Investigating Microphysical Properties of Mixed Phase Clouds in a Turbulent Laboratory Chamber.

Mixed-phase clouds are challenging to understand and describe theoretically due to uncertainties in ice nucleation, rate of conversion of liquid to ice and liquid to vapor, and conditions for coexistence of ice and supercooled water. To better understand the complex interactions, we have created steady state mixed phase cloud conditions in the Michigan Tech Pi Chamber and have used digital in-line holography to measure properties like ice fraction, phase relaxation time, etc. at spatial scales difficult to achieve in the atmosphere. Digital in-line holography allows an instantaneous and spatially localized measurement of ice and water droplet populations, so that their interactions can be inferred. Each hologram represents about 8 cubic centimeters of volume near the center bottom boundary with a minimum resolution of 7 μm. The holograms are then reconstructed to obtain particle properties such as size and position. The size of the ice particles, being non spherical, is expressed as an equivalent spherical diameter. They are then distinguished from the water droplets using the size and comparison with the phase Doppler interferometer, which characterizes water droplets easily but has difficulty with ice particle sizing.

The process of creating the mixed phase clouds is as follows: First a temperature gradient is achieved between the top and bottom boundaries of the Pi chamber, which are saturated with water vapor. In the experiments described here, the top boundary is maintained at -16 C and the bottom at 4 C. This gives a mean temperature of -6 C inside the chamber and liquid-water supersaturated conditions. Then NaCl aerosols are injected into the chamber until steady state cloud conditions are achieved, i.e. the rate of settling/loss of droplets balances the rate of activation of droplets and we get a steady droplet size distribution. Once water saturated conditions are achieved and the supercooled cloud is characterized, Snomax particles are injected into the chamber to create mixed phase conditions. The concentration of NaCl and Snomax is varied to generate mixing clouds with low as well as high particle concentrations in order to characterize different cloud types as found in the atmosphere. Figure 1 shows a look into the Pi-chamber with particles illuminated by a green laser sheet. (Top left) A liquid phase cloud created using only NaCl aerosols at 2.0*10⁴ cm^-3 (at 2 L min^-1 of air supply) injection rate into the chamber (low number concentration). (Top right) A mixed phase cloud created by adding Snomax particles of 3000 particles cm^-3 (at 4 L min^-1 of air supply) in addition to the continuous supply of NaCl aerosols. (Bottom left) A liquid phase cloud created using only NaCl aerosols at 1.25*10⁵ cm^-3 (at 2 L min^-1 of air supply) injection rate into the chamber (high number concentration). (Bottom right) A mixed phase cloud created by adding Snomax particles of 3000 cm^-3 (at 4 L min^-1 of air supply) to the chamber in addition to the continuous supply of NaCl aerosols. From the figure, we can see that once the injection of Snowmax begins, a visual decrease in the particle number concentration inside the chamber can be observed.

Using the holographic measurements, we find that ice particles and water droplets are not well mixed at the centimeter level. Certain holograms consist primarily of water droplets while others comprise primarily ice crystals. This is particularly evident during the low number concentration experiments. The measurements also allow us to calculate phase relaxation times both for cloud droplets and for ice particles. We observe conditions in which ice particles have a larger phase relaxation time compared to the coexisting water droplets, but also where the opposite is true. Furthermore, in some conditions ice particles show not only a large mean value, but also a broad distribution of phase relaxation times. Finally, mixing diagrams often show an increase in diameter with decrease in particle number concentration, which is similar to behavior observed in the cloud chamber for stochastic condensation in warm cloud conditions. For mixed phase clouds, the diameter increase corresponds to an increase in the ice fraction compared to warm clouds.