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<br />VOLUME 35 <br /> <br />-10000 <br />6000 <br /> <br />-6000 <br /> <br />-2600 <br /> <br />o <br /> <br />JOURNAL OF APPLIED METEOROLOGY <br /> <br />6000 <br /> <br />iT <br />it <br />t <br />i <br /> <br />if <br />j' <br />il <br />J,i <br />,$' <br /> <br />J, <br /> <br />1438 <br /> <br />-7600 <br /> <br />'000 <br /> <br />~ <br />.... <br />., <br />23000 <br />" <br />01 <br />'i <br />:t <br /> <br /> <br />2000 <br /> <br /> <br />1000 <br />-10000 -7600 -6000 -2600 <br /> <br />2600 <br /> <br />7600 10000 1 600 <br /> <br />16000 <br />5000 <br /> <br />'000 <br /> <br />3000 <br /> <br />2000 <br /> <br /> <br />o 2600 6000 <br />Olstance from JVL (m) <br /> <br />7600 <br /> <br />1000 <br />10000 12600 16000 <br /> <br />FIG. 5. Rawinsonde-determined vertical velocities measured along the path of the ascending <br />balloon for 10 February 1994. Magnitudes are scaled using the arrow shown in the insert. <br /> <br />winter storms released SF6 from the main Sierra ridge- <br />line and sampled for the tracer in the downwind valley <br />using either a mobile SF6 analyzer in a van or stationary <br /> <br />'<t <br /> <br />(J) <br />~N <br /> <br /> <br />>. <br />.> <br />~ <br /> <br />u <br />o <br /> <br />~0 <br /> <br />10000 15000 20000 <br />o ls Lon cs from JVL (m) <br /> <br />o <br />u <br />~ <br />.> <br />L <br />CD <br />>N <br />I <br /> <br />'<t <br />I <br /> <br />FIG. 6. Balloon-derived vertical velocities plotted with respect to <br />distance from Johnsville for 10 February 1994. Number plotted near <br />10 000 and 20 000 m is the elevation of the balloon at that distance <br />downwind. This technique allows an estimate of the horizontal wave- <br />length of the lee wave by measuring the distance between peaks and <br />valleys in the velocity data (after Shutts and Broad 1993). <br /> <br />time sequential samplers (Reynolds 1992). These re- <br />sults showed that in 9 of 10 experiments SF6 was ob- <br />served in the downwind valley at levels well above <br />background (10 ppt). These results suggested that <br />downward transport to the lee of the Sierra commonly <br />occurred during most winter storms. It seems reason- <br />able that lee waves may play an important role in this <br />transport. <br />It has been shown that the ascent rate of rawinsonde <br />balloons can be used to identify the presence of lee <br />waves [Lalas and Einaudi 1980; Reid 1972; Shutts and <br />Broad 1993 (hereafter SH); Shutts et al. 1994]. These <br />authors show that the ascent rate can be measured to <br />within 0.2 m s -1 using the change in pressure (altitude) <br />transmitted by the rawinsonde every 2 s. The ascent <br />rate was calculated from observed pressures at 20-s in- <br />tervals. Using the free lift (volume of a helium-filled <br />balloon) and the payload of the balloon, a nominal as- <br />cent rate of 5.2 m s -1 was calculated. This nominal <br />value of the ascent rate was subtracted from the ob- <br />served ascent rate to estimate free air vertical velocities <br />experienced by the balloon. A schematic of the hypo- <br />thetical path of the rawinsonde through a mountain lee <br />wave for this project area is shown in Fig. 4. An ex- <br />ample of how this methodology is applied in a real case <br />is shown in Fig. 5 for 10 February 1994. <br />What is the effect that these large upward and down- <br />ward vertical motions have on the transport of seeded <br />crystals across the target area? First, it must be empha- <br />sized that for ground-based seeding experiments we <br />