Practice Case 27 May 2013 Turbulence
Page 1, About the Case
It's 27 May 2013 and you're on the aviation forecast shift at 1200 UTC (14:00 Local Time). You need to
brief an airline pilot flying from Cape Town International airport at the south-western tip of South Africa to
King Shaka International airport in Durban, which is on the east coast of South Africa. [show map with
locations of airports and upper air stations marked]
First, you need to identify the dominant weather system affecting South Africa and then investigate
whether there's a threat of turbulence during the scheduled flight, and if so, where and at which level in
the atmosphere it can be expected. If turbulence is found, you will disseminate a SIGMET as part of your
meteorological watch activities. Note that the flight is set to fly at FL200.
We will evaluate past and present observational data and current NWP fields to determine the large scale
weather.
Page 1, Determine the Large Scale Pattern
Evaluating the large scale weather pattern helps the forecaster focus on the areas of possible turbulence.
With this knowledge, you can then further evaluate these areas for further indications of the presence of
turbulence.
Examine the water vapour imagery loops (from both the 6.2 µm and 7.3 µm channels), the infrared image
loop, the airmass imagery, and the NWP model data from 300 hPa and 250 hPa on 27 May 2013.
Tabbed interface with loops charts
Question: How would you describe the dominant weather situation over South Africa at 12 UTC?
a.
b.
c.
d.
A strong trough is present over the central part of the country
Zonal flow is indicated with west to east flow across all of South Africa **
A high pressure center is dominating the region
Low pressure of the west coast, and high pressure off the east coast is causing strong south
westerly winds over the region
Feedback: The correct answer is B. Both the satellite imagery and the upper air charts show that flow
across South Africa is from west to east.
Question: Locate the likely position of the jet axis draw an arrow indicating that location. (Drawing
question)
Display the 6.2 micron image from 1200 UTC and use drawing tools to locate the jet axis and then show
expert answer. On Expert answer, include location of Cape Town and Durban.
Expert answer:
Feedback: Compare your answer to the expert analysis. In determining the position of the jet axis, it is
important to evaluate all of the imagery available and compare it with the NWP output to find the jet
axis. The animated satellite imagery can help you identify the upper level flow and the evolution of it
over time. In this case, the flow seems to be mostly west to east across the country. The location
indicated by satellite is similar to the region of strongest winds in the NWP data. However, the jet seems
to turn cyclonically in the eastern part of the domain due to the trough offshore to the southeast. This
trough is not quite as pronounced in the NWP upper air fields. This is also true of the high pressure area
offshore to the northwest where pronounced clearing is present in the 10.8 micron infrared imagery.
Question: Is the flight path of the aircraft traveling from Cape Town to Durban likely to cross the jet
axis?
a. Yes
b. No
Display 6.2 micron image with jet axis and flight path.
Feedback: The correct answer is Yes. Compare the flight path to the location of the jet axis. You can see
that the flight is expected to cross the jet axis, and more importantly, is likely to fly through areas of
shear surrounding the jet axis.
Page 2 Cloud indicators of turbulence
So, we have seen that the jet moving across South Africa has very strong winds associated with it. We
know from the conceptual model previously discussed, turbulence can be found in the strong shear
areas surrounding the jet-strength winds (greater than 80 kts). Next, we take a look at the high
resolution visible (HRV) imagery to identify clouds that indicate turbulence.
Clear air turbulence (CAT) by definition occurs in cloud free regions. However, the presence of clouds
near these clear areas is associated with turbulence where they occur, and we can infer that turbulence
may be occurring in nearby clear areas.
The HRV imagery is good for detecting small scale cloud features associated with mountain waves,
gravity waves and turbulence.
Show tabbed interface with loops of HRV, IR, 7.3 micron and Airmass imagery
Question: Use the drawing tool to circle areas of clouds associated with turbulence.
Show HRV image at 1200 UTC
With Feedback, so image with circles…need to double check, this might not be 12UTC. Use the 12 UTC
from Vesa. Can we also mark Cape Town and Durban on the images?
Feedback: The corrugated pattern in the clouds over the northern parts of the Northern Cape, the
southern and central Free State, the Western Cape, and the Eastern Cape interior indicate the presence
of gravity waves and turbulence. Although best viewed on the HRV image, these clouds and related wave
patterns can also be seen on the other types of imagery.
Question: How do these cloud patterns change from 0600 UTC to 1200 UTC? Select all that apply
a. The corrugated wave patterns persist through the time period implying that turbulence has been
present since the early morning. **
b. The clouds remain stationary, so the regions of turbulence are predictable and static.
c. The clouds progress eastward with jet indicating that the turbulent areas are traveling in the
direction of the jet axis.**
d. Clouds increased steadily over time becoming overcast at 1200 UTC.
Page 3 Evaluate horizontal shear
Tabbed interface with 300 hPa chart and 200 hPa chart and satellite image from last question that
shows wave cloud areas circled, and Table 2 from the conceptual section. Note, graphics in script seem
to be better resolution than those in originals (the ones in originals are too hard to read to answer the
questions) Also need Cape Town and Durban marked. Place the flight path on them see
300mb_flightpath and 200mb_flightpath
In the NWP data, you can see that there is a wide region of jet strength (greater than 80 kt) winds along
the intended flight path.
Question: Use the NWP data to identify areas of shear along the flight path (use the regions of wave
clouds found previously as a first guess) and evaluate the horizontal wind shear in those areas. Which of
the flowing statements is true?
a.
b.
c.
d.
Greatest amount of shear is present over the Eastern Cape and corresponds moderate CAT.**
The strongest shear is present near Cape Town and corresponds to severe CAT.
No significant shear is present along the flight path, the aircraft will not experience turbulence.
The jet axis to too far north to have any influence on turbulence along the flight path.
Feedback: The correct answer is a. Shear is present along the flight path, and the largest values are seen
over the Eastern Cape. The values reach 20kt/1o latitude, which corresponds to regions of moderate CAT
as seen in Table 2. The areas of shear also correspond to the presence of clouds with corrugated
organization.
Page 4: Evaluate vertical shear
Tephigrams
We will now look at the aerological diagrams for the case study. The upper air ascent data is plotted on a
tephigram. Attention must be paid to the wind speeds at various levels in the vertical, as well as to the
general stability of the atmosphere. Examine the tephigrams from Cape Town (originating airport), Port
Elizabeth (represents the airmass enroute) and King Shaka Airport in Durban (destination airport).
Make tabbed interface with the three tephigrams and a mapped with each location marked.
NEED to LABEL tephigram locations may need to rewrite questions
PC_q14_68816tephi (CAPE TOWN); PC_q17_68842tephi (PORT ELIZABETH); PC_q20_68592tephi
(KING SHAKA)
Question:
Looking at the vertical wind profiles on each of the aerological diagrams. Between which pressure levels
is the Jetstream located? Select the best answer.
a) 100-70hPa
b) 150-70hPa
c) 150-300hpa**
d) 400-300hPa
NEED TO CONVERT TO HEIGHT LEVELS
Feedback: The correct answer is c. The wind speeds are greater than 80kt between 150hPa and
300hPa, with the strongest recorded wind reaching 100kt at 200hPa
Question 16
Between which flight levels do these pressure levels correspond.
a. 15,000-30,000 ft
b. 20,000-35,000 ft
c. 25,000-40,000 ft
d. 30,000-45,000 ft**
Feedback: The correct answer is d.
Question 18:
How does the layer of the jet speed winds correspond to the flight level of the aircraft traveling from Cape
Town to Durban (King Shaka airport)? The flight path is
a. much higher than the jet speed winds.
b. much lower than the jet speed winds. **
c. at the same level as the jet speed winds.
d. just above the jet speed winds.
Feedback: The correct answer is b. The flight level is 20,000 feet, which is below the jet level winds and
between 500 and 400 hPa.
Question 19:
Can you identify any vertical speed shear on the Port Elizabeth tephi wind profile that could generate
turbulence?
a) Yes
b) No
PC_q19_68842tephi_feedback
Feedback: The correct answer is Yes. Between 500hP /18000ft and 21000ft there is a 35kt change in
wind speed, which relates to 11.5kt/1000ft which can according to Table 2, is classified as severe
Page 5: Develop Mental Model
You have now looked at the information on the tephigrams for Cape Town, Port Elizabeth and King
Shaka. Each of the tephi’s position the jet stream between 150hPa and 300hPa. At Port Elizabeth, there
is vertical speed shear of 11.5kt/1000ft which is classified as severe turbulence.
You have also looked at the NWP output to determine wind speed and wind shear. The wind fields at
300hPa and 200hPa show a jet stream present over the central and western parts of South Africa, with
areas of horizontal wind speed shear of 20kt/1° latitude along the flight path. This information suggests
that moderate turbulence is possible in the areas where there horizontal wind shear is 20kt/1° latitude.
You have also looked at the satellite imagery and located significant cloud features and airmasses which
have guided you to a point where you understand the upper air flow patterns and synoptic pattern. The
flight path will include areas where wind shear is likely producing moderate turbulence.
Mental Model
Mental Model
Satellite
data
WV
Imagery
upper dry and
moist zones
location of
upper jet
HRV
small scale
features in the
cloud pattern
NWP wind
fields
Airmass
Location of
upper low and
upper high
location of
upper jet
200hPa
winds
Tephigrams/aerol
ogical diagrams
300hPa
winds
Jet Stream
found
Jet Stream
found
horizontal wind
shear classified
as moderate
turbuence
horizontal wind
shear classified
as moderate
turbuence
Jet Stream
Jet stream
found between
150hPa and
300hPa
Vertical speed
shear
Severe
turbulence
indicated
between 20000
and 21000ft at
King Shaka
The mental model of jet dynamic turbulence has been created by integrating satellite, NWP wind fields
and tephigrams, which gives insight to the weather systems, wind fields, vertical and horizontal speed
shear. Each data source provides information that combined can give a good understanding of what
mechanisms are in place to cause turbulence.
Page 6: Diagnose Turbulence Change this to a briefing for what the pilot can expect enroute. Redisplay 6.2 with jet and flight path and overlay areas where turbulence is expected due to shear and
wave clouds. Drawing tool question where they will circle areas based on different reasons (could we
use 3 different colors?
Question: On the map below, circle regions of turbulence expected along the flight path due to your
diagnosis of 1) cloud indicators, 2) horizontal wind speed shear, 3) vertical wind speed shear.
blankmapflightpath.png
blankmapflightpath_expertanswer.png
Need to add jet and fix areas
Question: Prepare a brief narrative for the pilot that outlines where you expect CAT to be an
issue during the flight from Cape Town to Durban. (Text box that requires them to enter
something)
Expert answer: The jet stream is particularly strong today and you can expect strong westerly
winds for the entire flight. Along with these strong winds, moderate turbulence is likely to be an
issue across the East Cape. Severe turbulence may also be a factor as the flight progresses to the
north of Port Elizabeth.
You need to disseminate your forecast. Each Meteorological institute will have its own methods
on how to disseminate aviation forecast product. Follow the standard operating procedure for
your specific organization in this regard. Make sure to send the products on time and to the
appropriate people.
Page 7: Summary
On the 27th May 2013 there was turbulence evident from the Satellite imagery, NWP data and the upper
air ascents (Tephi’s). The visible satellite imagery gives clear indications of turbulence, by the ripple-like
cloud features found over the southern and central parts of South Africa, extending from the west coast to
the east coast. The Jet stream was identifiable on the poleward side of the dry stratospheric air, depicted
by the red colour in the Airmass RGB and in the water vapor and infrared imagery. The occurrence of
cirrus cloud over the northern parts of South Africa and the southern parts of Namibia and Botswana are
resulting in evidence of the upper jet being located in that region. The NWP wind fields give evidence of
300hPa winds exceeding 80kt over the southern and central parts of South Africa, with horizontal speed
shear detected surrounding the jet axis. The upper air ascents also aid in identifying the level at which the
jet occurs and the height of the inversions. The tephi’s give evidence of vertical speed shear found on the
Port Elizabeth tephi (68842) between 18000ft and 21000ft. The speed shear detected on the tephi is high
enough to result in severe turbulence. The NWP winds at 300 hPa and 200 hPa do not predict any
significant horizontal speed shear in the Port Elizabeth area rather a bit farther north along the flight path.
From the satellite imagery, upper air ascents and the NWP, a forecaster should anticipate upper air
turbulence over the East Cape portion of the scheduled flight path due to the wave-like patterns on the
HRV imagery, the horizontal shear related to the position of the upper jet on the NWP and the vertical
wind shear from the Port Elizabeth tephi.
This case illustrates how integrating all available data with satellite data can provide confidence in making
turbulence forecasts. Especially in the case of Clear Air Turbulence (CAT) associated with the jet stream,
which often occurs without the presence of clouds, satellite imagery can be extremely helpful in both
placing the jet axis and finding surrounding indicators of turbulence.