# Section 2.1.1.2 Rationale for High Resolution

## Rationale for high resolution

Calculations become more accurate with higher resolution.  A high spatial resolution also enables a better representation of topographical fields (mountains, coasts, islands) and the effects they have on local weather and on large-scale flow.  It also produces a more accurate description of horizontal and vertical structures, which allows better assimilation of observations.   Increasing the resolution (i.e. making grid point spacing smaller) benefits the analyses and forecasts of both large-scale systems (such as large-scale blocking "omega" anticyclones, and "cut-off lows") and the small-scale systems often associated with severe weather.  The ability to accurately forecast the formation of large-scale blocking “omega” anticyclones and “cut-off lows” depends crucially on sufficient effective resolution.  However, a resolution of at least ~1km is required to capture mesoscale phenomena.

The smallest atmospheric features that can be resolved have wavelengths four to six times the nominal grid point distance.   Grid point spacing is currently about 9km in the medium range ensemble.  This implies features with wavelengths less than about 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.  Increasing the resolution (i.e. making grid point spacing smaller) benefits the analyses and forecasts of both large-scale systems (such as large-scale blocking "omega" anticyclones, and "cut-off lows") and the small-scale systems often associated with severe weather.  The ability to accurately forecast the formation of large-scale blocking “omega” anticyclones and “cut-off lows” depends crucially on sufficient effective resolution.  To capture mesoscale phenomena a resolution of at least ~1km is required.

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

Fig2.1.1.2-1: 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 (used by the extended range and seasonal (SEAS5) ensembles), 18km, 9km (used by medium range ensemble), 5km (a possible future resolution).

Fig2.1.1.2-2: Magnification of rainfall forecasts as in Fig2.1.1.2-1. 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 (e.g. Northeast England).

Fig2.1.1.2-1 and Fig2.1.1.2-2 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.