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Isaac Ginis, Alexander P. Khain, and Elena Morozovsky

Abstract

A model of the atmospheric boundary layer (BL) is presented that explicitly calculates a two-way interaction of the background flow and convective motions. The model is utilized for investigation of the formation of large eddies (roll vortices) and their effects on the structure of the marine boundary layer under conditions resembling those of tropical cyclones. It is shown that two main factors controlling the formation of large eddies are the magnitude of the background wind speed and air humidity, determining the cloud formation and latent heat release. When the wind speed is high enough, a strong vertical wind shear develops in the lower part of the BL, which triggers turbulent mixing and the formation of a mixed layer. As a result, the vertical profiles of velocity, potential temperature, and mixing ratio in the background flow are modified to allow for the development of large eddies via dynamic instability. Latent heat release in clouds was found to be the major energy source of large eddies. The cloud formation depends on the magnitude of air humidity.

The most important manifestation of the effects of large eddies is a significant increase of the near-surface wind speed and evaporation from the sea surface. For strong wind conditions, the increase of the near-surface speed can exceed 10 m s−1 and evaporation from the sea surface can double. These results demonstrate an important role large eddies play in the formation of BL structure in high wind speeds. Inclusion of these effects in the BL parameterizations of tropical cyclone models may potentially lead to substantial improvements in the prediction of storm intensity.

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Alexander P. Khain, Igor Sednev, and V. Khvorostyanov

Abstract

The interaction of the cold season land breeze with the background flow in the Eastern Mediterranean and its influence on the climatic distribution of convective precipitation is studied using a 2D nonhydrostatic cloud ensemble model with the spectral approach in the description of cloud microphysics. The model microphysics is based on solving two kinetic equations for the size distribution functions for water droplets and ice particles. Each function is described using 33 mass categories. The model takes into account the following microphysical processes: nucleation of cloud condensation nuclei; nucleation of ice nuclei., condensational growth/evaporation of drops; growth/sublimation of ice due to accretion; freezing of droplets; melting of ice particles; and coalescence of drops, drops and ice, and ice particles themselves. The computational domain (200 km by 12 km) is covered by a finite-difference grid consisting of 129 × 31 grid points. It is shown that the model is able to reproduce wind velocity and the distribution and intensity of precipitation. Results indicate that the interaction of the winter land breeze and the background flow determine to a great extent the climatic distribution of convective-type precipitation in the Eastern Mediterranean. The background wind substantially influences both the amount and distribution of precipitation. It determines the width of the zone of convective activity and its location relative to the seashore. It is also shown that latent heat release greatly increases both the intensity of thermally induced circulation and its vertical and horizontal spreading. It is indicated that deep convection triggered by the boundary layer circulation not only increases the intensity of breeze circulation but changes the thermodynamic structure through an increase of the temperature gradients between the areas of intense convection and surrounding areas. These gradients seem to maintain the breeze front location over the sea in case of moderate onshore winds. The results show that relative humidity over the land is one of the main factors determining the difference in the precipitation amount in the northern and southern regions of Israel.

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Alexander I. Falkovich, Alexander P. Khain, and Isaac Ginis

Abstract

The interaction of binary tropical cyclones (TC) is investigated using a coupled TC-ocean movable nested-grid model. The model consists of an eight-layer atmospheric model in the sigma coordinate system and a three-layer primitive equation ocean model. There are five meshes in the TC model. The outermost domain (3840 km × 3840 km) is motionless. For the description of each TC in a TC pair, two telescopically nested meshes of finer resolution are used. The pair of the middle (1600 km × 1600 km) and innermost (800 km × 800 km) meshes move with the center of a corresponding TC. The space increments of the outermost domain and the middle and finest meshes are 160, 80, and 40 km. The oceanic domain contains 107 × 107 grid points, with the spatial increment of 40 km. In all numerical experiments a pair of equal strength axisymmetric vortices was located at different separation distances.

Experiments show that the rate of development of interacting TCs is different, mainly due to the difference in the velocities of TC movement. There is a “critical” separation distance between the centers of TCs, so that in case the separation distance is less than this critical value, attraction and merger of the TCs were observed. The critical separation distance depends on the structure of the vorticity field created by the binary TCs. Because of the changes in the structure of a TC during its life cycle the critical separation distance should also change. Two mechanisms related to the mutual vorticity advection and to the activity of irrotational velocity components seem to contribute to the attraction and repulsion of binary TCs.

The impact of the TC-ocean interaction on the evolution and trajectory of binary TCs is much stronger than in the case of a single TC. A decrease in TC strength is related not only to a TC response to seawater cooling caused by the TC itself but also to the crossing of the cold water wakes created both by the other TC and by the TC itself. A decrease in strength loads to a decrease in the mutual rotation velocity and, consequently, to a marked change in the trajectories of each of the interacting TCs. Changes in the structure of binary TCs caused by the TC-ocean interaction lead to an increase of the critical separation distance. Binary TCs cause seawater cooling over vast ocean areas and lead to the formation of a spotted sea surface temperature pattern.

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Ji-Young Han, Jong-Jin Baik, and Alexander P. Khain

Abstract

The impacts of urban aerosols on clouds and precipitation are investigated using a spectral (bin) microphysics cloud model. For this purpose, extensive numerical experiments with various aerosol concentrations are performed under different environmental moisture conditions. To take into account the urban heat island and urban air pollution, it is considered that there is low-level heating in the urban area and that the aerosol concentration in the urban area is higher than that in the surrounding rural area. Simulation results show that a low-level updraft induced by the urban heat island leads to the formation of a low-level cloud and then a deep convective cloud downwind of the urban area. The onset of precipitation produced by the low-level cloud is delayed at higher aerosol concentrations. This is because when the aerosol concentration is high, a narrow drop size distribution results in a suppressed collision–coalescence process and hence in late raindrop formation. However, after the deep convective cloud develops, a higher aerosol concentration generally leads to the development of a stronger convective cloud. This is mainly due to increased release of latent heat resulting from the enhanced condensation process with increasing aerosol concentration. The low collision efficiency of smaller cloud drops and the resulting stronger updraft at higher aerosol concentrations result in higher liquid water content at higher levels, leading to the enhanced riming process to produce large ice particles. The melting of a larger amount of hail leads to precipitation enhancement downwind of the urban area with increasing urban aerosol concentration in all moisture environments considered.

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Kentaroh Suzuki, Teruyuki Nakajima, Takashi Y. Nakajima, and Alexander P. Khain

Abstract

This study investigates the correlation patterns between cloud droplet effective radius (CDR) and cloud optical thickness (COT) of warm clouds with a nonhydrostatic spectral bin microphysics cloud model. Numerical experiments are performed with the model to simulate low-level warm clouds. The results show a positive and negative correlation pattern between CDR and COT for nondrizzling and drizzling stages of cloud development, respectively, consistent with findings of previous observational studies. Only a positive correlation is simulated when the collection process is switched off in the experiment, whereas both the positive and negative correlations are reproduced in the simulation with collection as well as condensation processes. The positive and negative correlations can also be explained in terms of an evolution pattern of the size distribution function due to condensation and collection processes, respectively.

Sensitivity experiments are also performed to examine how the CDR–COT correlation patterns are influenced by dynamical and aerosol conditions. The dynamical effect tends to change the amplitude of the CDR–COT plot mainly through changing the liquid water path, whereas the aerosol amount significantly modifies the correlation pattern between CDR and COT mainly through changing the cloud particle number concentration. These results suggest that the satellite-observed relationships between CDR and COT can be interpreted as being formed through microphysical particle growth processes under various dynamical and aerosol conditions in the real atmosphere.

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Jeffrey C. Snyder, Alexander V. Ryzhkov, Matthew R. Kumjian, Alexander P. Khain, and Joseph Picca

Abstract

Observations and recent high-resolution numerical model simulations indicate that liquid water and partially frozen hydrometeors can be lofted considerably above the environmental 0°C level in the updrafts of convective storms owing to the warm thermal perturbation from latent heating within the updraft and to the noninstantaneous nature of drop freezing. Consequently, upward extensions of positive differential reflectivity (i.e., Z DR ≥ 1 dB)—called Z DR columns—may be a useful proxy for detecting the initiation of new convective storms and examining the evolution of convective storm updrafts. High-resolution numerical simulations with spectral bin microphysics and a polarimetric forward operator reveal a strong spatial association between updrafts and Z DR columns and show the utility of examining the structure and evolution of Z DR columns for assessing updraft evolution. This paper introduces an automated Z DR column algorithm designed to provide additional diagnostic and prognostic information pertinent to convective storm nowcasting. Although suboptimal vertical resolution above the 0°C level and limitations imposed by commonly used scanning strategies in the operational WSR-88D network can complicate Z DR column detection, examples provided herein show that the algorithm can provide operational and research-focused meteorologists with valuable information about the evolution of convective storms.

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Eyal Ilotoviz, Alexander P. Khain, Nir Benmoshe, Vaughan T. J. Phillips, and Alexander V. Ryzhkov

Abstract

A midlatitude hail storm was simulated using a new version of the spectral bin microphysics Hebrew University Cloud Model (HUCM) with a detailed description of time-dependent melting and freezing. In addition to size distributions of drops, plate-, columnar-, and branch-type ice crystals, snow, graupel, and hail, new distributions for freezing drops as well as for liquid water mass within precipitating ice particles were implemented to describe time-dependent freezing and wet growth of hail, graupel, and freezing drops.

Simulations carried out using different aerosol loadings show that an increase in aerosol loading leads to a decrease in the total mass of hail but also to a substantial increase in the maximum size of hailstones. Cumulative rain strongly increases with an increase in aerosol concentration from 100 to about 1000 cm−3. At higher cloud condensation nuclei (CCN) concentrations, the sensitivity of hailstones’ size and surface precipitation to aerosols decreases. The physical mechanism of these effects was analyzed. It was shown that the change in aerosol concentration leads to a change in the major mechanisms of hail formation and growth. The main effect of the increase in the aerosol concentration is the increase in the supercooled cloud water content. Accordingly, at high aerosol concentration, the hail grows largely by accretion of cloud droplets in the course of recycling in the cloud updraft zone. The main mechanism of hail formation in the case of low aerosol concentration is freezing of raindrops.

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Alexander V. Ryzhkov, Matthew R. Kumjian, Scott M. Ganson, and Alexander P. Khain

Abstract

Spectral (bin) microphysics models are used to simulate polarimetric radar variables in melting hail. Most computations are performed in a framework of a steady-state, one-dimensional column model. Vertical profiles of radar reflectivity factor Z, differential reflectivity Z DR, specific differential phase K DP, specific attenuation A h, and specific differential attenuation A DP are modeled at S, C, and X bands for a variety of size distributions of ice particles aloft. The impact of temperature lapse rate, humidity, vertical air velocities, and ice particle density on the vertical profiles of the radar variables is also investigated. Polarimetric radar signatures of melting hail depend on the degree of melting or the height of the radar resolution volume with respect to the freezing level, which determines the relative fractions of partially and completely melted hail (i.e., rain). Simulated vertical profiles of radar variables are very sensitive to radar wavelength and the slope of the size distribution of hail aloft, which is correlated well with maximal hail size. Analysis of relative contributions of different parts of the hail/rain size spectrum to the radar variables allows explanations of a number of experimentally observed features such as large differences in Z of hail at the three radar wavelengths, unusually high values of Z DR at C band, and relative insensitivity of the measurements at C and X bands to the presence of large hail exceeding 2.5 cm in diameter. Modeling results are consistent with S- and C-band polarimetric radar observations and are utilized in Part II for devising practical algorithms for hail detection and determination of hail size as well as attenuation correction and rainfall estimation in the presence of hail.

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Xiaowen Li, Wei-Kuo Tao, Alexander P. Khain, Joanne Simpson, and Daniel E. Johnson

Abstract

A two-dimensional cloud-resolving model is used to study the sensitivities of two microphysical schemes, a bulk scheme and an explicit spectral bin scheme, in simulating a midlatitude summertime squall line [Preliminary Regional Experiment for Storm-Scale Operational and Research Meteorology (PRE-STORM), 10–11 June 1985]. In this first part of a two-part paper, the developing and mature stages of simulated storms are compared in detail. Some variables observed during the field campaign are also presented for validation. It is found that both schemes agree well with each other, and also with published observations and retrievals, in terms of storm structures and evolution, average storm flow patterns, pressure and temperature perturbations, and total heating profiles. The bin scheme is able to produce a much more extensive and homogeneous stratiform region, which compares better with observations.

However, instantaneous fields and high temporal resolution analyses show distinct characteristics in the two simulations. During the mature stage, the bulk simulation produces a multicell storm with convective cells embedded in its stratiform region. Its leading convection also shows a distinct life cycle (strong evolution). In contrast, the bin simulation produces a unicell storm with little temporal variation in its leading cell regeneration (weak evolution). More detailed, high-resolution observations are needed to validate and, perhaps, generalize these model results. Interactions between the cloud microphysics and storm dynamics that produce the sensitivities described here are discussed in detail in Part II of this paper.

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Xiaowen Li, Wei-Kuo Tao, Alexander P. Khain, Joanne Simpson, and Daniel E. Johnson

Abstract

Part I of this paper compares two simulations, one using a bulk and the other a detailed bin microphysical scheme, of a long-lasting, continental mesoscale convective system with leading convection and trailing stratiform region. Diagnostic studies and sensitivity tests are carried out in Part II to explain the simulated contrasts in the spatial and temporal variations by the two microphysical schemes and to understand the interactions between cloud microphysics and storm dynamics. It is found that the fixed raindrop size distribution in the bulk scheme artificially enhances rain evaporation rate and produces a stronger near-surface cool pool compared with the bin simulation. In the bulk simulation, cool pool circulation dominates the near-surface environmental wind shear in contrast to the near-balance between cool pool and wind shear in the bin simulation. This is the main reason for the contrasting quasi-steady states simulated in Part I. Sensitivity tests also show that large amounts of fast-falling hail produced in the original bulk scheme not only result in a narrow trailing stratiform region but also act to further exacerbate the strong cool pool simulated in the bulk parameterization.

An empirical formula for a correction factor, r(q r) = 0.11q r −1.27 + 0.98, is developed to correct the overestimation of rain evaporation in the bulk model, where r is the ratio of the rain evaporation rate between the bulk and bin simulations and qr(g kg−1) is the rain mixing ratio. This formula offers a practical fix for the simple bulk scheme in rain evaporation parameterization.

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