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 in-cloud 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 low-level 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.
Highlights
Clouds are the manifestation of physical processes occurring over a wide range of spatial and temporal scales
The reason for exclusion is that w estimates can only be made in volumes containing liquid droplets, and we cannot be sure of the vertical distribution of microwave radiometer (MWR)-derived liquid water path (LWP) in multiple low-level clouds – the value is column-integrated
Despite the short duration of observations, the quality, temporal resolution and physical location of this study provide a wealth of detail regarding in-cloud vertical motion characteristics in an under-studied region
Summary
Clouds are the manifestation of physical processes occurring over a wide range of spatial and temporal scales. Shupe: Characteristic nature of vertical motions the ocean surface limit their vertical extent and control their microphysical nature (e.g., Paluch and Lenschow, 1991) These clouds exhibit a strong global climate signal, wherein their shortwave climate cooling effect outweighs their warming longwave greenhouse effect (e.g., Klein and Hartmann, 1993). Direction (up- and downdrafts) and dominant time– space scales of Arctic cloud vertical motions are often limited to experimental campaigns around the pan-Arctic continents (Pinto, 1998; Shupe et al, 2008a; McFarqhuar et al, 2011), or they are estimated numerically using large eddy simulations forced by “typical” Arctic conditions (Harrington et al, 1999; Solomon et al, 2011) Such “typical” conditions can span a large range due to the wide thermodynamic, surface and meteorological conditions influencing the Arctic over the annual cycle. This study is organized as follows: Sect. 2 contains a brief introduction to the ASCOS instrumentation and methods of analysis, Sect. 3 examines the characteristic profiles of velocity and thermodynamics, Sect. 4 examines the temporal frequency of w variance and covariance at 3 elevations within the cloud, Sect. 5 examines the relationships between cloudgenerated w variability and the coupling nature between the surface and the cloud, and a discussion of results and the following conclusions are presented in Sects. 6 and 7
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