In many cases, episodes of VH FFDI are short-lived, and not resolved by synoptic obser- vations (as suggested in Figure 3.2 and Figure 3.3). This assertion can be examined by checking the duration between the rst and last observation of VH FFDI on days of Very
(a)
(b)
Figure 3.3: Counts of FFDI >= 25 (Very High Fire Danger) by season for (a) Hobart Airport and (b) Launceston Airport. Note dierent y-axis scales on the plots. The same x-scale is used on both plots, to better allow comparison of activity between the stations over dierent seasons.
rating for periods. Nonetheless, it is valid to dene the VH event duration as the time interval between rst and last VH FFDI observation, even if that period is not continuous. Of course, a number of dierent measures of event duration are possible, each of which may be informative in particular contexts. For example, the longest period of continuous VH FFDI, or the total time above VH FFDI are both reasonable indicators of the level of re danger for a day. The time interval between rst and last observation of VH FFDI, however, provides a stringent upper limit to event duration and therefore the likelihood of detection of the VH FFDI event by a synoptic or 1500-only observation schedule. Figure 3.4 plots the time interval between rst and last VH FFDI observation for both Hobart (Figure 3.4(a)) and Launceston Airports (Figure 3.4(b)). VH FFDI events occurred very infrequently at Devonport Airport, and are not included here. Duration of events was binned into half- hourly categories. Those of duration less than one half-hour, most commonly single VH FFDI observations, were placed in the 0 hour bin. Also plotted are the durations of events where the FFDI exceeds 38. Again, a single observation of VH 38+ was placed in a 0 hour duration bin, together with, rarely, two or more such observations within a half-hour period (with no subsequent observation of FFDI greater than or equal to 38). In the case of both Hobart and Launceston Airports, there is a very rapid decrease in the number of VH FFDI events as duration increases, with a long tail of longer duration events, particularly at Hobart Airport. Interestingly, in both locations the decline in the number of VH 38+ events with increasing duration is much less abrupt.
Of considerable interest for both locations is the proportion of events lasting less than three hours, as these could potentially escape detection by a synoptic observing programme. For Hobart Airport, of the 301 VH FFDI events, 175 (58%) were of duration less than 3 hours and thus at risk of not being detected by a synoptic-only observation schedule. In the case of Launceston Airport 48 of a total 73 VH FFDI events (66%) lasted less than three hours.
Of the 100 cases of FFDI VH 38+ at Hobart Airport, 62 reached or exceeded FFDI 38 for less than three hours. When the criterion for FFDI was set at FFDI 70 (not displayed), some 17 events were detected in the METAR database for Hobart Airport, of which 15 (88%) were at or above FFDI 70 for less than three hours. At Launceston Airport, of the ten VH 38+ events, eight lasted less than three hours. The above suggests that the higher the value at which FFDI peaks, the less likely the peak is to be resolved by a synoptic observing network.
(a)
(b)
Figure 3.4: Duration of re danger at (a) Hobart and (b) Launceston Airports. Number of events refers to the total number of events recorded at each station for the respective study periods, binned by half-hour. Axes are again scaled dierently.
observation of FFDI ≥38 occurred after 1500 LCT on 16 days, and on ten of those days,
FFDI was still below 24 at 1500 LCT. Similarly, there are days in the Hobart Airport dataset on which the latest observation of FFDI≥38 occurred prior to 1500 LCT. Of the 100 days
on which FFDI reached or exceeded 38, conditions had eased by 1500 LCT on 37 days. Of these 37 events, conditions had eased below FFDI 24 by 1500 LCT on 15 days.
As an example of the rapid variability of re weather conditions around southeast Tas- mania, the event of 22 January 2006 is examined. On this day, Hobart Airport recorded a peak FFDI value of 64 at 1825 LCT. This FFDI was the highest of the 16 days in the Hobart Airport dataset which recorded FFDI below 38 until after 1500 LCT. The FFDI was 13, barely in the High range, at that time and had been lower than that until 1430 LCT. A weak seabreeze was pushed oshore after 1630 LCT as northwesterlies strengthened. By 1700 LCT, the FFDI had climbed to 44, but by 1901 LCT a cooler, moist southeasterly change had moved through Hobart Airport and the FFDI fell to 15. The time that the FFDI spent above 38 was less than two hours, and the observation record indicates that FFDI was in the range above 24 for only a few minutes longer than that. On this occasion, the synoptic observation schedule captured the event, but not the peak value. Had the cold front traversed the Hobart Airport region an hour earlier, however, the event would not have been captured at all by the synoptic schedule. Furthermore, even in this scenario, the event would not have been registered by an examination of 1500 LCT-only observations.
It is worth noting that the brevity of this event was not simply a function of the mi- croclimate of Hobart Airport, as a particular location in a complex coastal environment. Bushy Park, approximately 55 kilometres to the west-northwest of Hobart Airport in the inland Derwent Valley, also recorded elevated re danger on this day. The FFDI at Bushy Park peaked at 52 and exceeded 38 for about 75 minutes, spending about three and a half hours with FFDI above 24. Thus, while the transition to and from elevated re danger was not as abrupt at Bushy Park, it experienced comparable conditions to Hobart Airport. Other locations in southern Tasmania experienced broadly similar re weather on this day, although Hobart Airport did record the highest FFDI. During this time, a number of res were burning across a wide swathe of Tasmania, due to widespread lightning on 20 January. Most of these res became more dicult to control as a result of the weather conditions on 22 January.
Some events have occurred with very pronounced peaks early in the day, for which the 1500 LCT observation was below the High range. A peak FFDI of 115 was recorded at
1200 LCT on 11 January 1991, yet the passage of a cool, moist southeasterly change resulted in a 1500 LCT FFDI value of 9. On the other hand, early onset of seabreeze conditions, as in 22 January 2006, or the persistence of a maritime boundary layer until well into the afternoon can mask the approach of a change and typically hot, dry north to northwesterlies are not then experienced until after a 1500 LCT reading has occurred.
These real and not atypical events indicate strongly that climatologies based solely on 1500 LCT observations will miss many such signicant re weather events. In order to resolve the full range of possible re weather conditions, vital in an attempt to characterise current and future re weather climatology, weather conditions throughout the day need to be considered.