Warm Advection Wins Again: 2/24/01

Metr 698 Discussion Reading

Part A. Discussion of Dynamics

Part B: Graphics as Illustration

 

Part A. Discussion of Dynamics

What is the meaning of the term "Dynamics" that often appears in forecast discussions? For example, a forecaster may state "the dynamics are weak in this pattern." Well, we can't go over the various ways that the term "dynamics" is misused, but we can review how the term should be used.

The term "dynamics" relates to the synoptic-scale forcing for upwards motion in the mid-troposphere (and/or to the associated divergence in the upper troposphere). This "forcing" accounts for the lift associated with the development of synoptic-scale cloud and precipitation patterns AND (via continuity) to the surface pressure falls associated with "development" at sea-level.

Properly used, the term "synoptic-scale" EXCLUDES frontal lifting (either cold or warm), which is a mesoscale or subsynoptic phenomenon. For example, the following statement:

"...dynamics are strong in this case because cold advection is very evident behind the cold front.."

is incorrect because "cold advection" relates directly to the strength of a cold front and, more explicitly, to the strength of the mesoscale solenoidal lifting of the warm air ahead of the front.

The influence of "dynamics" is most properly assessed using the quasi-geostrophic omega equation. This is well-covered in Volume II of Bluestein's Synoptic-Dynamic Meteorology of Middle Latitudes, which is an updated version of Sverre Petterssen's treatment of the subject in Weather Analysis and Forecasting.

"Dynamics:" Poor Man's Qualitative Quasigeostrophic Omega Equation:

Upwards Motion
At A Given Level 

(Dynamics)
Proportional To Positive Vorticity Advection
Increasing With Height
1
Evaulated Across a Layer Centered at a Certain Level
and/or Warm Temperature Advection Evaluated At That Level

1Also true for negative vorticity advection decreasing with height.  Thus, an area of negative vorticity advection at 500 mb can be actually "forcing" upward motion if there is stronger negative vorticity advection below.

(Note: there are constants multiplying each term so the equation above cannot be used quantitatively)

A. Differential Vorticity Advection

The first term to the right is, in many instances, the ONLY term considered by forecasters when assessing "dynamics". This is an incorrect use of the equation.

The first term assesses the contribution of the geometry of the flow to the vertical motion fields. For example, in a short wave trough/ridge system in which curvature effects dominate, the upper tropospheric vorticity advection assesses the strength of the divergence at that level.

It is important to note that this term is the DIFFERENTIAL vorticity advection. For example, if through the layer 850-500 mb the positive vorticity advection increases with height, then this term returns "forcing" for upward motion at the centerpoint (say, at 700 mb) of the layer. Note that the vorticity advection does NOT cause the vertical motion, but is associated with it. Thus, the differential vorticity advection DIAGNOSES (does not cause it) the vertical motion field associated with the instantaneous geometry of the flow pattern.

Since vorticity advection close to the ground is normally negligible, this term is approximated by the vorticity advection at the top of the layer in question, say, at 500 mb. This gives forecasters "permission" to use the vorticity advection at one level (say, 500 mb) to assess the strength of the forcing from this term. Unfortunately, the resulting rule of thumb, "500 mb pva is associated with upward vertical motion" is misused because forecasters forget that there is another equally important term in the equation. Also, negative vorticity advection at, say, 500 mb, may be associated with forcing for upward motion by this term if the negative vorticity advection at lower elevations is a larger magnitude (as occurred for the 24 February 2001 case shown below).

B. Temperature Advection

The far right term estimates the forcing due to warm advection. This is the most neglected term in the equation, often not considered at all. Because it is often neglected or if forecasters simply do not consider it because they have a poor understanding of "dynamics", the reasoning of their forecasts will be poor and they can miss significant preciptation events in certain patterns on the West Coast.

You can tell when this happens because the forecast discussions will have bizarre explanations for the development of clouds and precipitation. There is an example below.

The temperature advection term also only diagnoses (does not indicate a cause and effect) the vertical motion field, in this case, due to temperature changes in the air column. Warm advection into an air column increases thicknesses locally, and raises the heights aloft (e.g., 300 mb level). In essence, this creates pressure rises at the top of the air column or layer in question (creates an isallobaric gradient), introduces an imbalance between the pressure gradient acceleration and Coriolis acceleration. To get back into geostrophic balance, the air at that level diverges, which continuity demands is associated with upward motion beneath (for the layer considered). Another way of looking at this is that since air motion is basically adiabatic (isentropic) and isentropes slope upward, air involved in warm advection MUST move upward.

In any event, warm air advection is associated with vertical motion SEPARATE from motions due to differential destabilization. I mention this because often forecasters refer to the vertical motion associated with warm advection as related to convective motions.

 

 

 

Part B: February 24, 2001: An Illustration

Consider the following snippets out of some Western Region forecast discussions for the morning of February 24, 2001:


"...OVERRUNNING RAIN HAS MOVED INTO THE DISTRICT EN MASSE BETWEEN 09 AND

10Z...WITH LIGHT RAIN DEVELOPING EVERYWHERE FROM MONTEREY N DURING

THAT HOUR. SATELLITE PIX SHOW ONGOING ENHANCEMENT OF THE CLOUDS

MOVING ONSHORE...A JET MAX AND SOME UPPER DIFFLUENCE* ARE WORKING

THEIR MAGIC...."

and

"...THE BEST EXAMPLE OF OVERRUNNING PRECIP YET THIS SEASON...."


*The forecaster alludes to the "enhancement of the clouds" and connects that with the divergence associated with the left-front quadrant of a jet strteak and "upper diffluence." As an aside, I remind students that "diffluence" is definitely NOT the same thing as divergence. Also, zonal jet streaks with divergence in their left front quadrants also show diffluence in the same region. Hence, in the discussion above, diffluence and the jet max are not separate "causes" for the cloud enhancement.


Here is an assessment of the "dynamics" based upon the QG-omega equation shown conceptually at the top of this page.

 

12 UTC 24 Feb 2001

Large area of enhancement develops in cloud band immediately adjacent to north-central and central California. By the way, this is a warm-front occlusion.

The sounding is pretty typical for such a feature. Note the frontal inversion. Also note the strong veering of the lower level wind field, consistent wiht strong warm advection.

By the way, although the air aloft has high relative humidity, the truly saturated air is in the lowest part of the troposphere, right where the warm advection is found

Radar (not shown) indicated the tops of the echoes to be all below 18000 feet (500 mb). This is not "overrunning" (precipitation falling from clouds above the frontal inversion).

 

 Surface Plot 12 UTC 24 Feb 2001

Large area of moderate rain is associated with development on the cloud band shown above.

 

 700 mb Vorticity Advection
12 UTC 24 Feb 2001

Negative vorticity advection is occuring in the area of enhancement. This is "forcing" for downward motion if the vorticity advection at the ground is considered neglible. I have exagerrated the vorticity advection by using a small contour interval. In reality it was very weak. This is emphasized by the 500 mb/vorticity chart shown at bottom.

 

 850 mb Temperature Advection
12 UTC 24 Feb 2001

Very strong warm advection is ongoing at 12 UTC. Note the center of the pattern is off the Central Coast. That is where radar indicated (not shown) VIP 4-5 echoes.

 

 500 mb Vorticity Advection
12 UTC 24 Feb 2001

Weak negative vorticity advection. Forecasters who only look at vorticity advection patterns at 500 mb would be thrown off guard and not anticipate the development that did occur associated with warm advection.