Tephigram

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

US 161200 TTAA 66121 71867 99003 35740 00000
00298 34539 00000 92848 32738 31524 85452 27757
32025 70846 30559 32033 50519 41162 32059 40668
49169 32072 30854 55766 32091 25969 57766
31592 20109 59568 32095 15288 63370 31588 10535
66768 31589 88286 57165 32092 88134 64968 31590
77110 31598
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 doesnt 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 SoundingStony Plain Jan
20, 2005 00Z
17
Sample Fog SoundingStony Plain Jan 20, 2005 12Z
18
Sample Heavy Rain SoundingQuillayute Jan 18,
2005 12Z
19
Sample Heavy Snow SoundingYarmouth 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 430-500 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 (1030 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.