Journal of Natural Disaster Science, Volume 33, Number 2, 2012, pp71-84

Active Forest Fire Occurrences in Severe Lightning Years in Alaska
Murad Ahmed FARUKH*
Hiroshi HAYASAKA***Department of Environmental Science, Bangladesh Agricultural University**Associate Professor, Graduate School of Engineering, Hokkaido University(Received August 17, 2011 Accepted March 21, 2012)
ABSTRACT
Severe lightning flashes, 120,000 a year or four times more than in the average year, started
around 300 fires in each of the three years of 2004, 2005, and 2007 but the burnt area of
each year differed considerably. The burnt areas in 2004 and 2005 were the largest andthe third largest in the last 55 years (1956-2010), respectively; the burnt area in 2007 wassmaller than the average 3,541 km2. Overall, a comprehensive understanding of active forest
fires in severe lightning years in Alaska was achieved based on various fire characteristics
including the newly introduced “number of live fires” and “daily fire activity by hotspots”
from NASA. These fire characteristics established the fact that severe lightning ignited
Alaska’s forests in June and July, and that most of the large fires in 2004 and 2005 started
and lasted until August. Large numbers of live fires became active during drought and highair-temperature conditions, which increased the burnt area in 2004. In 2005, continuous
precipitation started from June but a large burnt area was also created by a very active
forest fire period in August. The burnt area in 2007 was not so large due to continuous
precipitation from the beginning of June.
Keyword: active fire, hotspot, lightning, drought, precipitation

1. Introduction

Although lightning plays a very important role in forest fires as a natural igniter, there have been only few
studies associating lightning strikes with fire starts(Fuquay et al., 1972; Wierzchowski et al., 2002; Larjavaara et al., 2005). Wildfires are a major disturbancein interior Alaska, and ecological processes such as
post-fire succession, carbon cycling, and permafrost
degradation can be dramatically changed directly andindirectly by fires (Johnstone and Chapin, 2006; Kane
et al., 2007; Johnstone et al., 2008; O’Donnell et al.,2009). Morris (1934) and Barrows (1954) studied therelationship between thunderstorm days and lightning
fires in northwest US and the Rocky Mountains. The
authors have studied the weather conditions of anom-

Recent research on boreal forest fires including thosein Alaska is now being carried out for a number ofreasons such as “emission of global warming gases”
and the “fire return interval.” In Alaska, recent active
forest fires occurred since around 1990 suggesting ashort “fire return interval.” This fire trend in Alaska
appears to contradict recent research results for central Russia and other areas (Wallenius et al., 2011;
Flannigan et al., 2005). The authors are now trying tosubstantiate this by analyzing forest fire activity andlightning occurrence in Alaska.

71

M.AHMED FARUKH, H. HAYASAKA

alous lightning occurrences in 2005 in Alaska andproposed a lightning forecasting method for Alaska
using various weather indices (Farukh, Hayasaka,and Kimura, 2011a,b). Price and Rind (1991, 1994)
analyzed a global climate model and found that, in association with an increased global air temperature of
4ºC, global mean lightning activity increased by 26%,
and would ignite 44% more fires and burn a 78%
wider area than under current conditions.

extracted from fire data by NASA and the AFS (Alaska
Fire Service). The effects of weather condition on fire
activity are also examined by comparing hotspot datawith daily precipitation and air temperature data.
2. Data and Methods
2.1 Lightning Data
Daily lightning data from 1986 to 2010 were obtained
from the Alaska Interagency Coordination Center(AICC, 2011), Bureau of Land Management, andAlaska Fire Service (AFS, 2011) with data missing
for 1987 and 1989. The data from AFS include thecoordinates, dates, times, signal strength, polarity, etc.
for each lightning flash, and are detected over larger
areas from the Bering Strait to the Canadian mountainous areas (135ºW to 179ºW longitude), and fromthe tundra in the Arctic region to the forest areas on
the southeast coast (70ºN to 50ºN latitude). There arenine lightning detection sensors working in Alaska,
and the system was upgraded in 2000. This lightning
detection system had a positional accuracy of 2~4km and a detection efficiency of 40~80% up to 1999
(Dissing and Verbyla, 2003). The detection accuracy
and efficiency increased to 0.5~2 km and 80~90%,
respectively, from 2000.

Recent active fire trends in Alaska may be due to socalled global warming as global climate change models predict warmer, longer, and drier summers (Sojaet al., 2007) leading to greater fire occurrence andseverity (Flannigan et al., 1998; 2000; Gillett et al.,
2004; Kasischke and Turetsky, 2006) and a significant
change in carbon storage and emissions (Kasischke
et al., 2002; Tan et al., 2007). Sandberg et al. (2002)
reported that on average, Alaska contributes around
41% of the USA carbon emissions from wildland
fires. During the summer, the risk of fire in this regionis high due to the relatively small amount of precipitation (Kasischke, 2000). Campbell and Flannigan(2000) reported that the recent trend towards warmer
and drier summers is expected to increase the number
and size of boreal forest fires. Stevens and Dallison(2005) suggested that recent climate changes in boreal
forests could substantially impact the number and sizeof wildfires in Alaska. Hess et al. (2001) investigated
the influence of El Niño weather events on fires andconcluded that many of the large fire years occurredjust after a warm ENSO episode. The patterns of large
fires over the last 50 years in Alaska have been studied by Kasischke et al. (2002) by analyzing the Large
Fire Database (LFDB) and other information.

2.2 Fire Data (AFS data and MODIS hotspot data)
Fire and area burnt history data from 1956 to 2010
were provided by the University of Alaska Fairbanks(UAF) and Alaska Fire Service (AFS). The data included fire information regarding location, detection
date, out date, size (final burnt area), cause, and others. The data were divided into human-caused fires,lightning-caused fires, and other fires.

One of the purposes of this paper is to explain thesevere forest fires by lightning in 2004 and 2005 that
consumed 10% of the forest area of Alaska. A large
number of lightning flashes also occurred in 2007 but
the burnt area was only one-tenth that of 2004. This
may imply that forest fire activity in Alaska is strongly dependent on weather conditions. In this paper, theactivities of about 300 forest fires in 2004, 2005, and2007 occurring as a result of around 120,000 lightning flashes in those years are discussed using various
fire characteristics such as fire distribution, fire size,
fire duration, fire detection, and number of live fires

Moderate Resolution Imaging Spectroradiometer(MODIS) hotspot data were obtained using softwaredeveloped by NASA based on the NASA ATBDMOD 14 algorithm (Giglio et al., 2003) where MODIS WGS-84 geographic projection (the “latitude/
longitude projection”) data were used as input. Each
MODIS dataset represents active fire data (NASA,
2002) of the area of interest, i.e., Alaska. The data
may include fire detection from MODAPS, the definitive version of Collection 5 (version 5.1), and from
MODIS Rapid Response [MRR] (version 5.0). The
active fire products of MODIS obtained from the Web
72

ACTIVE FOREST FIRE OCCURRENCES IN SEVERE LIGHTNING YEARS IN ALASKA

Fire Mapper (Justice et al., 2002) also clearly depict
and describe hotspot occurrence in Alaska. The recent
daily hotspot data from January 2000 to December
2010 were processed and analyzed to determine fireoccurrence tendencies. Various satellite images acquired by Terra and Aqua (NASA: MODIS Rapid
Response System, 2011) from the Aeronet Bonanza
subset of North America were comprehensively visualized to compare fire locations with hotspots, andsometimes to obtain information on wind directions.
ArcView 3.3 and CAD (VectorWorks, 2010) software
were used for accurate plotting of the hotspots on adigital map of Alaska.

3. Lightning Trend and Wildland Fire in Alaska
A time series of the total number of yearly (May toAugust) lightning flashes from 1986 to 2010 (except
1987 and 1989 due to missing data) from north latitudes 62º to 68.5º and west longitudes 141º to 163º isshown in Figure 1. The recent years of 2004, 2005,
and 2007 were all extreme in terms of total annual
lightning flash numbers within this area. There isabout a fourfold difference between these anomalous
lightning numbers of around 120,000 each year andaverage lightning number of around 31,000. Compared to 2005, the largest contrast in the annual lightning flash count was found in 1996 with only around13,000 annual flashes.

2.3 Weather Data
Weather data from Fairbanks Airport were chosen
as those typical of Alaskan weather because it may
be expected to be the most reliable and has been
available for the longest period. Weather data wereobtained from the Alaska Climate Research Center,
Fairbanks (2011) and the summary of day data of the
National Climatic Data Center (2011), and these data
were analyzed. The weather components consideredwere average and maximum air temperatures, sealevel pressure, wind speed and direction, and precipitation. In this paper, the weather data from 1990 to2010 for Fairbanks were mainly used due to unavailability of “daily” data before this period.

The monthly lightning occurrence is also shown in
Figure 1 with different patterns for May, June, July,and August. As 90% of all lightning flashes occur inJune and July, most lightning-caused forest fires in
Alaska tend to start in these two months, which is evident from the accumulated curves for the number offires for 2004, 2005, and 2007 in Figure 6. The numbers at the top of the several bars show the rank of
the burnt area: 2004 was the largest fire year by burnt
area, 2005 was the third largest, and 2007 was thetwenty-first largest, though all these three years experienced very similar numbers of lightning flashes.

(I)
140,000

Detection accuracy 0.5~2 km
Detection efficiency 80~90%

Lightning detection
network updraded

80,000

*3

*1

*21

Detection accuracy 2~4 km
Detection efficiency 40~80%

August
July
June
May

100,000*Rank of burnt area

60,000*6
Ave. flash 43,000 (including '04,'05,'07)*8

40,000Ave. flash 31,000 (excluding '04,'05,'07)

*9

*5

*11

*10

20,000Year

Fig. 1. Yearly and monthly number of lightning flashes in Alaska from 1986 to 2010
73

2010

2009

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1986

0
1987

Number of lightning flashes

120,000

AlaskaMay~August:1986-2010
N:62º-68.5º,W:141º-163º

M.AHMED FARUKH, H. HAYASAKA

et al., 2005).

This may imply that the burnt area of each year may
be mainly influenced by weather conditions after thefire has started. Overall, lightning-caused forest fires
are responsible for more than 90% of the annual burnt
area in Alaska (Shulski et al., 2005).

4. Forest Fire Occurrences in Severe Lightning
Years (2004, 2005, and 2007)
4.1 Fire Distribution and Fire Size
Lightning-caused forest fire distribution maps of
2004, 2005, and 2007 are shown in Figure 3, which
also includes tundra fires of 2007. Each fire locationin this figure is shown by a black dot. The number oflightning-caused fires was 257, 326, and 259 in 2004,
2005, and 2007, respectively. In addition, circles withdifferent diameters are used to show the size and location of large fires. The diameter of each circle forlarge fires was determined by considering the final
burnt area although it does not show the actual firesize.

Alaska’s forest fire distribution by using wildland fire
history data from 1956 to 2010 provided by the UAF
and AFS is shown in Figure 2. In Figure 2, the burnt
area due to human-caused fire was not very large
except in 1969 and 1980. The difference between thehuman- and lightning-caused burnt area bars clearly
indicates that wildland burnt areas are mainly caused
by lightning.
In the last 55 years, the average area burnt by lightning-caused fires was calculated to be 3,541 km2.
Recently, large forest fires occurred consecutively in2004 and 2005. The burnt areas were 26,591 km2 in2004 and 18,822 km2 in 2005, the largest and thirdlargest, respectively, since 1956. Alaska lost 10% of
its total boreal forest in these two consecutive years.
The burnt area for 2007, the third most severe lightning year discussed here, was only 2,389 km2. The
standard deviations in burnt area (σba) for these three
years were 4.15 σba for 2004, 2.8 σba for 2005, and-0.21 σba for 2007. Large values of 4.15 for 2004 and
2.8 for 2005 may partly be due to the recent trend inboreal forest fires (Wallenius et al., 2011; Flannigan

In this paper, “very large,” “large,” and “small” areused to distinguish fires by size of burnt area. “Verylarge” is used for fires with a burnt area of more than
1,000 km2, “large” means a burnt area of more than
500 km2, and “small” is for burnt areas of less than
500 km2. The fire size distributions for 2004, 2005,
and 2007 are shown in Figure 4.
Figure 3 shows that most of the fires in 2004, 2005,
and 2007 were confined mainly to the center of
Alaska, or that fires were distributed uniformly in the

(II)

900
800

Alaska1956-2010

Human-caused burnt area

30,000Lightning-caused burnt area
1

4ı

25,000Fires

20,000

3ı

2

2

3

500

15,000

2ı

400
5

300

4

6

10

7

9

200

10,000
1ı

11

8
5,000

Average

100

Fig. 2. Burnt area distribution in Alaska from 1956 to 2010
74

2010

2007

2004

2001

1998

1995

1992

1989

1986

1983

1980

1977

1974

1971

1968

1965

1962

0
1959

0

Year

Burnt area (km )

600

1956

Number of total fires

700

ACTIVE FOREST FIRE OCCURRENCES IN SEVERE LIGHTNING YEARS IN ALASKA(III)

2004: 257 fires

2007: 259 fires

2005: 326 fires

Fig. 3. Fire distribution maps for 2004, 2005, and 2007

(IV)
2176

Total 257 fires
Total burnt area 26,591 km2

Total 326 fires
Total burnt area 18,822 km2

Total 259 fires
Total burnt area 2,389 km22

Size of final burnt area by fires (km )

2,000

1,500

1,000

500

2005

2004

90

80

70

60

50

40

30

20

0

10

90

80

70

60

50

40

30

20

0

10

90

80

70

60

50

40

30

20

10

0

0
2007

Fire size distribution

Fig. 4. Fire size distributions in 2004, 2005, and 2007

boreal forest areas of interior Alaska or south of the
Brooks Range and north of the Alaska Range. Fires
in the northern areas and near the coast were mainly
tundra fires. The fire distribution map of 2004 (Figure3) clearly shows several very large fires that occurredconcentrically in the eastern part of interior Alaska.
None of such large fires occurred in 2005 and 2007,
and the fire year 2004 could be termed a “very large
fire year.” In 2005, fires were distributed uniformly
in interior Alaska and the fire sizes were restricted toareas smaller than 1,000 km2. Thus, fire year 2005
could be termed a “large fire year.” The fires in 2007
were also distributed uniformly all over Alaska (Fig-ure 3) except on the northern slope of the Brooks
Range, near Fairbanks, and near Anchorage. We may
call the fire year 2007 a “small fire year.”
4.2 Fire Starting Date and Fire Duration
General information on fire sizes, burnt areas, and fire
durations is summarized in Table 1. The fire durations were calculated by the following equation:

Fire duration = Fire out date
– Fire discovery date +1………………………., (1)where “fire discovery date” and “fire out date” were
derived from the AFS fire data.
75

M.AHMED FARUKH, H. HAYASAKA

32 days (Table 1) with the mean fire duration of theten largest fires being 92 days or about three months(Table 2). These durations are shorter than the firesof 2004 but longer than those of 2007. The 2005 firestarting dates (Table 2) show that three large fires
started in the middle and the end of July. The durationof these fires was from 63 to 68 days, and these late
July fires may have increased the burnt area of 2005
in August.

Table 1: Fire size and fire duration in 2004, 2005, and
2007
2004
2005
Number of very large fires (>1,000 km2)
90
Number of large fires (500~1,000 km2)
812Number of small fires (<500 km2)
240
314
Total number of lightning-caused fires
257
326
Average burnt area (km2)
10358
Total burnt area (km2)
26,591 18,822
Average fire duration (days)
38
32
Maximum fire duartion (days)
159
146
Average fire starting date (DN)
170
175

2007
1
0
258
259
9
2,389
20
157
168

In 2007, 259 fires occurred and the total burnt area
was 2,389 km2; there was only one large fire, a tundra
fire (Anaktuvuk River fire, observed on July 16, witha burnt area of 1,039 km2). The burnt areas of the remaining 258 fires in 2007 were all smaller than 162
km2. Small fires in 2007 lasted 20 days on average
while the ten largest fires lasted longer, 82 days. This
was largely because some of the fires in 2007 started
early or from April and May (Table 2). This situation
implies that some of the fires in less active fire years
could last longer and become larger under certain
weather conditions.

Table 2 lists the ten largest fires of 2004, 2005, and2007 with the burnt areas, fire starting dates, and fire
durations. There was a total of 257 fires in 2004 (Table1), and 17 large fires (burnt area > 500 km2) were
responsible for 76.5% of the total burnt area in 2004
(26,591 km2). The total burnt area of nine very large
fires (burnt area > 1,000 km2) corresponded to 53.6%of the total. The reason for the high contribution offires with large burnt areas may be explained by thestarting dates and the long duration of these fires. The
average duration of all 2004 fires was 38 days (Table1), while the mean fire duration of the ten largest fires
was 133 days, more than four months (Table 2). The
nine very large fires started mainly in the middle ofJune and lasted longer, and the largest fire (a boundary fire) in 2004 started on June 12 and lasted 159
days.

4.3 Number of Live Fires
In this paper, the term “live fire” is used to explain
the overall “active fire” situation in 2004, 2005, and2007. Live fire was calculated using the “fire discovery date” and the “fire out date” as determined by
the AFS data, and the number of daily live fires was
summed up; the trend in the number of daily live fires
for each year is shown in Figure 5.

Among the 326 fires in 2005, the number of large
fires (burnt area > 500 km 2) was 12, which corresponded to 48.1% of the total burnt area (18,822
km2). The average fire duration of all 2005 fires was

In 2004, the number of live fires increased from the
beginning of June and reached 100 at the end of July.

Table 2: Ten largest fires in 2004, 2005, and 2007
Fire Rank
1
2
3
4
5
6
7
8
9
10
Average

2004
Burnt area Fire starting Fire duration Burnt area
(km2)
date (DN)(days)(km2)
2176
12 Jun (164)
159
990
1956
6 Jul (188)
104
958
1878
17 Jun (169)
144
938
1699
12 Jun (164)
131
785
1635
15 Jun (167)
125
783
1395
15 Jun (167)
125
775
1296
14 Jun (166)
147
773
1152
20 Jun (172)
141
697
1076
28 Jun (180)
125
674
987
14 Jun (166)126
625
18 Jun (170)
133
800
1525

2005
Fire starting Fire duration Burnt area
date (DN)(days)(km2)
13 Jun (164)1041039
19 Jul (200)68161
16 Jun (167)105
135
25 Jul (206)66
89
11 Jun (162)110
8724 Jul (205)6385
16 Jun (167)7777
30 May (150)122
73
20 Jun (171)97
72
11 Jun (162)10640
24 Jun (175)
92
186

76

2007
Fire starting Fire duration
date (DN)(days)
16 Jul (197)85
5 Jul (186)87
28 Apr (118)157
6 Jun (157)100
23 May (143)128
9 Jul (190)2018 Jun (169)84
22 May (142)29
8 Jul (189)94
9 Jul (190)39
17 Jun (168)82

ACTIVE FOREST FIRE OCCURRENCES IN SEVERE LIGHTNING YEARS IN ALASKA

(V)
140

2005

Number of live fires

120
100
80

2004

60

2007

40
20
0
120

May

150

Jun

180

Jul

210

Aug

Day number

240

Sep

270

Oct

300

Fig. 5. Number of live fires in 2004, 2005, and 2007of July in 2005. These discrepancies arise due to thedifferent detection systems for lightning and fires, aswell as to the so-called time lags mainly due to theschedules of survey flights by AFS and holdover fires(Hayasaka and Lynch, 2001).

In 2005, the largest number of daily live fires among
three years, 125 fires, was observed at the end ofJune. After the end of June, the number of live fires
decreased to 65. But in August, the number of live
fires was almost constant, about 100. In 2007, the decreasing trend in the number of live fires started justafter the number of fires reached 100, and the numberof live fires decreased to 20 at the end of August. Finally, the duration of the periods with large numbersof live fires, i.e., more than 60 consecutive live fires,must be noted: about 90 days in 2004, about 100 daysin 2005, and 25 days in 2007. These differences could
explain the differences in the total final burnt area of
each year.

The situation around lightning and fire occurrences
may be understood by considering the gradient of theline curves for the accumulated number of lightning
flashes and fires in Figure 6. Steep and large gradients of the curves show a “peak” and a “most intense
period” for lightning and fire occurrences, with low
and zero gradients of the curves suggesting no or few
occurrences of lightning and fires.
The accumulated curve for lightning shows four major lightning peaks in 2004. Two major peaks and one
continuous period of lightning in July are shown by
the curve for 2005. One continuous active lightning
period occurred in July of 2007. Under the different
lightning occurrence patterns in these three years, theaccumulated AFS fire curves showed different fire occurrence patterns. There is one sharp fire peak in the
middle of June in both 2004 and 2005, but only one
small peak in 2007. Fire occurrence in the middle ofJune is one of the major fire characteristics in Alaska.
In 2004, the one fire peak in June corresponds to thesecond lightning peak, and the number of fires continuously increased until the third fire peak in the middleof July. In 2005, the one sharp fire peak corresponded

4.4 Lightning and Fire Occurrences
Figure 6 shows the daily occurrences of lightning(from the upper x-axis) and fires (from the lower xaxis) from May to August in 2004, 2005, and 2007.
The two line curves from the upper and lower x-axes
represent the accumulated number of lightning flashes
and fires detected by AFS, respectively.
Figure 6 shows that the number of fires in 2004,
2005, and 2007 closely coincided with the lightning
occurrence tendencies. But several different characteristics of the lightning and fire occurrences arealso notable in this figure, such as at the third peakof lightning at the beginning of July in 2004, and at
the third and fourth peak of lightning in the middle
77

M.AHMED FARUKH, H. HAYASAKA

(VI)
300

20

Number of AFS fires / 10
Accumulated number of AFS fires (%)

250
Lightning

40

200

50%

60
150

Accumulated
lightning

Accumulated
No. of fires

100

80

Fires

100
50

0

50%

120

122 May 152 Jun 182

Jul

212 Aug 242 May 148 Jun 178

2004

Jul

208 Aug 238

May 1452005

Jun 175

Jul 205

Number of lightning flashes x 100
Accumulated number of lightning flashes (%)

0140
Aug 235

2007

Fig. 6. Number of lightning flashes and fires (AFS) in 2004, 2005, and 2007

to the first sharp peak of lightning, and fires occurred
continuously until the beginning of August. In 2007,
the occurrence of fires started from the beginning ofJune and lasted until the middle of July following avery similar fire occurrence rate as in the other two
years.

where DN is the “specified day number,” FBA is the“final burnt area,” and n is the number of fires detected on the same DN.
The bars from the lower x-axis in Figure 7 represent
daily burnt areas and show when the larger fires occurred. But with only this bar graph, it was not possible to identify the severity of the fires i.e., fire expansion, final burnt area, and overall fire activities. To
compensate for this, an accumulated curve (line curvefrom the lower x-axis) for the percentage of the total
burnt area was added to Figure 7. The steep gradientof this curve could show the tendency of severe fireoccurrence in each year.

4.5 Onset of Large Fires and Final Burnt Area
To show the behavior of each individual fire, anevaluation of the burnt areas was carried out by summing up the final burnt area of each fire for the same
fire detection date. The accumulated burnt area for thesame fire detection date was calculated by the following equation:n(Accumulated burnt area) DN = ¦ FBA DN, i

In 2004, one very severe fire occurred in the middleof June and the percentage of accumulated burnt areai =1………………………., (2)
(VII)

20

350

2

Burnt area (x 10 km )2

Accumulated burnt area (%, km )

400

40

Lightning

300

50%

60

250
200

100
50
0

80

Accumulated
lightning

150

100

Burnt area
Accumulated
burnt area

50%

122 May 152 Jun 182
2004

Jul

212 Aug 242 May 148 Jun 178
2005

Jul

208 Aug 238

May 145

Jun 175

Jul 205

120

Number of lightning flashes (x 100)
Accumulated number of lightning flashes (%)

0

450

140
Aug 235

2007

Fig. 7. Number of lightning flashes and fire occurrences (burnt areas) in 2004, 2005, and 2007
78

ACTIVE FOREST FIRE OCCURRENCES IN SEVERE LIGHTNING YEARS IN ALASKA

reached 50%. A severe fire was also found in the
middle of June 2005. But after reaching 50%, two different fire patterns for both of the years were found.
In 2004, the percentage of accumulated burnt area
reached 90% at the beginning of July but this percentage (90%) was reached at the beginning of August in
2005. This implies that severe fires occurred concentrically from the middle of June to the beginning ofJuly in 2004. But there were two severe fire periods
in 2005: in the middle of June and at the end of July.
In 2007, some small fires occurred at the beginningof July and the percentage of accumulated burnt area
reached 90% just after a severe fire in the middle ofJuly.

4.6.1 Fire Activation by Drought and Precipitation
The accumulated precipitation for 2004, 2005, and2007 (dotted curve), and the average accumulated
precipitation for 1990 to 2010 (solid curve), with bars
for the number of daily hotspots from May to August
are shown in Figure 8. The standard deviations (+1
σap and -1 σap) for the average accumulated precipitation with the σp values for 2004, 2005, and 2007
are shown on the y-axis for each year. The gradient(dashed line) of the accumulated precipitation curves
in Figure 8 expresses the overall precipitation pattern. Where the gradient is flat or zero, it shows theoccurrence of a rainless day or drought. The averageaccumulated precipitation curve in Figure 8 shows aconstant gradient from the beginning of June to theend of August, and the average precipitation rate during the fire season (May to August) is about 1.5 mm/
day in Alaska. The total accumulated precipitationin 2004, 2005, and 2007 was 114 mm (-0.86 σp), 223
mm (1.13 σp), and 195 mm (0.63 σp), respectively.

Finally, it is necessary to bear in mind that the above
severe fire occurrence trends only interpret large fireoccurrences, and are not directly related to fire activities or fire propagation after ignition by lightning.
This suggests that assistance from other data such as
daily hotspot data and weather data is needed so as toevaluate the fire activities of each year.

There were three distinct periods and peaks of daily
hotspots in 2004 (bars in Figure 8), and they occurred
during the two drought periods. This result suggests that the fires in 2004 became active in the two
drought periods, the one drought period being in June
and July and the other being in August. The fires occurring from the beginning of June were not so active
due to precipitation from the middle of May 2004.

4.6 Fire Activity and Weather
Daily hotspot data from NASA-MODIS are usedhere to evaluate fire activity and its interaction withweather conditions like precipitation and air temperature. For this purpose, weather data of the Fairbanks
station could not explain all Alaskan weather changes
but could be used as a weather indicator representative of weather conditions in the interior of Alaska.

The largest peak in the number of daily hotspots in
2004, 2005, and 2007 occurred in the middle of Au-(VIII)
250

4,500
Drought+ıap

Number of hotspots

+ıap

+ıap

200

3,500

+1.13ıp
+0.63ıp

3,000
Hotspots

150-0.86ıp

2,500
Drought

2,000

-ıap

-ıap

Accumulated
precipitation

-ıap

100

Drought

1,500

Average acc.
precipitation('90~'10)

1,000

50

Accumulated precipitation (mm)

4,000

500
0

0
121

May

151

Jun

181
2004

Jul

211

Aug

241

May

148

Jun

178

Jul
2005

208

Aug

238

May

145

Jun

175

Jul
2007

205

235

Aug

Fig. 8. Accumulated precipitation and daily hotspots (fires) in 2004, 2005, and 2007
79

M.AHMED FARUKH, H. HAYASAKA

Figure 9 shows that most of the hotspot periods in2004 and 2005 coincided with higher-Tmax periods anddays. The Tmax of most of these days exceeded 30ºC.
In addition, a few hotspot peaks like at the end ofJune and the middle of August 2004 and the middle ofAugust 2005 occurred at the final stage of the higherTmax periods, when the temperatures exceeded +1 σt.
These trends support the idea that forest fires become
active during high-temperature periods with rainlessor drought conditions.

gust 2005 (Figure 8). This active fire period occurred
due to drought from the middle of July. Other fires inJune and July 2005 may have been suppressed by thecontinuous precipitation that started from the middleof May. The precipitation rate from the middle of
May to the middle of July 2005 was 3.5 mm/day, andthis was around twice the average.
There was no noticeable hotspot peak in 2007. Thismay be explained by different lightning occurrence
tendencies in 2004, 2005, and 2007 found in Figures
6 & 7. Active lightning occurrences in the middle ofJune in 2004 and 2005 left fatal fires in number andburnt area as shown in Figures 6 & 7. On the other
hand, a small but notable lightning occurrence could
be found in the middle of June in 2007. Active lightning occurrence in 2004 and 2005 could be partiallyexplained by the high maximum air temperature in
Figure 9. Other weather factors related to active lightning occurrence in Alaska have already been clarified
by the authors (Farukh, Hayasaka, Kimura, 2011a,b).

Table 3: Factors for the normalized standard deviation(σt) of each monthin 2004, 2005, and 2007
2004
2005
2007
*Ave. Tmax (ºC)
1 σt

May
0.18
1.13
0.11
16.4
2.50

June
2.20
0.00
0.24
22.1
1.39

July
0.54
-0.36
0.51
22.7
1.77

*Average in the period from 1990 to 2010
The effect of Tmax on the fire behavior of each year
is shown in Table 3. Due to the large fluctuations indaily temperature, normalized standard deviations ofmonthly average temperatures are used here. Table 3
shows that the temperatures in June and August 2004
were significantly higher (+2.20 σt and +1.94 σt, respectively) than the average. These high temperatures
could be one of the causes of the largest fire occurrences in 2004. Although the monthly temperature ofAugust 2005 was not very much higher (+0.29 σt), thetemperatures at the beginning of August were significantly higher (Figure 9). These results suggest that air

4.6.2 Fire Activity and Air Temperature
To evaluate the effect of the “maximum air temperature” (Tmax) on fire activity, the daily Tmax change
along with hotspots is shown in Figure 9 for eachof the years 2004, 2005, and 2007. In this figure, theaverage Tmax is shown by the thick solid line curve
and the other two line curves (shown by dotted lines)
are for the standard deviations (+1 σ t and -1 σ t).
The monthly averages of Tmax for each year are also
shown, by heavy dotted horizontal lines.

(IX)
4,500
Average Tmax
(1990~2010)

Hotspots

Tmax

30

4,000
3,500

+ıt

3,000

25

2,500
20

-ıt

2,000
15

Drought

Drought

Drought

1,500

10

Number of hotspots

Daily maximum air temperature (Tmax , ºC)

40
35

1,000

5

500

0

0
121

151
May

181
Jun

211
Jul

2004

241
Aug

148
May

August
1.94
0.29
1.31
18.9
1.95

178
Jun

208
Jul

2005

238
Aug

145

175

205

235

May

Jun

Jul

Aug

2007

Fig. 9. Daily maximum air temperature (Tmax) and hotspots (fires) in 2004, 2005, and 2007
80

ACTIVE FOREST FIRE OCCURRENCES IN SEVERE LIGHTNING YEARS IN ALASKA

temperature is also a key factor in active forest firesin Alaska.

The arrows for the “accumulated final burnt area”(AFBA) in Table 4 show the approximate date of firestarting as well as the date of severe fires for 2004,
2005, and 2007. The AFBA clearly shows that severe
fires started mainly in June and July in the case of2004 (Table 4 & Figure 7). The hotspot numberssuggest severe fires in 2005 starting in June and Julyas well as in August. Fires in 2004 became active inthree major fire periods: at the end of June, the middleof July, and August (Table 4 & Figure 8). The most
severe fire activity occurred in the middle of August
2005, and there was only small fire activity in 2007.
The occurrence of two long drought periods (Table 4)of more than 30 days in 2004 may have assisted fire
expansion. The considerably high temperature (Tmax =+2.20 σt, Figure 9 & Table 3) in June 2004 may havebeen a factor in the active fires. The total precipitation from May to August 2005 was larger than average (+1.13 σp) but more than 100 live fires becamevery active during the long drought from the middleof July. In August 2007, the temperatures were high(+1.31 σt) but fires were not active due to the continuous precipitation from the beginning of June.

4.7 Summary of Fire Characteristics
A number of fire characteristics together with fire andlightning occurrences are listed in Table 4. In thistable, several kinds of downward arrow marks forlightning and precipitation, and upward arrow marks
for fire numbers and areas, hotspots, and drought, areused. The arrow sizes express the severity, and thepercentages show the accumulated value of listed firecharacteristics at that time point.
In Table 4, 50% of lightning tended to occur before
the beginning of July for all the years (shown by“↓50%”), and about 70% of the fires in 2004 and2005 had already started before the beginning of July
(Figure 6). This situation would occur due to differences in precipitation pattern and moisture conditionof the forest floor. Thus, fires starting before the beginning of July could possibly remain active if they
are surrounded by enough fuel, and the duration ofsome of these fires is more than 100 days (Table 2).

Table 4: Fire and lightning occurrences, and fire characteristics in 2004, 2005, and 2007
Year

June

Month
DN

150

2007

2005

2004

LightningFire (num.)Fire (area)
AFBA

160

July
170

180

190

50%

50%
70%

50%

70%

90%

50

65

August
200

210

220

230

240

4 major peaks
1 major peak
1 major peak
1 small peak
3 major peaks
3 major periods

90%
90%

Hotspot
Number oflive fires
Precipitation
Drought
Tmax (SD)LightningFire (num.)Fire (area)
AFBA

10

30

50

100

90

90

85

80
114 mm (-0.86ıp)
2 periods+2.20ıt

+0.54ıt
50%+1.94ıt

50%

90%

70%

90%

50%

70%

90%

80

105

2 major peaks
1 major peak
1 major peak
1 small peak
2 major peaks
2 major periods

Hotspot
Number oflive fires
Precipitation
Drought
Tmax (SD)LightningFire (num.)Fire (area)
AFBA
Hotspot
Number oflive fires
Precipitation
Drought
Tmax (SD)Remarks

10

50

90

110

105

105

105

90
223 mm (+1.13ıp)
1 period

0.00ıt-0.36ıt

+0.29ıt

50%
50%

1 continuous peak

90%
70%

1 peak90%

2 small peaks

50-90%

10

20

35

60

85

80

60

30

25

25
195 mm (+0.63ıp)+0.24ıt

+0.51ıt

+1.31ıt

Explanation of arrow markings:
used for lightning and precipitation,used for fire number, fire area (AFBA), hotspots, anddrought, and arrangement of arrows from left to right indicates the severity from high to low.

81

M.AHMED FARUKH, H. HAYASAKA

vice (AFS); 2. MODIS (Terra: EOS AM, and Aqua:
EOS PM) of NASA, USA; and 3. the Alaska Climate
Research Center, NOAA, for providing various reliable datasets and resultant stimulating findings, whichhave enabled the authors to research this issue.

5. Conclusions
Severe lightning flashes, 120,000 a year or more thanfour times the annual average, started around 300
fires in each year of 2004, 2005, and 2007. Several
fire characteristics introduced by the authors such as
fire duration, accumulated final burnt area (AFBA),
and number of live fires were found to be effective in
explaining forest fire activity in Alaska. Discussion
focusing on the various fire characteristics may reachthe following conclusions or causes of forest fires ineach of the years surveyed.
1. The 2004 forest fires left the historically largest
annual burnt area in Alaska mainly due to severe
fire occurrence under drought (-0.86 σp) and highair-temperature (+2.20 σ t) conditions in June.
Three active fire periods in June, July, and August
also triggered the largest burnt area under drought
conditions, with a large number of live fires (> 80)
in August.
2. The forest fires of 2005 were characterized by severe fire occurrence in June, July, and August. The
most active fire period in August started in Julyunder drought with a large numbers of live fires (>100) despite the relatively heavy precipitation from
May to the beginning of July (+1.13 σp). These
conditions resulted in the third-largest annual burntarea.
3. The 2007 forest fires left the twenty-first-largest
annual burnt area with no severe fire occurrence
under non-active lightning conditions in June, andlimited fire occurrence under severe lightning andprecipitation (+0.63 σp) in July. Although there was
one very large tundra fire, there were no August
fires even with drought and high-air-temperature
conditions (+1.31 σt).
Finally, it could be concluded that severe fires in
Alaska tend to occur from the middle of June to thebeginning of July, and that these severe fires could become active even in August under drought and highair-temperature conditions.

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