6.6.4 Guidance for Measurements in the Surface Layer

Monin-Obukhov (M-O) similarity theory is strictly applicable to steady-state horizontally homogeneous conditions in the surface layer. The temperature and wind speed measurements for use with M-O theory should be representative of a layer that is both high enough to be above the influence of the individual surface roughness elements and yet low enough to be within the surface layer; as a rule of thumb, the measurements should be made within the layer from 20z₀ to 100z₀ above the surface (2 - 10 m for a surface roughness of 0.1 m) [57].

Data quality objectives and, consequently, instrument specifications for monitoring of temperature and wind speed in the surface layer are determined by the limitations imposed during the extreme stability conditions; basically this requires a monitoring design with the capability to resolve the variable gradients in temperature and wind speed that can exist within the surface layer under various conditions.

The depth of the surface layer where M-O similarity theory applies ranges from about one tenth of the ABL depth (h) during neutral conditions (typically 500 - 600 m) to the lesser of | L | or 0.1 h during non-neutral conditions (less than 10 m during extreme stability conditions). This variability in the depth of the surface layer imposes limitations on what can be accomplished with a single fixed set of sensors. To ensure the availability of measurements representative of the entire surface layer during all stability conditions, one should employ a tall-tower (60 m or taller) equipped with wind and temperature sensors at several levels including, as a minimum, 2, 10 and 60 m. In the absence of a tall-tower, a standard 10-meter meteorological tower equipped with a single fixed set of sensors should be employed. Wind speed should be measured at the standard height of 10 m; the temperature difference should be measured between 2 and 10 m (for z₀ ~ 0.1 m). The usefulness of such a relatively low-lying measurement configuration lies in its applicability to both stable and unstable atmospheric conditions.

Application of M-O similarity should generally be restricted to low roughness sites located in relatively homogeneous terrain. For such sites, the reliability of the profile method for estimating surface layer parameters is primarily dependent on accurate temperature difference measurements (see Section 3.2.2 for siting and exposure of temperature sensors and Section 5.1 for sensor specifications).

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