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 Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, NY
Organization • Background • Double Trouble State Park (DTSP) wildfire event • WRF model configuration • Ingredients and indices • Results • Summary and future work
Background Overall goals: formulate diagnostics identify dry air in the lower troposphere diagnose processes that could transport this dry air to the surface This presentation: mixed-layer characteristics dry air aloft that modifies surface conditions time scales of a few hours mesoscale model simulations
Objectives • Using mesoscale model simulations of the 2 June 2002 Double Trouble State Park (DTSP) wildfire event, we will: • examine the Ventilation Index (VI) to assess whether the index is sensitive to differences between the mixed-layer depth (MLD) and PBL depth (PBLD); • determine whether Downdraft Convective Available Potential Energy (DCAPE) can diagnose the potential for low relative humidity to occur at the surface.
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. and Atmos. Phys., 102, 3–85.
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)
WRF Model Configuration • WRF version 3.1 • 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 and MYJ PBL schemes
Ingredients and Indices • Fire weather ingredients • wind speed • humidity (RH, mixing ratio) • temperature • Meteorological variables • mixed layer depth (MLD) • PBL depth (PBLD) (a parameter from the mesoscale model) • In a well-mixed boundary layer, the MLD and the PBLD would be expected to be similar. (Potter 2002)
Ingredients and Indices Ventilation Index (VI) Definition: the MLD multiplied by the “transport wind speed” The VI can be calculated from mesoscale model data using either the MLD or the PBLD. 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.
Results – VI We will now turn our attention to time series, at the fire location, of components of the VI: MLD MLD-average wind speed PBLD PBLD-average wind speed Note: the fire started to exhibit rapid spread between 1700 and 1800 UTC.
Results – VI The YSU simulation generally produces MLDs and PBLDs that are higher than those in the MYJ simulation. YSU MLDs and PBLDs track quite closely to each other, while MYJ MLDs and PBLDs differ more.
Results – VI The YSU VIs are higher than the MYJ VIs wherever the MLDs/PBLDs are higher. The apparent dependence of the VI on average wind speed is weaker.
Ingredients and Indices DCAPE DCAPE was originally formulated to estimate the potential strength of evaporatively cooled downdrafts beneath a convective cloud (Emanuel 1994). It has been suggested that the quantity 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.
Ingredients and Indices DCAPE DCAPE is calculated by: choosing a starting level for the “source air”; saturating that air with respect to water vapor; bringing the parcel to the surface while maintaining saturation; calculating the negative buoyancy of the parcel when it reaches the surface (or level of neutral buoyancy). The energy of the negative buoyancy at the surface is DCAPE. • For the starting point: • Potter (2004) proposes that a starting level of 3000 m be used; • we choose the top of the MLD as a starting point. • First, examine time series of simulated 3000 m and MLD DCAPE at the fire location using the YSU PBL scheme.
Results – DCAPE • The 3000 m DCAPE and the MLD DCAPE show similar evolutions in that they both reach a maximum at 1700 UTC. • The MLD DCAPE is more sensitive to the diurnal cycle, and exhibits a higher peak at the time when low relative humidity occurred at the surface during the DTSP wildfire.
Results – DCAPE We now examine an animation of horizontal plots of simulated MLD DCAPE using the YSU PBL scheme from 1300 UTC through 1800 UTC.
Results – DCAPE The horizontal plots of DCAPE suggest that south-central NJ and the Delmarva Peninsula might have been expected to experience anomalously low surface relative humidity as the mixed layer deepened between 1300 and 1800 UTC on 2 June 2002. (Charney and Keyser 2010)
Discussion and Future Work 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-layer dry 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.