Short-term mesoscale ensemble forecasts of precipitation for Arizona during the monsoon

The quality of MM5 ensembles is evaluated for short-range probabilistic quantitative precipitation forecasts over Arizona during the Southwest monsoon. The sensitivity of different ensemble constructs is examined with respect to analysis uncertainty, model parameterization uncertainty, and a combination of both. Model uncertainty is addressed through different cumulus and planetary boundary layer parameterizations, and through stochastic forcing representative of a component of subgrid-scale uncertainty. A first-order autoregression model adds a stochastic perturbation to the Kain-Fritsch cumulus scheme and MRF planetary boundary layer scheme. A sensitivity study is also conducted to determine the MM5 planetary boundary layer parameterizations capable of simulating the structure of the pre-convective, monsoon atmospheric boundary layer.; The results indicate that ensemble precipitation forecasts are skillful and may assist operational weather forecasters during the monsoon. The most skillful ensembles contain both analysis perturbations and mixed-model physics. The Blackadar or MRF planetary boundary layer schemes are recommended for MM5 simulations or forecasts of the Southwest monsoon.