Incorporation of aerosol particles between 25 and 850 nm into cloud elements: measurements with a new complementary sampling system

The interpretation of the physico-chemical processes in clouds is facilitated by segregating in situ cloud elements from their carrier gas and small particles (interstitial aerosol). Thus, the present study focuses on the quantitative phase segregation of interstitial air from cloud phase by two complementary samplers with microphysical on-line analysis of the separated phases. An improved counterflow virtual impactor (CVI) was developed for the collection and subsequent evaporation of the condensed phase, releasing dissolved gaseous material and residual particles. This sampler operates in the size range of few micrometers up to 50 μm in cloud element diameter and is matched by an interstitial Round Jet Impactor sampling the gas phase with interstitial particles. Calibrations of both samplers verified the calculated cut sizes D₅₀ of 4, 5, and 6 μm and quantified the slope of the collection efficiency curves. Until this study no direct CVI measurements of the residual particle sizes far below the diameter of 0.1 μm were available. For the first time a CVI was connected to a Differential Mobility Particle Sizer (DMPS) scanning between 25 nm and 850 nm, thus, including the entire Aitken mode in the residual size analysis. Cloud studies on the Puy de Dôme, France, revealed residual particle sizes including Aitken mode (diameter D<100 nm) and accumulation mode (D>100 nm). A major feature of the CVI data is expressed by the fact that despite incomplete incorporation of accumulation mode particles in cloud elements there are contributions of particles with diameters smaller than 0.1 μm to the number of residual particles. Cloud entrainment from height levels above the maximum supersaturation as wells as the size-dependent chemical composition of the aerosol population most likely produced the S-shaped size-dependent partitioning of residual particles. Compared to earlier studies the 50% partitioning diameters dropped significantly below 100 nm to roughly 70 nm.     

The effect of clouds on aerosol growth in the rural atmosphere

Measurements of accumulation mode aerosol in the atmospheric boundary layer under cloudy and cloud-free conditions, and in the lower free troposphere under cloud-free conditions, were conducted over the rural northwest of England. Normalised size distributions in the cloud-free boundary layer (CFBL) and the cloud-free free troposphere (CFFT) exhibited almost identical spectral similarities with both size distributions possessing a concentration peak mode-radius of ≈0.05 μm or less. By comparison, aerosol distributions observed in cloudy air exhibited a distinctive log-normal distribution with mode-radii occurring at ≈0.1 μm concomitant with a local minimum at ≈0.05 μm. The consistent and noticeable difference in spectral features observed between cloudy and cloud-free conditions suggest that a greater amount of gas-to-particle conversion occurs on cloudy days, presumably through in-cloud aqueous phase oxidation processes, leading to larger sized accumulation mode particles. Apart from the distinct difference between cloudy and cloud-free aerosol spectra on cloudy days, aerosol concentration and mass were observed to be significantly enhanced above that of the ambient background in the vicinity of clouds. Volatility analysis during one case of cloud processing indicated an increase in the relative contribution of aerosol mass volatile at temperatures characteristic of sulphuric acid, along with a smaller fraction of more volatile material (possibly nitric acid and/or organic aerosol). Growth-law analysis of possible growth mechanisms point to aqueous phase oxidation of aerosol precursors in cloud droplets as being the only feasible mechanism capable of producing the observed growth. The effect of cloud processing is to alter the cloud condensation nuclei (CCN) supersaturation spectrum in a manner which increases the availability of CCN at lower cloud supersaturations.     

Comment on “Glory phenomenon informs of presence and phase state of liquid water in cold clouds” by Anatoly N. Nevzorov

In a recent publication “Glory phenomenon informs of presence and phase state of liquid water in cold clouds” Nevzorov [Nevzorov, A., 2006. Glory phenomenon informs of presence and phase state of liquid water in cold clouds. Atmospheric Research 82, 367–378] claims that “the convincing evidence has been provided that this sort of glory forms as a first-order bow from spherical particles with a refractive index of 1.81–1.82 and diameter over 20 μm”. This is a highly unusual finding because the refractive index of liquid water and ice is between 1.30 and 1.35 in the visible spectral range. The author concludes that “once more corroboration is gained […] of droplets of liquid water in specific phase state referred to amorphous water, or A-water”. Here we show that the phenomena described by the author are easily explained assuming liquid water with a refractive index of 1.33 and a realistic droplet size distribution with an effective radius of around 10 μm. We conclude that this type of observations does not corroborate the existence of amorphous water in the atmosphere. In a recent publication we showed how to quantitatively derive cloud optical thickness, effective droplet radius, and even the width of the size distribution from observations of the glory [Mayer, B., Schröder, M., Preusker, R., Schüller, L., 2004. Remote sensing of water cloud droplet size distributions using the backscatter glory: a case study. Atmospheric Chemistry and Physics 4, 1255–1263].     

The Cloud Ice Mountain Experiment (CIME) 1998: experiment overview and modelling of the microphysical processes during the seeding by isentropic gas expansion

The second field campaign of the Cloud Ice Mountain Experiment (CIME) project took place in February 1998 on the mountain Puy de Dôme in the centre of France. The content of residual aerosol particles, of H₂O₂ and NH₃ in cloud droplets was evaluated by evaporating the drops larger than 5 μm in a Counterflow Virtual Impactor (CVI) and by measuring the residual particle concentration and the released gas content. The same trace species were studied behind a round jet impactor for the complementary interstitial aerosol particles smaller than 5 μm diameter. In a second step of experiments, the ambient supercooled cloud was converted to a mixed phase cloud by seeding the cloud with ice particles by the gas release from pressurised gas bottles. A comparison between the physical and chemical characteristics of liquid drops and ice particles allows a study of the fate of the trace constituents during the presence of ice crystals in the cloud.In the present paper, an overview is given of the CIME 98 experiment and the instrumentation deployed. The meteorological situation during the experiment was analysed with the help of a cloud scale model. The microphysics processes and the behaviour of the scavenged aerosol particles before and during seeding are analysed with the detailed microphysical model ExMix. The simulation results agreed well with the observations and confirmed the assumption that the Bergeron–Findeisen process was dominating during seeding and was influencing the partitioning of aerosol particles between drops and ice crystals. The results of the CIME 98 experiment give an insight on microphysical changes, redistribution of aerosol particles and cloud chemistry during the Bergeron–Findeisen process when acting also in natural clouds.     

Ice crystal shape effects on solar radiative properties of Arctic mixed-phase clouds—Dependence on microphysical properties

Based on 1-year cloud measurements with radar and microwave radiometer broadband solar radiative transfer simulations were performed to quantify the impact of different ice crystal shapes of Arctic mixed-phase clouds on their radiative properties (reflectance, transmittance and absorptance). The ice crystal shape effects were investigated as a function of microphysical cloud properties (ice volume fraction f_i, ice and liquid water content IWC and LWC, mean particle diameter D_m^I and D_m^W of ice/water particle number size distributions, NSDs).The required NSDs were statistically derived from radar data. The NSD was composed of a liquid and a solid mode defined by LWC, D_m^W (water mode) and IWC, D_m^I (ice mode). It was found that the ratio of D_m^I and D_m^W determines the magnitude of the shape effect. For mixed-phase clouds with D_m^I ≤ 27 μm a significant shape effect was obtained. The shape effect was almost insensitive with regard to the solar zenith angle, but highly sensitive to the ice volume fraction of the mixed-phase cloud. For mixed-phase clouds containing small ice crystals (D_m^I ≤ 27 μm) and high ice volume fractions (f_i > 0.5) crystal shape is crucial. The largest shape effects were observed assuming aggregates and columns. If the IWC was conserved the shape effect reaches values up to 0.23 in cloud reflectance and transmittance. If the ice mode NSD was kept constant only a small shape effect was quantified (≤ 0.04).     

Contribution of the MODIS instrument to observations of deep convective storms and stratospheric moisture detection in GOES and MSG imagery

Past studies based on the NOAA/AVHRR and GOES I-M imager instruments have documented the link between certain storm top features referred to as the “cold-U/V” shape in the 10–12 μm IR band imagery and plumes of increased 3.7/3.9 μm band reflectivity. Later, similar features in the 3.7/3.9 μm band have been documented in the AVHRR/3 1.6 μm band imagery.The present work focuses on storm top observations utilizing the MODIS data. The MODIS instrument (available onboard NASA's EOS Terra and Aqua satellites) provides image data with significantly better geometrical resolution (in some of its bands) and broader range of spectral bands as compared to that from AVHRR/3 observations. One of the goals of this study is to evaluate the contribution of this new instrument to observations of convective storm tops. Besides the cloud top features linked to storm top microphysics and morphology, the paper also addresses the possibility of detection of lower stratospheric water vapor above cold convective storm tops. This issue is explored utilizing MODIS as well as GOES and MSG imagery.In addition, the paper discusses an alternative interpretation of the “cold-U/V” patterns at the top of intense storms by a mechanism of “plume masking” as suggested by some of the observations.     

Microphysical and optical features of polluted cooling tower clouds

In November 1993 an airborne field study was performed in order to investigate the microphysical and radiative properties of cooling tower water clouds initiated by water vapour emissions and polluted by the exhaust from coal-fired power plants. The number-median diameter of the droplet size distributions of these artificial clouds was in the range of 13 μm. The concentration of smaller droplets (diameters d_D < 10 μm) increased with height and horizontal distance from the cooling towers. Close to the cooling towers, bimodal spectra were found with a second mode at 19 μm. The liquid water content (LWC) ranged between 2 and 5 g/m³ and effective droplet radii (R_e) between 6 and 9 μm were measured. LWC and R_e decreased with altitude, whereas the droplet concentration (N_D) remained approximately constant (about 2000 cm⁻³ ). An enrichment of interstitial aerosol particles with particle diameters (d_p) smaller 0.2 μm compared to the power plant plume in the vicinity of the clouds was observed. Particle activation for d_m > 0.3 μm. was evident, especially in cooling tower clouds further apart and separated from their sources. Furthermore, radiation measurements were performed, which revealed differences in the vertical profiles of downwelling solar and UV radiation flux densities inside the clouds.The effective droplet radius R_e was parameterized in terms of LWC and N_D using equations known from literature. The close agreement between measured and parameterized Re indicates a similar coupling of R_e, LWC and N_D as in natural clouds.By means of Mie calculations, volume scattering coefficients and asymmetry factors are derived for both the cloud droplets and the aerosol particles. For the cloud droplets, the optical parameters were described by parameterizations from the literature. The results show, that the link between radiative and microphysical properties of natural clouds is not changed by the extreme pollution of the artificial clouds.     

An experimental study of aerosol scavenging by hexagonal plate ice crystals

The scavenging efficiency, E, of small hexagonal plate ice crystals for aerosol particles has been measured in a series of laboratory cloud experiments. The ice crystal diameters, D μm ranged from 35 to 150 μm with aerosol particles in the range 4 to 6 μm. An ice crystal replication technique made possible the individual examination of more than 43m500 individual crystals from which the relation: log₁₀ E = 1.554 − 1.047 log₁₀D was established with values of E in the range 0.2–0.9 corresponding to the range of crystal sizes investigated. For some crystal collectors, values of E extended above unity and this was attributed to wake capture, oscillations and the extended sweep out path of the ice crystals associated with their spiral fall pattern.     

An airborne two-stage impactor for chemical analysis of size-segregated cloud drops

Aqueous concentrations of ionic species observed in cloud water studies often have been in conflict with expectations from model predictions. These inconsistencies result from the size-dependent chemical composition of cloud drops during different stages in the lifetime of a cloud. To study this phenomenon, droplets of clouds need to be collected in different size ranges with high resolution in space and time. The only possibility for this kind of study is the use of an aircraft. Therefore, during the last several years, an attempt was made to develop a mobile cascade impactor, which can be installed outside an aircraft. The cloud water sampled in different size fractions can be transferred into the interior of the aircraft during the measuring flight. The collector is able to sample two size fractions. For continental clouds, the cutoffs are chosen to be >5 and >13.5 μm in diameter. For maritime clouds, the cutoff for the first stage could be shifted to 18.6 μm by lowering the nozzle speed. Prior to field application, the collector was characterized with the aid of “calibration fogs” produced in the laboratory with different drop sizes and different chemical compositions. The characterization included the examination of the cutoffs and the reliability of the sampling procedure with regard to the subsequent chemical analysis. With a collection period of 2 min, collection rates in the order of 0.1–1 cm³ min⁻¹ can be obtained. The collector characterized in this manner was successfully used during measuring flights in clouds over northern Germany. Preliminary concentrations of NH₄⁺, SO₄²⁻ and Cl⁻ found in the two size fractions of the cloud drops are presented.     

Changes of cloud water chemical composition in the Western Sudety Mountains, Poland

The changing chemical composition of cloud water and precipitation in the Western Sudety Mountains are discussed against the background of air-pollution changes in the Black Triangle since the 1980s until September 2004. A marked reduction of sulphur dioxide emissions between the early 1990's and the present (from almost 2 million tons to around 0.2 million tons) has been observed, with a substantial decline of sulphate and hydrogen concentration in cloud water (SO₄²⁻ from more than 200 to around 70 μmol l^− 1; H⁺ from 150 to 50 μmol l^− 1) and precipitation (SO₄²⁻ from around 80 to 20–30 μmol l^− 1; H⁺ from around 60 to 10–15 μmol l^− 1) samples. At some sites, where fog/cloud becomes the major source of pollutants, deposition hot spots are still observed where, for example, nitrogen deposition can exceed 20 times the relevant critical load. The results show that monitoring of cloud water chemistry can be a sensitive indicator of pollutant emissions.     

The effects of in-cloud mass production on atmospheric light scatter

Direct physical measurements of particle mass and number concentration indicate an increase in overall aerosol mass resulting from cloud processing, most likely through aqueous-phase chemistry (e.g., SO₂ oxidation). Measurements conducted in the Pennines of Northern England reveal an average increase of 14 to 20% in dry aerosol mass (0.003<particle diameter<0.9 μm) after aerosol passage through an orographic cloud. The rate of in-cloud mass production is most sensitive to changes in upwind particle size distributions, SO₂ concentration, and cloud water acidity. Newly-formed mass appears in size range between 200 and 600 nm and enhances the bimodality of the particle number distribution after cloud processing. Furthermore, the cloud-produced mass is estimated to increase total light scattering, b_sp, by 18 to 24%. The scattering efficiency of the dry, cloud-generated aerosol is 5.0±0.3 m² g⁻¹ and increases to 7.4±0.7 m² g⁻¹ when adjusted to 90% relative humidity by incorporating particle hygroscopicity data.     

Changes in scavenging of particles by droplets due to weak electrification in clouds

Electrical charges on aerosol particles and droplets modify the droplet–particle collision efficiencies involved in scavenging, and the droplet–droplet and particle–particle collision efficiencies involved in coalescence of droplets and particles, even in only weakly electrified clouds and aerosol layers. This work places electrically enhanced scavenging, and the electrical inhibition of scavenging in the context of the microphysics of weakly electrified clouds.Collision efficiencies are calculated by numerical integration to obtain particle trajectories, that are determined by the complex interplay of electrical, gravitational and phoretic forces together with inertia. These modify the trajectory of a particle as it is carried by flow around the falling droplet. Conversely, the flow around the particle also modifies the trajectory of the droplet. The flows are specified analytically, using a hybrid of the Proudman–Pearson stream function for that region close to the droplet or particle, where it is accurate, merging into the exact Oseen stream function for larger distances, where that becomes accurate. The effect of the flow around the particle on the motion of the droplet was simulated using Langmuir's superposition technique on the hybrid stream functions. The treatment of inertia in the present calculations allows an extension of the scope of our previous work by a factor of 10 larger in particle size (10³ in mass). The coverage is extended to a wide range of atmospheric conditions and particle densities.The pressures and temperatures used in the models ranged from a representation of the lower troposphere at  1 km altitude (900 hPa, 10 °C) to that of the middle stratosphere at  30 km altitude (12 hPa, − 47 °C). The particles considered range from 0.1 μm to 10 μm radius; the droplet radii range from 4 μm to 50 μm; particle densities range from 300 kg m^− 3 to 2500 kg m^− 3; particle charges range from 2e to 100e with droplet charges of like sign of 100e; and relative humidities range from 10% to 100%.For the larger particles (radii greater than about 3 μm) interacting with the larger droplets (radii greater than about 15 μm) the effects of inertia increase with particle density and dominate at the larger densities. For particles with radii in the range 1–3 μm the ‘Greenfield Gap’ of very low collision efficiencies was found, and was determined to be due to the effects of the gravitational force causing a reduction of collisions of particles with the front of the droplet, and the effect of inertia overcoming the tendency for the weight to produce a collision in the slow velocity region in the rear. When the electrical or phoretic forces are sufficiently large the Greenfield Gap is closed.When the particles have radii < 3 μm inertial effects no longer dominate the collisions, although inertia modifies the weight effects for particles with radii down to about 0.5 μm. For charged aerosol particles with radii smaller than about 0.1 μm interacting with droplets or background aerosol particles smaller than a radius of about 15 μm, the long range electrical repulsive force is effective in opposing the phoretic forces and keeping the particle out of range of the short range attractive image force. Thus ‘electroscavenging’ gives way to ‘electroprotection’ against the scavenging or coagulation processes otherwise caused by Browninan diffusion or phoretic forces.In an atmosphere of temperature 10 °C and pressure 900 hPa the net phoretic force reduces to zero and becomes repulsive for particles with radii above about 2 μm (depending on particle conductivity). This enhances the development of the Greenfield Gap. However, the value of this radius (at which the net phoretic force is zero) increases strongly with decreasing temperature and pressure (increasing altitude) as expected from theory, and is about 5 μm in the middle troposphere and more than 10 μm in the stratosphere. Thus a net attractive phoretic force on particles extends into the 1–3 μm radius range in the upper troposphere; however, the weight and inertial effects can ensure the presence of the Greenfield Gap in that range for 2000 kg m^− 3 particles up to the middle stratosphere.     

Changes of cloud microphysical properties during the transition from supercooled to mixed-phase conditions during CIME

A ground-based seeding experiment using carbon dioxide and propane sprayed from pressurized bottles was carried out under supercooled cloud conditions on a small spatial and short time scale. Water vapor deposition on the artificially generated dry ice and propane ice germs as the main ice formation process (nucleation and growth) is consistent with the experimental results. After nucleation, diffusional growth of the ice particles, partly at the expense of evaporating small droplets, was identified during the mixing of the seeding line with the ambient supercooled cloud. Within the seeding plume, ice water contents up to 80% of the total condensed water are observed, although the size of the formed ice particles did not exceed 25 μm. From the changes of the ice and supercooled liquid phase with time under mixed-phase conditions, liquid water content (LWC) evaporation, ice water content (IWC) formation, and ice crystal growth rates are estimated, which are not affected by the artificial nucleation process. Thus, these rates are assessed to be applicable for a growing ice phase of small ice particles in a young mixed-phase cloud, where other growth mechanisms, like riming or aggregation, are negligible.     

Large aerosol particles in cirrus type clouds

Research flights in November 1990 over the central parts of the United States, Wyoming and Colorado, were aimed to the investigation of the properties and microstructure of cirrus clouds (mainly cirrocumulus lenticularis). Among the other parameters measured on board the NCAR Saberliner were the concentration and size distribution of submicron particles and, in some cases, the particle deliquescence. For coarse insoluble particles found inside and outside of cloud elements, size distributions and morphology information were obtained by evaluating inertial impactor samples with an optical microscope and scanning electron microscope. In addition, the coarse particle composition was determined by x-ray energy spectrum analysis. The following conclusions from these measurements are:The large and coarse particle size distribution can be roughly simulated by a log-normal function with the modus around r=0.5 μm. Particle concentrations are very variable between several tenths and several particles per cm³. Particle volume distribution features a distinct maximum around 0.75 μm without a broad plateau which was observed in the case of sampling at lower altitude. Aerosol composition heterogeneity at cirrus cloud level is well documented by the evaluation of the fine particle sampling taken with the UMR sampling system. This heterogeneity can be partly explained by the interaction between aerosol and cloud elements, which is documented by the measured particle size distribution curves inside and outside of cloud elements. Assuming that particle deliquescence is caused by H₂SO₄ and/or by (NH₄)₂SO₄, particle soluble mass fractions were found to be around 30% in the first case and about 40% in the second. The most frequently occurring elements in large and coarse particles at cirrus cloud level were Si, Cl, Ba, S, Ca and C.     

Theoretical design and preliminary tests of two new particle spectrometers for cloud microphysics research

Reliable cloud microphysical measurements can lead to a better understanding of the development of precipitation and cloud radiation. However, imaging of cloud particles with sizes < 200 μm has been limited to small (collected) samples of high-quality photographs or large quantities of poor-quality digital images. Here we introduce two new instruments with demonstrated abilities for improved measurements of cloud drop spectra, liquid water content and digital imaging of the size and shape of small ice crystals. The first instrument uses digital holography to increase the sample volume over that possible with in-focus imaging of small particles. The digital holograms can be processed automatically and are expected to reveal the size and shape of small ice crystals, and the three-dimensional concentration, spacing and liquid water content of cloud drops. The second instrument measures cloud drop spectra and liquid water content from high-resolution measurements of forward-scattered light from an ensemble of drops. This eliminates inherent problems of coincidence and the small sample volume associated with single-drop measurements of drops. The theory of measurement and results from tests of both instruments are presented.     

Cloud contribution to the daily and annual radiation budget in a mountainous valley

An automated-ventilated radiation station has been set up in a mountainous valley at the Logan Airport in northern Utah, USA, since mid-1995, to evaluate the daily and annual radiation budget components, and develop an algorithm to study cloudiness and its contribution to the daily and annual radiation. This radiation station (composed of pyranometers, pyrgeometers and a net radiometer) provides continuous measurements of downward and upward shortwave, longwave and net radiation throughout the year. The surface temperature and pressure, the 2-m air temperature and humidity, precipitation, and wind at this station were also measured. A heated rain gauge provided precipitation information. Using air temperature and moisture and measured downward longwave (atmospheric) radiation, appropriate formula (among four approaches) was chosen for computation of cloudless-skies atmospheric emissivity. Considering the additional longwave radiation during the cloudy skies coming from the cloud in the waveband which the gaseous emission lacks (from 8–13 μm), an algorithm was developed which provides continuous 20-min cloud information (cloud base height, cloud base temperature, percent of skies covered by cloud, and cloud contribution to the radiation budget) over the area during day and night. On the partly-cloudy day of 3 February, 2003, for instance, cloud contributed 1.34 MJ m^− 2 d^− 1 out of 26.92 MJ m^− 2 d^− 1 to the daily atmospheric radiation. On the overcast day of 18 December, 2003, this contribution was 5.77 MJ m^− 2 d^− 1 out of 29.38 MJ m^− 2 d^− 1. The same contribution for the year 2003 amounted to 402.85 MJ m^− 2 y^− 1 out of 9976.08 MJ m^− 2 y^− 1. Observations (fog which yielded a zero cloud base height and satellite cloud imaging data) throughout the year confirmed the validity of the computed data. The nearby Bowen ratio station provided the downward radiation and net radiation data. If necessary, these data could be substituted for the missing data at the radiation station. While the automated surface observing systems (ASOS) ceilometer at the Logan airport provides only the overhead cloud information, the proposed algorithm provides this information over the valley. The proposed algorithm is a promising approach for evaluation of the cloud base temperature, cloud base height, percent of skies covered by cloud, and cloud contribution to the daily and annual radiation budget at local and regional scales.     

Cloud drop spectra at different levels and with respect to cloud thickness and rain

Measurements on drop size were made in cumulus clouds over Pune (inland region) during the summer monsoon seasons. In this paper, the measurements of the cloud drop spectra made in non-raining clouds at different levels and for different thickness have been studied. Also, those on the days with rain and with no rain (the rain being observed within the clouds) have been compared. The average spectra broadened with height. The concentration of drops >50 μm (N_L), liquid water content (LWC), mean volume diameter (MVD) and dispersion increased with height. The concentration of drops <20 μm (N_S) and total concentration (N_T) decreased with height. The spectra were broader, while N_S and N_T are smaller and the other parameters are greater for thicker clouds as compared to those for thinner. The spectra were broader, while N_S and N_T are smaller and the other parameters are greater on the days with rain with respect to those on the days with no rain. The distributions were bimodal at higher levels, for thicker clouds and on the days with rain, while they were unimodal at lower levels, for thinner clouds and on the days with no rain. The variations of the cloud drop spectra, preceding rain, at initial stage of rain and following rain are discussed.     

Use of meteosat data for validation of the diurnal variation of the outgoing longwave radiation produced by ERBE

Radiative transfer calculations have been performed to demonstrate the usefulness of the Meteosat observations in the relative narrow-band of the water vapour absorption (WV, 5.7–7.1 μm) in addition to the observations in the atmospheric infrared window (IR, 10.5–12.5 μm) to deduce the integrated thermal outgoing longwave radiation (OLR). A statistical analysis of colocated and nearly simultaneous Meteosat and the Earth Radiation Budget Experiment (ERBE) data has yielded regression coefficients for estimating the OLR with Meteosat data during the months of April and July 1985. These results have been used to study the mean diurnal variation of the outgoing longwave radiation. The results show that in some cases, because of inadequate time sampling, the form (and especially the phase) of the longwave (LW) diurnal cycle is incorrectly determined by ERBE, but that Meteosat data can improve the determination. In nearly all cases, such errors have little or no influence on the determination of monthly mean LW flux fields.     

The microstructure of selected, small, isolated, cumulus clouds near Red Deer, Alberta

Physical experiments designed to explore the potential of rain augmentation through airborne glaciogenic seeding on small, isolated non-precipitating cumuliform clouds near Red Deer, Alberta were carried out during the period 1982–1985. The microstructure of 90 cumulus congestus clouds have been documented through repeated in-situ sampling using a cloud physics instrumented aircraft platform. Observations from the inspection passes of 57 clouds seeded with either dry ice pellets or silver iodide pyrotechnics, and all the passes of 33 natural clouds are presented.Measurements of the cloud droplet concentration indicate that Alberta cumulus clouds are typically continental in nature, with an average droplet concentration of 535 cm⁻³ and an average droplet diameter of 10.6 μm. Alberta clouds have average liquid water contents of 0.57 g m⁻³, with a peak 1-sec value of 3.17 g m⁻³. The 1-km average liquid water contents are 0.83 g m⁻³, with a peak value of 2.81 g m⁻³. Cloud lifetimes vary between 11 and 20 minutes. Concentrations of naturally occurring ice crystals are found to be low. The average maximum 1-km ice concentration was 31⁻¹, and the peak 1-km concentration was 73.11⁻¹ in the natural cloud dataset. Evidence of precipitation-sized particles was detected in 21% (7 of 33) of the clouds, and precipitation below cloud base was detected in 6% (2 of 33) of the clouds.A comparison of the Alberta cloud characteristics to the cumulus clouds from different locations showed that there are some distinct differences between Alberta clouds and the clouds from the other regions.     

A process oriented characterization of tropical oceanic clouds for climate model evaluation, based on a statistical analysis of daytime A-train observations

This paper aims at characterizing how different key cloud properties (cloud fraction, cloud vertical distribution, cloud reflectance, a surrogate of the cloud optical depth) vary as a function of the others over the tropical oceans. The correlations between the different cloud properties are built from 2?years of collocated A-train observations (CALIPSO-GOCCP and MODIS) at a scale close to cloud processes; it results in a characterization of the physical processes in tropical clouds, that can be used to better understand cloud behaviors, and constitute a powerful tool to develop and evaluate cloud parameterizations in climate models. First, we examine a case study of shallow cumulus cloud observed simultaneously by the two sensors (CALIPSO, MODIS), and develop a methodology that allows to build global scale statistics by keeping the separation between clear and cloudy areas at the pixel level (250, 330?m). Then we build statistical instantaneous relationships between the cloud cover, the cloud vertical distribution and the cloud reflectance. The vertical cloud distribution indicates that the optically thin clouds (optical thickness <1.5) dominate the boundary layer over the trade wind regions. Optically thick clouds (optical thickness >3.4) are composed of high and mid-level clouds associated with deep convection along the ITCZ and SPCZ and over the warm pool, and by stratocumulus low level clouds located along the East coast of tropical oceans. The cloud properties are analyzed as a function of the large scale circulation regime. Optically thick high clouds are dominant in convective regions (CF?>?80?%), while low level clouds with low optical thickness (<3.5) are present in regimes of subsidence but in convective regimes as well, associated principally to low cloud fractions (CF?<?50?%). A focus on low-level clouds allows us to quantify how the cloud optical depth increases with cloud top altitude and with cloud fraction.