Effects of Turbulent Mixing on the Structure and Macroscopic Properties of Stratocumulus Clouds Demonstrated by a Lagrangian Trajectory Model

Abstract

The final published version of this manuscript will replace the preliminary version at the above DOI once it is available.If you would like to cite this EOR in a separate work, please use the following full citation:Magaritz, L., M. Pinsky, and A. Khain, 2013: Effects of turbulent mixing on the structure and macroscopic properties of stratocumulus clouds demonstrated by a Lagrangian trajectory model. J. Atmos. Sci. Abstract 19The role of turbulent mixing in formation of the structure of stratocumulus clouds is 20 investigated using a Lagrangian-Eulerian parcel cloud model containing ~2000-5000 adjacent 21 parcels with the linear size of 25-40 m, moving with a turbulent-like velocity field with 22 observed energetic and statistical properties. The process of turbulent mixing of Lagrangian 23 parcels is parameterized using the k-epsilon theory extended to the case of mixing of non-24 conservative values. The model includes the interaction of cloud and the overlying inversion 25 layer. The stratocumulus clouds observed during flight RF01 of the DYCOMS-II field 26 experiment are simulated. 27 Effects of turbulent mixing are analyzed by comparing simulations with and without 28 mixing. When mixing between parcels is included, the thermodynamical and microphysical 29 structure of the measured stratocumulus clouds is properly reproduced. Mixing leads to a 30 more uniform cloud structure with well defined borders. Good agreement is found between 31 Paluch diagrams calculated in the model and those reproduced from measurements. The 32 radius of correlation of liquid water content and other variables calculated in the model is on 33 the order of several hundred meters and agrees well with observations. When mixing is not 34 included, the radius of correlation is on the scale of a single parcel and the cloud layer 35 contains dry entrained parcels making the microphysical structure unrealistic. It is also shown 36 that turbulent mixing leads to an increase in the effective radius, facilitates and accelerates 37 drizzle formation. The time in which a 40 m air parcel preserves its identification is estimated 38 from the results and is found to be on the order of 25 min. 39 40 Key words: stratocumulus clouds, effects of mixing on thermodynamics and statistical 41 properties of stratocumulus clouds, numerical modeling 42 43 3 65 mixing supersaturation in different parts of the mixing volumes is different and some fraction 66 of the droplets evaporates in dry air.67 4 There are contradictory opinions concerning the role played by extremely inhomogeneous 68 mixing in the formation of large droplets and first raindrops in cumulus clouds. Numerical 69 results of Cooper et al. (2013) seem to lend some support for the idea that inhomogeneous 70 mixing leads to an increase in drop size. In contrast, a numerical study by Schlüter (2006) have 71 shown that extremely inhomogeneous mixing does not result in formation of larger droplets, 72 but leads to spectrum broadening towards smaller drop sizes. These results were reached even 73 thou...

Theoretical investigation of mixing in warm clouds – Part 2: Homogeneous mixing

“…This fact brings up two questions: "What error is introduced when the spatial scale separating mixing types in models is much larger than 0.5 m?" and "Why are spectral microphysics models with a resolution of 40-50 m able 9268 M. Pinsky et al: Theoretical investigation of mixing in warm clouds -Part 2 to reproduce observed DSD and their moments with high accuracy (Benmoshe et al, 2012;Khain et al, 2013Khain et al, , 2015Magaritz-Ronen et al, 2014)? "…”

“…The first factor is that mixing leads to formation of cloud zones characterized by a spatial correlation scale (radius of correlation) of temperature, humidity and droplet concentration of about 150-250 m (Magaritz-Ronen et al, 2014). Numerical experiments with Lagrangian-Eulerian model of Sc (Magaritz-Ronen et al, 2014) have shown that the results are not sensitive to the choice of parcel size, if this size is substantially smaller than the spatial radius of correlation. Therefore, the mixing type has a minor effect on the results of mixing at scales lower than the radius of correlation.…”

Drizzle formation in stratocumulus clouds: effects of turbulent mixing

“…In order to make cloud structure realistic and represent processes resulting from interaction with the inversion layer, it was necessary to take into account processes of entrainment and mixing of adjacent parcels (Magaritz-Ronen et al, 2014). It was shown that turbulent mixing of parcels leads to realistic spatial variability of microphysical quantities characterized by a spatial correlation scale of ∼ 200 m. It was also shown that mixing increases the width of the droplet size distribution (DSD).…”

“…The characteristic time period during which an air parcel maintains its identity was found to be 15-20 min. Magaritz-Ronen et al (2014) successfully simulated the structure of a non-drizzling stratocumulus maritime cloud observed during research flight RF01 of the Second Dynamics and Chemistry of Marine Stratocumulus field study (DYCOMS-II).…”

Long-resident droplets at the stratocumulus top

“…Real parcels mix with the environment and lose their identity after a time comparable to the convective-eddy time, as most droplets that were initially in the parcels have already left. Neglecting mixing causes an overestimation of the fluctuations in the liquid field and a tooearly rain formation (Magaritz-Ronen et al, 2014). Second, most cases are based on a small number of parcels (∼ 1000), which do not allow the investigation of extreme behaviors that might be important for rain formation.…”