Toegepast Atmosferisch Onderzoek

[Dutch]

NOWCASTING CLOUD COVER

Introduction

Clouds are being observed by satellites already since several decades. In particular geostationary satellites are very suitable for nowcasting of clouds because they have a high temporal resolution. Polar satellites have a much higher spatial resolution, but their temporal resolution is too low for nowcasting. Inspite of this long history of observations clouds are still not assimilated in NWP models. However, since a few years very short range cloud forecasts using Meteosat imagery as an initialisation are made. One example is the 'MetCast' model, which is operational at the KNMI since the summer of 1998, and it forecasts until 12 hours ahead. This model resembles a complete numerical weather model, though dynamics is detached from the physics, and radiation computations are not made. The disconnection of dynamics provides an important advantage: the MetCast model has no spin up nor spin down of cloud water and degree of cloud cover. 
 

The MetCast model 

In MetCast (Meteosat Cloud Advection SysTem) clouds observed by Meteosat are advected with winds forecast by the HIRLAM model. Apart from advection also evaporation and/or condensation of liquid water is computed. In addition, turbulent diffusion is simulated. The initial 3-D cloud fields are determined by combining synoptic observations of cloud base heights and cloud top heights computed by the MetClock system. MetClock is a module which automatically computes cloud top temperatures and degree of total cloud cover from Meteosat images. Verification of the computed degree of cloud cover by the MetCast model has revealed that (on the average) the total cloud cover was forecast more precisely by MetCast, compared to the HIRLAM model; at least until the first 12 hours during daylight and until the first 5 hours at night. In particular during the first forecast hours the accuracy of the predictions was considerably higher than that of traditional weather models. The MetCast model makes plus 12 forecasts every 3 hours. On the hours in between plus 3 forecasts are made. The horizontal resolution of the model is 30 km, but this can be changed freely. The vertical resolution is 367 m, this can be changed as well. 

For the meteorologist at the KNMI the following 2-D fields are available: 

These 8 parameters are available as time series for the following stations:


Also, prognoses of Meteosat images are made for the ir and vis channels. 
 

Applications 

The MetCast model can be used in different applications:
  1. Combining the solar angle and the degree of total cloud cover can provide predictions of (short wave) global radiation.
  2. Predictions of clear skies at night can help the forecasting of fog.
  3. AuToTrend: At present experiments are carried out with a high resolution MetCast (30 m in the vertical, 20 km in the horizontal) in aid of aviation. In this high resolution version also several cloud layers are assimilated, whereas the operational model has only one cloud layer initially.
  4. We also have available an experimental version of MetCast in which a 1-D convection model is embedded. With this model we carry out nowcasting experiments for electric storms.

 

Additional information 

Starting from 2002 also images from the Meteosat Second Generation satellite will be available. These images will provide a higher spatial and temporal resolution of the cloud top temperature and degree of cloud cover. At present it is also attempted to apply the 3-D cloud analysis system (MetClock and the synoptic height of cloud base) to the Hirlam model. Possibly this application will help to improve predictions of Hirlam temperatures. 

References

  1. 'Cloud detection using Meteosat imagery and Numerical Weather Prediction model data', Arnout Feijt, Paul de Valk and Sibbo van der Veen, Journal of Applied Meteorology, July 2000. 
  2. 'Zeer korte termijn bewolkingsverwachting met behulp van MetCast',S. H. van der Veen, Technisch Rapprt TR-213, KNMI, september 1998 (in Dutch). 
  3. HIRLAM: High Resolution Limited Area Model. This is an operational numerical weather prediction model at the KNMI. 


Sibbo van der Veen