A. Khain

Notes on the state-of-the-art numerical modeling of cloud microphysics

Abstract Despite significant advances in cloud physics, many problems exist in the state-of-the-art microphysical cloud modeling. The progress is hampered by (1) many remaining gaps and uncertainties in our knowledge of cloud microphysics and (2) limitations of numerical approaches in representing some of known microphysical processes. In this paper, we attempt to give an assessment of several important problems of warm and ice microphysics and model limitations and identify areas where improvements are most urgently needed. Because of the complexity and broadness of the subject, the review does not offer an exhausted analysis of the field or provide solutions for all discussed problems. We are concerned with the spectral (bin) microphysical approach, which does not restrict the particle size spectra to any particular shape and, therefore, claims to reproduce formation of size spectra of cloud particles.

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 ...

Factors Determining the Impact of Aerosols on Surface Precipitation from Clouds: An Attempt at Classification

Abstract The simulation of the dynamics and the microphysics of clouds observed during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia—Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA–SMOCC) campaign, as well as extremely continental and extremely maritime clouds, is performed using an updated version of the Hebrew University spectral microphysics cloud model (HUCM). A new scheme of diffusional growth allows the reproduction of in situ–measured droplet size distributions including those formed in extremely polluted air. It was shown that pyroclouds forming over the forest fires can precipitate. Several mechanisms leading to formation of precipitation from pyroclouds are considered. The mechanisms by which aerosols affect the microphysics and precipitation of warm cloud-base clouds have been investigated by analyzing the mass, heat, and moisture budgets. The increase in aerosol concentration increases both the generation and the loss of the condensate mass. In the clouds developing in dry ai...

Role of atmospheric aerosol concentration on deep convective precipitation: Cloud‐resolving model simulations

[i] A two-dimensional cloud-resolving model with detailed spectral bin microphysics is used to examine the effect of aerosols on three different deep convective cloud systems that developed in different geographic locations: south Florida, Oklahoma, and the central Pacific. A pair of model simulations, one with an idealized low cloud condensation nuclei (CCN) (clean) and one with an idealized high CCN (dirty environment), is conducted for each case. In all three cases, rain reaches the ground earlier for the low-CCN case. Rain suppression is also evident in all three cases with high CCN. However, this suppression only occurs during the early stages of the simulations. During the mature stages of the simulations the effects of increasing aerosol concentration range from rain suppression in the Oklahoma case to almost no effect in the Florida case to rain enhancement in the Pacific case. The model results suggest that evaporative cooling in the lower troposphere is a key process in determining whether high CCN reduces or enhances precipitation. Stronger evaporative cooling can produce a stronger cold pool and thus stronger low-level convergence through interactions with the low-level wind shear. Consequently, precipitation processes can be more vigorous. For example, the evaporative cooling is more than two times stronger in the lower troposphere with high CCN for the Pacific case. Sensitivity tests also suggest that ice processes are crucial for suppressing precipitation in the Oklahoma case with high CCN. A comparison and review of other modeling studies are also presented.

Dominant role by vertical wind shear in regulating aerosol effects on deep convective clouds

[1] Aerosol-cloud interaction is recognized as one of the key factors influencing cloud properties and precipitation regimes across local, regional, and global scales and remains one of the largest uncertainties in understanding and projecting future climate changes. Deep convective clouds (DCCs) play a crucial role in the general circulation, energy balance, and hydrological cycle of our climate system. The complex aerosol-DCC interactions continue to be puzzling as more ‘‘aerosol effects’’ unfold, and systematic assessment of such effects is lacking. Here we systematically assess the aerosol effects on isolated DCCs based on cloud-resolving model simulations with spectral bin cloud microphysics. We find a dominant role of vertical wind shear in regulating aerosol effects on isolated DCCs, i.e., vertical wind shear qualitatively determines whether aerosols suppress or enhance convective strength. Increasing aerosols always suppresses convection under strong wind shear and invigorates convection under weak wind shear until this effect saturates at an optimal aerosol loading. We also found that the decreasing rate of convective strength is greater in the humid air than that in the dry air when wind shear is strong. Our findings may resolve some of the seemingly contradictory results among past studies by considering the dominant effect of wind shear. Our results can provide the insights to better parameterize aerosol effects on convection by adding the factor of wind shear to the entrainment term, which could reduce uncertainties associated with aerosol effects on climate forcing.

Spectral (Bin) microphysics coupled with a Mesoscale Model (MM5). Part I: Model description and first results

Abstract Considerable research investments have been made to improve the accuracy of forecasting precipitation systems in cloud-resolving, mesoscale atmospheric models. Yet, despite a significant improvement in model grid resolution and a decrease in initial condition uncertainty, the accurate prediction of precipitation amount and distribution still remains a difficult problem. Now, the development of a fast version of spectral (bin) microphysics (SBM Fast) offers significant potential for improving the description of precipitation-forming processes in mesoscale atmospheric models. The SBM Fast is based on solving a system of equations for size distribution functions for water drops and three types of ice crystals (plates, columns, and dendrites), as well as snowflakes, graupel, and hail/frozen drops. Ice processes are represented by three size distributions, instead of six in the original SBM code. The SBM uses first principles to simulate microphysical processes such as diffusional growth and collision...

Effects of in‐cloud nucleation and turbulence on droplet spectrum formation in cumulus clouds

SUMMARY Drop spectrum evolution is investigated using a moving mass grid microphysical cloud parcel model containing 2000 mass bins and allowing turbulent effects on droplet collisions. Utilization of precise methods of diffusion and collision drop growth eliminates any artie cial droplet spectrum broadening. Simulation of continental, intermediate and maritime clouds is conducted using different concentrations of cloud condensation nuclei and different vertical velocities at the cloud base. An increase of the collision kernel in turbulent surroundings is found to be an important factor in the acceleration of large droplet and raindrop formation. Droplet spectrum formation was found to be affected by three stages of in-cloud droplets’ nucleation: (a) nucleation near the cloud base, forming the primary mode of the droplet spectrum; (b) nucleation within a parcel, where supersaturation exceeds its maximum at the cloud base, this type of nucleation forming the secondary spectral mode; and (c) nucleation within the zone of intensive collisions, when a rapid decrease in drop concentration leads to an increase in supersaturation. It is shown that the secondary mode in the droplet spectrum contributes signie cantly to raindrop formation, therefore the absence of the secondary mode (the single-mode spectrum) can reduce or even inhibit formation of raindrops. The contributions of diffusion and collision growth to drop spectrum formation are compared. Effective collisions are found to start when the effective radius attains about 15 πm. The level where the effective radius attains 15 πm can be considered as the level of the e rst radar echo. This height is shown to crucially depend on cloud dynamics (in particular, on the vertical velocity at the cloud base) and on the concentration of aerosol particles.

Spectral (Bin) Microphysics Coupled with a Mesoscale Model (MM5). Part II: Simulation of a CaPE Rain Event with a Squall Line

Abstract Spectral (bin) microphysics (SBM) has been implemented into the three-dimensional fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The new model was used to simulate a squall line that developed over Florida on 27 July 1991. It is shown that SBM reproduces precipitation rate, rain amounts, and location, radar reflectivity, and cloud structure much better than bulk parameterizations currently implemented in MM5. Sensitivity tests show the importance of (i) raindrop breakup, (ii) in-cloud turbulence, (iii) different aerosol concentrations, and (iv) inclusion of scavenging of aerosols. Breakup decreases average and maximum rainfall. In-cloud turbulence enhances particle drop collision rates and increases rain rates. A “continental” aerosol concentration produces a much larger maximum rainfall rate versus that obtained with “maritime” aerosol concentration. At the same time accumulated rain is larger with maritime aerosol concentration. The scavenging of aerosols by nucleati...

Simulation of precipitation formation in the Eastern Mediterranean coastal zone using a spectral microphysics cloud ensemble model

Abstract The rain event at the end of November 1991 in the Eastern Mediterranean was simulated using a 2-D cloud ensemble model. The description of microphysical processes is based on solving kinetic equations for size distribution functions of water droplets and ice particles of six types: ice crystals (columnar, plate-like and dendrites), snowflakes, graupel and frozen drops. Each type is described by a special size distribution function containing 33 categories. Nucleation (activation) processes are described using the size distribution function for cloud condensation nuclei (33 size categories). The cloud model includes processes of ice nucleation from water vapor as well as by drop freezing. Ice processes, such as ice crystal diffusional growth, riming, aggregation, melting, etc. are included into the model based on the spectral approach. The main question we are addressing in the 2-D simulations is the impact of processes connected with ice generation on precipitation formation and distribution. The formation of ice particles does not lead to any significant dynamical changes in the location of persistent cloud generation or in the velocity fields. Moreover, the inclusion of the ice phase has little effect on the accumulated rain integrated over the whole computational area. On the other hand, ice formation was found to lead to a time delay in precipitation formation and a significant spatial redistribution of precipitation. Low density ice particles are transported inland by the background wind, so that the precipitation over the land at the distance of a few tens of kilometers from the sea shore is determined by these particles (mainly snowflakes). Because of the large size of melted raindrops, radar reflectivity turns out to be large (up to 35 dBZ) over the land under a comparably small rain rate of 1 mm/hour. In the experiment with the warm-rain microphysics only (no ice) precipitation forms faster and is concentrated mostly over the sea and within the coastal area.

Collision efficiency of drops in a wide range of Reynolds numbers : Effects of pressure on spectrum evolution

An approach is developed enabling one to calculate the collision efficiency and the collision kernel within a wide range of the Reynolds numbers (from 0 to 100) corresponding to drops up to 300-mm radii. The flow velocity field induced by falling drops is obtained by interpolation of two analytical solutions: the Stokes solution suitable for description of cloud droplets with radii below 30 mm (Re , 0.4) and the solution given by Hamielec and Johnson suitable for drops with radii ranging from 40 to 300 mm. The collision efficiency and the collision kernel are calculated at different heights of 1000, 750, and 500 mb. It is shown that both the collision efficiencies and the collision kernel significantly increase with height. This increase of the collision kernel is by 90% caused by the increase in the collision efficiency, and only by 10% is related to the increase of the swept volume. This is because of the high sensitivity of the collision efficiency to the relative drop‐drop velocity. The increase of the collision kernel with height is different for different drop pairs. It is maximal for droplets of 5‐10 mm colliding with comparably small drop collectors of 15‐25-mm radii. For these drop pairs the collision kernel at the 500-mb level is twice as large as (and even more than) that at the 1000-mb level. The collision efficiencies are calculated and presented in tables, with the high resolution required to describe sharp gradients for small droplets. The drop spectrum broadening and the rate of precipitation formation are found to be sensitive with respect to the variations of the collision rate with height. This is illustrated by solving the stochastic equation of collisions. The increase of the drop‐drop collision rate with height turned out to be significant and thus should be incorporated in numerical cloud models. The increase of the collision kernels with height for certain drop sizes can be of much importance in the context of the problem of the effect of ‘‘coalescence nuclei’’ arising on ultragiant cloud condensation nuclei, on the rain formation. This effect can also be important in rain enhancement by means of hygroscopic seeding. Possible effects of the density of colliding particles and the air density on the rate of riming are discussed.