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25th Operational Weather Squadron

LEADS Cross Sections Construction and Examples

Captain Brian YatesTraining Flight Commander

25th Operational Weather Squadron22 April 2010

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Outline

Introduction

Basic concept review

Cross Section Conceptual Model

Cross Section Generation and Pattern Recognition

Cold Fronts

Cold Air Damming/Shallow cold front

Warm Front Example

Occluded Front example

Trough Example

Other Uses

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Isentropic Cross Sections

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Through a consistent analysis of isentropic vertical cross-sections several clearly identifiable patterns begin to emerge these patterns add value to what we’re analyzing through our traditional methods.

Through the years in the academic community these cross-sections were often used to point out fronts at all levels in the atmosphere, the tropopause, regions of instability, and even used to diagnose areas of ascent or descent.

With the installation of LEADS 4.2 and subsequently 4.5.1 we now have the ability to make these cross-section charts (relatively quickly) from a myriad of forecast model data and display several parameters on one display window.

Introduction

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Thermal Wind Relationship in Isentropic Coordinates

cross sections are “cut” perpendicular to the feature being investigated.

In these areas of strong thermal contrast, look for a great tilt of the isentropic surface from high in the cold air to low in the warm air.

(1) When isentropic surfaces slope down, the thermal wind is positive; i.e., wind components into the plane of the cross section increase with height.

(2) When isentropic surfaces slope up, the thermal wind is negative; i.e., wind components out of the plane of the cross section increase with height.

(3) When isentropic surfaces are horizontal (quasi-barotropic), the thermal windis approximately zero and the normal wind component does not change withheight.

Concept Review

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Stability inferences made from Cross Sections

Vertical separation between consecutive isentropic surfaces is a measure ofthe static stability. Viewing a cross section and noting regions of strong (weak) stability wherever isentropic surfaces are close together (far apart).

- Useful in locating upper level frontal zones, inversions, and vertical changes in stability. In many ways this is much better than using standard stability indices which are

parcel-related or only computed using certain levels of the atmosphere.

Concept Review - StabilityDr. Jim Moore, Saint Louis University

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-The slope of an isentropic surface in the vertical is directly related to the thermal wind. -A vertically sloping isentropicsurface is indicative of a strong thermal contrast, where isentropicsurfaces tilt up or down significantly one can diagnose a frontal zone and a commensuratechange in the wind speed with height. - In a cross section analysis, even inthe absence of wind data, one could diagnose regions of strong or weak wind shear.

Surface Cold Front

Concept ReviewCont – modern display

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Results

Cross Sections take a few seconds to generate, but are well worth the time invested in generating them.

Notice on this chart the possible existence of a frontal system @ 39.13N, 86.62W

Jet Stream Analysis

The rest of the storyOverlap??

Upper Air Analysis

Jet Stream Analysis

The rest of the storyOverlap??

Jet Stream Analysis

The rest of the storyOverlap??

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Surface Analysis - Cold FrontsDr. Fred Sanders “Investigation of a Cold Front” circa 1955

· Frontal Analysis – Cold Front Review

10081002 1014

1020 1020 1014

FRONT

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Surface AnalysisDr. Fred Sanders “Investigation of a Cold Front” circa 1955

· Frontal Analysis – Cold Front Review

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Similar view of a cold front, present day

Red Circle – typical PFJ signature

Yellow Circle – “Y” Split between Tropopause and Stratosphere

Light Blue Circle – Upper Frontal zone

Black Circle – Surface and 850mb cold front

A

B

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Where is the Cold Front on this cross section?

Jet?

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Steeply stacked cold front – note the Jet front relationship as the front crosses mountainous terrain

KFSX – Flagstaff, AZ

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Cross Section of Theta contours and shaded temperatures. Items to note in the circles are the lowering of the colder temps across both cold fronts , with the front further to the east being the stronger of the two

W E

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Another steeply stacked cold front – Note the Jet front relationship as the front crosses mountainous terrain

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Appalachian Cold Air Damming

Shallow Cold Front

Surface AnalysisCold Air Damming

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Strong low level cold airmass from the SFC – 900mb. Evidence of another front in the area of the light blue circle.

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N S

Cross Section of Theta Contours and shaded lapse rates. Shaded regions of blue to purple show areas where temperature inversions are present.

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Warm Front Example

Surface Analysis Warm Front

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Warm Front – drawn from the warm air into the cold air. Note winds backing from southwest to east- southeast across the frontal zone. Also present is the drop in temperatures from south to north, almost a mirror image of what is present in a cold front.

A B

Note also the elevated warmlayer across Central Illinois

B

A

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Occluded Front Example

Surface Analysis

Occluded Front

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Occluded Front MoC Example

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Occlusion about to pass Fort Wayne, IN

Isentropes – solid black lines

Winds in Knots

Imaged Wind Speed

Very broad “U” shape of the warm air between the retreating cold airmass and the approaching cold airmass.

It becomes much more evident on the next slide.

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TROWAL and Occlusion

Temperatures in degrees C

Note the Trough Of Warm-air Aloft (TROWAL) beginning at KFWA and sloping up and NE’ward evident to about 500mb.

A B

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TROWAL and Occlusion

Temperatures in degrees C

Zoomed in view of the TROWAL.

Below 400 mb

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TROWAL and Occlusion

Lapse rates with the Occluded Front

Negative Lapse Rates start with the transition in shading from green to dark blue

Warm frontal inversion

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Surface AnalysisTrough or Front

· Trough or Frontal boundary???

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MetSat shows what could be a weak frontal system or simply a trough. Looking at the Theta and Temperature cross section there are 3 items that stand out on this chart1) Distinct warming in the upper

level temps on the left side of the chart.

2) The trough through the middle of the chart with no appreciable change in temperature

3) The remnants of what could be a weak cold front or occluded front in the black circle

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Cross Sections used to indicate possible Orographically induced turbulence and downslope wind event.

Cross Sections other uses

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Vertical velocity “couplet” pattern from 700 mb into the Stratosphere denotes possible terrain induced turbulence pattern. To date have not been able to find any reference trying Vertical Velocity values to turbulence intensities. But can recognize patterns that indicate possible terrain induced turbulence.

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Cross Sectionsother uses

· Freezing precipitation at Cannon AFB, NM · Cross section shows warm layer aloft

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Warm Layer ~2300ft thick

METAR KCVS 281155Z 00000KT 4SM -FZRA OVC008 M01/M01

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Cross Sectionsother uses

· Possible Mountain Wave Turbulence and strong downslope winds at Malmstrom AFB, MT · Cross section shows Vertical Velocity signature and strong winds

aloft

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Summary

Previously used outside of the operational environment Isentropic Cross Sections have long been a valuable tool when looking at the 3-D aspect of the atmosphere.

As computing power continues to increase, these tools are becoming available with not only forecast data, but with real-time observed data

When employed correctly, the patterns recognized on a cross section can quickly add clarity to what may have been a confusing synoptic pattern, this is important in a work environment where time is critical

Identifiable features – Jet streams, shortwave troughs, warm, cold, occluded fronts, inversions, warm layers aloft, strong winds just above the surface, turbulent flow across mountains (inferred)

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References

AWS TN 87-002, “Isentropic Analysis and Interpretation”, 1987, pgs 37-44 , Dr. James T. Moore, August 1987

Palmen E. and Newton C.W., “Atmospheric Circulation Systems”, 1969, Academic Press

Martin, J. E., “Quasi-Geostrophic Forcing of the Assent in the Occluded Sector of Cyclones and the TROWAL Airstream”, 1999, M.W.R., 17, pgs 70-88

Saucier, Walter J., “Principles of Meteorological Analysis”,

Sanders, F., 1955: An investigation of the structure and dynamics of an intense surface frontal zone. J. Meteor., 12, 542{552.1955, Dover Phoenix Editions