The dynamics of stratocumulus: Aircraft observations and comparisons with a mixed layer model

Abstract

Results are presented from a detailed case study of daytime stratocumulus over the North Sea using an instrumented aircraft. The measurements include turbulence fluctuation data, radiation fluxes and droplet spectra and were made both in and out of cloud. The mean structure is discussed and a diagnostic, onedimensional, mixed layer model is formulated to predict the variation of turbulent fluxes with height and to assess the sensitivity of these solutions to various physical processes. These predictions are compared with observations enabling the effectiveness of some widely used model assumptions to be tested.Water transport by gravitational settling is found to be important throughout the depth of the cloud. Mixing within the cloud is driven by convection generated primarily by radiative effects: strong cooling from cloud top with warming beneath although the net heating of the cloud layer is close to zero. Turbulent kinetic energy is exported downwards via negatively buoyant elements from the zone of intense cooling near cloud top into the body of the cloud. This appears to be achieved by the combined action of the turbulent transport and velocity-pressure correlation terms in the turbulent kinetic energy balance equation which are therefore important in maintaining mixing in the lower part of the cloud.The cloud and sub-cloud layers are found to be decoupled, i.e. they appear as two separated mixed layers. Reasons for this are examined and some further consequences for cloud evolution are investigated. In the particular case studied, potential instability could be generated in the lower layer leading to low-level cumulus formation. These rise into the stratocumulus layer thereby reconnecting the two previously separated regions. The implications for stratiform cloud modelling are discussed and some recommendations for future work made. 783

Effect of Wind Speed on Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth over the North Pacific

“…In addition, decoupled layers frequently occur over the ocean [21][22][23][24][25] that create vertical gradients in sea-salt aerosol concentrations in the lower troposphere [12,14,[26][27][28][29][30]. Relatively strong vertical gradients have also been observed and modeled for coarse mode aerosols (>0.5 µm radius), and modeled for submicron aerosols, in otherwise well-mixed marine boundary layers (MBLs) without thermodynamic evidence of decoupling [12].…”

The thermodynamic structure of summer Arctic stratocumulus and the dynamic coupling to the surface

“…Rapid changes in the Arctic climate during the past decades (Serreze et al, 2000;Overland et al, 2004;ACIA, 2005) have led to widespread attention in the global climate research community. Annual average near-surface temperatures in the Arctic have increased by over a factor of 2 compared to the rest of the world (ACIA, 2005;Richter-Menge, 2010) and the sea-ice extent has been declining at an accelerating rate, especially during summer and early fall (Comiso, 2002;Nghiem et al, 2007;Stroeve et al, 2012).…”

“…These processes are discussed in Morrison et al (2012). For example, turbulence generated by cloud-top cooling and in-cloud upward air motion play a critical role; the layer with largest liquid concentrations near cloud top emits longwave radiation to space (Pinto, 1998), which decreases static stability in the clouds and leads to a buoyant overturning circulation (e.g., Nicholls, 1984). These cloud-driven turbulent motions promote the growth of both liquid and ice, rather than just ice growing at the expense of the liquid (Korolev, 2007) as would intuitively be expected in an ice/liquid mixture.…”

Evolution of non-uniformly seeded warm clouds in idealized turbulent conditions

“…[17] and the references therein) to (ii) threedimensional (3D) large eddy simulations and observations of a given cloud (see [18]- [21] and the references therein) to (iii) micro-physical modelling of droplet condensation and collisions in a turbulent cloud, see [22]- [34]. These approaches span the scales from thousands of kilometres to microns.…”