Simulations of Atmospheric Flows in the Boundary Layer over Inhomogeneous Surface Conditions

Abstract. Using a Large-Eddy Simulation model, we have systematically studied the inability of boundary layer turbulence to efficiently mix reactive species. This creates regions where the species are accumulated in a correlated or anti-correlated way, thereby modifying the mean reactivity. We quantify this modification by the intensity of segregation, IS, and analyse the driving mechanisms: heterogeneity of the surface moisture and heat fluxes, various background wind patterns and non-uniform isoprene emissions. The heterogeneous surface conditions are characterized by cool and wet forested patches with high isoprene emissions, alternated with warm and dry patches that represents pasture with relatively low isoprene emissions. For typical conditions in the Amazon rain forest, applying homogeneous surface forcings and in the absence of free tropospheric NOx, the isoprene-OH reaction rate is altered by less than 10%. This is substantially smaller than the previously assumed IS of 50% in recent large-scale model analyses of tropical rain forest chemistry. Spatial heterogeneous surface emissions enhance the segregation of species, leading to alterations of the chemical reaction rates up to 20%. The intensities of segregation are enhanced when the background wind direction is parallel to the borders between the patches and reduced in the case of a perpendicular wind direction. The effects of segregation on trace gas concentrations vary per species. For the highly reactive OH, the differences in concentration averaged over the boundary layer are less than 2% compared to homogeneous surface conditions, while the isoprene concentration is increased by as much as 12% due to the reduced chemical reaction rates. These processes take place at the sub-grid scale of chemistry transport models and therefore need to be parameterized.

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Combined effects of surface conditions, boundary layer dynamics and chemistry on diurnal SOA-evolution

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-9331-2012 ◽

2012 ◽

Vol 12 (4) ◽

pp. 9331-9375 ◽

Cited By ~ 2

Author(s):

R. H. H. Janssen ◽

J. Vilà-Guerau de Arellano ◽

L. N. Ganzeveld ◽

P. Kabat ◽

J. L. Jimenez ◽

...

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Land Surface ◽

Large Scale ◽

Organic Aerosol ◽

Layer Growth ◽

Combined Effects ◽

Free Troposphere ◽

Surface Conditions ◽

Boundary Layer Dynamics

Abstract. We study the combined effects of land surface conditions, atmospheric boundary layer dynamics and chemistry on the diurnal evolution of biogenic secondary organic aerosol in the atmospheric boundary layer, using a model that contains the essentials of all these components. First, we evaluate the model for a case study in Hyytiälä, Finland, and find that it is able to well reproduce the observed dynamics and gas-phase chemistry. We show that the exchange of organic aerosol between the free troposphere and the boundary layer (entrainment) must be taken into account in order to explain the observed diurnal cycle in organic aerosol (OA) concentration. An examination of the budgets of organic aerosol and terpene concentration shows that the former is dominated by entrainment, while the latter is mainly driven by emission and chemical transformation. We systematically examine the role of the land surface, which governs both the surface energy balance partitioning and terpene-emissions, and the large-scale atmospheric process of vertical subsidence. Entrainment is especially important for the dilution of organic aerosol concentrations under conditions of dry soils and low terpene-emissions. Subsidence suppresses boundary layer growth while enhancing entrainment. Therefore it influences the relationship between organic aerosol and terpene-concentrations. Our findings indicate that the diurnal evolution of SOA in the boundary layer is the result of coupled effects of the land surface, dynamics of the atmospheric boundary layer, chemistry, and free troposphere conditions. This has potentially some consequences for the design of both field campaigns and large-scale modeling studies.

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Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-17815-2010 ◽

2010 ◽

Vol 10 (7) ◽

pp. 17815-17851 ◽

Cited By ~ 1

Author(s):

N. A. Brunsell ◽

D. B. Mechem ◽

M. C. Anderson

Keyword(s):

Boundary Layer ◽

Heat Flux ◽

Latent Heat ◽

Latent Heat Flux ◽

Land Surface ◽

Surface Heterogeneity ◽

Length Scales ◽

Surface Conditions ◽

Boundary Layer Dynamics

Abstract. The role of land-atmosphere interactions under heterogeneous surface conditions is investigated in order to identify mechanisms responsible for altering surface heat and moisture fluxes. Twelve coupled land surface – large eddy simulation scenarios with four different length scales of surface variability under three different horizontal wind speeds are used in the analysis. The base case uses Landsat ETM imagery over the Cloud Land Surface Interaction Campaign (CLASIC) field site for 3 June 2007. Using wavelets, the surface fields are band-pass filtered in order to maintain the spatial mean and variances to length scales of 200 m, 1600 m, and 12.8 km as lower boundary conditions to the model. The simulations exhibit little variation in net radiation. Rather, a change in the partitioning of the surface energy between sensible and latent heat flux is responsible for differences in boundary layer dynamics. The sensible heat flux is dominant for intermediate surface length scales. For smaller and larger scales of surface heterogeneity, which can be viewed as being more homogeneous, the latent heat flux becomes increasingly important. The results reflect a general decrease of the Bowen ratio as the surface conditions transition from heterogeneous to homogeneous. Air temperature is less sensitive to surface heterogeneity than water vapor, which implies that the role of surface heterogeneity in modifying the local temperature gradients in order to maximize convective heat fluxes. More homogeneous surface conditions, on the other hand, tend to maximize latent heat flux. Scalar vertical profiles respond predictably to the partitioning of surface energy. Fourier spectra of the vertical wind speed, air temperature and specific humidity (w, T and q) and associated cospectra (w'T', w'q' and T'q'), however, are insensitive to the length scale of surface heterogeneity, but the near surface spectra are sensitive to the mean wind speed.

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Relation of the Formation of Secondary Flow Between Atmospheric Flow and Turbomachinery Flow

Volume 2, Parts A and B ◽

10.1115/ht-fed2004-56033 ◽

2004 ◽

Author(s):

Joseph T. Hamrick

Keyword(s):

Boundary Layer ◽

Research Program ◽

Ground Plane ◽

Ground Surface ◽

Vortex Generator ◽

State University ◽

Iowa State University ◽

Atmospheric Flow ◽

Atmospheric Flows ◽

Transfer Of Technology

Researchers at Iowa State University instituted a research program in which they built a vortex generator in an effort to simulate atmospheric mesocyclones and to determine their relationship to tornado formation. In their research it was observed that when a vortex touched down on a ground surface there was a build up of a concentrated vortex at the center of the parent vortex. It was also observed that the concentrated vortex cannot be formed without contact with a ground surface. They attributed the build up of the concentrated vortex to radially inward flows in the boundary layer adjacent to the ground plane but provided no analysis of the flow in the boundary layer. To perform the analysis provided herein an attempt was made to apply a transfer of technology developed at the National Advisory Committee for Aeronautics (NACA), Lewis Flight Propulsion Laboratory at Cleveland, Ohio. In the NACA research the mechanisms of radially inward flows in boundary layers were evaluated. The transfer of that technology to atmospheric flows is presented herein. In this paper it is illustrated that the transfer of technology can be an effective tool in attempts to resolve issues in related fields of endeavor.

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