IDENTIFICATION OF METEOROLOGICAL REGIMES
CORRESPONDING WITH HIGH OBSERVED OZONE
The following is a procedure that may be used to assist in
selecting modeling episodes. Other techniques may be considered on
a case-by-case basis; they should be described in the Modeling
Protocol and approved by the appropriate EPA Regional Office.
Identification of meteorological regimes for a given area
under review begins with constructing a climatological windrose of
high ozone days. The windrose is constructed by first selecting
all days from the period 1987 to present during which at least one
ozone monitor within the area recorded an exceedance of the ozone
NAAQS or some other cutoff level (e.g., 100 ppb). Additional years
of data are encouraged in constructing the climatological windrose
(e.g., 1980-1991). Next, for each exceedance day, calculate the
morning (i.e., 7:00 a.m. - 10:00 a.m.) resultant wind velocity.
Then group the resultant wind velocities for all of the exceedance
days into eight compass directions plus calm, to establish a
climatic windrose of high-ozone days for the area under review.
Calm winds are defined as those with speeds less than 1.5 m/s and
referred to as the null wind direction. The windrose will include
nine bins (0-8); place the wind directions corresponding to the
eight compass points into bins 1-8, and the calm or null wind
direction into bin 0. The bins with frequencies significantly
higher than the average frequency for all bins should be defined as
the "predominant wind directions" (PWD).
Next, compare the morning (i.e., 7:00 a.m. - 10:00 a.m.)
resultant wind velocity for each exceedance day during 1987-89 and
more recent years with the climatic windrose of high-ozone days.
Categorize exceedance days with wind directions corresponding to
previously identified climatic PWD's as belonging to that PWD.
Lump all other exceedance days occurring during 1987-89 and later
into a category called "other." Rank each exceedance day within
each PWD category and within the "other" category according to its
areawide daily maximum ozone observation. Within each category,
the day with the highest areawide daily maximum concentration is
After the steps described in the two preceding paragraphs are
completed, meteorological regimes can be defined for use in the
attainment demonstration test described in Section 6.4. This may
be done as follows:
n1201 - n1202 - n1203 - n1204 - n1205 - n1206 - n1207 - n1208 - n1209 - n1210 - n1211 - n1212 - n1213 - n1214 - n1215 - n1216 - n1217 - n1218 - n1219 - n1220 - n1221 - n1222 - n1223 - n1224 - n1225 - n1226 - n1227 - n1228 - n1229 - n1230 - n1231 - n1232 - n1233 - n1234 - n1235 - n1236 - n1237 - n1238 - n1239 - n1240 - n1241 - n1242 - n1243 - n1244 - n1245 - n1246 - n12247 - n1248 - n1249 - n1250
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