An initial examination of the areas of intense fire activity was conducted on LANDSAT archival imagery from USGS. One image that is both relatively cloud-free and free of scan-line correction problems showed the post-fire scars. An excerpt of the image is shown in Fig. 1d and clearly indicates leafless eucalypt canopy left behind by intense fire activity.

Weather data were supplied by the Australian Bureau of Meteorology. This included automatic weather station (AWS) half-hourly observations and daily vertical profiles from radiosonde flights. Relevant weather data were recorded at Mt Boyce AWS (WMO Station Number 94743), Nullo Mountain AWS (94754), Penrith Lakes AWS (94763) and Cessnock Airport AWS (95771), and radiosonde data were recorded at Williamtown (94776). The radar data were recorded by the Newcastle radar. The locations of all of the above recording stations are shown in Fig. 1a.

Figure 2 shows time series plots of key weather measurements and fire indices for these stations. Fire behaviour estimates, using the equations of Noble et al. (1980), are based on the Forest Fire Danger Index. As no direct measurements of fuel moisture content (FMC) were available, FMC was described using the dimensionless fuel moisture index (FMI), which has been shown to produce reliable estimates of fuel moisture content in a variety of fine fuel types (Sharples et al., 2009; Sharples and McRae, 2011; Sharples and Matthews, 2011). In particular, Sharples et al. (2009) showed that FMI ≤5 consistently corresponded to eucalypt litter moisture contents below 3–4 %, a predictor of elevated fire intensity and energy release.

Fromm et al. (2012, especially Fig. 15c) discussed the synoptic situation from the west, dominated by the passage, of a pressure trough linked to a cold front further south.

The reflectivity data forming a full volume scan are available at 10 min resolution on 22 November. Computations from scan azimuth, elevations and range permit estimation of the maximum height of radar returns (the “echotop”; Fromm et al., 2012). We also employ vertical aggregated reflectivity, an indication of overall convective activity. The vertical aggregate of reflectivity is a simple summation of individual reflectivity values in the column at each reported range/azimuth location. Note that echoes below the nominal cloud-base altitude (taken to be 4 km, based on the radiosonde profile at Williamtown) are attributable to non-hydrometeors such as debris lofted by the active fire (Lindley et al., 2011). Above the cloud base, stronger echoes are returned by hydrometeors, although these tend to be anomalously small (Rosenfeld, et al., 2007). In Fig. 6 we use a rain-rate product, which is derived from reflectivity.

The New South Wales Rural Fire Service aircraft used the Daedalus 1268 ATM multispectral line scanner supplied by Air Target Services Pty Ltd, then under contract (Cook et al., 2009, Sharples et al., 2012). For each run, flown at heights of 7 to 8 km, data acquisition took generally 10 min to complete. In some cases a number of contiguous passes were merged to provide coverage of the whole fire area.

Indicative colour samples are provided in the legend for Fig. 3; however, they relate to dynamically calibrated multispectral line scan products, taken in varying smoke scattering and absorption environments, which alter the pseudo-colour rendering.

The Grose Valley fire began as two ignitions generally 10 km north of Mt Boyce that were detected on 14 November. The exact date and time of the ignitions are unknown (Cronstedt, 2006).

Prior to 21 November at 02:07 (line scan 116), the fire was spreading with a single bounding fire line (as seen in Fig. 3b) in a manner that could be deduced from fuel, terrain and weather data. It had gone out or been extinguished in places.

A breakaway (AFAC, 2012) can be seen in Fig. 3b, north of Anvil Rock (AR). As a result of this, the containment line for the fire in the north-east had to be extended to the south-east of Explorers Range (ER). Figure 3c shows this as cold edge (line scan 117M, 22 November at 00:20) but also shows active fire edge extending past Banks Ridge (BR) to the south-east.

The fire burnt parallel to the cliff line, bypassed containment and became established on the plateau around ER, as might be expected given the terrain and the low FMI inferred from the weather observations (Fig. 2b).

The fire slowed significantly on the steep downslope runs off the range on 21 November at about 08:30, when the fire weather conditions began to ameliorate. At approximately this time the prevailing wind direction also changed from west-south-west to north-west (Mt Boyce, Fig. 2). The apparent lack of intense wind-driven fire activity at lower elevations in the Grose Valley suggests that these winds did not penetrate significantly down into the gorge; however, the fire on ER spread as a head fire down BR. On 21 November by 09:00 this part of the fire complex had burnt approximately 350 ha, with some lesser runs later in the night indicated by radar returns.

Observations at Penrith Lakes AWS indicated the presence of a pronounced foehn effect over parts of the region, starting on 21 November at about 17:00, the night before the pyroCb blow ups. Indeed, the synoptic weather pattern was consistent with that associated with the development of foehn winds in south-east Australia (Sharples et al., 2010). As they noted, the approach of a synoptic front caused winds to align nearly perpendicular to parts of the Great Dividing Range. This primarily produced mechanical foehns and resultant elevated temperatures and depressed humidity levels in the lee of the terrain. An abrupt increase in temperature from 21.4 to 30.3 ∘C with a complementary drop in relative humidity from 46 to 19 % was recorded on 21 November over the period 16:30–17:00 at Penrith Lakes AWS (Fig. 2). Coincident with these changes was a change in wind direction from south-west to north-west and an increase in wind speed from 9 to about 30 km h-1. This nocturnal foehn event would have significantly inhibited the usual moisture absorption by fuels that takes place overnight (Sharples, 2009; Sharples et al., 2010). It should be noted that while Mt Boyce AWS is close to the Grose Valley fire, it is on part of the terrain that would be generating, not experiencing, the foehn effect. The easterly AWSs (Penrith Lakes and Cessnock) are where the full effects of adiabatic processes would be expected. The fire ground, being up to 800 m lower than Mt Boyce AWS, might be expected to experience the effects of such a foehn wind event.

The fire also spread to the south, from the end of the containment line near AR. At the white arrow in Fig. 3c the fire crossed a creek line at the cliff top and moved uphill towards the south. The fire also dropped below the cliff line near Clarke Head (CH) and began to spread slowly downslope to the east towards Govetts Creek.

Figure 3c also shows the fire crossing the Grose River, approximately 1.5 km WSW of Mt Banks, after backing down the slopes below AR and Perrys Lookdown (PL). On 22 November at 01:00 the wind at Mt Boyce AWS shifted towards the west and increased in speed to above 35 km h-1. The change also brought the winds into closer alignment with the valley axis. On 22 November until 02:41 (line scan 126, Fig. 3d) the fire produced dense medium-range spot fires that extended 3.5 km downwind (in the area labelled “GV-C” in Fig. 1b), with a crosswind extent of approximately 2 km. After merging, these spot fires covered an area of approximately 600 ha, as indicated in Fig. 3d. An extent of about 100 ha of this was on the plateau to the north-west of Edgeworth David Head, where the fire had merged with the right flank of the previous fire run over ER. Figure 3d shows that the entire area burnt with high intensity, based on the relatively uniform spectral signature for active or decaying flame (Fig. 3, legend).

At a similar time, the left flank of the fire on the plateau (near King Georges Brook and Banks Ridge) exhibited spread with a notable northerly component, despite the west-north-west direction of the prevailing winds. This part of the fire complex is labelled “GV-N” in Fig. 1b. The relative intensity of the spectral signals indicates that this lateral spread produced higher rates of energy release compared to those inferred from Fig. 3c, 2.5 h earlier. The lateral fire propagation evident at GV-N is consistent with and best explained by the fire channelling phenomenon discussed by Sharples et al. (2012). Indeed, the steep lee slopes of the numerous side spurs leading north to Carmarthen Ridge satisfy the preconditions necessary for fire channelling to occur (Sharples et al., 2012). The downwind extension of the flaming zone extending towards Mt Caley is also consistent with that seen in other fire channelling events (Sharples et al., 2012).

There was a spatial alignment between the inferred fire channelling event at GV-N, which was captured on 22 November at 02:41 in Fig. 3d, and the strong radar returns recorded after 02:20 (Fig. 4b), the convective core of which has been outlined in Fig. 3d. Figure 4b indicates that the pyroconvective activity in the vicinity of GV-N started at approximately 01:30, and Fig. 4a shows that the pyroconvective top exceeded an altitude of 9 km for approximately 40 min between 02:30 and 03:10. In this period the intense lateral spread associated with fire channelling can be inferred to have been underway. This establishes a compelling spatial and temporal connection between the intense lateral spread associated with fire channelling and the violent pyroconvective activity detected by the radar. As stated by Finney and McAllister (2011), a large fire source will produce a plume which experiences less entrainment and is thus able to reach greater heights. This is consistent with the GV-N situation. The significance of this linkage between fire activity and convection is further demonstrated in the later event at CH, which will be discussed below.

Figure 3e (line scan 127M, 22 November 05:00) shows that, after traversing the heavily dissected cliff line between PL and CH, the fire exhibited rapid lateral spread towards the south-south-west along a 1.3 km length of cliff line. This part of the fire complex is labelled “GV-S” in Fig. 1b. The pattern of fire propagation depicted here is consistent with fire channelling, and the terrain in this vicinity meets the criteria necessary for fire channelling occurrence (Sharples et al., 2012). In addition to the lateral spread, the fire also spread downwind for 2.6 km across Govetts Creek, producing an active flaming area of approximately 400 ha. This expanse of active flame was constrained at the cliff line below Du Faur Head with some spotting for a further 1.7 km onto the plateau above. The southern flank of this part of the fire complex may be seen to be a series of merging spot fires, which is again consistent with other instances of fire channelling (Sharples et al., 2012).

The crosswind dimension of the flaming zone depicted to the south-south-west of CH in Fig. 3e was 1.3 km, and assuming that the lateral spread was initiated immediately after 02:41, the inferred minimum rate of lateral spread is 0.6 km h-1. However, it is unlikely that a spectral signature corresponding to active flaming (i.e. bright yellow) would persist for over 2 h without cooling and so it is far more likely that the fire covered the crosswind distance over a time of the order of the burnout time for the dominant vegetation type in the area, i.e. ∼30 min or less. Indeed, Fig. 4b indicates peak radar returns in the vicinity of GV-S sometime after 04:30, and if this is taken as an indicator of the onset of the lateral spread then the estimate for the rate of lateral spread can be revised to 2.6 km h-1. This latter estimate of the lateral rate of spread is of a similar order to that inferred in other cases of fire channelling using overlapping line-scan data (Sharples et al., 2012).

Figure 3e illustrates the spatial alignment between the fire channelling event at GV-S and the location of the strong radar returns used to populate Fig. 4. Figure 4 indicates that this plume developed into a significant pyroCb that exceeded 9 km in height between 05:00 and 05:40. Moreover, Fig. 4a confirms that the radar detected an abrupt increase in the echotop altitude at approximately the same time as the fire channelling event at GV-S was underway. This pyroCb activity associated with the fire channelling event ultimately registered echotops near the tropopause at ∼11.5 km.

It is estimated that 650 ha burnt within 2 h at GV-S, yielding an estimated energy release of around 2×1011 kJ, broadly comparable to values estimated for the Canberra fire in 2003 (Fromm et al., 2006). This and the entrainment concepts in Finney and McAllister (2011) support the connection between the fire and the pyroCb.