Assessment of the vertical exchange of heat, moisture, and momentum

above a wildland fire using observations and mesoscale

simulations

Joseph J. CharneyUSDA Forest Service, Northern Research Station, East Lansing, MI

Michael T. KieferDepartment of Geography, Michigan State University

Daniel KeyserDepartment of Atmospheric and Environmental Sciences, University at Albany

1. Background

2. Observed meteorological conditions

3. WRF simulation

4. ARPS simulation

5. Discussion and synthesis

Organization

Fire weather is primarily concerned with surface meteorological conditions

• wind speed and direction

• humidity (usually relative humidity)

• temperature

Background

Vertical exchanges of heat, moisture, and momentum affect and are affected by the fire

At large spatial scales (~ 1–10km), mesoscale processes can be associated with variations in surface winds and humidity that affect the evolution of the fire

At fine spatial scales (~ 0.1–100 m), the combustion process produces heat, moisture, super-heated gasses, and solid material (smoke) that perturb the environment in the vicinity of the fire

Background

How can we use NWP models to better understand and anticipate these effects?

Case studies of fire events

Investigate the processes associated with the observed fire behavior and atmospheric perturbations

Develop indices and diagnostics that highlight the relevant processes for fire weather forecasters and fire managers

Background

Double Trouble State Park Wildfire• June 02, 2002 fire in south-central NJ• A campfire abandoned the night before turned into a rapidly spreading wildfire at 1800 UTC• Burned 1,300 acres• Forced the closure of the Garden State Parkway• Damaged or destroyed 36 homes and outbuildings• Directly threatened over 200 homes• Forced the evacuation of 500 homes• Caused an estimated $400,000 in property damage

Double Trouble Observations

Double Trouble Observations

Double Trouble Observations

MODIS visible satellite imagery

Double Trouble Observations

Meteogram from KWRI (McGuire AFB)

Double Trouble Observations

New Brunswick wind profiler

WRF3.1 model simulation of the meteorological conditions associated with the fire

Examine, at the fire location, the surface and vertical distribution of:

temperaturemoisturemomentum

Link the local evolution of these quantities to mesoscale processes

04km simulation, 51 vertical levels, initialized with NARR, MYJ PBL, NOAH LSM, RRTM radiation

Double Trouble WRF Simulation

Relative Humidity (%)

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WRF3.1 MYJ

OBS - McGuire AFB

Surface Wind Speed (m/s)

0.001.002.003.004.005.006.007.008.009.00

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WRF3.1 MYJ

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Double Trouble WRF Simulation

Simulated skew-T log-P at the fire location at 1800 UTC

Double Trouble WRF Simulation

Simulated 700hPa relative humidity at 1800 UTC

Double Trouble WRF Simulation

Double Trouble WRF Simulation

Simulated vertical cross section of relative humidity (color shaded) and vertical velocity (contours) at 1700 UTC

Simulated vertical cross section of relative humidity (color shaded) and vertical velocity (contours) at 1800 UTC

Double Trouble WRF Simulation

ARPS model simulation using 1800 UTC potential temperature and wind speed profiles from the WRF as a base state

A prescribed 28.8 kW/m2 surface heat flux parameterizes the heat released by the fire

50m horizontal grid spacing

Stretched vertical grid to 9 km with 2.5 m separation at the surface

120 minute simulation

Idealized 2-D ARPS simulation

Kiefer et al. (2009) employ the ARPS model to investigate the impact of the environmental wind profile on dry convection above a prescribed heat source

Identified two primary modes of convection:

intense plumemulti-cell

Idealized 2-D ARPS simulation

Double Trouble Fire

Surface wind speed (buoyancy imparted by the fire)

Mixed-layer average wind speed

(advection of updrafts away from the fire)

Idealized 2-D ARPS simulation

white squares = intense plume

grey squares = multi-cell

Idealized 2-D ARPS simulation

wind speed potential temperature

Base state profiles

Idealized 2-D ARPS simulation

perturbation potential temperature (shaded) and vertical velocity (contours)

Idealized 2-D ARPS simulation

total horizontal wind speed (shaded) and vertical velocity (contours)

Discussion and Synthesis

The two models simulate fire-atmosphere interactions on different scales

WRF: tongue of dry air aloft connects with dry air at the ground coincident with the time when the fire exhibited rapid growth and erratic fire behavior

ARPS: the prescribed heat source induces convective perturbations within the mixed layer and across the boundary between the mixed layer and the free atmosphere

The simulations demonstrate how the potential for vertical exchanges of heat, moisture, and momentum above a wildland fire can be diagnosed

Short-term (~0.5 to 2 hours) fluctuations in critical fire-weather ingredients are important to users of fire-weather and fire-behavior forecasts

This study addresses the inherent difficulties in observing and simulating these ingredients on the fine spatial and temporal scales that are important for wildland fire management decisions

Discussion and Synthesis