The default value of 1 meter for urban surface roughness length, assumed if the parameter is

omitted, is considered appropriate for most applications. Any application of AERMOD that

utilizes a value other than 1 meter for the urban roughness length should be considered as a nonregulatory

application, and would require appropriate documentation and justification as an

alternative model, subject to Section 3.2 of the Guideline on Air Quality Models (EPA, 2005b).

The use of a value other than 1 meter for the urban surface roughness length will be explicitly

treated as a non-DFAULT option in the next update to the AERMOD model.

 

5.4 METEOROLOGICAL DATA SELECTIONS FOR URBAN APPLICATIONS

(01/09/08)

 

5.4.1 Urban applications using NWS meteorological data (01/09/08)

 

When modeling urban sources, the urban algorithms in AERMOD are designed to enhance the

turbulence levels relative to the nearby rural setting during nighttime stable conditions to account

for the urban heat island effect (Cimorelli, et al., 2004). For urban applications using

representative NWS meteorological data the AERMOD urban option (URBANOPT) should be

selected (EPA, 2004b), regardless of whether the NWS site is located in a nearby rural or an

urban setting. This is due to the fact that the limited surface meteorological measurements

available from NWS stations will not account for the enhanced turbulence or other

meteorological characteristics of the urban boundary layer included in the AERMOD urban

algorithms. The determination of surface characteristics for processing NWS meteorological

data for urban applications should conform to the recommendations presented in Section 3.1.

 

 

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