THE RETRIEVAL OF CIRRUS CLOUD PROPERTIES FROM AIRCRAFT MULTISPECTRAL REFLECTANCE MEASUREMENTS DURING EUCREX93

We present details of a scheme for retrieving cirrus cloud optical thi ckness and effective particle size from nadir-viewing reflectance meas urements. Two near-infrared wavelengths are used, one with negligible ice absorption (1.04 mu m) and one with significant ice absorption (1. 55 mu m). Four ice crystal shapes are used in the scheme: ice spheres, hexagonal columns, hexagonal plates and randomized polycrystals. We h ighlight the sensitivity of the retrievals to the shape of the phase f unction, which follows from the particular ice crystal shape assumed. Five aircraft-based retrievals are presented, all cases occurring duri ng the 1993 intensive field campaign of the European Cloud Radiation E xperiment (EUCREX'93). The retrieved effective sizes are compared with the in situ measurements taken from the same cases. In these comparis ons, account has to be taken of the fact that (a) in situ replicator d ata indicate that the presence of small (<100 mu m) ice crystals can h ave a significant effect on the effective crystal size, and (b) the me asured crystal sizes consistently show a systematic decrease with heig ht in all cases, the effective sizes at cloud base being typically two or three times larger than at cloud top. Taking these considerations into account, we conclude that the assumption of polycrystals gives th e most consistent agreement with the in situ measurements.