Searching for the equilibrium raindrop size distribution: Observations and simulations

Equilibrium rain DSD (drop size distribution) is the steady state size distribution achieved by raindrops as they fall through the atmosphere. When the equilibrium rain DSD is achieved, Z-R relation becomes linear. This has important implications in radar rainfall retrieval. Although the equilibrium rain DSD has long been established by 1-D rain shaft model through collision/coalescence/breakup, as well as evaporation, the time required for achieving it remains uncertain, especially when cloud dynamics are included. If and to what extent the equilibrium DSD exists (and detectable) in nature remains controversial. Here we present new efforts in searching for the equilibrium DSD using both surface and airborne observations, as well as the Goddard Cumulus Ensemble (GCE) model with a spectral bin microphysical scheme. The unique aspect of our modeling study is that it is a fully coupled dynamic model which simulates storm structures realistically. Three precipitation systems are simulated in this study: a tropical maritime convective system (TOGA COARE), a mid-latitude continental squall line (PRE-STORM), and a sea-breeze convection (CRYSTAL-ICE). The rainfall structures of all cases, and especially the PRE-STORM case, have been validated extensively using surface and satellite observations. In the meantime, comparisons of rain DSDs serve as additional validations of the model simulations. It is shown through both observations and simulations that the equilibrium DSD may be more ubiquitous than previously considered and cloud dynamics do play a role in determining how fast the equilibrium state can be approached in different rainfall systems. Uncertainties in both model simulations and observations are also discussed.