Towards Meter-Scale Characterization of Cloud Properties from Airborne Platforms Using Phase-Doppler Interferometry

Accurate and precise measurements of cloud drop characteristics at high resolution are essential for advancing our understanding of cloud microphysical processes, improving cloud parameterizations, and validating weather and climate models. While single-particle counters can provide well-resolved, accurate measurements of cloud drop properties, many of the commonly used instruments, often referred to as legacy probes, face significant challenges. These challenges include large uncertainties in the cross-sectional sampling area, a limited size range over which they are capable of measuring drops, and potential artifacts from shattering and splashing. Additionally, these instruments often suffer from coincidence errors, which limit the effective sampling area and, consequentially, compromise the spatial resolution of cloud characterization which can be achieved. Although some of these issues may be partially resolved in certain cases, they frequently lead to measurement bias. Despite these well-known and fundamental limitations, legacy probes are routinely deployed.

One technique which is capable of overcoming the obstacles facing other single-particle counters is phase-Doppler interferometry (PDI). PDI is a nonintrusive measurement technique which utilizes the wavelength of scattered light as the measurement scale, as opposed to the intensity of scattered light, and simultaneously measures drop size, velocity, time-of-arrival, duration of time in the probe volume, and the scattered signal intensity record with very high accuracy and resolution. The technique is capable of measuring drops ranging from 0.5 μm to approximately 2 mm in diameter or greater, with a working dynamic size range of 50 to 1 for a given instrument setup (i.e., 2-100 μm, 3-150 μm, etc.). It also provides a well-defined probe volume, accounting for both prevailing conditions and drop size dependency. While PDI is a well-established, robust technique which is considered a worldwide standard for spray characterization, its utilization in the cloud physics community has not yet been fully exploited, with only a limited number of research institutions and universities using PDI for airborne field campaigns.

Here, we present novel developments to the PDI technique which we have incorporated into a new flight probe termed the Precision Droplet Analyzer Flight Probe (PDAF). We discuss innovations that enable a well-characterized sample volume approximately two orders of magnitude larger than other single-particle counters and encompassing a wide drop size range. The PDAF is a dual-range system composed of two essentially independent instruments, or channels, in the same instrument housing. Each individual channel is based on the PDI technique and is intended to measure a different drop size range. Measurements from each channel are analyzed and merged into a single comprehensive drop size distribution in real-time.

Among the various innovations presented, we will discuss a novel signal parsing strategy which is capable of resolving measurement errors associated with drop coincidence. With this approach, the scattered signal from individual drops during coincidence may be parsed so that individual drops can be accurately sized without measurement bias. This innovation enables us to increase the effective sample volume by approximately two orders of magnitude to allow for high-efficiency data collection and characterization of cloud properties at a spatial scale of approximately 1-meter.

We will also discuss significant improvements made in defining and controlling the effective probe volume. Methods have been developed and refined to more accurately characterize each dimension of the effective cross-sectional sampling area using a combination of laboratory and in situ measurements. These developments ensure that population-level statistics such as number concentration and liquid water content can be derived with much greater certainty. Additionally, we have incorporated a range of apertures into each receiver to dynamically change the size of the probe volume while sampling in-cloud. The appropriate aperture is selected automatically in real-time based on prevailing conditions, allowing us to control and limit the probability of coincidence.

Lastly, we will present developments which have been implemented to extend the dynamic range of the PDI technique well beyond the historical 50 to 1 range, reaching 100 to 1 or greater, per channel. By incorporating this extended dynamic range into the dual-range approach, the working range of the PDAF can cover the entire static range of the PDI technique from 0.5 μm to 2 mm or greater, with substantial detection overlap between each channel. This allows us to significantly increase the number of samples obtained as well as assess the level of agreement in the overlap region to ensure the quality of measurements collected.

With these innovations, we present an airborne cloud instrument with a well-characterized probe volume approximately two orders of magnitude larger than other single-particle counters, capable of resolving the long-standing issue of drop coincidence, and covering an extensive drop size range from 0.5 μm to 2 mm or greater. These advancements enable the collection of sufficient particle counting statistics to generate drop size distributions and population-level statistics at approximately 1-meter resolution during in-flight acquisition, marking a significant advancement in cloud microphysical measurement capabilities.