If no valid observation of the reference wind speed or direction exists between these limits

the hour is considered missing and a message is written to the AERMET message file. For the

wind speed to be valid its value must be greater than or equal to the threshold wind speed.

AERMOD processes hours of invalid wind speed, e.g. calms, in the same manner as ISC (EPA

calms policy).

All observed wind speeds in a measured profile that are less than uth are set to missing and are

therefore not used in the construction of the wind speed profile (profiling of winds is

accomplished in AERMOD).

7.2.4 CALCULATING THE POTENTIAL TEMPERATURE GRADIENT ABOVE THE

MIXING HEIGHT FROM SOUNDING DATA

AERMET calculates d2/dz for the layer above zi as follows:

- If the sounding extends at least 500 m above zi the first 500 m above zi is used to

determine d2/dz above zi.

- If the sounding extends at least 250 m above zi (but not 500 m) then the available

sounding above zi is used to determine d2/dz above zi.

- AERMET limits d2/dz above zi to a minimum of 0.005 K m-1.

- If the sounding extends less than 250 m above zi then set d2/dz = 0.005 K m-1 (a default

value).

7.2.5 MEASURED TURBULENCE

All measured turbulence values are passed to AERMOD if the hour is non-missing. This is

true even for those levels where the wind speed is below uth. Based on measurements with

research grade instruments, reasonable minimum turbulence levels in non-calm conditions for

vertical turbulence (Fw ) and lateral turbulence ( F

v) values are set by AERMOD to 0.02 m s-1 and

0.2 m s-1, respectively. Although these lower limits are applied to the measured values of the

turbulence the calculated profile values of Fw & F

v are not subjected to any lower limits. We do

not restrict these estimated profiles because it would bias the calculation of the effective values of

turbulence, which are averages through the layer between the receptor and the plume height, in

determining the dispersion of the plume. However, as discussed in Section 7.9 these limits are

applied to the effective values of turbulence and wind speed.

7.2.6 DATA SUBSTITUTION FOR MISSING ON-SITE DATA

If on-site data are missing for an hour, the hour is considered missing unless the user specifies

a substitute data set. AERMET does not default to NWS (or any other offsite) data.

n1001 - n1002 - n1003 - n1004 - n1005 - n1006 - n1007 - n1008 - n1009 - n1010 - n1011 - n1012 - n1013 - n1014 - n1015 - n1016 - n1017 - n1018 - n1019 - n1020 - n1021 - n1022 - n1023 - n1024 - n1025 - n1026 - n1027 - n1028 - n1029 - n1030 - n1031 - n1032 - n1033 - n1034 - n1035 - n1036 - n1037 - n1038 - n1039 - n1040 - n1041 - n1042 - n1043 - n1044 - n1045 - n1046 - n1047 - n1048 - n1049 - n1050

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