6.2.3 Treatment of Calms

Calms, periods with little or no air movement, require special consideration in air quality evaluations; one of the more important considerations involves model selection. If the limiting air quality conditions are associated with calms, then a non-steady-state model, such as CALPUFF [27], should be used. The use of a time varying 3-dimensional flow field in this model enables one to simulate conditions which are not applicable to steady-state models; e.g., recirculations and variable trajectories. Guidance for preparing meteorological data for use in CALPUFF is provided in the user’s guide to the meteorological processor for this model [28].

Steady-state models may be used for regulatory modeling applications if calms are not expected to be limiting for air quality. Calms require special treatment in such applications to avoid division by zero in the steady-state dispersion algorithm. EPA recommended steady-state models such as ISCST accomplish this with routines that nullify concentrations estimates for calm conditions and adjust short-term and annual average concentrations as appropriate. The EPA CALMPRO [29] program post-processes model output to achieve the same effect for certain models lacking this built-in feature. 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 odel specific flagging conventions.

For the purposes of this guidance and for the objective determination of calm conditions applicable to in situ monitoring, a calm occurs when the wind speed is below the starting threshold of the anemometer or vane, whichever is greater. For site-specific monitoring (using the recommended thresholds for wind direction andwind speed given in Table 5-2) a calm occurs when the wind speed is below 0.5 m/s. One should be aware that the frequency of calms are typically higher for NWS data bases because the sensors used to measure wind speed and wind direction have a higher threshold - typically 2 kts (1 m/s) - see Section 6.7.

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