Characteristic nature of vertical motions observed in Arctic mixed-phase stratocumulus

Abstract

Abstract. Over the Arctic Ocean, little is known on cloud-generated buoyant overturning vertical motions within mixed-phase stratocumulus clouds. Characteristics of such motions are important for understanding the diabatic processes associated with the vertical motions, the lifetime of the cloud layer and its micro-and macrophysical characteristics.In this study, we exploit a suite of surface-based remote sensors over the high-Arctic sea ice during a weeklong period of persistent stratocumulus in August 2008 to derive the incloud vertical motion characteristics. In-cloud vertical velocity skewness and variance profiles are found to be strikingly different from observations within lower-latitude stratocumulus, suggesting these Arctic mixed-phase clouds interact differently with the atmospheric thermodynamics (cloud tops extending above a stable temperature inversion base) and with a different coupling state between surface and cloud. We find evidence of cloud-generated vertical mixing below cloud base, regardless of surface-cloud coupling state, although a decoupled surface-cloud state occurred most frequently. Detailed case studies are examined, focusing on three levels within the cloud layer, where wavelet and power spectral analyses are applied to characterize the dominant temporal and horizontal scales associated with cloud-generated vertical motions. In general, we find a positively correlated vertical motion signal amongst vertical levels within the cloud and across the full cloud layer depth. The coherency is dependent upon other non-cloud controlled factors, such as larger, mesoscale weather passages and radiative shielding of lowlevel stratocumulus by one or more cloud layers above. Despite the coherency in vertical velocity across the cloud, the velocity variances were always weaker near cloud top, relative to cloud middle and base. Taken in combination with the skewness, variance and thermodynamic profile characteristics, we observe vertical motions near cloud top that behave differently than those from lower within the cloud layer. Spectral analysis indicates peak cloud-generated w variance timescales slowed only modestly during decoupled cases relative to coupled; horizontal wavelengths only slightly increased when transitioning from coupling to decoupling. The similarities in scales suggests that perhaps the dominant forcing for all cases is generated from the cloud layer, and it is not the surface forcing that characterizes the time-and space scales of in-cloud vertical velocity variance. This points toward the resilient nature of Arctic mixed-phase clouds to persist when characterized by thermodynamic regimes unique to the Arctic.

The free troposphere as a potential source of arctic boundary layer aerosol particles

“…Such layers are common in the high Arctic [Shupe et al, 2013;Sotiropoulou et al, 2014;Sedlar and Shupe, 2014] and one was present in the case being simulated (Figure 2a), but the simulation tended to become more coupled over time. Such layers are common in the high Arctic [Shupe et al, 2013;Sotiropoulou et al, 2014;Sedlar and Shupe, 2014] and one was present in the case being simulated (Figure 2a), but the simulation tended to become more coupled over time.…”

“…Stratocumulus clouds were present most of the time and were often found to be decoupled from the surface [Shupe et al, 2013;Sedlar and Shupe, 2014;Sotiropoulou et al, 2014]. Stratocumulus clouds were present most of the time and were often found to be decoupled from the surface [Shupe et al, 2013;Sedlar and Shupe, 2014;Sotiropoulou et al, 2014].…”

Confronting Arctic Troposphere, Clouds, and Surface Energy Budget Representations in Regional Climate Models With Observations

“…However, instantaneous errors in modeled minus observed SHF and LHF were frequently as large as the observed fluxes. These fluxes are dependent upon the surface characteristics, winds, and the lower tropospheric stratification, which is very closely linked to low‐level Arctic mixed‐phase clouds (Brooks et al, ; Persson et al, ; Sedlar et al, ; Sedlar & Shupe, ; Shupe et al, , ; Sotiropoulou et al, ; Tjernström et al, ). Known biases in modeled clouds can therefore directly influence biases in modeled turbulent heat fluxes.…”

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

“…During ASCOS, low-level mixed-phased clouds were decoupled from the surface about 75 % of the time Sedlar and Shupe, 2014;Sotiropoulou et al, 2014). The cloud layer shown in Fig.…”