Remote Sensing of Cirrus Radiative Parameters during EUCREX’94. Case Study of 17 April 1994. Part I: Observations

Abstract

During the intensive European Cloud and Radiation Experiment 1994 (EUCREX?94) conducted off the coast of Brittany (France) over the Atlantic Ocean during April 1994, natural cirrus have been analyzed from in situ and remote sensing measurements. The authors have particularly studied the case of 17 April 1994. For this day a cirrus bank is described by a complete dataset, that is, classic airborne thermodynamical measurements, microphysical (forward scattering spectrometer probe) and OAP-2D2-C (optical array probe-cloud) probes manufactured by Particle Measuring System, and radiative (Barnes Precision Radiation Thermometer, Eppley pyranometers, and upward- and downward-looking pyrgeometers) measurements above and below the cloud. More specific airborne instruments were used such as upward backscatter lidar with polarization capabilities (LEANDRE) on board the Avion de Recherches Atmosphériques et Télédétection and the Polarization and Directionality of the Earth?s Reflectances (POLDER) radiometer on board the Falcon for measurement of bidirectional and polarized reflectances. The scene was also documented by NOAA-12/Advanced Very High Resolution Radiometer data. However, the nonsphericity of cirrus ice crystals is clearly demonstrated by the lidar backscattering depolarization ratio measurements (?p = 24%) and by the absence of any rainbow in POLDER bidirectional reflectances. A specular reflection of the solar light observed on POLDER images indicates the presence of horizontally oriented ice particles in the cloud. All these optical properties will be studied in a companion paper (Part II) and compared with optical properties derived from microphysical models in order to evaluate the radiative impact of natural cirrus clouds.