Philip H. Austin

Pockets of open cells and drizzle in marine stratocumulus

Data from recent field studies in the northeast and southeast Pacific are used to investigate pockets of open cells (POCs) that are embedded in otherwise uniform stratocumulus. The cellular structure within a POC resembles broader regions of open cellular convection typically found further offshore. In both regions, cells are composed of precipitating cell walls and cell interiors with depleted cloud water and even clearing. POCs are long lived and embedded in broader regions of stratocumulus where average droplet sizes are relatively large. In contrast, stratiform, or unbroken, cloud formations tend to be accompanied by less, or no, drizzle, suggesting that precipitation is necessary for the sustenance of the open cellular structure. Because, by definition, open cells are associated with a reduction in cloud cover these observations provide direct evidence of a connection between cloudiness and precipitation—a linchpin of hypotheses that posit a connection between changes in the atmospheric aerosol and c...

Cloudiness and Marine Boundary Layer Dynamics in the ASTEX Lagrangian Experiments. Part II: Cloudiness, Drizzle, Surface Fluxes, and Entrainment

Abstract The Analysis of the Atlantic Stratocumulus Transition Experiment (ASTEX) Lagrangians started in Part I is continued, presenting measurements of sea surface temperature, surface latent and sensible heat fluxes from bulk aerodynamic formulas, cloud fraction, and drizzle rate for the two Lagrangians, mainly using data from horizontal legs flown by the Electra and C130. Substantial drizzle, averaging 1 mm day−1 at the surface, was measured during the first Lagrangian. The surface fluxes increased rapidly as the air mass advected over rapidly increasing SST. Cloud fraction remained high throughout. During the second Lagrangian, drizzle formed in the stratocumulus layer but mainly evaporated in the deep, dry cumulus layer and the subcloud layer before reaching the surface. Stratocumulus cloud cover was thickest when moist air lay above the inversion and then it dissipated to leave only cumuli once dry air advected over the inversion. Three methods are compared for determining entrainment rate (European...

Variability of Optical Depth and Effective Radius in Marine Stratocumulus Clouds

Abstract Radiance measurements made by the Advanced Very High Resolution Radiometer (AVHRR) at 1-km (nadir) spatial resolution were used to retrieve cloud optical depth (τ) and cloud droplet effective radius (reff) for 31 marine boundary layer clouds over the eastern Pacific Ocean and the Southern Ocean near Tasmania. In the majority of these scenes (each roughly 256 × 256 km2 in extent) τ and reff are strongly correlated, with linear least squares yielding a regression curve of the form reff ∝ τ1/5. This relationship is consistent with an idealized model of a nonprecipitating layer cloud in which 1) the average cloud liquid water content increases linearly with height at some fraction of the adiabatic lapse rate in a 1 km2 vertical column, and 2) the normalized horizontal variability of the cloud liquid water path exceeds the variability of a scaled measure of the cloud droplet number concentration. In contrast, other scenes of similar horizontal extent show little or no correlation between retrieved val...

Precipitation in Stratocumulus Clouds: Observational and Modeling Results

Abstract The spatial and temporal variability of precipitating stratocumulus layers is examined using aircraft observations, satellite retrievals of cloud optical depth, and one-dimensional models that include coalescence and a simple representation of layer turbulence. The aircraft observations show large horizontal variations in cloud thickness and precipitation, with local rain rates 4–5 times larger than the replacement moisture flux, and evidence for precipitation scavenging of small cloud droplets. The satellite observations show that, despite this local water loss, the distribution of cloud optical thickness remains nearly constant over the course of a day, indicating that on larger scales precipitation removal and cloud-top entrainment are in approximate balance with the vapor flux. The authors apply analytic and numerical models of steady-state precipitation to the observed microphysical conditions, and find that the models can match the drop size distributions observed during both heavy and ligh...

Linear eddy modeling of droplet spectral evolution during entrainment and mixing in cumulus clouds

Abstract The explicit mixing parcel model (EMPM) of Krueger et al. [Krueger, S.K., Su, C.-W., McMurtry, P.A., 1997. Modeling entrainment and fine-scale mixing in cumulus clouds. J. Atmos. Sci. 54, 2697–2712.] is extended to explicitly account for individual droplet growth. The model provides a 1D representation of physical properties within a cloud parcel and incorporates the rising of the cloud parcel, entrainment of environmental air into the parcel, finite-rate turbulent mixing parameterized by Kolmogorov inertial range scalings, and droplet growth. Because the simulation is performed in a 1D domain, all relevant length scales of the mixing process can be resolved. For the typical domain size used (20 m) in this study, the growth of approximately 2000 droplets is explicitly calculated based on their local environments. Comparisons of computed droplet spectra with spectra obtained from airplane penetration exhibit good agreement. Further results showing effects of entrained CCN, droplet sedimentation and mixing rates on droplet spectral evolution are provided.

Homogeneous nucleation of supercooled water: Results from a new equation of state

A series of laboratory and aircraft measurements have indicated that supercooled liquid water exists to temperatures as low as −70°C. These measurements also show that classical nucleation theory, using standard values for the thermodynamic properties of supercooled water, underestimates the nucleation rate of ice in liquid water at large supercoolings. New theoretical estimates for this homogeneous nucleation rate are presented, based on a new analytic equation of state for liquid water. The new equation of state, which is accurate over a pressure range of 3000 atmospheres and a temperature range of 1200K, is used to infer the latent heat of melting, liquid water density, and ice-water surface energy of supercooled water. Predictions of the nucleation rate and the homogeneous freezing temperature made by this equation of state are in agreement with observations at temperatures as cold as −70°C and at pressures as high as 2000 atmospheres. These results indicate that it is not necessary to invoke a phase transition at −45°C to explain aircraft and laboratory observations of homogeneous ice nucleation in supercooled water clouds.

Life Cycle of Numerically Simulated Shallow Cumulus Clouds. Part II: Mixing Dynamics

This paper is the second in a two-part series in which life cycles of six numerically simulated shallow cumulus clouds are systematically examined. The six clouds, selected from a single realization of a largeeddy simulation, grow as a series of pulses/thermals detached from the subcloud layer. All six clouds exhibit a coherent vortical circulation and a low buoyancy, low velocity trailing wake. The ascending cloud top (ACT), which contains this vortical circulation, is associated with a dynamic perturbation pressure field with high pressure located at the ascending frontal cap and low pressure below and on the downshear side of the maximum updrafts. Examination of the thermodynamic and kinematic structure, together with passive tracer experiments, suggests that this vortical circulation is primarily responsible for mixing between cloud and environment. As the cloud ACTs rise through the sheared environment, the low pressure, vortical circulation, and mixing are all strongly enhanced on the downshear side and weakened on the upshear side. Collapse of the ACT also occurs on the downshear side, with subsequent thermals ascending on the upshear side of their predecessors. The coherent core structure is maintained throughout the ACT ascent; mixing begins to gradually dilute the ACT core only in the upper half of the cloud’s depth. The characteristic kinematic and dynamic structure of these simulated ACTs, together with their mixing behavior, corresponds closely to that of shedding thermals. These shallow simulated clouds, however, reach a maximum height of only about four ACT diameters so that ACT mixing differs from predictions of self-similar laboratory thermals.

Life Cycle of Numerically Simulated Shallow Cumulus Clouds. Part I: Transport

This paper is the first in a two-part series in which the life cycles of numerically simulated shallow cumulus clouds are systematically examined. The life cycle data for six clouds with a range of cloud-top heights are isolated from an equilibrium trade cumulus field generated by a large-eddy simulation (LES) with a uniform resolution of 25 m. A passive subcloud tracer is used to partition the cloud life cycle transport into saturated and unsaturated components; the tracer shows that on average cumulus convection occurs in a region with time-integrated volume roughly 2 to 3 times that of the liquid-water-containing volume. All six clouds exhibit qualitatively similar vertical mass flux profiles with net downward mass transport at upper levels and net upward mass flux at lower levels. This downward mass flux comes primarily from the unsaturated cloud-mixed convective region during the dissipation stage and is evaporatively driven. Unsaturated negatively buoyant cloud mixtures dominate the buoyancy and mass fluxes in the upper portion of all clouds while saturated positively buoyant cloud mixtures dominate the fluxes at lower levels. Small and large clouds have distinct vertical profiles of heating/cooling and drying/moistening, with small clouds cooling and moistening throughout their depth, while larger clouds cool and moisten at upper levels and heat and dry at lower levels. The simulation results are compared to the predictions of conceptual models commonly used in shallow cumulus parameterizations.

The Influence of the Cloud Shell on Tracer Budget Measurements of LES Cloud Entrainment

AbstractDirect measurements of rates of entrainment into and detrainment from cumulus cloud cores obtained from LES model cloud fields produce values twice as large as those produced from tracer budget calculations. This difference can be explained by two effects: the presence of a shell of air around the cloud cores that is moister than the mean environment and air at the edge of the cloud core that is drier than the mean core, and the tendency for the mean tracer values of the entrained fluid to be greater than the mean tracer values of the cloud shell. Preferential entrainment of shell air that is moving upward faster than the mean shell creates strong vertical momentum fluxes into the cumulus cloud core, thereby making the assumption that cumulus cloud cores entrain fluid with zero vertical momentum incorrect. Variability in the properties of the moist cloud shell has strong impacts on entrainment values inferred from tracer budget calculations. These results indicate that the dynamics of the cloud sh...

Interpolation of LES Cloud Surfaces for Use in Direct Calculations of Entrainment and Detrainment

Abstract Direct calculations of the entrainment and detrainment of air into and out of clouds require knowledge of the relative velocity difference between the air and the cloud surface. However, a discrete numerical model grid forces the distance moved by a cloud surface over a time step to be either zero or the width of a model grid cell. Here a method for the subgrid interpolation of a cloud surface on a discrete numerical model grid is presented. This method is used to calculate entrainment and detrainment rates for a large-eddy simulation (LES) model, which are compared with rates calculated via the direct flux method of Romps. The comparison shows good agreement between the two methods as long as the model clouds are well resolved by the model grid spacing. This limitation of this technique is offset by the ability to resolve fluxes on much finer temporal and spatial scales, making it suitable for calculating entrainment and detrainment profiles for individual clouds.