Amplification of the pattern

[Andy Wehrle]

Forecasts from the SPC and CPC often mention an "amplifying" or "deamplifying" pattern. What does this mean and what is its bearing on severe thunderstorm potential? :?

 

[Robert Dewey]

While I haven't looked it up, I have always assumed it to mean;

The height gradient is tightening and wind speeds are increasing (usually in the mid levels). This would be the case if a shortwave was moving into the region, or a strong jet max (or even large scale jet stream) was pushing in.

I'm sure Mike Geukes, Glen Romine, or someone else will correct me :lol:

 

[Nic Wilson]

Forecasts from the SPC and CPC often mention an "amplifying" or "deamplifying" pattern. What does this mean and what is its bearing on severe thunderstorm potential? :?

"Amplification" when used in its technical meteorology definition refers to the exisistence of long waves in the synoptic upper-level wind pattern. Lately, we have see a majority of zonal flow (mid-latitude winds from west to east across the Northern Hemisphere). This largely favors a summertime ridge as has been seen across the central United States and is not favorable for strong upper-level winds. This zonal flow is known as "deamplification" and is not favorable for widespread severe outbreaks. The introduction of long waves into the synoptic pattern enhances geostrophic imbalance and produces jet maxes as they round the base of the long wave trofs. For a chaser, amplification of the pattern is a great thing to hear as the presence of a longwave enhances the formation of a powerful low pressure system and typically brings in the other features such as strong upper-level winds and directional shear that are necessary for severe weather.

One more thing of note: Amplification of the pattern typically needs more than an imbedded shortwave. The only recent weather events have been caused by small shortwaves imbedded in the zonal flow.

 

[Glen Romine]

I'll elaborate on this a bit - wouldn't want to let down my fan club . As Nic started to say - with amplification/deamplification we are talking about the upper level pattern of troughs and ridges - but with amplification we are referring to an increase in the amount of north-south extent of the trough/ridge pattern, which can mean a digging trough (lowering heights within the base of the trough), a building ridge (heights rising within the crest of the ridge) or both. Since intensifying surface cyclones are associated with increasing cold air advection to the west of the surface wave (which leads to lowering heights in the base of the trough) and warm advection leading the cyclone (causing increasing heights in the ridge leading the cyclone), and as the pattern becomes more amplified the upper level divergence increases (often called the curvature effect), lowering the pressure of the surface cyclone and in turn increasing the winds around the cyclone leading to even greater cold/warm advection and pattern amplification. De-amplification is therefore the opposite, namely a tendency for weakening of the upper air patern toward a more zonal (E-W) flow as opposed to the more meridional (N-S) flow pattern. As Nic said, the typical springtime severe weather outbreaks are often associated with an amplifying upper-level wave pattern. However, most of the best severe weather events are from smaller waves emerging in only weakly amplified patterns, as highly amplified patterns usually leads to unfavroable deep layer shear profiles near the surface low (such as southerly winds at 500 mb - rarely a good thing).

Glen