Modeling Cirrus Clouds. Part I: Treatment of Bimodal Size Spectra and Case Study Analysis

Abstract

Abstract A model has been developed that predicts the evolution of bimodal size spectra in cirrus clouds. This was done by predicting two size distributions: one for ice particles less than about 150 µm and another for larger particles. The sum of these two distributions yielded the composite, bimodal size distribution, which was predicted from the growth processes of vapor deposition and aggregation. Predicted size spectra were directly compared with size spectra measured during a cirrus cloud case study sampled during a Lagrangian spiral descent. Favorable agreement was obtained between predicted and measured size distributions, especially at ice particle sizes < 150 µm. The aircraft sampling technique and conditions characterizing the case study were well suited for evaluating the effect of aggregation on ice particle growth. Model calculations indicated that aggregation growth increased mean ice particle sizes by up to 50 µm and reduced concentrations by up to 60% during the case study. The cloud-aver...