The sensitivity of fire-behavior and smoke-dispersion indices to the

1. The sensitivity of fire-behavior and smoke-dispersion indices to the diagnosed mixed-layer depth Joseph J. Charney USDA Forest Service, Northern Research Station, East Lansing, MI and Daniel Keyser Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, NY

2. Organization • Background • Objective • Double Trouble State Park (DTSP) Wildfire Event • WRF Model Configuration • Indices and Diagnostics • Results • Conclusions

3. Background The goal of this project is to diagnose the spatial and temporal variability of meteorological quantities in the planetary boundary layer that can affect fire behavior and smoke dispersion. Meteorologists and fire and smoke managers are currently debating the manner in which the mixed-layer depth is and should be diagnosed.

4. Background While fire-behavior and smoke-dispersion indices are sensitive mixed-layer depth (MLD) diagnostics, the potential for sensitivities in the indices to affect fire- and smoke-management decisions is not well-understood. A quantitative assessment of these sensitivities can help inform the ongoing discussions and enable fire and smoke managers to anticipate whether the implementation of a given MLD diagnostic could affect their ability to fulfill burn program requirements.

5. Objective • We will assess the sensitivity of a fire-behavior index and a smoke-dispersion index to three MLD diagnostics, using mesoscale model simulations of the 2 June 2002 DTSP wildfire event. • Indices: • fire-behavior index: Downdraft Convective Available Potential Energy (DCAPE) • smoke-dispersion index: Ventilation Index (VI) • MLD diagnostics: • surface CAPE • potential temperature(z) = potential temperature(sfc) • potential temperature(z)=potential temperature(z/2) • z = height above ground level

6. DTSP Wildfire Event • Occurred on 2 June 2002 in east-central NJ • Abandoned campfire grew into major wildfire by 1800 UTC • Burned 1,300 acres • Forced closure of the Garden State Parkway • Damaged or destroyed 36 homes and outbuildings • Directly threatened over 200 homes • Forced evacuation of 500 homes • Caused ~$400,000 in property damage • References:  • Charney, J. J., and D. Keyser, 2010: Mesoscale model simulation of the meteorological conditions during the 2 June 2002 Double Trouble State Park wildfire. Int. J. Wildland Fire, 19, 427–448. • Kaplan, M. L., C. Huang, Y. L. Lin, and J. J. Charney, 2008:  The development of extremely dry surface air due to vertical exchanges under the exit region of a jet streak.  Meteor. Atmos. Phys., 102, 3–85.

7. DTSP Wildfire Event "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." (Charney and Keyser 2010)

8. WRF Model Configuration • WRF version 3.4 • 4 km nested grid • 51 sigma levels, with 21 levels in the lowest 2000 m • NARR data used for initial and boundary conditions • Noah land-surface model • RRTM radiation scheme • YSU PBL scheme

9. Indices and Diagnostics DCAPE • DCAPE calculation: • choose a starting level for the parcel • saturate the parcel • bring the parcel to the surface while maintaining saturation • evaluate the negative buoyancy of the parcel as it passes the “level of free sink” and reaches the surface or the level of neutral buoyancy • The integrated energy of the negative buoyancy when the parcel reaches the surface is DCAPE. • For the starting level: • Potter (2005) proposes 3000 m • We choose the top of the MLD

10. Indices and Diagnostics DCAPE DCAPE was originally formulated to estimate the maximum potential strength of evaporatively cooled downdrafts beneath a convective cloud (Emanuel 1994). It has been suggested that DCAPE could be applied to wildland fires (Potter 2005). We hypothesize that in the case of a mixed layer produced by dry convection, large DCAPE may correlate well with low surface relative humidity when the mixed-layer is deep and the top of the mixed layer is dry.

11. Indices and Diagnostics Ventilation Index (VI) Definition: the MLD multiplied by the “transport wind speed” The transport wind speed can be interpreted in several different ways: • mixed-layer average wind speed • surface wind speed (usually 10 m) • 40 m wind speed For the purposes of this study, the mixed-layer averaged wind speed will be used. From Hardy et al. (2001)

12. Indices and Diagnostics • MLD Diagnostics • 1) The MLD is diagnosed by determining the height to which near-surface eddies can rise freely. • A PBL parameterization-independent formulation is used to calculate the parcel exchange potential energy (PEPE) as proposed by Potter (2002). • The lowest height at which PEPE is zero is identified as the top of the surface-based mixed layer.

13. Indices and Diagnostics MLD Diagnostics Dr. Margaret LeMone and coauthors in their presentation at the 12th Annual WRF Users’ Workshop (20–24 June 2011, National Center for Atmospheric Research, Boulder, CO) proposed a number of mixed-layer diagnostics for use with mesoscale model output. 2) The MLD is diagnosed by finding the highest level above the ground where the potential temperatureequals the surface potential temperature: Ɵ mixed layer height height potential temperature

14. Indices and Diagnostics MLD Diagnostics 3) The MLD is diagnosed by finding the highest level above the ground where the potential temperature equals the potential temperature at one-half that distance above the ground: mixed layer height height z/2 z potential temperature

15. Results Time series of MLD1, MLD2, and MLD3:

16. Results Time series of DCAPE using MLD1, MLD2, and MLD3

17. Results Time series of VI using MLD1, MLD2, and MLD3

18. Results Correlation of DCAPE with surface and near-surface moisture variables from 1200 UTC to 2100 UTC 2 June 2002.

19. Results Time-height cross section of RH with DCAPE time series and correlations

20. Lemone1 MLD

21. Lemone2 MLD

22. Conclusions