Results and Conclusion

1. Observational and theoretical investigations of turbulent structures generated by low-Intensity prescribed fires in forested environments • X. Bian, W. Heilman, J. Charney, J. Hom, K. Clark, N. Skowronski, M. Gallagher, and M. Patterson S. Zhong, M. Kiefer, and N. Duan • USDA Forest Service, East Lansing, MI Michigan State University, East Lansing, MI Introduction Low-intensity prescribed fires (LIPF) can be a viable tool for managing forest ecosystems. However, LIPF may radically modify the atmospheric environment within canopy layers by inducing strong fire-atmosphere interactions. These interactions can lead to intense turbulence production in and around the fire front, and this turbulence effects the dispersion of smoke. As part of a broad Joint Fire Science Program (JFSP) project to develop modeling tools for predicting smoke dispersion from LIPFs, the USDA Forest Service - Northern Research Station conducted two LIPFs in 2011 and 2012 on forested plots in the New Jersey Pine Barrens (NJPB) to collect meteorological and air-quality data that can be used for model validation due to the fire-atmosphere interactions. Experimental investigations of the turbulence mixing structures from the 2011 and 2012 LIPFs have been undertaken to provide further insight into the complex physical processes that govern turbulent energy production, turbulent eddy sizes, and turbulent diffusion of momentum and heat in the vicinity of LIPFs within forest vegetation layers. Using high frequency (10 Hz) measurements of temperatures and wind speeds using fine-wire thermocouples and sonic anemometers at multiple levels during the LIPFs, a quantitative description of LIPF flux-profile relationships are developed, with new information contained in the correlations and turbulence quantities. Characteristic turbulent structures before, during, and after fire-front passage are described through the LIPF flux-profile relationships. Theory A Non-local Turbulent Closure Model (NTCM) for vertical mixing in Fire Induced Convective Boundary Layers (FICBL) has been developed specifically for development and application in micro-scale atmospheric-fire interaction models. The NTCM is based on the concept that vertical mixing/transport within the FICBL is inherently asymmetrical. The non-local upward/downward mixed/transported by buoyant fire-smoke plumes originating in the surface layer is simulated by mixing from the lowest model layer directly to all other layers (Fig 1) in the FICBL, which is similar to the model developed by Blackadar (1978, 4th Symp. on Atmospheric Turbulence, Diffusion and Air Quality, pp. 443–447, Reno, Am. Meteorol. Soc.) Following Holtslag and Moeng (1991 J.A.S: Eddy Diffusivity and Countergradient Transport in Convective Atmospheric Boundary), the vertical tuburlent flux of a quantity can be written as W’C’ = - Kc (dC/dz – Gn) (eqn. 1) Where Kc is local In an attempt to provide more realistic eddy diffusivities in the planetary boundary layer, particularly for daytime conditions, a parameterization based directly on boundary layer Theory. Parameterizations of eddy diffusivities within the PBL are based on various length scales and velocity scales which characterize the turbulence in several distinct regimes. The various turbulent regimes are characterized by two dimensionless parameters, h/L, where h is the FICBL height and L is the Monin~)bukov length, and z/h, where z is the height above ground The surface layer is defined as the bottom 10% of the planetary boundary layer (PBL) where vertical fluxes are nearly constant with height and turbulence is mostly caused by eddy shear stress (Reynolds' stress) between the wind and the ground. The relevant length scales in the surface layer are the height above ground, which describes the size of the dominant eddies, the Monin-Obukov length (L), which defines the relationship between buoyant and shear production of TKE. The velocity scale in the surface layer is the friction velocity (u*), which is defined as u,=z~/2, where To is the surface Reynolds' stress or momentum flux (uw2+vw2) **.5 Km in the surface Monin--Obukov similarity theory According to well-known empirical functions for local-mixing nondimensional surface layer flux-profiles depending on atmospheric stability (z/L) K z = ku. z(1 - z/h)20(z/L). (A4) Note that when z/h ~ 1 Equation (A4) reverts to the surface layer expression (Equation A1). Also, Kz is constrained to go to zero at the top of the mixed layer. In the mixed-layer portion of the convective boundary layer, buoyant production of TKE is much more important than shear production. Therefore, friction velocity (u.) in Equation (A1) is replaced by convective velocity (w*) as the scaling velocity during convectively unstable conditions (h/L< -10) in the mixed layer to give Kz= kw*z(1 - z/h), (A5) where Kz is again constrained to equal zero at the top of the PBL. It is this dependence on the convective velocity, which is a function of surface heat flux rather than shear stress, which enables this model to produce more realistic K, values for the mixed layer than the formulation of Louis (1979). A disadvantage of this model, however, is that both expressions for K, in the PBL are very sensitive to the PBL height which may be poorly estimated. Observed data The data used in the analyses were measurements of the three-dimensional wind components (U, V, and W) and temperature (T) from sonic anemometers on the three flux towers within the plot during the LIPF on March 20, 2011. The sampling rate was10 Hz and data in this study from two levels (3 and 10 m AGL) among 3 towers (See Heilman 2013) were used to assess the ambient and fire-induced atmospheric turbulence regimes. Analysis Method LIPF may induce or change the behavior of turbulent eddies in the lower atmosphere. The amount of energy in these turbulent eddies is defined as turbulent kinetic energy or TKE. TKE can be produced mechanically by wind shear and/or thermally by buoyancy effects, both of which can be calculated from the data. The TKE budget equation can be written as Pr = Pb + Ps – Pt, where Ps is wind shear production, Pb represents buoyancy production or dissipation, and Pt is the local time rate of change of TKE or TKE storage that can also be estimated using the data. Pr represents the residual of the TKE budget equation, which includes TKE diffusion, advection, dissipation, and other higher-order terms. Using the data, we estimated the four terms in the vicinity of the fire front during the LIPF experiment. A small Pr indicates that local change of TKE is mainly a balance between the two production terms ( Ps and Pb) and a large residual would suggest the importance of other factors that should not be neglected. • Results and Conclusion • These tests demonstrate quicker upward transport of ground-level heat and momentum fluxes by the FIPFs as compared to the eddy diffusion scheme currently used … • This is the model used in the comparisons described in • Based on field experiment data • The analyses of turbulence data collected before, at, and post fire front indicate that • The LIPF can generate turbulence • that is much greater than • turbulence in the ambient • atmosphere. • The production of turbulence at • the surface near the fire front was • associated with increased • variance in the 3D winds, • specially in vertical direction, • while buoyancy was strongest at • higher levels within the fire plume. • Turbulence at 10-m level nearly • doubled that at 3-m level, • indicating that the assumption of • ‘constant flux layer’ near the • surface is no longer valid in the • LIPF environment. • The residual term is generally • small in the ambient environment, • but quite large at the fire front • and somewhat large immediate • before and after the fire front, • indicating that other factors in • addition to shear and buoyancy • are important to local turbulence • behavior. Further analyses are • needed to identify these specific • factors. Acknowledgments. Funding for this research was provided by the U.S. National Fire Plan , the U.S. Joint Fire Science Program (Project # 09-1-04-1), and the USDA Forest Service (Research Joint Venture Agreement # 09-JV-11242306-089). Figure 1. High frequency sonic anemometer data collected on March 20, 2011 from the LIPF experiment conducted in the New Jersey Pine Barrens. There are 3-level sonic anemometers (10-Hz U,V,W,T) on a 30-m-tower at heights of 3, 10, and 30 m AGL; 3-level sonic anemometers (10-Hz U,V,W,T) on a 20-m-tower at heights of 3, 10, and 20 m AGL; and 2-level sonic anemometers (10-Hz U,V,W,T) on a 10-m-tower at heights of 3 and 10 m AGL.