisture fluxes are used to produce precipitation. Therefore, precipitation efficiency is defined to evaluate how efficiently the convective system produces precipitation. This quantity is a crucial factor in a number of research topics. For example, precipitation efficiency is a key parameter in the closure assumptions in some cumulus parameterization schemes (Kuo 1965, 1974; Fritsch and Chappell 1980; Kain and Fritsch 1993; Grell 1993). Doswell et al. (1996) proposed an ingredient approach to forecasting the potential for heavy rainfall and flash floods using the precipitation efficiency as an essential conversion rate. In addition to the weather-scale topics, precipitation efficiency is also an important parameter for cloud-climate feedback processes. For example, Lau and Wu (2003) analyzed satellite data from the Tropical Rainfall Measuring Mission (TRMM; Simpson et al. 1988) and found that there is a substantial increase in precipitation efficiency of light warm rain as the sea surface temperature (SST) increases, but precipitation efficiency of heavy rain associated with deep convection is independent...