Below will be a look at the variability due to seasonality in the EOF components. This look is based on the fact that through my event classification scheme the preponderance of events occur later in the year, late August through September, then seen in previous works, July through August.
Figure 1: Principal Component 1 for both seasons
Figure 2: Principal Component 2 for both seasons Figure 3: Variance of the first 10 components for both seasons
With PC #1 a well defined trough (lower heights region) is present over central Alberta that stretches southward into southern California. But, with a shift in the time frame from July/August to mid-August/September we see this feature become much more well defined. This makes physical sense (we are moving closer to a time of year where more mid-latitude 'action' is found further south) and it also falls in line with our findings of trough interactions being important components or drivers of surge events in the late Summer.
Comparing the first to the second principal component shows that the July/August season is picking up more of a trough signature than that of the later season. This trough feature is associated with roughly 20% of the variance for mid-Aug/September and roughly 14% for July/August. It is reassuring to see that this trough feature is the leading component for the mid-Aug/September season in that it lends more foundation to our findings of the importance of these features.
The one issue I am having qualms with is the absence of the monsoon ridge over the Four Corners region. I believe we could be seeing some indication of this feature in PC #1 for July/August, but it appears very weak in nature. The problem could be that we are simply looking slightly too high in the atmosphere to pick out a strong signal with this feature, or perhaps my domain is slightly too large to pick out what might be considered a subtle feature in context of the overall synoptic and planetary patterns.
Here is a composite of the 500 hPa height pattern associated with strong events. Though we were questioning whether the 500 hPa height anomalies were showing weakening of the monsoonal ridge or an actual trough, it appears quite clear from the actual height patterns that the anomaly feature is indeed associated with a strong trough moving in from the Pacific.
Ridging, though relatively weak in nature, does appear to be in place slightly east of the Four Corners (i.e., Four Corners' High) on Day -4 and -3 to the event. At this time we are already seeing signs of a deepening trough off Northern California. As we progress closer to the event day the trough deepens dramatically, nearly explosively, and progrades into regions where it could potentially affect the characteristics and development of the NAM system. The actual height patterns and the anomaly fields display a strong spatial relationship to one another, and thus, I feel confident say that the anomalies we are seeing are associated with a trough feature.
There are still quite a few questions regarding the origin of this trough feature though. More on that to come soon.
Following the same outline as laid-out in the most recent post regarding composite analysis, I will now show composites for large scale fields in relation to defined surge events.
Weak Surge
Weak Surge: Non-common with Strong Surges
Again, plots are shown in three day prior through 3 days after a surge event (middle column).
For these figures the top row is precipitation anomalies (0 to 5 mm/day), the second is 200 hPaisotachs (50 to 65 mph= Light Blue, 66 to 80 mph = Yellow), the third is 500 hPa height anomalies (-120 to 120 meters, blues = negative anomalies, yellows = positive anomalies) and the fourth row is 850 to 500 hPa lapse rate anomalies (-5 to 5 degrees Celsius, same color conventions as height anomalies).
Though I showed both for comparison, I will limit my discussion to only the non-common weak event plot since I feel it is more indicative of the true weak events. Isotachs show relatively zonal flow. From the 500 hPa height anomalies we notice pronounced ridging (positive anomalies) over the northern Pacific, and from event day on we notice troughing (negative anomalies) over the western United States. The ridging over the northern Pacific could potentially be a related to a slightly northwestward shifted North Pacific High. The ridging followed by troughing over the western United States is most likely related to surface warming then cooling (associated with precipitation) ahead and behind, respectively, of the surge event.
850 to 500 lapse rate anomalies show a clear, and persistent pattern throughout the composite. A couplet of positive and negative anomalies appear adjacent to one another over northern Mexico during the preceding days till the day after the event. This feature I am proposing is associated with some voriticty feature, either an easterly wave or some smaller scale vorticity lobe (TUTT?), that is causing upward and downward motions in the atmosphere. The upward (downward) motions are causing a stabilizing (destabilizing) of the atmosphere, i.e., negative (positive) lapse rate anomalies.
The patterns seen in these figures fit well into the classical model of how a 'typical' NAM surge event should happen.
And now onto the interesting stuff...
Strong Surge
First, let's point out what appears to be similar. From the 850-500 hPa lapse rate anomalies we are still seeing the stabilizing of the atmosphere post surge. From the 500 hPa height anomalies we see the increased heights (surface warming) over the western United States associated with some thermal ridging. From that point on things seem different...very different.
200 hPaisotachs are now less zonal and are showing some signs of a trough progressing inland from the Pacific Ocean. Most striking off all are the results shown on the 500 hPa height anomalies that stand out from previous examples. The composite shows a dramatic and well-defined area of negative height anomalies (up to -120 meters!) moving in off the west coast. The feature is most certainly an indication of a strong trough feature, the exact type of extratropical control feature we have been hoping to see with our research. By comparing the precipitation anomalies with the 500 hPa height anomalies you can see that the precipitation stays just downwind adjacent to the incoming trough. Clearly, the trough feature is playing some role in progressing the NAM surge event system.
Here are some of my thoughts so far on how things are starting to look: (1) Weak events, as we have defined them, might end up being the ones tied to the tropical, e.g., easterly wave, controls. (2) Strong events where precipitation is seen further north into the Great Basin regions, i.e., Andy's work, might be the ones intimately tied to the extratropics.
As you can tell, I am more than a little excited by the results from the strong events.
To provide a prospective on the time-series of IWVF over the JJAS 'season,' a figure showing the day of maximum IWVF (daily average for 28 years) is shown. Days are broken down as follows: June: 1-30 July: 31-61 August: 62-92 September:93-122
The data shows that for the region most associated with NAM surge precipitation, i.e., AZ, western NM, southern UT and NV, and southwestern CO, the peak occurence of IWVF typically falls later in the season between mid-August and mid-September. Our data has been pointing towards this fact, but it is still striking to see how much of the region is dominated by later-season IWVF than most previous research has accounted for. What I am specifically revering to is that most NAM surge research is limited to only July and August analysis, thereby ignoring the vast majority of IWVF events, and most likely surge events, that are taking place.
To back up this point, draw your attention to the Yuma, AZ location (extreme southwestern AZ; Data point where most previous research bases 'surge event' definitions on). The peak for this region appears around day 70 (early August). So, if data analysis were limited to this region a season of only July and August observations might make sense. But, as data points out over the larger domain, this is merely an artifact of where you gather data, not actually physical manifestations for the region.
There are some swearly data areas where very early season peaks are shown directly adjacent to very late season peaks, e.g., northern Mexico. I believe this is an indication of regions with little variability in IWVF amounts during the period analyzed, therefore, resulting in drastic changes in date but not necesarrily amounts.
I believe there were some issues inherent to the way I was previously defining two day events in the IWVF field. For example, in a situation where there were four days straight above 90th percentile, then my code would have registered 3 events. To combat this flaw I went through and defined a two day event as:
2 consecutive days above 90th percentile for a zone where event day - 1 must not meet criteria
This appears to have helped out very well, and thus, I will show new and improved (and shiny, I might add) results.
Note: Since MATLAB disagrees with you about how large certain subplots should be when you start adding in colorbars, I will simply state data ranges here:
These restrictions are based on observed values in the data and which value range best described the data.
(1) Event for SAZ Zone The figure includes IWVF and anomalies of Dewpoint, Precipitable Water and Precipitation. Anomalies also helped to weed-out patterns present in these fields that were not easily decernded from the basic field values. We clear see a northward flux of dewpoint, precipitable water and precipitation in relation to IWVF events. Precipitable water shows the most concurrent signature with IWVF, which should be expected as these fields are based on similar data. We can now see the slightly delayed, by one or two days, flux of precipitation and dewpoint into the AZ, NM and UT domains. As described more fully in a previous post, this is a feature we expect to see. So, things are looking good.
(2) Event for SAZ and NAZ Zones
Features are similar in this figure as with (1) except that we are seeing a stronger signal in all fields compared to (1). This should be expected since we are further refining our events to cases where IWVF moved more northward, thus a strong flux.
(3) All Zones (1 Day Criteria)
So, the final figure is for an event that shows up in all three zones on a given day. This event is not restricted to 2 consecutive days criteria as with above examples. Most of the signatures appear before event day for this criteria. What we are pretty much looking at in this 'event day' is the height of activity, i.e., furthest northward flux. If we were defining events of 2 consecutive days for SAZ and NAZ then it would probably show up on Day-1 here. Again, signals are very amplified here, as we should expect.
Well, I am pretty to call events defined. So, Events = Defined:
Weak Event = 2 consecutive days of IWVF at or above 90th percentile for SAZ Moderate Event = 2 consecutive days of IWVF at or above 90th percentile for SAZ and NAZ Strong Event = 1 day of IWVF at or above 90thpercentile for SAZ, NAZ and SUT
I will be showing composites of three different surge criteria:
(1) SAZ Zone IWVF above 90th ( 342 Total: 12 per Year: 3 per month) (2) SAZ Zone IWVF above 90th for 2 consecutive days ( 146 Total: 5 per Year: 1.3 per month) (3) All Zones IWVF above 90th ( 47 Total: 1.7 per year: 0.4 per month)
These criteria were chosen empirically, and simply based on which results I found interesting or best told a story.
All composites will be shown for 3 Days Before through 3 Days After an event. ==================================================================== First, let's look at plot (1) I suppose my choosing this criteria was slightly less empirical than the other two simply because I felt as though looking at results for southern Arizona might provide some strong signals. The strongest signal composite appears to be between IWVF and precipitation. A clear northward flux of precipitation over the Great Basin can be seen to correlate temporally with Day -1 through Day +1. Precipitable Water values do appear larger during this same time period, but since the general spatial pattern of values for this parameter does not appear to be dynamic I am wondering if IWV results should be suspect. Having tried to use IWV in the past for defining surges and looking at other composites this issue has come up before. So, I believe that these values might be a little shifty in the model output. Again, there does seem to be a clear signal in the precipitation field in relation to these IWVF events.
==================================================================== Next, I wanted to look at events of two consecutive days of the above criteria. I felt that by further restricting my dataset, i.e., number of events, that I would get a clearer picture of patterns in the system. Patterns here are very similar to what was seen in the previous composite. There is still a strong temporal agreement between IWVF and precipitation. Something I found interesting in this plot though is that on Day +1 there seems to be a more intense region of precipitation further north into southern Utah and Colorado. I believe that this is indicative a stronger surge event, which would agree with our criteria of needing two consecutive days to reach or exceed the 90th percentile in SAZ.
==================================================================== Finally, I looked at events of all three zones meeting or exceeding their 90th percentile on the same day. This criteria should allow us to look at the very strong surge events, and hopefully show us heightened signals compared to the previous two plots. Indeed, we are seeing signals that one would suspect to be associated with strong surges of the NAM. Precipitation appears to be moving quite far northward now, and it also seems augmented compared to previous plots. At first, I felt as if my calculation might have been off based on the fact that precipitation seemed so heavy in Arizona for Day -1, and even Day -2. I don't believe this is an error, but is simply the first stages of the northward flux of moisture. If we looked at events for SAZ we would see that the majority of them are centered around Day -1 for this criteria, in place of the event day. This is shown in Figure 1 from 5 Sept 2009.
Again, the IWV values seem rather questionable. They do show some sign of being heigthened on Day -3 through Day -1, but I still have my reservations about trusting this data. One feature I have noticed in all plots is the elevated values of IWV in southeastern Arizona. I realized after looking at numerous plots that this region of elevated values of IWV is spatial correlated to a region of very low, for the region, precipitation values. This only makes me question this data more as I feel like this just does not make solid physical sense. The only connect I could make between the two is that the model is representing moisture that has not been precipitated out yet in this region. I am baffled.
You may have noticed that I left out dewpoint from this figures. Plots of dewpoint for all figures were ineffective at showing any association with the larger pattern or with IWVF events. I assumed, again, that my calculations were off, but after looking back through codes I found that dewpoint was calculated no different than any other parameter, and thus should not be showing signs of bad data manipulation. Once again, I am baffled.
I feel that the strong composites between IWVF and Precipitation is good, and expected, sign. This is showing that we are not only picking up events of IWVF, but that those events are associated with elevated precipitation patterns for the desert southwest. Seeing further northward progression of precipitation appear as we further restricted IWVF events to tougher criteria is also a good sign as it shows we are able to classify the stronger surges through IWVF values.
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.
Figure 2: Principal Component 2 for both seasons
Figure 3: Variance of the first 10 components for both seasons
