A. Pokrovsky

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...

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...

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...

Possible Aerosol Effects on Lightning Activity and Structure of Hurricanes

According to observations of hurricanes located relatively close to the land, intense and persistent lightning takes place within a 250–300-km radius ring around the hurricane center, whereas the lightning activity in the eyewall takes place only during comparatively short periods usually attributed to eyewall replacement. The mechanism responsible for the formation of the maximum flash density at the tropical cyclone (TC) periphery is not well understood as yet. In this study it is hypothesized that lightning at the TC periphery arises under the influence of small continental aerosol particles (APs), which affect the microphysics and the dynamics of clouds at the TC periphery. To show that aerosols change the cloud microstructure and the dynamics to foster lightning formation, the authors use a 2D mixed-phase cloud model with spectral microphysics. It is shown that aerosols that penetrate the cloud base of maritime clouds dramatically increase the amount of supercooled water, as well as the ice contents and vertical velocities. As a result, in clouds developing in the air with high AP concentration, ice crystals, graupel, frozen drops and/or hail, and supercooled water can coexist within a single cloud zone, which allows collisions and charge separation. The simulation of possible aerosol effects on the landfalling tropical cyclone has been carried out using a 3-km-resolution Weather Research and Forecast (WRF) mesoscale model. It is shown that aerosols change the cloud microstructure in a way that permits the attribution of the observed lightning structure to the effects of continental aerosols. It is also shown that aerosols, which invigorate clouds at 250–300 km from the TC center, decrease the convection intensity in the TC center, leading to some TC weakening. The results suggest that aerosols change the intensity and the spatial distribution of precipitation in landfalling TCs and can possibly contribute to the weekly cycle of the intensity and precipitation of landfalling TCs. More detailed investigations of the TC–aerosol interaction are required.

The influence of time-dependent melting on the dynamics and precipitation production in maritime and continental storm clouds

Abstract Simulations of one maritime and four continental observed cases of deep convection are performed with the Hebrew University Cloud Model that has spectral bin microphysics. The maritime case is from observations made on 18 September 1974 during the Global Atmospheric Research Program’s Atlantic Tropical Experiment (GATE). The continental storm cases are those of summertime Texas clouds observed on 13 August 1999, and green-ocean, smoky, and pyro-clouds observed during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia–Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA–SMOCC) campaign on 1–4 October 2002. Simulations have been performed for these cases with a detailed melting scheme. This scheme allows calculation of liquid water fraction within each mass bin for the melting of graupel, hail, snowflakes, and crystals, as well as alteration of the sedimentation velocity of ice particles in the course of their melting. The results obtained with the detailed melting scheme are compared with ...

The Role of Small Soluble Aerosols in the Microphysics of Deep Maritime Clouds

Some observational evidence—such as bimodal drop size distributions, comparatively high concentrations of supercooled drops at upper levels, high concentrations of small ice crystals in cloud anvils leading to high optical depth, and lightning in the eyewalls of hurricanes—indicates that the traditional view of the microphysics of deep tropical maritime clouds requires, possibly, some revisions. In the present study it is shown that the observed phenomena listed above can be attributed to the presence of small cloud condensation nuclei (CCN) with diameters less than about 0.05 mm. An increase in vertical velocity above cloud base can lead to an increase in supersaturation and to activation of the smallest CCN, resulting in production of new droplets several kilometers above the cloud base. A significant increase in supersaturation can be also caused byadecreaseindropletconcentrationduringintensewarmrainformationaccompaniedbyanintensevertical velocity. This increase in supersaturation also can trigger in-cloud nucleation and formation of small droplets. Another reason for an increase in supersaturation and in-cloud nucleation can be riming, resulting in a decrease in droplet concentration. It has been shown that successive growth of new nucleated droplets increases supercooled water content and leads to significant ice crystal concentrations aloft. The analysis of the synergetic effect of thesmallest CCNand giantCCNon production ofsupercooledwater andice crystals in cloud anvils allows reconsideration of the role of giant CCN. Significant effects of small aerosols on precipitation and cloud updrafts have been found. The possible role of these small aerosols as well as small aerosols with combination of giant CCN in creating conditions favorable for lightning in deep maritime clouds is discussed.

Comparison of collision velocity differences of drops and graupel particles in a very turbulent cloud

The motion of water drops and graupel particles within a turbulent medium is analyzed. The turbulence is assumed to be homogeneous and isotropic. It is demonstrated that the inertia of drops and graupel particles falling within a turbulent flow leads to the formation of significant velocity deviations from the surrounding air, as well as to the formation of substantial relative velocity between drops and graupel particles. The results of calculations of the continuous growth of raindrops and graupel particles moving within a cloud of small droplets are presented both in a non-turbulent medium and within turbulent flows of different turbulence intensity. Continuous growth of a drop-collector was calculated with the coalescence efficiency Ee=1, as well as using Ee values provided by Beard and Ochs [Beard, K.V., Ochs, H.T., 1984. Collection and coalescence efficiencies for accretion. J. Geophys. Res., 89: 7165–7169.] ranging from 0.5 to about 0.75 for different droplet sizes. In the case of graupel–droplet interaction Ee was assumed equal to 1. It is shown that in the case Ee=1 in a non-turbulent medium, the growth rates of graupel and raindrops are close. Under turbulent conditions typical of mature convective clouds, graupel grows much faster than a raindrop. In the case Ee<1 the growth rate of a water drop slows down significantly, so that graupel grows faster than raindrops even under non-turbulent conditions. Turbulence greatly increases the difference between the growth rates of graupel and drop-collectors. Possible consequences of the faster growth of graupel in terms of cloud microphysics are discussed.

A spatial shift of precipitation from the sea to the land caused by introducing submicron soluble aerosols: Numerical modeling

[1] Precipitation in the eastern Mediterranean takes place during the cold season, when sea surface temperature is higher than the land surface temperature by 5°C-10°C. This temperature difference leads to the formation of the land breeze-like circulation, which interacts with dominating westerlies and leads to an intense cloud formation over the sea ~10-20 km from the coastal line. As a result, most of the precipitation falls on the sea without reaching the land. At the same time the eastern Mediterranean region experiences a lack of freshwater. For investigating a possibility to shift the release of precipitation from sea to land, numerical simulations were performed using the Hebrew University 2-D cloud model and the 3-D Weather Research and Forecasting model, both operating with spectral bin microphysics, and the 3-D COSMO model of the German Weather Service applying a two-moment bulk parameterization for cloud physics. The respective results indicate that an increase in concentration of small aerosols leads to a delay in raindrop formation and fosters the formation of extra ice particles with low settling velocity. This ice is advected inland by the background wind. As a result, precipitation over land increases at the expense of precipitation over sea by 15%-20%. The spatial shift of precipitation from sea to land can be as large as 50-70 km depending on the wind speed of the background flow. These results suggest a new possibility to enhance precipitation in a particular region by cloud seeding with small aerosols.