Vitaly I. Khvorostyanov

A New Double-Moment Microphysics Parameterization for Application in Cloud and Climate Models. Part I: Description

A new double-moment bulk microphysics scheme predicting the number concentrations and mixing ratios of four hydrometeor species (droplets, cloud ice, rain, snow) is described. New physically based parameterizations are developed for simulating homogeneous and heterogeneous ice nucleation, droplet activation, and the spectral index (width) of the droplet size spectra. Two versions of the scheme are described: one for application in high-resolution cloud models and the other for simulating grid-scale cloudiness in larger-scale models. The versions differ in their treatment of the supersaturation field and droplet nucleation. For the high-resolution approach, droplet nucleation is calculated from Kohler theory applied to a distribution of aerosol that activates at a given supersaturation. The resolved supersaturation field and condensation/deposition rates are predicted using a semianalytic approximation to the three-phase (vapor, ice, liquid) supersaturation equation. For the large-scale version of the scheme, it is assumed that the supersaturation field is not resolved and thus droplet activation is parameterized as a function of the vertical velocity and diabatic cooling rate. The vertical velocity includes a subgrid component that is parameterized in terms of the eddy diffusivity and mixing length. Droplet condensation is calculated using a quasi-steady, saturation adjustment approach. Evaporation/deposition onto the other water species is given by nonsteady vapor diffusion allowing excess vapor density relative to ice saturation.

The Theory of Ice Nucleation by Heterogeneous Freezing of Deliquescent Mixed CCN. Part I: Critical Radius, Energy, and Nucleation Rate

Abstract This paper extends previous work on the theory of heterogenous ice nucleation. The goals of this analysis are to explain empirical observations of ice nucleation and to provide a suitable framework for modeling and parameterizing the ice nucleation process in cloud-scale and large-scale atmospheric models. Considered are the processes of heterogeneous freezing of deliquescent mixed cloud condensation nuclei that may serve as ice nuclei, and the properties of an ice germ critical radius, energy, and nucleation rate of ice crystals are examined as functions of temperature and supersaturation. Expressions for nucleation in a polydisperse aerosol for the deliquescence-freezing mode are developed. Equations are derived for the threshold and critical saturation ratios as functions of temperature and nucleation rate, and for the threshold and critical temperatures as functions of saturation ratio. Equivalence of the new formulation for the freezing point depression with traditional expressions is shown ...

Terminal velocities of droplets and crystals: Power laws with continuous parameters over the size spectrum

Abstract This paper presents a unified treatment of cloud particle fall velocities for both liquid and crystalline cloud particles over the entire size range observed in the atmosphere. The fall velocity representation is formulated in terms of the Best (or Davies) number X, and the Reynolds number Re. For the power-law representations used in many applications, the coefficients are found as the continuous analytical functions of X (or diameter) over the entire hydrometeor size range. Analytical asymptotic solutions are obtained for these coefficients for the two regimes that represent large and small particles and correspond to potential and aerodynamical flows, respectively. The new formulation is compared with experimental data and previous formulations for small drops, large nonspherical drops, and various ice crystal habits. For ice crystals, published mass–dimension and area–dimension relationships are used. The advantage of the new representation of fall velocities over previous representations is ...

Cirrus Cloud Simulation Using Explicit Microphysics and Radiation. Part I: Model Description

Abstract A mesoscale 2D/3D cloud model complex with explicit account for the water and ice cloud microphysics and radiative processes is described. The model has several versions suitable for the simulation of various cloud types, including, of particular concern in the current version, cirrus clouds. Model computations are based on the two kinetic equations for droplet and crystal size distribution functions, with division of droplet and crystal size spectra into 30 bins from 1 μm to 3.5 mm, and with account for the various mechanisms of cloud condensation and ice nuclei activation, condensation–deposition, and coalescence–accretion growth. These equations are solved along with supersaturation and radiative transfer equations for longwave and solar radiation. Simple yet accurate analytical expressions are presented for the scattering and absorption coefficients, and the single-scattering albedo. This allows detailed calculations of the optical and radiative characteristics of clouds (i.e., fluxes, diverg...

Cirrus Cloud Simulation Using Explicit Microphysics and Radiation. Part II: Microphysics, Vapor and Ice Mass Budgets, and Optical and Radiative Properties

Abstract The 2D/3D cloud model complex with explicit microphysics and radiation described in Part I is used to simulate the development of a midlatitude cirrus cloud, including interaction with radiation. To account for the effects of the interaction of various scales of motion on cloud development, a synoptic-scale vertical velocity field is superimposed on the mesoscale velocity field generated by the model, mimicking the effects of an upper-level shortwave trough. The main results under the conditions simulated here are the following. Cirrus cloud growth is much slower than assumed previously, because the process of vapor deposition to ice crystals is far from instantaneous: the crystal phase relaxation time (i.e., the characteristic time of vapor absorption by crystals) takes 0.5–2.0 h. Even after 1 h of cloud development, supersaturation with respect to ice can remain 5%–10%, while the condensed ice is only 40%–60% of the amount that would be realized assuming that all excess vapor is transformed int...

A new theory of heterogeneous ice nucleation for application in cloud and climate models

A new formulation is presented of the thermodynamical theory of heterogeneous ice crystal nucleation in clouds by freezing. This theory unifies and explains the empirical ice nuclei dependence on temperature and supersaturation, predicts crystal formation via condensation-freezing at a subsaturation over water. The theory also explains observations of high nucleation rates and crystal concentrations at warm (-5 > -12 °C) temperatures when the splintering mechanism may be not effective. This theory can be applied to parameterizations for use in cloud and climate models.

Cirrus Cloud Ice Water Content Radar Algorithm Evaluation Using an Explicit Cloud Microphysical Model

Abstract A series of cirrus cloud simulations performed using a model with explicit cloud microphysics is applied to testing ice water content retrieval algorithms based on millimeter-wave radar reflectivity measurements. The simulated ice particle size spectra over a 12-h growth/dissipation life cycle are converted to equivalent radar reflectivity factors Ze and visible optical extinction coefficients σ, which are used as a test dataset to intercompare the results of various algorithms. This approach shows that radar Ze-only approaches suffer from significant problems related to basic temperature-dependent cirrus cloud processes, although most algorithms work well under limited conditions (presumably similar to those of the empirical datasets from which each was derived). However, when lidar or radiometric measurements of σ or cloud optical depth are used to constrain the radar data, excellent agreement with the modeled contents can be achieved under the conditions simulated. Implications for the satelli...

The Theory of Ice Nucleation by Heterogeneous Freezing of Deliquescent Mixed CCN. Part II: Parcel Model Simulation

Abstract The new theory of ice nucleation by heterogeneous freezing of deliquescent mixed cloud condensation nuclei (CCN) presented in Part I is incorporated into a parcel model with explicit water and ice bin microphysics to simulate the process of ice nucleation under transient thermodynamic conditions. Simulations are conducted over the temperature range −4° to −60°C, with vertical velocities varying from 1 to 100 cm s−1, for varying initial relative humidities and aerosol characteristics. These simulations show that the same CCN that are responsible for the drop nucleation may initiate crystal nucleation and can be identified as ice nuclei (IN) when crystals form. The simulated nucleation rates and concentrations of nucleated crystals depend on temperature and supersaturation simultaneously, showing good agreement with observations but with noticeable differences when compared with classical temperature-only and supersaturation-only parameterizations. The kinetics of heterogeneous ice nucleation exhib...

Toward the Theory of Stochastic Condensation in Clouds. Part I: A General Kinetic Equation

Abstract In order to understand the mechanisms of formation of broad size spectra of cloud droplets and to develop a basis for the parameterization of cloud microphysical and optical properties, the authors derive a general kinetic equation of stochastic condensation that is applicable for various relationships between the supersaturation relaxation time τf and the timescale of turbulence τL. Supersaturation is considered as a nonconservative variable, and thus additional covariances and a turbulent diffusion coefficient tensor that is dependent on the supersaturation relaxation time, kij(τf), are introduced into the kinetic equation. This equation can be used in cloud models with explicit microphysics or can serve as a basis for development of parameterizations for bulk cloud models and general circulation models.

Cloud effects from boreal forest fire smoke: evidence for ice nucleation from polarization lidar data and cloud model simulations

Polarization lidar observations from the interior of Alaska have revealed unusual supercooled altocumulus cloud conditions in the presence of boreal forest fire smoke from local and regional fires. At temperatures of about −15 °C, the lidar data show ice nucleation prior to liquid cloud formation (i.e. below water saturation), as well as the occasional glaciation of the liquid layer. Thus the smoke aerosol appears to act as ice nuclei that become activated in updrafts before the liquid cloud forms, as the concentrated aqueous organic solutions are diluted sufficiently to allow them to freeze heterogeneously. This haze particle freezing process is similar to the production of cirrus ice crystals homogeneously at much colder temperatures. To test this hypothesis, cloud microphysical model simulations constrained by the measurements were performed. They indicate that this heterogeneous ice nucleation scenario can be supported by the cloud model. Although ice formation in this manner may generally act in the atmosphere, the boreal smoke particles produce an unusually dramatic effect in the lidar data. We conclude that smoke-induced ice nucleation occurs at moderate supercooled temperatures either through the effects of raised soil/dust particles embedded in the smoke droplets, coated soot aerosol or through the nucleation via certain organic solutions.