Cloud Drop Concentrations Research Articles

Parameterization of the nitric acid effect on CCN activation

Abstract. In this paper we present a parameterization of the nitric acid effect on cloud droplet formation. The new parameterization is intended to be used in large scale models in order to obtain regional and global estimates of the effect of nitric acid on cloud drop concentrations and the radiative balance. The parameterization is based on numerical air parcel model simulations and can be applied for unimodal and bimodal lognormal aerosol particle size distributions in a large variety of different conditions. In addition to the aerosol particle distribution and gas-phase HNO3 concentration, the parameterization requires temperature, total pressure, updraft velocity, and the number concentration of cloud droplets formed at zero nitric acid concentration, as input parameters. The parameterization is also suitable for describing the effect of hydrochloric acid on the cloud drop concentrations, and in practice, the HNO3 and HCl concentrations can be summed up to yield the total effect. The comparison between the parameterization and the results from numerical air parcel model simulations show good consistency.

Remotely Sensed Microphysical and Thermodynamic Properties of Nonuniform Water Cloud Fields

Abstract Visible and near-infrared reflected radiances have been used to estimate the cloud optical depth and effective radius of cloud-filled global area coverage (GAC) pixels from the Advanced Very High Resolution Radiometer (AVHRR) for two cases in the North Atlantic Ocean. One is representative of clouds having low concentrations of cloud condensation nuclei (CCN), while the other is an example of maritime clouds forming in continental air, in this case, intruding from Europe around a cutoff low pressure system. It is shown that an estimate of the cloud drop concentration can be obtained from remotely sensed cloud radiative properties and standard meteorological analyses. These concentrations show very clearly the influence of enhanced CCN on cloud microphysics. However, conclusions regarding the indirect radiative effect of aerosol on cloud must wait for the development of a framework for analyzing changes in cloud liquid water path (LWP). It is shown that estimates of LWP are greatly influenced by the scheme that is used to identify cloudy pixels at the AVHRR GAC resolution. Application of a very strict thermal channel spatial coherence criterion for identifying cloud-filled pixels yields mean LWP estimates for cloudy pixels alone that are 40%–75% higher than mean LWP estimates for the much larger sample of possibly cloudy pixels identified by a reflectance threshold criterion.

Aerosol–cloud drop concentration closure in warm cumulus

Our understanding of the activation of aerosol particles into cloud drops during the formation of warm cumulus clouds presently has a limited observational foundation. Detailed observations of aerosol size and composition, cloud microphysics and dynamics, and atmospheric thermodynamic state were collected in a systematic study of 21 cumulus clouds by the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during NASA's Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL‐FACE). An “aerosol‐cloud” closure study was carried out in which a detailed cloud activation parcel model, which predicts cloud drop concentration using observed aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against observations. On average, measured droplet concentration in adiabatic cloud regions is within 15% of the predictions. This agreement is corroborated by independent measurements of aerosol activation carried out by two cloud condensation nucleus (CCN) counters on the aircraft. Variations in aerosol concentration, which ranged from 300 to 3300 cm−3, drives large microphysical differences (250–2300 cm−3) observed among continental and maritime clouds in the South Florida region. This is the first known study in which a cloud parcel model is evaluated in a closure study using a constraining set of data collected from a single platform. Likewise, this is the first known study in which relationships among aerosol size distribution, CCN spectrum, and cloud droplet concentration are all found to be consistent with theory within experimental uncertainties much less than 50%. Vertical profiles of cloud microphysical properties (effective radius, droplet concentration, dispersion) clearly demonstrate the boundary layer aerosol's effect on cloud microphysics throughout the lowest 1 km of cloud depth. Onboard measurements of aerosol hygroscopic growth and the organic to sulfate mass ratio are related to CCN properties. These chemical data are used to quantify the range of uncertainty associated with the simplified treatment of aerosol composition assumed in the closure study.

Cloud susceptibility and the first aerosol indirect forcing: Sensitivity to black carbon and aerosol concentrations

Present‐day global anthropogenic emissions contribute more than half of the mass in submicron particles primarily due to sulfate and carbonaceous aerosol components derived from fossil fuel combustion and biomass burning. These anthropogenic aerosols increase cloud drop number concentration and cloud albedo. Here, we use an improved version of the fully coupled climate/chemistry models to investigate cloud susceptibility and the first indirect effect of anthropogenic aerosols (the Twomey effect). We examine the correspondence between the model simulation of cloud susceptibility and that inferred from satellite measurements to test whether our simulated aerosol concentrations and aerosol/cloud interactions give a faithful representation of these features. This comparison provides an overall measure of the adequacy of cloud cover and drop concentrations. We also address the impact of black carbon absorption in clouds on the first indirect forcing and examine the sensitivity of the forcing to different representations of natural aerosols. We find that including this absorption does not change the global forcing by more than 0.07 W m−2, but that locally it could decrease the forcing by as much as 0.7 W m−2 in regions where black carbon emissions are pronounced. Because of the nonlinear relationship between cloud drop number and aerosol number concentrations, the total forcing does not equal the sum of the forcing from each individual source. Our estimated total first indirect forcing is −1.85 W m−2, with −0.30 W m−2 associated with anthropogenic sulfate, −1.16 W m−2 associated with carbonaceous aerosols from biomass burning, and −0.52 W m−2 associated with carbonaceous aerosols from fossil fuel combustion. Estimates of forcing by sulfate and total carbonaceous aerosols increase to −0.31 and −1.67 W m−2, respectively, if natural emissions of organic aerosols are only 8.4 Tg yr−1, but decrease to −0.26 and −1.27 W m−2 if they are as large as 42 Tg yr−1. Even larger estimates of forcing are derived if dust and sea‐salt emissions are not included. The effect of aerosol abundance on cloud life cycle may be important but is not treated in this study.

A case study on microphysical and radiative properties of power-plant-originated clouds

Cloud microphysical characteristics and solar cloud albedo of a stratiform cloud field downwind a coal-fired power plant and of naturally formed stratiform continental clouds are compared. In situ measurements of aerosol, drop microphysical, and solar radiative parameters of both cloud types are complemented by one- and three-dimensional radiative transfer calculations. The aerosol particle concentrations in and around the power-plant-originated clouds were significantly enhanced and the cloud drop concentrations were nearly doubled in comparison with the naturally formed clouds, whereas the effective cloud drop radius remained roughly unchanged. The radiation measurements as well as the modeling revealed a higher solar albedo of the power-plant-originated cloud compared to the surrounding natural clouds. Even when high amounts of absorbing material were assumed in the drops of the power plant cloud, the calculations showed a higher albedo of this cloud. The possibility that the power plant's water vapor emissions significantly alter the cloud microphysics downwind the power plant is discussed.

A numerical model of the cloud‐topped marine boundary layer with explicit treatment of supersaturation–number concentration correlations

AbstractA numerical model of the cloud‐topped marine boundary layer is presented. The model, based on ensemble‐averaged conservation equations and containing detailed aerosol microphysics, differs from previous models in one major respect: the mean supersaturation–cloud drop number concentration correlation term, generally neglected in previous numerical mean cloud models, is parametrized using a traditional boundary‐layer flux‐gradient relationship. The present work highlights the importance of this correlation term and shows how a simple representation of this correlation can lead to dramatically different, and improved, simulations of cloud behaviour. Simulations are presented for a well‐studied marine boundary‐layer stratocumulus case. Inclusion of the supersaturation–cloud drop number concentration correlation leads to more physically realistic cloud drop spectra and cloud drop concentrations throughout the height of the cloud. Although the parametrization presented must be viewed as a first approximation, the crudeness of the approach should be seen as a justification and starting point for future work. Copyright © 2002 Royal Meteorological Society

Parameterization of drop effective radius for drizzling marine stratocumulus

We describe a parameterization of cloud drop effective radius Re developed for drizzling marine stratocumulus clouds. The parameterization was derived using data from the CIMMS LES model with explicit formulation of drop size resolving microphysics. We showed that in the case of moderate drizzle an accurate parameterization of Re in the 8 ÷ 20 μm radius range can be obtained on the basis of two microphysical variables: liquid water content and drop concentration. For heavy drizzle, only a three‐variable parameterization proves to be sufficiently accurate. We developed two versions of the parameterization. The first one is based on traditional cloud microphysical prognostic variables representing partial moments of the drop distribution function: cloud water content, drizzle water content, and cloud drop concentration. The second one is based on the full moments of the drop size distribution function: drop concentration, total liquid water content, and radar reflectivity. The first version of the parameterization is rather sensitive to the value of the threshold radius that divides cloud and drizzle water. As a result, the errors are larger than in the second full moment version. It is concluded that a three‐variable full moment parameterization provides the most accurate representation of effective radius in precipitating stratocumulus cloud layers.

Numerical Comparison of Two Ice Crystal Formation Mechanisms on Snowfall Enhancement from Ground-Based Aerosol Generators

Two mechanisms of ice crystal formation, contact freezing and very rapid condensation freezing, were applied to numerical simulations of ground-based seeding with the Guide Model, an orographic cloud model, to study whether different mechanisms of ice crystal formation substantially affect precipitation patterns and intensities. Although the numerical model has limitations, it was expected to indicate how different ice crystal formation rates lead to differences in precipitation patterns and intensities between the two mechanisms. Numerical simulations of two case studies are presented. One is characterized by moderate wind speeds and colder cloud temperatures, the other by stronger winds and warmer cloud temperatures. The moderate wind field and colder cloud temperatures yielded nearly half an order of magnitude more precipitation than the strong wind field and warmer cloud temperatures. Sensitivity analysis showed that snowfall as a result of forced condensation freezing is strongly dependent on the ambient temperature at the ground-based generator site, while generator site temperature had less effect on the precipitation as a result of contact freezing. Snowfall resulting from contact freezing, however, was found to be strongly dependent on the cloud drop concentration. Liquid water content did not significantly affect the precipitation resulting from ice crystal formation by either mechanism. Precipitation rates induced by forced condensation freezing are about two orders of magnitude greater than those induced by contact freezing in the cases simulated, over the Sierra crest, because of the limited time available for ice particles to grow and precipitate.

Remote sensing the susceptibility of cloud albedo to changes in drop concentration

The role of clouds in reflecting solar radiation to space and thereby reducing surface heating is of critical importance to climate. Combustion processes that produce green-house gases also increase cloud condensation nuclei (CCN) concentrations which in turn increase cloud drop concentrations and thereby cloud albedo. A calculation of cloud susceptibility, defined in this work as the increase in albedo resulting from the addition of one cloud drop per cubic centimeter (as cloud liquid water content remains constant), is made through satellite remote sensing of cloud drop radius and optical thickness. The remote technique uses spectral channels of the Advance Very High Resolution Radiometer (AVHRR) instrument on board the NOAA polar orbiting satellites. An approximation for the effect of the atmosphere on the signal received by the AVHRR is included in the analysis. Marine stratus clouds, as well as being important modifiers of climate, are cleaner than continental clouds an so likely to be of higher susceptibility. Analysis of several stratus scenes, including some containing ship tracks, supports this expectation.

Measurement of cloud integrated backscattering coefficient with a nephelometer

We describe the application of a nephelometer to in‐situ, real‐time measurement of the integrated backscattering coefficient σbs of cloud drops. For thin and moderately thick clouds, this quantity is related to the cloud albedo A, for 0° zenith solar radiation, through the relation A=σbsL, where L is the geometric cloud thickness. The instrument was used in a field experiment on El Yunque peak in Puerto Rico. The measured σbs values are in agreement with results of Mie theory calculations, using independently measured cloud drop concentration and size distribution.

Aerosol over the Southern Ocean

In a first attempt to assess a proposed climatic change feedback process involving cloud condensation nuclei (CCN) and cloud albedo, CCN concentrations N as a function of supersaturation S were measured on a voyage from latitude 43 to 65°S in October–November 1988. The usual relationship N= CS k , with k=0.5 and C a constant was a fair apprraximation for S in the range 0.3–0.7% implying that CCN concentrations should largely determine cloud drop concentrations and hence albedo for clouds with S in that range. South of latitude 50°S and at smaller S, k was 1 or larger on average, which would lead to reduced dependence of albedo on CCN for the relevant clouds. N varied very widely for separations of the order of 100 km or 6 hours in time, particularly when the sea was partly ice-covered, suggesting strong local influences. During a large increase in N 60°S, unaccompanied by an increase in condensation nuclei (CN), cloud drops grew more rapidly than usual. In a subsidiary experiment particles were collected and examined by transmission electron microscopy. For particles less than 0.2 μm diameter, 80–90% appeared to consist of ammonium sulfate, the remainder being sea salt or an unknown substance which was more liquid and heat-resistant. Dialysis showed that the sulfate particles contained a few percent of insoluble material. Particles which formed cloud drops in vapours other water, were also studied. Comparison of these and water CCN and the rates of droplet suggested that the water insoluble portion of the particles was ethanol-soluble and surface-active. CN concentrations decreased by a factor of about 2 between 43 and 65°S, a change closely paralleled by ethanol CCN concentrations.

Aerosols in Polluted versus Nonpolluted Air Masses: Long-Range Transport and Effects on Clouds

To assess the influence of anthropogenic aerosols on the physics and chemistry of clouds in the northeastern United States, aerosol and cloud-drop size distributions, elemental composition of aerosols as a function of size, and ionic content of cloud water were measured on Whiteface Mountain, NY, during the summers of 1981 and 1982. In several case studies, the data were cross-correlated with different air mass types - background continental, polluted continental, and maritime - that were advected to the sampling site. The results are the following: (1) Anthropogenic sources hundreds of kilometers upwind cause the small-particle (accumulation) mode number to increase from hundreds of thousands per cubic centimeter and the mass loading to increase from a few to several tens of micrograms per cubic meter, mostly in the form of sulfur aerosols. (2) A significant fraction of anthropogenic sulfur appears to act as cloud condensation nuclei (CCN) to affect the cloud drop concentration. (3) Clouds in Atlantic maritime air masses have cloud drop spectra that are markedly different from those measured in continental clouds. The drop concentration is significantly lower, and the drop size spectra are heavily skewed toward large drops. (4) Effects of anthropogenic pollutants on cloud water ionic composition are an increase of nitrate by a factor of 50, an increase of sulfate by more than one order of magnitude, and an increase of ammonium ion by a factor of 7. The net effect of the changes in ionic concentrations is an increase in cloud water acidity. An anion deficit even in maritime clouds suggests an unknown, possibly biogenic, source that could be responsible for a pH below neutral, which is frequently observed in nonpolluted clouds.

Aircraft Flow Effects on Cloud Drop Images and Concentrations Measured by the NAE Twin Otter

A theoretical model for the local flow velocity ahead of the wing quarter-chord point on the National Aeronautical Establishment Twin Otter has been developed and experimentally verified by flight measurements using a pitot-static equipped Particle Measuring Systems (PMS) canister. This flow velocity model was used to calculate drop trajectories and the resulting airflow effects on drop images and concentrations. Results indicate that drop images and concentrations measured by the Twin Otter are distorted in the worst case by no more than 25% for an unusually high aircraft lift coefficient (CL) of 0.79, and by only a few percent for lower values of CL typical of normal flight. A method to correct water drop images and concentrations is described and results for a PMS probe mounted at one underwing station are presented.

Intercomparison of Instruments used for Measurement of Cloud Drop Concentration and Size Distribution

Measurements of drop concentration and size distribution with the Clague droplet gun and the Forward Scattering Spectrometer Probe in 21 cumulus clouds on five days show no evidence of systematic differences between the two instruments. Though average drop concentrations and liquid water contents agree well within the estimated probable errors, values of these parameters in individual clouds occasionally show large discrepancies. Ways of reducing probable errors in the FSSP are discussed.

Some Factors Governing Ice Particle Multiplication in Cumulus Clouds

Abstract Field evidence of high ice crystal concentrations in cumulus clouds at temperatures warmer than −10°C is consistent with the operation of a Hallett-Mossop “multiplication” process. This mechanism requires the generation of large drops in the cloud below the −8°C level, which is governed in turn by the cloud base temperature and cloud drop concentration. These two parameters are shown to he useful in delineating a “multiplication boundary,” separating cloud conditions in which multiplication takes place from those in which it does not.

Another “Time Lag” in the Activation of Atmospheric Ice Nuclei

If most atmospheric ice nuclei activate by contacting or immersing themselves inside a water droplet, then ice nucleus cloud chambers may seriously underestimate the concentration of these particles. Ice nuclei diffuse toward cloud droplets due to Brownian and turbulent motion. In a natural cloud of 0.5 gm m−3 with a drop concentration of 1000 cm−3, ten times more particles 50–104 Å in radius enter the drops in 1 hr than collide in 2 min. In an ice nucleus counter with a similar cloud, a normal air sample residence time of under 2 min would not successfully model a stable atmospheric cloud, which might last several hours. In a specific time, the fraction collected by the above mechanisms increases with liquid water content and cloud drop concentration. Before ice nucleus counter measurements are used for theoretical computations, some attempt should be made to adjust the concentration to the parameters existing in the particular cloud under consideration. The relative proportion of nuclei which activate with or without collision with cloud drops should at least be estimated in the laboratory.