Analysis of Diurnal, Interdiurnal and Interannual Variations during Northern Hemisphere Summers Using METEOSAT Infrared Channels

Abstract

Using METEOSAT data in the ISCCP B2 format, we study the mean radiation fields and their fluctuations during Northern Hemisphere summer (June, July, August) of 1983, 1984 and 1985, for regions of 5°?5° located from 50°N to 50°S and from 60°E to 60°W. The study is performed for the IR atmospheric window channel (10.5?12.5 ?m) and the water vapor band (5.7?7.1 ?m). The year-to-year differences of the mean fields delineate large regions of positive and negative anomalies with principally a zonal distribution. This suggests that interannual perturbations in the large-scale meridional circulation have a strong influence on the radiation field, principally by way of convective activity. To study diurnal variations, we separate coherent diurnal variance obtained by compositing over the 3 months, and the total intradiurnal variance obtained by integration of the power spectra over periods lower than 1 day. In the IR window, the coherent diurnal variance, expressed as a percentage of the total intradiurnal variance, is stronger over subsidence areas, reaching values greater than 98% over desert regions due to surface temperature and values near 70% over ocean regions due to diurnal variations of stratiform cloudiness. Over ITCZ or midlatitude regions, this percentage is lower. In the WV band, the latitudinal distribution presents maximum values of this percentage between the equator and 10°N (>35%) with a progressive decrease up to 30° of latitude (<5%) in both hemispheres. The coherent diurnal variation is larger (up to 50%) over central and eastern Africa and related to convective activity over highlands. Spectral analysis of interdiurnal fluctuations reveals a progressive shift of the dominant time scales from short time scales (1?2.5 day band) over convective zones to periods longer than 9.2 days over subsidence areas. Regional aspects are revealed by mapping the spectral variance in selected frequency bands as a percentage of the total interdiurnal variance. Over subsidence areas and many other regions where the IR signal depends strongly on surface temperature and lower atmospheric levels inaccessible to the WV channel, the strong coherence between the two channels suggests that the same time scales dominate over the entire vertical extent from low to middle troposphere.