Cloud droplet growth due to turbulent coagulation
Clouds have a major influence on the entire atmosphere, and the understanding of their climate impact is closely linked to the microphysical properties of the cloud. In this regard, our knowledge is still fragmentary and the treatment of clouds leads to large uncertainties in weather and climate predictions. Hence, understanding how cloud droplet distributions evolve in space and time is a major task in cloud physics.
One of the central unknowns in cloud physics is the source of the large droplets required to initiate the rapid production of warm rain in both maritime and continental clouds. A related and also unexplained phenomenon is the observed broadening of the cloud droplet spectrum with increasing height from cloud base at both small and large ends of the spectrum. A long history of attempts exists to solve these problems, but they remain open.
Experimental and numerical evidence shows that in-cloud turbulence can significantly enhance the collision kernel and hence the droplet growth rate, as well as increase the heterogeneity and complexity of the system. Air turbulence can impact droplet collision by at least four mechanisms. First, particle inertia leads to increased relative velocities and less correlated velocity directions (acceleration effect). Second, the wind field shear produces collisions between particles even with the same inertia (shear effect). The acceleration and the shear effect are often referred to as the transport effect. Third, coagulation rates are enhanced due to local concentration increases if the particle response times are on the or- der of the Kolmorgorov scale. For this phenomenon the terms "preferential concentration" or "accumulation effect" are used. Fourth, turbulence can also impact the local droplet-droplet hydrodynamic interactions.
To model the effect of turbulence on the coagulation process, we are using collision kernels derived from direct numerical simulations for particulates in turbulent fluids. In Riemer and Wexler [2005] we showed that turbulent coagulation enhancement can explain warm rain-drop formation and the observed broadening on the right side of the size distribution, and that thermodynamic effects explain the observed broadening on the left side of the size distribution. The results have far-reaching implications for all processes that depend on the microphysical properties of the clouds, such as heterogeneous chemistry and the interaction of clouds with radiation. The effect of turbulence on the coagulation of particles is not limited to atmospheric phenomena, but is present in a great range of engineering and geophysical applications. Turbulence also affects industrial processes such as dust separation in cyclones, TiO2 production and fine spray combustion.
Current Work
In collaboration with Mark Miller's group from BNL, we are validating the model results with a comprehensive set of in-situ and remote sensing measurements collected during the Marine Stratus Radiation Aerosol and Drizzle Experiment (MASRAD).Publications
J. Ching, N. Riemer, M. Miller, M. Dunn [2009] In-cloud turbulence structure of marine stratocumulus at Point Reyes, CA, during MASRAD, Geophysical Research Letters, submitted.
N. Riemer, A.S. Wexler, K. Diehl [2007] Droplet growth by gravitational coagulation enhanced by turbulence – Comparison of theory and measurements, Journal of Geophysical Research, 112, D07204, DOI: 10.1029/2006JD007702.
N. Riemer and A.S. Wexler [2005] Droplets to drops by turbulent coagulation, Journal of the Atmospheric Sciences 62, 1962-1975. (pdf)