Digital Holography for In Situ Determination of the Sizes of Hydrometeors and Precipitation Formation

The determination of the portion of the lower tropospheric water content in the condensed phase is fundamental to the measurement of precipitation.  Furthermore, determining the onset of precipitation development and the conversion of condensed water to precipitation is a basic requirement, but one that is experimentally difficult to achieve.   Traditional methods, such as single-particle light scattering techniques and optical array instruments, are limited either by a small sample volume (which is needed to avoid coincidence problems with scattered light) or by a small and difficult-to-measure depth of field for small particles in the case of optical array instruments.   The recent development of compact, high-speed digital holographic cameras together with digital reconstruction techniques allows for an improved sample volume and better determination of the numbers of particles in the size range of 50 to 100 microns, which is a critical size range for the transition between the condensed phase (e.g. cloud water) and precipitation.   This technique is used to improve the coverage of airborne precipitation measurements, for example, to better identify where the onset of precipitation occurs.   High-speed computing makes holographic reconstruction of millions of individual cloud and precipitation particles possible.  This allows for the concentration, size and shape of each particle in the range of ~6 microns to mm sizes to be determined in a well-defined sample volume.   The time and resources required for this processing remains an unmet need to fully exploit this technology.  Improvements in algorithms and further improvements in code efficiency offer good prospects for making this observation technique easier and more available for routine use.