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Tephigram Hodograph Analysis

The Forecasting Problem. Three stepsAnalysisDiagnosisPrognosisAnalysis: understanding what is happeningDiagnosis: understanding why it is happeningPrognosis: understanding what's going to happen next.. Data Visualization. Data is the start of the analysis process.As meteorologists, we need to

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Tephigram Hodograph Analysis

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    1. Tephigram & Hodograph Analysis A Forecasting Perspective

    2. The Forecasting Problem Three steps Analysis Diagnosis Prognosis Analysis: understanding what is happening Diagnosis: understanding why it is happening Prognosis: understanding what's going to happen next.

    3. Data Visualization Data is the start of the analysis process. As meteorologists, we need to be able to synthesize data from multiple sources quickly and easily. Satellite, radar, upper air observations, surface observations, aircraft reports, ... Often, there is too much information and the meteorologist runs the risk of being overwhelmed by it. This can have an impact on the entire forecast process. If the analysis is compromised, so will the diagnosis and the prognosis. The key to managing this information is to present it in forms that allow diagnosis to take place in the most efficient manner possible

    4. Tephis and Hodos These are the primary tools for visualizing temperature, moisture, wind and height information through a column. How vertical the sounding is depends on the winds aloft. From the sounding we can determine convective potential and storm severity, freezing rain likelihood, advection, frontal positions, snow microphysics, snow-liquid water ratios, wind shear, and a host of other critical parameters, often at a glance.

    5. Location of Canadian Upper Air Sites

    6. The Raw Data Imagine having to evaluate a dozen or so soundings. You have 30 minutes to do a thorough analysis. Here's what part of the raw data looks like:

    7. A Little Better?

    8. The Tephigram While it is possible to determine a host of thermodynamic quantities from the tephi, in practice, only three are considered: Dry bulb temperature Dewpoint temperature Wet bulb temperature Other parameters may be considered, but they will depend on the situation. It is the relationship between these quantities (i.e. the pattern they show) that is important. From those three quantities you can determine: Cloud bases and heights Precipitation type and intensity Surface wind speed and direction Atmospheric stability and wind shear

    9. Kelowna: January 16, 12Z

    10. What Really Happened? METAR CYLW 161200Z AUTO 00000KT 9SM OVC025 OVC053 M14/M17 A3049 RMK SLP366 METAR CYLW 161300Z 27002KT 12SM FEW008 OVC024 M12/M17 A3049 RMK SF1SC7 /S04 AFT 06Z/ SLP365 METAR CYLW 161400Z 00000KT 10SM -SN FEW008 OVC025 M13/M17 A3048 RMK SF1SC7 SLP362 METAR CYLW 161500Z 00000KT 3SM -SN SCT007 OVC023 M13/M16 A3047 RMK SN2SF2SC4 SLP358 METAR CYLW 161600Z 00000KT 4SM -SN FEW007 SCT014 OVC022 M12/M16 A3047 RMK SF1SF2SC5 SLP357 METAR CYLW 161700Z 00000KT 3SM -SN OVC022 M12/M15 A3047 RMK SN3SC5 /S01 AFT 13Z/ SLP358 SPECI CYLW 161709Z 18002KT 1/2SM SN VV004 RMK SN8

    11. The Hodograph A visual representation of the vertical distribution of the horizontal wind. This is a vector representation. Some concepts: Veering – a change in wind direction in the clockwise sense. Backing – a change in wind direction in the counter-clockwise sense. Advection – the transport of a property by the wind. Veering winds indicate warm advection** Backing winds indicate cold advection

    12. Vertical Wind Shear Thermal wind equation If there is a temperature gradient on a constant pressure surface, the winds must change with height and vice-versa. An airmass with no temperature gradients is said to be barotropic. An airmass where only the speed changes with height is said to be equivalent barotropic. An airmass where the direction changes with height is said to be baroclinic.

    13. Baroclinicity In simple terms, baroclinicity provides the energy that drives storm development, whether it be thunderstorms or large synoptic low pressure systems. Strong horizontal temperature gradients are a characteristic of fronts. Strong shear should be expected near them. Wind shear, easily identified on the hodograph, is a measure of the baroclinicity of the airmass and provides an indication of the airmass' ability to support storm development. In general terms, the real atmosphere is always baroclinic but how baroclinic depends on the situation. There is a caveat: how well does the real wind conform to the geostrophic wind? The planetary boundary layer, usually the 1 km of the troposphere nearest the Earth's surface, is the layer where friction must be considered.

    14. The Ekman Spiral Friction is strongest closest to the ground and decreases until the top of the PBL is reached. Above that point, the real wind and the geostrophic wind are the same. With a little math, you can show that the impact of friction causes the wind to veer with height. The Ekman Spiral is not an indication of baroclinicity or of warm advection. Most soundings should show veering with height in the PBL. If veering is observed off the surface, how much of it is due to friction and how much due to warm advection? A non-trivial question and one that doesn’t have a satisfactory answer as yet.

    15. Charleston, SC: January 16, 12Z What is the surface temperature? What are the winds aloft? Is the hodograph veering or backing? How many cloud layers are there? What are their bases and tops?

    16. Sample Freezing Rain Sounding Stony Plain: Jan 20, 2005 00Z

    17. Sample Fog Sounding Stony Plain: Jan 20, 2005 12Z

    18. Sample Heavy Rain Sounding Quillayute: Jan 18, 2005 12Z

    19. Sample Heavy Snow Sounding Yarmouth: Jan 23, 2005 12Z

    20. Reality Bites The previous cases were for events that actually occurred at the upper air site or immediately upstream. Reality is rarely this clear cut. It's a whole lot messier.

    21. 00Z Sounding for Stony Plain, Alberta Dec 18, 2004

    22. 00Z Sounding for Fort Smith, NT Dec 18, 2004

    23. 00Z Sounding for Prince George, BC Dec 18, 2004

    24. 00Z Sounding for Fort Nelson, BC Dec 18, 2004

    25. Convective Assessment Need three things for convective development: Lift Instability Moisture (Wind Shear?) In a tephigram analysis, instability and moisture are assessed separately. Then we proceed to the lifting process, determine convective inhibition, CAPE, ... Remember, convection and instability are not the same thing. Instability is a necessary but not sufficient condition for convection. Convection develops when we realize the instability. If free convection isn't likely, then we have to assess mechanical lifting to see if free convection can be achieved. Finally, we assess shear and its possible impacts.

    26. Static Stability: A Simple Overview

    27. Local Instability

    28. Atmospheric Instability In the atmosphere, we conceptually isolate a parcel of air from the environment and perturb it to see what it does. Instability in the atmosphere is directional. Perturbations in one direction may show stability while perturbations in another may be very unstable. In practice, when evaluating instability with a tephigram, we are concerned with vertical instability. Remember, though, that a real sounding is not necessarily vertical.

    29. The Lifting Process In its simplest terms, we take a parcel at some level having temperature, T, and dewpoint, Td, and lift it to saturation adiabatically. Imagine lifting a parcel of moist yet unsaturated air. What happens? The pressure around the parcel drops and it expands. The expansion takes energy from the parcel and it cools We aren't changing the dewpoint at all, so eventually saturation occurs (i.e. There are no moisture inputs to our parcel) The temperature follows Poisson's Equation:

    30. LCL's and LFC's, Oh My! The point where this occurs is called the Lifting Condensation Level (LCL). The temperature and pressure at this level are called the Condensation Temperature and the Condensation Pressure respectively. Once we have reached saturation, condensation continues to occur through the lifting process. That releases energy into parcel which slows the expansion of the parcel and the temperature doesn't fall as rapidly. The slope of the pseudo-adiabats aren't as great as for the dry adiabats. Eventually, we reach the Level of Free Convection (LFC), the point at which are parcel will continue rising without the need for energy input from the environment. The LFC says nothing about the extent of convection. That is determined by the environment curve above the LFC.

    31. The CCL Revisited The CCL is the approximate location of the cloud bases for your convective cloud. Helps define the convective temperature. In practice, it isn't used operationally!! We do mix the lowest 50 to 100 mb of the boundary layer to get a more accurate representation of the moisture supply. Ask the public forecaster what the high is today and perform parcel ascents with that information. Interested in assessing the state of any capping inversion that might be present. Essentially, this the reverse of the CCL process.

    32. Entrainment Parcels never travel precisely up the pseudo-adiabat. Our parcel is never truly “isolated” from the environment it is moving through. As it rises, drier air from the environment is entrained into the rising parcel. This reduces latent heat release. The impact on the sounding is minor. The parcel still more or less follows the pseudo-adiabat. It's real path is tipped slightly toward the dry adiabat. This means that the CAPE we calculate is really a maximum value. The real value will be slightly less, sometimes by 100 J/kg or so. In some cases, where you have a very marginal positive area, entrainment can mean the difference between getting convection and getting none.

    33. A Real Convective Case Sounding for July 18, 2004 at 00Z Stony Plain, Alberta west of Edmonton 00Z in Alberta is just after max-heating time, which occurs about 4:30-5:00 PM local time Sounding is about as unstable as it is going to get.

    34. The Raw Sounding

    35. After Mixing the Lowest 100 mb

    36. Lift a Surface Parcel

    37. Colour in the Positive and Negative Areas

    38. What Do We Know? We don't appear to have any directional shear, but there is speed shear. It is not terrific, so a good first guess is that if storms develop, we will be dealing with pulse storms. The wind shear through the 0-6 km layer is approx. westerly at 40 kt. This is a good estimate of the storm's motion. That is the lower limit for supercell formation. The base of the thunderstorms is at approx 7000 ft AGL. Fairly high-based. Maximum tops are around 48,000 ft. There is a small negative area (-48 J/kg) which means that we will need a small amount of mechanical lifting to get free convection. CAPES are just over 2100 J/kg

    39. We Know Even More... Look at the shape of the positive area. Is it skinny or fat? Remember CAPE is related to the strength of the updraft. In this case, maximum updraft speeds were over 230 km/h. Entrainment would likely reduce this to about 165 km/h which is still significant. What type of severe weather, if any, would you expect from a thunderstorm developing in that environment?

    40. What Really Happened At about 0430Z (10:30 local time) multiple reports of golf-ball sized (40 mm+) hail were received from the south and southeast side of Edmonton. Reports of dented cars from Millwoods. Moderate to heavy rains in the southeast. No reports of strong winds. Entire event lasted 30-45 minutes. This storm was a “hailer”. As a general rule of thumb, pulse storms usually produce hail about 1% of the CAPE. In other words, our 2100 J/kg CAPE should have produced hail about 21 mm (nickel sized). It was twice this size. It could well be that we were dealing with a supercell, which would account for the larger hail, even though shears were borderline.

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