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Types of Precipitation - charts and diagrams
It is important to assess the likelihood of different types of precipitation, particularly those of a hazardous nature. The method of assessment of the type of surface precipitation by the IFS models depends critically upon the temperature structure of the model atmosphere, which includes the layers through which the model precipitation falls. But modelling of this can be difficult, especially if the structure of the model atmosphere is imprecisely defined.
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- the kind of surface in question and its probable temperature before assessing the result of any indicated freezing precipitation.
- the various physical effects of precipitation types when deposited on surfaces and the way IFS handles them.
Histograms of Types of Precipitation
HRES/Ensemble Control gives no information on the probability of a type of precipitation and should not be used on its own; reference should always be made to ensemble output, particularly by use of the histogram, to assess the probability of other types of precipitation. Histograms of the ensemble "Types of Precipitation" are available as an option on ecCharts and on Opencharts. This gives easy to use information about the probability of alternative types of precipitation from the results of all ensemble members.
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Fig8.1.8.1: Method to display histograms of probability of types of precipitation from ecCharts. The location of the histogram may be selected using the probe tool on the chart.
Instantaneous, single member HRES/Ensemble control
A layer is availableon ecCharts to show the type of precipitation for precipitation rates greater than 0.1mm/hr. The precipitation types are shown by colouration.
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Fig8.1.8.3: ecChart forecast chart showing HRES/Ensemble Control forecast of instantaneous type of precipitation forecast DT 00UTC 20 February 2018, T+36 VT 12UTC 21 February 2018. A type of precipitation is shown wherever the forecast rate of precipitation is greater than 0.1mm/hr. The types of precipitation are represented by colours: Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets. The type of precipitation is also given in the "probe" information (top of diagram) according to a code - in this case for Quebec City (shown by the pin), 3 represents freezing rain at this time.
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An indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
Instantaneous most probable type of precipitation
A more informative ecChart display of instantaneous type of precipitation is based upon ensemble output and shows the most probable type of precipitation but gives no information on any, possibly hazardous, precipitation having a lower probability. Such information can be obtained by reference to the instantaneous type of precipitation meteogram/histogram product.
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It should be remembered that an indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
Variation of probability with forecast time
The "Probability of Type of Precipitation" chart clearly shows the areas at greater risk for precipitation of any kind and also the areas where there is greater uncertainty. However, the uncertainty also varies with forecast lead-time:
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Fig8.1.8.6: Multiplots of ensemble probability of type of precipitation forecasts all verifying at 12UTC 21 February 2018 from a series of ensemble forecast runs at 24hr intervals. As lead-time increases the more hazardous, less common types of precipitation are less prominent or do not appear, and greys (total probability <50%) are more prominent.
Interpretation of colouring on charts
It is important to note that the colour scales on type of precipitation charts and the accompanying histograms indicate different things - they are not interchangeable.
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- Colours show precipitation types
- Shading shows precipitation rates (scale above histograms).
Further examples
Freezing rain is a very hazardous type of precipitation which brings a significant risk to aviation, transport, communication and indeed to life. There are outputs to highlight the probability and amounts of freezing rain. It should be remembered that an indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
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Fig:8.1.8.9: ecCharts presentation of HRES forecast freezing rain accumulations over the previous 6hr, DT 00UTC 16 February 2023, T+42 VT 18UTC 17 February 2023. The vertical profile at Bangor, Maine, USA shows the classic freezing rain structure of a very moist and precipitating frontal zone overlying the sub-zero layer near the surface.
Modulation of Precipitation Type Probabilities by Sea Track and Topography
The following plots illustrate how the handling of precipitation type by ensemble forecasts in a marginal rain-snow situation can make physical sense; and specifically how user-relevant subtle changes are predicted with some precision.
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histograms of precipitation type in conjunction with charts of most probable precipitation type to assess the most likely precipitation type and the probability of alternatives. The histograms are readily available on ecCharts.
Freezing drizzle
Freezing drizzle can be as much of a hazard as freezing rain. The "Probability of Freezing Rain" option on ecCharts can show 0% when the default threshold is used. Freezing drizzle can be seen if a very low threshold (say >0.01mm in previous 6hr) is applied.
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