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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).
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