Plans for week 3

I have finished the sounding composite program. The averaging tends to smooth much of the detail out of the soundings, so to supplement the composite, I'll include each case sounding overlaid on the same plot. This will give some idea of the spread and common features. I will do this for the top 10 strong cases and bottom 10 weak cases.

Download NARR data for top 5-10 cases and bottom 5-10 cases to initialize WRF. Model should be run 18 hours from the time of event to 6 hours after, each for 24 hours.

From the model data, I am interested in finer time resolution soundings and locations of features that I have pointed out in the synoptic composites.

From this information, combined with the Anchorage-Cordova pressure gradient, and the sounding parameter data, I can construct an empirical predictive formula. I have already ran a few tests that return a greater than .50 correlation and only only a 7% error.

Sounding Parameters

Using a program to pull sounding data from the U Wyoming archive, I was able to isolate certain characteristics of each case sounding. I focused on three primary supporting features of high wind events:

• Low level cross-mountain flow
• Critical level where cross-mountain flow drops to zero
• Inversion or stable layer aloft

To produce quantitative data, I averaged the sfc-850mb wind for cross-mountain flow, yielding an average direction, average speed, and the component crossing the mountain from the southeast. I calculated the height of the critical level by the level at which the component of the wind cross the mountain dropped below 5kt. I tested for the existence of an inversion layer between 850mb and 500mb, and determined the height of the bottom of the inversion as well as the vertical depth of the inversion.

These six variables were then compared to the magnitude of the wind report in each event. The inversion and critical level data was promising: The stronger events more frequently had an inversion present, with the bottom at a lower height than for weak events, and the depth of the inversion larger than for weak events.

The critical level was also generally at a lower height for strong events and higher, or not present at all during weak events.

Interestingly, the cross-mountain flow shows a small negative correlation to the magnitude of the wind event, which I'm not sure what to make of ... weaker events tended to have stronger low level cross-mountain flow...

Quantitative Data

Beginning with the Anchorage-Cordova pressure gradient, I want to look at more quantitative data in order to develop an empirical forecast method. The composite maps give excellent qualitative information on the synoptic situation. One possibility is to pull the prevalent features from the composites to produce usable data. My current project is to examine sounding data, generating quantitative information for each case on the important features (Case Soundings). I have begun writing a program to pull this data from the UWyoming archive.

Anchorage-Cordova Pressure Gradient

The gradient at the time of the report shows little correlation to the magnitude of the report. The maximum gradient within 18 hours of the report exhibits more of a positive correlation (+0.40).



The time difference shows a positive correlation to the magnitude of the report (+0.31). This suggests that the maximum pressure gradient tends to lead weaker events and lag stronger events.



Possible plan:
Develop an empirical formula for forecasting high wind events.


Important, quantitative factors...

Analysis of change composites

This is analysis for the composite maps in the previous post

These change composites returned some interesting results.

In the strong cases, the greatest magnitude SLP change is -22hPa over southwest Alaska associated with the low. The maximum change is +12hPa around Juneau associated with the downstream high. This produces a strong isallobaric wind from the east centered around Anchorage. In the weak cases, the greatest magnitude SLP change is +15hPa near 50N/170W associated with the upstream high. The pressure tendency with the low is much weaker and more diffuse than in the strong cases, and the downstream high only has a +5hPa change. The strongest isallobaric wind in this domain is thus over the Aleutians between the upstream high and the low, rather than around Anchorage between the low and the downstream high.

The 500hPa has similar results. The strong class has the greatest change couplet from the trough and downstream ridge, while the weak class has the greatest change couplet from the upstream ridge and trough. The couplet is also much stronger in the strong class: -180m to +150m versus +90m to -80m in the weak class. The differences in height tendency with the downstream ridge is the most significant result. The strong class features a +150m change in the northern GOA, while the weak class has only a +90m change further southeast. As a result, there is also a significant contrast in height change specifically over Anchorage. In the strong class, heights rise 80m, and in the weak class, heights are about constant.

Another point is that the strong class exhibits well defined maxima and minima in height change, with a predominant negative tilt. The weak class has more elongated maxima and minima that are predominantly positively tilted. What this and the above observations suggest is an amplifying wave pattern in the strong class and a more progressive pattern in the weak class.

Much of the evidence here points to the strong cases featuring a much more intense low that remains the dominant feature in the domain. The weak cases exhibit a weaker low that is clearly also undergoing secondary development to the southeast. This weakens the pressure gradient and re-orients it more toward the NE-SW over Anchorage. The dominant feature becomes the upstream high over the northern Pacific.

The tendency in the lifted index shows a significant increase in stability in the strong class versus only a small rise in the weak class. Over Anchorage, the LI increases 5C in the strong class and 1C in the weak class.

Composites of variable derivatives

The top ten strongest wind events in the NCDC storm reports were compared to the weakest ten events. Composites were made of the 24-hour change in: 500hPa height, sea level pressure, SB lifted index, and 850hPa omega.

850hPa Wind

Sea level pressure

500hPa heights

SB lifted index

Plans for next week:

• Analyze variable derivatives ✓
• Quantify gradients: Cordova to Anchorage for each case
• Case by case SLP and LLJ
• More in depth with soundings

Case soundings

ANC Soundings from the nearest 12-hour increment PRIOR to the high wind report. These four soundings are from some of the strongest events, and include well defined specific features favorable for windstorms. These features include:

• Strong low level southeasterly winds
• Winds veering with height, becoming southwesterly in the mid levels
• Conditionally unstable mixed boundary layer
• Mid level temperature inversion or stable layer
• Conditionally unstable layer above the inversion

Future ideas: Create composite soundings for strong versus weak classes. Compare these specific features in the composites.