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In this study, the super droplet-method (SDM) is used in large-eddy simulations of an isolated cumulus congestus observed during the 1995 Small Cumulus Microphysics Study field project in order to ...

Aim The anastomosis technique used following right-sided colonic resection is widely variable and may affect patient outcome. This study aimed to assess the association between leak and anastomosis technique (stapled vs handsewn). Method This was a prospective, multicentre, international audit including patients undergoing elective or emergency right hemicolectomy or ileo-caecal resection operations over a 2-month period in early 2015. The primary outcome measure was the presence of anastomotic leak within 30 days of surgery, determined using a prespecified definition. Mixed effects logistic regression models were used to assess the association between leak and anastomosis method, adjusting for patient, disease and operative cofactors, with centre included as a random-effect variable. Results This study included 3208 patients, of whom 78.4% (n = 2515) underwent surgery for malignancy and 11.7% (n = 375) underwent surgery for Crohn's disease. An anastomosis was performed in 94.8% (n = 3041) of patients, which was handsewn in 38.9% (n = 1183) and stapled in 61.1% (n = 1858). Patients undergoing hand-sewn anastomosis were more likely to be emergency admissions (20.5% handsewn vs 12.9% stapled) and to undergo open surgery (54.7% handsewn vs 36.6% stapled). The overall anastomotic leak rate was 8.1% (245/3041), which was similar following handsewn (7.4%) and stapled (8.5%) techniques (P = 0.3). After adjustment for cofactors, the odds of a leak were higher for stapled anastomosis (adjusted OR = 1.43; 95% CI: 1.04-1.95; P = 0.03). Conclusion Despite being used in lower-risk patients, stapled anastomosis was associated with an increased anastomotic leak rate in this observational study. Further research is needed to define patient groups in whom a stapled anastomosis is safe.

In this survey we consider the impact of turbulence on cloud formation from the cloud scale to the droplet scale. We assess progress in understanding the effect of turbulence on the condensational and collisional growth of droplets and the effect of entrainment and mixing on the droplet spectrum. The increasing power of computers and better experimental and observational techniques allow for a much more detailed study of these processes than was hitherto possible. However, much of the research necessarily remains idealized and we argue that it is those studies which include such fundamental characteristics of clouds as droplet sedimentation and latent heating that are most relevant to clouds. Nevertheless, the large body of research over the last decade is beginning to allow tentative conclusions to be made. For example, it is unlikely that small-scale turbulent eddies (i.e. not the energy-containing eddies) alone are responsible for broadening the droplet size spectrum during the initial stage of droplet growth due to condensation. It is likely, though, that small-scale turbulence plays a significant role in the growth of droplets through collisions and coalescence. Moreover, it has been possible through detailed numerical simulations to assess the relative importance of different processes to the turbulent collision kernel and how this varies in the parameter space that is important to clouds. The focus of research on the role of turbulence in condensational and collisional growth has tended to ignore the effect of entrainment and mixing and it is arguable that they play at least as important a role in the evolution of the droplet spectrum. We consider the role of turbulence in the mixing of dry and cloudy air, methods of quantifying this mixing and the effect that it has on the droplet spectrum. Copyright © 2012 Royal Meteorological Society and British Crown Copyright, the Met Office

In bacterial cells, processing of double-stranded DNA breaks for repair by homologous recombination is dependent upon the recombination hotspot sequence Chi and is catalysed by either an AddAB- or RecBCD-type helicase–nuclease. Here, we report the crystal structure of AddAB bound to DNA. The structure allows identification of a putative Chi-recognition site in an inactivated helicase domain of the AddB subunit. By generating mutant protein complexes that do not respond to Chi, we show that residues responsible for Chi recognition are located in positions equivalent to the signature motifs of a conventional helicase. Comparison with the related RecBCD complex, which recognizes a different Chi sequence, provides further insight into the structural basis for sequence-specific ssDNA recognition. The structure suggests a simple mechanism for DNA break processing, explains how AddAB and RecBCD can accomplish the same overall reaction with different sets of functional modules and reveals details of the role of an Fe–S cluster in protein stability and DNA binding.

The physical principle of convective quasi-equilibrium proposed by Arakawa and Schubert states that the atmosphere is effectively adjusted to equilibrium by an active role of convective heating against large-scale forcing (physical convective quasi-equilibrium, or PCQ). A simple consequence of this principle is that the rate of change of the thermodynamic field (typically measured by the convective available potential energy (CAPE)) is much smaller than the rate of change of the large-scale forcing (diagnostic convective quasi-equilibrium, or DCQ). Such a diagnostic state is generally observed in the tropical atmosphere at the synoptic-scale, and this is often taken as a proof for the physical mechanisms behind Arakawa and Schubert's convective quasi-equilibrium: however, theoretically, there are several alternative physical mechanisms that are also able to establish this diagnostic state. The paper examines the approach of the tropical atmospheric system to DCQ with increasing time-scale in order to distinguish various alternatives to PCQ. The latter predicts that the system approaches DCQ exponentially with a time-scale characteristic of convection. However, the alternatives considered in the paper predict algebraic asymptotes to DCQ with increasing time-scale. First it is demonstrated that PCQ is not required to achieve DCQ by considering a linear primitive-equation system with arbitrary convective heating, in which the roles of convective heating and large-scale forcing are completely reversed; algebraic asymptotes are achieved. An even simpler analogue is to assume that the rate of generating CAPE is controlled by white-noise forcing. More generally, such an algebraic asymptote is obtained by any system with a power-law spectrum both for CAPE and large-scale forcing, although a restriction must be applied to ensure a decreasing asymptote with increasing time-scale. The approach to DCQ is examined for both the Maritime Continent Thunderstorm Experiment data and cloud-resolving model simulation data, and both indicate no tendency for exponential adjustments in the short time limit.

This note discusses the results of numerical simulations in which a prescribed-flow model is merged with a double-moment warm-rain and ice microphysics scheme to investigate the impact of microphysical processes on precipitation from deep organized convection. The prescribed two-dimensional flow mimics an idealized squall line, with a narrow region of strong convective updraft and much wider region of weak stratiform updraft overlaying a stratiform downdraft. To cover the broad range of conditions that are possible for a squall line as well as to explore precipitation dependence on different parameters, 25 pairs of simulations are performed. Simulated cases differ in the dynamics (e.g. changing the updraft strength or large-scale shear), thermodynamics (e.g. changing the inflow sounding) or microphysics (e.g. changing the collision efficiencies). Each pair features cloud condensation nuclei (CCN) in either a pristine or polluted environment. Total surface precipitation and partitioning between convective and stratiform precipitation in each pair appears to be almost the same, with the difference being typically a few tenths of 1%. However, the dynamical and thermodynamical parameters do affect the precipitation significantly. It follows that the surface precipitation from organized convection can differ between pristine and polluted environments only through the feedback of CCN on cloud dynamics. In this feedback, small differences in the latent heating for the same flow pattern lead to different flow patterns in subsequent squall-line evolution. Details of this feedback need to be investigated using a dynamical model. Copyright © 2009 Royal Meteorological Society

Quantitative parameterization of turbulent collision of cloud droplets represents a major unsolved problem in cloud physics. Here a hybrid direct simulation tool is used specifically to quantify the turbulent enhancement of the gravitational collision-coalescence. Simulation results show that air turbulence can enhance the collision kernel by an average factor of about 2, and the observed trends are supported by scaling arguments. An impact study using the most realistic collection kernel suggests that cloud turbulence can significantly reduce the time for warm rain initiation. Areas for further development of the hybrid simulation and the impact study are indicated. Copyright © 2009 Royal Meteorological Society

Entrainment into the stratocumulus-topped boundary layer (STBL) is investigated by means of large-eddy simulations. Set-up of the numerical experiment is based on the research flight RF-01 in the DYCOMS-II field campaign. We focus on the stability of the flow in the cloud-top region known as the Entrainment Interface Layer (EIL). We calculate the local gradient Richardson number, Ri, at the surface of maximum static stability and at the material top of the STBL defined by a threshold of the total water content. We found that regions in which updraughts impinge upon the inversion and diverge horizontally are characterized by small values of Ri. Resulting turbulence is responsible for entrainment and formation of the EIL. Volumes of the STBL air and the free-tropospheric air from above it, mixed in proportion resulting in negative buoyancy and typically void of cloud water, form ‘cloud holes’ -- trenches of descending cloud-free air which surround updraught areas. The entrainment process is further analyzed using a passive scalar introduced after three hours of the simulation above the layer of maximum static stability. The mixing fraction of this scalar in the air within the cloud holes falls within the range corresponding to the buoyancy reversal. Some of the negatively buoyant mixtures sinking through the cloud holes are wrapped around the edge of cloudy regions and recirculated into the cloud, which causes a local increase of the cloud-base height. The rest of the entrained free-tropospheric air sinks slowly into the STBL and leads to its gradual dilution. Copyright © 2008 Royal Meteorological Society

This paper investigates daytime convective development over land and its representation in single-column models (SCMs) and cloud-resolving models (CRMs). A model intercomparison case is developed based on observations of the diurnal cycle and convection during the rainy season in Amazonia. The focus is on the 6 h period between sunrise and early afternoon which was identified in previous studies as critical for the diurnal cycle over summertime continents in numerical weather prediction and climate models. This period is characterized by the formation and growth of a well-mixed convective boundary layer from the early morning temperature and moisture profiles as the surface sensible- and latent-heat fluxes increase after sunrise. It proceeds with the formation of shallow convective clouds as the convective boundary layer deepens, and leads to the eventual transition from shallow to deep precipitating convection around local noon. To provide a benchmark for other models, a custom-designed set of simulations, applying increasing in time computational domain and decreasing spatial resolution, was executed. The SCMs reproduced the previously identified problem with premature development of deep convection, less than two hours after sunrise. The benchmark simulations suggest a possible route to improve SCMs by considering a time-evolving cumulus entrainment rate as convection evolves from shallow to deep and the cloud width increases up to an order of magnitude. The CRMs featuring horizontal grid length around 500 m are capable of capturing the qualitative aspects of the benchmark simulations, but there are significant differences among the models. Two-dimensional CRMs tend to simulate too rapid a transition from shallow to deep convection and too high a cloud cover. Copyright © 2006 Royal Meteorological Society

This paper discusses the large-scale moisture–convection feedback in the tropics, where spatial fluctuations of deep convection cause perturbations of free-tropospheric moisture which, in turn, affect the spatial distribution of deep convection. A simple heuristic argument using the timescale of free-tropospheric humidity change explains why moisture–convection feedback is particularly relevant for tropical intraseasonal oscillations. The large-scale dynamical context for moisture–convection feedback is investigated in idealized rotating constant-sea-surface-temperature (‘tropics everywhere’) aquaplanet using cloud-resolving convection parametrization (CRCP; super-parametrization) and a traditional convective parametrization (the Emanuel scheme). The large-scale organization of convection within the equatorial waveguide takes the form of MJO-like (Madden–Julian Oscillation) coherent structures. First, CRCP simulations are performed in which development of large-scale free-tropospheric moisture perturbations is artificially suppressed using relaxation with a timescale of one day. As in previous simulations where much shorter relaxation timescale was used, MJO-like coherences do not develop and, if already present, they disintegrate rapidly. Second, CRCP simulations that start from planetary-scale moisture perturbation in the free troposphere are conducted. The ensuing large-scale velocity perturbations have e-folding times of five and seven days, respectively, for interactive and prescribed radiation simulations. This supports the conjecture that interactive radiation enhances moisture–convection feedback; an enhanced large-scale circulation results from differences in radiative cooling between areas having enhanced and suppressed convectively-generated moisture and cloudiness. Additional support for the role of moisture–convection feedback in intraseasonal oscillations is seen in simulations that apply the Emanuel scheme. The standard configuration of the Emanuel scheme is insensitive to free-tropospheric humidity and results in weak MJO-like coherences. A simple modification of the Emanuel scheme that enhances its sensitivity to free-tropospheric humidity dramatically improves the simulated MJO-like coherences. Copyright © 2004 Royal Meteorological Society

The Escherichia coli open reading frame YbdK encodes a member of a large bacterial protein family of unknown biological function. The sequences within this family are remotely related to the sequence of gamma-glutamate-cysteine ligase (gamma-GCS), an enzyme in the glutathione biosynthetic pathway. A gene encoding gamma-GCS in E. coli is already known. The 2.15 A resolution crystal structure of YbdK reveals an overall fold similar to that of glutamine synthetase (GS), a nitrogen metabolism enzyme that ligates glutamate and ammonia to yield glutamine. GS and gamma-GCS perform related chemical reactions and require ATP and Mg2+ for their activity. The Mg2+-dependent binding of ATP to YbdK was confirmed by fluorescence spectroscopy employing 2'(or 3')-O-(trinitrophenyl)adenosine 5'-triphosphate, and yielding a dissociation constant of 3 +/- 0.5 microM. The structure of YbdK contains a crevice that corresponds to the binding sites of ATP, Mg2+ and glutamate in GS. Many of the GS residues that coordinate the metal ions and interact with glutamic acid and the phosphoryl and ribosyl groups of ATP are also present in YbdK. GS amino acids that have been associated with ammonia binding have no obvious counterparts in YbdK, consistent with a substrate specificity that is different from that of GS. Ligase activity between glutamic acid and each of the twenty amino acid residues was tested on high performance liquid chromatography (HPLC) by following the hydrolysis of ATP to ADP. Catalysis was observed only with cysteine. A pyruvate kinase/lactic acid dehydrogenase coupled assay was used to rule out GS activity and to determine that YbdK exhibits gamma-GCS activity. The catalytic rate was found to be approximately 500-fold slower than that reported for authentic gamma-GCS.

The crystal structures of YibK from Haemophilus influenzae (HI0766) have been determined with and without bound cofactor product S-adenosylhomocysteine (AdoHcy) at 1.7 and 2.0 A resolution, respectively. The molecule adopts an alpha/beta fold, with a topology that differs from that of the classical methyltransferases. Most notably, HI0766 contains a striking knot that forms the binding crevice for the cofactor. The knot formation is correlated with an alternative arrangement of the secondary structure units compared with the classical methyltransferases. Two loop regions undergo conformational changes upon AdoHcy binding. In contrast to the extended conformation of the cofactor seen in the classical methyltransferase structures, AdoHcy binds to HI0766 in a bent conformation. HI0766 and its close sequence relatives are all shorter versions of the more remotely related rRNA/tRNA methyltransferases of the spoU sequence family. We propose that the spoU sequence family contains the same core domain for cofactor binding as HI0766 but has an additional domain for substrate binding. The substrate-binding domain is absent in HI0766 sequence family and may be provided by another Haemophilus influenzae partner protein, which is yet to be identified.

This paper documents the impact of ice microphysics on the multiscale organization of tropical convection as simulated in two-dimensional cloud-resolving model simulations. Results from simulations which apply warm-rain microphysics are compared with results from a simulation which includes ice processes presented previously in this journal. In these simulations, moist convection and large-scale dynamics interact on an unprecedented range of spatial and temporal scales, albeit within a limited two-dimensional dynamical framework. In general, the large-scale organization of convection is similar in all simulations, although the scale and propagation speed of convectively-coup led gravity waves are different between the warm-rain and ice simulations. The most striking differences are on the mesoscale. The warm-rain simulations feature mesoscale convective systems with a reduced stratiform component and shorter life cycle than mesoscale systems in the simulation with ice microphysics. These impacts are consistent with previous modelling studies of the impact of ice microphysics on organized convection. It is hypothesized that the shorter life cycle of mesoscale convective systems in warm-rain simulations influences scale selection of the large-scale convectively-coup led gravity waves. Moreover, the vertical transport of horizontal momentum is affected as well. To the author's knowledge this is the first time the impact of cloud microphysics on mesoscale convective systems is shown to affect the coupling between deep convection and large-scale perturbations in the tropics. Copyright © 2003 Royal Meteorological Society

The crystal structure of HI0074 from Haemophilus influenzae, a protein of unknown function, has been determined at a resolution of 2.4 A. The molecules form an up-down, four-helix bundle, and associate into homodimers. The fold is most closely related to the substrate-binding domain of KNTase, yet the amino acid sequences of the two proteins exhibit no significant homology. Sequence analyses of completely and incompletely sequenced genomes reveal that the two adjacent genes, HI0074 and HI0073, and their close relatives comprise a new family of nucleotidyltransferases, with 15 members at the time of writing. The analyses also indicate that this is one of eight families of a large nucleotidyltransferase superfamily, whose members were identified based on the proximity of the nucleotide- and substrate-binding domains on the respective genomes. Both HI0073 and HI0074 were annotated "hypothetical" in the original genome sequencing publication. HI0073 was cloned, expressed, and purified, and was shown to form a complex with HI0074 by polyacrylamide gel electrophoresis under nondenaturing conditions, analytic size exclusion chromatography, and dynamic light scattering. Double- and single-stranded DNA binding assays showed no evidence of DNA binding to HI0074 or to HI0073/HI0074 complex despite the suggestive shape of the putative binding cleft formed by the HI0074 dimer.

SUMMARY This paper reports an intercomparison study of midlatitude continental cumulus convection simulated by eight two-dimensional and twothree-dimensional cloud-resolving models (CRMs), driven by observed large-scale advective temperature and moisture tendencies, surface turbulent euxes, and radiative-heating proe les during three sub-periods of the summer 1997 Intensive Observation Period of the US Department of Energy’s Atmospheric Radiation Measurement (ARM) program. Each sub-period includes two or three precipitation events of various intensities over a span of 4 or 5 days. The results can be summarized as follows. CRMs can reasonably simulate midlatitude continental summer convection observed at the ARM Cloud and Radiation Testbed site in terms of the intensity of convective activity, and the temperature and specie c-humidity evolution. Delayed occurrences of the initial precipitation events are a common feature for all three sub-cases among the models. Cloud mass e uxes, condensate mixing ratios and hydrometeor fractions produced by all CRMs are similar. Some of the simulated cloud properties such as cloud liquid-water path and hydrometeor fraction are rather similar to available observations. All CRMs produce large downdraught mass euxes with magnitudes similar to those of updraughts, in contrast to CRM results for tropical convection. Some inter-model differences in cloud properties are likely to be related to those in the parametrizations of microphysical processes. There is generally a good agreement between the CRMs and observations with CRMs being signie cantly better than single-column models (SCMs), suggesting that current results are suitable for use in improving parametrizations in SCMs. However, improvements can still be made in the CRM simulations; these include the proper initialization of the CRMs and a more proper method of diagnosing cloud boundaries in model outputs for comparison with satellite and radar cloud observations.

This paper discusses results from idealized simulations using a non-hydrostatic general circulation model with Cloud-Resolving Convection Parameterization (CRCP, the 'super parameterization') pertinent to the large-scale organization of tropical convection. The essence of tropical dynamics is the intricate balance between large-scale processes-such as radiative transfer, large-scale waves, monsoons, Hadley and Walker circulations- and the convective dynamics. Traditional approaches to this problem consider either large-scale models using convection parameterization or cloud-resolving models in which large-scale effects are prescribed. The CRCP merges the two approaches. It uses a 2D cloud-resolving model to represent the impact of cloud-scale processes-such as convective motions, precipitation formation and fallout, interaction of clouds with radiative and surface processes-in every column of a large-scale or global model. The global CRCP model is applied to the idealized problem of a rotating constant-SST aquaplanet in convective-radiative equilibrium. The aquaplanet has the size and rate of rotation of the Earth. The global CRCP simulations feature pronounced large-scale organization of convection within the equatorial waveguide. Prominent equatorial 'super cloud clusters' spontaneously develop in CRCP global simulations and bear a strong resemblance to the Madden-Julian Oscillation observed in the terrestrial tropics. Model results suggest that convective transport of zonal momentum, supposedly due to the impact of organized convection, plays a significant role in the large-scale organization of tropical deep convection.

Cloud systems observed during 1–7 September of GATE are examined in a hierarchical approach, namely: a two-dimensional cloud-resolving simulation using a 2 km grid length; two- and three-dimensional simulations using the Kain-Fritsch convective parametrization and 10, 15 and 25 km grid lengths; and coarse-grid simulations without any convective parametrization. All simulations are forced by the same objectively analysed time-varying large-scale advection of temperature and moisture. The domain-mean winds are relaxed to the observed wind profiles. Both the cloud-resolving modelling and the lower-resolution modelling with parametrized convection realize the three observed cloud system categories (squall line, non-squall cluster and scattered convection) and transitions among them. In particular, the well-organized fast-moving squall-type cloud system observed on 4 September is realized in a three-dimensional experiment with parametrized convection. In contrast, the lower-resolution modelling without any convective parametrization fails to produce the squall-type convective system during the weakly forced period but successfully represents the non-squall cluster during strong forcing. This lack of success is mostly attributed to convective triggering and the absence of resolved downdraught-enhanced surface fluxes. These issues are not as critical during strong large-scale forcing. The observed evolutions of temperature, water vapour mixing ratio, precipitation and surface moisture flux are realized in all simulations. A common deficiency is the overprediction of upper-level relative humidity. The simulation with parametrized convection features a comparatively large low-level water vapour mixing ratio, a surface and upper-level cold temperature bias and a mid-tropospheric warm bias. This is mainly attributed to deficiencies in how the Kain-Fritsch scheme represents convective mass flux, detrainment and entrainment by cumulus congestus.

The large-scale organization of tropical deep convection is investigated in idealized two-dimensional cloud-resolving simulations. A 20 000 km periodic computational domain allows interactions among moist convection, mesoscale organization, and surface exchange on a wide range of scales. A uniform 10 m s−1 easterly wind and a uniform sea surface temperature of 30°C are assumed. A prescribed temperature tendency mimicking the mean radiative cooling of the tropical troposphere is used in this pilot study. The large-scale organization of convection resembles the observed tropical atmosphere as well as results from some previous numerical studies using parametrized convection. The simulation highlights mesoscale convective systems with horizontal scales of several hundred kilometres moving east to west with speeds similar to the mean flow, and an envelope of convection with a horizontal extent of a few thousand kilometres and propagating west to east. The propagation speed of the large-scale envelope compares well with the phase velocitites of convectively coupled Kelvin waves observed in the equatorial waveguide. Convective momentum transport and the impact of convective systems on temperature and moisture near the surface are key processes responsible for the large-scale organization of convection. These results are discussed in the context of recent observations and numerical studies of convection organization in the tropics.

Results of a 24-day, two-dimensional integration of tropical cloud systems forced by large-scale ascent, surface fluxes and radiation in a typical sheared tropical environment are presented. A non-hydrostatic, cloud-resolving numerical model containing sophisticated microphysical parametrizations as well as turbulence, surface flux and short/long-wave radiative representations was used. A predominant cloud-system hierarchy was identified: fast westward-moving mesoscale convective systems, producing extensive cirrus anvils and a strong radiative effect; and slow-moving regions of enhanced precipitation, causing a significant modification of the fast-moving cloudsystem behaviour on the time-scale of about one day. The experimental set-up was similar to that used by Sui et al. The demonstrated episodic convective activity and the fundamental role of organized deep convection by and large agrees with their analysis. However, despite many similarities, the results for the mean thermodynamic statistical equilibrium are dramatically different: a warm and humid regime, as opposed to the cold and dry regime of Sui et al. High relative humidities and very high upper-tropospheric cirrus cloud amount led to a strong greenhouse effect with low outgoing long-wave radiation at the top of the atmosphere (120–150 W m−2). At the same time, these clouds screened the ocean surface from the solar radiation and caused very low solar radiation absorption at the surface (only about 30 W m−2 when averaged over a diurnal cycle). The surface net cloud forcing was very large, about −200 W m−2 which is in accord with earlier findings of Ramanathan and Collins. The above numbers should be considered as an upper limit of effects of deep convection on radiative fluxes in the tropics because of the warm and moist regime demonstrated in the experiment.

A consensus seems to exist throughout the cloud-physics community that buoyancy reversal associated with evaporative cooling affects not only the global (cloud-scale) dynamics of a convective cloud, but also the rate of mixing between the cloud and its environment. The latter effect is associated with the concept of the so-called ‘cloud-top entrainment instability’ (CTEI), which assumes a positive feedback between buoyancy reversal and the rate of entrainment. In this paper, effects of buoyancy reversal on cloud dynamics are discussed in the context of an unstratified anelastic two-fluid system. Convection in this system mimics some essential features of cumulus convection. Two-dimensional numerical experiments, with and without the effects of buoyancy reversal, have been performed. It was found that buoyancy reversal has a dramatic impact on the overall flow evolution, but that its effect on the rate of mixing between the two fluids is small, i.e. flows which differed dramatically depending on whether there was buoyancy reversal or not still resulted in similar amounts of mass being mixed. This result casts doubt on the concept of CTEI in cumulus dynamics. The distinction between strong global effects of buoyancy reversal and minor effects on the rate of mixing in buoyancy-reversing convection is supported by laboratory experiments with classical and buoyancy-reversing thermals.

The formation of buoyancy reversal due to entrainment of dry environmental air, and its implication for cumulus dynamics, are discusses. Concepts originating from laboratory experiments with reacting turbulent flows, and from numerical simulations of homogeneous and isotropic turbulence, are applied to distinguish between large-scale entraining eddies developing at the interface, subsequent development of smaller-scale motions, and final homogenization by microscale processes. The physics of the microscale homogenization by molecular diffusion and sedimentation of cloud droplets is discussed. It is shown that, as a result of droplet sedimentation, much smaller negative buoyancy (buoyancy undershoots) may be generated on cloud microscale as compared with the value predicted by a classical nonlinear mixing diagram of cloudy and cloud-free air. Highly idealized numerical experiments aimed at simulating the temporal evolution of a small convective cloud were performed with and without the effects of molecular mixing that results in buoyancy reversal. It is argued that these experiments provide two limiting cases, and that the dynamics of a real cumulus cloud is located somewhere between them. The major effect of buoyancy reversal as suggested by these experiments is to increase the intermittency associated with cloud evolution. The temporal variation of cloud-top height, maximum undraughts, and minimum downdraughts increases significantly when buoyancy reversal is allowed. It is argued that results of numerical experiments, together with other laboratory and theoretical studies, cast serious doubts on the concept of cumulus entrainment being driven by the cloud-top entrainment instability.

A simple model of the influence of small-scale topography on precipitation via the ‘seeder-feeder’ mechanism is presented. In the model the large-scale stratiform seeder cloud and the subcloud layer are modified by the airflow perturbations caused by small-scale topography. Two separate scales of motion are considered; the large scale, with the updraught maintaining the seeder cloud, and the small scale tied to topography. In the diagnostic numerical model the interaction of these two scales is simulated. Two basic situations are considered: rain with a ground temperature of 12°C and snow with –5°C. Results of the model application to a bell-shaped solitary hill are presented and compared with previous theoretical studies and field observations in the Welsh hills. The influence of orographic airflow perturbations on snow growth above the melting level may explain the observed strong dependence of rain enhancement on the horizontal wind speed. Simple considerations for the complex topography are also presented with reference to observations made near Uppsala, Sweden. It is suggested that the observed rain distribution may again result from influences on snow growth, with both the amplitude of airflow perturbations near the melting level and its phase relative to the topography beneath being very important.