Moisture Surge Events

Building onto work that was shown on my last blog post, I will present my current work on describing poleward moisture surges of the NAM system.

I am still working with the sub-domains, or zones, that were defined in the previous post. First, I will show a figure of IWVF values (blue line) from 1984 for each of the three zones. Again, these values are the aggregated value for each zone for each day of the monsoon season (1 June through 30 Sept). On top of each zones IWVF will be plotted the 95th (magenta), 90th (cyan) and 80th (red) percentile values for each specific zone. The quantitative value for each threshold level will be indicated in the title of each subplot.

Interesting to note is the fact that the 95th percentile is larger for Southern Utah (SUT) compared to Northern Arizona (NAZ), 10.5 and 10.2 respectively. The percent difference is only 2.9% which is a neglagiable difference and doesn't necessarily constitute or reflect bad data or bad calculations.

Event Correlation Between Zones:

Lead/Lag correlations between zones will provide an understand of northward progression of moisture surges, as well as a hint at surge intensity. My calculation is based on events in Southern Arizona (SAZ) and looks at the correlation between the other two zones for Days -2 through Days +2 relative to the event in SAZ. Therefore, a 0.44 value for NAZ at Day +1 would mean that 45 percent of the events in SAZ had an event in NAZ the next day. Below will be three figures: The first will be 80th percentile threshold calculation for events, the second will be 90th and the third will be 95th.

There appears to be quite good correlation between all through zones on event days for SAZ. All figures also appear to have higher percentage values for post-event days, which makes physical sense when thinking about the typical northward progression of these surge events. At first I was a little surprised to see the left side of each probability curve having such high values. Once I spent a little time thinking about this I came to the conclusion that given the close temporal proximity of many of the surge events during active years, some of these high values could be attributed to surge events on previous days and not the one defined for the correlation.

Event Classification Scheme:

Onwards to defining what a surge event 'is' for this study. Based on these results, I propose the following classification algorithm for surge events in this study.

Foundation to this Criteria: 90th percentile threshold for each zone

Further refinements:
As has been shown in many recent papers, there is a push towards defining not only 'surge events' but also surge intensity, i.e., weak, moderate and strong.

Weak Surge: Only SAZ shows an event for a given day
Moderate Surge: SAZ and NAZ both show an event for a given day
Strong Surge: tSAZ, NAZ and SUT all show an event for a given day

With this criteria, I get the following numbers:
Weak Surge: 342 events over 28 yrs; avg 12.2 per yr; avg 3.1 per month
Moderate Surge: 217 events over 28 yrs; avg 7.8 per yr; avg 2.0 per month
Strong Surge: 150 events over 28 yrs: avg 5.4 per yr; avg 1.4 per month

Therefore, 63% of surges are 'moderate' and 44% are classified 'strong.'

I feel confident about this scheme, or as confident as I will ever feel about defining such dynamic weather phenomena.

-jamie