The diurnal cycle observed by Meteosat-8 and simulated by a climate model

We present an analysis of the diurnal cycle of outgoing longwave radiation (OLR) from the Geostationary Earth Radiation Budget (GERB) instrument on the Meteosat-8 satellite. The GERB data were averaged and interpolated to create a mean diurnal cycle based on Local Solar Time (LST) with a fifteen minute temporal resolution. Principal Component analysis was used to separate the significant diurnal signals, which were found to correspond well to physical modes of forcing. The first principal component describes the land surface response to solar heating, peaking shortly after local noon. The second principal component is significant in areas with strong diurnal cloud variations and reveals both convective cloud over land and stratocumulus over the ocean. The convective cloud forms preferentially over mountain ranges and we also see propagating convective systems over North-East Brazil. These two principal components describe over 90% of the total diurnal variation.

To extend this analysis, we use narrowband radiances from the SEVIRI (Spinning Enhanced Visible and InfraRed Imager) instrument on the same satellite. SEVIRI provides radiances with high spatial and temporal resolution and these data may be used to derive the diurnal variation of deep convective cloud and of upper tropospheric humidity (UTH). By applying a technique similar to that of Soden (Geophysical Research Letters, Vol 27, 2173-2176, 2000) to the 6.7 micron water vapour channel from SEVIRI, the diurnal cycle in UTH may be derived, and the relationships between the diurnal cycles of solar heating, cloud response, UTH and the OLR investigated.

These data may also be used to evaluate the simulation of the diurnal cycle in climate models. In this study, we evaluate the new HiGEM model (see http://www.higem.nerc.ac.uk/), developed by a consortium led by the Centre for Global Atmospheric Modelling at Reading. The model performs well in cloud-free conditions, but in common with most models HiGEM produces deep convective cloud too early in the day, particularly in the Inter-Tropical Convergence Zone.