Exploring parameterization for turbulent entrainment‐mixing processes in clouds

Abstract

[1] Different turbulent entrainment-mixing processes (e.g., homogeneous and inhomogeneous) occur in clouds; accurate representation of these processes is critical for improving cloud-related parameterizations in large-scale models, but poorly understood and quantified. Using in situ aircraft observations over the U. S. Department of Energy's Atmospheric Radiation Measurement Southern Great Plains site during the March 2000 Cloud Intensive Observation Period and numerical simulations with the Explicit Mixing Parcel Model (EMPM), here we explore the potential of using degree of homogeneous mixing as a measure to quantify these different mechanisms and examine various microphysical measures of homogeneous mixing degree and their relationships to entrainment-mixing dynamics as measured by transition scale numbers. Three different microphysical measures for the homogeneous mixing degree are newly defined and each is coupled with one of two different transition scale numbers. Both observations and simulations show that all the combinations have positive correlated relationships; simulations further show that the tightest relationship is between the measure of homogeneous mixing degree considering adiabatic number concentration and the transition scale number accounting for mixing fraction of dry air. A parameterization of the entrainment-mixing processes is advanced according to the relationships of homogeneous mixing degree measures to transition scale numbers.

On Which Microphysical Time Scales to Use in Studies of Entrainment‐Mixing Mechanisms in Clouds

“…Two points are noteworthy. First, this study focuses on the commonly used r ‐ n mixing diagram that is highly relevant to representing the mixing mechanisms in the commonly used two‐moment microphysics parameterization (Lu et al, ). The effect of time scales on the spectral shape of cloud droplet size distribution deserves further study in view of their potentially significant impacts on parameterizations of effective radius (e.g., Liu et al, ; Liu & Daum, ) and development of precipitation (e.g., Liu et al, ; Wood, ; Xie & Liu, ).…”

“…If initial RH is equal to 90%, S f is around −4%, which is much closer to saturation than the previous case. It is well known that RH is critical to entrainment‐mixing processes (e.g., Lu et al,), but τ phase is not able to reflect the variation of RH. To overcome this weakness, we introduce saturation time scale ( τ satu ) that corresponds to S f = −0.5% (RH = 99.5%) …”

“…In calculations of τ phase , τ satu , and τ react , n can be either adiabatic droplet number concentration ( n a ) or number concentration after entrainment but before mixing and evaporation ( n 0 ). Only the time scales calculated with n 0 are hereafter included, because n 0 is a better choice than n a in entrainment‐mixing studies (Kumar et al, ; Lu et al, ). Our analysis shows that the results are similar if n a is used.…”

Variation in entrainment rate and relationship with cloud microphysical properties on the scale of 5 m

“…The first one is the mixing fraction of adiabatic cloud (v), which can be calculated based on the conservations of total water and energy during the entrainment-mixing at z; the same method was widely used in other studies [25,[44][45][46]. See the Electronic Supplementary Material for details.…”

Impacts of Free-Tropospheric Temperature and Humidity on Nocturnal Nonprecipitating Marine Stratocumulus

“…Since cloud drop size and number greatly affect the cloudtop evaporative cooling, which plays an important role in cloud-top entrainment (Sandu et al 2008;Caldwell and Bretherton 2009;Lu et al 2013), sensitivity tests with regard to the aerosol concentration are conducted by repeating our experiments over an aerosol concentration range from 50 to 500 mg 21 . Starting from a base case, three MSc cases with a same k are simulated and analyzed.…”