Mesoscale model simulations
While the observed weather conditions documented in the previous section
provide evidence that unusually dry and windy conditions developed in southern NJ on
the afternoon of 2 June 2002, the observational record cannot resolve the conditions in
the immediate vicinity of the fire, nor provide sufficient spatial and temporal detail to
associate the observed fire behavior with specific atmospheric phenomena. To address
these shortcomings in the observational record, the MM5, described in the mesoscale
model description section above, is employed with greater spatial and temporal resolution
than is available from the observations. The MM5 provides a fine-scale analysis of the
sequence of events that occurred during the DT wildfire, and helps diagnose the
meteorological conditions at the wildfire location, as well as the features that precede and
accompany the observed weather and fire behavior. All of the model results discussed
below derive from a simulation executed on a 4-km domain covering the northeastern
United States (Domain 4 in Fig. 1 of Zhong et al. 2005). The MM5 is initialized at 1200
UTC 1 June 2002 using NARR data for initial conditions and boundary conditions.
Figure 9 shows the surface winds and relative humidity from the MM5 simulation
valid at 1800 UTC on 2 June. The simulated surface winds reproduce the observed westnorthwesterly and northwesterly flow at 1800 UTC 2 June in eastern PA and NJ (Fig.
3b). Simulated wind speeds are between 5 m s−1 and 10 m s−1, with higher wind speeds
evident across central NJ. The wind shift diagnosed in the observations (Fig. 3a–b) is
present offshore in the simulation at 1800 UTC 2 June, with north-northwesterly flow off
the NJ coast changing to west-northwesterly and westerly farther to the east. The
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simulated surface relative humidity distribution coincides with the observed pattern of
dry air in Fig. 3b, with the driest conditions evident in central and southern NJ and more
moist air present to the north and west. It is noteworthy that isolated areas of extremely
low relative humidity (less than 30% in the simulation) have developed by 1800 UTC in
southern NJ, as well as in Delaware and across Maryland to the south and west. The
local relative humidity minima embedded into the broader-scale low-relative humidity air
across the region suggest that mesoscale processes could be playing a role in the
development of these features.
Simulated relative humidity at 700 hPa valid at 1800 UTC 2 June 2002 (Fig. 10)
reveals a broad area of very dry air extending westward from southern NY through NJ
and PA, with relative humidities below 10% evident in a broad swath across this region.
This 700 hPa swath of low relative humidities establishes the horizontal extent of the dry
air above 750 hPa identified in the 0000 UTC 3 June observed sounding at OKX (Fig.
6b). Simulated skew T–log p soundings at OKX (not shown) reproduce the observed
thermodynamic structure evident at 1200 UTC 2 June in the observations. Specifically,
the simulated thermal profile indicates the presence of a shallow surface-based mixed
layer beneath an inversion between 950 and 900 hPa, a second mixed layer extended into
the middle troposphere, and considerably drier air above the second mixed layer. The
model likewise reproduces the observed light surface winds just above the ground, which
increase in speed with height and become west-northwesterly above 600 hPa. The
greater spatial and temporal resolution of the MM5 allows us to examine the simulated
skew T–log p sounding valid at 1800 UTC 2 June at the wildfire location (39.9°N,
74.1°W) (Fig. 11). The sounding reveals a dry layer that extends from 500 hPa down to
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850 hPa, with the base of the dry layer situated just above the surface-based mixed layer.
Although the overall thermodynamic structure of the MM5 sounding at the fire location
at 1800 UTC 2 June resembles the observed sounding at 0000 UTC 3 June at OKX (Fig.
6b), the simulated sounding suggests that dry air has descended closer to the ground
above the wildfire, with the driest air occurring between 790 and 650 hPa. The profile of
simulated winds at the wildfire location show northwesterly winds in the mixed layer,
transitioning to west-northwesterly winds at 750 hPa. Wind speeds increase from 5 to 10
m s−1 in within the mixed layer, to 15 to 35 m s−1 in the driest layer between 790 and 650
hPa, and 40 to 50 m s−1 in the upper troposphere. The vertical distribution of simulated
winds at 1800 UTC 2 June closely corresponds with the wind profiler observations from
New Brunswick, NJ (Fig. 7), where a layer of northwesterly winds from the surface to
about 2200 m was observed at 1800 UTC.
A northwest-southeast vertical cross section (see Fig. 10 for the cross section
location) of simulated wind speed and relative humidity through the wildfire location at
1700 and 1800 UTC 2 June identifies two locations where 100–200 hPa deep “shafts” of
low relative humidity overlay minimums in surface relative humidity (Fig. 12). At 1700
UTC (Fig. 12a) one of these “shafts” is located directly above the DT wildfire, with the
surface relative humidity minimum appearing directly over the fire location.
Approximately 200 km to the northwest, coincident with the tallest Appalachian peaks in
this cross section, another shaft of locally low relative humidity is in place above a
somewhat weaker surface relative humidity anomaly. The upstream feature appears to be
generated by a combination of orography-induced subsidence at the surface and
downward vertical advection of dry air due to the generation of mountain waves. The dry
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air at the wildfire location, however, is not clearly the result of either of these processes,
although evidence of waves propagating downstream from the mountains to the wildfire
location can be seen in the simulated wind vectors at this time. By 1800 UTC (Fig. 12b),
the dry shaft near the wildfire location has propagated towards the southeast.
Interestingly, a secondary relative humidity minimum has formed at the ground
approximately 20 km southeast of the fire, directly below the location of the 1700 UTC
relative humidity minimum at 900 hPa (Fig. 12a). These analyses strongly suggest that
coupling between the surface air and the pool of dry air aloft is associated with the
formation of these surface-based relative humidity minimums.
Time series from 0000 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) at
the wildfire location reveal that the onset of rapid PBL growth between 1300 and 1400
UTC coincides with a rapid reduction in surface relative humidity and an increase in
surface wind speed. By 1700 UTC, when the initial attack occurred on the DT wildfire
(Table 1), the surface relative humidity reached its minimum value, while wind speeds
continued to climb through the afternoon. Time-height cross sections of simulated
relative humidity (Fig. 14a) and wind speed (Fig. 14b) at the wildfire location show the
vertical distribution of the simulated wind speed and relative humidity throughout the
day. These analyses confirm that the atmosphere 100-200 hPa above the fire location
was both considerably drier and exhibited higher wind speeds (momentum) than at the
ground. During the afternoon, two different episodes of vertical coupling occur, one at
1700 UTC and the other at 2000 UTC. In the first episode, at 1700 UTC, the dominant
effect at the surface is drying, although a modest increase in surface wind speed occurs as
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well. At 2000 UTC, the increase in wind speed is more evident while the drying signal is
weaker. These episodic variations in wind speed in the simulation occur during the same
time period (1800 – 2000 UTC) when the firefighters reported multiple wind shifts while
fighting the fire.
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