Mesoscale model simulation of the meteorological conditions during the
2 June 2002 Double Trouble State Park wildfire
Joseph J. Charney A,C and Daniel Keyser B
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A
USDA Forest Service, 1407 S. Harrison Road, Room 220, East Lansing, MI 48823,
USA.
B
Department of Atmospheric and Environmental 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 (526 ha) 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 high-resolution 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 UTC1 (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
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meteorological conditions contributed to erratic fire behavior and rapid fire growth.
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 (526 ha) (Fig. 2), forced the closure of the Garden State
Parkway, damaged or destroyed 36 homes and outbuildings, directly threatened over 200
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UTC = EDT + 4 h, where EDT (Eastern Daylight Time) corresponds to local time at the fire location.
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homes, forced the evacuation of 500 homes, and caused an estimated US$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,
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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
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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
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firefighters were forced to abandon their initial attack on the wildfire. These observations
highlight a profound change in surface meteorological conditions that occurred 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 land-
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surface features, the development of the moisture and wind anomalies is hypothesized to
result from a local drying process at the ground and aloft.
Kaplan et al. (2008) employ high-resolution numerical weather prediction (NWP)
model simulations to diagnose and analyze multiscale interactions among dynamical
processes that culminated in meteorological conditions conducive to the outbreak and
rapid growth of the DTSP wildfire. The results of the simulations are used to formulate a
multistage conceptual model describing the sequence of meteorological events and
processes that led to pronounced surface drying prior to the occurrence of the wildfire.
Our paper focuses on the diagnostic capabilities of the available observations and a
mesoscale NWP model to establish connections between atmospheric phenomena and
processes aloft and at the surface that may have impacted the evolution of the DTSP
wildfire. We do not attempt to simulate the evolution of the wildfire itself. Instead, we
utilize available observations in conjunction with a mesoscale NWP model simulation to
examine the meteorological conditions that contribute to the observed fire behavior in the
case of the DTSP wildfire. In the next section we describe the mesoscale NWP model
employed to study the wildfire. Subsequent 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, including potential implications of the
results of this study for real-time fire-weather forecasting.
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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
(MM5) (Grell et al. 1995). The MM5 is a nonhydrostatic, primitive-equation, mesoscale
model that uses a terrain-following pressure-coordinate (i.e., sigma) system. The model
includes 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
(Black 1994) for initial and boundary conditions. Our preliminary 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
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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 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 Palmer Drought Severity Index
for the week ending 1 June 2002 indicates a long-term drought at the “severe” level in
central and southern New Jersey (National Oceanic and Atmospheric
Administration/Climate Prediction Center,
http://www.cpc.noaa.gov/products/monitoring_and_data/drought.shtml). 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
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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
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 surface
analysis 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 west and
north, a wind shift extending from southwestern PA through northeastern New York
(NY) 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 extending from southeastern
Virginia (VA) northeastward off the Mid-Atlantic coast. Wind speeds have increased
from 5 m s −1 at 1200 UTC to 13 m s −1 at 1800 UTC in southern NJ. By 0000 UTC (Fig.
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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
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and NJ between 1200 UTC and 1800 UTC, consistent with the wind shift and
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
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higher mixing ratios evident immediately to the west and northwest at both times. The
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presence of a mixing ratio minimum over central and southern NJ at 1800 UTC suggests
that a local drying process augmented 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
and a sharp temperature gradient extends northeastward through NY 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 shift in surface wind direction from west-southwesterly to
northwesterly that occurs over 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 advancing 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
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middle-to-upper troposphere moves over 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).
A skew T–log p sounding from 1200 UTC 2 June at Upton, NY (OKX) (Fig. 6a),
suggests that a shallow surface-based mixed layer is forming 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,
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a mixed layer extends to 550 hPa with west-southwesterly winds of 18 m s −1 at the
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bottom of the layer, shifting to west-northwesterly winds of 36 m s −1 at the top. Above
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550 hPa, the air is considerably drier while wind speeds vary between 36 m s −1 at 550
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hPa and 46 m s −1 at the thermodynamic tropopause, which is located just above 200 hPa.
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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,
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wind speeds increase with height, from 21 m s −1 at 750 hPa to 59 m s −1 at 350 hPa.
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Notably, the soundings also reveal that dry, high-momentum air has descended from just
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above 550 hPa to 750 hPa between 1200 UTC and 0000 UTC. The sharp contrast
between surface-based mixed-layer 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 winds at the lowest level of the profiler record (~200 m) of 5 m s −1 at 1200
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UTC veer to northwesterly by 1500 UTC and remain northwesterly through 2100 UTC.
Wind speeds at this level increase to 15 m s −1 at 1600 UTC, and then vary between 10 m
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s −1 and 15 m s −1 for the remainder of the profiler record. This sequence of wind
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observations coincides with the wind shift over eastern PA and NJ shown in the surface
analyses (Figs. 3a–c). A layer of northwesterly winds from the lowest indicated level to
~1000 m appears by 1500 UTC and subsequently deepens to ~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
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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
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
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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
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
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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
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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 drying process could be playing a role in the development of these
minima.
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
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to 11 m s −1 between the surface at 1001 hPa and the top of the mixed layer at 850 hPa,
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and from 18 m s −1 to 39 m s −1 in the overlying dry layer between 780 hPa and 510 hPa.
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A northwest–southeast-oriented vertical cross section passing through the fire
location of simulated relative humidity and pressure-coordinate vertical velocity at hourly
intervals between 1500 UTC and 1800 UTC 2 June is shown in Figs. 12a–d. The crosssectional perspective extends the depiction of the lower-to-middle troposphere provided
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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 time interval between 1500 UTC and 1800 UTC corresponds to the appearance and
deepening of the layer of northwesterly winds documented in the profiler record (Fig. 7)
and to surface drying observed at WRI and ACY (Figs. 8a,b); the respective times shown
in Figs. 12c,d 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 the cross section as a downward-directed tongue of relative
humidity values less than 10% extending below 800 hPa northwest of the fire location at
1500 UTC and 1600 UTC (Figs. 12a,b), immediately to the northwest of the fire location
at 1700 UTC (Fig. 12c), and directly above the fire location at 1800 UTC (Fig. 12d).
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 1600 UTC and
1700 UTC (Figs. 12b,c), a narrow filament protrudes downward from the overlying
tongue of low relative humidity, reaching 900 hPa directly above the fire location at 1600
UTC and 930 hPa between 40 km and 50 km southeast of the fire location at 1700 UTC.
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. 12d), the filament has lowered to 950 hPa, consistent with
the signature of subsidence evident at 1600 UTC and 1700 UTC (Fig. 12b,c), and has
progressed towards the southeast but still appears to retain a connection to the surfacebased relative humidity minimum at the fire location. The filament continues to coincide
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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
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. 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
simulation (Fig. 13c). Nevertheless, the increase in the PBL depth between 1200 UTC
and 1800 UTC is accompanied by an increase in the mixed-layer depth, inferred visually
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from a sequence of simulated soundings during this time interval (not shown), 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 directly above and southeast of the fire location, all of which appear in the
vertical cross sections between 1600 UTC and 1800 UTC (Figs. 12b–d), are evident in
Fig. 14a during this time 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 wind speed at 1000 hPa 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 appearance
of the narrow filament directly above the fire location (1600 UTC), the downward
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progression of the tongue of low relative humidity (1600–1800 UTC), the occurrence of
the surface-based relative humidity minimum (1700–1800 UTC), and the presence of a
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.
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 (526 ha), forced the closure of the Garden
State Parkway, and caused an estimated US$400 000 in property damage. While dead
and dry fuels 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 during the morning and early afternoon of 2 June. Upper-air analyses,
satellite imagery, skew T–log p soundings, and wind profiler observations show the
evolution of an upper-level trough and jet streak, a ribbon of dry air in the middle-to-
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upper troposphere, a deepening mixed layer, and a deepening layer of northwesterly
winds during this time period. Based on the available observational evidence, 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
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.” A northwest–southeast-oriented
vertical cross section reveals a downward-directed tongue of low relative humidity values
immediately to the northwest of the fire location at 1700 UTC, the estimated start time of
the fire, 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
19
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 lends additional evidence to support a
linkage between the surface-based 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
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 dry, high-momentum air in the middle troposphere 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
20
available to fire-weather 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 VA. 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.
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
may point 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 and firefighters
as an indicator of 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 meteorological conditions at the surface conducive to erratic fire
behavior and rapid fire growth. The present study, along with case studies by Mills and
Pendlebury (2003), Mills (2005a, 2005b), and Zimet et al. (2007), employ a combination
21
of observations and simulations of the meteorological environments associated with large
wildfires, and are leading to the better understanding, improved diagnosis, and enhanced
prediction of fire–atmosphere interactions. These advances provide a pathway for the
formulation and design of a new generation of fire-weather indices and diagnostics
capable of identifying specific locations where meteorological conditions are anticipated
to be conducive to erratic fire behavior and rapid fire growth.
22
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.
23
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25
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 (value indicated in g kg−1 at station location; 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. Adapted from surface
analyses generated and archived in the Department of Atmospheric and Environmental
Sciences, University at Albany, State University of New York.
Fig. 4. NARR 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.
P
P
P
P
P
P
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 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 ).
TU
UT
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 ).
TU
UT
26
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.
P
P
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 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) 1500 UTC 2
June 2002, (b) 1600 UTC 2 June 2002, (c) 1700 UTC 2 June 2002, and (d) 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.
P
P
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.
P
P
Fig. 15. Simulated (a) Haines Index (by category, color shaded as indicated in legend)
and (b) Ventilation Index (m 2 s −1 , color shaded as indicated in legend) valid at 1800 UTC
2 June 2002.
P
P
P
P
27
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
P
P
P
P
P
P
P
P
P
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)]
28
P
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
P
P
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
29
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).
30
Fig. 2. Aerial photograph showing the fire scar due to the Double Trouble State Park
wildfire.
31
Fig. 3. Surface analyses of potential temperature (contour interval 4°C, solid), mixing
ratio (value indicated in g kg−1 at station location; 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. Adapted from surface
analyses generated and archived in the Department of Atmospheric and Environmental
Sciences, University at Albany, State University of New York.
32
Fig. 4. NARR 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.
P
P
P
P
P
P
33
Fig. 4. NARR 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.
P
P
P
P
P
P
34
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.
35
Fig. 6. Skew T–log p 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 ).
TU
UT
36
Fig. 7. Wind profiler observations at New Brunswick, NJ, from 1100 UTC to 2100 UTC
2 June 2002.
37
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 ).
TU
UT
38
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.
P
P
39
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.
40
Fig. 11. Simulated skew T–log p sounding at the fire location valid at 1800 UTC 2 June
2002. The fire location is indicated by the fire icon in Fig. 10.
41
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) 1500 UTC 2
June 2002, (b) 1600 UTC 2 June 2002, (c) 1700 UTC 2 June 2002, and (d) 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.
42
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) 1500 UTC 2
June 2002, (b) 1600 UTC 2 June 2002, (c) 1700 UTC 2 June 2002, and (d) 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.
43
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.
P
P
44
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.
P
45
P
Fig. 15. Simulated (a) Haines Index (by category, color shaded as indicated in legend)
and (b) Ventilation Index (m 2 s −1 , color shaded as indicated in legend) valid at 1800 UTC
2 June 2002.
P
P
P
P
46