Aerosol Microphysical and Radiative Effects on Continental Cloud Ensembles

Aerosol-cloud-radiation interactions represent one of the largest uncertainties in the current climate assessment. Much of the complexity arises from the non-monotonic responses of clouds, precipitation and radiative fluxes to aerosol perturbations under various meteorological conditions. In this study, an aerosol-aware WRF model is used to investigate the microphysical and radiative effects of aerosols in three weather systems during the March 2000 Cloud Intensive Observational Period campaign at the US Southern Great Plains. Three simulated cloud ensembles include a low-pressure deep convective cloud system, a collection of less-precipitating stratus and shallow cumulus, and a cold frontal passage. The WRF simulations are evaluated by several ground-based measurements. The microphysical properties of cloud hydrometeors, such as their mass and number concentrations, generally show monotonic trends as a function of cloud condensation nuclei concentrations. Aerosol radiative effects do not influence the trends of cloud microphysics, except for the stratus and shallow cumulus cases where aerosol semi-direct effects are identified. The precipitation changes by aerosols vary with the cloud types and their evolving stages, with a prominent aerosol invigoration effect and associated enhanced precipitation from the convective sources. The simulated aerosol direct effect suppresses precipitation in all three cases but does not overturn the aerosol indirect effect. Cloud fraction exhibits much smaller sensitivity （typically less than 2%） to aerosol perturbations, and the responses vary with aerosol concentrations and cloud regimes. The surface shortwave radiation shows a monotonic decrease by increasing aerosols, while the magnitude of the decrease depends on the cloud type.

Numerical Simulation of the Scavenging Rates of Ice Crystals of Various Microphysical Characteristics

Numerical models of trajectories of small aerosol spheres relative to oblate spheroids were used to determine ice crystal scavenging efficiencies. The models included the effects of aerodynamic flow about the ice particle, gravity, aerosol particle inertia and drag and electrostatic effects. Two electric configurations of the ice particle were investigated in detail. The first applied a net charge to the ice particle, of magnitude equal to the mean thunderstorm charge distribution, while the second applied a charge distribution, with no net charge, to the ice particle to model the electric multipole charge distribution. The results show that growing ice crystals with electric multipoles are better scavengers than single ice crystals with net thunderstorm charges, especially in the Greenfield gap (0.1 to 1.0 um), and that larger single crystals are better scavengers than smaller single crystals. The results also show that the low density ice crystals are more effective scavengers with net charges than they are with charge distribution.

Particle number concentration, size distribution and chemical composition during haze and photochemical smog episodes in Shanghai

The aerosol number concentration and size distribution as well as size-resolved particle chemical composition were measured during haze and photochemical smog episodes in Shanghai in 2009. The number of haze days accounted for 43%, of which 30% was severe （visibility 〈 2 km） and moderate （2 km 〈 visibility 〈 3 km） haze, mainly distributed in winter and spring. The mean particle number concentration was about 17,000/cm3 in haze, more than 2 times that in clean days. The greatest increase of particle number concentration was in 0.5-1μm and 1-10 μm size fractions during haze events, about 17.78 times and 8.78 times those of clean days. The largest increase of particle number concentration was within 50-100 nm and 100-200 nm fractions during photochemical smog episodes, about 5.89 times and 4.29 times those of clean days. The particle volume concentration and surface concentration in haze, photochemical smog and clean days were 102, 49, 15 μm3/cm3 and 949, 649, 206 μm2/cm3, respectively. As haze events got more severe, the number concentration of particles smaller than 50 nm decreased, but the particles of 50-200 nm and 0.5-1μm increased. The diurnal variation of particle number concentration showed a bimodal pattern in haze days. All soluble ions were increased during haze events, of which NH4, SO24- and NO3 increased great/y, followed by Na＋, IC, Ca2＋ and CI-. These ions were very different in size-resolved particles during haze and photochemical smog episodes.

Species abundance distributions as a proxy for the niche-neutrality continuum

Aims Species abundance distributions(SADs)are often used to verify mechanistic theories underlying community assembly.However,it is now accepted that SADs alone are not sufficient to reveal biological mechanisms.Recent attention focuses on the relative importance of stochastic dispersal processes versus deterministic processes such as interspecific competition and environmental filtering.Here,we combine a study of the commonness and rarity of species(i.e.the SAD)with mechanistic processes underlying community composition.By comparing the occurrence frequen-cies of each and every species with its abundance,we quantify the relative contributions of common and rare species to the maintenance of community structure.Essentially,we relate the continuum between commonness and rarity with that of niches and neutrality.Methods An individual-based,spatially explicit model was used to simu-late local communities in niche spaces with the same parameters.We generated sets of assemblages from which species were elim-inated in opposing sequences:from common to rare and from rare to common,and investigated the relationship between the abundance and frequency of species.We tested the predictions of our model with empirical data from a field experiment in the environmentally homogeneous alpine meadows of the Qinghai-Tibetan plateau.Important Findings Our simulations support the widespread notion that common species maintain community structure,while rare species maintain species diversity,in both local and regional communities.Our results,both from theoretical simulations and from empirical observations,revealed positive correlations between the abundance of a particular species and its occurrence frequency.SAD curves describe a continuum between commonness and rarity.Removing species from the‘rare’end of this continuum has little effect on the similarity of communities,but remov-ing species from the‘common’end of the continuum causes significant increases in beta diversity,or species turnover,between communities.In local communities distributed in a homogenous habitat,species located at the‘common’end of the continuum should be selected by environmental filtering,with niche space partitioning governed by interspecific competition.Conversely,species located at the‘rare’end of the continuum are most likely subject to stochastic dispersal pro-cesses.Species situated at intermediate locations on this continuum are therefore determined by niche and neutral processes acting together.Our results suggest that,in homogeneous habitats,SAD curves describ-ing the common-rare continuum may also be used to describe the con-tinuum between niches and neutrality.