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Table of Contents

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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IFS gives information about the intensity and probability of different types of precipitation.  It is especially important to assess the likelihood of hazardous precipitation and its impact.  Even a small probability of hazards such as heavy snowfall or freezing rain/drizzle can be useful information forecaster and customer, even at quite short lead-times.  The type of surface precipitation critically depends upon the temperature structure of the model atmosphere.  Especially important are the layers in the lower atmosphere through which model precipitation falls. However, modelling these can be difficult, especially if the structure of the model atmosphere is imprecisely defined.  However, the ensemble gives possible variations in the temperature and humidity structure of the lower troposphere.   The distribution of types of precipitation among the ensemble members gives an assessment of probability of the types of precipitation that may occur.  

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Fig8.1.10-1A: HRES forecast type of precipitation chart and ensemble most probable type of precipitation. HRES DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024.  At the arrowed location the HRES forecast shows freezing rain (red) but ensemble shows most probable precipitation is snow (50-70%).

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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Fig8.1.10-1B: Vertical profile at the arrowed location in Fig8.1.10-1.  DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024.  The HRES forecast (solid line) shows warm layer above the surface typical of freezing rain.  The ensemble shows a broad range of temperatures but no clear indication of the structure of the warm layer.  

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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Fig8.1.10-2A: HRES forecast type of precipitation chart and ensemble most probable type of precipitation.  HRES DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024.  At the arrowed location the HRES forecast shows ice pellets (orange) but ensemble shows most probable precipitation is uncertain (30-50%), but see also Fig8.1.10-2C.  

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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Fig8.1.10-2B: Vertical profile at the arrowed location in Fig8.1.10-2.  DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024.  The HRES forecast (solid line) shows temperature fringes the 0ºC isotherm layer above the surface typical of ice pellets.  The ensemble shows a broad range of temperatures but no clear indication of the structure of the warmer layer.  

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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Detail of precipitation types: 4% heavy freezing rain (dark red, >1.0mm/hr), 15% freezing rain (red, 0.2-1.0mm/hr), 2% light freezing rain (pale red, 0.05-0.2mm/hr), 5% heavy ice pellets (brown, 0.2-1.0mm/hr), 10% ice pellets (orange, 0.2-1.0mm/hr), 32% heavy snow (dark mauve, >1.0mm/hr), 5% snow (mauve, 0.2-1.0mm/hr), 4% light snow (light mauve, 0.04-0.2mm/hr), 7% heavy sleet (dark cyan, >1mm/hr), 8% sleet (cyan green,0.2mm-1mm/hr), 8% heavy rain (dark green,>1mm/hr).  The probability of freezing rain of any intensity is about 21%.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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Fig8.1.10-7: 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.  

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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Fig8.1.10-8: Sequence of precipitation types DT:00UTC 04 March 2023.   VTs:00UTC 04 Mar (T+00) to 12UTC 06 Mar (T+60).  Precipitation type shown by colours.  

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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For more information on freezing precipitation see:

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