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 hPa isotachs (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...
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 hPa isotachs (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 hPa isotachs 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.
-jamie
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 hPa isotachs 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.
-jamie
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