IOP 8: Extended IOP 1800 UTC March 24 - 1800 UTC March 26
(1000 PST March 24 - 1000 PST March 26)
Core IOP 1200 UTC March 25 - 1200 UTC March 26
(0400 PST March 25 - 0400 PST March 26)

Southern Owens Valley; west of Independence
1500 PST
by Vanda Grubisic

Report written by Vanda Grubisic, Thursday, April 1

Synoptic overview:

With the offshore low-pressure system beginning to move inland, models (Eta, GFS, COAMPS) were predicting an extended 48-hour period of strong W to SW winds over the project area, with all three models being in good agreement for this event. Westerly winds, gradually increasing in strength, were forecast to reach two maxima, one at 12 UTC March 25 (Thursday) and the other at 12 UTC March 26 (Friday). Forecast winds peaked at 35 knots at 700 mb and 55 knots at 500 mb. After the frontal passage on Friday, winds were predicted to shift to a NW direction, ending the event.


The plan of operations for IOP 8 on March 24-26:

1. MAPR: on all the time, multiple radiosonde launches during the core 24-hour period, additionally a radiosonde launch at 00 UTC March 25 and 18 UTC March 26
2. MISS: to be located at Independence airport, multiple radiosonde launches during the core 24-h period
Total of 9 radiosonde launches from the valley during the core 24-h period, and 11 sondes during the extended IOP
3. MGLASS: radiosonde launches from Fresno every three hours starting at 12 UTC March 25, ending 12 UTC March 26 (9 sondes total)
4. Lemoore: radiosonde launches every six hours starting at 00 UTC March 25, ending at 18 UTC March 26 (6 sondes total)

Radiosonde launch schedule for IOP 8 from 18 UTC March 24 to 18 UTC March 26
March 24March 25March 26
180006091215182100030609121518
LemooreXXXXXX
MGLASSXXXXXXXXXX
MAPRXXXXXXX
MISSXXXX

Summary of operations and observations:

This was a strong wave event with rotors. Extended periods of westerly winds were documented by the ground stations during this IOP. Brief periods of easterlies, within the extended period of westerly winds, documented at a few stations in the middle of the valley during the core IOP. The wind profilers documented extended periods of strong positive and negative vertical motions at or below mountain-top height and strong westerly wind. Several radiosondes in the valley were caught in the strong updrafts and downdrafts making downward excursions (as large as 1.5 km downward) during their ascent. Wave and rotor clouds were present paralleling the Sierra. Rotor clouds were substantial, in places having the appearance of a convective cumulus with a flat base.

The event started around 18 UTC March 24 (10 PST) with wind at station 1 of the ground network turning westerly with gusts up to 16 m/s. It is possible to clearly define this beginning as wind at station 1 at this time of the day is typically easterly.

MISS was located at Independence airport, where it had been working continuously during the past two IOPs. During this two day event, winds documented by MISS were westerly from the ground up for most of March 25 (except for a few hours around 15 UTC with northerly winds below 1 km) and a rather complicated wind field from 01 to 06 UTC on March 26 with some easterly winds around 1 km (an elevated rotor?). Toward the end of the IOP, winds turned more into a N direction. The strongest winds of close to 20 m/s were measured around 13 UTC March 26 at 3 km and higher up. Strong vertical velocities were measured by MISS throughout this period.

Two periods of strong W winds at the ground were recorded at the MISS site, one in the afternoon of March 24 (from 21 UTC March 24 to 04 UTC March 25) with the maximum speed of 9 m/s, and the other during the afternoon of March 25 (from 22 UTC March 25 to 07 UTC March 26) with the maximum speed up to 10 m/s. Weak E to SE winds of less than 2 m/s at the MISS site were measured from 16 to 18 UTC March 25 (a footprint of a rotor?).

The sounding data from the MISS site show a stable layer with the base around 600 mb, except for the last one (09 UTC March 26) where the base of the stable layer was somewhat higher. The sonde launched at 03 UTC was twice caught in strong downdrafts near 2 and 4.5 km. Winds were generally westerly increasing in strength above 2 km.

At the MAPR site, two episodes of westerlies at the ground were also observed by the MAPR AWS, one with low speeds around 2 m/s between 03 and 09 UTC March 25 (later afternoon/evening/night March 24), and the other between 01 and 02 UTC March 26 (afternoon of March 25) with speeds up to 8 m/s. Westerly winds of 15 m/s reaching down to 1.5 km above ground were documented at this site for the most part of March 26 (note that there was a change in the MAPR mode around 00 UTC on March 26, which improved the wind retrievals). During most of this IOP, MAPR was recording hour-long periods of positive and negative vertical velocity at or above 3 km. The most intense vertical motions were recorded by MAPR around 12 UTC on March 26, with a dipole of 6 m/s updrafts/downdrafts within half an hour (evidence of subrotor structures?).

Owens Lake; west of Independence
1620 PST
by Vanda Grubisic

Five radiosondes were launched from the MAPR site during the core IOP period from 12 UTC March 25 to 12 UTC March 26. The sonde released at 00 UTC March 26 was caught in a strong downdraft at 5 km, which took it down to 3.5 km, at which point it started rising again. The sonde released at 12 UTC March 26 was also caught in a strong downdraft at 5 km (another sonde was released an hour later that wen straight up). All soundings show a stable layer with a base near 600 mb. All sounding during the core IOP period show WSW winds. The last sounding carried out from the MAPR site during this IOP at 18 UTC March 26 already showed NW winds above 500 mb.

Nine sondes were released during the core IOP period from the MGLASS site. The 03 UTC sonde launch was attempted twice, but was unsuccessful both times due to freezing rain. These upwind soundings are characterized by strong WSW winds, and layered temperature and moisture structures. Between 00 UTC and 09 UTC March 26, the air below 2 km had become saturated.

During the entire IOP, five radiosondes were launched from Lemoore. 00 UTC March 26 sounding was lost due to problems with their computer equipment.

Foehn Wall, Sierra Nevada; north of Bishop
1130 PST