The Observed Relationship Between Precipitation Efficiency and the Large-Scale Environment for a Location in the Tropics

The rainfall reaching the surface for a given amount of net condensation can vary due to factors such as re-evaporation as well as spatial and temporal spreading of precipitation. The ratio of surface precipitation to net condensation is one way to define the precipitation efficiency. Precipitation efficiency is often a key parameter in the representation of convection in weather and climate models. However despite its importance, there is little agreement on the value of precipitation efficiency and how it scales with environmental conditions. Additionally, much of the literature focuses on precipitation efficiency estimates using models (e.g. large eddy simulations), presumably due to a lack of suitable observations. Understanding how precipitation efficiency should vary is of utmost importance to interpreting the behaviour of rainfall extremes in models, particularly in a warming climate.

A commonly applied precipitation scaling for extreme events can be used to decompose convective rainfall events into three terms representing efficiency, dynamic and thermodynamic effects. The necessary observations required to apply this scaling are rainfall, as well as profiles of vertical motion, temperature and pressure.

A C-Band polarimetric radar dataset (CPOL) over Darwin, Australia, has recently been developed spanning the wet seasons over the period 1998 to 2017. We combine the radar observations of rainfall with retrievals of vertical velocity from a co-located pair of vertical wind profilers operating at two different frequencies. Combining the rainfall and vertical velocity with sounding information allows us, for the first time, to estimate the precipitation efficiency of convective events from observations for a location in the tropics. The availability of concurrent estimates of the large-scale environment achieved through a variational analysis approach enables us to investigate the relationship between precipitation efficiency and environmental conditions.