Orographic precipitation and flash flood occurrence frequency in Southern California

Flash flooding is a serious natural hazards due to the number of people affected and proportion of fatalies. In Southern California, flash flooding, and other hazards such as landslides and debris flows, result as moisture-laden Pacific air masses interact with the local mountain slopes of the Transverse and Peninsular Mountain Ranges. A case study aimed at advancing the understanding of flash flood occurrence climatology over the Southern California region has been undertaken through interdisciplinary modelling of orographically-induced precipitation, stream geomorphology and hydrologic response. This modeling approach is undertaken because the reporting of flash floods occurrence does not exist with high spatial resolution nor with high spatial and temporal consistency, and records of precipitation observations from which to derive the climatology are sparse relative to the spatial scales of flash flooding.

Estimates of orographically-driven precipitation at scales relevant for flash flooding is examined and intercompared from various modelling approaches: (a) an interpolation of station observations employing PRISM climatology, (b) a simplified orographic model, and (c) available regional model output from the California Reanalysis Downscaling (CaRD10; Kanamitsu & Kanamura, 2007). Precipitation estimates are produced on a 5-km horizontal grid through the modelling of method (b) and via interpolation for methods (a) and (c) and over wet seasons for the historical period 1950 – 2005. The simplified orographic model employs potential theory flow to estimate steady state three-dimensional wind fields for given free stream velocity forcing winds, atmospheric moisture advection, and cloud and precipitation microphysics proposed by Kessler (1969). NCEP Reanalysis I is used to provide free stream wind forcing and large scale atmospheric moisture conditions. Summary statistics including monthly volumes and climatological averages show good agreement in the topographically-forced regions, with additional spatial detail allowed by the high resolution, simplified orographic model.

The implication of these different precipitation products on the estimation of regional flash flood occurrence frequency over the 1950 to 2005 period is explored. Flash flood occurrence is based on a hydro-geomorphologic response threshold for small-scale watersheds of the order of 25km2 and hydrologic soil moisture accounting modelling. The regional focus allows for comment on the spatial variability in flash flood occurrence and the use of various precipitation products provides insight on the uncertainties of flash flood occurrence climatology estimation.