Examination of HIGH-RESOLUTION NWP Model QPF and PQPF For The American River Basin From Time-lagged and Multiple Model Ensembles During Hmt-west 2006

Abstract

Methods of generating NWP ensemble QPF include using alternative initial conditions, multiple models, or a combination of both. We examine an approximation to the method of alternative initial conditions that involves reinitializing a forecast model after a short time period, during which the initial state has evolved. This is known as a time-lagged ensemble. Time-lagged ensembles are a very efficient way to use computer resources in that a single model can be run in succession to generate the ensemble. By comparison, the other approaches require multiple models or multiple simulations from the same model to be run simultaneously. A practical test of a time-lagged ensemble is conducted during the 2006 field campaign of the Hydrometeorological Testbed that focused on the American River Basin (ARB) during 2005 December through 2006 March (HMT-WEST 2006). Precipitation in the ARB is largely produced by upslope winds and synoptic- and meso-scale forcing mechanisms which may evolve over time periods of hours. In addition, data is sparse over the ocean, which is upstream of the ARB. It is reasonable, therefore, to expect that significantly different initial states may be obtained after just a few hours. As a result of initializing from different conditions, forecasts may diverge. The real-time forecasting experiment run concurrently with HMT-WEST 2006 examined the utility of high- resolution NWP and methods of generating NWP ensembles. The forecasts consisted of three NWP models (RAMS, MM5, WRF), having grid-point spacing of 3-km. Each model was reinitialized every three hours with a diabatic initialization that permits clouds and supportive circulation and produced a forecast of 30 hours in length. At the conference, we will present results of a comparison of WRF time-lagged and multi-model ensembles for QPF and PQPF during the intesive observing periods of the HMT-WEST 2006 field project.