Improved Measurements of the Ice Water Content in Cirrus Using a Total-Water Probe

TLDR

Earlier IWC measurements of cirrus assumed cloud air was saturated with ice, a simplification that limited accuracy. This study presents an improved method for measuring cirrus ice water content using a total‑water probe. The method replaces the saturation assumption with direct water‑vapor measurements from a fast‑response Lyman‑α fluorescence sensor, coupled to the total‑water probe. The new approach eliminates earlier anomalies, yields IWC values that agree closely with a 2D optical array probe, and enables more stringent validation of crystal‑mass algorithms.

Abstract

This note describes an improved method for the measurement of the ice water content (IWC) of cirrus cloud using a total water content probe. A previous version of this technique assumed that the air in cloud-containing regions was saturated with respect to ice. This assumption has now been replaced with measurements of the water vapor content from a fast-response Lyman-α fluorescence water vapor sensor. The improved measurement of the vapor phase resolves anomalies in the earlier measurements that were due to the assumption of saturation with respect to ice everywhere within cloud. The comparison of IWC measurements made by this new method with those from a 2D optical array probe is greatly improved. The new measurements may now be used to provide much more stringent tests of the algorithms used for the derivation of crystal mass from measured size in 2D probe data.