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

Abstract

The commonly used time scales in entrainment‐mixing studies are examined to seek the most appropriate one, based on aircraft observations of cumulus clouds from the RACORO campaign and numerical simulations with the Explicit Mixing Parcel Model. The time scales include the following: τevap, the time for droplet complete evaporation; τphase, the time for saturation ratio deficit (S) to reach 1/e of its initial value; τsatu, the time for S to reach −0.5%; and τreact, the time for complete droplet evaporation or S to reach −0.5%. It is found that the proper time scale to use depends on the specific objectives of entrainment‐mixing studies. First, if the focus is on the variations of liquid water content (LWC) and S, then τreact for saturation, τsatu and τphase are almost equivalently appropriate, because they all represent the rate of dry air reaching saturation or of LWC decrease. Second, if one focuses on the variations of droplet size and number concentration, τreact for complete evaporation and τevap are proper because they characterize how fast droplets evaporate and whether number concentration decreases. Moreover, τreact for complete evaporation and τevap are always positively correlated with homogeneous mixing degree (ψ); thus, the two time scales, especially τevap, are recommended for developing parameterizations. However, ψ and the other time scales can be negatively, positively, or not correlated, depending on the dominant factors of the entrained air (i.e., relative humidity or aerosols). Third, all time scales are proportional to each other under certain microphysical and thermodynamic conditions.

Scale Dependence of Cloud Microphysical Response to Turbulent Entrainment and Mixing

“…They are therefore complementary to the DNS of cloud core regions (Ayala et al, ; Onishi et al, ; Sardina et al, ; Saito & Gotoh, ). We anticipate that DNS of entrainment and mixing will help shed light on questions related to the appropriate microphysical time scale (Lehmann et al, ; Lu et al, ) and how to parameterize the microphysical response to homogeneous versus inhomogeneous mixing. Finally, as spatial and temporal scales are increased, the collision‐coalescence process likely becomes more relevant, and extension to that regime is a promising area of investigation.…”

Aerosol–Cloud Closure Study on Cloud Optical Properties using Remotely Piloted Aircraft Measurements during a BACCHUS Field Campaign in Cyprus

“…Recent works have been published, investigating the role of entrainment and turbulence for broadening the cloud droplet spectra with an adiabatic parcel model (Grabowski and Abade (2017); Abade et al (2018)), aiming to improve subgrid- 15 scale representation for a large eddy simulation cloud model. Studies of entrainment-mixing mechanisms in cumulus clouds used manned aircraft observations (CIRPAS Twin Otter) to highlight the scale dependence of the mixing processes (Lu et al, 2018). However, as the scale of entrainment processes ranges from km to mm, Lu et al (2014) point out the limitation of a 10 Hz sampling rate with a manned aircraft flying at 50 m s -1 (spatial resolution 5 m).…”

Cloud Edge Properties Measured by the ARM Shortwave Spectrometer Over Ocean and Land

“…These observations have deep roots in the physics of the cloud‐air mixing process. Recent studies (Gao et al, ; Korolev et al, ; Lu et al, ; Pinsky, Khain, & Korolev, ; Pinsky, Khain, Korolev, & Magaritz‐Ronen, ; Pinsky & Khain, ) show that mixing includes two processes: mechanical mixing caused by turbulent diffusion and droplet evaporation accompanied by an increasing RH. The variation of droplet size during mixing depends on the ratio of the saturation deficit to the liquid water content in the cloud volume, and the ratio of the characteristic mixing time to the phase relaxation time (e.g., Pinsky et al, ).…”