Rationale for high resolution

Accurate forecasting of the formation of systems depends crucially on sufficient effective resolution.  Calculations become more accurate with higher resolution (i.e. smaller grid point spacing).  A high spatial resolution produces a more accurate description of horizontal and vertical structures, which in turn allows better assimilation of observations.  It gives better representation of mountains, coasts, islands, etc. and the effects they have on local weather and on large-scale flow.  Large-scale systems (e.g. blocking "omega" anticyclones or "cut-off lows") benefit from higher resolution.  But Increasing resolution particularly benefits the analyses and forecasts small-scale systems that are often associated with severe weather.   However, the smallest features and wavelengths that can be resolved are four to six times nominal grid length.  A resolution of at least ~1km is required to capture mesoscale phenomena.     

Grid point spacing is currently ~9km in the medium range and sub-seasonal ensembles so wavelengths less than ~45km may not be captured.    Such small atmospheric systems have a predictability of relatively few hours, but their representation is important for energy exchanges between different atmospheric scales.

An illustration of the effect of resolution upon forecast rainfall distribution and intensity. 

Fig2A.1.1.2-1: Rainfall forecasts run for the same period (24hr rainfall to T+69 verifying at 06UTC 6 Dec 2015) at several resolutions.    Resolutions: 80km , 36 km (seasonal and sub-seasonal ensembles), 18km, 9km (medium range ensemble), 5km (a possible future resolution).

Fig2A.1.1.2-2: Magnification of rainfall forecasts as in Fig2A.1.1.2-1. The bottom right plot shows the observed rainfall for comparison.  Higher resolution gives better representation of the rain shadow effect but it still is underestimated in places (e.g. Northeast England).

Fig2A.1.1.2-1 and Fig2A.1.1.2-2 illustrate the impact of resolution on rainfall, particularly orographic enhancement and the rain shadow effect.  The effect of increased model resolution on:

• orographic precipitation:
  • better capture and detail of the intensity of precipitation.
  • more detail related to individual mountains/groups of mountains.
• rain shadow effects:
  • better representation of the rain shadow.  However, all runs underestimate the rain shadow, but more so at lower resolution (the model allows more moisture to spill over the hills because of the smoother orography).
  • the effect can be far reaching.  There was a strong southwesterly wind in this case and at higher resolution the rain shadow extends right across the North Sea into Scandinavia while at lower resolution the rainshadow is less extensive (i.e. probably showing too much rain is shown over North Jutland).
  • better representation of the detail within the rain shadow.  At high resolution the rain shadow is limited east of Edinburgh (because the Central Lowlands of Scotland do not provide a significant barrier to the southwesterly airflow) and rain is shown formed into a narrow band reaching southwest Norway where flooding was reported.  At low resolution the rain shadow from the Central Lowlands of Scotland seems to be over-estimated (because the smoother orography does not capture the Central Lowland valley).

Note:   An estimate of the range of totals likely within each gridbox is made by the Point Rainfall product.

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