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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.  

A major weather hazard is freezing rain (and freezing drizzle) which requires a particular and relatively rare type of temperature and humidity structure in the vertical.  It is important to appreciate that diagnosis of freezing rain in IFS takes no account of the temperature of the surface, only on the temperature structure of the boundary layers.  IFS indication of freezing rain/drizzle indicates only that super-cooled droplets are likely to exist in the boundary layer but gives no guidance upon the likely formation of glaze or glazed ice on exposed surfaces.  Nevertheless, the forecast rate of precipitation does give some indication of the amount of ice accretion that may be expected.  The user should consider the kind of surface in question and its probable temperature before assessing the result of any indicated freezing precipitation.

Rain at the surface can sometimes change to snow perhaps because of evaporative cooling of the airmass as the precipitation falls.     

The ensemble gives an effective way of assessing possible variations in the temperature and humidity structure of the lower troposphere and hence the probabilities of the types of precipitation that may occur.  Probabilities are naturally assigned according to the distribution of types of precipitation among the ensemble members.  Even a small probability of hazards such as heavy snowfall or, more especially, freezing rain can be useful information for the forecaster and ultimately for the customer, even at quite short lead-times.

ECMWF products, currently available in ecCharts, aim to help with these forecasting challenges in different ways.  One can examine charts that relate to the instantaneous type of precipitation, and its instantaneous intensity (in map and meteogram/histogram formats), and also to maps that display accumulations of a certain type of precipitation over a period, or probabilities thereof.  Freezing drizzle may often be identified on ecCharts freezing rain charts by setting the accumulation over 6hr to a very low threshold (0.01mm).  However, the user should also consider the various physical effects of precipitation types when deposited on surfaces and the way IFS handles them.

These ecCharts products incorporate some post-processing related to the minimum permissible precipitation rate, which varies according to type, and which has been incorporated to try to deliver relatively "bias-free" products for the user.  In tests this helped greatly to minimise net under- or over-prediction of the occurrence of the different types of precipitation.  This aspect is not discussed directly in this user guide, but more information can be found in the Additional Sources of Information links for this section.  

There are a number of additional useful options related to different types of precipitation in ecCharts which are not directly discussed in this section, but which the user should be aware of; notably:

  • total snowfall probability,
  • total snowfall rate probability,
  • probability of combined events of wind gust and total snowfall.

CONTROL-10 Instantaneous Type of Precipitation Charts

Within ecCharts a layer is available to show CONTROL-10 type of precipitation for precipitation rates greater than 0.1mm/hr (shown by coloration).  However, CONTROL-10 type of precipitation charts give no information on probability of the type of precipitation shown, or on any alternative types.  For this, it is important to assess the temperature structure of the lower layers of the atmosphere to enable a tentative probability, or at least a risk, of potential severe weather types.


Fig8.1.8.1: ecChart CONTROL-10 forecast instananeous type of precipitation chart with CONTROL-10 orography DT 00UTC 20 January 2018, T+60 VT 12UTC 22 January 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 location of Straubing, Lower Bavaria is indicated by the pin where, as the precipitation area moves east, CONTROL-10 indicates initial snow will turn to freezing rain followed by rain.  However, CONTROL-10 type of precipitation charts give no information on probability of the type of precipitation shown or on any alternative types of precipitation. 

Ensemble Instantaneous Type of Precipitation Charts

A more informative ecChart display of instantaneous type of precipitation is based upon ensemble output and shows the most probable type of precipitation.  The indicated type of precipitation, however, gives no information on any hazardous precipitation having a lower probability.   Such information can be obtained by reference to the instantaneous type of precipitation meteogram/histogram product.

Fig8.1.8.2: Ensemble forecasts instantaneous type of precipitation chart, 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 charts show the most probable type of precipitation based on the proportion of types of precipitations forecast by the ensemble members.  These are represented by colours:- Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets - darker shades indicate greater probability.  Where the total precipitation probability is less than 50% the type of precipitation is not shown, but  the total probability of any precipitation (between 10%-30% or 30%-50%) is shown by the shades of grey.  The probability of each type of precipitation (taken as the proportion of ensemble members showing this type) is also given in the "probe" information (top of diagram) according to a code - in this case for Quebec City, 3 represents >70% probability of rain at this time.  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. 


Fig8.1.8.2A: Ensemble forecast instantaneous type of precipitation chart, DT 12UTC 19 February 2018, T+45 VT 09UTC 21 February 2018 (run time 12hours earlier than in Fig8.1.8.2). The histogram shows the distribution among ensemble members of types of precipitation at Quebec City at 09UTC 21 February 2018 (location is indicated by the pin).  The ensemble chart suggests that the area of freezing rain lies to the northeast of Quebec at this time and the most probable type of precipitation is rain.  However, the histogram for Quebec shows the probability of freezing rain overnight was very high but that the risk has greatly reduced by 09UTC.  However, the probability of freezing rain at 09UTC is still about 21% - heavy (dark red) 8%, moderate (red) 10% – so a risk of occurrence of a potentially dangerous phenomenon persists. It is important to inspect the histogram of precipitation types and not rely only on the probability of precipitation charts alone.  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.


CONTROL-10 is the unperturbed member of the ensemble and may or may not be the same as all or any of the ensemble member solutions.  CONTROL-10 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 to assess the probability of some other type of precipitation.


Fig8.1.8.3: CONTROL-10 forecast instananeous type of precipitation chart, DT 00UTC 20 February 2018, T+36 VT 12UTC 21 February 2018 (the same time as Fig8.1.8.2).  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 location of Quebec City is indicated by the pin.  The type of precipitation is also given in the "probe" information (top of diagram) according to a code - in this case for Quebec City, 3 represents freezing rain at this time.  Note this is different to ensemble guidance for this time (Fig8.1.8.2) where the greatest probability is for rain at this time at Quebec at 12UTC 21 February 2018 (but there may be a lower probability of freezing rain).  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. 



Fig8.1.8.4: An example of the most probable instantaneous type of precipitation product displayed over a larger area.  Type of precipitation is shown by the colour scale.  To help users to have a better understanding of the situation, where the total precipitation probability is less than 50% the type of precipitation is not shown, but  the total probability of any precipitation (between 10-30 or 30-50%) is shown by the shades of grey.


The most probable type of precipitation product clearly shows the areas at greater risk for precipitation and also the areas where there is greater uncertainty.  However, the uncertainty also varies with forecast lead-time.  At shorter lead-times the ensemble solutions will usually be fairly similar and probability of the types of precipitation are likely to be high and charts will show a lot of detail.   However as lead-time increases the spread of ensemble solutions will be greater and the development, timing and location of precipitation events will become less certain with time and charts will show larger areas of grey (<50% probability).

 

Fig8.1.8.5: 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. 

Ensemble Instantaneous Type of Precipitation Histograms

More information regarding the probability of alternative types of precipitation may gained from inspection of the results from all ensemble members.  Histograms of the ensemble types of precipitation that show this are available by use of a option on ecCharts.

Fig8.1.8.6:  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.


 Fig8.1.8.7:  (Same ecChart as Fig8.1.8.1).  DT 00UTC 20 January 2018, T+60 VT 12UTC 22 January 2018.  The most probable type of precipitation is shown by the colours but gives no information on any alternative types of precipitation.  The histogram, however, shows probabilities of each type of precipitation as measured by the proportion among the ENS members of each type of precipitation forecast at the selected location.  The ENS grid point altitude is displayed at the top of the meteogram.

Fig:8.1.8.7 shows that at Straubing at 12UTC 22 January 2018 several possibilities of types of precipitation are forecast by the ENS members: ~26% rain (of which ~4% light, ~10% moderate, ~12% heavy intensity), ~2% sleet, ~2% wet snow, ~14% Snow (varying intensities), ~18% ice pellets (varying intensities), ~34% freezing rain (of which 4% light, ~20% moderate, 10% heavy intensity), ~6% no precipitation.  Thus freezing rain has the greatest probability and would be shown as red areas in the probability of type of precipitation product in ecCharts.  Nevertheless, the chances of rain falling at this particular time are only slightly lower which in this particular instance happens to match the forecast precipitation type provided by CONTROL-10, as shown on the chart.  Without viewing the histograms the user would not know of this significant alternative.  Freezing rain is such a significant hazard that even a small probability (as at 03UTC 24 January 2018 - see histogram) might be worth a warning to customers sensitive to this type of precipitation even though rain might be more probable and shown as such on the charts.  It is very important that the histograms for a location of interest are consulted to capture occasions when hazardous precipitation is forecast with a lower probability, as often occurs at longer lead-times.  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. 

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.  The colours in the chart show the most probable type of precipitation; the colours in the histogram show the probability of each intensity of precipitation at the chosen probe location.

Fig8.1.8.8: An example type of precipitation chart.  Ensemble forecast charts, DT 00UTC 23 February 2018, T+83 VT 21UTC 26 February 2018.  It is important to note that the colour scales on chart and histogram indicate different things - they are not interchangeable.  The colours in the chart show the most probable type of precipitation represented by colours:- Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets - darker shades indicate greater probability (not the intensity of the precipitation).  The colours in the histogram show the probability of each intensity of precipitation at the location of the pin (the colours are given in the scale above the histogram - darker shades indicate greater intensity).  In this case at the location of the pin in the English Channel the dark blue area implies >70% pobability of wet snow.  This agrees with the histogram where, at 21UTC 26 February 2018, there is ~82% probability of wet snow composed of ~37% light intensity (pale blue), ~41% moderate intensity (mid-blue), ~4% heavy inensity (dark blue).

Further Examples related to Types of Precipitation

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.


    

Fig8.1.8.9A (left): CONTROL-10 forecast freezing rain (total over 12hr).  DT 00UTC 23 February 2018, T+72 VT 00UTC 26 February 2018.  Colour scale: Light Blue 0.0-0.2mm, Mid Blue 0.2-0.5mm, Dark Blue 0.5-1.0mm, Dark Red 1.0-2.0mm, Red 2.0-5.0mm, Pink 5.0-10.0mm, Orange >10.0mm.  The pin marks the location of Quebec City with the forecast 12hr total of freezing rain given in the probe information frame.  Freezing rain total values do not the same as accumulation of glaze or glazed ice although some proportional accretion must be expected.

Fig8.1.8.9B (right): ENS forecast freezing rain probability (>1mm over 12hr).  DT 00UTC 23 February 2018, T+72 VT 00UTC 26 February 2018.  Colour scale: Yellow 5-35%, Green 35-65%, Darker Green 65-95%, Blue >95%.  The pin marks the location of Quebec City with the forecast probability of >1mm 12hr total of freezing rain given in the probe information frame.

Histograms of Types of Precipitation

CONTROL-10 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 ENS output, particularly by use of the histogram, to assess the probability of other types of precipitation.  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.


   

Fig8.1.8.10A (left): Type of precipitation forecast chart from CONTROL-10.  DT 00UTC 31 December 2017, T+84 VT 12UTC 03 January 2018.  The CONTROL-10 forecast type of precipitation for a location in central Slovakia (shown by pin) is sleet (grey).

Fig8.1.8.10B (right): Histogram for the same location in central Slovakia (shown by pin in left hand frame) from ensemble DT 00UTC 31 December 2017, T+84 VT 12UTC 03 January 2018.  The most probable precipitation shown on the histogram is sleet (~20), but the histograms show a low probability of rain (~5%), but a significant probability of moderate or heavy snow (~15%).  Thus neither CONTROL-10 type of precipitation charts nor ENS probability of type of precipitation charts should not be used without reference to the accompanying histogram.  Users should also note the height of the grid point used (always shown in the histogram heading) and assess the relevancy of the precipitation types to the precise location for the forecast which may be at a very different altitude.  Overnight between the 3rd and 4th of January there is a low probability (~5%) of freezing rain which nevertheless may be of significance to customers.


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.

Fig8.1.8.11: Illustration of the probabilities of different instantaneous precipitation types.  The flow is mainly easterly over England and illustrates the impact of different length sea tracks over the relatively warm sea, and the impact of topography.  Consider an approximate low level trajectory from Belgium to Wales (dashed arrow).  Colour is used wherever the probability of some precipitation falling is 50% (from the ensemble).  The colour itself illustrates the most likely type, whilst the darkness of the shading indicates how likely that type is.  Dry snow over Belgium, wet snow and sleet over the relatively warm southern North Sea, wet snow over colder southeastern England, dry snow further away from the sea (most likely as the air re-cools partly via evaporation), then over Welsh mountains the probability of dry snow is very high (because the high probability of temperatures being below zero and of high probability of precipitation falling due to orographically forced ascent).


Fig8.1.8.12: Typical histograms of probabilities of instantaneous precipitation type for locations within the airstream.  Colours in Fig8.1.8.11 represent the most likely (i.e. produced by the greatest percentage of ensemble members) but don't show other types, even if they are marginally less likely.  The use of histograms gives a better overview of precipitation types and enables a more confident forecast to be made by the user.  Considering 12UTC 31 Jan 2019: In Belgium there is high confidence of dry snow with very small probability of any alternative precipitation type.  

Sometimes the probabilities of two or more precipitation types may be very similar.  For example, the difference in probability between problem-causing wet snow and less hazardous sleet or rain may be small and it may be may be difficult to decide between them.  The charts of most probable precipitation type only show the precipitation type that has the greatest probability, no matter how small the probability is greater than the next most probable precipitation type.

Therefore it is wise to use 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.


Probability of freezing precipitation charts modified to show drizzle???


Fig:8.1.8.13: ecCharts presentation of ensemble forecast of probability of freezing rain accumulations, threshold 0.01 mm in previous 6 hr, DT 12UTC 24 January 2023, T+21 VT 09UTC 25 January 2023.  The vertical profile at Reading (located by the pin) shows a saturated layer of stratus at about 950 hPa with temperature above 0°C.  Very slight precipitation produced from the stratus by the model falls through an underlying sub-zero layer near the surface. The model gave above 65% probability of very light freezing precipitation.  Freezing drizzle was observed at Reading during the period.


Fig:8.1.8.14: ecCharts presentation of CONTROL-10 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.



Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)







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