6.9 Recommendations

The hourly scalar mean wind speed and wind direction should be used in steady-state Gaussian dispersion models. These statistics should be processed using the methods provided in Section 6.2.1; unit vector processing (Section 6.2.2) may also be used to estimate the hourly scalar mean wind direction. The standard deviation of the wind direction should be calculated using the techniques described in Section 6.2.1. Hourly statistics may be obtained by processing samples over an entire hour or by averaging sub-hourly statistics. The recommended sub-hourly averaging interval for wind data processing is 15 minutes; two valid 15-minute averages are required for a valid hourly average. 

For the purposes of this guidance, a calm occurs when the wind speed is below the starting threshold of the anemometer or vane, whichever is greater. Calms require special treatment in such applications to avoid division by zero in the steady-state dispersion algorithm. For similar reasons, to avoid unrealistically high concentration estimates at low wind speeds (below the values used in validations of these models - about 1 m/s) EPA recommends that wind speeds less than 1 m/s be reset to 1 m/s for use in steady-state dispersion models; the unaltered data should be retained for use in non-steady-state modeling applications. Calms should be identified in processed data files by flagging the appropriate records; user’s guides for the model being used should be consulted for model specific flagging conventions.

Recommended sampling and processing strategies for the primary meteorological variables for various applications are given in Table 6-1. 

The Pasquill-Gifford (P-G) stability category should be determined with Turner's method (Section 6.4.1) using site-specific wind speed measurements at or near 10 m and representative cloud cover and ceiling height. Other approved methods for estimating the P-G stability category, for use when representative cloud cover and ceiling observations are not available, include the solar radiation delta-T (SRDT) method described in Section 6.4.2, and turbulence-based methods using site-specific wind fluctuation statistics: _E (Section 6.4.3) or_A (Section 6.4.4). Alternative methods for determining stability category should be evaluated in consultation with the Regional Office.

Emperical relationships for use in models employing boundary layer scaling techniques should be selected in accordance with a von Karmam constant of 0.4; recmmended empirical relationships are given in reference [59].

Missing data should be flagged or replaced as appropriate depending on the model to be used. Isolated one-hour gaps in meteorological data bases used in regulatory modeling should be filled with estimates bases on persistence or interpolation. Application specific procedures should be used to fill longer gaps

If the recommendations in this section cannot be achieved, then alternative approaches should be developed in consultation with the EPA Regional Office.

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