Assessment of cloudless-to-cloud transition zone from downwelling longwave irradiance measurements

The transition zone has been defined as corresponding to atmospheric conditions between cloud-free (albeit containing a suspension of dry particles, i.e. aerosol) and clouds (air highly hydrated droplets or ice crystals). Since significantly affect the infrared radiation reaching Earth's surface, broadband measurements, which are usually performed with pyrgeometers, implicated in some methods proposed for detecting clouds. present work uses one these characterize cloudy conditions. Two years downwelling longwave irradiance measured pyrgeometer located Girona (in northeast Iberian Peninsula) have used determine values cloud detection index. method must be previously tuned against set selected cases detect cloudiness certain confidence level. effect changing level was studied derive occurrence performance method, support observations by shortwave instruments, ceilometer retrievals images captured hemispherical camera, then analyzed short time periods. duration periods is presented here, showing that 41% very (10 min, temporal resolution method), median distribution about 20 that, although more than hour observed, longer durations steadily decrease frequency. It found 10–15% included may correspond zone, thus strengthening estimations from other shortwave-based only suitable daylight