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<br />1448
<br />
<br />JOURNAL OF APPLIED METEOROLOGY
<br />
<br />VOLUME 35
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<br />FIG. 17. Temporal plot of meteorological variables measured at
<br />Mount Hough for 17 February 1993. Variables from the top down
<br />are as follows: derived hourly averaged liquid water content from ice
<br />detector trips and wind speed, ice detector trips, l-min average wind
<br />speed, l-min average wind direction, relative humidity, and temper-
<br />ature.
<br />
<br />2) RIDGE TRACK PLUME INTERSECTIONS
<br />
<br />Table 5 summarizes SF6 plume intersections from
<br />site 7 along the ridge track. A little over half the number
<br />of intersections were noted compared to the valley
<br />track. The average distance from site 7 to the ridge
<br />plume intersection was almost 22 km. Based on an av-
<br />erage wind speed of 10m s -1 , the plume was about 36
<br />min old. Although the plumes were more than twice
<br />the age of those over the valley, the plumes' horizontal
<br />dimensions were only slightly wider-3-4 km over
<br />valley-but the spread angle was less-lOo versus 130
<br />for the valley. The best day of vertical transport of SF6
<br />is still 5 February.
<br />Based on an average of five ridge passes per flight
<br />when SF6 would have time to reach the ridge position,
<br />only 40% of the passes indicated plumes at flight alti-
<br />tude, which averaged 2562 m. In fact, 11 of 18 plume
<br />intersections came on three days: 5 February, 17 Feb-
<br />ruary, and 9 March.
<br />Table 6 summarizes the ridge plume intersections for
<br />site 9. These plumes were about 45 min old when sam-
<br />
<br />pled. For 17 March, on which no valley interceptions
<br />were noted, two separate plume interceptions were
<br />noted over the ridge. This would indicate that the
<br />downward portion of the wave transported the tracer
<br />below aircraft altitudes over the valley, and the upward
<br />portion 'of the wave transported the tracer to higher al-
<br />titudes over the ridge. This seems to be confirmed by
<br />Fig. 13. On average, plume characteristics for the ridge
<br />versus valley interceptions for site 9 are, respectively,
<br />plume width 6.6 versus 5.2 km and average SF6 con-
<br />centrations of 24 versus 36 ppt. Again, it would appear
<br />that at the nominal in-cloud flight altitude of 2650 m
<br />the aircraft is just skimming the top of the plumes.
<br />Comparing site 7 and 9 SF6 intercepts along the
<br />ridge pass for the same days, (17 February, 9 March,
<br />17 March, and 19 March), the following plume char-
<br />acteristics are found for sites 7 and 9, respectively:
<br />plume widths of 4.7 versus 4.2 km, average SF6 con-
<br />centrations of 16 versus 19 ppt, and average L WC of
<br />0.02 versus 0.04 g m-3. These results indicate little
<br />difference in tracer transport over the target ridge from
<br />these two sites.
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<br />FIG. 18. Temporal plots of measurements made by the NOAA
<br />aircraft on pass 13 for 17 February 1993. Panels from top to bot-
<br />tom are as follows: calculated precipitation rate from the ice par-
<br />ticle probe, temperature and dewpoint, cloud liquid water, aircraft
<br />altitude and ice particle concentrations (total ice, solid; particles
<br />smaller than 250 pm, dashed), and SF6 concentration. Sulfur hex-
<br />aflouride analyzer has an ll-s lag. Vertical lines denote the true
<br />position of SF6.
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