Investigation of a Low-Cost Hot-Wire Liquid Water Content Probe for Dense Fog Measurements

The measurement of vertical and horizontal variations of LWC is essential for quantifying cloud microphysical processes, including mass advection, latent heat release, and turbulence flux divergence, but existing devices are bulky, costly, and less sensitive to tiny droplets. Low-cost liquid water content (LWC) sensors help to measure the fine spatial scales (1 cm) and high frequencies associated with fluctuations in LWC to understand fog and cloud processes, as well as provide additional data to validate numerical weather prediction (NWP) models. The proposed hot-wire probe is made of a platinum wire with a 125 µm diameter and 5 cm length and uses an assumed droplet deposition efficiency and sensitivity. The temperature of the sensor wire plays a significant role in the estimation of latent heat of vaporization, stored energy in fog droplets, and convection heat loss. The sensor has a highly sensitive circuit that measures wind velocities from centimeters per second to 50 m/s. The time response was estimated using amplifier characteristics, wire geometry, and thermal properties. At a dry sensor wire temperature of, a response time of 0.02 s was obtained. Laboratory tests were conducted to develop calibration curves by measuring the electrical power delivered to the wire when changing wind speeds, humidity, temperature and masses of droplets in the air stream. These data allowed for the regression of electrical power as a function of these variables with a coefficient of determination (R²) of over 0.99.

The hot-wire LWC probe measurement was validated using an in-situ DMT FM120 fog probe during the Fog and Turbulence Interactions in the Marine Atmospheric (Fatima) campaign, which was conducted on Sable Island in Canada during 1-31 July 2022. The hot-wire LWC probe was positioned within the sampling volume of a three-dimensional sonic anemometer and adjacent to relative humidity and ambient temperature sensors, enabling in-situ calibration. Results suggested that the FM120 LWC and the hot-wire probe measurements match well during weak fog events (i.e. with low number concentrations) but deviate during periods of high LWC. To attempt to account for this deviation, we conducted a lab experiment that investigated heat transfer from the wire during high LWC cases. In laboratory calibrations, the convective heat transfer coefficient was estimated when the sensor wire was fully covered by water droplets, which improved LWC estimation with the probe. We propose a correction to the hot-wire LWC probe calculation procedure to account for this phenomenon. This should allow for improved estimates of LWC during dense fog events.