A meteorolgical investigation of the 'springtime bump' : an early season peak in the fire danger experienced in Tasmania

Datasets of Tasmanian fire weather observations are analysed to investigate the existence of an anecdotal \springtime bump\" an early fire weather peak in the Tasmanian fire season. Such a phenomenon is not well-documented in comparison to the usual summer to early autumn peak. The existence of a springtime fire danger peak is confirmed for eastern and southeastern Tasmania approximately one year in two. It is also shown that there has been a substantial increase over recent decades in the number of springtime fire weather events in southeast Tasmania with peak McArthur Forest Fire Danger Index (FFDI) in excess of 40. Diurnal variations in fire danger between regions of Tasmania and between high- and low elevation sites are examined highlighting differences in typical diurnal fire danger behaviour in different regimes. For example fire danger peaks are generally experienced during the morning at the (high-level southeastern) summit of Mt Wellington around noon on the west coast and later in the day at low level southeastern sites. High temporal resolution observations illustrate that the peaks of fire danger can be quite short-lived and frequently occur at times other than mid-afternoon however. Thus 50% of daily fire danger peaks at Hobart Airport (in the southeast) occur at times other than 1500 Local Time (LT). This suggests that many climate studies which use widely available 1500 LT observations may substantially underestimate the level of fire danger in their areas of study. Case studies of two individual springtime fire weather events indicate that at least two mechanisms operate in the generation of severe fire weather. Hot dry air can be advected from continental Australia and/or dry high-momentum air can be transported from high in the troposphere to the surface through front/trough vertical circulations resulting in abrupt increases in fire danger during already severe events. In both cases there is evidence of a foehn effect contributing to the warmth of the airmasses passing over the central Tasmanian topography. Further study of dangerous fire weather days throughout the fire season using high temporal resolution fire weather observations shows that the differences evident between the two case studies occur more generally and that there are distinct synoptic differences between the two types of fire weather event. Abrupt fire danger increases are associated with the presence of jet streaks close to Tasmania and with negatively-tilted upper tropospheric troughs. Such features may be evident in numerical weather guidance some days in advance of events allowing for early notice to fire and land managers. The seasonality of these events suggests that they have contributed to springtime fire danger over recent decades but are not the sole cause of dangerous springtime fire weather. The short length of record of high temporal resolution observations does not permit an assessment of changes in frequency or seasonality. Investigation of projected future Tasmanian fire danger using regional climate modelling suggests that current trends are likely to continue with a gradual increase in cumulative fire danger and in 99th percentile FFDI. Of particular note the proportion of Tasmania subject to 99th percentile springtime fire danger in excess of FFDI 24 is projected to increase from 6% (1961-80) to 21% (2081-2100). This is a more rapid increase than is projected for summer with very little change projected for autumn trends which have been observed over recent decades."