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The diagnosis of mixed-layer characteristics and their relationship to meteorological conditions above eastern U.S. wildland fires Joseph J. Charney USDA Forest Service, Northern Research Station, East Lansing, MI and Daniel Keyser
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Joseph J. Charney
USDA Forest Service, Northern Research Station, East Lansing, MI
Department of Atmospheric and Environmental Sciences, University at Albany, Albany, NY
Our overarching investigation concerns diagnosing the presence of dry air aloft and the potential for this dry air to be transported to the surface such that it can influence a fire.
This presentation focuses on atmospheric mixed layer and its potential importance for diagnostics that attempt to detect the influence of dry air aloft on a fire.
We are investigating methodologies by which the potential for dry air in the lower troposphere to modify surface conditions over time scales of a few hours can be detected using mesoscale model simulations.
"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."
"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."
(from Charney and Keyser, 2010)
Examine the impact of the differences between the mixed-layer depth and the PBL depth on a selection of fire-weather indices to assess the sensitivity of the indices to the difference between the diagnosed mixed-layer depth and parameterized PBL depths.
Determine whether DCAPE diagnoses the potential for low relative humidity and high winds at the surface as the MLD increases in a mesoscale simulation of the DTSP wildfire.
(from Potter 2002)
Ventilation Index (VI)
Definition: the MLD multiplied by the “transport wind speed”
Can be calculated using either the MLD or the PBLD from a mesoscale model.
The transport wind speed can be interpreted in several different ways
• surface (usually 10 m) wind speed
• 40 m wind speed
• mixed-layer average wind speed
For the purposes of this study, the mixed-layer averaged wind speed will be used.
Downdraft Convective Available Potential Energy (DCAPE)
DCAPE was originally formulated to estimate the potential strength of rain and evaporatively cooled downdrafts beneath a precipitating convective cloud (Emanuel 1994)
However, it has been suggested that the quantity could be applied to wildland fires (Potter 2005).
“…in the case of a mixed layer produced by dry convection, large DCAPE may correlate well with large surface dewpoint depression (i.e., low surface relative humidity) and surface gustiness when the mixed-layer is deep and the top of the mixed layer is dry.”
(from Brian Potter's DCAPE web page: http://airfire.org/firewx/dcape )
DCAPE is calculated by choosing a starting level for an atmospheric parcel. The parcel is then saturated with water vapor, and kept saturated as it descends to the ground. The downward buoyancy of the parcel when it reaches the surface is DCAPE.
DCAPE strongly depends on the starting point for the “source air” and on how dry the air is below that starting point.
Potter (2004) recommended that a starting level of 3000 m be used.
For the purposes of this investigation, we chose the MLD/PBLD as a starting point.
We will compare the DCAPE results from these three calculation (3000 m, MLD, and PBLD) for the DTSP wildfire case.
For the DTSP event, temporal variations in the VI appear to be more strongly dependant upon variations in the MLD/PBLD than on variations in wind speed within the near-surface layer.
The YSU scheme produces PBLDs that are very similar to the MLD for this event. The MYJ, however, exhibits more substantial differences. YSU is the operational WRF scheme while MYJ is often used for research purposes. When using the MYJ scheme, the difference between the MLD and the PBLD should be taken into account.
DCAPE is able to diagnose conditions wherein dry air aloft is transported to the surface as the mixed layer deepens.
A precise determination of the MLD is vitally important for getting the numbers right for fire and smoke indices. In mesoscale models, there are a number of sensitivities that need to be considered, including vertical level distribution, surface layer characteristics, entrainment at the top of the PBL, etc.
DCAPE shows some promise as a mixed-layerdry air diagnostic. Could it also be used to address entrainment of dry air at the top of the mixed layer, and the potential for this dry air to impact the surface conditions? Additional DCAPE formulations, such as starting the DCAPE parcel at the MLD + 500 m (for example), should be investigated.