Matthew C. Wyant

Moisture Transport, Lower-Tropospheric Stability, and Decoupling of Cloud-Topped Boundary Layers

Abstract Decoupling during the “Lagrangian” evolution of a cloud-topped boundary layer advected equatorward by the trade winds in an idealized eastern subtropical ocean is studied using a mixed-layer model (MLM). The sea surface temperature is gradually warmed while the free tropospheric sounding remains unchanged, causing the boundary layer to deepen, the surface relative humidity to decrease, and surface latent heat fluxes to increase. Diurnally averaged insolation is used. For entrainment closures in which entrainment rate is related to a large-eddy convective velocity scale w*, the MLM predicts an increasingly prominent layer of negative buoyancy fluxes below cloud base as the sea surface temperature warms. Decoupling of the mixed layer can be inferred when the MLM-predicted negative buoyancy fluxes become too large for the internal circulations to sustain. The authors show that decoupling is mainly driven by an increasing ratio of the surface latent heat flux to the net radiative cooling in the cloud...

Numerical Simulations and a Conceptual Model of the Stratocumulus to Trade Cumulus Transition

Abstract A two-dimensional eddy-resolving model is used to study the transition from the stratocumulus topped boundary layer to the trade cumulus boundary layer. The 10-day simulations use an idealized Lagrangian trajectory representative of summertime climatological conditions in the subtropical northeastern Pacific. The sea surface temperature is increased steadily at 1.5 K day−1, reflecting the southwestward advection of the subtropical marine boundary layer by the trade winds, while the free tropospheric temperature remains unchanged. Results from simulations with both a fixed diurnally averaged shortwave radiative forcing and a diurnally varying shortwave forcing are presented. A two-stage model for the boundary layer evolution consistent with these simulations is proposed. In the first stage, decoupling is induced by increased latent heat fluxes in the deepening boundary layer. After decoupling, cloud cover remains high, but the cloudiness regime changes from a single stratocumulus layer to sporadic...

A GCSS Boundary-Layer Cloud Model Intercomparison Study Of The First Astex Lagrangian Experiment

Three single-column models (all with an explicit liquid water budget and compara-tively high vertical resolution) and three two-dimensional eddy-resolving models (including one with bin-resolved microphysics) are compared with observations from the first ASTEX Lagrangian experiment. This intercomparison was a part of the second GCSS boundary-layer cloud modelling workshop in August 1995.In the air column tracked during the first ASTEX Lagrangian experiment, a shallow subtropical drizzling stratocumulus-capped marine boundary layer deepens after two days into a cumulus capped boundary layer with patchy stratocumulus. The models are forced with time varying boundary conditions at the sea-surface and the capping inversion to simulate the changing environment of the air column.The models all predict the observed deepening and decoupling of the boundary layer quite well, with cumulus cloud evolution and thinning of the overlying stratocumulus. Thus these models all appear capable of predicting transitions between cloud and boundary-layer types with some skill. The models also produce realistic drizzle rates, but there are substantial quantitative differences in the cloud cover and liquid water path between models. The differences between the eddy-resolving model results are nearly as large as between the single column model results. The eddy resolving models give a more detailed picture of the boundary-layer evolution than the single-column models, but are still sensitive to the choice of microphysical and radiative parameterizations, sub-grid-scale turbulence models, and probably model resolution and dimensionality. One important example of the differences seen in these parameterizations is the absorption of solar radiation in a specified cloud layer, which varied by a factor of four between the model radiation parameterizations.

Clouds, Aerosol, and Precipitation in the Marine Boundary Layer: An ARM Mobile Facility Deployment

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A single-column model intercomparison of a heavily drizzling stratocumulus-topped boundary layer

Received 12 February 2007; revised 11 July 2007; accepted 2 August 2007; published 27 December 2007. [1] This study presents an intercomparison of single-column model simulations of a nocturnal heavily drizzling marine stratocumulus-topped boundary layer. Initial conditions and forcings are based on nocturnal flight observations off the coast of California during the DYCOMS-II field experiment. Differences in turbulent and microphysical parameterizations between models were isolated by slightly idealizing and standardizing the specification of surface and radiative fluxes. For most participating models, the case was run at both typical operational vertical resolution of about 100 m and also at high vertical resolution of about 10 m. As in prior stratocumulus intercomparisons, the simulations quickly develop considerable scatter in liquid water path (LWP) between models. However, the simulated dependence of cloud base drizzle fluxes on LWP in most models is broadly consistent with recent observations. Sensitivity tests with drizzle turned off show that drizzle substantially decreases LWP for many models. The sensitivity of entrainment rate to drizzle is more muted. Simulated LWP and entrainment are also sensitive to the inclusion of cloud droplet sedimentation. Many models underestimate the fraction of drizzle that evaporates below cloud base, which may distort the simulated feedbacks of drizzle on turbulence, entrainment, and LWP.

Toward low-cloud-permitting cloud superparameterization with explicit boundary layer turbulence

Systematic biases in the representation of boundary layer (BL) clouds are a leading source of uncertainty in climate projections. A variation on superparameterization (SP) called “ultraparameterization” (UP) is developed, in which the grid spacing of the cloud-resolving models (CRMs) is fine enough (250m × 20m) to explicitly capture the BL turbulence, associated clouds and entrainment in a global climate model capable of multi-year simulations. UP is implemented within the Community Atmosphere Model using 2° resolution (∼14,000 embedded CRMs) with one-moment microphysics. By using a small domain and mean-state acceleration, UP is computationally feasible today and promising for exascale computers. Short-duration global UP hindcasts are compared with SP and satellite observations of top-of-atmosphere radiation and cloud vertical structure. The most encouraging improvement is a deeper BL and more realistic vertical structure of subtropical stratocumulus (Sc) clouds, due to stronger vertical eddy motions that promote entrainment. Results from 90-day integrations show climatological errors that are competitive with SP, with a significant improvement in the diurnal cycle of offshore Sc liquid water. Ongoing concerns with the current UP implementation include a dim bias for near-coastal Sc that also occurs less prominently in SP and a bright bias over tropical continental deep convection zones. Nevertheless, UP makes global eddy-permitting simulation a feasible and interesting alternative to conventionally parameterized GCMs or SP-GCMs with turbulence parameterizations for studying BL cloud-climate and cloud-aerosol feedback.

DNS and LES for Simulating Stratocumulus: Better Together

We argue that combining direct numerical simulation (DNS) with large-eddy simulation (LES) and field studies could accelerate current lines of stratocumulus research. LES allows for a faster and more holistic study of the parameter space, but LES is sensitive to details of its formulation because the energetics are tied to unresolved processes in the cloud top region. One way to assess this sensitivity is through field studies. Another way is through DNS. In particular, DNS can be used to test the hypothesis that LES, even with an inadequate representation of the physics of cloud top entrainment, properly quantifies the sensitivity of cloud-topped boundary layers to changing environmental conditions. We support this argument by contrasting theoretical aspects of both techniques, by presenting first DNS results of a stratocumulus-topped boundary layer and discussing their convergence toward Reynolds number similarity, and by showing the consistency of DNS results with LES results and field measurements.

The Sensitivity of Numerical Simulations of Cloud-Topped Boundary Layers to Cross-Grid Flow

File is a gzipped tar file containing 74 netcdf files and descriptive notes. The associated research paper can be found at doi:10.1002/2017MS001241