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EGOWS 2008 Systematic forecasting of weather “type” in the GFE. John Bally CAWCR. GFE. Snow. Rain. How do we describe the weather?. Isolated. Drizzle. Patchy. Heavy. Widespread. Sleet. Light. Thunderstorms. Scattered. Showers. Precipitating Weather Forecast Process Map.

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Presentation Transcript
slide2
GFE

Snow

Rain

How do we describe the weather?

Isolated

Drizzle

Patchy

Heavy

Widespread

Sleet

Light

Thunderstorms

Scattered

Showers

slide3

Precipitating Weather Forecast Process Map

Deep and Shallow

Instability

Upper Level

Moisture

Snow

Level

Cloud

Cover

Potential Type of

Precipitating Weather

Probability of

Precipitation

Expected Intensity

of Precipitation

Weather Grid

slide4
GFE

Will the precipitation be convective ?

slide5
GFE

The model upper RH fields have skill.

Is there enough moisture for rain?

slide6
GFE

Diagnose weather type from instability, cloud cover and upper RH

slide7
GFE

Start with the Probability of Precipitation from a multi model ensemble.....

slide8
GFE

Match the statistics of ensemble PoP to climatology.......Limit PoP to 70%

slide9
GFE

Use PoP to delineate areas of weather and assign coverage

slide10
GFE

Make sure that the PoP and the most expected precipitation amount match.....

slide11
GFE

What effect will the wind field have on Precipitation ?

slide12
GFE

Increase forecast precipitation where convergence will trigger showers.....

slide13
GFE

And set the weather intensity from the expected precipitation rate.....

slide14
GFE

The previous day, with a cold front crossing the west Australian coast .....

09 Jun 12Z

08 Jun 18Z

>>

09 Jun 00Z

>>

09 Jun 06Z

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slide15
GFE

Forecasters expect more instability than the model shows......

09 Jun 12Z

08 Jun 18Z

>>

09 Jun 00Z

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09 Jun 06Z

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slide16

Feature Based Precipitating Weather Process

Define Feature

Track Feature

Associate Weather

Type with Feature

Potential Type of

Precipitating Weather

Probability of

Precipitation

Expected Intensity

of Precipitation

Weather Grid

slide17
GFE

Draw in the axis of the cold front...

slide18
GFE

And again 12 and 24 hours later....

slide19
GFE

Make contours from these lines... Value is the time of wind change .......

slide20
GFE

Use select tool to dynamically pick out the area from 3 hrs ahead of the change to 6 hours behind it .....

slide21
GFE

The cold frontal zone moving over western Australia .....

09 Jun 12Z

08 Jun 18Z

>>

09 Jun 00Z

>>

09 Jun 06Z

>>

slide22
GFE

Use this “edit area” to assign “showers and storms” potential weather type near the front .....

09 Jun 12Z

08 Jun 18Z

>>

09 Jun 00Z

>>

09 Jun 06Z

>>

slide23
GFE

Again, calculate the forecast weather from potential weather type, PoP and expected precipitation intensity ...

09 Jun 12Z

08 Jun 18Z

>>

09 Jun 00Z

>>

09 Jun 06Z

>>

slide24
GFE

Also increase wind speed near the change line... often under-forecast by model......

09 Jun 12Z

08 Jun 18Z

>>

09 Jun 00Z

>>

09 Jun 06Z

>>

gfe now we produce some words
GFE Now we produce some words....

Weather Types are described by........

Coverages : How much of specified area is impacted ?

Intensities : how strong ?

Attributes : optional features of the weather type

Coverage, intensity and attributes combined .... e.g. “Widespread heavy showers with hail”

gfe sample and summarise the grids
GFE Sample and summarise the grids ....
  • Take a time-series of grids and perform mathematical operations, including:
    • Averaging, Min/Max, deciles etc
    • Collection of weather keys
  • These operations produce samples, which represent the best numerical description over the space & time in question.
  • e.g. Wind : ((5, 10), (300, 340)) MaxT : (22, 28)‏Weather: ((SctSH+ 32%), (PaRa- 20%), (AreasTS 10%))
gfe describe the situation over time
GFE Describe the situation over time.....
  • Examines the grids at the minimum time resolution, e.g. 3 hourly resolution
  • Identify times in the sequence where the samples change significantly
  • Choose significant transition points and produce one sub-phrase for each time chunk.
  • e.g. “Light wind becoming northerly 15-20 knots during the morning then tending southwesterly in the evening.
gfe combine weather within a type
GFE Combine weather within a type.....

Look at similarity of weather between subphrases

Is it worth describing a transition from the chance of light rain to the chance of moderate rain?

Generally combine across one coverage OR intensity

Combined more aggressively in complex situations

gfe describe the stats of each period
GFE Describe the stats of each period...
  • Take the statistics for each chunk (time period)
  • Produce a scalar, vector, or weather description
  • Wind ((5, 20), (300, 340)) “north to northwest wind up to 20 knots”
  • Wx (isolSh-) “isolated light showers” or “chance of a light shower”
gfe talking about changes
GFE Talking about changes.....
  • Transition style for trends while over-time, e.g. “becoming light around midday”.
  • Over-time style for isolated events, e.g. “Rain during the evening”.
  • Use transition words......eg
    • Wx (isolSh-, WideRa) “isolated light showers increasing to widespread rain”
    • Wx (isolSh-, WideSh) “isolated light showers becoming more widespread”
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