Sensitivity of Orographic Moist Convection to Landscape Variability: A Study of the Buffalo Creek, Colorado, Flash Flood Case of 1996

Abstract

A number of numerical experiments with a high-resolution mesoscale model were conducted to study the convective rainfall event that caused the 1996 Buffalo Creek, Colorado, flash flood. Different surface conditions and treatments of land surface physics were utilized to assess the sensitivity of this orographic moist convection to local and regional landscape forcing. Given accurate large-scale synoptic conditions at the lateral boundaries, the mesoscale model with a convection-resolving grid shows reasonably good skill in simulating this convective event with a lead time of up to 12 h. Sensitivity experiments show that a primary reason for this success is the use of an advanced land surface model that provides time-varying soil-moisture fields. This land surface model plays an important role in capturing the complex interactions among the land surface, the PBL, cloud-modulated radiation, and precipitation. For the case simulated, such interactions contribute to the temporal and spatial distribution of surface heating at small scales, and the convective triggering and development. Tests show that the landscape variability at small and large scales significantly affects the location and intensity of the moist convection. For example, on timescales of 6 to 12 h, differences in initial soil moisture associated with irrigation in the plains affect the evolution of the convection near the Continental Divide. Also, the surface modification by a wildfire burn influences the path of the major convective event that caused the flash flood. A watershed-based quantitative-precipitation-forecast skill score is proposed and employed. The relative success with which this severe thunderstorm is simulated over complex terrain provides some hope that the careful treatment of land surface physics in convection-resolving models can perhaps provide some useful level of predictability.