Uphill From Discharge to Spatial Rainfall - Toward a Generalized Topology of Physically-Based Orographic QPE Corrections

Abstract:

The relationship between the travel time probability density function (TTpdf) of solute transport in a 3D hillslope and the instantaneous unit hydrograph was examined by Fiori and Russo (2008) who found that the distribution of length paths within the hillslope was the key factor controlling the shape of the unit hydrograph (and the TTpdf). This finding suggests that it should be possible to map a basin's runoff flow net in response to a rainfall event from discharge observations (i.e. the streamflow hydrograph) at the basin's outlet by backward Lagrangian tracking of water fluxes in a 3D hydrologic model. Indeed, we pursue this approach to develop space-time varying corrections of radar rainfall fields in complex terrain based on the backward propagation of model streamflow errors at the basin's outlet, hereafter referred to as IRC (inverse Rainfall Correction) for simplicity. The underlying premise is that 3D basin geometry is adequate to capture the transient evolution of water storage and transport across the basin. Quantitative Precipitation Estimates (QPEs) from remote-sensing observations (e.g. ground or space-based radars) are especially challenging in mountainous regions. Here, we demonstrate the systematic application of the methodology for several rainfall events and in different basins in the IPHEx 2014 domain first (Barros et al. 2014). Second, we hypothesize that rainfall measurement artifacts that translate into QPE errors are not event specific. Rather, orographic QPE errors are organized by landform, and they can be categorized and learned over time from the observing geometry conditional on weather regime. Consequently, we apply a data-driven mapping algorithm to reconcile the space-time climatologies of QPE errors and IRC corrections and demonstrate how a generalized correction model emerges that can applied to regional QPE. Whereas we demonstrate this approach for the IPHEx 10-year combined radar-raingauge reference precipitation product, its application to other precipitation products such as IMERG is straightforward.