January 31, 1991


MEMORANDUM

SUBJECT:  Georgia Power Plant Yates GEP Modeling

FROM:     James L. Dicke, Chief
          Techniques Evaluation Section, SRAB (MD-14)

TO:       Brenda Johnson, Meteorologist
          Region IV


     In response to your memorandum of January 11, 1991, the
Model Clearinghouse has reviewed your position regarding the
Yates GEP modeling and offers the following comments.  You have
stated the case correctly for using scalar wind speed, in terms
of dilution speed, and scalar average or unit vector direction,
to model plume transport.  In addition, EPA's point source models
apply a wind speed profile for determining the change of speed
with height, e.g. from the 10m measurement level to stack top,
and this requires input of scalar mean or unit vector wind speed.

     The report from the 1980 workshop does recommend reporting
hourly resultant vector directions and unit vectors are not
mentioned.  However, in succeeding years, the recommendations now
found in EPA's on-site guidance document were developed and peer
reviewed.  In capsule form they appear on page 6-42 of the
guidance document and EPA received no adverse public comments on
them following presentation of the document at the Fourth
Conference on Air Quality Modeling.  Thus those recommendations
should be followed for straight line Gaussian dispersion model
applications unless the user's guide for a particular model
states differently.

     If you have further questions, please contact me or Desmond
Bailey at FTS 629-5248.

cc:  D. Bailey 
     B. Miller
     J. Tikvart
     D. Wilson