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. By covering about 30 % of the Earth and by exerting astrong greenhouse effect, high-level clouds play an important role in theenergy balance of our planet. Their warming and cooling effects within theatmosphere strongly depend on their emissivity. The combination of clouddata from two space-borne infrared sounders, the Atmospheric InfraRedSounder, AIRS, and the Infrared Atmospheric Sounding Interferometer, IASI,which observe the Earth four times per day, allows us toinvestigate the diurnal variation of these high-level clouds bydistinguishing between high opaque, cirrus, and thin cirrus clouds. Wedemonstrate that the diurnal phase and amplitude of high-level clouds can beestimated from these measurements with an uncertainty of 1.5 h and 20 %,respectively. By applying the developed methodology to AIRS and IASI cloudobservations for the period of 2008–2015, we obtained monthly geographicaldistributions of diurnal phase and amplitude at a spatial resolution of1∘ latitude ×1∘ longitude. In agreement with otherstudies, the diurnal cycle of high-level clouds is the largest over land inthe tropics. At higher latitudes, their diurnal cycle is the largest duringthe summer. For selected continental regions we found diurnal amplitudes ofcloud amount of about 7 % for high opaque clouds and for thin cirrus, and9 % for cirrus. Over ocean, these values are 2 to 3 times smaller. Thediurnal cycle of tropical thin cirrus seems to be similar over land and overocean, with a minimum in the morning (09:00 LT) and a maximum during the night (01:00 LT). Tropical high opaque clouds have a maximum in the evening (21:00 LT overland), a few hours after the peak of convective rain. This lag can beexplained by the fact that this cloud type includes not only the convectivecores, but also part of the thicker anvils. Tropical cirrus show maximumcoverage during the night (01:00 LT over land). This lag indicates that they are partof the deep convective cloud systems. However, the peak local times alsovary regionally. We are providing a global monthly database of detecteddiurnal cycle amplitude and phase for each of these three high-level cloudtypes.

Diurnal variation of high-level clouds from the synergy of AIRS and IASI space-borne infrared sounders