Effects of free‐tropospheric thermodynamic properties and large‐scale subsidence on the nocturnal growth of marine stratocumulus clouds are investigated through large‐eddy simulation based on Flight 5 of the Physics of Stratocumulus Top research campaign. It was characterized by a weak inversion and moist troposphere. Sensitivity simulations are performed using variations in the subsidence and free‐tropospheric temperature and humidity. The results confirm that the cloud‐top entrainment instability parameter κ alone cannot unambiguously control the behaviors of marine stratocumulus due to the opposite response of liquid water path (LWP) to changes in the humidity and temperature jumps with the same variation in κ , thereby both jumps should be considered separately. However, sometimes it could be considered as a joint factor of the humidity and temperature jumps controlling LWP even though cloud‐top entrainment instability does not occur or κ < 0. An obvious role of subsidence is to push the inversion layer down nearly identically and at the same rate under different ambient conditions. Enhanced subsidence, as expected, diminishes the entrainment rate, cloud‐top height, cloud thickness, and LWP. However, it has a small influence on the inversion thickness and the dynamic instability across the inversion. An LWP budget analysis shows that the direct effect of subsidence is a small reduction in the LWP, but the indirect effect of subsidence is large due to contributions from other physical processes. Therefore, subsidence is an important physical process controlling the LWP budget, even though its direct contribution is significantly smaller than the largest contributions from both radiation and entrainment.

The effects of wind shear and radiative cooling on the stratocumulus‐topped boundary layer (STBL) were investigated via a set of large‐eddy simulations. The set‐up of the numerical experiments was based on Flight TO13 from the Physics of Stratocumulus Top (POST) field campaign, in which sensible and latent heat fluxes at the surface were small and thermodynamic conditions prevented cloud‐top entrainment instability.The results demonstrate that the presence of radiative cooling invigorated convective circulations across the STBL and sharpened the inversion above the cloud, while wind shear at the top of the STBL was a source of turbulence in the capping inversion and caused dilution of the cloud top. The flux and gradient Richardson numbers in the capping inversion and in the topmost layer of the cloud were nearly critical.Analysis of the turbulent kinetic energy (TKE) budget and its transport indicated that turbulence in the inversion capping the cloud was produced locally by wind shear and was dynamically decoupled from the turbulence driven by convective circulations across the STBL. Similar conclusions were derived for the topmost part of the cloud.

Turbulence observed during the Physics of Stratocumulus Top (POST) research campaign is analyzed. Using in-flight measurements of dynamic and thermodynamic variables at the interface between the stratocumulus cloud top and free troposphere, the cloud top region is classified into sublayers, and the thicknesses of these sublayers are estimated. The data are used to calculate turbulence characteristics, including the bulk Richardson number, mean-square velocity fluctuations, turbulence kinetic energy (TKE), TKE dissipation rate, and Corrsin, Ozmidov and Kolmogorov scales. A comparison of these properties among different sublayers indicates that the entrainment interfacial layer consists of two significantly different sublayers: the turbulent inversion sublayer (TISL) and the moist, yet hydrostatically stable, cloud top mixing sublayer (CTMSL). Both sublayers are marginally turbulent, i.e., the bulk Richardson number across the layers is critical. This means that turbulence is produced by shear and damped by buoyancy such that the sublayer thicknesses adapt to temperature and wind variations across them. Turbulence in both sublayers is anisotropic, with Corrsin and Ozmidov scales as small as ∼ 0.3 and ∼ 3 m in the TISL and CTMSL, respectively. These values are ∼ 60 and ∼ 15 times smaller than typical layer depths, indicating flattened large eddies and suggesting no direct mixing of cloud top and free-tropospheric air. Also, small scales of turbulence are different in sublayers as indicated by the corresponding values of Kolmogorov scales and buoyant and shear Reynolds numbers.

A range of large-eddy simulations of the stratocumulus-topped boundary layer is performed to study the influence of grid resolution on selected parameters including liquid water path and second and third moments of vertical velocity fluctuations. The simulations are based on two sets of aircraft measurements, which are also used to evaluate the results of the simulations. The specific case presented here indicates that simulations with a grid aspect ratio accounting for the anisotropic nature of the turbulence near the surface and at the top of the boundary layer lead to better agreement with measurements than simulations with an isotropic grid. SIMULATIONS Stratocumulus clouds cover approximately one fifth of the Earth's surface in the annual mean (7). Thus, they play an important role in the radiative balance of the Earth and understanding of this type of cloud is essential for e.g. accurate climate prediction. Large-eddy simulation (LES) is a widely used tool for stratocumulus studies, but how to best account for the influence of unresolved turbulence is still an open question. In the model intercomparison study of (6), simulations with no explicit subgrid-scale (SGS) term in the equations for the scalar variables showed - at least in some respects - better agreement with measurements than simulations employing an explicit SGS model. Here we continue along the lines of (6) and perform a range of large-eddy simulations (ILESs) of the stratocumulus-topped boundary layer (STBL) to study the influence of grid resolution and aspect ratio on the temporal evolution and spatial structure of the simulated STBL. The performed simulations are based on two sets of aircraft measurements: research flight 1 of the second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study (5) and research flight 10 of the Physics of Stratocumulus Top (POST) experiment (1). The measurements provide realistic initial conditions and forcing applied in the simulations, as well as means of evaluating the quality of the results. We use an ILES code based on the EULAG model (3). It solves the governing anelastic equations for liquid water and water vapour mixing ratios, momentum and potential temperature using finite differences and the MPDATA (Multidimen- sional Positive Definite Advection Transport Algorithm) advection scheme (4). The longwave radiation scheme applied in the simulations is similar to the one used in (6). As described by e.g. (2), the MPDATA scheme has the ability to implicitly account for the effect of the unresolved turbulence on the resolved flow through the truncation terms associated with the scheme. Thereby the need for an explicit subgrid-scale model is removed and hence the name implicit LES. RESULTS

A modified UFT-M version of the ultrafast airborne thermometer UFT, aimed at in-cloud temperature measurements, was designed for the Physics of Stratocumulus Top (POST) field campaign. Improvements in its construction resulted in the sensor's increased reliability, which provided valuable measurements in 15 of the 17 flights. Oversampling the data allowed for the effective correction of the artefacts resulting from the interference with electromagnetic transmissions from on-board avionic systems and the thermal noise resulting from the sensor construction. The UFT-M records, when averaged to the 1.4 and 55 m resolutions, compared to the similar records of a thermometer in a Rosemount housing, indicate that the housing distorts even low-resolution airborne temperature measurements. Data collected with the UFT-M during the course of POST characterise the thermal structure of stratocumulus and capping inversion with the maximum resolution of ~1 cm. In this paper, examples of UFT-M records are presented and discussed.

High spatial resolution measurements of temperature and liquid water content, accompanied by moderate-resolution measurements of humidity and turbulence, collected during the Physics of Stratocumulus Top experiment are analyzed. Two thermodynamically, meteorologically and even optically different cases are investigated. An algorithmic division of the cloud-top region into layers is proposed. Analysis of dynamic stability across these layers leads to the conclusion that the inversion capping the cloud and the cloud-top region is turbulent due to the wind shear, which is strong enough to overcome the high static stability of the inversion. The thickness of this mixing layer adapts to wind and temperature jumps such that the gradient Richardson number stays close to its critical value. Turbulent mixing governs transport across the inversion, but the consequences of this mixing depend on the thermodynamic properties of cloud top and free troposphere. The effects of buoyancy sorting of the mixed parcels in the cloud-top region are different in conditions that permit or prevent cloud-top entrainment instability. Removal of negatively buoyant air from the cloud top is observed in the first case, while buildup of the diluted cloud-top layer is observed in the second one.

High resolution measurements of temperature and cloud water, collected during Physics of Stratocumulus Top experiment are investigated. Two case studies presented here illustrate differences between "classical" stratocumulus capped with a sharp inversion and dry layer above and one type of "nonclassical" stratocumulus with weak inversion with moist air above. Entrainment and tranport into the cloud deck are investigated by means of statistical analysis of LWC and temperature fluctuations. It comes, that in "classical" case downdrafts with depleted water content are characterized with reduced temperature (effect of evaporative cooling, presumably negative buoyancy), while in this "non classical" case such downdrafts are of increased temperature, suggesting that in this case evaporative cooling is not a driving mechannism of downward transport.