Possible roles of ice nucleation mode and ice nuclei depletion in the extended lifetime of Arctic mixed‐phase clouds

Abstract

[1] The sensitivity of Arctic mixed phase clouds to the mode of ice particle nucleation is examined using a 1-D cloud model. It is shown that the lifetime of a simulated low-level Arctic mixed-phase stratus is highly sensitive to the number concentration of deposition/condensationfreezing nuclei, and much less sensitive to the number of contact nuclei. Simulations with prognostic ice nuclei concentration exhibit rapid depletion of deposition/ condensation-freezing nuclei due to nucleation scavenging which significantly extends the mixed-phase cloud lifetime. In contrast, scavenging has little impact on the number of contact nuclei. Thus, contact mode nucleation generally dominates in the cloud layer when both modes are simultaneously considered. The dominance of contact nucleation in Arctic mixed-phase clouds is consistent with a number of in situ observations, remote retrievals, and laboratory experiments. A conceptual model of long-lived Arctic mixed-phase clouds is developed that explains their persistence through the rapid depletion of deposition/ condensation-freezing ice nuclei and a self-regulating drop-contact freezing feedback.

Modelling micro- and macrophysical contributors to the dissipation of an Arctic mixed-phase cloud during the Arctic Summer Cloud Ocean Study (ASCOS)

“…Rogers et al (2001) found that for thin, low-level stratus clouds, the IN concentration at −15 to −20 • C was around 1 L −1 . Nevertheless, ice crystal concentrations in the Arctic may vary over 3 orders of magnitude (Morrison et al, 2005) and a maximum ice crystal concentration of 0.25 L −1 has been observed in a similar season and geographic region to that of ASCOS (Bigg, 1996). Considering IN concentration of 0.25 L −1 or lower from a past field campaign in the high Arctic (Bigg, 1996) and taking the absence of IN measurements above the instrument detection limit during ASCOS into account, such a large increase in ice crystal number concentration seems an unlikely mechanism responsible for the observed cloud dissipation during ASCOS.…”

“…The concentration of measured IN can be uncertain due to different instrumentation and measurement techniques or due to natural variability of IN. Concentrations of IN can range as low as 0.01 to 1 L −1 , but concentrations can also reach even two to three orders of magnitude higher (Morrison et al, 2005;Rogers et al, 2001). Rogers et al (2001) found that persistent lowlevel stratus clouds contain low ice crystal concentrations, which indicates a low IN concentration in these clouds.…”

On the representation of immersion and condensation freezing in cloud models using different nucleation schemes

“…However, parcel model simulations have shown that IN concentrations of ∼ 10 l −1 < N IN < 500 l −1 lead to the immediate glaciation of clouds (Eidhammer et al, 2009;Ervens et al, 2011), which contradicts observations of persistent mixed-phase clouds. The presence of higher IN concentrations in the atmosphere might be due to the separation of the two phases by precipitation of ice particles, which would limit the influence of the Bergeron-Findeisen process, or result from other selfregulating mechanisms (e.g., Harrington et al, 1999;Morrison et al, 2005Morrison et al, , 2012. Since the parcel model does not include these additional processes, the sensitivity of IN concentration to the onset of cloud glaciation might be overestimated.…”

Relative importance of acid coating on ice nuclei in the deposition and contact modes for wintertime Arctic clouds and radiation

“…Therefore, contact freezing is likely to be altered even in the inside-out contact freezing. Morrison et al (2005) have assessed the relative importance of contact freezing and deposition ice nucleation using a 1D model. They found that contact freezing was the dominant ice nucleation mechanism in mixedphase clouds due to the rapid depletion of deposition IN.…”