Mesoscale model simulation of the meteorological conditions during the
2 June 2002 Double Trouble State Park wildfire
Joseph J. CharneyA,C and Daniel KeyserB
A
USDA Forest Service, 1407 S. Harrison Road, Room 220, East Lansing, MI 48823,
USA.
B
Department of Earth and Atmospheric Sciences, University at Albany, State University
of New York, Albany, NY 12222, USA.
C
Corresponding author. Email: jcharney@fs.fed.us
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Abstract. On the morning of 2 June 2002, an abandoned campfire grew into a wildfire in
the Double Trouble State Park in east-central New Jersey. The wildfire burned 1300
acres and forced the closure of the Garden State Parkway for several hours due to dense
smoke. In addition to the presence of dead and dry fuels due to a late spring frost prior to
the wildfire, the meteorological conditions at the time of the wildfire were conducive to
erratic fire behavior and rapid fire growth. Observations indicate the occurrence of a
substantial drop in relative humidity at the surface accompanied by an increase in wind
speed in the vicinity of the wildfire during the late morning and early afternoon of 2 June.
The surface drying and increase in wind speed are hypothesized to result from the
downward transport of dry, high-momentum air from the middle troposphere occurring in
conjunction with a deepening mixed layer. This hypothesis is addressed using a highresolution mesoscale model simulation to document the structure and evolution of the
planetary boundary layer and lower-tropospheric features associated with the arrival of
dry, high-momentum air at the surface coincident with the sudden and dramatic growth of
the wildfire.
Additional keywords: fire weather, fire-weather forecasting, wildfire.
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Introduction
As a thick morning mist cleared on 2 June 2002, the New Jersey Forest Fire
Service (NJFFS) lookout at the Cedar Bridge fire tower in east-central New Jersey (NJ)
reported a smoke plume in the northeastern corner of the Double Trouble State Park
(DTSP), near the Jakes Branch of the Toms River (Figs. 1a,b) at 1709 UTC (NJFFS
2003). Firefighters from two NJFFS units responded immediately to the wildfire,
arriving on the scene within five minutes of the initial report, at 1714 UTC (Table 1).
Radio logs from the Lakewood fire tower indicate that the fire was advancing at
approximately 0.5 m s−1 during the initial attack, when a sudden change in surface
meteorological conditions contributed to the onset of extreme fire behavior. Eleven
minutes later, at 1725 UTC, the firefighters were forced to abandon their initial attack and
pull back to establish defensible positions around the town of Beachwood, NJ, less than 3
km east-southeast of the fire front. The crews lit backfires ahead of the fire front in an
effort to deprive the fire of fuel. A combination of a surface wind shift from westerly to
northwesterly and the actions of the firefighters prevented the fire from sweeping through
the town. Continued efforts were required throughout the day to protect homes,
outbuildings, and other structures to the south of the town as the fire continued to spread
rapidly, with multiple surface wind shifts and spot fires occurring ahead of the fire front.
By sunset that evening, 361 firefighters had been called in to fight a wildfire that burned
1300 acres (Fig. 2), forced the closure of the Garden State Parkway, damaged or
destroyed 36 homes and outbuildings, directly threatened over 200 homes, forced the
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evacuation of 500 homes, and caused an estimated $400 000 in property damage (NJFFS
2003).
Prior to the initial attack, weather observations at the Lakewood fire tower at 1415
UTC indicated a surface wind of 4 m s−1 with gusts to 9 m s−1, a temperature of 24°C,
and a relative humidity of 62%. Approximately two hours after the initial attack (1900
UTC), a spot weather forecast prepared by the Mount Holly, NJ, National Weather
Service (NWS) Forecast Office predicted a wind of 7 m s−1 with gusts to 13 m s−1, a
temperature of 28°C, and a relative humidity of 28% (Table 2). The Lakewood fire tower
reported a wind in excess of 18 m s−1 at 1725 UTC, coinciding with the time when the
firefighters were forced to abandon their initial attack on the wildfire. These observations
highlight the profound change that occurred in the atmosphere between the late morning
and the early afternoon of 2 June, resulting in a substantial drop in relative humidity
accompanied by an increase in surface wind speed at the wildfire location, and
contributing to the decision to abandon the initial attack and defend Beachwood, NJ.
The Eastern Area Modeling Consortium (EAMC), in East Lansing, MI, operates a
real-time mesoscale modeling system that simulates meteorological conditions
throughout the north-central and northeastern United States (Heilman et al. 2003; Zhong
et al. 2005). At the time of the wildfire, the EAMC real-time simulations indicate that
dry, high-momentum air at the surface was most evident over southeastern Pennsylvania
(PA) and southern NJ (Charney et al. 2003). Since this horizontal distribution of
moisture and momentum cannot be attributed to a land–sea boundary or other local landsurface characteristics, the generation mechanism for the moisture and wind anomalies is
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hypothesized to derive from larger than local scale (e.g. mesoscale) atmospheric
conditions at the ground and aloft.
Kaplan et al. (2008) employ high-resolution simulations run in a research mode to
analyze the dynamics of the mesoscale circulations associated with the generation of dry
air aloft during the DTSP wildfire, emphasizing the physical processes that contribute to
the development of the observed surface meteorological conditions. Our paper focuses
on the diagnostic capabilities of the observations and numerical weather prediction
(NWP) models that are routinely available in real time during a wildfire. We highlight
connections between atmospheric conditions aloft and the evolution of the surface
conditions that may have impacted the observed fire behavior. We do not attempt to
simulate the evolution of the wildfire itself. Instead, we focus on the utility of the
available observations and the ability of a mesoscale NWP model to diagnose the
meteorological conditions that contributed to the observed fire behavior during the DTSP
wildfire. In the next section we describe the mesoscale atmospheric model employed to
study the DTSP wildfire. Following sections investigate the observed fire behavior and
meteorological conditions, and present a mesoscale model simulation of the
meteorological conditions that contributed to the observed fire behavior. The final
section consists of discussion and conclusions.
Mesoscale model description
The mesoscale NWP model employed for this study is the Pennsylvania State
University/National Center for Atmospheric Research Mesoscale Model version 5.3
5
(MM5) (Grell et al. 1995). The MM5 is a nonhydrostatic, primitive-equation, mesoscale
model that uses a terrain-following pressure-coordinate system. The model includes
physical subgrid-scale parameterizations to account for radiative transfer (Mlawer et al.
1997), mixed-phase cloud microphysics (Reisner et al. 1998), cumulus convection (Kain
and Fritsch 1990), boundary-layer turbulence (Janjić 1990), and land-surface exchange
processes (Chen and Dudhia 2001). For this study, we produce 48-h simulations on a 36km domain, and on one-way nested 12-km and 4-km domains, with hourly model output
retained for validation and analysis on each domain The outermost, 36-km domain
covers the entire continental United States; the 12-km domain covers New England, the
Mid-Atlantic states, the Ohio Valley, and the Great Lakes region; and the 4-km domain
covers most of New England and the Mid-Atlantic states (see Domains 1, 2, and 4 in Fig.
1 of Zhong et al. 2005). The model contains 35 sigma levels, with 15 sigma levels
located in the lowest 2000 m to better resolve planetary boundary layer (PBL) structures
that have the most direct impact on fire behavior.
Real-time simulations on the aforementioned domains are produced by the EAMC
using the National Centers for Environmental Prediction (NCEP) operational Eta model
for initial and boundary conditions. Our analyses indicate that while simulations using
the Eta model for initial and boundary conditions reproduce many of the observed surface
phenomena associated with the DTSP wildfire, the simulations fail to adequately
represent the PBL depths observed at the time of the wildfire. We hypothesize that
interactions between dry, high-momentum air aloft and a deepening afternoon PBL are
important to the observed evolution of the DTSP wildfire, which requires a simulation of
PBL depths that closely correspond to observations. In order to address the shortcomings
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of the MM5 simulations using the Eta model for initial and boundary conditions, we
performed additional simulations using the NCEP North American Regional Reanalysis
(NARR) (Mesinger et al. 2006) for initial and boundary conditions. The simulations
using the NARR for initial and boundary conditions produce PBL depths that more
closely match observations, so a simulation on the 4-km domain using the NARR is
adopted for the remainder of this study.
Observed fire behavior and meteorological conditions
The DTSP wildfire started from a campfire that had been left smoldering on the
night of 1 June and was small enough to escape detection at fire observation towers until
1709 UTC (Table 1) due to poor visibility and high relative humidity in the morning
(Steve Maurer, NJFFS, personal communication). The Keetch–Byram Drought Index
(Keetch and Byram 1968) in southern NJ at the time of the fire was 105, which indicates
a long-term, climatological drought at the “moderate” level. Other indices suggest that
the fuel conditions were at moderate risk for fire. However, NJFFS (2003) reports that
frost damage was present in the area due to unusual freezing temperatures that occurred
some weeks before the fire. As a result, the drought and fuel indices for the area may
have underestimated the amount of dead and dry fuels that existed at the time and
location of the DTSP wildfire.
As discussed in the Introduction, a profound change in surface meteorological
conditions between the late morning and early afternoon of 2 June contributed to the
decision to abandon the initial attack on the wildfire. The weather observations available
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at the time of the wildfire give ample evidence of evolving meteorological conditions
throughout the 12-h period from 1200 UTC 2 June to 0000 UTC 3 June 2002. A synoptic
analysis of the observed surface winds reveals that winds in eastern PA and NJ are
generally light, with westerly and west-southwesterly winds dominating at 1200 UTC
(Fig. 3a). Farther to the north and west, a wind shift indicates a transition to
northwesterly or north-northwesterly flow. At 1800 UTC (Fig. 3b), which coincides with
the time when the fire was declared a “major fire” and within 18 min of a change in wind
direction being reported on the fire line (Table 1), the surface analysis indicates the
aforementioned wind shift passing through central NJ, with westerly winds occurring in
southern NJ. Also note that wind speeds increase from 5 m s−1 at 1200 UTC to 13 m s−1
at 1800 UTC in southern NJ. By 0000 UTC (Fig. 3c), winds in eastern PA and NJ are
generally northwesterly or north-northwesterly, with speeds ranging from 3 m s−1 to 8 m
s−1. An overall drying trend is evident in eastern PA and NJ between 1200 UTC and
1800 UTC, consistent with the wind shift and the accompanying dry advection from the
northwest. In central and southern NJ, mixing ratios fall from 10 g kg−1 at 1200 UTC
(Fig. 3a) to 5 g kg−1 at 1800 UTC (Fig. 3b), with higher mixing ratios evident
immediately to the west and northwest at both times. The presence of a mixing ratio
minimum over central and southern NJ at 1800 UTC suggests that a local drying process
augmented the advective drying during the 6-h period ending at this time.
The conditions aloft broadly indicate the influence of a trough moving through the
northeastern United States between 1200 UTC 2 June and 0000 UTC 3 June 2002. At
1200 UTC, northwesterly flow prevails throughout most of the northeastern United States
behind a sharp temperature gradient that extends northeastward through New York (NY)
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and New England at 850 hPa (Fig. 4a). Northwesterly and westerly winds occur at 850
hPa over PA and southern New England along and east of the trough axis. At 300 hPa, a
jet streak northwest of the Great Lakes is approaching NY and New England (Fig. 4b).
At 0000 UTC, the sharp temperature gradient at 850 hPa has progressed into PA and
southern New England, and northwesterly flow and lower temperatures have penetrated
into NY and New England, in conjunction with the southeastward movement of the
trough during the previous 12 h (Fig. 4c). At 300 hPa, the jet streak has advanced
southeastward and extends across NY and southern New England at 0000 UTC (Fig. 4d).
The movement and evolution of these upper-air features between 1200 UTC 2 June and
0000 UTC 3 June 2002 coincides with the southeastward progression of the surface wind
shift that passes through eastern PA and NJ during this 12-h period (Figs. 3a–c).
Geostationary Operational Environmental Satellite (GOES) water vapor imagery
for 1215 UTC 2 June and 0015 UTC 3 June 2002 (Figs. 5a,b) delineates a ribbon of dry
air sagging southeastward during this time period. This ribbon of dry air coincides
closely with the axis of the jet streak at 1200 UTC 2 June and 0000 UTC 3 June 2002
depicted in Figs. 4b,d, respectively. The GOES imagery suggests that dry air in the
middle-to-upper troposphere moves into southern New England in conjunction with the
progression and evolution of the previously documented upper-air features between 1200
UTC 2 June and 0000 UTC 3 June 2002. A Moderate-Resolution Imaging
Spectroradiometer (MODIS) visible satellite image valid at 1558–1611 UTC 2 June
shows clear-sky conditions, suggestive of the presence of dry air, over southeastern PA
and southern NJ two hours prior to the time when the DTSP wildfire was declared a
“major fire” (Fig. 5c).
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A skew T–log p sounding from 1200 UTC 2 June at Upton, NY (OKX) (Fig. 6a),
indicates a shallow surface-based mixed layer beneath an inversion between 940 hPa and
900 hPa. Surface winds are light and from the southwest, while winds just above the
ground are 13 m s−1 from the west-northwest. Above the inversion, a mixed layer extends
to 550 hPa with west-southwesterly winds of 18 m s−1 at the bottom of the layer, shifting
to west-northwesterly winds of 36 m s−1 at the top. Above 550 hPa, the air is
considerably drier while wind speeds vary between 36 m s−1 at 550 hPa and 46 m s−1 at
the thermodynamic tropopause, which is located just above 200 hPa. The 0000 UTC 3
June sounding at OKX (Fig. 6b) reveals that while the surface temperature of 20°C is
nearly unchanged from the 1200 UTC observation of 21°C the dewpoint has dropped
from 15°C to 4°C. The surface-based mixed layer has deepened to 780 hPa, and the drier
air aloft is now located above the top of the inversion at 750 hPa. Winds throughout the
sounding are now from the northwest, with surface wind speeds of 5 m s−1 increasing to
13 m s−1 at the top of the mixed layer. Above the mixed layer, wind speeds increase with
height, from 21 m s−1 at 750 hPa to 59 m s−1 at 350 hPa. Notably, the soundings also
reveal that dry, high-momentum air has descended from just above 550 hPa to 750 hPa
between 1200 UTC and 0000 UTC. The sharp contrast between surface-based mixedlayer air and the air immediately above the inversion apparent in the 0000 UTC sounding
(Fig. 6b) is hypothesized to be characteristic of the environment in which the DTSP
wildfire occurred.
Wind profiler observations from New Brunswick, NJ (Fig. 7), show the evolution
of the winds below 3000 m between 1100 UTC and 2100 UTC 2 June. Westsouthwesterly surface winds of 5 m s−1 at 1200 UTC veer to northwesterly by 1500 UTC
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and remain northwesterly through 2100 UTC. Surface wind speeds increase to 15 m s−1
at 1600 UTC, and then vary between 10 m s−1 and 15 m s−1 for the remainder of the
profiler record. This sequence of surface wind observations coincides with the passage of
the wind shift through eastern PA and NJ shown in the surface analyses (Figs. 3a–c).
Above the ground, a layer of northwesterly winds from the surface to about 1000 m
appears by 1500 UTC, and subsequently deepens to about 2700 m, coinciding with the
top of the profiler record, by 2100 UTC. This deepening layer of northwesterly winds
suggests that the southeastward-moving trough identified in the 850 hPa analyses (Figs.
4a,c) and the deepening mixed layer diagnosed in the OKX skew T–log p soundings
(Figs. 6a,b) are influencing the evolution of the winds aloft during the period of the
profiler record. Surface meteograms from McGuire Air Force Base, NJ (WRI) (Fig. 8a),
and Atlantic City, NJ (ACY) (Fig. 8b), indicate that surface winds start to shift from
west-southwesterly to west-northwesterly between 1300 UTC and 1400 UTC, dewpoints
begin to decrease at 1400 UTC, and maximum wind gusts and minimum dewpoints occur
at 1800 UTC. The wind shift and decrease in dewpoint observed at WRI and ACY
between 1300 UTC and 1800 UTC are consistent with the wind and mixing ratio
evolution over central and southern NJ documented between 1200 UTC and 1800 UTC in
the surface analyses (Figs. 3a,b).
The dewpoint reduction and the shift in wind direction from west-southwesterly
to northwesterly documented in the surface observations, the observed evolution of the
upper-level trough and jet streak, and the ribbon of dry air in the middle-to-upper
troposphere evident in the satellite imagery could be critical factors for explaining the
surface drying and wind variability observed at the time of the outbreak and rapid growth
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of the DTSP wildfire. This surface drying and wind variability is hypothesized to result
from the downward transport of dry, high-momentum air from the middle troposphere
occurring in conjunction with a deepening mixed layer during the late morning and early
afternoon of 2 June. Since strong winds (Byram 1954; Fahnstock 1965; Brotak and
Reifsnyder 1977; Simard et al. 1987) and low relative humidity (Lansing 1939; Davis
1969; Simard et al. 1987) are known to be conducive to large fire development and rapid
fire spread, the meteorological events documented in this section could help explain the
observed fire behavior in this case.
Mesoscale model simulation
The observational documentation presented in the previous section of the
meteorological conditions over the northeastern United States on 2 June 2002, the day of
the DTSP wildfire, demonstrates that unusually dry and windy conditions developed over
central and southern NJ during the afternoon on this day. These conditions are
hypothesized to result from the downward transport of dry, high-momentum air from the
middle troposphere occurring in conjunction with a deepening mixed layer during the late
morning and early afternoon of 2 June. Nevertheless, the observational record does not
resolve the meteorological conditions in the immediate vicinity of the DTSP wildfire, nor
does it possess sufficient spatial and temporal detail to allow this hypothesis to be
assessed. To address these shortcomings in the observational record, the MM5 is
employed to examine the meteorological conditions over the northeastern United States
with finer spatial and temporal resolution than is available from the observations. The
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MM5 provides a high-resolution, three-dimensional analysis of the meteorological
conditions in the vicinity of the DTSP wildfire, as well as of the atmospheric features that
precede and accompany erratic fire behavior and rapid fire growth. The model results
discussed in the remainder of this section derive from a 48-h simulation executed on the
4-km domain covering the northeastern United States that was introduced in the
mesoscale model description section. The MM5 simulation starts at 1200 UTC 1 June
2002 using NARR data for initial and boundary conditions.
Figure 9 shows the surface winds and relative humidity from the MM5 simulation
valid at 1800 UTC 2 June, corresponding to the time when the DTSP wildfire was
declared to be a “major fire.” The simulated surface winds reproduce the observed westnorthwesterly and northwesterly flow at 1800 UTC 2 June over eastern PA and NJ (Fig.
3b). Simulated surface wind speeds are between 5 m s−1 and 10 m s−1 over eastern PA
and NJ, with the higher wind speeds in this interval evident across central NJ. The
simulated surface relative humidity distribution in Fig. 9 reflects the observed pattern of
dry air evident in Fig. 3b, with the lowest simulated relative humidity values located over
central and southern NJ and higher values present to the north and west. It is noteworthy
that an isolated area of low relative humidity (i.e. less than 30%) is present in the
simulation at 1800 UTC over southern NJ, with additional areas found to the south and
west. As in the previous section, the presence of these relative humidity minima within
the broader-scale pattern of low-relative humidity across the Mid-Atlantic region
suggests that a local (i.e. mesoscale) drying process could be playing a role in the
development of these minima.
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The simulated relative humidity distribution at 700 hPa valid at 1800 UTC 2 June
(Fig. 10) reveals a broad area of extremely low values extending westward from southern
NY through NJ and PA, with relative humidities below 10% evident in a broad swath
across this region. This simulated swath of extremely low relative humidity delineates
the horizontal extent of the dry air at 700 hPa found above the location of the DTSP
wildfire. The vertical profiles of temperature, dewpoint, and wind passing through this
dry air swath at the fire location (39.927°N, 74.225°W) are portrayed in the simulated
skew T–log p sounding valid at 1800 UTC 2 June (Fig. 11). The sounding reveals a dry
layer above the surface-based mixed layer, the top of which is located at 850 hPa. The
profile of simulated wind at the fire location shows northwesterly winds in the mixed
layer, transitioning to west-northwesterly at 750 hPa. Wind speeds increase from 7 m s−1
to 11 m s−1 between the surface at 1001 hPa and the top of the mixed layer at 850 hPa,
and from 18 m s−1 to 39 m s−1 in the overlying dry layer between 780 hPa and 510 hPa.
A northwest–southeast-oriented vertical cross section passing through the fire
location of simulated relative humidity and pressure-coordinate vertical velocity at 1700
UTC and 1800 UTC 2 June is shown in Figs. 12a,b. The cross-sectional perspective
extends the depiction of the lower-to-middle troposphere provided by the simulated skew
T–log p sounding to the horizontal dimension approximately parallel to the airflow in the
lower troposphere (see Fig. 10 for cross-section location). The respective times shown
correspond to the estimated start time of the DTSP wildfire and to the time when it was
declared to be a “major fire” (Table 1). The simulated swath of extremely low relative
humidity at 700 hPa identified in Fig. 10 is manifested in both cross sections as a
downward-directed tongue of relative humidity values less than 10% extending below
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800 hPa immediately to the northwest of the fire location at 1700 UTC (Fig. 12a) and
directly above the fire location at 1800 UTC (Fig. 12b). This tongue corresponds to the
dry layer above the surface-based mixed layer, which extends to 850 hPa, shown in the
sounding at 1800 UTC (Fig. 11). At 1700 UTC (Fig. 12a), a narrow filament protrudes
downward from the overlying tongue of low relative humidity, reaching 930 hPa at a
position between 40 km and 50 km southeast of the fire location. This filament, which
coincides with a region of localized subsidence, appears to be connected to, or coupled
with, a surface-based relative humidity minimum at the fire location. At 1800 UTC (Fig.
12b), the aforementioned filament has lowered to 950 hPa, consistent with the signature
of subsidence evident at 1700 UTC (Fig. 12a), and has progressed towards the southeast,
but still appears to retain a connection to the surface-based relative humidity minimum at
the fire location. The filament continues to coincide with a region of subsidence at 1800
UTC, although the spatial extent and strength of this feature have decreased relative to
1700 UTC.
The foregoing examination of the cross sections suggests the following scenario
in support of the hypothesis for the development of unusually dry and windy conditions
over central and southern NJ during the afternoon of 2 June stated at the beginning of this
section. The surface-based relative humidity minimum at the fire location during the
time period bracketing the initiation and rapid growth of the DTSP wildfire may be
linked by a narrow filament of low relative humidity air to a reservoir of dry air in the
middle troposphere. This reservoir, which corresponds to the swath of extremely low
relative humidity at 700 hPa (Fig. 10), provides the source of dry, high-momentum air
15
that is transported downward from the middle troposphere to the surface as the mixed
layer deepens during the late morning and early afternoon of 2 June.
Time series at the fire location valid from 1200 UTC 2 June to 0000 UTC 3 June
of simulated surface relative humidity (Fig. 13a), surface wind speed (Fig. 13b), and PBL
depth (Fig. 13c) reveal that a rapid decrease in surface relative humidity and an increase
in surface wind speed coincide with the onset of rapid PBL growth between 1300 UTC
and 1400 UTC. By 1800 UTC, the time of “major fire” declaration, the surface relative
humidity reaches its minimum value and the PBL depth reaches its maximum value,
while wind speeds continue to increase during the afternoon, reaching a maximum two
hours later, at 2000 UTC. Although the mixed-layer depth inferred visually from the
skew T–log p sounding (Fig. 11) is not necessarily equivalent to the PBL depth diagnosed
in the MM5 (Fig. 13c), the increase of the latter between 1200 UTC and 1800 UTC
suggests that the former also has increased during this time interval as stipulated in the
hypothesis for the development of exceptionally dry and windy conditions in the vicinity
of the fire location.
Time–height cross sections at the fire location of simulated relative humidity (Fig.
14a) and wind speed (Fig. 14b) from 1200 UTC 2 June to 0000 UTC 3 June between
1000 hPa and 700 hPa extend the depiction of the evolution of these respective quantities
at the surface in the time series (Figs. 13a,b) upward into the middle troposphere. The
surface-based relative humidity minimum at the fire location, the tongue of low relative
humidity in the vicinity of the fire location, and the narrow filament of low relative
humidity southeast of the fire location identified in the vertical cross sections at 1700
UTC and 1800 UTC (Figs. 12a,b), are evident in Fig. 14a prior to and during this time
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interval. The increase in surface wind speed at the fire location documented in the time
series between 1300 UTC and 2000 UTC in Fig. 13b is evident at 1000 hPa in Fig. 14b.
This increase in surface wind speed is accompanied by the downward development of a
wind speed maximum in the middle and lower troposphere and by an increase in wind
speed within a mixed layer in which the wind speed is approximately uniform with height
above 975 hPa. The increase in mixed-layer depth between 1300 UTC and 1800 UTC
inferred from Figs. 13c and 14b is manifested in Fig. 14a by the increasing height of the
region of maximum relative humidity in the lower troposphere during this time period,
where this region is assumed to indicate the top of the mixed layer.
Further inspection of Fig. 14a reveals a close coincidence between the arrival of
the narrow filament above the fire location (1600–1700 UTC), the downward progression
of the tongue of low relative humidity (1600–1800 UTC), the occurrence of the surfacebased relative humidity minimum (1700–1800 UTC), and the appearance of the relative
humidity maximum at 850 hPa, corresponding to the top of the mixed layer in the
simulated sounding (1800 UTC). This coincidence, along with the evolution of the
simulated relative humidity and wind speed documented in the time series and time–
height cross sections, lend additional evidence in support of: (1) a linkage between the
surface-based relative humidity minimum and a reservoir of dry air in the middle
troposphere via the narrow filament of low relative humidity; (2) the hypothesis that dry,
high-momentum air is transported downward from the middle troposphere to the surface
as the mixed layer deepens during the late morning and early afternoon of 2 June.
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Discussion and conclusions
On 2 June 2002, an abandoned campfire grew into a wildfire in the DTSP in eastcentral NJ. The wildfire burned 1300 acres, forced the closure of the Garden State
Parkway, and caused an estimated $400 000 in property damage. While dead fuels on the
ground due to a late spring frost contributed to the severity of the wildfire, the
meteorological conditions at the time of the fire were conducive to erratic fire behavior
and rapid fire growth. Surface meteorological observations in NJ document an increase
in wind speed, a decrease in mixing ratio, and a wind shift from west-southwesterly to
northwesterly in the vicinity of the wildfire during the morning and early afternoon of 2
June. Upper-air observations, satellite imagery, skew T–log p soundings, and wind
profiler data show the evolution of an upper-level trough and jet streak, a ribbon of dry
air in the middle-to-upper troposphere, a deepening mixed layer, and a deepening layer of
northwesterly winds during this time period. Based on the available observations, we
hypothesize that the documented surface drying and wind variability result from the
downward transport of dry, high-momentum air from the middle troposphere occurring in
conjunction with a deepening mixed layer.
Since the observational record does not resolve the meteorological conditions in
the immediate vicinity of the DTSP wildfire, the MM5 is employed to examine the
meteorological conditions over the northeastern United States with sufficient spatial and
temporal resolution to allow the foregoing hypothesis to be addressed. The simulation
produces an increase in surface wind speed and a reduction in surface relative humidity at
the wildfire location, and reveals a broad area of extremely low relative humidity values
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at 700 hPa extending westward from southern NY through NJ and PA. Vertical profiles
of temperature, dewpoint, and wind at the fire location highlight the simulated mixed
layer structure and the conditions in the overlying dry layer at 1800 UTC 2 June, the time
when the DTSP wildfire was declared a “major fire.” Northwest–southeast-oriented
vertical cross sections reveal a downward-directed tongue of low relative humidity values
immediately to the northwest of the fire location at 1700 UTC and directly above the fire
location at 1800 UTC. At both of these times, a narrow filament of low relative humidity
coincides with a region of localized subsidence above a surface-based relative humidity
minimum at the fire location. Time series at the fire location of simulated surface relative
humidity, surface wind speed, and PBL depth reveal a decrease in surface relative
humidity and an increase in surface wind speed coinciding with PBL growth through the
morning and early afternoon. Time–height cross sections at the fire location of simulated
relative humidity and wind speed document the evolution of the surface-based relative
humidity minimum, the tongue and narrow filament of low relative humidity aloft, the
increase in surface wind speed, and the downward development of a wind speed
maximum accompanying an increase in wind speed within the mixed layer. The
simulation results lend additional evidence to support a linkage between the surfacebased relative humidity minimum and a reservoir of dry air aloft, and the hypothesis that
dry, high-momentum air aloft is transported to the surface as the mixed layer deepens
during the late morning and early afternoon of 2 June.
At the time of the DTSP wildfire, the operational fire-weather products available
to the NJFFS consisted of fire-weather forecasts prepared by the Mount Holly, NJ, NWS
Forecast Office. The NWS fire-weather forecast, as one of the tools available to the
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firefighters, contributed to the ability of the NJFFS to plan and execute firefighting
activities effectively and helped prevent loss of life and catastrophic loss of property.
Routine NWS fire-weather forecasts include near-surface meteorological quantities, such
as surface wind speed, relative humidity, and temperature, and above-ground quantities,
such as the Haines Index (HI), mixing height, transport wind speed (the mean wind speed
within the mixed layer), and the Ventilation Index (VI, the mixed-layer depth multiplied
by the transport wind speed; e.g. Hardy et al. 2001). The observational evidence and
mesoscale simulation presented in this study of the DTSP wildfire suggest that a linkage
between middle-tropospheric, dry, high-momentum air and dry air at the surface results
in meteorological conditions conducive to erratic fire behavior and rapid fire growth. It
is instructive, therefore, to examine the simulated HI (Fig. 15a) and VI (Fig. 15b) at 1800
UTC 2 June to determine whether these indices, which are routinely available to fireweather forecasters and firefighters, reveal the potential for this linkage during the DTSP
wildfire. Simulated HI values of 4 and 5, which correspond respectively to “moderate”
and “high” potential for fires to become large or exhibit erratic fire behavior (Haines
1988), appear over NJ, eastern PA, Delaware, Maryland, and northern Virginia.
However, the HI distribution indicates an elevated potential for erratic fire behavior and
rapid fire growth over a broad area and does not exhibit sensitivity to the mesoscale
features aloft described in this study (e.g. those characterized by low relative humidity in
the vicinity of the fire location that are reviewed earlier in this section). Similarly, the
simulated VI (Fig. 15b) does not differentiate between high values in central and southern
NJ and high values over other parts of the northeastern United States.
20
Whereas the HI and the VI are sensitive to variations in mixed-layer depth and
atmospheric quantities within the mixed layer, the meteorological conditions documented
in this study occur in a layer extending from the surface to the middle troposphere, which
points to why these fire-weather indices do not highlight the potential for erratic fire
behavior and rapid fire growth at the DTSP wildfire location. Also, since the VI is
designed and typically employed as a smoke dispersion index, high VI values over a
certain location are not routinely interpreted by fire-weather forecasters or firefighters as
an indicator of extreme fire behavior. These properties of the HI and the VI suggest that
new fire-weather indices sensitive to mesoscale meteorological conditions aloft could
provide early warning of atmospheric conditions conducive to extreme fire behavior.
The present study, along with case studies by Mills and Pendlebury (2003), Mills (2005a,
2005b), and Zimet et al. (2007), employ a combination of observations and simulations
of the meteorological environments associated with large wildfires. These studies are
leading to the better understanding, improved diagnosis, and enhanced prediction of fireatmosphere interactions. These advances provide a framework for the formulation and
design of a new generation of fire-weather indices and diagnostics capable of identifying
specific locations where atmospheric conditions are anticipated to be conducive to
extreme fire behavior.
21
Acknowledgements
This research was supported by Research Joint Venture Agreement 03-JV11231300-101 between the USDA Forest Service, Northern Research Station, and the
University at Albany, State University of New York. The authors thank Xindi Bian and
Lesley Fusina for their help in preparing mesoscale simulations of the DTSP wildfire
event. Horace Somes and John Hom were instrumental in obtaining the NJFFS (2003)
fire report. The aerial photograph of the fire reproduced in Fig. 2 was provided by Bert
Plante of the NJFFS.
22
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24
Table captions
Table 1. Sequence of events during the Double Trouble State Park wildfire from
1415 UTC to 2148 UTC 2 June 2002 [adapted from NJFFS (2003)]
Table 2. Spot forecast issued by the Mount Holly, NJ, NWS Forecast Office for the
period from 1900 UTC 2 June 2002 to 1000 UTC 3 June 2002
Figure captions
Fig. 1. Visible satellite imagery showing (a) the state of NJ and (b) the Double Trouble
State Park. The fire icon indicates the approximate location of the origin of the wildfire.
The locations of Upton, NY (OKX), New Brunswick, NJ, McGuire Air Force Base, NJ
(WRI), and Atlantic City, NJ (ACY), are also indicated in (a).
Fig. 2. Aerial photograph showing the fire scar due to the Double Trouble State Park
wildfire.
Fig. 3. Surface analyses of potential temperature (contour interval 4°C, solid), mixing
ratio (contour interval 5 g kg−1, dashed, shaded as indicated in legend), and wind (full
barb 5 m s−1) valid at (a) 1200 UTC 2 June 2002, (b) 1800 UTC 2 June 2002, and (c)
0000 UTC 3 June 2002.
Fig. 4. Upper-air analyses of (a) geopotential height (contour interval 30 m, solid),
temperature (°C, color shaded as indicated in legend), and wind (maximum vector 25 m
s−1) at 1200 UTC 2 June 2002 at 850 hPa, (b) geopotential height (contour interval 120
m, solid), wind speed (m s−1, color shaded as indicated in legend), and wind (maximum
vector 75 m s−1) at 1200 UTC 2 June 2002 at 300 hPa, (c) as in (a) except for 0000 UTC
3 June 2002, and (d) as in (b) except for 0000 UTC 3 June 2002.
Fig. 5. GOES water vapor images valid at (a) 1215 UTC 2 June 2002 and (b) 0015 UTC
3 June 2002; (c) MODIS visible satellite image valid at 1558–1611 UTC 2 June 2002.
Fig. 6. Skew T–log p atmospheric soundings at Upton, NY (OKX), valid at (a) 1200
UTC 2 June 2002 and (b) 0000 UTC 3 June 2002. Adapted from the University of
Wyoming weather web page (http://weather.uwyo.edu/upperair/sounding.html).
Fig. 7. Wind profiler observations at New Brunswick, NJ, from 1100 UTC to 2100 UTC
2 June 2002.
Fig. 8. Surface meteograms from 0000 UTC to 2300 UTC 2 June 2002 for (a) McGuire
Air Force Base, NJ (WRI), and (b) Atlantic City, NJ (ACY). Adapted from the Plymouth
State Weather Center web page (http://vortex.plymouth.edu/statlog-u.html).
25
Fig. 9. Simulated surface relative humidity (%, color shaded as indicated in legend) and
surface wind (full barb 5 m s−1) valid at 1800 UTC 2 June 2002.
Fig. 10. Simulated relative humidity (%, color shaded as indicated in legend) at 700 hPa
valid at 1800 UTC 2 June 2002. The fire icon indicates the fire location and the thick
black line shows the orientation of the vertical cross section in Fig. 12.
Fig. 11. Simulated skew T–log p atmospheric sounding at the fire location valid at 1800
UTC 2 June 2002. The fire location is indicated by the fire icon in Fig. 10.
Fig. 12. Northwest–southeast-oriented vertical cross section of simulated relative
humidity (%, color shaded as indicated in legend) and pressure-coordinate vertical
velocity (contour interval 10 dPa s−1, solid, starting at 10 dPa s−1) valid at (a) 1700 UTC 2
June 2002 and (b) 1800 UTC 2 June 2002. The fire location is indicated by the fire icon
at 208 km on the abscissa of the cross section. The location of the cross section is
indicated by the thick black line in Fig. 10.
Fig. 13. Time series at the fire location valid from 1200 UTC 2 June 2002 to 0000 UTC
3 June 2002 of simulated (a) surface relative humidity (%), (b) surface wind speed (m
s−1), and (c) PBL depth (m). The fire location is indicated by the fire icon in Fig. 10.
Fig. 14. Time–height cross section at the fire location valid from 1200 UTC 2 June 2002
to 0000 UTC 3 June 2002 of simulated (a) relative humidity (%, color shaded as
indicated in legend) and (b) wind speed (m s−1, color shaded as indicated in legend). The
fire location is indicated by the fire icon in Fig. 10.
Fig. 15. Simulated (a) Haines Index (by category, color shaded as indicated in legend)
and (b) Ventilation Index (m2 s−1, color shaded as indicated in legend) valid at 1800 UTC
2 June 2002.
26
Time (UTC)
1415
1700
1709
1714
1725
1726
1731
1735
1747
1751
1800
1801
1808
1818
1823
1851
1857
1936
1937
1938
1953
1959
2001
2004
2010
2024
2055
2113
2136
2148
Activity reported
Lakewood fire tower weather: wind west at 4 m s−1, gusting to 9 m s−1;
temperature 24°C; relative humidity 62%
Estimated start time of fire
Fire reported by Cedar Bridge fire tower
Direct attack on fire by NJFFS firefighters begins
Lakewood fire tower reports winds greater than 18 m s−1; direct attack
abandoned
Backfiring operations begin north and east of the fire
Request submitted for Garden State Parkway to be closed
Backfiring operations begin south of the fire
Request submitted for aerial support for fighting the fire
Fire jumps Double Trouble Road and approaches the Garden State
Parkway
Fire is officially declared to be a major fire
Fire has crossed the Garden State Parkway
First report of a house being burned
Wind shift reported on the fire line
Fire crews prepare for structure protection
Wind shift to the north reported; former right flank of the fire becomes
the head fire
Lakewood fire tower reports winds north at 16 m s−1
Big wind shift reported on the fire line
Lakewood fire tower reports winds shifting to the east, northeast
Right flank becomes head fire
Fire has been diverted south of the line of homes located just east of the
Garden State Parkway
Wind shift reported on fire line; electric lines down on roadway
House on fire
Evacuation order issued for homes in the area
Wind shift reported on fire line
Wind shift reported on fire line; wind shift causes fire to spread rapidly
towards the south directly towards a crew
East flank of fire reported to be growing; fire crews respond to quell
Fire west of the Garden State Parkway reported to be contained
Fire declared to be under control
Lakewood fire tower reports winds diminishing to less than 9 m s−1
Table 1. Sequence of events during the Double Trouble State Park wildfire from
1415 UTC to 2148 UTC 2 June 2002 [adapted from NJFFS (2003)]
27
Time
(UTC)
1900
2100
2300
0100
0400
0700
1000
Temperature
(°C)
28
27
25
22
17
14
13
Relative
humidity (%)
28
27
30
35
45
55
65
Wind direction and
speed (m s−1)
WNW 7 GUST 13
WNW 7 GUST 12
NW 6 GUST 10
NW 5 GUST 8
NW 4
NNW 2
NNW 2
Table 2. Spot forecast issued by the Mount Holly, NJ, NWS Forecast Office for the
period from 1900 UTC 2 June 2002 to 1000 UTC 3 June 2002
28