A Mesoscale Modeling Study of the 1996 Saguenay Flood

Abstract

A mesoscale simulation of the 19?21 July 1996 Saguenay flood cyclone was performed using the Canadian Mesoscale Compressible Community (MC2) model to study the processes leading to the explosive development and the large amount of precipitation. The performance of the simulation is verified by careful comparison with available observations with particular emphasis on the quantitative forecast of precipitation. It was shown that the model accurately simulates the wind, temperature, and humidity fields. Using the Kong and Yau microphysics scheme, the model performs quite well in the threat scores over a broad range of precipitation thresholds. Comparison of model precipitation against an objective analysis from rain gauge measurements and against the time evolution of accumulated precipitation at specific sites indicates generally good agreement except that the magnitude of the maxima is about 10% lower in the simulation. Potential vorticity (PV) inversion and sensitivity experiments show that the rapid deepening of the cyclone results from a combination of upper-level forcing from two shortwave troughs that partially merge, an upper-level jet streak, latent heat release, and low-level thermal advection. Condensational heating was integral for the establishment of a phase lock between the surface cyclone and a strong, upper-level trough that steers the cyclone. The flow field associated with a weaker trough, located downstream of the stronger trough, acted to retard the progression of the stronger trough, ultimately causing the cyclone to be located in a favorable position to interact with orography. It was shown that in the middle of the explosive deepening period, the contributions to the 900-hPa geopotential height anomaly from the upper-level dry PV anomaly, the low-level moist PV anomaly, and the surface potential temperature anomaly were 47%, 41%, and 12%, respectively. The contribution to the precipitation from orographic variation is quantified through sensitivity experiments in which aspects of the orography field are altered in the model conditions. It was found that orographic variation contributed to approximately 15% of the 48-h accumulated precipitation in the region of the flooding and up to over 25% in other local areas.