6.6.3 Surface Roughness Length

The roughness length (z₀ ) is related to the roughness characteristics of the terrain. Under near-neutral conditions and with a homogeneous distribution of obstacles, a local value of z₀ can be determined from the logarithmic wind profile.

For general application, since typical landscapes almost always contain occasional obstructions, one should attempt to estimate an effective roughness length. The recommended method for estimating the effective roughness length is based on single level gustiness measurements _u [60]:

Wind measurements for use in (6.6.11) should be made between 20 z₀ and 100 z₀ ; toselect the appropriate measurement level, an initial estimate of the effective roughness length must first be made based on a visual inspection of the landscape (see roughness classifications provided in Table 6-10). The sampling duration for _u and should be between 3 and 60 u minutes. Data collected for use in estimating the effective surface roughness should be stratified by wind speed (only data for wind speeds greater than 5 m/s should be used) and wind direction sector (using a minimum sector arc width of 30 degrees). Median z₀ values should be computed for each sector; results should then be inspected to determine whether the variation between sectors is significant. An average of the median values should be computed for adjacent sectors if the variation is not significant. Estimates of the effective surface roughness using these procedures are accurate to one significant figure; i.e., a computed value of 0.34 m should be rounded to 0.3 m. Documentation of the successful application of these procedures is provided in reference [61].

Table 6-10 Terrain Classification in Terms of Effective Surface Roughness Length, ZO
Terrain Description	ZO (m)
Open sea, fetch at least 5km	0.0002
Open flat terrain; grass, few isolated obstacles	0.03
Low crops, occasional large obstacles; x'/h > 20*	0.10
High crops, scattered obstacles, 15 < x'/h < 20*	0.25
Parkland, bushes, numerous obstacles, x'/h 10*	0.50
Regular large obstacle coverage (suburb, forest)	0.50 - 1.0

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