Broadband and Narrowband Infrared Emittance Derived From ASTEX Measurements

The evaluation of the role of the radiative effects of low level stratocumulus clouds as deduced by Global Circulation Models (GCMs) requires an accurate parameterization of the cloud's IR emittance. During the ASTEX Phase ll IFO (Intensive Field Observation) in June, l992, Colorado State University sampled the downwelling infrared radiation field under marine stratocumulus cloud layers using an Eppley pyrgeometer, a Barnes PRT6 bolometer and a Bomem mid-infrared interferometer. Using these data and the supporting information from a cloud laser ceilometer operating at 0.904 um, a rawinsonde system and infrared radiation transfer models, the infrared emittance of the overlying cloud layer may be determined. In addition, the radiation data alone allow determination of the "cloud forcing" on the surface energy budget. The analysis techniques and results will be described in case study, time series and statistical summaries.

Initial evaluation indicates a high degree of correlation between the window intensity measurements and the broadband IR irradiance measurements. Thus, it is likely that a relationship may be established between the corresponding IR emittances.