Inhomogeneous Mixing and Drizzle in Stratus Clouds

Abstract:

Inhomogeneous mixing was introduced to explain warm cloud precipitation (Baker et al. 1980) by reducing droplet concentrations to thus allow growth of drops large enough to precipitate. For a similar entity mixing theory Telford (1995) stated, “...big drops occur in parcels with lower liquid water mixing ratios.” Thus, Yum et al. (2015) stated that inhomogeneous mixing should lead to negative correlation coefficients, R, between cloud droplet liquid water content, LWC_c (L_c) and cloud droplet mean volume, V, or mean droplet diameter (MD).

A variety of mixing scenarios between extreme inhomogeneous and extreme homogeneous have been observed. In the Physics of Stratocumulus Tops (POST, July-Aug. 2008 central California coast) we find 5 negative R between L_c and MD for 1-s data of 146 horizontal cloud penetrations (3.4%). But a fast R computer program provided linear regressions over 50-s intervals of R(L_c-MD) at every second, except the necessary extreme edges. For these 34,915 s, 1617 were negative (4.6%). Therefore, R(L_c-MD) numerical values that ranged from 0.995 to -0.84 characterize the type of mixing at every second; high positive values for homogeneous or no mixing and low or negative R(L_c-MD) for inhomogeneous mixing.

The figure shows R and two-tailed probabilities, P2, between mean MD and mean concentrations of the smallest drizzle drops (40-64 µm diameter) with the relative frequency of R(L_c-MD) within 21 R(L_c-MD) bins. Relative frequency is the number of seconds within each 0.05-wide R(L_c-MD) bin compared to the total number of seconds within each of the 146 cloud passes. All negative R(L_c-MD) s comprise a single bin. Contrary to expectations the largest cloud droplets and the greatest drizzle amounts do not occur at negative or very low positive R(L_c-MD) at the left of the panels. But consistent with inhomogeneous mixing predictions high R(L_c-MD) at the right of the panels, which represent extreme homogeneous mixing or no mixing, show negative R that indicate only small cloud droplets and minimal drizzle. Nearly all R(L_c-MD) bins < 0.9 seem to contribute to larger droplets and greater drizzle. These results are in accord with original inhomogeneous mixing theory.

Baker, M.B., Corbin, R.G., & Latham, J. (1980). QJRMS, 108, 871-898.

Telford, J.W. (1995). JAM. 34, 2098-2099.

Yum et al. (2015). JGRA, 120, 5047-5069.