ADAPTATION AND APPLICATION OF A QUANTITATIVE RAINFALL FORECASTING-MODEL IN A MOUNTAINOUS REGION

This work addresses operational forecasting of orographic rainfall for hydrologic modeling. The forecast model formulation builds on existin g work and proposes a moisture source parameterization which accounts for the primary mechanisms of precipitation processes and conditions i n mountainous regions. Specifically, the issue of orographic rainfall is addressed utilizing two components to represent the water vapor sou rces. One component is associated with the mesoscale rain system in th e absence of orography. This component is defined during a particular rain event using radar observations and a discrete water budget analys is over a fixed region of the rain field. The second source term is or ographic in nature and is a function of the average wind vector and th e ground slope at the location of interest. The orographic moisture so urce term is additive to the contribution from the mesoscale source. T he result is a rainfall forecasting model with an explicit physics-bas ed orographic component and a relatively straightforward parameterizat ion. Model performance is illustrated using two rainfall events observ ed by radar in the Cevennes region of France; 1 h lead-time rainfall f orecasts for three watersheds in this mountainous region are evaluated . One event is used for calibration of the model and the second event is used for validation. The proposed model formulation is shown to per form either as well as or slightly better than the simplified approach es of persistence and advection forecasts. Additional evaluations of t he approach with data from other regions remains necessary for making comprehensive conclusions about the proposed formulation. This work sh ows the importance of obtaining information about the moisture input s ources so as to increase the information defining the rain water sourc e. Observing and forecasting rainfall rate alone does not explicitly a ccount for the source of moisture that is transformed into rainfall at a future time. This work shows that a limit on forecast lead-time may be related to the response time of the precipitating cloud system.