Ultimate Meteorological Question from Observational Astronomers: How Good is the Cloud Cover Forecast?

TL;DR

This study evaluates the accuracy of the GFS model in forecasting total, layer, and convective cloud cover compared to satellite observations, finding it reliable for total cloud but less so for layer and convective clouds.

Contribution

It provides a comprehensive validation of GFS cloud forecasts against satellite data, highlighting strengths and limitations for astronomical observation planning.

Findings

GFS model predicts total cloud cover with within +/-15% error in most regions.

Forecast skill for total cloud cover remains significant up to 120 hours ahead.

Layer and convective cloud forecasts are less accurate, especially for low and high clouds.

Abstract

To evaluate the capability of numerical cloud forecast as a meteorological reference for astronomical observation, we compare the cloud forecast from NCEP Global Forecast System (GFS) model for total, layer and convective cloud with normalized satellite observation from the International Satellite Cloud Climatology Project (ISCCP), for the period of July 2005 to June 2008. In general, the model forecast is consistent with the ISCCP observation. For total cloud cover, our result shows the goodness of the GFS model forecast with a mean error within +/-15% in most areas. The global mean probability of < 30% forecast error (polar regions excluded) declines from 73% to 58% throughout the 180h forecast period, and is more skilled than ISCCP-based climatology forecast up to tau~120h. The comparison on layer clouds reveals a distinct negative regional tendency for low cloud forecast and a questionable positive global tendency for high cloud forecast. Fractional and binary comparisons are performed on convective cloud forecast and revealed the GFS model can only identify less than half of such cloud. In short, our result suggests that the GFS model can provide satisfactory worldwide total cloud forecast up to a week ahead for observation scheduling purpose, but layer and convective cloud forecast is less reliable than the total cloud forecast.