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 existing work and proposes a moisture source parameterization which accounts for the primary mechanisms of precipitation processes and conditions in mountainous regions. Specifically, the issue of orographic rainfall is addressed utilizing two components to represent the water vapor sources. One component is associated with the mesoscale rain system in the absence of orography. This component is defined during a particular rain event using radar observations and a discrete water budget analysis over a fixed region of the rain field. The second source term is orographic in nature and is a function of the average wind vector and the ground slope at the location of interest. The orographic moisture source term is additive to the contribution from the mesoscale source. The result is a rainfall forecasting model with an explicit physics-based orographic component and a relatively straightforward parameterization. Model performance is illustrated using two rainfall events observed by radar in the Cevennes region of France; 1 h lead-time rainfall forecasts 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 perform either as well as or slightly better than the simplified approaches of persistence and advection forecasts. Additional evaluations of the approach with data from other regions remains necessary for making comprehensive conclusions about the proposed formulation. This work shows the importance of obtaining information about the moisture input sources so as to increase the information defining the rain water source. Observing and forecasting rainfall rate alone does not explicitly account 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.