LEADS Cross Sections Construction and Examples

1. LEADS Cross Sections Construction and Examples Captain Brian Yates Training Flight Commander 25th Operational Weather Squadron 22 April 2010

2. 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

3. Isentropic Cross Sections

4. Introduction 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.

5. Concept Review • 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.

6. Concept Review - Stability Dr. Jim Moore, Saint Louis University Stability inferences made from Cross Sections Vertical separation between consecutive isentropic surfaces is a measure of the 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.

8. Concept Review Cont – modern display -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

9. 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

10. Upper Air Analysis Jet Stream Analysis The rest of the story Overlap??

11. Jet Stream Analysis The rest of the story Overlap??

12. Jet Stream Analysis The rest of the story Overlap??

13. Surface Analysis - Cold FrontsDr. Fred Sanders “Investigation of a Cold Front” circa 1955 Frontal Analysis – Cold Front Review 1020 1020 1014 FRONT 1008 1014 1002

14. Surface AnalysisDr. Fred Sanders “Investigation of a Cold Front” circa 1955 Frontal Analysis – Cold Front Review

15. A B 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 B A

16. Where is the Cold Front on this cross section? Jet?

17. Steeply stacked cold front – note the Jet front relationship as the front crosses mountainous terrain KFSX – Flagstaff, AZ

19. 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

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

21. Surface AnalysisCold Air Damming Appalachian Cold Air Damming Shallow Cold Front

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

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

25. Surface Analysis Warm Front Warm Front Example

26. B Note also the elevated warm layer across Central Illinois A 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. B A

27. Surface Analysis OccludedFront Occluded Front Example

28. Occluded Front MoC Example B A

29. 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. A B

30. B A 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

31. B A TROWAL and Occlusion Temperatures in degrees C Zoomed in view of the TROWAL. Below 400 mb A B

32. B A 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 A B

33. Surface AnalysisTrough or Front Trough or Frontal boundary???

34. 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 chart • Distinct warming in the upper level temps on the left side of the chart. • The trough through the middle of the chart with no appreciable change in temperature • The remnants of what could be a weak cold front or occluded front in the black circle

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

36. 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.

37. Cross Sectionsother uses Freezing precipitation at Cannon AFB, NM Cross section shows warm layer aloft

38. Warm Layer ~2300ft thick METAR KCVS 281155Z 00000KT 4SM -FZRA OVC008 M01/M01

39. 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

41. 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)

42. 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