Constantin Andronache's Research Topics



Mechanisms and effects of ice nucleation in tropical cyclones

In July, 2005 the National Aeronautics and Space Administration (NASA) investigated tropical cyclogenesis, hurricane structure and intensity change in the eastern Pacific and western Atlantic using its ER-2 high altitude research aircraft. The campaign, called the Tropical Cloud Systems and Processes (TCSP) experiment, was conducted in conjunction with the National Oceanographic and Atmospheric Administration (NOAA) Hurricane Research Division (HRD) Intensity Forecasting Experiment (IFEX). A number of in situ and remote sensor datasets were collected inside and above four tropical cyclones (Hurricanes Dennis and Emily, Tropical Storm Gert and the pre-genesis stages of Tropical Storm Eugene). These four storms represent a broad spectrum of tropical cyclone intensity and development in diverse environments. While the TCSP datasets directly address several key hypotheses governing tropical cyclone formation, the campaign also sampled two unusually strong, early season storms.

We are interested to investigate the interactions between tropospheric aerosols and clouds. The main goals are:- (1) to analyze the correlations between satellite- and aircraft-data related to aerosols and cloud-cover properties, using TCSP observations; (2) to improve the EMM, which already has the unique capability of predicting particle properties (shape, bulk density, size) without categorization assumptions and to predict the particle size distributions; (3) to improve the bulk microphysics scheme of the CRM, enabling it to predict the mass, concentration and possibly certain properties of particles, with dependences of their nucleation and coagulation processes on the ambient turbulence and in-cloud electric fields; and (4) to answer scientific questions related to the role of cloud dynamics, electric fields, turbulence and other ambient conditions in the nucleation processes that provide the linkage between aerosol and ice particle properties in cirrus. Particular focus will be given to the competition between the homogeneous freezing of aerosol and that of cloud-droplets.

Related links:

  • TCSP
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    The link between aerosols and cloud ice crystals.

    The coverage of cirrus in the tropics and their radiative effects depend on anvil lifetimes and spreading in the upper troposphere (UT). One of the goals of the CRYSTAL FACE (CF) experiment is to understand the cirrus anvil evolution processes. Cirrus anvil properties can be linked to intensity of convection in the generating cumulonimbus and on UT ambient conditions. This work uses CF measurements to document the dependence of cirrus anvil ice crystal concentration and effective size on aerosol concentration, ambient relative humidity and vertical velocity. It is shown that the presence of significant ice crystal concentration (diameter larger than 50 micrometers) in cirrus anvil is linked to ambient water vapor supersaturated with respect to ice, predominant mesoscale updraft motion and large number concentration of aerosol particles (with diameter less than 1 micrometer). The study presents cases with time evolution of anvil characteristics and a discussion of factors that affect aerosol and ice crystal concentration during cirrus anvil aging. With the aim of diagnosing the dependence of ice crystal concentration on vertical motion in the cumulonimbus core, and possible dependence on atmospheric aerosol loading, an explicit microphysical model is applied.

    Related links:

  • CRYSTAL FACE
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    In-cloud aerosol scavenging mechanisms.

    A study of the in-cloud aerosol scavenging mechanisms was undertaken to develop a microphysical modeling tool with application to aerosol wet removal at mesoscale. The model considers the collision efficiency between aerosols and warm cloud particles (droplets, raindrops), raindrop size distribution, and vertical distribution of cloud variables. The model estimations compare well with scavenging rates determined from long term records of acid rain variables. Future work will address the problem of aerosol chemistry and cloud electrification impact on the scavenging rate. Another extension of this work will focus on scavenging by snow and ice particles.

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    Role of electric charge in below-cloud scavenging of ultra-fine particles.

    The study is concerned with the role of diffusion and electric charge in the below-cloud scavenging of ultra-fine particles. The developed module is suitable for boundary layer aerosol modeling during rainfall.

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    Below-cloud scavenging of boundary layer aerosol.

    Below-cloud scavenging (BCS) coefficients of aerosols by rainfall are estimated for reported aerosol size distributions measured during field experiments in various environments. The method employed is based on explicit calculations of the efficiency of collision between a raindrop and aerosol particles. Such BCS coefficients can be used in numerical models that describe: 1) the detailed evolution of aerosol size distribution and, 2) the evolution of total aerosol mass concentration. The effects of raindrop size distribution and aerosol size distribution variability on BCS coefficients are illustrated using observed data. Results show that BCS coefficient increases with rainfall rate and has a significant dependence on aerosol size distribution parameters. Thus, BCS is important for very small particles (with diameters less than 0.01 Ám) and for coarse particles (with diameters larger than 2 Ám). For rainfall rate R ~ 1 mm hr-1, the 0.5-folding time of these particles is of the order of one hour. It is shown that BCS is negligible for aerosol particles in the range [0.1-1] Ám if compared with in-cloud scavenging rates for low and moderate rainfall rates ( R ~ 0.1-10 mm hr-1). The results indicate that a boundary layer aerosol size distribution with coarse mode is drastically affected very shortly after rain starts (in a fraction of one hour) and consequently, the below-cloud aerosol size distribution becomes dominated by particles in the accumulation mode.

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    Interactions between ITCZ and tropospheric aerosol.

    We report model simulations of the effect of deep convection on aerosol under typical Intertropical Convergence Zone (ITCZ) conditions in the tropical Indian Ocean as encountered during the Indian Ocean Experiment (INDOEX). Measurements taken during various phases of INDOEX showed significant aerosol mass concentrations of nss-sulfate, carbonaceous, and mineral dust over the Northern Indian Ocean. During the winter dry season these aerosol species accumulate and are transported long distances to the tropical regions. In contrast, aerosol measurements South of the ITCZ exhibit significantly lower aerosol concentrations, and the convective activity, mixing and wet removal in the ITCZ are responsible for their depletion. Our results, based on a cloud-resolving model, driven by NCEP analysis, show that convection and precipitation can remove significant amounts of aerosol, as observed in the Indian Ocean ITCZ. The aerosol lifetime in the boundary layer (BL) is of the order of hours in intense convection with precipitation, but on average is in the range of 1-3 days for the case studied here. Since the convective events occur in a small fraction of the ITCZ area, the aerosol lifetime can vary significantly due to variability of precipitation. Our results show that the decay in concentration of various species of aerosols is comparable with in situ measurements and that the ITCZ can act to reduce the transport of polluted air masses into Southern Hemisphere especially in cases with significant precipitation. Another finding is that aerosol loadings typical to North of ITCZ tend to induce changes in cloud microphysical properties. We found that a difference between clean air masses as those encountered South of the ITCZ to aerosol polluted air masses as encountered North of the ITCZ is associated with a slight decrease of the cloud droplet effective radius (average changes of about 2 um) and an increase in cloud droplet number concentration (average changes by about 40 to 100 cm-3) consistent with several in situ measurements. Thus, polluted air masses from the Northern Indian Ocean are associated with altered microphysics and the extent of these effects is dependent on the efficiency of aerosol removal by ITCZ precipitation and dilution by mixing with pristine air masses from Southern Hemisphere.

    Related links:
  • Indian Ocean Experiment (INDOEX)
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    Indirect effect of aerosols in a high resolution cloud resolving model.

    A high-resolution limited area nonhydrostatic model was used to simulate sulfate-cloud interactions during the convective activity in a case study from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment, December 20-25, 1992. The model includes a new detailed sulfate-cloud microphysics scheme designed to estimate the effects of sulfate on cloud microphysics and radiative properties and the effects of deep convection on the transport and redistribution of aerosol. The data for S02 and SO4 species were taken from the Pacific Exploratory Mission West B observations during February-March 1994. Results show that a change in sulfate loading from the minimum to the maximum observed value scenarios (i.e., from about 0.01 to 1 ug m-3) causes a significant decrease of the effective radius of cloud droplets (changes up to 2 um on average) and an increase of the diagnostic number concentration of cloud droplets (typical changes about 5-20 cm-3). The change in the average net shortwave (SW) radiation flux above the clouds was estimated to be on average -1.5 W m-2, with significant spatial and temporal variations. The horizontal average of the changes in the net SW radiation fluxes above clouds has a diurnal cycle, reaching typical values approximately -3 W m-2. The changes in the average net longwave radiation flux above the clouds were negligible, but they showed significant variations, typically between -10 W m-2 and 10 W m-2 near the surface. These variations were associated mainly with the changes in the distribution of cloud water, which showed typical relative changes of cloud water path of about 10-20%. Other notable changes induced by the increase of aerosol were the variations in air temperature of the order of 1░C. The case study presented here suggests that characteristics of convective clouds in tropical areas are sensitive to atmospheric sulfate loading, particularly during enhanced sulfate episodes.

    Related links:
  • Global Tropospheric Experiment (GTE)
  • Tropical Ocean Global Atmospheres/Coupled Ocean Atmosphere Response Experiment (TOGA/COARE)

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    The impact of gas-to-particle conversion on tropospheric aerosol.

    Aircraft observations during the Pacific Exploratory Mission in the western Pacific Ocean, phase B (PEM-West B), taken in February-March 1994, have been used to constrain a numerical model that calculates local concentrations of gaseous H2SO4, rates of homogeneous nucleation, and concentrations of newly formed, nanometer-sized particles. The data was selected from 13 flights over the western Pacific Ocean that covered an altitude range from the boundary layer (BL) to the upper troposphere (UT) and latitudes from 100S to 600N. The largest nucleation rates were calculated for the data from the flights over the temperate latitudes (L>300N). Within these latitudes, homogeneous nucleation rates averaged about 1-100 particles cm-3s-1. Significantly smaller nucleation rates were calculated for the tropical (L<200N)and subtropical (200N - 300N) regions. The relatively large nucleation rates calculated for the temperate latitudes could be largely attributed to the cold temperatures encountered in this region during the PEM-West B flights. For the data from the tropical and subtropical flights, little or no homogeneous nucleation was calculated for the average conditions encountered in the BL and midtroposphere (MT). Instead, significant nucleation was limited either to the UT or to several small-scale events. These enhanced nucleation events were generally characterized by spikes in relative humidity and low aerosol surface density. However, the strongest nucleation events, with homogeneous nucleation rates of about 10 particles cm-3s-1, were associated with high concentrations of SO2, most likely as a result of pollution from the Asian continent. Our results imply that in regions in which homogeneous nucleation is dominated by small-scale fluctuations, approaches that attempt to infer nucleation rates using average or typical conditions will grossly underestimate the actual average rate of nucleation.

    Related links:
  • Global Tropospheric Experiment (GTE)

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    The effect of aircraft emission on atmospheric aerosol, contrails and cirrus clouds.

    A numerical model of the time evolution of subsonic aircraft exhaust is used to evaluate the possible activation of soot particles by collisions with S02 and H2SO4 gas molecules and Brownian coagulation with H2SO4/H2O aerosol formed by homogeneous nucleation. The soluble mass fraction accumulated on soot by the three processes is estimated for emission indices of sulfur from 0.001 to 3 g kg-1. The calculations indicate that the soluble mass fraction of sulfate added to soot particles (assumed to be totally hydrophobic at the point of exhaust) can be large enough to form activated particles within the exhaust plumes of aircraft operating on fuels with typical sulfur contents. However, for emissions from aircraft operating on extremely low sulfur fuels, the soluble material added to soot particles is not sufficient to activate them within the time frame observed for contrail formation. This result, coupled with the Busen and Schumann [1995] observations of contrail formation from an aircraft using 0.004 g S kg-1 fuel, suggests that heterogeneous interactions between soot and sulfur within the exhaust plume are not sufficient to explain the presence of activated particles and contrails in the wakes of high altitude aircraft if the emitted sulfur is in the form of SO2 only. It is probable that soot particles already have enough soluble material when emitted from the engine exhaust, or/and a higher conversion of sulfur into H2S04 enable them to act as cloud condensation nuclei (CCN) for contrails.

    Related links:
  • Aviation and the Global Atmosphere
  • Atmospheric Effects of Aviation: A Review of NASA's Subsonic Assessment Project (1999)
  • SUbsonic aircraft: Contrail & Clouds Effects Special Study (SUCCESS)
  • Pollution from Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT)

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    Influence of dynamics and chemistry on atmospheric isoprene.

    An analysis is presented of vertical profiles of isoprene concentration and meteorological parameters measured in the boundary layer (BL) during the daylight hours at a rural site in Alabama and an urban site in Atlanta, Georgia, during the summer of 1990, as part of the Southern Oxidants Study. Of the 37 isoprene profiles recorded at the sites, 16 exhibited complex vertical structure with local maxima within the BL. This complex vertical structure appears to arise from a variety of turbulent processes fostered by horizontal inhomogeneities hi the surface emissions of isoprene and by the transient appearance of layers of strong wind shear and/or vertical stability within the BL. A statistical analysis of the data suggests that the complex features observed hi the individual profiles are stochastic hi nature and tend to cancel out upon averaging over all profiles. Nevertheless, these complex structures can confound attempts to infer the BL abundance of a short-lived hydrocarbon like isoprene from a set of measurements at a single height. Our calculations suggest that measurements made at a height of 40 - 100 m above the surface will yield the most reliable measure of average BL concentrations of reactive hydrocarbons.

    Related links:
  • Southern Oxidants Study (SOS)
  • Comparison of Modeled versus Observed Isoprene
  • Air Resources Laboratory (ARL)
  • Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET)

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