Fire growth patterns in the 2017 mega fire episode of October 15, central Portugal

Abstract

Increasingly large fires are occurring in southern Europe, especially at its westernmost part, where a humid Mediterranean climate combines with rough topography, flammable vegetation types, undermanaged forest and high ignition rates. These fires have huge socioeconomic and environmental impacts, including the potential loss of human lives. Portugal was struck by an exceptionally severe fire season in 2017 that included a multiple mega fire event (~200 kha) in mid-October, unprecedented in Europe in terms of season and the extent of area burned in a very short period. This new phenomenon, arguably caused by climate change, needs to be characterized and understood to inform future fire management. We describe how five mega fires (or fire complexes) have developed in central Portugal on October 15-16, 2017, and assess their major drivers. We described fire weather through the Canadian FWI System, the C-Haines index and atmospheric profiles, retrieved weather data, and estimated fine dead fuel moisture content. Seven extremely large fire scars resulted from the October 15 event. We selected five of them (18,503-48,462 ha) for fire growth reconstruction. To describe fire spread we compiled data from various sources, including satellite imagery, official reports and fire suppression data, photos and videos, and people accounts. When satellite hotspots presented high density, the Fire Radiative Power of these points was interpolated to identify the approximate location and shape of the active fire front. We built a fire location chronology and interpolated the resulting points to support the final production of a map of hourly fire isochrones. For each hour period we calculated burned area and fire growth rate. Fire danger rating was Extreme, with most weather stations displaying FWI>50, expressing the joint effects of critical fuel aridity and hurricane Ophelia and its advection of warm and dry air from Africa. C-Haines index reached 10-11 (on a maximum of 13) and the moisture content of fine dead fuels was 3-6%. Forests comprised most (78%) of the burned surface, with Pinus pinaster generally dominating. Among fires, an average of 73% of the area burned for the first time since 1975 or had not experienced fire for the last 19 years or more, indicating potentially high fuel load. Fires spread followed the SW-NE axis and often crowned, with spotting as an important spread mechanism that allowed fire percolation through fragmented forest landscapes to impact important wildland-urban interfaces. Estimates of maximum hourly rates of spread varied from 5 to 9 km hr-1 between fires, corresponding to 50-90 MW m-1 for a typical fuel load. Initial fire growth was fast and dominated by strong winds and very low fuel moisture content. Wind subsided subsequently, relative humidity increased, and a sequence of pyro-convection events developed, coinciding with the fastest, and also erratic, fire spread periods. Overall, an average of 10,000 ha burned per hour between 16:00 and 05:00 of the following day. The findings from this reconstruction and the explanation of the events will assist in improved fire management policies and operational guidelines, from fuel management to fire preparedness and suppression.