Application Of Microholography for Ground-based In Situ Measurements in Stratus Cloud Layers: A Case Study

Abstract

A holographic droplet and aerosol recording system (HODAR) has been designed and operated in situ in a low-level stratus cloud layer adopting the Fraunhofer in-line technique to measure sizes and velocity vectors of cloud droplets as well as to determine sizes and shapes of other hydrometeors (e.g., graupel). The particle-size radii covered by this ground-based instrument range from three to several hundred micrometers. A case study conducted on the Kleiner Feldberg Mountain Observatory near Frankfurt, Germany, during November 1990 documents the temporal evolution of the cloud-droplet size distribution over a period of 19 h during which the size distribution changed from monomodal to bimodal. The velocity vectors of individual droplets, assessed by means of a double pulsed hologram, can be used to prove that the apparatus does not distort the airflow at wind speeds smaller than 1 m s?1. During a different event holographic images of biogenic particles and graupel grains were recorded to demonstrate the system's capability of imaging the shapes of hydrometeors down to a few micrometers in size. Based on this capability, a cautious conclusion on the sizes of cloud interstitial aerosol can also be drawn. The various technical details of the recording and viewing setups of HODAR and improvements with respect to previously published designs are discussed and a thorough error analysis is provided.