Boundary-Layer Meteorol (2007) 124:291–304 DOI 10.1007/s10546-006-9148-3 O R I G I NA L PA P E R
Local boundary-layer development over burnt and unburnt tropical savanna: an observational study Chris K. Wendt · Jason Beringer · Nigel J. Tapper · Lindsay B. Hutley
Received: 27 July 2006 / Accepted: 5 December 2006 / Published online: 19 June 2007 © Springer Science+Business Media B.V. 2007
Abstract Fire scars have the ability to radically alter the surface energy budget within a tropical savanna by reducing surface albedo, increasing available energy for partitioning into sensible and latent heat fluxes and increasing substrate heat flux. These changes have the potential to alter boundary-layer conditions and ultimately feedback to local and regional climate. We measured radiative and energy fluxes over burnt and unburnt tropical savanna near Howard Springs, Darwin, Australia. At the burnt site a low to moderate intensity fire, ranging between 1,000 and 3,500 kW m−1 , initially affected the land surface by removing all understorey vegetation, charring and blackening the ground surface, scorching the overstorey canopy and reducing the albedo. A reduction in latent heat fluxes to almost zero was seen immediately after the fire when the canopy was scorched. This was then followed by an increase in the sensible heat flux and a large increase in the ground heat flux over the burnt surface. Tethered balloon measurements showed that, despite the presence of pre-monsoonal rain events occurring during the measurement period, the lower boundary layer over the burnt site was up to 2◦ C warmer than that over the unburnt site. This increase in boundary-layer heating when applied to fire scars at the landscape scale can have the ability to form or alter local mesoscale circulations and ultimately create a feedback to regional heating and precipitation patterns that may affect larger-scale processes such as the Australian monsoon. Keywords Albedo · Fire scar · Northern Territory · Surface energy budget · Tethered balloon · Tropical savanna
C. K. Wendt (B) · J. Beringer · N. J. Tapper School of Geography and Environmental Science, Monash University, Building 11, Melbourne, VIC 3800, Australia e-mail:
[email protected] L. B. Hutley School of Science and Primary Industries, Charles Darwin University, Darwin, NT 0909, Australia
292
C. K. Wendt et al.
1 Introduction The mesic or wet/dry tropical savannas of far northern Australia are an important and extremely fire prone ecosystem. Covering roughly two million square kilometres of northern Australia (Williams et al. 2004) the tropical savanna biome is subject to frequent, yet relatively low intensity, burning of the grassy understorey (Russell-Smith et al. 2000). A 4- to 5-month wet season (austral summer), during which almost all growth occurs, especially grass, is then followed by a 7- to 8-month season (austral winter) during which the grassy understorey cures to provide large quantities of fine fuels. Currently up to 250,000 km2 of savanna burns annually within Australia (Beringer et al. 1995) with an expected increase under predicted climate change (Williams et al. 2001). The ability of fire to influence climate directly through the emission of smoke and trace gases from the combustion of fuels is well documented (Wotawa and Trainer 2000; Honrath et al. 2004; Miranda et al. 2005). However, fire also has the ability to alter the atmosphere through indirect effects such as changing land surface properties and the surface energy budget, and initiating feedbacks to the atmosphere at local and regional scales (Beringer et al. 2003). Changes in albedo have been shown to result in direct effects on the surface energy fluxes, which ultimately feedback to the overlying atmosphere. In their Bunny Fence Experiment (BuFEX) Lyons et al. (1993) showed the importance of surface property changes and the resultant climatic feedbacks from areas of contrasting albedo over native and agricultural vegetation in an area of outback Western Australia. Native and agricultural vegetation are separated by a defined boundary, which developed due to the construction of a “bunny” fence to protect crops against rabbits, and exhibited different evapotranspiration regimes and albedo values. Their study showed that an area of native vegetation (albedo 0.09) that was adjacent to an area of agricultural land (albedo 0.13–0.25) absorbed more incident solar radiation and in turn generated greater levels of heating and convection (up to two to three times), even despite the increased evaporation from the irrigated agricultural land. These differences in surface characteristics and energy partitioning were then shown to result in local circulation changes and a possible decrease in winter rainfall. Furthering this study Huang et al. (1995) and Lyons (2002) were able to use a one-dimensional modelling approach to further support their BuFEX observations of increased cloud formation over the native vegetation, suggesting that a “land-sea breeze” mechanism induced by the stark contrast in surface energy balance was most likely in place as a result of the heating of the boundary layer over the native vegetation. A similar mechanism has also been proposed as an explanation for observed differences in the convective boundary layer found during the Boardman Atmospheric Radiation measurement Regional Flux Experiment (BARFEX) (Segal et al. 1991; Doran et al. 1995). Although in a very different biome, Chambers and Chapin (2003) and Chambers et al. (2005) looked at boreal forest stands in Alaska and found a decrease in net radiation, enhanced ground heat flux and a reduction in Bowen ratio values following fire, which lead to a reduction of sensible heat fluxes when compared to an unburnt stand. They found that across a fire-scar boundary, the changes in surface properties and energy partitioning were sufficiently large to induce mesoscale circulations and possibly trigger convective development (Chambers and Chapin 2003). They also pointed out that, due to the size and abundance of fire scars throughout interior Alaska, their effects might contribute to regional climate patterns.
Local boundary-layer development over burnt and unburnt tropical savanna
293
Fire scars, which refer to the charred and blackened landscape after a fire event, can radically alter the surface energy budget by reducing surface albedo, changing aerodynamic properties so that available energy for partitioning into convective fluxes is increased and by increasing substrate heat flux. These factors, combined with other physical changes such as decreasing surface roughness and increasing net radiation, affect the surface–atmosphere coupling and have the potential to influence atmospheric motion and moist convection at a range of scales. In northern Australia, Kilinc and Beringer (2007) showed increased lightning strike densities over fires scars due to heating differences. Recent studies illustrating the role of fire in the land–atmosphere system emphasise the significance of such a disturbance and highlight the need for the investigation of fire prone ecosystems more generally (Eugster et al. 2000; Beringer et al. 2003; Chambers and Chapin 2003; Hoffmann et al. 2003). With vegetation and the atmosphere so closely coupled, understanding the indirect feedbacks associated with fire becomes increasingly important, especially under predicted climate change and altered fire regimes. Such information is important in order to adequately plan for future management of tropical savanna regions, especially with most areas of northern Australia experiencing intervals between burning that are measured in years rather than decades or centuries (Williams et al. 2002). Such frequent burning is also likely to change species composition and increase the frequency of more intense and destructive late dry season fires. Climate modelling work by Hoffmann and Jackson (2000) showed that a 10% reduction in precipitation occurred when savanna was converted to grassland. This lead to increased mean surface air temperatures and increases in dry periods during the wet season that could be particularly damaging to shallow rooted crops. This study is an initial investigation into the effect of fire scars on the atmospheric boundary layer within a tropical savanna ecosystem. It will provide the basis for further modelling studies of the larger-scale effects of tropical savanna fire scars on local to regional climate, including possible impacts on the Australian monsoon.
2 Site description To investigate the effect of fire on the heat and moisture fluxes to the atmosphere and subsequent feedbacks to the boundary layer, two field sites were chosen on the Gunn Point Peninsula approximately 35 km south-east of Darwin near the township of Howard Springs in the coastal, humid zone of the Northern Territory, Australia. One site (12◦ 29.712 S, 131◦ 09.003 E) was burnt savanna and the other site (12◦ 18.113 S, 131◦ 06.002 E) unburnt savanna. Howard Springs has a mean annual rainfall of 1,750 mm, with approximately 95% falling during the pronounced wet season from December to March (Hutley et al. 2000). This is followed by an essentially rainless dry season, lasting from May to September. A transitional period occurs during the months of October and November, which is characterised by increased humidity and temperature with occasional thunderstorms and the onset of canopy flushing by many of the tree and shrub species (Williams et al. 1997). Daily maximum temperatures at Darwin Airport range from 30.4◦ C (July) to 33.1◦ C (October and November) (Australian Government Bureau of Meteorology 2004). The daily maximum and minimum temperatures have an approximate range of 7◦ C in the wet season and 11◦ C in the dry season (Australian Government Bureau of Meteorology 2004).
294
C. K. Wendt et al.
The vegetation on the Gunn Point peninsula consists of Eucalypt-dominated woodlands, open and closed forests, and seasonally flooded swamps and wetlands (Hutley et al. 2000). The burnt and unburnt sites consisted of open forest and were representative of the mesic tropical savannas of Northern Australia. A previous study by Hutley et al. (2000) at the same site showed there are approximately 600–750 stems/ha with Eucalyptus tetrodonta and Eucalyptus miniata the dominant species, accounting for approximately 80% of total basal area and 50% of the total canopy cover in the region et al. (Hutley et al. 2000). O’Grady (1999) found that the overstorey leaf area index (LAI) varied seasonally between 0.6 in the dry season and 0.95 in the wet season for this site. The understorey at both sites consists of a mosaic of small trees and shrubs with a seasonally continuous cover of annual grasses, with some perennial grasses also present (Hutley et al. 2000). Probably of most significance to this study is the presence of green C4 grass (Sorghum spp.) during the wet season. Hutley et al. (2000) showed that at ground level Sorghum spp. is responsible for a dramatic change in LAI, from a wet season value of 2–3 to a dry season value of 0.2, when the grass senesces. It is the abundance and annual drying of the Sorghum, along with the presence of course woody debris, that creates the available fuel for many of the wildfires in the area. These fires are mostly surface fires, not crown fires, and range in intensity from 20,000 kW m−1 , depending on fuel load, climatic conditions and the time of year. The soils of the region are generally extensively weathered and laterised, weakly acidic and low in nutrients. A detailed description of the soils can be found in Cole (1986).
3 Fire scars Fire scars throughout the Northern Territory can range from a few m2 to hundreds of km2 , and the fire scar investigated in this study was centred at approximately 12◦ 29 S, 131◦ 09 E and was approximately 100 km2 in area (Fig. 1). Burning occurred on day 241 (August 29, 2003), with the fire being patchy and ranging between low (5 km.
4 Methods 4.1 Meteorological and flux measurements In order to investigate the impact of fire on the surface heat and moisture fluxes we installed an instrument tower to collect meteorological and eddy-covariance data before, during and after the fire event. It has been previously described by Beringer et al. (2003) and will only be briefly described here. The tower was instrumented with a three-dimensional (3D) ultrasonic anemometer (Campbell Scientific Inc., model CSAT3) at a height of 25 m (canopy height of 12–14 m), with all three-dimensional wind vectors co-ordinate rotated (McMillen 1998). An open path infrared gas analyser
Local boundary-layer development over burnt and unburnt tropical savanna
295
Fig. 1 Location and timing of fire scars in the Gunn Point region, including the location of the Burnt and Unburnt experimental sites (modified from Tropical Savannas Cooperative Research Centre map maker—North Australian Fire Information http://savanna.ntu.edu.au/information/savanna_mapmaker.html)
(Licor, model LI-7500) was used to measure turbulent fluctuations of CO2 and H2 O. Paired pyranometers (Middleton Instruments, model EP-07) and pyrgeometers (Eppley Labs Inc., model PIR) were used to measure incoming and outgoing shortwave radiation and longwave radiation fluxes, respectively. Net radiation above the surface was also measured using a Frischen type net radiometer (REBS, model Q∗ 7.1) with a wind-speed-dependant dome cooling correction applied to the results. Soil heat flux plates (REBS, model HFT3) and soil temperature probes (Campbell Scientific Inc., model TCAV) were used to measure ground heat flux (Fuchs and Tanner 1968) at four representative locations. All climatic variables were measured every 10 s and 30-min averages recorded to a data logger (Campbell Scientific Inc., model CR5000). Hereafter, all times are given as local Central Standard Time (CST = UTC +9.5 h), while the term “daily” refers to the 24-h period from midnight to midnight. Solar noon at Howard Springs was approximately 1300 CST during the period of measurements. The tower flux data were recorded during 2003 from day 214 to day 299 and are split into four data blocks (chosen on the basis of rainfall events), pre-fire (214–240), post-fire (time to first significant rainfall) (242–248), recovery (249–276) and recovered (277–299), with the day of the fire (241) excluded from the data and treated as a discrete event. 4.2 Boundary-layer measurements Two identical tethered balloon systems were used to take vertical profiles within the boundary layer. Simultaneous soundings were taken at the burnt and unburnt site, with a tethered balloon (Vaisala TB3), winch, radiosonde (manufactured by the University of Canterbury Physics and Astronomy Department) and receiving system
296
C. K. Wendt et al.
(SALCOM, model 15–12–0000). The radiosondes measured temperature, pressure, humidity, wind speed and direction, with the data being received and logged directly to a laptop using a PC-Windows based software package (University of Canterbury, New Zealand). Due to the Australian Civil Aviation Safety Authority (CASA) operating restrictions, measurements were restricted to a maximum height of approximately 530 m above ground level. The tethered balloon profiling was undertaken in the middle of the burnt and unburnt patches in order to minimise edge effects. Each sounding took approximately 1 h to complete, and soundings were conducted simultaneously at both sites for the following times: 0100, 0600, 0900, 1200, 1500, 1800 and 2100 CST during days 247–249, 254–255, 261–262, 267 and 274–276 of 2003. In total, 172 soundings were made between the burnt and unburnt sites. Intensive observations were conducted on day 267 (September 24, 2003) with soundings being conducted every hour between 0600 and 2100. All sounding observations were quality controlled using predetermined thresholds for each parameter, processed and combined to form single profiles for each time period. Profiles were averaged for each 25-m layer through the atmosphere; for example, the 25-m data represent the mean of all data between 0 and 25 m. Prior to measurements being made, absolute and cross-calibrations were performed. A dew point generator was used to calibrate relative humidity, while temperaturecontrolled rooms and a controlled wind source were used to calibrate temperature and wind speed, respectively. Both sondes were then field tested on the same tethered balloon flight, and derived corrections were applied to the data.
5 Results and discussion The wildfire on day 241 immediately affected the land surface by significantly decreasing the albedo through the consumption of the dry grassy understorey, course woody debris and the charring of the ground surface. Due to instrumentation problems albedo values for 2003 were unavailable. However, previous studies from 2002 at the same site showed that the albedo almost halved from 0.12 pre-fire to 0.07 post-fire during a similar intensity late dry-season fire (Beringer et al. 2003). A change in albedo is reflected in net radiation measurements, which increased only slightly in the post-fire period (up by 2.2%). This increase was smaller than expected, based on the albedo changes, since longwave emissions from the surface also increased (Beringer et al. 2003). Net radiation did, however, significantly increase during the recovery period (up 20.2%) as the trees began the process of recovery by shedding their leaves, reducing canopy LAI, and exposing the barren ground surface to the atmosphere. The heat from the understorey fire caused substantial canopy scorch and, as a result, significant leaf-drop occurred in the weeks following fire. This canopy damage reduced evapotranspiration to almost zero and altered the partitioning of energy, with a reduction in latent heat flux (LE) and an increase in sensible heat flux (H). This increase is likely to result in an increase in the heating of the overlying boundary layer. The ratio between sensible and latent heat fluxes is given as the Bowen ratio (β = H/LE), and the mean daytime β for the pre-fire period of 1.9 (Fig. 2) indicates that the majority of available energy was being converted to sensible heating. These values are typical of tropical savannas during the mid to late dry season, when the understorey grasses have senesced, the canopy LAI has decreased and there is low soil moisture (Hutley et al. 2000; Beringer et al. 2003). It should be noted that daily
Local boundary-layer development over burnt and unburnt tropical savanna
297
Fig. 2 Bowen ratio (β = H/LE) and daily total rainfall (mm) for the burnt site. Fire occurred on day 241 (29 August 2003) and is excluded from the analysis. Rain events correlate with clear decreases in β as evaporation increased Table 1 Daily 30-min averaged temperatures recorded from the burnt site climate/flux tower
Pre-fire (214–240) Post-fire (242–248) Recovery (249–276) Recovered (277–299)
Minimum temperature (◦ C)
Maximum temperature (◦ C)
Mean Temperature (◦ C ± standard error)
25.79 25.52 29.01 30.01
35.44 35.84 37.68 39.06
30.33 ±0.45 30.43 ± 0.48 33.13 ± 0.45 34.22 ± 0.55
weather conditions at the study site were very consistent during the pre-fire, post-fire, recovery and recovered periods but show some seasonal trends of increasing temperature and humidity into the summer (Table 1). Immediately following the fire β dramatically increased due to a scorching of the canopy and reduction of transpiration to near zero, with an increase in the daily average to 3.8 for the post-fire period (Fig. 2). Although some leaf-drop or shedding was visually observed during the post-fire period, and so affecting albedo, such an event would not be reflected in β, since the canopy had already shut down immediately after the fire and no longer contributed to latent heating. After the first significant rainfall event (22 mm on day 249) the daily β value decreased to 1 with relatively high surface evaporation. In the following few days the surface quickly dried and β increased to previous values. The mean daytime β value over this period fell slightly to 2.4 (recovery period), still much higher than the pre-fire value. Interestingly, the Bowen ratio was seen to settle to just below its pre-fire value (recovered period: β = 1.8) more quickly than previous studies in the same area (Hutley et al. 2000; Beringer et al. 2003). Although the fire was of similar intensity to those of previous studies, the presence of rainfall soon after the fire may have aided the understorey and canopy recovery. Rainfall also increased the amount of water available for evaporation and subsequently increased the amount of energy that was dissipated as latent heat, shown as a reduction in the Bowen ratio. It was not possible to distin-
298
C. K. Wendt et al.
Fig. 3 Daily mean volumetric soil moisture (m3 m−3 ) and latent heat flux (W m−2 ) for the burnt site
guish between evaporation and transpiration sources during this study. The overstorey canopy (post-fire) was observed to flush faster following rainfall and early germination of understorey grasses and shrubs was evident during the recovery period. Volumetric soil water content, which averaged 5.1% pre-fire (maximum of 10%), increased to 6.5% during the post-fire period and 6.9% during the recovery/recovered periods (maximum of 13.5%) due to the sporadic, pre-monsoon rainfall typical of this time of the year. Figure 3 shows a clear increase in latent heating following the increases in soil moisture due to rain. In terms of the overall magnitude of the heat fluxes (Table 2), the pre-fire daily sensible heat flux was 68 W m−2 which is moderate due to the dry soils, low LAI, high canopy resistance and relatively high levels of incoming radiation during the day at this time of the year (Beringer et al. 2003). Sensible heat fluxes increased slightly to 69 W m−2 during the post-fire period, during which time there was a significant reduction in latent heat fluxes from 36 W m−2 pre-fire to 18 W m−2 . Sensible heat fluxes increased again during the recovery period to 85 W m−2 before decreasing slightly during the recovered period, indicating a seasonal trend consistent with a progression toward summer. Latent heat flux increased to 35 W m−2 during the recovery period, with vegetation growth after rainfall. The most significant change in energy partitioning was the increase in the ground heat flux, with a near three-fold increase from 18 W m−2 pre-fire to 53 W m−2 during the post-fire period, which was caused by the charring and blackening of the soil surface, decreased albedo and increased radiant load on the soil due to the significantly reduced overstorey cover due to leaf-drop. Ground heat storage also increased up to 2.5 times post-burn with high daily variability due to cloud cover and soil moisture changes. Beringer et al. (2003) reported a moderate fire during 2001 (3550 ± 640 kW m−1 ) at the same site, resulting in a change in daily sensible heat flux of 57 W m−2 pre-burn to 88 W m−2 post-burn. Although not as large as previous flux changes reported at the same site, the impact of energy partitioning during the 2003 fire is still evident. Feedbacks from these changes to the overlying atmosphere were still significant and
Local boundary-layer development over burnt and unburnt tropical savanna
299
Table 2 Average heat flux values for pre-fire, post-fire, recovery and recovered periods recorded from the burnt site climate/flux tower Data set (day)
Sensible heat flux (H) (W m−2 )
Latent heat flux (LE) (W m−2 )
Ground heat flux (G) (W m−2 )
Pre-fire (214–240) Post-fire (242–248) Recovery (248–276) Recovered (277–299)
68.40 ± 0.35 69.33 ± 0.35 84.61 ± 0.46 82.87 ± 0.35
36.11 ± 0.12 18.17 ± 0.12 35.30 ± 0.23 45.95 ± 0.23
17.71 ± 0.23 52.66 ± 0.23 69.79 ± 0.35 69.33 ± 0.35
evident in boundary layer heating profiles reported below, even accounting for the effect of rainfall shortly after the fire event. Boundary-layer profiles taken in the weeks following the fire indicate that during the day the atmosphere over the fire scar is warmer and more effectively mixed. To investigate the effect of the fire scar on the lower atmosphere, individual profiles for a specific time of day (e.g. all 0600 profiles) were combined and the mean (± standard error) taken for each 25 m layer of the atmosphere, with days being used as replicates. All data within each atmospheric layer are represented by a mean value at the top of that layer. For example, the data between the surface and 25-m are represented by an averaged data point at 25 m. The tethersonde data showed that at 0100 there was a clear temperature inversion in the first 100 m at both sites (Fig. 4), this is a typical nighttime profile for a clear sky, high radiant energy input/output system that has relatively high daytime surface temperatures and relatively low nighttime surface temperatures. Interestingly air above the burnt site was still warmer up to a height of approximately 300 m where the two profiles converged. Ground and air temperature analysis showed that the burnt savanna reached higher ground surface and screen-level temperatures than the unburnt savanna and also remained warmer for longer after sunset (Fig. 5). The burnt site showed a more rapid increase in ground temperature during the morning, as the blackened land surface was exposed to the incoming solar radiation. Soil temperature at the burnt site remained higher than the overlying air temperature until 2300, indicating that the overlying air mass at the burnt site is in contact with the warmer ground surface for a longer period of time after sunset than the unburnt site, contributing to the increased temperatures seen in the 0100 profiles. The 0600 soundings showed that a surface-based temperature inversion was still evident, with both sites showing similar temperatures, with high variability throughout the profiles. By 0900 surface heating from incoming solar radiation is evident and a clear temperature lapse is seen at both sites (Fig. 4). Both sites show similar temperatures throughout the profiles. Maximum near-surface air temperature differences were seen between 1200 and 1800 when the difference in the sensible heating between sites was largest. The burnt site was also found to have slightly higher mean wind speed (not shown) throughout the profiles during these times, although there is much variability. Perhaps surprisingly, surface roughness length, as calculated from wind speed profiles, was slightly higher at the burnt site (z0 = 0.49 m) compared to the unburnt site (z0 = 0.46 m). The influence of the heating contrasts between sites appeared to extend up to a height of approximately 275 m in this study, and it should be noted that the fetch for the burnt site was approximately 5 km and for the unburnt site >5 km. Therefore it is
300
C. K. Wendt et al.
Fig. 4 Mean layered temperature profiles from 13 days of simultaneous soundings from the burnt and unburnt sites (between days 247 and 276). Profiles show mean (±standard error) temperature data for each 25-m layer. The burnt site is shown as a solid line and the unburnt site by a dashed line. Time is indicated in the upper left hand of each plot (CST)
Local boundary-layer development over burnt and unburnt tropical savanna
301
Fig. 5 Diurnal mean air and soil temperature recorded for unburnt savanna (days 214–240) and burnt savanna (days 242–299)
likely that this has limited the height of influence, since the thermal internal boundary layer may not have been fully developed. The fire scar in this study was of small to medium size for this area of northern Australia and therefore the boundary layer effect reflects the scale of the observed phenomenon on the surface. By 2100 surface temperatures had decreased by up to 4◦ C and the burnt site still exhibited a warmer profile up to 200 m, above which there was a crossover (Fig. 4). A more in-depth analysis of the data revealed that this was not the result of any single profile used to calculate the mean rather it is a recurring. Analysis of wind direction and humidity data failed to accurately reveal the cause of this phenomenon, but the prevailing wind direction (north-east) suggests that this is the product of a coastal air mass, possibly a sea breeze influencing the site. Individual profile analysis revealed that lower level temperatures at both sites increased rapidly as solar energy was input into the system during the morning. Depending on daily conditions (such as cloud cover and incoming solar radiation), the burnt site was consistently warmer throughout the entire measured profile by up to 2◦ C at some levels. The sounding on day 267 (not shown) revealed that air above the burnt site remained warmer than that over the unburnt site by 1.5◦ C, to a maximum height of 375 m. Due to operating restrictions it was not possible to extend profiles to the top of the daytime convective boundary layer (estimated from Bureau of Meteorology Darwin Airport radiosonde data to be 1.5–3 km depending on daily conditions). However, increased heating of the boundary layer was evident over the burnt site to the maximum height of 625 m during one individual sounding. With no rainfall after the fire, or with increased fire intensity, it could be assumed that heating of the boundary layer will also increase. Overall the tethered balloon soundings showed that, despite rainfall increasing the latent heat flux to the overlying atmosphere, there was still enough increased sensible heat over the burnt savanna surface to result in significant differential heating in the lower levels of the boundary layer.
302
C. K. Wendt et al.
The higher sensible and ground heat fluxes after fire in tropical savannas and the subsequent increase in lower boundary layer heating, provides the potential for increased convection and enhanced mixing that would lead to a deeper boundary layer. If these differences between burnt and unburnt savanna occur on sufficiently large spatial scales, then thermally driven local circulations may possibly affect larger-scale circulations such as the Australian monsoon (Beringer et al. 2003; Wardle and Smith 2004).
6 Summary and conclusion Once burnt, tropical savannas show a significant decrease in albedo and a fundamental change in the partitioning of available energy. Bowen ratio analysis showed that more energy was being partitioned into sensible heat than latent heat before fire events, which is a typical pattern during the dry season when there is low available soil moisture and low canopy LAI (Hutley et al. 2000). Fire and the resultant fire scar caused a substantial increase in sensible heat flux to the atmosphere, which is associated with increased convection and mixing. At times of maximum heating, air temperatures above the savanna fire scar were up to 2◦ C higher than unburnt areas. Depending on local conditions such as aerodynamic changes to savanna vegetation, the size and intensity of the fire and enhanced sensible heat fluxes over scars, it is possible that these areas could produce localized areas of convergence and divergence and associated mesoscale circulations (Knowles 1993). The possible formation of mesoscale circulations may lead to an increase in convective cloud development and precipitation following a fire event, altering local conditions until pre-fire conditions are restored (Beringer et al. 2003). This is likely to be dependent on the availability of atmospheric water vapour at the synoptic scale, which is highly seasonal. The intensity of late dry-season fires are often an order of magnitude greater than early or mid dry-season fires (Williams et al. 1998) and occur at times when there is greater atmospheric moisture available (pre-monsoonal period of August– October). With a shift in fire regimes to more intense late dry season fires, especially in remote areas of Northern Australia, possible climatic impacts and feedbacks should be assessed. Our observational study shows that even relatively small and moderately intense fires have an impact on the surface energy balance that in turn affects the overlying atmosphere. It has been suggested that large-scale fire scars could significantly modify regional precipitation patterns (Beringer et al. 2003). There has been a documented shift in fire regimes from an early-dry season (April to May) aboriginal patch burning regime, to a more destructive late-dry season fire regime (Braithwaite 1991; Williams et al. 1998), with large intense landscape scale fires more likely to occur. Some changes in composition (tree/grass mix) have been shown to alter local and regional climate (Hoffmann and Jackson 2000), which demonstrates the importance of the vegetation-climate interactions in tropical savanna regions. Recent studies such as Görgen et al. (2006), Wardle and Smith (2004), Williams et al. (2001) and Beringer et al. (2003) further emphasise the importance of understanding vegetation-atmosphere interactions under changing fire and weather regimes and also point to possible feedbacks between these changes and regional climate, such as the Australian monsoon and future rainfall patterns. Future work in this study is to apply a mesoscale climate model to quantify the effect of fire scars on local and regional scale climate.
Local boundary-layer development over burnt and unburnt tropical savanna
303
Acknowledgments This project was made possible by the support of the Australian Research Council (ARC DP0344744), Civil Aviation Safety Authority, Bushfires Council Northern Territory and the Howard Springs Country Fire Authority.
References Australian Government Bureau of Meteorology (2004) Climatic averages, Northern Territory. http://www.bom.gov.au/climate/averages/ Cited May 2004 Beringer J, Packham D, Tapper NJ (1995) Biomass burning and resulting emissions in the Northern Territory, Australia. Int J Wildland Fire 5(4):229–235 Beringer J, Hutley LB, Tapper NJ, Coutts A, Kerley A, O’Grady PO (2003) Fire impacts on surface heat, moisture and carbon fluxes from a tropical savanna in northern Australia. Int J Wildland Fire 12:1–8 Braithwaite RW (1991) Aboriginal fire regimes of monsoonal Australia in the 19th century. Search 22:247–249 Chambers SD, Chapin III FS (2003) Fire effects on surface-atmosphere energy exchange in Alaskan black spruce ecosystems:Implications for feedbacks to regional climate. J Geophys Res D:Atmos 108:1–17 Chambers SD, Beringer J, Randerson JT, Chapin III FS (2005) Fire effects on net radiation and energy partitioning:Contrasting responses of tundra and boreal forest ecosystems. J Geophys Res 110:D09106 Cole NM (1986) The Savannas:biogeography and geobotany. Academic Press, London, 438 pp Doran JC, Shaw WJ, Hubbe JM (1995) Boundary-layer characteristics over areas of inhomogeneous surface fluxes. J Appl Meteorol 34:559–571 Eugster W, Rouse WR, Pielke R, McFadden JP, Baldocchi DD, Kittel TGF, Chapin III FS, Liston G, Vidale PL, Vaganov E, Chambers S (2000) Land-atmosphere energy exchange in arctic tundra and boreal forest:available data and feedbacks to climate. Glob Change Biol 6(1):84–115 Fuchs M, Tanner CB (1968) Calibration and field test of soil heat flux plates. Soil Sci 32:326–328 Görgen K, Lynch AH, Marshall AG, Beringer J (2006) The impact of abrupt land cover changes by savanna fire on northern Australian climate. J Geophys Res D:Atmos 111:D19106 Hoffmann WA, Jackson RB (2000) Vegetation-climate feedbacks in the conversion of tropical savanna to grassland. J Climate 13:1593–1602 Hoffmann WA, Schroeder W, Jackson RB (2003) Regional feedbacks among fire, climate, and tropical deforestation. J Geophys Res D:Atmos 108:4–11 Honrath RE, Owen RC, Val Martín M, Reid JS, Lapina K, Fialho P, Dziobak MP, Kleissl J, Westphal DL (2004) Regional and hemispheric impacts of anthropogenic and biomass burning emissions on summertime CO and O3 in the North Atlantic lower free troposphere. J Geophys Res 109:D24310 Huang X, Lyons TJ, Smith RCG (1995) Meteorological impact of replacing native perennial vegetation with annual agricultural species. Hydrol Process 9:645–654 Hutley LB, O’Grady AP, Eamus D (2000) Evapotranspiration from Eucalypt open-forest savanna of Northern Australia. Functional Ecol 14:183–194 Johnson BJ, Miller GH, Fogel ML, Magee JW, Gagan MK, Chivas AR (1999) 65,000 Years of vegetation change in central Australia and the Australian summer monsoon. Science 284:1150–1152 Kilinc M, Beringer J (2007) Spatial and temporal distribution of lightning strikes and their relationship with vegetation type, elevation and fire scars. J Climate 20:1161–1173 Knowles J (1993) The influence of forest fire induced albedo differences on the generation of mesoscale circulations. Dissertation, Colorado State University, 84 pp Lyons TJ, Schwerdtfeger P, Hacker JM, Foster IJ, Smith RCG, Huang X (1993) Land–atmosphere interaction in a semiarid region:the Bunny Fence experiment. B Amer Meteorol Soc 74:1327–1334 Lyons TJ (2002) Clouds prefer native vegetation. Meteorol Atmos Phys 80:131–140 McMillen RT (1988) An eddy correlation technique with extended applicability to non-simple terrain. Bound-Lay Meteorol 43:231–245 Miranda AI, Ferreira J, Valente J, Santos P, Amorim JH, Borrego C (2005) Smoke measurements during Gestosa-2002 experimental field fires. Int J Wildland Fire 14:107–116 O’Grady AP, Eamus D, Hutley LB (1999) Transpiration increases during the dry season:patterns of tree water use in eucalypt open-forests of northern Australia. Tree Physiol 19:591–597 Russell-Smith J, Allan G, Thackway R, Rosling T, Smith R (2000) Fire management and savanna landscapes in Northern Australia. In:Russell-Smith J, Hill GE, Djoeroemana S, Myers BA (eds)
304
C. K. Wendt et al.
Fire and sustainable agricultural and forestry development in eastern Indonesia and Northern Australia. Australian Centre for International Agricultural Research, Canberra, 237 pp Segal M, Cramer JH, Pielke RA, Garratt JR, Hildebrand P (1991) Observational evaluation of the snow breeze. Mon Weather Rev 119:412–424 Wardle R, Smith I (2004) Modeled response of the Australian monsoon to changes in land surface temperatures. Geophys Res Lett 31:L16205, doi:10.1029/2004GL020157 Williams A, Cook GD, Ludwug JA, Tongway D (1997) Torch, trees, teeth and tussocks:disturbance in the tropical savannas of the Northern Territory. In:Klomp N, Lunt ID (eds) Frontiers in ecology:building the links. Elsevier Science, Oxford, 334 pp Williams RJ, Gill AM, Moore PHR (1998) Seasonal changes in fire behaviour in a tropical savanna in Northern Australia. Int J Wildland Fire 8(4):227–239 Williams AAJ, Karoly DJ, Tapper NJ (2001) The sensitivity of Australian fire danger to climate change. Climatic Change 49:171–191 Williams RJ, Griffiths AD, Allan G (2002) Fire regimes and biodiversity in the wet-dry tropical savanna landscapes of northern Australia. In:Bradstock RA, Williams JA, Gill AM (eds) Flammable Australia:the fire regimes and biodiversity of a continent. Cambridge University Press, Cambridge, UK, 425 pp Williams RJ, Hutley LB, Cook GD, Russell-Smith J, Edwards A, Chen X (2004) Assessing the carbon sequestration potential of mesic savannas in the Northern Territory, Australia:approaches, uncertainties and potential impacts of fire. Functional Plant Biol 31:415–422 Wotawa G, Trainer M (2000) The influence of Canadian forest fire on pollutant concentration in the United States. Science 288:324–328
