26/27 Nov 01. Note the canyon-looking (BRP) feature over the central US.

First thing to remember: TROWALs are canyons on isentropic surfaces -

that is, they are regions of local warmth. We will be considering

TROWALs that are associated with occluded cyclones. Of course, a local

canyon on an isentropic surface need not be associated with a synoptic

cyclone, but for the purposes of this talk, TROWALs accompany occlusions.

Note that D3D is a very handy way to visualize TROWAL features.

may not help you get a leg up on the model forecast - but it will help

you understand why the model forecast is doing what it is doing. Also,

you'll better understand a structure observed in extratropical cyclones.

May be a different way of looking at already-known structures in a storm.

How can you use AWIPS to find TROWALs?

the 1950s by Canadians, then the concept was neglected/forgotten;

Reintroduced by Martin. Original TROWAL literature: focus on structure,

not dynamics.

concepts weren't widely taught. Observations showed tropical air aloft

moving poleward from the point of occlusion. Red boxes highlight

observations of warm air in each study. A lot of observational studies.

models: Norwegian Cyclone Model, warm conveyor belt [WCB] and cold

conveyor belt [CCB] of Carlson. The benefit of these models is that they

give an intuitive grasp of where to look for strongest rising motion,

and how the storm will develop. Also gives a 1st guess of airflow

through the system. The Norwegian cyclone model describes the evolution

of a family of storms along the Polar Front. Incipient->developing->

mature->occluded storm along a front.

belt, cold conveyor belt and dry airstream are described. Observations

suggest that the TROWAL airstream is west of the warm conveyor belt (in

some but not all storms) so there can be ascent west of the limiting

streamline. The TROWAL airstream is characterized by strong ascent over

the cold conveyor belt, and cyclonic turning towards the surface

cyclone. Despite the Limiting Streamline (LS), air moves from the warm

sector to near the Low, contrary to Carlson's conceptual model. Note

that the TROWAL airstream turns cyclonically with rising motion over

cold air; the warm conveyor belt turns anticyclonically downstream

as it rises.

Precipitation is aligned with the trough of warm air aloft, which is

represented by the notch in the (isentropic) surfaces

Height of TROWAL increases steadily as you move from its roots in the

warm air

Note that there is implied vertical motion in this figure, implied both

by the precipitation and the slopes of the isentropic surfaces.

A TROWAL ascends - therefore it moves through isobaric surfaces. The

correct surface to see a TROWAL on is an isentropic one (either theta or

theta-e, depending on the moisture present).

module at least are associated with occluded cyclones. That's not to say

that they aren't found elsewhere in the atmosphere, but the structure

we're talking about today is linked to occlusions.

TROWALs are manifest by a canyon on an isentropic surface. A region of

locally warm, moist air. Think about what this means for static

stability and moisture availability.

Because the isentropic surface is strongly slanted, it is a region of

potential horizontal frontogenesis because a strong horizontal

temperature gradient is in place

Warm moist air north of the warm front. No need to worry about

convection to the south robbing your moisture in a winter storm (as

happens so often in central Illinois for WI storms!) Warm moist air is

already present if a TROWAL is there.

Frontogenesis + warm moist air + strong front: Strong vertical motion

and lots of precipitation

Knowing where the TROWAL is: where to focus on development of important

weather.

What causes the vertical motion? Could be isentropic upglide. More

likely, since this is a dying, occluded storm: frontogenesis.

in the canyon on the title slide) can lead to vigorous ascent if the

wind is blowing up the surfaces. Also, strongly sloped isentropes mean a

strong horizontal front - potential for strong frontogenesis and the

attendant circulations. If you know where the TROWAL is, focus on that

region for a big response from any incoming atmospheric forcing. TROWALs

correspond to regions of low stability, more moisture, and strong fronts.

And remember, since this is in an occluded system, you might be expecting

things to be spinning down, but if a TROWAL is present, that isn't

necessarily going to be the case.

"regular" format and Q-vector notation.

along-front and cross-front. Change in Qn affects the magnitude of the

temperature gradient, changing Qs alters the orientation of the

isotherms. In other words, Qn forcing strengthens the front; Qs forcing

changes the orientation of the front. Qn forcing is something like

deformation, with the deformation axis parallel to the front. Qs forcing

is caused by cyclonic thermal vorticity advection by the thermal wind,

one of the forcing mechanisms in the Sutcliffe Development Equation.

Consider an initial straight front with convergent Qs at some time. How

will this evolve with time? Last slide: Forcing Qn will changed the

packing of the isotherms, so this front should not get stronger/weaker.

The isotherms will not become more closely packed. Their orientation

will change. Along-front forcing, which is not always what you think

about in frontogenesis.

could be how TROWALs form - because TROWALS are also canyons on

isentropic surfaces. Note that because there is no Qn forcing, the

isentropes do not become more tightly packed.

them on a surface or upper air chart or in a satellite image?

intrusion wrapping around the occlusion. S-shape to isotherms. Also,

since we've been referencing theta-e surfaces - make sure the atmosphere

is saturated so that theta-e is the relevant dynamic surface.

coding is the pressure at which the theta-e surface occurs. The red

pressures are near 950 mb, the bluest are near 600, the purple near 400.

Nice wrapped system. In a live session, a student would be asked to

identify where the low pressure should be, and where the warm air has

wrapped around the storm. (This should be an Easy Question). The TROWAL

is a tongue of warm air/high pressures on theta-e surface.

Consider this storm over the Pacific. The GFS data shows the surface

pressure overlaps where the clouds put the storm. Well-mixed temperature

structure at the surface, but a nice tongue of warm air at 700mb from WA

northwestward into the G. of AK.

Note that the terminus of the TROWAL seems to be where there is active

precipitation - very steeply slanting isentropes in that region, a

strong front.

Remember: the 700-mb temperature is on an isobaric surface; TROWALs

follow isentropic surfaces. The isentropic surface may move through the

isobaric surface.

Low Sun-angle shots or animation that allow you to discern different

airstreams.

sun-angle shots. Note all the different cloud heights in the eastern

system. The WCB is clearly visible - could one of the other edges also

be a TROWAL airstream edge? Note that the WCB turns anticyclonically,

and the TROWAL airstream will turn cyclonically. Look for cloud edges

that turn this way.

cold cloud tops to the west of the WCB in the landfalling Pacific

cyclone. It appears that this warm air is not moving downstream with the

conveyor belt - it's trapped, wrapping a little around the occluding

cyclone. (This also happens in the very occluded system at the western

edge of the loop). The TROWAL region is where surface winds can be quite

intense - over the Ocean, this can have dire shipping consequences. In this

case, look at the very strong ship-sinking convection that develops near

the Queen Charlottes as the TROWAL approaches.

Other things to mention: if it is not the TROWAL airstream, this could

be convection in the destabilized airmass as dry air overruns the low

level moisture. Not enough data over the ocean to make a definitive

statement. In either case, however, satellite imagery lets you keep

track of things.

give a guess at what's up. Sometimes you need to combine satellite

imagery (WV image over AK, nice swirl of an occluded system) with model

data (700-mb temperatures from the GFS). Note that the 700-mb

temperatures show a clear warm prod - the 850 mb temperatures did not

have this feature (not in a 5K contouring, at least). Sometimes you have

to look at more than one level to find a TROWAL. Better yet, look at an

isentropic surface! In this particular case, the TROWAL may not exist

at 850 mb, being confined instead to regions around 700mb. Even if it

is not visible at 700, it may exist above or below that level.

criteria into a cookbook format. Look at the surface map: Are there any

thickness ridges associated with the SLP minimum (if you're looking at

model forecast output - do they have continuity in time?). Take a cross

section perpendicular to the thermal ridge - use that to see if the

isentropic surfaces display a local minimum at the thickness ridge. DO

this to find the relevant isentropic surface to examine forcing on. More

important for an intensive post-storm analysis than during a forecast.

level to find the most pronounced ridge-y-ness in the isotherms. Show

where the cross section would be taken - note that it should be

perpendicular to the thermal ridge. Again, the TROWAL and the occlusion

are not parallel.

done in D3D with its theta-e surfaces. Or in D2D. Or you can plot single

theta-e values on a pressure surface in awips. (Dan Baumgardt, SOO at

LaCrosse sent me the code to do this -- this is like the TROWAL

diagnostic in AWIPS 5.2.2). Find the isentropic surface at the warm edge

of the warm/cold fronts - something like a limiting streamline. This is

going to be the surface that the warmest, wettest air parcels will

follow into the mid-troposphere.

cross-section allows you to choose wisely the isentropic surface. The

"best" isentropic surface is fairly constant with time - in other words,

if it's 304 K at 1200 UTC, it's not going to be far from that at 1800

UTC or 0000 UTC.

TROWAL in this image? These are surface MSLP contours (4-mb interval)

and 840-mb temperatures (5 C contour interval). Should be able to see

the warm tongue at 840mb extending from the lower Ohio Valley to

northern IA. The TROWAL is the line through the crest of the isotherms.

This is what a TROWAL would look like on an isobaric surface.

look at the QG forcing of the ascent, particularly in frontal

coordinates (Qn and Qs). Examine the frontogenesis if there is strong

banding - it should be parallel to the TROWAL, which is the axis of the

warmest air. There "should" be frontogenesis here, because the

isentropic surface are so strongly sloped in the TROWAL canyon. An

example of this during the case study is shown in just a bit.

isolate an airstream that is the so-called TROWAL airstream. These

typically start close to the cold front in the warm air, and not far

south of the warm front. You would expect them to be in the warm

conveyor belt, but they do not move anticyclonically downstream as they

ascend. This is one aspect of D3D that really shines as far as

TROWAL-finding goes.

to copious precipitation amounts. This is a Repeat/reminder slide.

Wilmarr MN: 30.8 inches. Sioux Falls about 1/15th of annual

precip in one storm. Second biggest storm on record.

LES helped give 30+" in the UP (although the snow was not all Lake Effect --

Lots of it was dynamically produced by the storm) Madison: Up to this storm,

no snow at all in Madison, then this storm gave a 10-minute flurry, denied the first snowless November ever! Had to settle for yet another November

with a trace of snow.

black Hills as well. Foot of snow widespread

hours. Note the filling of the storm, and the filling also of the tongue

of warm air (TROWAL). Also, note how the southern part of the TROWAL

races eastward and the western edge is stuck in central MN around a

pivot point in the atmosphere.

words, this is fairly typical of storms that are fairly vigorous as they

emerge from the Rocky Mountains, but once they lose the column

stretching there, they fill - but they still are energetic enough to put down

a lot of snow eastward to Iowa and Minnesota. (Although I note they

usually don't dump on WI).

comments: Big storm coming, well-organized and deep. I Love the Sioux

Falls comment as the storm winds down. So this was an expected snow

Storm, and it was pretty well-predicted.

figure shows eta forecasts valid at 0600 UTC 27 November, 12 hours

apart, and the forecasts show good run-to-run consistency, and paint

pretty much the same picture - lots of snow from South Dakota towards

the UP of Michigan. [Toggle between the two model runs] One difference

that I notice: seeming connection of convection to storm in one run

[30-h forecast] that lacks in the 18-h forecast. Not much precipitation

into southern WI with this storm - Madison got less than a tenth. SO

again, the trend here towards dryness, and a disconnect between the

southern convection and the northern precipitation verified. Can't tell

the true implications of that change unless there's more study. Also,

the position of the rain/snow line looks like it moves about 1 county

between model runs. This is key to a good forecast - how well is the

model developing the TROWAL that is in the forecast? Are the effects of

the TROWAL being manifest as warm air making it closer to NW Iowa? How

well is the model developing the TROWAL compared to reality? Knowledge

of where the TROWAL is is key to understanding and reacting to the

storm as it evolves.

43. Note especially the tongue of warm air poking into Iowa and how the

sharpness of this tongue increases as you get closer to verification

time. Warmth and moisture is close by for the snowfall. Later model run

shows deeper penetration of warmth/moisture towards cold air. A better

representation of the TROWAL? Note how the position and the width of the

warm air change with the model runs. Subtle changes, but potentially

very important. Caveat: temperature is NOT theta-e. As shown later,

theta-e may match TROWAL better than temperature

thunder in Nebraska, i.e., this system still is packing a punch even

though it is occluded. Note the development of enhanced cloudiness in a

SW-NE band over central MN at the end of the period - this region is at

the terminus of the TROWAL and where the model QPF forecasts were most

enthusiastic. Note that the enhancement in the IR that is crossing into

IA from Nebraska at frame 17 expands greatly as it rotates up into MN

where the TROWAL exists. This is an example of why TROWAL knowledge is

important: The very innocuous feature rotates up towards the TROWAL

and simply explodes one it reaches it as the dynamics of the satellite

cloud line feature and the TROWAL interact.

tongue, i.e., close to the TROWAL. Suggestive of strong vertical

motions, probably frontogenesis. Again, the TROWAL is where to focus for

the most interesting weather. In this example, a vigorous impulse, as

indicated by the lightning, is approaching a TROWAL. Beware!

is just north of the dry air in the WV channel. Difficult to relate

water vapor structures to the TROWAL.

TROWAL is shown as 304K isopleth on individual pressure surfaces (400,

500, 700, 850, 925 mb). Again, this is code from Dan Baumgardt @ ARX -

TROWAL diagnostic in 5.2.2. These two WV images demonstrate the

challenges of identifying the TROWAL location using just WV imagery.

expect the Qs to be behaving associated with these contours? Recall that

convergence of Qs will lead to localized warming. You should expect

Qs to be relaxing with time here because the nose of warm air is relaxing

with time.

the atmosphere. A whole load of things, but we will focus on

frontogenesis. 925-mb frontogenesis and the 304K isentrope (Theta-e).

Note how they are related. The TROWAL ends at the region of

frontogenesis. Not unexpected - strongly sloping isentropic surfaces are

primed for frontogenesis.

to the frontogenesis. Note that the contour interval of

frontogenesis is different here than in the 925-mb plot (4 vs. 12)

determined to be one that showed the TROWAL structure well. The canyon

initially extends from central IL (warm air) NWward to SD. (Figure

produced in D3D) Note how easily the TROWAL is identifiable using D3D.

The canyon really shows up!

Again, highlight the TROWAL canyon. (Another D3D figure)

canyon over Iowa/SD. Yet another D3D figure

upper left part of the map. "Warmer" colors are farther down in the

troposphere, bluer colors are higher up. This figure contains a series

of streamlines (from 0-42 h in a forecast run) that all originate at the

same lat/lon point, but at different levels. Note that some follow the

warm conveyor belt: rising motion, anticyclonic curvature, eventually

sinking downstream of the downstream ridge. Some of the parcels from

down low also enter the warm conveyor belt. But there are several that

ascend, but never make the anticyclonic turn to the east - these are

parcels "stuck" in the TROWAL airstream.

convergence leads to a local minimum in the laplacian of the temperature

-- meaning there's a local max in temperature.

contours of Qs in the same layer. 5 different frames - this is 24 hours

of model output. Note the correlation between the Qs and the layer theta

curvature - convergence at local maxima, divergence at local minima. The

values appear to be insufficient to sharpen the trowal - it propagates

along and weakens. Sanity check: as a TROWAL moves along, do the Qs

fields support it? If a region of Qs develops, should see a thermal

perturbation develop as well.

Baumgardt, SOO at ARX. Note how the tongue of warm/moist air overlaps

nicely with the cloud edge (All of Canada has low Sun-angle in Dec, so

it's easy to pick ou the cloud edges). Theta-e ridge corresponds very

nicely to cloud edges. If a TROWAL exists - it will move through this

isobaric surface. That's probably why there isn't a great correspondence

between the cloud edge and the nose of the warm air.

Note GOES 8/10 boundary - GOES 8 significantly dimmer.

temperature fields overlain, as well as weather. Note the development of

a TROWAL in the temperature fields that lags the presence in the theta-e

field. Do not rely simply on temperature to find TROWALs - you may need

to look at theta-e as well. The metars show moderate snow associated

with the cloud band (not many stations, however). The temperature

deformation occurs in the region of strong cyclonic thermal vorticity

advection by the thermal wind, as suggested by theory. One other thing

that is especially noteworthy here: Look how fast the nice "S"-shape

develops in the isotherms.

delineating where a TROWAL is gives you a BIG clue of where the heavy

snow/rain is going to fall, especially if there is also frontogenesis

occurring.

TROWAL marked over the NY/PA border. However, the isobaric charts

thermal analysis did not support this feature, so I'm not sure why it's

been put there.