User:Nellie Sommer/sandbox

=Precipitation Efficiency= Precipitation efficiency is defined as the fraction of condensed water in a cloud that reaches the surface as precipitation. Effects such as re-evaporating precipitation and local atmospheric moistening result in the precipitation rate not being directly proportional to the condensation rate. Therefore precipitation efficiency is a complex characteristic of a convective system. It has an impact on cloud paremetrisation, heavy rainfall and flash floods, and climate feedback of water vapor and clouds.

=Theoretical Framework= The precipitation efficiency $$\epsilon$$ is a measure of how much of the condensed water in a cloud reaches the surface as precipitation:

$$\epsilon=\frac{P}{C}$$

P is the surface precipitation rate (with unit kg m−2 s−1) and C is the column-integrated condensation rate (also with units kg m−2 s−1). . Precipitation efficiency can be made up of two factors: conversion efficiency and sedimentation efficiency. Conversion efficiency is defined as the fraction of cloud condensate, that is converted to precipitation rather than remaining suspended as a cloud or leaving the convective region through entrainment. Sedimentation efficiency describes the probability that a precipitating hydrometeor will reach the surface and not evaporate as it falls. Precipitation efficiency can be defined at different scales. One such definition is through large-scale hydrometeor budgets, which account for source and sink terms of the water cycle. Using this definition, the precipitation efficiency can be estimated based on currently available assimilation data of satellite and sounding measurements. Another definition is through small-scale cloud microphysical budgets, which define precipitation sources and sinks in terms of condensation and deposition rates and can be applied in models with explicit parameterization of cloud microphysics.ref: buch This is referred to as cloud-microphysic These definitions are highly correlated when analysed with cloud-resolving simulations. When all source and sink terms are accounted for, the precipitation efficiency ranges from 0 to 100%.

=Key processes= Precipitation efficiency is affected by multiple microphysical and larger scale processes. On the microphysical level these processes include the evaporation of rain before reaching the surface and the phase changes of water from and to vapor, liquid and ice. ...(more to come)

=Precipitation Extremes= The local precipitation rate in an extreme precipitation event is found to be proportional to the precipitation efficiency. ...(more to come)

=Precipitation Efficiency and Climate Change= The change in precipitation efficiency with warming is poorly understood and seems to depend heavily on the definition of precipitation efficiency. Large scale precipitation efficiency, which is averaged over space and time is per definition 1 since what goes up must come down. For predicting changes with warming one uses the microphysical or a highly correlated definition of precipitation efficiency. In a warming climate precipitation is generally expected to increase and convective mass flux to decrease. These effects would seem to imply an increase in precipitation efficiency, but global climate models and observations do not confirm this. This is due to to the sensitivity of cloud condensation to environmental conditions such as tropospheric temperature, humidity and stratification, which are also impacted by warming. Global climate models predict precipitation efficiency to increase or decrease with warming depending on which factors they account for when calculating precipitation. After 2 °C of warming, the predicted change by the ensemble mean for precipitation efficiency is 2.5%, with models ranging from –7% to +12%. Models assuming an increase in precipitation efficiency with global warming, predict a stronger weakening of the Pacific Walker and the Hadley circulation and a stronger increase in precipitation extremes.

=Other useful sources= LANGHANS, WOLFGANG & Yeo, Kyongmin & ROMPS, DAVID. (2015). Lagrangian Investigation of the Precipitation Efficiency of Convective Clouds. Journal of the Atmospheric Sciences. 72. 1045-1061. 10.1175/JAS-D-14-0159.1.