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  • Author or Editor: Alexander P. Khain x
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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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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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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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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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Takamichi Iguchi, Teruyuki Nakajima, Alexander P. Khain, Kazuo Saito, Toshihiko Takemura, Hajime Okamoto, Tomoaki Nishizawa, and Wei-Kuo Tao

Abstract

Numerical weather prediction (NWP) simulations using the Japan Meteorological Agency Nonhydrostatic Model (JMA-NHM) are conducted for three precipitation events observed by shipborne or spaceborne W-band cloud radars. Spectral bin and single-moment bulk cloud microphysics schemes are employed separately for an intercomparative study. A radar product simulator that is compatible with both microphysics schemes is developed to enable a direct comparison between simulation and observation with respect to the equivalent radar reflectivity factor Ze, Doppler velocity (DV), and path-integrated attenuation (PIA). In general, the bin model simulation shows better agreement with the observed data than the bulk model simulation. The correction of the terminal fall velocities of snowflakes using those of hail further improves the result of the bin model simulation. The results indicate that there are substantial uncertainties in the mass–size and size–terminal fall velocity relations of snowflakes or in the calculation of terminal fall velocity of snow aloft. For the bulk microphysics, the overestimation of Ze is observed as a result of a significant predominance of snow over cloud ice due to substantial deposition growth directly to snow. The DV comparison shows that a correction for the fall velocity of hydrometeors considering a change of particle size should be introduced even in single-moment bulk cloud microphysics.

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Barry H. Lynn, Alexander P. Khain, Jian Wen Bao, Sara A. Michelson, Tianle Yuan, Guy Kelman, Daniel Rosenfeld, Jacob Shpund, and Nir Benmoshe

Abstract

Hurricane Irene (2011) moved northward along the eastern coast of the United States and was expected to cause severe wind and flood damage. However, the hurricane weakened much faster than was predicted. Moreover, the minimum pressure in Irene occurred, atypically, about 40 h later than the time of maximum wind speed. Possible reasons for Irene’s weakening and the time shift between maximum wind and minimum central pressure were studied in simulations using WRF with spectral bin microphysics (WRF-SBM) with 1-km grid spacing and ocean coupling. Both ocean coupling and aerosol distribution/concentration were found to influence Irene’s development. Without ocean coupling or with ocean coupling and uniform aerosol distribution, the simulated maximum wind occurred at about the same time as the minimum pressure. With ocean coupling and nonuniform spatial aerosol distributions caused by aerosols from the Saharan air layer (band) and the continental United States, the maximum wind occurred about 40 h before the simulated minimum pressure, in agreement with observations. Concentrations of aerosols of several hundred per cubic centimeter in the inner core were found to initially cause convection invigoration in the simulated eyewall. In contrast, a weakening effect dominated at the mature and the decaying stages, when aerosols from the band and land intensified convection at the simulated storm’s periphery. Simulations made with 3-km instead of 1-km grid spacing suggest that cloud-scale processes interactions are required to correctly simulate the timing differences between maximum wind and minimum pressure.

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