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Snow forecasting Techniques

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Snow forecasting Techniques

- Understanding of the impact of snow on aviation operations
- Understanding of the strengths and weaknesses of a variety of snow forecasting techniques
- Be able to apply the techniques to a real case study.

- Poor visibility and low cloud base
- Snow ‘packing’ restricting:
- airflow into engines
- preventing retraction of landing gear
- Blocking or Pitot tubes
- ‘Wet’ snow (T>0°C) will result in airframe icing.

- Poor visibility and low cloud base
- Snowfall accumulation on airframe:
- Aerodynamics
- all up weight of aircraft
- windshield obscured
- Runway contamination:
- degrading braking action.
- obscuring runway and runway lights
- 1mm of rain = 1cm of snow.

Probability of snow90%70%50%30%10%

Thickness (gpm)51805238525852925334

Advantages:

- Easy to use
Disadvantages:

- Not necessarily representative of the lowest levels of atmosphere

SNOW PROBABILITY (AMSL):

528.0 DM ≈ 40%

522.0 DM ≈ 80%

516.0 DM ≈ 95%

534.0 DM <10%

WHAT IS THE SNOW PROBABILITY AMSL AT POINTS:

A?

B?

C?

B

A

C

30-40%

>95%

<10%

Probability of snowMainlyReadily turnsMainlySnow

snow to snowrainvery rare

Height of wet-bulb<300 M<600 M600 M900 M

0 °C level AGL

Advantages:

- Easy to use
- Takes account of evaporative cooling (though not precipitation intensity)
Disadvantages:

- Too course in borderline situations

Watch for cold surface air undercutting warm air!

Wet-bulb freezing level ≈900m AGL

Snow unlikely

850

3

5

2

0 C

7

900

950

9

1000

10

0

Wet-bulb freezing level ≈600m AGL

Rain readily turning to snow

850

3

5

2

0 C

7

900

950

9

1000

10

0

Probability of snow90%70%50%30%10%

Height of 0 °C isotherm AGL (hPa)1225354561

Advantages:

- Easy to use
Disadvantages:

- Too coarse in borderline situations
- Takes no account of precipitation intensity or evaporative cooling if low level air is dry.

0°C isotherm level ≈110hPa AGL

<10% probability of snow

850

3

5

2

0 C

7

900

950

9

1000

10

0

0°C isotherm level ≈45hPa AGL

30% probability of snow

850

3

5

2

0 C

7

900

950

9

1000

10

0

Probability of snow90%70%50%30%10%

Surface temp (°C)0.3+1.2+1.6+2.3+3.9

Advantages:

- Easy to use
Disadvantages:

- Takes no account of warm air aloft
- Takes no account of precipitation intensity

Surface temperature

90% 70% 50% 30% 10%

0.3 +1.2 +1.6 +2.3 +3.9

WHAT IS THE SNOW PROBABILITY AMSL AT POINTS:

A?

B?

C?

aa

70%

C

40%

20%

A

B

90%70%50%30%10%

C12811290129312981303

Where:

C is the corrected value of the 1000–850 hPa thickness (gpm)

A is the actual 1000–850 hPa thickness (gpm)

H1000 is the height of the 1000 hPa surface AMSL

HGR is the height of the station AMSL

Probability of snow

90%70%50%30%10%

C12811290129312981303

Example

1000-850: 128.7DMMSLP: 992hPaHGR: 100M

1hPa ≈ 10m

C = 1287 + (-80-100)/30= 1287 – 180/3= 1287 – 6= 1281

= 90%

Advantages:

- Samples crucial low levels of atmosphere
- Gives precise values
Disadvantages:

- Inaccurate if there is a cold or warm undercut near surface
- Takes no account of precipitation intensity

DRY Low level air temperature initially above freezing

Snow falls into the lower levels of this atmosphere

Snow falling into a layer with an above freezing temperature melts and may evaporate if layer is unsaturated

Large amounts of latent heat required

3

5

2

Melting starts as 0°C isotherm is reached

850

7

9

1000

0 C

10

0

Cooling occurs as snow melts just below this level

Temperature profile changes

Profile starts to follow the 0°C isotherm down toward the surface

Dew point increases slightly

3

5

2

850

7

9

1000

10

0

0 C

Rough guide

1hr continuous melting snow - 600 feet of isothermal

4hrs continuous melting snow - 1200 feet of isothermal

Rain increasingly turning to snow at surface

3

5

2

0 C

850

7

9

1000

10

0

If melting snow is of heavy intensity then isotherm can reach surface

LESSON: In borderline snow situations, if precipitation is forecast to be heavy and prolonged, forecast snow.

3

5

2

0 C

850

7

9

1000

10

0

- Each snow forecasting technique has strengths and weaknesses
- Crucial forecasting points:
- Temperature and humidity of the lowest 1500M of the atmosphere
- Intensity and duration of precipitation
- Height of airfield
- Small changes in 1 to 3 above can lead to big forecast errors
- Each technique is a probability forecasting assuming that precipitation is occurring
- If it is dry then probability of snow = 0 no matter how cold it is!

Any questions?

UK, 25th November 2005

Snow case study

Newquay Airport

- Please write down the following:
- Newquay Airport: EGDG
- 51°N 05°W
- Height 150M
- You will be given snow forecasting information for 0300Z, 0900Z and 1500Z
- Calculate the snow probability using the techniques taught this morning
- Use the tephigrams to forecast the intensity of of any precipitation eg TEMPO +SHRA.

1000-850: 129.9DM

1000-500: 525.6

MSLP: 1006hPa

T: +4.0°C

TEMPO -SHRA

Probability of snow90%70%50%30%10%

1000-500hPa Thickness (gpm)51805238525852925334

Probability of snowMainly snow Rain turns to snow Mainly rain Snow rare

Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M

Probability of snow90%70%50%30%10%

Height of 0 °C isotherm AGL (hPa)1225354561

Probability of snow90%70%50%30%10%

Surface temp (°C)0.3+1.2+1.6+2.3+3.9

Prob of snow 90%70%50%30%10%

Boyden C12811290129312981303

50%

300m

60hPa

<10%

1296

1000-850: 130.1DM

1000-500: 519.4

MSLP: 1000hPa

T: +4.0°C

TEMPO SHRASN

Probability of snow90%70%50%30%10%

1000-500hPa Thickness (gpm)51805238525852925334

Probability of snowMainly snow Rain turns to snow Mainly rain Snow rare

Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M

Probability of snow90%70%50%30%10%

Height of 0 °C isotherm AGL (hPa)1225354561

Probability of snow90%70%50%30%10%

Surface temp (°C)0.3+1.2+1.6+2.3+3.9

Prob of snow 90%70%50%30%10%

Boyden C12811290129312981303

80%

<300m

35hPa

<10%

1296

1000-850: 128.3DM

1000-500: 515.9

MSLP: 995hPa

T: +0.0°C

TEMPO +SHSN

Probability of snow90%70%50%30%10%

1000-500hPa Thickness (gpm)51805238525852925334

Probability of snowMainly snow Rain turns to snow Mainly rain Snow rare

Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M

Probability of snow90%70%50%30%10%

Height of 0 °C isotherm AGL (hPa)1225354561

Probability of snow90%70%50%30%10%

Surface temp (°C)0.3+1.2+1.6+2.3+3.9

Prob of snow 90%70%50%30%10%

Boyden C12811290129312981303

>90%

<300m

12hPa

90%

1276

Now lets’ see what really happened!