A Technique to Measure Entrainment in Cloud by Dual-Polarization Radar and Chaff

Abstract

A new method for studying the entrainment of air into clouds has been developed and tested, and initial results demonstrating the technique are presented here. This new method is superior to other chaff techniques that allow air parcels to be followed only until the chaff reflectivity becomes indistinguishable from cloud reflectivity. Frequently this is won after the chaff enters the cloud. The new technique uses circular depolarization ratio (CDR) measurements to avoid this limitation. Since the CDR of chaff is typically 20 dB greater than that of most hydrormeteors, the signature provided by CDR allows chaff to be followed into moderately reflective regions of the cloud long after the reflectivity signature of the chaff has disappeared. Circular polarization is superior to linear polarization in measurements such as these because the chaff signatures in circular depolarization do not require assumptions about chaff filament orientation, whereas interpretation of ZDR signatures (from linear polarization) does require such assumptions. In this paper, we discuss the technique, and present initial 8.6 mm wavelength radar measurements of chaff released near the base of a cumulus cloud. These measurements demonstrate that the chaff can be distinguished from its CDR signature well after its reflectivity has been lost in the reflectivity of the cloud. Possible applications as well as limitations of the technique are discussed. An extension of the technique that offers the potential for qualitatively observing electrical fields in cloud is also discussed.