6.5.1 The Holzworth Method

The Holzworth method [44] provides twice-per-day (morning and afternoon) mixing heights based on calculations using routine NWS upper-air data. The morning mixing height is calculated as the height above ground at which the dry adiabatic extension of the morning minimum surface temperature plus 5 °C intersects the vertical temperature profile observed at 1200 Greenwich Mean Time (GMT). The minimum temperature is determined from the regular hourly airways reports from 0200 through 0600 Local Standard Time (LST). The “plus 5 °C “ was intended to allow for the effects of the nocturnal and early morning urban heat island since NWS upper-air stations are generally located in rural or suburban surroundings. However, it can also be interpreted as a way to include the effects of some surface heating shortly after sunrise. Thus, the time of the urban morning mixing height coincides approximately with that of the typical diurnal maximum concentration of slow-reacting pollutants in many cities, occurring around the morning commuter rush hours.

The afternoon mixing height is calculated in the same way, except that the maximum surface temperature observed from 1200 through 1600 LST is used. Urban-rural differences of maximum surface temperature are assumed negligible. The typical time of the afternoon mixing height may be considered to coincide approximately with the usual mid-afternoon minimum concentration of slow-reacting urban pollutants.

Hourly mixing heights, for use in regulatory dispersion modeling, are interpolated from these twice per day estimates. The recommended interpolation procedure is provided in the user’s guide for the Industrial Source Complex (ISC) dispersion model [47].

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