Joanna Slawinska

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.

Droplet Activation and Mixing in Large-Eddy Simulation of a Shallow Cumulus Field

AbstractThis paper presents the application of a double-moment bulk warm-rain microphysics scheme to the simulation of a field of shallow convective clouds based on Barbados Oceanographic and Meteorological Experiment (BOMEX) observations. The scheme predicts the supersaturation field and allows secondary in-cloud activation of cloud droplets above the cloud base. Pristine and polluted cloud condensation nuclei (CCN) environments, as well as opposing subgrid-scale mixing scenarios, are contrasted. Numerical simulations show that about 40% of cloud droplets originate from CCN activated above the cloud base. Significant in-cloud activation leads to the mean cloud droplet concentration that is approximately constant with height, in agreement with aircraft observations. The in-cloud activation affects the spatial distribution of the effective radius and the mean albedo of the cloud field. Differences between pristine and polluted conditions are consistent with the authors’ previous study, but the impact of th...

Optical Properties of Shallow Convective Clouds Diagnosed from a Bulk-Microphysics Large-Eddy Simulation

Large-eddy simulation (LES) models provide an indispensable tool to study processes within cloudtopped subtropical and trade wind boundary layers (e.g., Siebesma et al. 2003; Stevens et al. 2005, and references therein). Typically, LES models use bulk representation of cloud microphysics. Bulk approach assumes that warm (ice free) clouds are exactly at water saturation and that a cloud cannot exist in undersaturated conditions. It is well established from cloud observations (e.g., Stommel 1947; Warner 1955; Blyth 1993; Wang and Albrecht 1994) and cloud modeling (e.g., Brenguier and Grabowski 1993; Carpenter et al. 1998; Siebesma et al. 2003; Chosson et al. 2007) that shallow cumulus and stratocumulus clouds are strongly diluted by entrainment and that such dilution affects not only bulk thermodynamic properties [e.g., the liquid water content (LWC)] but also cloud microphysics (i.e., the spectrum of cloud droplets). For nonprecipitating clouds, conservation of total water (vapor plus liquid) and moist static energy determines bulk thermodynamic properties of cloud volumes diluted by entrainment of environmental air. Predicting changes of the cloud droplet spectra resulting from entrainment, on the other hand, requires additional constraints because situations where cloud water after homogenization is distributed over either the same number of smaller droplets (i.e., the homogeneous mixing scenario) or smaller number of droplets with the initial size (i.e., the extremely inhomogeneous mixing scenario; Baker and Latham 1979; Baker et al. 1980) are equally possible (see discussion in Andrejczuk et al. 2006). The impact of entrainment and mixing on cloud droplet spectra has been shown to significantly affect mean radiative properties of a field of stratocumulus and cumulus clouds. Chosson et al. (2004) were first to show this for the case of stratocumulus. They examined the impact of microphysical transformation following entrainment and mixing by considering separately the homogeneous and extremely inhomogeneous mixing scenarios and showed that the mean cloud optical thickness derived by applying the homogeneous scheme was about 35% larger than for the extremely inhomogeneous mixing (see Chosson et al. 2007 for further discussion). This result prompted an investigation reported in Grabowski (2006, hereafter G06), where a similar issue was investigated in the context of Corresponding author address: Wojciech W. Grabowski, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307-3000. E-mail: grabow@ncar.ucar.edu

Multiscale Interactions in an Idealized Walker Circulation: Mean Circulation and Intraseasonal Variability

AbstractA high-resolution cloud-resolving model (CRM) simulation is developed here for a two-dimensional Walker circulation over a planetary-scale domain of 40 000 km for an extended period of several hundred days. The Walker cell emerges as the time-averaged statistical steady state with a prescribed sinusoidal sea surface temperature (SST) pattern with a mean temperature of 301.15 K and a horizontal variation of 4 K. The circulation exhibits intraseasonal variability on a time scale of about 20 days with quasi-periodic intensification of the circulation and broadening of the convective regime. This variability is closely tied to synoptic-scale systems associated with expansion and contraction of the Walker circulation. An index for the low-frequency variability is developed using an empirical orthogonal function (EOF) analysis and by regressing various dynamic fields on this index. The low-frequency oscillation has four main stages: a suppressed stage with strengthened midlevel circulation, an intensifi...

Multiscale Interactions in an Idealized Walker Cell: Simulations with Sparse Space-Time Superparameterization

This paper discusses the sparse space‐time superparameterization (SSTSP) algorithm and evaluates its ability to represent interactions between moist convection and the large-scale circulation in the context of a Walker cell flow over a planetary scale two-dimensional domain. The SSTSP represents convective motions in each column of the large-scale model by embedding a cloud-resolving model, and relies on a sparse sampling in both space and time to reduce computational cost of explicit simulation of convective processes. Simulations are performed varying the spatial compression and/or temporal acceleration, and results are compared to the cloud-resolving simulation reported previously. The algorithm is able to reproduce a broad range of circulation features for all temporal accelerations and spatial compressions, but significant biases are identified. Precipitation tends to be too intense and too localized over warm waters when compared to the cloud-resolving simulations. It is argued that this is because coherent propagation of organized convective systems from one large-scale model column to another is difficult when superparameterization is used, as noted in previous studies. The Walker cell in all simulations exhibits low-frequency variability on a time scale of about 20 days, characterized by four distinctive stages: suppressed, intensification, active, and weakening. The SSTSP algorithm captures spatial structure and temporal evolution of the variability. This reinforces the confidence that SSTSP preserves fundamental interactions between convection and the large-scale flow, and offers a computationally efficient alternative to traditional convective parameterizations.

Indo-Pacific Variability on Seasonal to Multidecadal Time Scales. Part I: Intrinsic SST Modes in Models and Observations

AbstractThe variability of Indo-Pacific SST on seasonal to multidecadal time scales is investigated using a recently introduced technique called nonlinear Laplacian spectral analysis (NLSA). Through this technique, drawbacks associated with ad hoc prefiltering of the input data are avoided, enabling recovery of low-frequency and intermittent modes not previously accessible via classical approaches. Here, a multiscale hierarchy of spatiotemporal modes is identified for Indo-Pacific SST in millennial control runs of CCSM4 and GFDL CM3 and in HadISST data. On interannual time scales, a mode with spatiotemporal patterns corresponding to the fundamental component of ENSO emerges, along with ENSO-modulated annual modes consistent with combination mode theory. The ENSO combination modes also feature prominent activity in the Indian Ocean, explaining a significant fraction of the SST variance in regions associated with the Indian Ocean dipole and suggesting a deterministic relationship between these patterns. A p...

Impact of Volcanic Eruptions on Decadal to Centennial Fluctuations of Arctic Sea Ice Extent during the Last Millennium and on Initiation of the Little Ice Age

AbstractThis study evaluates different hypotheses of the origin of the Little Ice Age, focusing on the long-term response of Arctic sea ice and oceanic circulation to solar and volcanic perturbations. The authors analyze the Last Millennium Ensemble of climate model simulations carried out with the Community Earth System Model at the National Center for Atmospheric Research. The authors examine the duration and strength of volcanic perturbations, and the effects of initial and boundary conditions, such as the phase of the Atlantic multidecadal oscillation. They evaluate the impacts of these factors on decadal-to-multicentennial perturbations of the cryospheric, oceanic, and atmospheric components of the climate system. The authors show that, at least in the Last Millennium Ensemble, volcanic eruptions are followed by a decadal-scale positive response of the Atlantic multidecadal overturning circulation, followed by a centennial-scale enhancement of the Northern Hemispheric sea ice extent. It is hypothesiz...

Northern Hemisphere winter warming and summer monsoon reduction after volcanic eruptions over the last millennium: Winter Warming in the Last Millennium

Observations show that all recent large tropical volcanic eruptions (1850-present) were followed by surface winter warming in the first Northern Hemisphere (NH) winter after the eruption. Recent studies show that climate models produce a surface winter warming response in the first winter after the largest eruptions, but require a large ensemble of simulations to see significant changes. It is also generally required that the eruption be very large, and only two such eruptions occurred in the historical period: Krakatau in 1883 and Pinatubo in 1991. Here we examine surface winter warming patterns after the 10 largest volcanic eruptions between 850 and 1850 in the Paleoclimate Modeling Intercomparison Project 3 last millennium simulations and in the Community Earth System Model Last Millennium Ensemble. These eruptions were all larger than those since 1850. Though the results depend on both the individual models and the forcing data set used, we have found that models produce a surface winter warming signal in the first winter after large volcanic eruptions, with higher temperatures over NH continents and a stronger polar vortex in the lower stratosphere. We also examined NH summer precipitation responses in the first year after the eruptions, and find clear reductions of summer Asian and African monsoon rainfall.

Multiscale Interactions in an Idealized Walker Cell: Analysis with Isentropic Streamfunctions

AbstractA new approach for analyzing multiscale properties of the atmospheric flow is proposed in this study. For that, the recently introduced isentropic streamfunctions are employed here for scale decomposition with Haar wavelets. This method is applied subsequently to a cloud-resolving simulation of a planetary Walker cell characterized by pronounced multiscale flow. The resulting set of isentropic streamfunctions—obtained at the convective, meso-, synoptic, and planetary scales—capture many important features of the across-scale interactions within an idealized Walker circulation. The convective scale is associated with the shallow, congestus, and deep clouds, which jointly dominate the upward mass flux in the lower troposphere. The synoptic and planetary scales play important roles in extending mass transport to the upper troposphere, where the corresponding streamfunctions mainly capture the first baroclinic mode associated with large-scale overturning circulation. The intermediate-scale features of...