Synoptic-scale and Mesoscale environments conducive to forest fires during the October 2003 extreme fire event in southern California

This study has employed both observational data and numerical simulation results to diagnose the synoptic-scale and mesoscale environments conducive to forest fires during the October 2003 extreme fire event in southern California. A three-stage process is proposed to illustrate the coupling of the synoptic-scale forcing, such as the high pressure ridge and the upper-level jet streak, which leads to meso-α scale subsidence in its exit region, and the mesoscale forcing, such as wave breaking and turbulence and the wave-induced critical level, which leads to severe downslope (Santa Ana) winds. Two surges of dry air were found reaching the surface in southern California. The first dry air surge arrived as a result of moisture divergence and isallobaric adjustments behind a surface cold front. The second dry air surge reached southern California as the meso-α to meso-β scale subsidence and the wave-induced critical level over the coastal ranges phased together to transport the dry air from the upper-level jet streak exit region toward the surface and mix the dry air down to the planetary boundary layer on the lee side of the coastal ranges in southern California. The wave breaking region on the lee side acted as an internal boundary to reflect the mountain wave energy back to the ground and created severe downslope winds through partial resonance with the upward propagating mountain waves.

The widely used Haines Index (HI) and the newly developed NCSU3 and NCSU4 fire indices were calculated and compared during this period using the numerical model data. The results show that all three indices predicted reasonably well the potential for large fire growth in southern California. The patterns predicted by the NCSU3 and NCSU4 indices were very similar until the phasing between the vertically increasing sinking motions, a deep well-mixed boundary layer and a strong vertical wind shear zone developed. A temperature inversion above a near surface well-mixed boundary layer in southern California provided an environment favorable for the development of plume-dominated fires, for which the HI was originally designated to predict the potential for large fire growth. This particular type of environment is part of the reason why the HI was successful in assessing the potential for large fire growth in southern California before the Santa Ana winds became the major controlling factor.