Vaughan T. J. Phillips

Simulation of effects of atmospheric aerosols on deep turbulent convective clouds using a spectral microphysics mixed-phase cumulus cloud model. Part I: Model description and possible applications

Abstract An updated version of the spectral (bin) microphysics cloud model developed at the Hebrew University of Jerusalem [the Hebrew University Cloud Model (HUCM)] is described. The model microphysics is based on the solution of the equation system for size distribution functions of cloud hydrometeors of seven types (water drops, plate-, columnar-, and branch-like ice crystals, aggregates, graupel, and hail/frozen drops) as well as for the size distribution function of aerosol particles playing the role of cloud condensational nuclei (CCN). Each size distribution function contains 33 mass bins. The conditions allowing numerical reproduction of a narrow droplet spectrum up to the level of homogeneous freezing in deep convective clouds developed in smoky air are discussed and illustrated using as an example Rosenfeld and Woodleys case of deep Texas clouds. The effects of breakup on precipitation are illustrated by the use of a new collisional breakup scheme. Variation of the microphysical structure of a ...

An empirical parameterization of heterogeneous ice nucleation for multiple chemical species of aerosol

A novel, flexible framework is proposed for parameterizing the heterogeneous nucleation of ice within clouds. It has empirically derived dependencies on the chemistry and surface area of multiple species of ice nucleus (IN) aerosols. Effects from variability in mean size, spectral width, and mass loading of aerosols are represented via their influences on surface area. The parameterization is intended for application in largescale atmospheric and cloud models that can predict 1) the supersaturation of water vapor, which requires a representation of vertical velocity on the cloud scale, and 2) concentrations of a variety of insoluble aerosol species. Observational data constraining the parameterization are principally from coincident field studies of IN activity and insoluble aerosol in the troposphere. The continuous flow diffusion chamber (CFDC) was deployed. Aerosol species are grouped by the parameterization into three basic types: dust and metallic compounds, inorganic black carbon, and insoluble organic aerosols. Further field observations inform the partitioning of measured IN concentrations among these basic groups of aerosol. The scarcity of heterogeneous nucleation, observed at humidities well below water saturation for warm subzero temperatures, is represented. Conventional and inside-out contact nucleation by IN is treated with a constant shift of their freezing temperatures. The empirical parameterization is described and compared with available field and laboratory observations and other schemes. Alternative schemes differ by up to five orders of magnitude in their freezing fractions (30°C). New knowledge from future observational advances may be easily assimilated into the scheme’s framework. The essence of this versatile framework is the use of data concerning atmospheric IN sampled directly from the troposphere.

A New Parameterization of Cloud Droplet Activation Applicable to General Circulation Models

Abstract A new parameterization is proposed to link the droplet number concentration to the size distribution and chemical composition of aerosol and updraft velocity. Except for an empirical assumption of droplet growth, the parameterization is formulated almost entirely on first principles to allow for satisfactory performance under a variety of conditions. For a series of updraft velocity ranging from 0.03 to 10.0 m s−1, the droplet number concentrations predicted with the parameterization are in good agreement with the detailed parcel model simulations with an average error of −4 ± 26% (one standard deviation). The accuracy is comparable to or better than some existing parameterizations. The parameterization is able to account for the effects of droplet surface tension and mass accommodation coefficient on activation without adjusting the empirical parameter. These desirable attributes make the parameterization suitable for being used in the prognostic determination of the cloud droplet number concent...

Nucleation Processes in Deep Convection Simulated by a Cloud-System-Resolving Model with Double-Moment Bulk Microphysics

A novel type of limited double-moment scheme for bulk microphysics is presented here for cloud-systemresolving models (CSRMs). It predicts the average size of cloud droplets and crystals, which is important for representing the radiative impact of clouds on the climate system. In this new scheme, there are interactive components for ice nuclei (IN) and cloud condensation nuclei (CCN). For cloud ice, the processes of primary ice nucleation, Hallett–Mossop (HM) multiplication of ice particles (secondary ice production), and homogeneous freezing of aerosols and droplets provide the source of ice number. The preferential evaporation of smaller droplets during homogeneous freezing of cloud liquid is represented for the first time. Primary and secondary (i.e., in cloud) droplet nucleation are also represented, by predicting the supersaturation as a function of the vertical velocity and local properties of cloud liquid. A linearized scheme predicts the supersaturation, explicitly predicting rates of condensation and vapor deposition onto liquid (cloud liquid, rain) and ice (cloud ice, snow, graupel) species. The predicted supersaturation becomes the input for most nucleation processes, including homogeneous aerosol freezing and secondary droplet activation. Comparison of the scheme with available aircraft and satellite data is performed for two cases of deep convection over the tropical western Pacific Ocean. Sensitivity tests are performed with respect to a range of nucleation processes. The HM process of ice particle multiplication has an important impact on the domain-wide ice concentration in the lower half of the mixed-phase region, especially when a lack of upper-level cirrus suppresses homogeneous freezing. Homogeneous freezing of droplets and, especially, aerosols is found to be the key control on number and sizes of cloud particles in the simulated cloud ensemble. Preferential evaporation of smaller droplets during homogeneous freezing produces a major impact on ice concentrations aloft. Aerosols originating from the remote free troposphere become activated in deep convective updrafts and produce most of the supercooled cloud droplets that freeze homogeneously aloft. Homogeneous aerosol freezing is found to occur only in widespread regions of weak ascent while homogeneous droplet freezing is restricted to deep convective updrafts. This means that homogeneous aerosol freezing can produce many more crystals than homogeneous droplet freezing, if conditions in the upper troposphere are favorable. These competing mechanisms of homogeneous freezing determine the overall response of the ice concentration to environmental CCN concentrations in the simulated cloud ensemble. The corresponding sensitivity with respect to environmental IN concentrations is much lower. Nevertheless, when extremely high concentrations of IN are applied, that are typical for plumes of desert dust, the supercooled cloud liquid is completely eliminated in the upper half of the mixed phase region. This shuts down the process of homogeneous droplet freezing.

The dependence of aerosol effects on clouds and precipitation on cloud-system organization, shear and stability

Precipitation suppression due to an increase of aerosol number concentration in stratiform cloud is well-known. It is not certain whether the suppression applies for deep convection. Recent studies have suggested increasing precipitation from deep convection with increasing aerosols under some, but not all, conditions. Increasing precipitation with increasing aerosols can result from strong interactions in deep convection between dynamics and microphysics. High cloud liquid, due to delayed autoconversion, provides more evaporation, leading to more active downdrafts, convergence fields, condensation, collection of cloud liquid by precipitable hydrometeors, and precipitation. Evaporation of cloud liquid is a primary determinant of the intensity of the interactions. It is partly controlled by wind shear modulating the entrainment of dry air into clouds and transport of cloud liquid into unsaturated areas. Downdraft-induced convergence, crucial to the interaction, is weak for shallow clouds, generally associated with low convective available potential energy ( CAPE). Aerosol effects on cloud and precipitation can vary with CAPE and wind shear. Pairs of idealized numerical experiments for high and low aerosol cases were run for five different environmental conditions to investigate the dependence of aerosol effect on stability and wind shear. In the environment of high CAPE and strong wind shear, cumulonimbus- and cumulus-type clouds were dominant. Transport of cloud liquid to unsaturated areas was larger at high aerosol, leading to stronger downdrafts. Because of the large vertical extent of those clouds, strong downdrafts and convergence developed for strong interactions between dynamics and microphysics. These led to larger precipitation at high aerosol. Detrainment of cloud liquid and associated evaporation were less with lower CAPE and wind shear, where dynamically weaker clouds dominated. Transport of cloud liquid to unsaturated areas was not as active as in the environment of high CAPE and strong shear. Also, evaporatively driven differences in downdrafts at their level of initial descent were not magnified in clouds with shallow depth as much as in deep convective clouds as they accelerated to the surface over shorter distances. Hence the interaction between dynamics and microphysics was reduced, leading to precipitation suppression at high aerosol. These results demonstrate that increasing aerosol can either decrease or increase precipitation for an imposed large-scale environment supporting cloud development. The implications for larger-scale aspects of the hydrological cycle will require further study with larger-domain models and cumulus parameterizations with advanced microphysics. (Less)

Modeling the Interactions between Aerosols and Liquid Water Clouds with a Self-Consistent Cloud Scheme in a General Circulation Model

To model aerosol–cloud interactions in general circulation models (GCMs), a prognostic cloud scheme of cloud liquid water and amount is expanded to include droplet number concentration (Nd) in a way that allows them to be calculated using the same large-scale and convective updraft velocity field. In the scheme, the evolution of droplets fully interacts with the model meteorology. An explicit treatment of cloud condensation nuclei (CCN) activation enables the scheme to take into account the contributions to Nd of multiple aerosol species (i.e., sulfate, organic, and sea-salt aerosols) and to consider kinetic limitations of the activation process. An implementation of the prognostic scheme in the Geophysical Fluid Dynamics Laboratory (GFDL) AM2 GCM yields a vertical distribution of Nd with a characteristic maximum in the lower troposphere; this feature differs from the profile that would be obtained if Nd is diagnosed from the sulfate mass concentration based on an often-used empirical relationship. Prognosticated Nd exhibits large variations with respect to the sulfate mass concentration. The mean values are generally consistent with the empirical relationship over ocean, but show negative biases over the Northern Hemisphere midlatitude land, perhaps owing to the neglect of subgrid variations of large-scale ascents and inadequate convective sources. The prognostic scheme leads to a substantial improvement in the agreement of model-predicted present-day liquid water path (LWP) and cloud forcing with satellite measurements compared to using the empirical relationship. The simulations with preindustrial and present-day aerosols show that the combined first and second indirect effects of anthropogenic sulfate and organic aerosols give rise to a steady-state global annual mean flux change of 1.8 Wm 2 , consisting of 2.0 Wm 2 in shortwave and 0.2 W m 2 in longwave. The ratios of the flux changes in the Northern Hemisphere (NH) to that in Southern Hemisphere (SH) and of the flux changes over ocean to that over land are 2.9 and 0.73, respectively. These estimates are consistent with the averages of values from previous studies stated in a recent review. The model response to higher Nd alters the cloud field; LWP and total cloud amount increase by 19% and 0.6%, respectively. Largely owing to high sulfate concentrations from fossil fuel burning, the NH midlatitude land and oceans experience strong radiative cooling. So does the tropical land, which is dominated by biomass burning–derived organic aerosol. The computed annual, zonal-mean flux changes are determined to be statistically significant, exceeding the model’s natural variations in the NH low and midlatitudes and in the SH low latitudes. This study reaffirms the major role of sulfate in providing CCN for cloud formation.

Representation of microphysical processes in cloud-resolving models: Spectral (bin) microphysics versus bulk parameterization

Most atmospheric motions of different spatial scales and precipitation are closely related to phase transitions in clouds. The continuously increasing resolution of large-scale and mesoscale atmospheric models makes it feasible to treat the evolution of individual clouds. The explicit treatment of clouds requires the simulation of cloud microphysics. Two main approaches describing cloud microphysical properties and processes have been developed in the past four and a half decades: bulk microphysics parameterization and spectral (bin) microphysics (SBM). The development and utilization of both represent an important step forward in cloud modeling. This study presents a detailed survey of the physical basis and the applications of both bulk microphysics parameterization and SBM. The results obtained from simulations of a wide range of atmospheric phenomena, from tropical cyclones through Arctic clouds using these two approaches are compared. Advantages and disadvantages, as well as lines of future development for these methods are discussed. (Less)

Improvements to an Empirical Parameterization of Heterogeneous Ice Nucleation and Its Comparison with Observations

AbstractA framework for an empirical parameterization (EP) of heterogeneous nucleation of ice crystals by multiple species of aerosol material in clouds was proposed in a 2008 paper by the authors. The present paper reports improvements to specification of a few of its empirical parameters. These include temperatures for onset of freezing, baseline surface areas of aerosol observed in field campaigns over Colorado, and new parameters for properties of black carbon, such as surface hydrophilicity and organic coatings. The EP’s third group of ice nucleus (IN) aerosols is redefined as that of primary biological aerosol particles (PBAPs), replacing insoluble organic aerosols. A fourth group of IN is introduced—namely, soluble organic aerosols.The new EP predicts IN concentrations that agree well with aircraft data from selected traverses of shallow wave clouds observed in five flights (1, 3, 4, 6, and 12) of the 2007 Ice in Clouds Experiment–Layer Clouds (ICE-L). Selected traverses were confined to temperatur...

Boundary layer control on convective available potential energy: Implications for cumulus parameterization

[1] Convective available potential energy (CAPE), frequently regarded as an indicator of the potential intensity of deep convection, is strongly controlled by the properties of the planetary boundary layer (BL). Variations in CAPE observed during field experiments in midcontinent North America, the tropical east Atlantic, and the tropical west Pacific, can be accounted for mostly by changes in the temperature and humidity in the BL. The coupling between CAPE and the BL holds for both convective and nonconvective conditions. The coupling under conditions of deep convection implies a constraint on the intensity of deep convection which can be used as a closure for cumulus parameterization. This constraint requires equilibrium in the environment of the parcel used as a basis for calculating CAPE. Over many cases, parcel-environment equilibrium is observed to hold more robustly than equilibrium of CAPE itself. When observational uncertainties are considered, it is uncertain whether quasi-equilibrium, in which the rate of change of CAPE is substantially less than the rate at which mean advection and BL fluxes change CAPE, holds at subdiurnal timescales in the eastern Atlantic and the western Pacific. Quasi-equilibrium is a poor approximation at subdiurnal timescales in midcontinent North America. At timescales approaching diurnal, quasi-equilibrium holds in all cases. Cumulus parameterizations based on quasi-equilibrium may be limited in their ability to model diurnal cycles as a result. CAPE fluctuations related to large, subdiurnal variations in surface fluxes are much sharper than CAPE fluctuations related to changes in mean advection above the BL, especially over land. The strong BL control on CAPE indicates that deep convection does not equilibrate rapid, high-amplitude variations in CAPE originating there. (Less)

The influence of aerosol concentrations on the glaciation and precipitation of a cumulus cloud

The response of the glaciation and precipitation of U multi-thermal cumulus cloud to changes in the aerosol concentration has been assessed in a series of sensitivity tests with the UMIST Explicit Microphysics Model (EMM). A simulation of this cloud from the Met Office cloud-resolving model (CRM) has been utilized in these tests. This cumulus cloud was observed h aircraft during the initial stage of its growth over New Mexico on 10 August 1987. The growth of the simulated cloud is divided into two parts: a shallow phase followed by a deep phase. Maximum values of the cloud depth in these two phases were 5 and 9 km. respectively. In the EMM simulations including only the shallow phase, the precipitation efficiency was found to decrease substantially with increasing atmospheric concentrations of cloud condensation nuclei (CCN). Also, the graupel mixing ratio and total ice concentration ere found to decrease as normalized CCN concentration are increased above typical continental values. These changes are explicable in terms of: (I) the Hallett-Mossop (H-M) process at -3 to -8 degreesC and the freezing of supercooled raindrops in collisions with ice splinters dominating the glaciation and (2) the warm-rain process being more significant for the overall precipitation production than the ice process in these particular Simulations. The almost complete suppression of precipitation by extreme CCN concentrations corresponding to a forest-fire plume in the EMM simulation is consistent with the analysis by Rosenfeld of satellite images of Indonesian cumuli engulfed by smoke from biomass burning. A clear tendency for ice crystals to be smaller and more numerous in the anvil as found with increasing CCN concentrations beyond typical continental values in long-term simulations that included the deep phase. The sensitivity of the precipitation rate to the normalized CCN concentration was found to be relatively low in these deep cases. (Less)