Meso-Scale Forecasts – do they help?

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Meso-scale forecast apply Numerical Weather Prediction to limited areas using high resolution.


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Introduction

Global Numerical Weather Prediction models run by National Met Services mostly use grid lengths of about 13 – 20 km. Such models can only represent weather on a scales of about 60 1000 km.

Meso-scale (Limited area) NWP models are used to predict smaller scale detail. These forecasts come into two broad categories; those run by National Met Services and those run by private sector firms.


 

National Weather Service Meso-scale Models

These are run on a range of grid lengths down to 1.5 km but the computing power limits the area covered. The Met Office runs a model on a 1.4 km grid over the small area over the British Isles. It runs a model with a 4 km grid over the larger squar and within a global model having a grid length of about 17 km. All at 70 levels in the vertical. That requires as much computing power as their global model with a 17 km grid.

Courtesy of the UK Met Office.

The Danish Met Service, DMI, runs a model with a grid length of 5 km at 40 levels.

There are several European meso-scale modelling groups and countries producing meso-scale forecasts relevant to their national areas of interest.


National Met Service Model Inputs

Global input

  • All meso-scale models have to run nested within a global model and the first input is on that scalel. That can be between 13 and 20 km.

Observational input

  • To represent small detail, weather information on a smaller scale than possible using conventional in situ observing networks. Sensors carried on satellites . Depending on the instrument, resolutions vary from 1 km up to about 40 km. Surface wind data have a resolution of about 25 km. An additional, rather limited source is from rainfall radar.
  • Major problems are that the data are at many different times and are of varying quality and quantity. There are always uncertainties in the data analysis.

Topographic data

  • These data sets are available at high resolution and include information on land use. In addition, information is included on soil moisture.

Lateral boundaries

  • Weather moves and that has to be taken into account. This is done by updating boundaries from the global model.

Commercial Model Inputs

Mostly, these use the WRF (Weather research Forecast) model. The coding for this is freely available. This should be as good or similar to models used by national Met Services.

As far as I am aware, the inputs are

Model input

  • The US GFS, 0.25 or 0.5 degree grid output.

Observational data

  • None.

Topographic data

  • As for National Met Services.

Lateral boundaries

  • The GFS 0.25 or 0.5 degree data.

These models should take topography into account but forecasts will be compromised by lack of meteorological detail at the data analysis stage.


Limiting factors for Meso-Scale Models

Quality of the initial analysis

  • Shower clouds and similarly sized features will not be well represented. This will affect all models but more so for the commercial ones.

Short lifetimes of small weather features.

  • Those small features that are analysed may not exist long enough to be predicted usefully.

Lateral boundary input

  • The models “know” nothing about small detail entering the model area. In mobile weather situations, the benefits of meso-scale will decrease rapidly in time.

Chaos

  • Small weather detail cannot be explicitly predicted.
  • Models can only take chaos into account through the use of model ensembles.

Meso-scale forecasts

These are some of the meso-scale forecasts readily available and of most interest to UK sailors. A more extensive list can be found at Grib-And-Objective-Forecasts-Reviewed

Model[[,& Forecast Period

Computational Grid km

Sample fprecast

Internet Source

UKV
T+36

1.4

Rain area

iPad
and
Android Apps
Met Office site

The presentation is good for rain areas but poor for winds

Denmark
T+54

5 km

Winds

 DMI Website

This is a good and useful presentation but cannot show detail smaller than about 25 m size.

Norway
T=48

2.5 km

YR.NO Website

Zooming in provides moredata. Tapping on a location anywhere on the map gives a meteogram including sea

Spain
T+72

5 km

 AEMet Website

Not such a good presentation as the Danish service. A little clumsy in operation. Show island wind shadows well.

Many other sources of meso-scale and Global models will be found on a GRIB Services list page.


Summary

Meso-scale models are very useful to National Weather Services for short term warning purposes. Risk can be accessed through use of ensembles.

They should help sailors because of better topographic representation than in global models. However, the complexity of the atmosphere and chaos limits their value in deterministic use.


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