High altitude clouds such as Cirrus have a substantial impact on the global radiation budget and their climatic impact depends on their micro-structure. Clouds composed of small crystals with effective radii less than 16 Pm have a cooling effect, whilst clouds made up of larger crystals have a warming effect (Lynch et al., 2002; p 397). The success of cirrus microphysical modelling depends largely on the right choice of microphysical parameters and the diffusivity of water vapour (D,) is one such crucial parameter. In this study we have explored the sensitivity of cirrus microphysics to the specification of D, in large eddy model (LEM) simulations of cirrus case studies. We analysed important processes where vapour diffusion plays a role in the evolving microphysics. Cirrus clouds form at high altitudes where the molecular mean free path and hence D, is significantly larger than the ground level value. To date some LEMs (e.g. the UK Met Office LEM) do not consider the height-dependence of the diffusivity of water vapour. Although this may not pose much of a problem for warm clouds formed in the lower boundary layer, for high altitude cirrus clouds, using ground level values could affect the microphysical development. In this study we have shown that crystal growth rates, the ice water mixing ratios, crystal number concentrations, auto-conversion rates of ice particles to form aggregates, the deposition rates of water vapour on aggregates, and the long-wave radiative cooling rates depend sensitively on the choice of the diffusivity of water vapour. The most widely used empirical formulation on the height-dependence of the diffusion coefficient is that described by Pruppacher and Klett (1997) and is valid over a temperature regime between -40 degrees C and 40 degrees C. Since many cirrus clouds form at temperatures colder than -40 degrees C, it becomes imperative to use sophisticated formulations for an accurate prescription of D, for cirrus studies. In this first LEM study we have used a Lennard-Jones (L-J) model to estimate D, and applied it to two cirrus case studies. This formulation is accurate and valid over cirrus forming altitudes and is effective even when temperatures are colder than -40 degrees C. First, we have shown that the L-J model can be easily adapted within LEMs to study cirrus clouds and thence we examined the resulting microphysics through simulations with and without the L-J update. We observed that the overall microphysical development was sensitive to the choice of the diffusion coefficient of water vapour. We believe that this study will aid cirrus modellers worldwide who are often constrained by the availability of microphysical observational data. Copyright (C) 2007 Royal Meteorological Society.

From theoretical, numerical and experimental studies of small inertial particles with density equal to β (>1) times that of the fluid, it is shown that such particles are ‘centrifuged’ out of vortices and eddies in turbulence. Thus, in the presence of gravitational acceleration g , their average sedimentation velocity V T in a size range just below a critical radius a cr is increased significantly by up to about 80%. We show that in fully developed turbulence, a cr is determined by the circulation Γ k of the smallest Kolmogorov micro-scale eddies, but is approximately independent of the rate of turbulent energy dissipation ϵ , because Γ k is about equal to the kinematic viscosity ν . It is shown that a cr varies approximately like and is about 20 μm (±2 μm) for water droplets in most types of cloud. New calculations are presented to show how this phenomena causes higher collision rates between these ‘large’ droplets and those that are smaller than a cr , leading to rapid growth rates of droplets above this critical radius. Calculations of the resulting droplet size spectra in cloud turbulence are in good agreement with experimental data. The analysis, which explains why cloud droplets can grow rapidly from 20 to 80 μm irrespective of the level of cloud turbulence is also applicable where a cr ∼1 mm for typical sand/mud particles. This mechanism, associated with unequal droplet/particle sizes is not dependant on higher particle concentration around vortices and the results differ quantitatively and physically from theories based on this hypothesis.

Any population of cloud droplets forming on polydisperse condensation nuclei is thermodynamically unstable. There is no value of the supersaturation for which the growth rate of all the droplets is zero, so that if some droplets are in equilibrium, then some must have positive and some negative growth rates. Droplets with positive growth rates will continue to grow at the expense of those with negative growth rates. This effect has been termed the ripening process, and has been postulated to be a potential mechanism to explain broad droplet size distributions in stratiform clouds. In this paper multiple parcel trajectories are used, derived using a simple representation of the turbulent dynamics, to examine the time evolution of the droplet size distribution in a nonentraining stratiform cloud. It is shown that the magnitude of the effect is critically dependent upon the mean parcel in-cloud residence time. The simulations suggest that, for a stratiform clouds of h=400 m thickness, and a vertical wind standard deviation of sigma(w) = 0.6 m s(-1) (typical for stratocumulus clouds in a fairly vigorous, well-mixed boundary layer), the ripening effect is negligible, in that the droplet size distribution changes little with time. However, clouds with-low sigma(w) = 0.2 m s(-1) (typical of weaker strains clouds) show a marked spectral ripening effect over a period of several hours. Ripening is observed in the numerical model in both clean and polluted aerosol distributions. Autoconversion rates calculated from the droplet size distributions increase markedly with time as ripening takes place. It is suggested that to accurately model droplet size distributions in stratus cloud, it may be necessary to take into account the distribution of in-cloud parcel residence time.

Recent research on the interaction of inertial particles with turbulent vortices show that inertial bias causes particles to settle faster in turbulence than in still air as a consequence of particle accumulation in downward fluid velocity regions [Hainaux et al., 2000; Davila and Hunt 2001; Fevrier et al., 2000]. To date, these effects have not been included in models of droplet collection growth. In addition earlier models have also neglected the dependence of the collection growth on the radius of the smaller collected drops assuming that the collected drops are stationary. We have considered all these effects and have still been able to solve the collection equation analytically and the earlier Baker[1993] results are retrievable as a special case when these effects are turned off. The results presented in this study are in closed form and are therefore extremely attractive for precipitation parametrizations in large scale models.

A Monte Carlo model is used to calculate the short-wave reflectance of cloud fields in which the liquid water content varies in space. It is shown that, even in unbroken cloud layers of constant geometrical thickness, variations of liquid water content on horizontal scales of a few hundred meters significantly reduce the short-wave reflectance of the cloud layer. The droplet concentration has been varied and the sensitivity of the cloud reflectance to these changes has been calculated. It is shown that the sensitivity of the reflectance of broken and inhomogeneous cloud fields to droplet concentration is significantly less than for plane-parallel clouds, for small values of the solar zenith angle.

In this conclusion paper, remote sensing retrievals of cloud optical thickness performed during the EUCREX mission 206 are analyzed. The comparison with estimates derived from in situ measurements demonstrates that the adiabatic model of cloud microphysics is more realistic than the vertically uniform plane parallel model (VUPPM) for parameterization of optical thickness. The analysis of the frequency distributions of optical thickness in the cloud layer then shows that the adiabatic model provides a good prediction when the cloud layer is thick and homogeneous, while it overestimates significantly the optical thickness when the layer is thin and broken. Finally, it is shown that the effective optical thickness over the whole sampled cloud is smaller than the adiabatic prediction based on the mean geometrical thickness of the cloud layer. The high sensitivity of the optical thickness on cloud geometrical thickness suggests that the effect of aerosol and droplet concentration on precipitation efficiency, and therefore on cloud extent and lifetime, is likely to be more significant than the Twomey effect.

Two major recent field programmes—the European Cloud Radiation Experiment (EUCREX) and the Aerosol Characterization Experiment II (ACE‐2)‐have extensively analysed the dynamical, microphysical and radiative attributes of stratocumulus clouds contaminated by continental air. Although an extensive set of dynamical and microphysical data are now available, there are no accounts of any matching theoretical modelling studies. To fully understand and numerically model the interplay between the dynamics, microphysics, radiative and chemical properties of the two clouds chosen for our case‐studies would require a full three‐dimensional large‐eddy simulation (LES) model coupled to a full‐size resolving microphysical model where the computational costs would be prohibitive. In this study we have ‘optimized’ the classic Kessler parametrization scheme so that it is effectively able to distinguish between clean and contaminated clouds. We perform LES runs with the optimized scheme to study the morphology and the dynamics of the clouds and then use a second one‐dimensional microphysical parcel model run with identical environmental conditions to study the effects of pollution on the clouds as well as the droplet spectral evolution. This procedure yields extremely good agreement with observations at modest computational expense. It is shown that nitric acid (HNO3) vapour in the parts per billion by volume (p.p.b.v.) range affects cloud formation by increasing the number of cloud droplets and decreasing the mean size compared to an acid‐free simulation. The effects of HNO3 contamination on the EUCREX case‐study is evident owing to the proximity of this cloud to sources of air pollutants. With 10 p.p.b.v. of HNO3, we are able to achieve good agreement with the observations of the droplet effective radii as well as with observations of the optical‐depth variation. For the ACE‐2 cloud which formed further away from sources of pollutants, even on a typical ‘polluted’ day when the ambient HNO3 was ∼5 parts per trillion by volume, the drop concentration was found to be insensitive to changes in the HNO3.

Detailed aircraft observations of sea-breeze frontal structure and dynamics are presented for two cases of well defined sea-breeze fronts near the east coast of England. In the first case the sea-breeze was advancing into a well mixed convective boundary layer with strong turbulence and an offshore breeze of around 3 m s(-1). In the second case the sea-breeze was penetrating into a convective boundary layer characterized by weaker turbulence and an offshore wind speed of 2-3 m s(-1). Indeed, during the course of the measurements a stable internal boundary layer was forming in the early evening. Cross-sections of the frontal structure are derived from aircraft traverses at a range of heights along a fixed line normal to the coast. A typical head-like structure is observed in both cases, with a region of strong mixing immediately seawards of the head. An approximately exponential fall-off in dissipation rate with distance seaward of the leading edge of the front is found. Heat and momentum fluxes are used to derive turbulent kinetic energy (TKE) budgets for the mixing region and indicate that turbulence in this mixing region is maintained by TKE generated by strong shear at the top of the cold-air inflow. In the first case the shear production of TKE is almost twice that of the second case, suggesting that the presence of ambient turbulence in the convective boundary layer has a significant effect upon the frontal dynamics. In the second case, as the turbulence decayed, a curious wave-like structure appeared behind the leading edge of the front. The waves have wavelengths of 1-3 km and it is unlikely that they were caused by Kelvin-Helmholtz instability. It is suggested that the waves might be a solitary wave-train emerging as the sea-breeze interacts with a low-level stable layer forming in the early evening.

The dominant role of clouds in modulating and interacting with radiative energy transports within the atmosphere, in providing precipitation, transporting water and influencing air-chemical processes is still not understood well enough to be accurately represented within atmospheric circulation and climate models over all regions of the globe. Also the extraction of real-world cloud properties from satellite measurements still contains uncertainties. Therefore, various projects have been developed within the Global Energy and Water Cycle Experiment (GEWEX), to achieve more accurate solutions for this problem by direct measurements within cloud fields and other complementary studies. They are based on the hypothesis, that most relevant properties of cloud fields can be parametrized on the basis of the prognostic field variables of atmospheric circulation models, and that the cloud microphysical properties can directly be related – with additional parameters on the particle shapes etc. – to the radiative transfer properties. One of these projects has been the European Cloud and Radiation Experiment (EUCREX) with its predecessor ICE (International Cirrus Experiment). The EUCREX and ICE provided a common platform for research groups from France, Germany, Sweden and the United Kingdom to concentrate their efforts primarily on high, cold cirrus. They showed, with data from satellites, that this cloud species enhances the atmospheric greenhouse-effect. Numerical mesoscale models were used in sensitivity studies on cloud developments. In-situ measurements of cloud properties were made during more than 30 aircraft missions, where also in-flight comparisons of various instruments were made to ensure the quality of data sets measured from different aircraft. The particle sampling probes, used for in-cloud measurements, showed a disagreement in total number density in all ranges between about 20–50%, while all other instruments agreed quite satisfactorily. A few measured holographic data provided information on typical ice-crystal shapes, which were used in numerical simulations of their absorption and scattering properties. Several new instruments for both in-situ and remote measurement, such as a polar nephelometer, a chopped pyrgeometer and an imaging multispectral polarimeter (POLDER) for cloud and radiation measurements were tested and improved. New algorithms were developed for cloud classifications in multispectral satellite images and also for simulations of the scattering of radiation by non-spherical particles. This paper primarily summarizes the EUCREX results obtained between 1989 and 1996, and provides examples of the many results which have been obtained so far. It is not a complete review of the world-wide state in this field, but it tries to place the EUCREX results into the world-wide development. Therefore many references are made to the results of other groups, which in turn influenced the work within EUCREX.

Many Large Eddy Simulation (LES) models use the classic Kessler parameterisation either as it is or in a modified form to model the process of cloud water autoconversion into precipitation. The Kessler scheme, being linear, is particularly useful and is computationally straightforward to implement. However, a major limitation with this scheme lies in its inability to predict different autoconversion rates for maritime and continental clouds. In contrast, the Berry formulation overcomes this difficulty, although it is cubic. Due to their different forms, it is difficult to match the two solutions to each other. In this paper we single out the processes of cloud conversion and accretion operating in a deep model cloud and neglect the advection terms for simplicity. This facilitates exact analytical integration and we are able to derive new expressions for the time of onset of precipitation using both the Kessler and Berry formulations. We then discuss the conditions when the two schemes are equivalent. Finally, we also critically examine the process of droplet evaporation within the framework of the classic Kessler scheme. We improve the existing parameterisation with an accurate estimation of the diffusional mass transport of water vapour. We then demonstrate the overall robustness of our calculations by comparing our results with the experimental observations of Beard and Pruppacher, and find excellent agreement.

Simulations of fields of warm (ice‐free) cumulus clouds are made using a large‐eddy model based on observational soundings. Comparisons between numerical and experimental results show good agreement.

Two flights of the UK Meteorological Office's Hercules aircraft through daytime frontal cirrus around Scotland have been analysed using wavelet analysis on the vertical velocity time-series from the horizontal runs. It is shown that wavelet analysis is a useful tool for analysing the turbulence data in cirrus clouds. It finds the largest scales involved in producing turbulence, as does Fourier analysis, such as the 2-km spectral peaks corresponding to convective activity during flight A283. Wavelet spectra have the added advantage that the position is shown, and so they identify smaller-scale, highly localised processes such as the production of turbulent kinetic energy by the breaking of Kelvin-Helmholtz waves due to the vertical shear in the horizontal wind. These may be lost in Fourier spectra obtained for long time-series, though they contribute something to the average spectral density at the appropriate scale. The main disadvantage of this technique is that only octave frequency bands are resolved.

Decoupling of the marine boundary-layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface-based mixed layer (SML) have frequently been observed and modelled. Observations of the cumulus-stratocumulus interaction during a Lagrangian study as part of the Atlantic Stratocumulus Transition Experiment (ASTEX), detailed in Martin et al. (1995), were made in an airmass which, although it was over the sea, was highly polluted, since it had recently come from industrial Europe. These observations suggested that the interaction was associated with significant changes in the stratocumulus thickness, microphysics and radiative properties. However, it was suggested that such changes may vary according to the type of airmass in which the cumulus-stratocumulus interaction was taking place. In this paper, two further case studies of the interaction in airmasses from different locations, exhibiting different thermodynamic properties and different levels of pollution, but being generally cleaner than that in the ASTEX Lagrangian study, are analysed to try to assess the influence of airmass type on the interaction. Although the cloud liquid-water content in the stratocumulus generally increases locally in a region of cumulus cloud penetration, the changes in the droplet spectrum which result from mixing between the cumulus and stratocumulus droplets depend on the individual droplet spectra in the two cloud types. This, in turn, is influenced by the aerosol characteristics in the boundary layer, the updraught velocities associated particularly with the cumulus clouds, and the actual and relative cloud thickness of the cumulus and stratocumulus. The effects of penetrating cumulus clouds on the droplet-effective-radius in the stratocumulus, and hence on the cloud radiative properties, may therefore differ significantly between boundary layers in which the interaction is occurring.

A system has been developed for use on a light aircraft for the measurement of the turbulent wind vector components that does not rely on the use of either an inertial navigation system (INS) or Doppler radar. The system described here uses a five-hole probe to measure the wind vector relative to the aircraft. A GPS system, a vertical gyroscope for aircraft pitch and roll angles, a gyrocompass system, and a strap-down three-axis accelerometer system are used to obtain aircraft motion. Flight tests and results of an intercomparison with the United Kingdom Meteorological Office C-130 are presented. Under conditions of straight and level flight, the estimated rms errors are 0.3 m s(-1) for the vertical wind component and 2 m s(-1) for the horizontal components.

Decoupling of the marine boundary layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface-based mixed layer have frequently been observed and modelled. However, little is known of the details of the interaction between the two cloud types, how this affects the microphysical and radiative properties of the stratocumulus, and how the cloud-topped marine boundary layer evolves in such cases. Recent observational studies suggest that the microphysical and geometrical characteristics of the stratocumulus layer can be modified significantly by interaction with cumulus clouds. In this paper, a preliminary assessment of the relative contribution of different processes and initial boundary-layer conditions, as suggested by the observations, to the outcome of the interaction between cumulus and stratocumulus clouds is carried out using a one-dimensional entraining parcel model, which has been modified to simulate a cumulus cloud which penetrates a stratocumulus layer. The results suggest that the change in droplet size at the stratocumulus top as a result of mixing with the penetrating cumulus clouds is particularly sensitive both to the amount of pollution in the air mass and to the vertical structure of the boundary layer, and that these may be more important in determining the outcome of the interaction than the local temperature. A simple coalescence calculation is then used to show that the introduction of cumulus cloud droplets into a stratocumulus layer may alter the potential for drizzle formation in the cloud layer by introducing droplets which are of a different size from those already existing in the stratocumulus. However, it is clear that more detailed dynamical, microphysical and radiative-transfer models are required, in conjunction with further observations, if these effects are to be parametrized for use in large-scale numerical models.

Cloud fields from a Large‐Eddy Simulation (LES) combined with a Monte Carlo radiative‐transfer model have been used to examine the behaviour of clouds with regard to the monochromatic albedo of short‐wave solar radiation. the LES cloud fields have an inhomogeneous structure in two dimensions and contain highly irregular concentrations of liquid water. the calculations show that the albedo sensitivity to the cloud‐field orientation becomes notably important for solar zenith angles above 30° for the LES clouds used. Replacing each cloud point with the in‐cloud mean water content, increases the cloud‐field albedo, compared to the original cloud‐field albedo, in many cases by more than 30%. Calculations have also been made with differing droplet concentrations throughout the cloud fields. It is found that changes in the cloud‐field albedo with droplet concentration are influenced by the general degree of inhomogeneity in the cloud structure. the original irregular LES cloud fields are more sensitive to changes in the droplet concentration than their equivalent plane‐parallel clouds with the same total water content and mean vertical water‐content profiles.