Hail Formation And Its Effect On A Model Updraft

Abstract

The simulation of hailgrowth and its effects on cloud liquid water and energy balance is achieved by injecting hail embryos at the freezing level of a model updraft. Steady-state one-dimensional numerical models in which hail is only ascending or where it is ascending and descending are considered. The problems associated with the hail accumulation zones of former similar models are overcome by introducing a size distribution of hail embryos. Moderate concentrations of hail embryos, H to 2 per cubic meter, grow to deplete a significant fraction of the cloud's liquid water content. If this significant depletion of liquid water occurs, a hail accumulation zone is formed wherein the downward force on the cloud's updraft due to the hailstones is of the same order of magnitude as the upward buoyant force and further these zones are found to contain a maximum hail mass only three or four times greater than the vet-adiabatic liquid water content. The growth of hailstones from 0.25 or 0.50cm embryos to 3cm while ascending or U to 6cm after ascending and descending takes about lOmin in the former case and 20-30min in the latter. These growth rates are consistent with radar observations and they require only adiabatic liquid water contents (3*5gn/m* at maximum). Even larger stones are grown when they are non-spherical. The growth of large hailstones is therefore not dependent on an elaborate model of hailstone recycling within the cloud. Hail surface icing conditions and heat and mass exchange between the hailstones and the cloud air are found to be important to hailstone characteristics, the updraft's energy balance and liquid water content. Therefore, any future models of hailclouds should include the feedback mechanisms between growing hail and cloud air if moderate to heavy hail showers are to be numerically simulated. Altering the number of hail embryos or the cloud's liquid water content for the purpose of weather modification can produce positive or negative effects on hail production which are highly dependent on the hailcloud's parameters. For this reason any methods used to suppress hail should be allied with a definitive model of hail formation.