Evaporation Of Cloud Droplets Research Articles

The effects of mineral dust particles, aerosol regeneration and ice nucleation parameterizations on clouds and precipitation

Abstract. This study focuses on the effects of aerosol particles on the formation of convective clouds and precipitation in the Eastern Mediterranean Sea, with a special emphasis on the role of mineral dust particles in these processes. We used a new detailed numerical cloud microphysics scheme that has been implemented in the Weather Research and Forecast (WRF) model in order to study aerosol–cloud interaction in 3-D configuration based on 1° × 1° resolution reanalysis meteorological data. Using a number of sensitivity studies, we tested the contribution of mineral dust particles and different ice nucleation parameterizations to precipitation development. In this study we also investigated the importance of recycled (regenerated) aerosols that had been released to the atmosphere following the evaporation of cloud droplets. The results showed that increased aerosol concentration due to the presence of mineral dust enhanced the formation of ice crystals. The dynamic evolution of the cloud system sets the time periods and regions in which heavy or light precipitation occurred in the domain. The precipitation rate, the time and duration of precipitation were affected by the aerosol properties only at small spatial scales (with areas of about 20 km2). Changes of the ice nucleation scheme from ice supersaturation-dependent parameterization to a recent approach of aerosol concentration and temperature-dependent parameterization modified the ice crystals concentrations but did not affect the total precipitation in the domain. Aerosol regeneration modified the concentration of cloud droplets at cloud base by dynamic recirculation of the aerosols but also had only a minor effect on precipitation. The major conclusion from this study is that the effect of mineral dust particles on clouds and total precipitation is limited by the properties of the atmospheric dynamics and the only effect of aerosol on precipitation may come from significant increase in the concentration of accumulation mode aerosols. In addition, the presence of mineral dust had a much smaller effect on the total precipitation than on its spatial distribution.

Volatility and Yield of Glycolaldehyde SOA Formed through Aqueous Photochemistry and Droplet Evaporation

Aqueous hydroxyl radical (∼10−12 M) oxidation of glycolaldehyde (1 mM), followed by droplet evaporation, forms secondary organic aerosol (SOA) that exhibits an effective liquid vapor pressure and enthalpy of vaporization of ∼10−7 atm and ∼70 kJ/mol, respectively, similar to the mix of organic acids identified in reaction samples. Salts of these acids have vapor pressures about three orders of magnitude lower (e.g., ammonium succinate ∼10−11 atm), suggesting that the gas–particle partitioning behavior of glycolaldehyde SOA depends strongly on whether products are present in the atmosphere as acids or salts. Several reaction samples were used to simulate cloud droplet evaporation using a vibrating orifice aerosol generator. Samples were also analyzed by ion chromatography (IC), electrospray ionization mass spectrometry (ESI-MS), IC-ESI-MS, and for total carbon. Glycolaldehyde SOA mass yields were 50–120%, somewhat higher than yields reported previously (40–60%). Possible reasons are discussed: (1) formation of oligomers from droplet evaporation, (2) inclusion of unquantified products formed by aqueous photooxidation, (3) differences in gas–particle partitioning, and (4) water retention in dried particles. These and similar results help to explain the enrichment of organic acids in particulate organic matter above clouds compared with those found below clouds, as observed previously in aircraft campaigns. Copyright 2012 American Association for Aerosol Research

Resolving both entrainment-mixing and number of activated CCN in deep convective clouds

Abstract. The number concentration of activated CCN (Na) is the most fundamental microphysical property of a convective cloud. It determines the rate of droplet growth with cloud depth and conversion into precipitation-sized particles and affects the radiative properties of the clouds. However, measuring Na is not always possible, even in the cores of the convective clouds, because entrainment of sub-saturated ambient air deeper into the cloud lowers the concentrations by dilution and may cause partial or total droplet evaporation, depending on whether the mixing is homogeneous or extreme inhomogeneous, respectively. Here we describe a methodology to derive Na based on the rate of cloud droplet effective radius (Re) growth with cloud depth and with respect to the cloud mixing with the entrained ambient air. We use the slope of the tight linear relationship between the adiabatic liquid water mixing ratio and Re3 (or Rv3) to derive an upper limit for Na assuming extreme inhomogeneous mixing. Then we tune Na down to find the theoretical relative humidity that the entrained ambient air would have for each horizontal cloud penetration, in case of homogeneous mixing. This allows us to evaluate both the entrainment and mixing process in the vertical dimension in addition to getting a better estimation for Na. We found that the derived Na from the entire profile data is highly correlated with the independent CCN measurements from below cloud base. Moreover, it was found that mixing of sub-saturated ambient air into the cloud at scales of ~100 m and above is inclined towards the extreme inhomogeneous limit, i.e. that the time scale of droplet evaporation is significantly smaller than that for turbulent mixing. This means that ambient air that entrains the cloud is pre-moistened by total evaporation of cloud droplets before it mixes deeper into the clouds where it can hardly change the droplet size distribution, hence Re remains close to its adiabatic value at any given cloud depth. However, the tendency towards the extreme inhomogeneous mixing appeared to slightly decrease with altitude, possibly due to enhanced turbulence and larger cloud drops aloft. Quantifying these effects, based on more examples from other projects and high resolution cloud models is essential for improving our understanding of the interactions between the cloud and its environment. These interactions may play an important role in cloud dynamics and microphysics, by affecting cloud depth and droplet size spectra, for example, and may therefore influence the cloud precipitation formation processes.

Comment on “‘Elevated heat pump’ hypothesis for the aerosol-monsoon hydroclimate link: ‘Grounded’ in observations?” by S. Nigam and M. Bollasina

In their recent paper, Nigam and Bollasina [2010] (hereinafter NB) claimed to have found observational evidences that are at variance with the elevated heat pump (EHP) hypothesis regarding the possible impacts of absorbing aerosols on the South Asian summer monsoon [Lau et al., 2006; Lau and Kim, 2006]. We found NB's arguments and inferences against the EHP hypothesis flawed, stemming from their own out of context interpretation of the hypothesis. NB argued that the simultaneous negative correlation of aerosol with rainfall, and correlations with other quantities in May, are evidence against the EHP hypothesis. Their argument cannot be justified. First, Lau and Kim [2006] (hereinafter LK06) never stated that the main rainfall response to EHP is in May. Second, the EHP is about responses of the entire Indian monsoon system that are nonlocal in space and time with respect to the aerosol forcing. As shown in Figure 4 of LK06, while the aerosol anomalies are strongest in April-May, the strongest rainfall response is in June-July, with the enhanced rainfall fed by an induced thermally driven circulation which brings additional moisture from the ocean to the Indian subcontinent. Third, the increased rainfall over the Bay of Bengal as shown in Figure 1a of NB and the increased low-level convergence in Figure 1f of NB do not necessarily reflect responses associated with EHP but rather the large ]scale circulation that provides the buildup of the aerosols before the onset of the monsoon rainfall over India. Because aerosol can only accumulate where there is little or no washout by rain, the negative correlation is a necessary condition for increased atmospheric loading of aerosols. For the same reason, the spatial distributions of rainfall and aerosol generally are offset with each other, i.e., high aerosol in regions of low rainfall. This is evident in Figure 1, which shows the climatological mean of the MODIS aerosol optical depth (AOD), and TRMM rainfall over India in May. The maximum AOD is found over the Indo-Gangetic Plain and the desert regions of northwest India and Pakistan

Influence of cloud condensation and giant cloud condensation nuclei on the development of precipitating trade wind cumuli in a large eddy simulation

To investigate the effects of both cloud condensational nuclei (CCN) and giant CCN (GCCN), the Regional Atmospheric Modeling System was used to investigate the effects of various CCN and GCCN concentrations on the development of precipitating trade wind cumuli in a large eddy simulation (LES) framework. The sounding to initialize the LES was taken from the Rain in Cumulus over the Ocean Experiment archive for 11 January 2005. Several sensitivity experiments were performed in which two levels of CCN (GCCN) concentrations were used: 100 (0.01) and 1000 (0.1) cm−3 corresponding to low and high values, respectively. Both CCN and GCCN can affect the precipitation processes. With low GCCN concentration, raising the CCN concentration from low to high reduced the precipitation rate as well as the accumulated precipitation due to the presence of a large number of small cloud droplets that are inefficient in forming drizzle. However, GCCN can have a greater response in increasing the precipitation rate and accumulation when the cloud system has a high CCN concentration. The total cloud coverage (TCC) was reduced for the higher CCN concentration experiments because of the susceptibility of evaporation of cloud droplets in the upper parts of the cloud as a result of entrainment. On the other hand, the TCC was increased for the higher GCCN concentration experiments. For this trade wind cumuli case, the time‐ and domain‐averaged albedo changed very slightly with increased [CCN] and/or [GCCN] because of a compensating increase/decrease among the optical depth, liquid water path, cloud coverage, and cloud droplet concentration.

Condensation/Evaporation Calculation Method for Cloud Droplets Using the CIP-CSLR Scheme

We present a number-density advection based method designed to calculate the condensation/ evaporation of cloud droplets. An Eulerian-in-radius discretization scheme is adopted, making the present model suitable for use in the spectral bin method for cloud microphysics. The mass advection equation is solved using the constrained interpolated profile-conservative semi-Lagrangian with rational function (CIP-CLSR) scheme. In order to evaluate the present method based on the CIP-CSLR scheme, we performed a condensation growth calculation using the present method and conventional methods, and then compared the results. The comparison revealed the advantages of the present method in numerical accuracy, numerical stability, conservation and computational cost.

Evaporation of Water Droplets by Radiation: Effect of Absorbing Inclusions

Numerical solutions are obtained for transient response of water droplets to radiative heating. Results are presented for evaporation of polydispersed droplets in the solar radiation field. The evolution of the temperature and size of the droplet are determined. The results are discussed based on cloud droplet evaporation in the presence of absorbing inclusions.

Satellite‐based assessment of possible dust aerosols semi‐direct effect on cloud water path over East Asia

The semi‐direct effects of dust aerosols are analyzed over eastern Asia using 2 years (June 2002 to June 2004) of data from the Clouds and the Earth's Radiant Energy System (CERES) scanning radiometer and MODerate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite, and 18 years (1984 to 2001) of International Satellite Cloud Climatology Project (ISCCP) data. The results show that the water path of dust‐contaminated clouds is considerably smaller than that of dust‐free clouds. The mean ice water path (IWP) and liquid water path (LWP) of dusty clouds are less than their dust‐free counterparts by 23.7% and 49.8%, respectively. The long‐term statistical relationship derived from ISCCP also confirms that there is significant negative correlation between dust storm index and ISCCP cloud water path (CWP). These results suggest that dust aerosols warm clouds, increase the evaporation of cloud droplets and further reduce the CWP, the so‐called semi‐direct effect. The semi‐direct effect may play a role in cloud development over arid and semi‐arid areas of East Asia and contribute to the reduction of precipitation.

Mixing of cloud and clear air in centimeter scales observed in laboratory by means of Particle Image Velocimetry

Cloudy air, containing small (∅ 14 μm) water droplets, undergoes mixing with the unsaturated environment inside the cloud chamber in the process resembling smallest scales of entrainment and mixing in real clouds. Particle Image Velocimetry (PIV) applied to images from the chamber interior is used to investigate dynamics of the process in scales from 1.2 mm to few centimeters. A special algorithm, allowing for investigation of droplets motion, is developed, tested and adapted to the experimental data. Two velocity components retrieved in the vertical cross-section through the chamber interior indicate anisotropy of small-scale turbulent motions, with the preferred vertical direction. This result confirms earlier numerical studies, indicating that evaporation of cloud droplets at the cloud–clear air interface may substantially influence the small-scale turbulence in clouds.

Numerical Simulation of Cloud–Clear Air Interfacial Mixing

Abstract This paper discusses results from a series of direct numerical simulations of the microscale cloud–clear air mixing, set forth in the idealized scenario of decaying moist turbulence. In the moist case, kinetic energy of microscale motions comes not only from the classical downscale energy cascade, but it can also be generated internally due to the evaporation of cloud droplets. Three sets of numerical simulations are performed for three intensities of initial large-scale eddies. In each set, a control dry simulation is performed, as well as two moist simulations applying either bulk or detailed representation of cloud microphysics. Model results suggest that, as far as the evolutions of enstrophy and turbulent kinetic energy are concerned, the most significant impact of moist processes occurs at the low intensity of initial large-scale eddies (the input turbulent kinetic energy of 2 × 10−4 m2 s−2 resulting in the maximum eddy dissipation rate of 5 × 10−4 m2 s−3). In such a case, mixing and homoge...

Can the direct and semi‐direct aerosol effect compete with the indirect effect on a global scale?

During the Indian Ocean Experiment (INDOEX) Ackerman et al. [2000] found that black carbon (BC) aerosols can reduce cloud cover and liquid water path (“semi‐direct” aerosol effect). Absorption of solar radiation by BC leads to a heating of the air which can result in an evaporation of cloud droplets. This warming can partially offset the cooling due to the indirect aerosol effect. We use the ECHAM4 general circulation model to investigate whether this effect is discernible on a global scale. The results of this sensitivity study, where the absorption of BC only takes place in clear air and interstitially, indicate that the zonal mean reduction in cloud cover and liquid water path in highly polluted regions of the Northern Hemisphere can amount to 4% and 10 g m−2, respectively. However, the indirect aerosol effects of increasing cloud lifetime and cloud albedo dominate.

Effects of Image Charges on the Scavenging of Aerosol Particles by Cloud Droplets and on Droplet Charging and Possible Ice Nucleation Processes

Previous calculations of the rate at which falling droplets in clouds collide with aerosols have led to the conclusion that except in thunderclouds any electrical charges on the aerosols or droplets have little effect on the collision rate. However, it had been assumed that the aerosols would have only a few elementary charges on them, whereas it is now known that at the tops of nonthunderstorm clouds the evaporating droplets may have several hundred elementary charges on them and that much of this charge remains on the residual aerosol for 5 min or so after the evaporation. Also, most previous calculations neglected image charge forces that provide strong attraction at close range even when droplet and aerosol have charges of the same sign and of comparable magnitude. The authors present numerical calculations showing that electrical effects dominate collision rates for charged evaporation aerosols. The calculations are for the size range of 0.1- to 1.0-μm radius with the collision efficiency compared to that for phoretic and Brownian effects being greater by up to a factor of 30 greater for droplets from 18.6- to 106-μm radius with relative humidity in the range 95%–100% and only 50 elementary charges on the aerosol. The results imply that electrical effects can be important for the scavenging of evaporation aerosol particles in the size range of the Greenfield gap. The authors call this process “electroscavenging.” Electroscavenging of charged particles, when the particles are mostly of the same sign, is a previously unrecognized droplet charging process. Electroscavenging also provides a pathway for contact ice nucleation when charged aerosol particles from evaporated charged droplets collide with supercooled droplets. Ice nucleation can occur because aerosol particles from the evaporation of cloud droplets have been found to be more effective as ice forming nuclei than other aerosol particles that have not been processed through droplets.

The generation of turbulence and entrainment by buoyancy reversal

AbstractIn the stratocumulus‐capped atmospheric boundary layer, the evaporation of cloud droplets in mixtures of cloudy and entrained air can, under certain circumstances, reduce their buoyancy to less than that of the surrounding unmixed cloudy air. With a negative buoyancy excess, these parcels will sink away from cloud‐top and therefore generate turbulence within the boundary layer. Dimensional arguments are used to formulate a scaling for the generation of turbulence by this mechanism, referred to as buoyancy reversal, which is found to agree remarkably well with results from idealized high‐resolution three‐dimensional large‐eddy simulations (LES). Furthermore, simple arguments relating the turbulence generation by buoyancy reversal to entrainment at cloud‐top give a simple parametrization for the entrainment rate which is also found to agree very well with the LES.

A STUDY OF PROCESSES THAT GOVERN THE MAINTENANCE OF AEROSOLS IN THE MARINE BOUNDARY LAYER

We systematically evaluate the relative influence of sources and sinks of particles in the remote marine boundary layer (MBL) to elucidate the principal factors that govern MBL aerosol behavior. Processes considered include: (1) surface flux of dimethyl sulfide (DMS); (2) gas-phase oxidation of DMS to SO 2; (3) gas-phase oxidation of SO 2 to H 2SO 4; (4) mass transfer of SO 2 and H 2SO 4 to pre-existing particles; (5) homogenous nucleation of H 2SO 4/H 2O; (6) entrainment of air from the free troposphere; (7) deposition to the sea surface; (8) cycling of air through clouds and rain scavenging; (9) oxidation of SO 2 in sea salt aerosols and cloud droplets; and (10) sea-salt particle production at sea surface. The average aerosol number concentration is found to be quite sensitive to the rate of entrainment of aerosol-containing air from the free troposphere. The path that leads to the greatest accumulation of non-sea-salt (nss) sulfate involves SO 2 (rather than H 2SO 4) absorption into existing particles. Because of scavenging of SO 2 and H 2SO 4 by sea-salt aerosol, a considerable fraction of nss-sulfate is internally mixed with sea-salt aerosol. Under the conditions assumed in this study, MBL aerosol number concentration is dominated by free tropospheric aerosol under virtually all conditions, 89% in the base case, and even 69% at a 17 m s -1 wind speed. Aerosol mass, on the other hand, is dominated by sea-salt particles, 62% in the base case and 98% at a wind speed of 17 m s -1. Evaporation of cloud droplets provides 4.6% of the particle number in the base case, but 28% of the particle mass. At the high nucleation rate case considered here, there is notably little change in the overall contributions to aerosol number and mass from the base case; only about 5% of the total particle number is provided by nucleation events. Variation in precipitation frequency also has only a minor effect on the overall contributions. One concludes that the MBL aerosol is remarkably robust in the face of ever-changing conditions. Free tropospheric aerosol entrainment tends to sustain particle number concentrations, and sea-salt emissions maintain most of the aerosol mass. Cloud processing, while not a major contributor to aerosol number, does provide, except under high wind conditions, the order of 20% of the aerosol mass. Although nucleation occurs only infrequently and does not contribute appreciably to long-term average aerosol number or mass, nucleation is, nonetheless, the mechanism that replenishes aerosol number in brief, intense episodes when aerosol surface area levels are substantially reduced by precipitation.

Cloud condensation nuclei as a real source of ice forming nuclei in continental and marine air masses

Cloud condensation nuclei (CCN) constitute a source of ice-forming nuclei (IFN) in continental and marine air masses. The mode of ice nucleation studied was condensation-followed-by-freezing. Concentrations of IFN formed by evaporation of cloud droplets and subsequent condensation of water vapor was found to be for marine aerosol particles 40 to 110 m −3 at S w = 0.2% and 80 to 700 M −3 at S w = 5%; for continental air masses it was 40 to 300 m −3 at S w=0.2% and 5%. Concentrations of IFN of marine origin increased with increasing S w, at a constant temperature; they were constant over a wide temperature range at a constant S w. IFN concentrations in continental air masses were found to be independent of S w at a constant temperature; they were constant for the S w range of 0.2 to 5% (70% of data) and 0.5 to 5% (30% of data). The IFN produced through the evaporation-condensation cycle of marine aerosol particles have properties that are identical to IFN present in a marine atmosphere—their concentrations are independent of temperatures at S w≥0.25%.

Sensitivity of a general circulation model to parameterizations of cloud-turbulence interactions in the atmospheric boundary layer

Several approaches to parameterize the turbulent transport of momentum, heat, water vapour and cloud water for use in a general circulation model (GCM) have been tested in 1-dimensional and 3-dimensional model simulations. The schemes differ with respect to their closure assumptions (conventional eddy diffusivity model versus turbulent kinetic energy closure) and also regarding their treatment of cloud–turbulence interactions. The basic properties of these parameterizations are discussed first in relation to column simulations of a stratocumulus–topped atmospheric boundary layer (ABL) under a strong subsidence inversion during the KONTROL experiment in the North Sea. It is found that the K-models tend to decouple the cloud layer from adjacent layers above and below because the turbulent activity is calculated from local variables. The higher-order scheme performs better in this respect because internally generated turbulence can be transported up and down through the action of turbulent diffusion. Thus, the TKE-scheme provides not only a better link between the cloud and the sub-cloud layer but also between the cloud and the inversion as a result of cloud-top entrainment. In the stratocumulus case study, where the cloud is confined by a pronounced subsidence inversion, increased entrainment favours cloud dilution through enhanced evaporation of cloud droplets. In the GCM study, however, additional cloud-top entrainment supports cloud formation because indirect cloud generating processes are promoted through efficient ventilation of the ABL, such as the enhanced moisture supply by surface evaporation and the increased depth of the ABL. As a result, tropical convection is more vigorous, the hydrological cycle is intensified, the whole troposphere becomes warmer and moister in general and the cloudiness in the upper part of the ABL is increased. DOI: 10.1034/j.1600-0870.1995.t01-1-00004.x

Development of a new real-time method for measuring S(IV) in cloud water using a counter-flow virtual impactor

A new method of analysis for S(IV) is described incorporating a counter-flow virtual impactor for collection and evaporation of cloud droplets with an SO2 analyzer. This technique allows analysis of aqueous S(IV) concentration in real-time and overcomes some sampling or analysis problems of conventional sampling with bulk cloud water collectors and wet chemical analysis methods. The technique is demonstrated by the correlated responses of the S(IV) instrument and instruments measuring physical properties of clouds while passing in and out of clouds. Measured aqueous S(IV) concentrations ranged from the detection limit (0.1 nmol per m3 of air) to 1.8 nmol m-3 in clouds in Ohio. S(IV) molar concentrations (moles/l of cloud water) were not calculable from these measurements, but can be calculated when a measurement of the liquid water content of the collected cloud water is made simultaneously.

Microphysical Effects on Cloud Water Acidity: A Case Study in a Nonprecipitating Cloud Event Observed at Mt. Mitchell, North Carolina

There is increasing recognition that deposition of cloud water may significantly contribute to the decline of forest. In this paper, chemical and microphysical data obtained at Mt. Mitchell, North Carolina during one orographic, non-precipitating cloud event are examined to investigate the relationship between temporal variation of cloud acidity and cloud microphysics. The cloud acidity was substantially higher than in precipitating cloud events. Sulfate, nitrate, ammounium and hydrogen ions were the major constituents. The results suggest that water vapor condensation and evaporation of cloud droplets are the dominant processes in determining the total concentration of cloud water. 18 refs., 6 figs.

Chemical composition of clouds at Mt. Mitchell, North Carolina, USA

Using a recently developed cloud and rain acidity/conductivity (CRAC) real time analyzer, clouds atop Mt. Mitchell State Park, North Carolina — the highest peak (∼ 2038 m MSL) in the eastern US — were investigated from May to October during 1987 to 1989. Measurements for chemical and physical climatology were performed to estimate exposure of pollutant at the forest canopy resulting from direct capture of cloud droplets. The mountain peak is exposed on the average to cloud episodes about 70% days per year. Results are reported for integrating times of cloud collection, approximately 5 min, during several precipitating and non-precipitating cloud events. Cloud water was found to have higher acidity and higher concentrations of sulfate, nitrate and ammonium than previously observed in rain droplets. The pH of the cloud water was in the range of 2.4 to 4.9 during 1987 through 1989 field seasons, while the pH of the rain water was in the range of 3.5 to 5.5. The cloud water collected in summer season was more acidic than those collected in any other seasons (spring and fall). The maximum of average cloud pH value (i.e., low acidity) was found when the predominant wind direction was from south-east. On the other hand, the low average cloud pH values (i.e., high acidity) were found when the predominant wind direction was west through northeast. Total concentrations of major chemical species in cloud water at Mt. Mitchell State Park are significantly higher (∼ 1250 to ∼ 3020 µeq/1) than those reported for other rural locations. According to the time-sequential chemical concentration, a concave trend with total concentration decreasing at the beginning and rising toward the end of event was observed. It was also observed that changes in the individual ionic concentration are proportional to the changes in total concentration. It suggests that condensation and evaporation of cloud droplets are the important processes, during various stages of a cloud event, in determining the total pollutant concentration of cloud water. DOI: 10.1034/j.1600-0889.1992.00004.x

Entrainment instability and vertical motion as causes of stratocumulus breakup

AbstractEntrainment instability is thought to be a cause of stratocumulus breakup. At the interface between the cloud and the overlying air, mixtures may form which are negatively buoyant because of cloud droplet evaporation. Quantities devised to predict breakup, δ2, X and δm, are obtained from aircraft observations and are tested against cloud observations from satellite. Often, the parameters indicate that breakup should occur but the clouds remain, sometimes for several days. One possible explanation for breakup is vertical motion from passing synoptic cyclones. Several cases suggest that breakup is associated with the downward vertical motion from the cold air advected behind an eastward moving cyclone.