6.6 Boundary Layer Parameters

This section provides recommendations for monitoring in support of air quality dispersion models which incorporate boundary layer scaling techniques. The applicability of these techniques is particularly sensitive to the measurement heights for temperature and wind speed; the recommendations for monitoring, given in Section 6.6.4, consequently, focus on the placement of the temperature and wind speed sensors. A brief outline of boundary layer theory, given in the following, provides necessary context for these recommendations. The references for this section [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59] provide more detailed information on boundary layer theory.

The Atmospheric Boundary Layer (ABL) can be defined as the lower layer of the atmosphere, where processes which contribute to the production or destruction of turbulence are significant; it is comprised of two layers, a lower surface layer, and a so-called “mixed” upper layer. The height of the ABL during daytime roughly coincides with the height to which pollutants are mixed (the mixing height, Section 6.5). During night-time stable conditions, the mixing height (h) is an order of magnitude smaller than the maximum daytime value over land; at night, h is typically below the top of the surface-based radiation inversion [57].

The turbulent structure of the ABL is determined by the amount of heat released to the atmosphere from the earth’s surface (sensible heat flux) and by interaction of the wind with the surface (momentum flux). This structure can be described using three length scales: z (the height above the surface), h (the mixing height ), and L (the MONIN - Obukhov length). The MONIN - Obukhov length is defined by:

• the surface fluxes of heat H = pC_p 
• and momentum , 

and  reflects the height at which contributions to the turbulent kinetic energy from buoyancy and shear stress are comparable; the Obukhov length is defined as:

where k is the von Karman constant, is the mean potential temperature within the surface layer, g/ is a buoyancy parameter, and u* is the friction velocity. The three length scales define two independent non-dimensional parameters: a relative height scale (z/h), and a stability index (h/L)[56].

Alternatives to the measurement of the surface fluxes of heat and momentum for use in (6.6.1) involve relating turbulence to the mean profiles of temperature and wind speed. 

The Richardson number, the ratio of thermal to mechanical production (destruction) of turbulent kinetic energy, is directly related to another non-dimensional stability parameter (z/L) and, thus, is a good candidate for an alternative to 6.6.1. The gradient Richardson number (Rg) can be approximated by:

Large negative Richardson numbers indicate unstable conditions while large positive values indicate stable conditions. Values close to zero are indicative of neutral conditions. Use of (6.6.2) requires estimates of u based on measurements of wind speed at two levels in the surface layer; however, the level of accuracy required for these measurements is problematic (u is typically the same order of magnitude as the uncertainty in the wind speed measurement). The bulk Richardson number (R_b ) which can be computed with only one level of wind speed is a more practical alternative:

6. METEOROLOGICAL DATA PROCESSING
  6.1 Averaging and Sampling Strategies 
  6.2 Wind Direction and Wind Speed 
      6.2.1 Scalar Computations 
      6.2.2 Vector Computations 
      6.2.3 Treatment of Calms  
      6.2.4 Turbulence 
      6.2.5 Wind Speed Profiles  
  6.3 Temperature 
      6.3.1 Use in Plume-Rise Estimates  
      6.3.2 Vertical Temperature Gradient 
  6.4 Stability 
      6.4.1 Turner's method  
      6.4.2 Solar radiation/delta-T (SRDT) method 
      6.4.3  _E method 
      6.4.4 _Amethod 
      6.4.5 Accuracy of stability category estimates 
  6.5 Mixing Height 
      6.5.1 The Holzworth Method  
  6.6 Boundary Layer Parameters  
      6.6.1 The Profile Method 
      6.6.2 The Energy Budget Method  
      6.6.3 Surface Roughness Length 
      6.6.4 Guidance for Measurements in the Surface Layer 
  6.7 Use of Airport Data 
  6.8 Treatment of Missing Data  
      6.8.1 Substitution Procedures 
  6.9 Recommendations