Water vapor imagery and impulses

[Rob H]

Something I've been wondering about for some time... I often see SPC outlook text and some individuals' forecasts mentioning impulses and disturbances that are visible in the water vapor imagery. How do you identify them on WV loops, what causes them, and how do they affect the "ingredients" of severe weather (lift/moisture/instability/shear)?

 

[rdale]

http://www.meted.ucar.edu/norlat/sat_features/

Part of:

http://www.meted.ucar.edu/topics_satellite.php

Also from VISIT --

http://rammb.cira.colostate.edu/visit/sat_signatures.html

from:

http://rammb.cira.colostate.edu/visit/topic_sat.html

 

[L.B. LaForce]

May I suggest this book also:

http://www.amazon.com/Weather-Analy...r_1_10?s=books&ie=UTF8&qid=1280254900&sr=1-10

 

[Jeff Duda]

When meteorologists speak of impulses, disturbances, or pieces of energy, they are generally referring to a perturbation in a constant pressure surface, the most commonly used is 500 mb. All of those terms mean the same thing, and they imply the existence of a trough (I suppose a ridge is a disturbance, too, but no one really refers to ridges as "disturbances"). Impulses carry along with them some relative vorticity (cyclonic for a trough, anticyclonic for a ridge), and by quasi-geostrophic theory, differential vorticity advection leads to large scale vertical motion (differential cyclonic vorticity advection leads to upward motion). This rising motion helps destabilize the atmosphere by lifting layers and either releasing potential instability, eroding a cap, or just increasing lapse rates. This is what makes impulses visible on water vapor satellite imagery - the presence of moistening or drying due to upward or downward large scale motion. Since adiabatic upward motion causes increased RH, you will see higher brightness temperatures (brighter) where there is upward motion immediately downstream of an impulse, and lower brightness temperatures where there is downward motion (drying) just behind the impulse.

 

[Patrick Marsh]

This is what makes impulses visible on water vapor satellite imagery - the presence of moistening or drying due to upward or downward large-scale motion. Since adiabatic upward motion causes increased RH, you will see higher brightness temperatures (brighter) where there is upward motion immediately downstream of an impulse, and lower brightness temperatures where there is downward motion (drying) just behind the impulse.

Jeff, you've posted a pretty decent synopsis here. However, I have a couple of points to caution you, any anyone else examining water vapor imagery for troughs.

1. As air enters the upstream side of a trough, it typically descends. This is why we see a darkening in water vapor imagery, adiabatic descent warms and dries the parcel. As this parcel continues to move through the trough, it begins to encounter the area of rising motion downstream. As it initially beings to rise, it will still appear as a dark spot in water vapor imagery because the parcel being lifted is so dry that the mid-level moisture is not sufficient to be sampled. So, in the presence of strong troughs, keep in mind that ascent might actually extend farther upstream that water vapor imagery suggests.

2. In the event of tropospheric folds and couple jets, it is possible to have ascent through a substantial portion of the troposphere, without observing high moisture values in water vapor imagery. This is because water vapor channel essentially samples the air in the tropospheric fold, which is dry, and misses the moist air beneath the tropospheric fold.

Again, please don't take these comments as criticisms; what you've written is very good overview. I just wanted to draw attention to these two situations where the "obvious" answer might be wrong. As great as water vapor imagery is, to be able to use it effectively, it must be analyzed with an understanding of the associated theory! In any event, I'm sure there are other cases as well that require additional interrogation, but these two jumped out at me as they are relatively quite common.