Changes in Summer-Nighttime Stability, Wind Speed and Fuel Moisture over the Western US

Reports from western US firefighters that nighttime fire activity has been increasing during the spans of many of their careers have recently been confirmed by satellite measurements over the 2003-2020 period (Freeborn et al. 2022). The hypothesis that increasing nighttime fire activity has been caused by increased nighttime vapor pressure deficit (through its effect on fuel moisture) is consistent with findings of recent studies that have documented significant positive trends in nighttime vapor pressure deficit over the western US during the last 40 years (Chiodi et al. 2021; Balch et al. 2022). Other meteorological conditions besides vapor pressure deficit, however, also typically exhibit strong diurnal changes that should be expected to help quell fire activity at night, compared to day. These include nocturnally increased near-surface atmospheric stability and decreased wind speed. We are interested in better understanding the extent to which all these factors have been changing over recent decades. Here, we quantify the extent to which the summer-nighttime distributions of Equilibrium Moisture Content (EMC), Planetary Boundary Layer Height (PBLH) and near-surface wind speed (WSPD) have been changing over the western US based on hourly data from the European Centre’s ERA5 data set. We consider changes between the most recent decade and the 1980s and 1990s, when many present managers and firefighters began their careers, and since the 2000s, when the satellite-based measurements of fire activity became available. We examine the increased (or decreased) likelihood of experiencing, in the recent period, nighttime conditions that would have registered as unusually conducive to fire activity (e.g. hot, dry and windy) in the previous period. Each variable’s behavior is considered on its own, and in conjunction with one another (For example, where have recent nighttime conditions exhibited both lower EMC and higher WSPD?). A main intent of this work is to inform further study of the reasons for the observed increases of nighttime fire activity.