Holographic Cloud Particle Imager (HCPI) Phase IIB Update

Description

Most cloud microphysical, weather forecast and climate models assume a homogeneous and uncorrelated spatial distribution of the particles within clouds.  However, in situ measurements point toward small-scale (mm to cm) correlations between particles due to droplet inertia, turbulence, and phase relaxation.  The spatial distribution of hydrometeors in a cloud influences the radiative properties of the cloud as well as processes controlling the growth and evolution of those hydrometeors.  Adjusting models to account for the inhomogeneity of clouds requires more detailed measurements of hydrometeor spatial distributions and could increase the accuracy of weather and climate predictions.

This work presents a validation study of a prototype holographic cloud particle imager (HCPI) that measures both the 3D spatial distribution and size distribution of hydrometeors in sequences of 20 cm³ volumes.  The HCPI uses in-line holography to generate cloud particle holograms.  Diffraction theory enables a numerical reconstruction of particle positions and sizes within the sample volume, allowing reconstruction of particles ranging from ~14 um to several mm in diameter.  From the determined 3D spatial distribution, the “patchiness” of the particle spacing relative to a Poisson distribution can be quantified with the pair correlation function. The presented measurements were collected while the HCPI was installed on the University of Wyoming (UW) King Air research aircraft together with a suite of in situ cloud and precipitation sensors. 

In collaboration with the King Air team at UW and Droplet Measurement Technologies (DMT), we compare the performance of the HCPI with existing commercial instruments flown side-by-side.  These include a DMT Cloud Droplet Probe (CDP-2), a SPEC Two-Dimensional Stereo (2D-S) probe, and a SPEC High Volume Precipitation Spectrometer (HVPS), which together provide concentration, size, and shape information over the same range of particle sizes as the HCPI.  Cloud particle holograms and particle size and spatial distributions will be presented, together with comparisons of measurements from the commercial instruments under various cloud conditions.  We hypothesize about differing results in certain droplet size ranges and show results from simulated holograms.  While these flights utilized a manned aircraft, the final version of this instrument will be made for use on a mid-sized unmanned aircraft system (UAS), such as a TigerShark/ArcticShark.