Report written by Vanda Grubisic, Sunday, March 21
Synoptic overview:
Starting Sunday afternoon (March 14) and through Monday morning (March 15), the models (Eta, GFS) were forecasting moderately strong NW flow over Owens Valley and the Sierra Nevada. The maximum of that NW flow was forecast to occur early on Monday morning, March 12, between 12 UTC and 15 UTC. It was this episode of NW flow that was to be documented during this IOP.
The plan of operations for IOP 4 on March 15:
1. MAPR: a radiosonde launch at 12 UTC March 15
2. MISS: located at its base at Independence airport, if possible run throughout the night from Sunday to Monday
3. MGLASS: no activity
4. Lemoore: no activity
Summary of operations and observations:
The expectation for this IOP was that the vertical shear in this NW flow might be sufficiently strong to make the atmosphere conducive to resonance and wave trapping, recreating conditions that had occurred earlier in the project on March 4, with large trapped waves being generated by topography further upstream in Oregon (?). (In this latter case, the vertical shear of the NW wind appears to have been much stronger, producing strong resonant trapped waves that had reached as far south as Owens Valley; see notes on March 4 case). That expectation was based in part on the COAMPS forecast, which predicted vertical wave motions at 4 km with phase lines perpendicular to the NW flow on the order of 1.5 m/s achieving a maximum at 12 UTC March 15. No waves were visually observed nor documented by wind profilers in this case.
MISS was located at Independence airport where it had operated continuously for several previous days. Close to the beginning of the IOP around 02 UTC on March 15, the winds near the ground were southerly close to 13 knots, turning into the W-NW winds above 1.5 km. At 02 UTC, the maximum of that NW flow was around 15 knots at 2.5 km. After 06 UTC, between 2.5 and 3 km, the winds were more westerly, with the maximum of 20 knots reached at 13 UTC. Soon after that the event died out. The maximum altitude from which the winds were obtained by MAPR was 2.5 km at the beginning of this IOP, dropping down to 1.5 km by the end of the IOP, with a large gap in between 05 and 11 UTC (contamination of the signal by birds flying over?).
The radiosonde was launched in the valley at 12 UTC from the MAPR site. The winds documented by the radiosonde were weak below 700 mb. Between 700 and 200+ mb NW winds increased with height. There was an inversion and a shallow moist layer at the inversion base close to 680 mb.
One radiosonde was also launched upwind at the MGLASS site in Fresno (to train the personnel). That launch was at 22:16 UTC March 14 showing NW winds, increasing significantly in strength above 600 mb, and a layered moisture and temperature structure.