Analysis of Windstorm Criteria

From Hopkins (1994) ...

Colman and Dierking (1992), three criteria for windstorms:

1. strong cross-barrier flow at mountain-top level
2. an inversion at or just above the ridge top
3. a critical level (the cross-barrier flow decreases to zero) above the ridge top

The windstorms are driven by downslope winds, gap winds, or a combination of the two. The force balance in this situation becomes nearly antitriptic, with the wind at a large angle to the isobars.

For the first criteria, this requires a strong pressure gradient across the mountain axis. The pressure gradient increases with a deepening low approaching from the west, and a quasi-stationary high in the eastern Gulf of Alaska.

Relating this to the mid and upper level pattern, a sharpening wave pattern is favorable. A deep trough, tilting negative into the Aleutians supports a rapidly deepening surface cyclone. The downstream ridge must remain nearly stationary, and amplify. Downstream of the ridge axis, falling heights then sharpen / amplify the ridge, and increase upper level confluence supporting an intensifying surface high locked over the eastern GOA.

Comparing the 24-hour change in 500hPa heights for the strong cases versus the weak cases, the strong class exhibits all of these characteristics. The weak class amplifies the trough and ridge system, but it increases heights slightly over western Canada, keeping the ridge a little more broad, with weaker upper level confluence. As a result, surface pressures in the eastern GOA increase more in the strong class than in the weak class.

--> The second criteria point is related to stability. An inversion ... layer of high stability ... is required at or just above the ridge top. The layer is found between 800hPa and 500hPa, and must be near the critical level (third criteria point). One measure of stability that applies in this situation is the lifted index, which is the difference between the temperature of a surface-based parcel lifted to 500hPa and the environmental 500hPa temperature. A higher LI indicates higher stability.

Composites of the strong and weak classes shows that the strong cases feature an area of much higher LI over eastern Alaska toward ANC.

--> The third criteria point is related to vertical wind shear. Strong cross-barrier winds are required at mountain-top level (per the first point), and a level with zero cross-barrier flow is required further above the mountain-top (per the third point). The SW-NE orientation of the Chugach Mountains to the east of Anchorage require southeast winds at mountain-top, and southwest winds at the critical level. So strong WAA - veering winds with height (90 deg optimal) - are necessary ... with a resulting westerly shear vector.

The WAA also acts to decrease surface pressure on the west side of the mountains. In addition, the WAA helps amplify the downstream ridging, supporting strengthening surface high pressure on the east side of the mountains. Both of these results increase the cross-barrier pressure gradient

The strong class features a much stronger north-south temperature gradient than the weak class, which by the thermal wind balance supports stronger westerly shear.

850hPa temperature

One necessary note about the two classes:
The month of each of the weak cases averages +/-2.5 months from January, whereas the strong cases average only +/-1.2 months from January ... so the strong cases occur more frequently during the favorable winter season, which makes sense.

Starting with NCDC cases

Using the NCDC storm events database, reports were sorted for "high wind" in the "Anchorage Muni to Bird Creek, Alaska" area. The time period was very limited; only including October 2006 through December 2011. However this still returned 41 events with wind reports of at least 50kts in the area. These 41 events were then sorted by magnitude, and split into strong (20) and weak (21) cases. The median was 72.5 which was used as the benchmark for evenly splitting the cases. From this point, the date and time of each report was rounded to the nearest 6-hour increment.

Date         Time     Wind   Class
--------------------------------------
20061003     1200     65     Weak
20061008     1800     71     Weak
20061208     1200     89     Strong
20070125     1800     83     Strong
20070129     1800     73     Strong
20070201     1200     76     Strong
20070407     0600     61     Weak
20070907     0600     67     Weak
20071025     1800     73     Strong
20071108     1800     74     Strong
20071122     1200     67     Weak
20071125     0600     64     Weak
20071208     0000     77     Strong
20080120     0600     65     Weak
20080121     1800     74     Strong
20080219     1800     74     Strong
20080305     1200     71     Weak
20080401     0600     63     Weak
20081009     1800     87     Strong
20090113     1800     91     Strong
20090115     1800     65     Weak
20090329     0600     64     Weak
20091111     1200     71     Weak
20091130     1800     72     Weak
20100305     0600     69     Weak
20100510     1800     71     Weak
20100816     1800     63     Weak
20100924     1200     51     Weak
20101203     1200     72     Weak
20110101     0600     75     Strong
20110407     0000     87     Strong
20111014     0600     68     Weak
20111024     1800     68     Weak
20111025     0600     62     Weak
20111103     1200     77     Strong
20111203     1200    103     Strong
20111207     1200     75     Strong
20111210     1200     78     Strong
20111211     1800     84     Strong
20111217     1800     90     Strong
20111220     1200     73     Strong

Two files containing the date and time of the cases were created for the strong set and the weak set. 6-hour composites were then created from the NCEP/NCAR reanalysis using ESRL. The following variables were examined at both the report time and 24 hours prior:

• 850hPa wind 
• Sea level pressure 
• 500hPa heights 
• SB Lifted index 

850hPa wind 0hr

850hPa wind -24hr

Sea level pressure 0hr

Sea level pressure -24hr

500hPa height 0hr

500hPa height -24hr

SB Lifted index 0hr

With regards to the time shift concern:
Averaging the difference between the actual time of the wind report and the rounded time yielded an average of a 29 minute lag for the strong class, and a 33 minute lead for the weak class. In the composites above, the strong class does appear to lag the weak class in a number of features, however the 62-minute difference is too small of a magnitude to consider the differences on a synoptic scale. Therefore, analysis of the results will assume that all of the cases are perfectly synchronized by the time of the wind event, and that any differences in the composites are significant to the differences between strong and weak events.