Anning Cheng

Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO

Twelve large-eddy simulations, with a wide range of microphysical representations, are compared to each other and to independent measurements. The measurements and the initial and forcing data for the simulations are taken from the undisturbed period of the Rain in Cumulus over the Ocean (RICO) field study. A regional downscaling of meteorological analyses is performed so as to provide forcing data consistent with the measurements. The ensemble average of the simulations plausibly reproduces many features of the observed clouds, including the vertical structure of cloud fraction, profiles of cloud and rain water, and to a lesser degree the population density of rain drops. The simulations do show considerable departures from one another in the representation of the cloud microphysical structure and the ensuant surface precipitation rates, increasingly so for the more simplified microphysical models. There is a robust tendency for simulations that develop rain to produce a shallower, somewhat more stable cloud layer. Relations between cloud cover and precipitation are ambiguous.

CGILS: Results from the First Phase of an International Project to Understand the Physical Mechanisms of Low Cloud Feedbacks in Single Column Models

CGILS—the CFMIP-GASS Intercomparison of Large Eddy Models (LESs) and single column models (SCMs)—investigates the mechanisms of cloud feedback in SCMs and LESs under idealized climate change perturbation. This paper describes the CGILS results from 15 SCMs and 8 LES models. Three cloud regimes over the subtropical oceans are studied: shallow cumulus, cumulus under stratocumulus, and well-mixed coastal stratus/stratocumulus. In the stratocumulus and coastal stratus regimes, SCMs without activated shallow convection generally simulated negative cloud feedbacks, while models with active shallow convection generally simulated positive cloud feedbacks. In the shallow cumulus alone regime, this relationship is less clear, likely due to the changes in cloud depth, lateral mixing, and precipitation or a combination of them. The majority of LES models simulated negative cloud feedback in the well-mixed coastal stratus/stratocumulus regime, and positive feedback in the shallow cumulus and stratocumulus regime. A general framework is provided to interpret SCM results: in a warmer climate, the moistening rate of the cloudy layer associated with the surface-based turbulence parameterization is enhanced; together with weaker large-scale subsidence, it causes negative cloud feedback. In contrast, in the warmer climate, the drying rate associated with the shallow convection scheme is enhanced. This causes positive cloud feedback. These mechanisms are summarized as the “NESTS” negative cloud feedback and the “SCOPE” positive cloud feedback (Negative feedback from Surface Turbulence under weaker Subsidence—Shallow Convection PositivE feedback) with the net cloud feedback depending on how the two opposing effects counteract each other. The LES results are consistent with these interpretations.

Intercomparison and interpretation of single-column model simulations of a nocturnal stratocumulus-topped marine boundary layer

Ten single-column models (SCMs) from eight groups are used to simulate a nocturnal nonprecipitating marine stratocumulus-topped mixed layer as part of an intercomparison organized by the Global Energy and Water Cycle Experiment Cloud System Study, Working Group 1. The case is idealized from observations from the Dynamics and Chemistry of Marine Stratocumulus II, Research Flight 1. SCM simulations with operational resolution are supplemented by high-resolution simulations and compared with observations and large-eddy simulations. All participating SCMs are able to maintain a sharp inversion and a mixed cloud-topped layer, although the moisture profiles show a slight gradient in the mixed layer and produce entrainment rates broadly consistent with observations, but the liquid water paths vary by a factor of 10 after onl y1ho fsimulation at both high and operational resolution. Sensitivity tests show insensitivity to activation of precipitation and shallow convection schemes in most models, as one would observationally expect for this case.

Cloud-Resolving Simulation of Low-Cloud Feedback to an Increase in Sea Surface Temperature

This study investigates the physical mechanisms of the low cloud feedback through cloud-resolving simulations of cloud-radiative equilibrium response to an increase in sea surface temperature (SST). Six pairs of perturbed and control simulations are performed to represent different regimes of low clouds in the subtropical region by specifying SST differences (DSST) in the range of 4 and 14 K between the warm tropical and cool subtropical regions. The SST is uniformly increased by 2 K in the perturbed set of simulations. Equilibrium states are characterized by cumulus and stratocumulus cloud regimes with variable thicknesses and vertical extents for the range of specified DSSTs, with the perturbed set of simulations having higher cloud bases and tops and larger geometric thicknesses. The cloud feedback effect is negative for this DSST range (20.68 to 25.22 W m 22 K 21 ) while the clear-sky feedback effect is mostly negative (21.45 to 0.35 W m 22 K 21 ). The clear-sky feedback effect contributes greatly to the climate sensitivity parameter for the cumulus cloud regime whereas the cloud feedback effect dominates for the stratocumulus regime. The increase of liquid water path (LWP) and cloud optical depth is related to the increase of cloud thickness and liquid water content with SST. The rates of change in surface latent heat flux are much higher than those of saturation water vapor pressure in the cumulus simulations. The increase in surface latent heat flux is the primary mechanism for the large change of cloud physical properties with 12 K SST, which leads to the negative cloud feedback effects. The changes in cloud fraction also contribute to the negative cloud feedback effects in the cumulus regime. Comparison of these results with prior modeling studies is also discussed.

The Liquid Water Oscillation in Modeling Boundary Layer Cumuli with Third-Order Turbulence Closure Models

A hierarchy of third-order turbulence closure models are used to simulate boundary layer cumuli in this study. An unrealistically strong liquid water oscillation (LWO) is found in the fully prognostic model, which predicts all third moments. The LWO propagates from cloud base to cloud top with a speed of 1 m s21. The period of the oscillation is about 1000 s. Liquid water buoyancy (LWB) terms in the third-moment equations contribute to the LWO. The LWO mainly affects the vertical profiles of cloud fraction, mean liquid water mixing ratio, and the fluxes of liquid water potential temperature and total water, but has less impact on the vertical profiles of other second and third moments. In order to minimize the LWO, a moderately large diffusion coefficient and a large turbulent dissipation at its originating level are needed. However, this approach distorts the vertical distributions of cloud fraction and liquid water mixing ratio. A better approach is to parameterize LWB more reasonably. A minimally prognostic model, which diagnoses all third moments except for the vertical velocity, is shown to produce better results, compared to a fully prognostic model.

Changes in clouds and atmospheric circulation associated with rapid adjustment induced by increased atmospheric CO2: a multiscale modeling framework study

The radiative heating increase due to increased CO2 concentration is the primary source for the rapid adjustment of atmospheric circulation and clouds. In this study, we investigate the rapid adjustment resulting from an instantaneous doubling of CO2 and its physical mechanism using a multiscale modeling framework (MMF). The cloud-resolving model component of this MMF includes a sophisticated third-order turbulence closure and the MMF simulates realistic shallow and deep cloud climatology and boundary layer turbulence. Although the simulated cloud adjustment and its mechanism generally agree with earlier studies with conventional global climate models and another MMF with a lower-order turbulence closure, this MMF simulates an increase in the global-mean shortwave and net cloud radiative cooling and a negative cloud radiative effect change due to cloud adjustment. This result is related to the large increase in low-level clouds over the extratropical and subtropical oceans, resulting from reduced cloud-top entrainment implied from strengthened inversion. The downshift of planetary boundary layer and low-level clouds is generally weaker than that simulated by other models, which is due to reduction of shallow cumulus in the ascending and weak subsidence circulation regimes but to increase of stratocumulus in the strongest subsidence regime. Optically thicker stratocumulus compensates for reduced cooling by shallow cumulus. The reduced strength of all oceanic circulation regimes, which may be contributed by weakened energy transport resulting from water vapor and cloud CO2 masking effects, not only reduces optical depth of convective clouds but also shifts cloud coverage to lands where deep convection is enhanced.

Single‐Column Model Simulations of Subtropical Marine Boundary‐Layer Cloud Transitions Under Weakening Inversions

Results are presented of the GASS/EUCLIPSE single-column model inter-comparison study on the subtropical marine low-level cloud transition. A central goal is to establish the performance of state-of-the-art boundary-layer schemes for weather and climate models for this cloud regime, using large-eddy simulations of the same scenes as a reference. A novelty is that the comparison covers four different cases instead of one, in order to broaden the covered parameter space. Three cases are situated in the North-Eastern Pacific, while one reflects conditions in the North-Eastern Atlantic. A set of variables is considered that reflects key aspects of the transition process, making use of simple metrics to establish the model performance. Using this method some longstanding problems in low level cloud representation are identified. Considerable spread exists among models concerning the cloud amount, its vertical structure and the associated impact on radiative transfer. The sign and amplitude of these biases differ somewhat per case, depending on how far the transition has progressed. After cloud breakup the ensemble median exhibits the well-known “too few too bright” problem. The boundary layer deepening rate and its state of decoupling are both underestimated, while the representation of the thin capping cloud layer appears complicated by a lack of vertical resolution. Encouragingly, some models are successful in representing the full set of variables, in particular the vertical structure and diurnal cycle of the cloud layer in transition. An intriguing result is that the median of the model ensemble performs best, inspiring a new approach in subgrid parameterization.

The Response of Simulated Arctic Mixed‐Phase Stratocumulus to Sea Ice Cover Variability in the Absence of Large‐Scale Advection

This study examines the responses of Arctic Mixed-Phase Stratocumulus (AMPS) boundary layer to sea ice cover variability near the sea ice margins using large eddy simulations. The simulations are conducted for two different atmospheric conditions, based on observations from the Surface Heat Budget of the Arctic Ocean Experiment (SHEBA) (100% sea ice-covered) and the Mixed-Phase Arctic Cloud Experiment (M-PACE) (open ocean). The effect of sea ice cover variability is investigated for both atmospheric conditions by conducting a series of simulations prescribed with varying amounts of sea ice cover and no large-scale advection. As sea ice cover amount decreases, the SHEBA boundary layer deepens and becomes decoupled. The relative strength of turbulence driven by surface heating to that driven by cloud top radiative cooling increases. Cloud ice and snow grow more efficiently than cloud liquid with moisture transported from the lower boundary layer. On the other hand, as sea ice cover amount increases, the M-PACE boundary layer becomes shallower and more coupled with the surface as turbulence mainly driven by cloud top radiative cooling. Moisture supply from the surface is reduced while cloud droplets are generated from turbulence at cloud top with little ice formation. In both atmospheric conditions, the boundary layer turbulence structure is modified according to change in the relative strength of boundary-layer turbulent sources as sea ice amount changes, resulting in the growth/decay of the cloud layer. Simulations with smaller sea ice cover amounts are associated with more cloud ice but not necessarily more cloud liquid.

Differences in the hydrological cycle and sensitivity between multiscale modeling frameworks with and without a higher-order turbulence closure: MMF HYDROLOGICAL SENSITIVITY

Xu was supported by the NASA Interdisciplinary Study program (Grant NNH12ZDA001N-IDS). The computational resources were provided by Argonne National Laboratory, DOE’s Office of Science and the local computation clusters: K-cluster and Icluster. Li acknowledges the support of NASA Postdoctoral Program. Blossey and Stan acknowledge support from the NSF Science and Technology Center for Multi-Scale Modeling of Atmospheric Processes (CMMAP), led by David Randall and managed by Colorado State University under cooperative agreement No. ATM-0425247.

Final Report to the U.S. Department of Energy for studies of Evaluation of Turbulence Parameterizations for Cloud-Resolving Models

The intermediately-prognostic higher-order turbulence closure (IPHOC) introduces a joint double-Gaussian distribution of liquid water potential temperature (θl ), total water mixing ratio (qt ), and vertical velocity (w ) to represent any skewed turbulence circulations .The distribution is inferred from the first-, second-, and third-order moments of the variables given above, and is used to diagnose cloud fraction and grid-mean liquid water mixing ratio, as well as the buoyancy and fourth-order terms in the equations describing the evolution of the second- and third-order moments. Only three third-order moments (those of θl , qt , and w ) are predicted in the IPHOC.