Investigation of the characteristics of the reflectivity of clouds

This paper investigates some of the reflectivity characteristics that clouds (when modelled as solid bodies) must exhibit to be compatible with observations that the reflecting surface of a cloud (i) appears almost equally bright across its face, (ii) is brightest when the cloud is opposite to the Sun but decreases in brightness as the cloud moves to other positions and (iii) increases in brightness with increasing optical thickness of the cloud in the observer's line of sight. These observations, respectively, are shown to imply that the peak value of the bidirectional total reflectivity from a cloud surface (i) increases in inverse proportion to the cosine of the angle between the Sun and the normal to the cloud surface, as the incident angle increases, (ii) appears to be directed back in the direction of the incident radiation, and (iii) increases as optical thickness of the cloud in the observer's line of sight increases. The results could have application in many fields (e.g. modelling diffuse radiance distributions for cloudy skies).     

Measurements of the scavenging of sulfate and nitrate in clouds

Measurements are presented on the scavenging of sulfate and nitrate by cumulus, stratus and strato-cumulus clouds. Assuming that all of the particulate sulfate was in the size range 0.1–1.0 μm radius and that sulfate was scavenged with the same efficiency as sub-micrometer particles in general, the nucleation scavenging coefficient of sulfate in these clouds was deduced to be 0.7 ±0.2 and evidence for sulfate production (1.0±0.3 μgm⁻³) within cloud water was also obtained. Evidence for nitrate scavenging, by nitrates serving as cloud condensation nuclei or by the absorption of HNO₃ by cloud droplets, is also presented. The data suggest that either gaseous nitrogen compounds in the air other than HNO₃ can dissolve and contribute to the nitrate concentration in cloud water or that nitrate can be produced within cloud droplets.     

Cloud condensation nuclei associated with arctic haze

Measurements are reported of the cloud condensation nucleus spectrum of Arctic haze from which it has been possible to deduce that a large fraction of the cloud active nuclei are soluble salts. On the basis of these findings, maximum supersaturation during the formation of Arctic stratiform clouds is expected to be around a third of a percent (for updrafts of 10 cm s⁻¹). Soluble nuclei down to 3–4 × 10⁻⁶ radius would be nucleated under these conditions. These inferences suggest that clouds forming in polluted Arctic air may contain relatively small (i.e. 10–30 cm⁻³) cloud droplet concentrations.     

A model study of smoke-haze influence on clouds and warm precipitation formation in Indonesia 1997/1998

In the last few decades, fire and smoke-haze occurrence increased in Indonesia by intentionally set land clearing fires and higher fire susceptibility of disturbed forests. Particularly, during El Niño years with prolonged droughts in Indonesia, land clearing fires become uncontrolled wildfires and produce large amounts of gaseous and particulate emissions. This paper investigates the influence of smoke-haze aerosols from such fires on clouds and precipitation over Indonesia during the El Niño event 1997/1998 by numerical modelling. Warm precipitation formation in both layered and convective clouds is calculated dependent on the atmospheric aerosol concentration. In the smoke-haze affected regions of Indonesia, aerosol–cloud interactions induce events with both precipitation suppression and increase compared to a reference simulation without aerosol–cloud interactions. The effect of precipitation suppression is found to dominate with about 2/3 of all precipitation modification events pointing to a prolongation of smoke-haze episodes. The corresponding convective cloud top height of shallow clouds is increased whereas distinct lower deep convective cloud top heights are found. The remaining about 1/3 events are characterised by increased precipitation and cloud liquid water content, accompanied by lower convective cloud top heights of shallow clouds and higher deep convective clouds.     

Air parcel model simulations of a convective cloud: Bacteria acting as immersion ice nuclei

The effects of bacteria acting as immersion ice nuclei were investigated in numerical sensitivity studies and compared to the efforts of other ice nuclei such as mineral dust and soot particles. An adiabatic air parcel model was employed simulating convective situations with different initial aerosol particle distributions. The maximum fractions of active ice nuclei were based on field measurements of the proportioning of atmospheric aerosol particle types in continental and marine air masses. Recent field measurements of bacteria concentrations in cloud water and in snow samples were used. From the concentrations in bulk samples the concentration in mean sized cloud droplets was estimated. Immersion freezing was described based on laboratory measurements to constrain the freezing fraction versus temperature. The results indicated that the effects of diminutive amounts of bacteria on ice formation in convective clouds, while being significantly less than the effects of mineral dust particles, might be comparable to the expected effects of soot particles acting as ice nuclei. It can be predicted that bacterial ice nuclei would have to be enriched by at least 10⁴ times reported concentrations in cloud water in order to equate to the impact of mineral dust ice nuclei present in 20–25% of all cloud droplets.     

A preliminary study of the chemical modification of cloud condensation nuclei in stratiform clouds at Whiteface Mountain,New York

Results are presented for pH, nitrate and sulfate ion content of cloud water collected at Whiteface Mountain in northern New York. Evidence exists for the production of sulfate in possibly one of the case studies and for nitrate production in all three of the case studies examined. The results are not entirely consistent with the hypothesis that chemical modification of cloud condensation nuclei (CCN) takes place in stratiform clouds. For only one case is CCN production evident and total aerosol number concentration is enhanced.The measured nitrate production cannot be accounted for by the soluble fraction of the CCN aerosol and may result from the absorption of interstitial HNO₃(g). This might account for the change in cloud condensation nuclei, indicated by the results, when sulfate production was not evident. The CCN spectrum can also be altered by agglomeration of captured particles upon cloud droplet evaporation. It is not possible to separate the two mechanisms (chemical vs physical) at this time.     

Chemical Dynamics of Clouds at Mt. Mitchell,North Carolina

The role of clouds as the primary pathway for deposition of air pollutants into ecosystems has recently acquired much attention. Moreover, the acidity of clouds is highly variable over short periods of time. Cloud water collections were made at Mt. Mitchell State Park, North Carolina, using a real-time cloud and rain acidity/ conductivity (CRAC) analyzer during May to September 1987, 1988 and 1989 in an effort to explore extremes of chemical exposure. On the average, the mountain peak was exposed to cloud episodes about 70 percent of experimental days. The lowest pH of cloud water in nearly real-time (～10 min.) samples was 2.4, while that in hourly integrated samples was 2.6. The cloud pH during short cloud events (mean pH 3.1), whjch results from the orographic lifting mechanism, was lower than that during long cloud events (mean pH 3.5), which are associated with mesoscale or synoptic atmospheric disturbances. On the average, the pH values in nonprecipitating cloud events were about 0.4 pH unit lower than those in precipitating cloud events. Sulfate, nitrate, ammonium and hydrogen ions were found to be the major constituents of cloud water, and these accounted for -90 percent of the ionic concentration. Total ionic concentrations were found to be much higher in non-precipitating clouds (670-3,010 μeq/L) than those in precipitating clouds (220-370 μeq/L). At low acidity, ionic balance is sometimes not obtained. It is suggested that organic acids may provide this balance.

The profile of cloud water ionic concentration versus time was frequently observed to show decrease at the beginning and rising toward the end during short cloud events. Before the dissipation of clouds, a decrease in cloud water pH and an increase in ionic concentration were found. At the same time, temperature and solar radiation increased, and relative humidity and microphysical parameters (liquid water content, average droplet size, and droplet concentration) decreased. These observations suggest that evaporative dissipation of cloud droplets leads to acidification of cloud water. Mean pH of cloud water was 3.4 when the prevailing wind was from the northwest direction, and it was 3.9 when the wind was from the west direction. The effects of variations in cloud liquid water content have been separated from variations in pre-cloud pollutant concentrations to determine the relationship between source intensity and cloud water concentrations.     

Effect of different types of clouds on surface UV-B and total solar irradiance at southern mid-latitudes: CMF determinations at Córdoba,Argentina

The effect of clouds on total and UV-B irradiance in Córdoba, Argentina, was studied employing the TUV 4.1 model and measurements obtained with YES UVB-1 and YES TSP-700 radiometers, and a spectral radiometer Ocean Optics USB-4000. The experimental measurements were selected from a 10 years dataset (1999–2008). Clouds were classified by direct observation as cirrus, cumulus, and stratocumulus. The broadband Cloud Modification Factors (CMFs) have been calculated in the range of the total and the UV-B radiation for these types of clouds. The relations between them were analyzed for a significant number of days. The broadband CMF values range from around 0.1 up to 1.25, depending on the wavelength interval and on the cloud type. The CMF_UVB versus CMF_T plots for different clouds have shown good adjustments and significant differences, which allows the distinction between them.Stratocumulus clouds show large attenuations and a linear relation with larger slopes as the solar zenith angle (SZA) increases. For this type of clouds an average slope of (1.0 ± 0.2) was found. The relation between the CMF for cumulus clouds is linear with an average slope of (0.61 ± 0.01). No dependence with the SZA was observed. Cirrus clouds plots show an exponential behavior with fit parameters equal to (0.48 ± 0.08) and (0.68 ± 0.15). However, when small SZA intervals are analyzed a linear relation is found. When the relations between the CMF were similar (cumulus and cirrus), the spectral variation in the UV range (320–420 nm) of a modified CMF (CMF_m) was used to distinguish them. Hence, the spectral differences among the three types of clouds have been also analyzed for several days and SZA. Here, it was found that the effect of cirrus is essentially wavelength independent while cumulus and stratocumulus clouds show exponential decay relations but with different ordinates.In the analyzed relations the microphysical properties of the clouds seem to determine its behavior while the optical thickness leads to the different degrees of attenuation.The results obtained in this work are in agreement with those found for other authors.     

Non-dissipative cloud transport in Eulerian grid models by the volume-of-fluid (VOF) method

The formation of clouds is coupled to the vapour saturation condition. Cloud modelling is therefore dramatically disturbed by dilution processes, which are induced by recurrent interpolations on the fixed (Eulerian) grid. The numerical diffusion gives rise to degeneration and premature disappearance of the modelled clouds. The difficulties increase, if sectional mass representation in the drop microphysics and aerosol chemistry is considered. To tackle this problem, stringently defined and tracked phase boundaries are required.The numerical diffusion of clouds can be totally suppressed by the volume-of-fluid (VOF) method, which is applied here in connection with an atmospheric model. The cloud phase is distinguished by prognosing the partial cloud volume in all grid cells near the cloud boundary. Adopting elementary geometrical forms for the intracellular cloud volume and simple diagnostic rules of their alignment, the standard transport fluxes can be used in the new equation. Separate variables for the cloud and environmental phase complete the transport scheme.The VOF method and its realisation are described in detail. Advection, condensation, evaporation, and turbulent diffusion are considered within the VOF framework. The variation of the grid resolution and turbulence conditions for a rising thermal leads to striking arguments in favour of the VOF method, resulting in higher intensity, lifting, and lifetime as well as clear boundaries of the simulated clouds (even for low grid resolution).     

The Rossby Centre Regional Atmospheric Climate Model part II: application to the Arctic climate

The Rossby Centre regional climate model (RCA2) has been integrated over the Arctic Ocean as part of the international ARCMIP project. Results have been compared to observations derived from the SHEBA data set. The standard RCA2 model overpredicts cloud cover and downwelling longwave radiation, during the Arctic winter. This error was improved by introducing a new cloud parameterization, which significantly improves the annual cycle of cloud cover. Compensating biases between clear sky downwelling longwave radiation and longwave radiation emitted from cloud base were identified. Modifications have been introduced to the model radiation scheme that more accurately treat solar radiation interaction with ice crystals. This leads to a more realistic representation of cloud-solar radiation interaction. The clear sky portion of the model radiation code transmits too much solar radiation through the atmosphere, producing a positive bias at the top of the frequent boundary layer clouds. A realistic treatment of the temporally evolving albedo, of both sea-ice and snow, appears crucial for an accurate simulation of the net surface energy budget. Likewise, inclusion of a prognostic snow-surface temperature seems necessary, to accurately simulate near-surface thermodynamic processes in the Arctic.     

A model of sulphate production in a cap cloud and subsequent turbulent deposition onto the hill surface

A model has been constructed of the dynamics and microphysics of a hill cap cloud. This has been used to investigate the aqueous phase oxidation of SO₂ in the cloud droplets and the subsequent turbulent deposition of chemical species onto the hill surface. It is suggested that the dominant oxidant is H₂O₂ in these clouds and that therefore the process is likely to be oxidant limited. The amount of sulphate produced is comparable to that found in cloud condensation nuclei typically found over the U.K. and elsewhere away from strong local sources of sulphate aerosol. Ammonia concentrations are very important as they alter the cloud water pH and hence the solubility of SO₂.Turbulent or ‘occult’ deposition is very sensitive to wind speed, the stability profile of the atmosphere and to the surface roughness. In a supercritical flow regime the occult deposition is a maximum just on the lee of the hill.     

Sources of hydrogen peroxide in cloudwater

Results of a theoretical investigation of H₂O₂ formation in cloud droplets arising from gaseous HO₂ radical scavenging are presented. It is shown that this process is pH dependent with the maximum rate of H₂O₂ production occurring below pH 3. This dependence arises as a result of the dissociation of HO₂ in water (pK_a = 4.9) and the subsequent disproportionation reaction of HO₂ and O₂⁻ to form hydrogen peroxide. O₂⁻ is also removed by reaction with O₃ to produce OH radicals and this process becomes more competitive as both the pH and $\text{O}{}_2{}^{\mathrm{-}}\text{HO}{}_2$ ratio increase. The presence of soluble organic species, such as aldehydes, in cloudwater counteracts the effect of ozone by converting OH back to HO₂. For low pHs (< 3) the net contribution of organic solutes of H₂O₂ production is predicted to be relatively small, being limited by the availability of OH radicals scavenged from the gas phase. Existing cloud chemistry models may overestimate the rate of aqueous oxidation of formaldehyde by OH radicals.Under conditions where scavenging of gas-phase free radicals by cloud droplets is efficient, uptake of HO₂ radicals may be reversible. The aqueous concentration of OH is unlikely to approach thermodynamic equilibrium with the gas phase (H ∼-30 M atm⁻¹ and can be treated as irreversible. In clouds with a small mean droplet radius, efficient scavenging of precursor OH radicals should result in a decrease in gas-phase HO₂ production with a reduction in the yield of aqueous H₂O₂, although this is offset by the presence of soluble organic species. A similar effect is predicted for clouds with a high liquid water content.The supply of HO₂ and OH radicals to cloud droplets is controlled by gas-phase ozone chemistry which is in turn dependent on the solar u.v. radiation intensity. The u.v. density in clouds may be higher than in clear air when the solar zenith angle is small, thus enhancing H₂O₂ production, but falls off markedly as the solar zenith angle becomes larger. Predicted rates of H₂O₂ formation in clouds based on midday conditions are likely to be considerably higher than the average daytime value, particularly in summer. Diurnal and seasonal effects on H₂O₂ generation are expected to be more marked in clouds than in clear air.     

Atomization and Cloud Behavior In Venturi Scrubbing

The exact collection mechanism of a venturi scrubber has been unknown up to this time. Photographic stop-action techniques and glass venturi scrubbers have made it possible to establish where and how particles are captured and to speculate on possible gas removal possibilities. This report extends the knowledge of pneumatic atomization which is used in gas scrubbing and many other applications by providing further information on cloud-type atomization. Cloud-type atomization which is produced by pneumatic atomization of liquid streams (not drops) results in the formation of liquid droplets which appear to be less than 10 microns in diameter. These droplets coalesce and form clouds which move as single entities. Effective overall cloud diameters are determined to be a function of the velocity of the atomizing gas stream. The effective cloud diameters start at 170 [a and increase as throat gas velocities increase from 150 ft/sec. Throat velocities and liquid inlet nozzle diameters necessary to obtain water clouds of specific effective diameters can be estimated.

These large clouds are efficient impaction targets and stop most of the particulate matter within 0.5 cm from the throat scrubbing liquid inlets. High gas absorption is expected for the clouds of droplets because turbulent gas movement can exist inside and outside the clouds and the 10 μ droplets provide exceptional surface area.     

The Evolution of Visibility Deterioration and Aerosol Spectra in the St. Louis Urban Plume

Under the auspices of Project METROMEX, studies of visibility de-teoration downwind of St. Louis were conducted during July-August 1974-1975. Estimates of horizontal visual range, standard meteorological data, and aerosol characteristics within the mixing layer were acquired upwind, over, and downwind of the metropolitan area by means of airborne transects. Aerosol number, surface, and volume distributions for particles between 0.025-2.5 µm were generated from the airborne measurement of Aitken nucleus concentrations, cloud condensation nuclei, and aerosols detected in situ with optical probes. Visibility reduction amounting to 50% of prevailing regional upwind visibilities consistently occurs at a distance corresponding to 2-3 hours travel time downwind for an air parcel moving with the mean transport wind. The regions of visibility minimum do not coincide with locations of maximum Aitken nucleus concentrations, but rather correspond in space and time to increased values of cloud condensation nuclei and increased numbers of particles in the 0.1-2.5 µm diameter range. Comparisons of observed aerosol evolution with similar laboratory studies suggest that most of the light scattering aerosols are of secondary origin.     

Attenuation of UV-B radiation in the atmosphere: Clouds effect,at Qena (Egypt)

The effects of clouds (amount, type and height) on the surface UV-B radiation have been investigated at Qena, Egypt (26°17′, 32°10′, 96 m asl) using 2 years data (2004–2005) carried out by South Valley University (SVU)-meteorological research station. Thus, the characteristics of cloud's statistical property during the study period were employed to evaluate the general feature of the region of this study. However, ≈86% of all the observations were ⩽2 octas and the overcast conditions (8 octas) were very rarely over the study region (only 0.2% of all cases). These observations included 10% low-level clouds, 3.16% mid-level clouds and 7.59% high-level clouds. The dominated types of these clouds are stratocumulus (8.9%) and cirrus (5.8%).The hourly values of cloudless sky UV-B radiation (UV-B₀) and consequently the cloud modification factor (CMF) were estimated. An empirical model was developed for CMF as a function of the amount of cloud at low- and mid-level and high-level clouds. The correlation coefficients were equal to 0.985 and 0.987, respectively. In addition, a general expression of the CMF for situations those are considered as the effect of different clouds was found. The efficiency of this model has been tested in combination with a cloudless sky empirical model using independent data set. For this purpose, the hourly values of UV-B at selected cloudless and cloudy days were estimated. A good agreement was observed between the measured and the predicted values of our model. The mean value of the correlation coefficients of these selected days was 0.98.In addition, the attenuation of UV-B radiation could be determined by considering low- and mid-level and high-level clouds. The reduction of UV-B radiation as a function of cloud amount was non-linear for the both cases. At cloud amount of 100%, UV-B radiation was reduced by 83% on average by the high-level clouds.     

Cumulus cloud transport,scavenging and chemistry: Observations and simulations

Observational and numerical investigations of cumulus cloud scavenging, transport and chemical processes are presented. The experimental data set includes surface and aircraft measurements of the chemistry and microphysics of aerosol, cloud and precipitation. To help in the interpretation of these experimental data fully three-dimensional simulations of cloud chemistry and scavenging are performed. After adjusting several unmeasured model parameters, reasonable agreement could be obtained between the simulated and observed cloud chemistry and aerosol distribution in clouds. The rate at which the simulated clouds transported and transformed pollutants did not exceed a few per cent per hour.     

Simulating chemistry–aerosol–cloud–radiation–climate feedbacks over the continental U.S. using the online-coupled Weather Research Forecasting Model with chemistry (WRF/Chem)

The chemistry–aerosol–cloud–radiation–climate feedbacks are simulated using WRF/Chem over the continental U.S. in January and July 2001. Aerosols can reduce incoming solar radiation by up to ?9% in January and ?16% in July and 2-m temperatures by up to 0.16 °C in January and 0.37 °C in July over most of the continental U.S. The NO₂ photolysis rates decrease in July by up to ?8% over the central and eastern U.S. where aerosol concentrations are high but increase by up to 7% over the western U.S. in July and up to 13% over the entire domain in January. Planetary boundary layer (PBL) height reduces by up to ?23% in January and ?24% in July. Temperatures and wind speeds in July in big cities such as Atlanta and New York City reduce at/near surface but increase at higher altitudes. The changes in PBL height, temperatures, and wind speed indicate a more stable atmospheric stability of the PBL and further exacerbate air pollution over areas where air pollution is already severe. Aerosols can increase cloud optical depths in big cities in July, and can lead to 500–5000 cm^?3 cloud condensation nuclei (CCN) at a supersaturation of 1% over most land areas and 10–500 cm^?3 CCN over ocean in both months with higher values over most areas in July than in January, particularly in the eastern U.S. The total column cloud droplet number concentrations are up to 4.9 × 10⁶ cm^?2 in January and up to 11.8 × 10⁶ cm^?2 in July, with higher values over regions with high CCN concentrations and sufficient cloud coverage. Aerosols can reduce daily precipitation by up to 1.1 mm day^?1 in January and 19.4 mm day^?1 in July thus the wet removal rates over most of the land areas due to the formation of small CCNs, but they can increase precipitation over regions with the formation of large/giant CCN. These results indicate potential importance of the aerosol feedbacks and an urgent need for their accurate representations in current atmospheric models to reduce uncertainties associated with climate change predictions.     

Impact of clouds and aerosols on ozone production in Southeast Texas

A radiative transfer model and photochemical box model are used to examine the effects of clouds and aerosols on actinic flux and photolysis rates, and the impacts of changes in photolysis rates on ozone production and destruction rates in a polluted urban environment like Houston, Texas. During the TexAQS-II Radical and Aerosol Measurement Project the combined cloud and aerosol effects reduced j(NO₂) photolysis frequencies by nominally 17%, while aerosols reduced j(NO₂) by 3% on six clear sky days. Reductions in actinic flux due to attenuation by clouds and aerosols correspond to reduced net ozone formation rates with a nearly one-to-one relationship. The overall reduction in the net ozone production rate due to reductions in photolysis rates by clouds and aerosols was approximately 8 ppbv h^?1.     

Chemical climatology of high elevation spruce-fir forests in the southern Appalachian mountains

The physical and chemical climatology of high elevation (> 1500 m) spruce-fir forests in the southern Appalachian mountains was studied by establishing a weather and atmospheric chemical observatory at Mt Mitchell State Park in North Carolina (35 degrees 44' 05" N, 82 degrees 17' 15"W). Data collected during the summer and autumn (May-October) of 1986, 1987, and 1988 are reported. All measurements were made on or near a 16.5 m walk-up tower extending 10 m above the forest canopy on Mt Gibbes (2006 m msl), which is located approximately 2 km SW of Mt Mitchell. The tower was equipped with standard meteorological instrumentation, a passive cloud water collector, and gas pollutant sensors for O3, SO2, NOx. The tower and nearby forest canopy were immersed in clouds 25 to 40% of the time. Non-precipitating clouds were very acidic (pH 2.5-4.5). Precipitating clouds were less acidic (pH 3.5-5.5). The dominant wind directions were WNW and ESE. Clouds from the most common wind direction (WNW) were more acidic (mean pH 3.5) than those from the next most common wind direction (ESE, mean pH 5.5). Cloud water acidity was related to the concentration of SO4(2-), and NO3- ions. Mean concentration of H+, NH4+, SO4(2-), and NO3- ions in the cloud water varied from 330-340, 150-200, 190-200 and 120-140 micromol litre(-1) respectively. The average and range of O3 were 50 (25-100) ppbv (109) in 1986, 51 (26-102) ppbv in 1987, and 66 (30-140) during the 1988 field seasons, respectively. The daily maximum, 1-h average, and 24-h average concentrations were all greatest during June through mid-August, suggesting a correlation with the seasonal temperature and solar intensity. Throughfall collectors near the tower were used to obtain a useful estimate of deposition to the forest canopy. Between 50-60% of the total deposition of SO4(2-) was due to cloud impact.