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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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EGOWS 2008Systematic forecasting of weather “type” in the GFE John BallyCAWCR
GFE Snow Rain How do we describe the weather? Isolated Drizzle Patchy Heavy Widespread Sleet Light Thunderstorms Scattered Showers
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
GFE Will the precipitation be convective ?
GFE The model upper RH fields have skill. Is there enough moisture for rain?
GFE Diagnose weather type from instability, cloud cover and upper RH
GFE Start with the Probability of Precipitation from a multi model ensemble.....
GFE Match the statistics of ensemble PoP to climatology.......Limit PoP to 70%
GFE Use PoP to delineate areas of weather and assign coverage
GFE Make sure that the PoP and the most expected precipitation amount match.....
GFE What effect will the wind field have on Precipitation ?
GFE Increase forecast precipitation where convergence will trigger showers.....
GFE And set the weather intensity from the expected precipitation rate.....
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 >>
GFE Forecasters expect more instability than the model shows...... 09 Jun 12Z 08 Jun 18Z >> 09 Jun 00Z >> 09 Jun 06Z >>
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
GFE Draw in the axis of the cold front...
GFE And again 12 and 24 hours later....
GFE Make contours from these lines... Value is the time of wind change .......
GFE Use select tool to dynamically pick out the area from 3 hrs ahead of the change to 6 hours behind it .....
GFE The cold frontal zone moving over western Australia ..... 09 Jun 12Z 08 Jun 18Z >> 09 Jun 00Z >> 09 Jun 06Z >>
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 >>
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 >>
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.... 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 .... • 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..... • 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..... 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... • 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..... • 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”
