Impacts of aerosol chemical composition on microphysics and precipitation in deep convection

Aerosols affect precipitation by modifying cloud properties such as cloud droplet number concentration (CDNC). Aerosol effects on CDNC depend on aerosol properties such as number concentration, size spectrum, and chemical composition. This study focuses on the effects of aerosol chemical composition on CDNC and, thereby, precipitation in a mesoscale cloud ensemble (MCE) driven by deep convective clouds. The MCE was observed during the 1997 department of energy's Atmospheric Radiation Measurement (ARM) summer experiment. Double-moment microphysics with explicit nucleation parameterization, able to take into account those three properties of aerosols, is used to investigate the effects of aerosol chemical composition on CDNC and precipitation. The effects of aerosol chemical compositions are investigated for both soluble and insoluble substances in aerosol particles. The effects of soluble substances are examined by varying mass fractions of two representative soluble components of aerosols in the continental air mass: sulfate and organics. The increase in organics with decreasing sulfate lowers critical supersaturation (S_c) and leads to higher CDNC. Higher CDNC results in smaller autoconversion of cloud liquid to rain. This provides more abundant cloud liquid as a source of evaporative cooling, leading to more intense downdrafts, low-level convergence, and updrafts. The resultant stronger updrafts produce more condensation and thus precipitation, as compared to the case of 100% sulfate aerosols. The conventional assumption of sulfate aerosol as a surrogate for the whole aerosol mass can be inapplicable for the case with the strong sources of organics. The less precipitation is simulated when an insoluble substance replaces organics as compared to when it replaces sulfate. When the effects of organics on the surface tension of droplet and solution term in the Köhler curve are deactivated by the insoluble substance, S_c is raised more than when the effects of sulfate on the solution term are deactivated by the insoluble substance. This leads to lower CDNC and, thus, larger autoconversion of cloud liquid to rain, providing less abundant cloud liquid as a source of evaporative cooling. The resultant less evaporative cooling produces less intense downdrafts, weaker low-level convergence, updrafts, condensation and, thereby, less precipitation in the case where organics is replaced by the insoluble substance than in the case where sulfate is replaced by the insoluble substance. The variation of precipitation caused by the change in the mass fraction between the soluble and insoluble substances is larger than that caused by the change in the mass fraction between the soluble substances.     

Two luminosity classes of quasars

Observations of quasars suggest that compact quasars with flat radio spectra belong to the class of dwarf galaxies and extended quasars with steep spectra to the one of bright galaxies. This can be seen also in the associations of quasars with clusters of galaxies, compact quasars occurring in nearby clusters and extended quasars in distant clusters.     

Dynamical friction on a satellite of a disk galaxy: The circular orbit

In a previous paper, we have studied dynamical friction during a parabolic passage of a companion galaxy past a disk galaxy. This paper continues with the study of satellites in circular orbits around the disk galaxy. Simulations of orbit decay in a self gravitating disk are compared with estimates based on two-body scattering theories; the theories are found to give a satisfactory explanation of the orbital changes. The disk friction is strongly dependent on the sense of rotation of the companion relative to the rotation of the disk galaxy as well as on the amount of mass in a spherical halo. The greatest amount of dynamical friction occurs in direct motion if no spherical halo is present. Then the infall time from the edge of the disk is about one half of the orbital period of the disk edge. A halo twice as massive as the disk increases the infall time four fold. The results of Quinn and Goodman, obtained with a non-self-gravitating method, agree well with our experiments with massive halos (Q ₀ 1.5), but are not usable in a more general case. We give analytic expressions for calculating the disk friction in galaxies of different disk/halo mass ratios.     

Global sediment core-top calibration of the TEX86 paleothermometer in the ocean

The TEX₈₆ (TetraEther indeX of tetraethers consisting of 86 carbon atoms) paleothermometer is based on the relative distribution of archaeal lipids, i.e. isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs), and is increasingly used to reconstruct past sea water temperatures. To establish a more extensive, global calibration of the TEX₈₆ paleothermometer, we analyzed GDGTs in 287 (in comparison with 44 in currently used calibration) core-top sediments distributed over the world oceans and deposited at different depths. Comparisons of TEX₈₆ data with (depth-weighted) annual mean temperatures of the overlying waters between 0 m and 4000 m as well as with different seasonal mean temperatures at 0 m water depth showed that the TEX₈₆ proxy reflects mostly annual mean temperatures of the upper mixed layer. The relationship between TEX₈₆ values and sea-surface temperatures (SSTs) was non-linear mainly because below 5 °C the change in TEX₈₆ values was minor with temperature. This suggests that the TEX₈₆ proxy might not be directly applicable for the Polar Oceans. Nevertheless, between 5 °C and 30 °C, the TEX₈₆ proxy has a strong linear relationship with SSTs. Here, we, therefore, propose a new linear calibration model (T = −10.78 + 56.2 ∗ TEX₈₆, r² = 0.935, n = 223) for past SST reconstructions using the TEX₈₆ palaeothermometer.     

Patterns,puzzles and people: implementing hydrologic synthesis

There have been several calls made for hydrologic synthesis research: namely activities which unify diverse data sources across sites, scales and disciplines to uncover new connections and to promote a holistic understanding of water science. This paper draws on the NSF‐funded Hydrological Synthesis Project (HSP) run by the University of Illinois at Urbana‐Champaign to elucidate mechanisms, benefits and challenges of implementing hydrologic synthesis research from the perspectives of participants in a pilot research study. Two broadly different mechanisms of implementing synthesis were adopted in the HSP: 6‐week Summer Institutes in which Ph.D. students conducted team‐based research under the guidance of faculty mentors, and focused workshops which disseminated knowledge and shared experiences between scientists at many different career levels. The Summer Institutes were a test bed in which new ideas could be explored, assisted students in developing a wide range of skills, and were highly productive, but posed challenges for mentors and students because the ‘new’ research topics initiated during the Institutes' programmes needed to be completed in competition with students' ongoing Ph.D. research or mentor's existing research programs. The workshop‐based model circumvented this conflict and was also highly productive, but did not offer the same opportunity to experiment with new ideas as part of the synthesis research. Leadership, trust, flexibility and long gestation times were all important to bringing synthesis research to a positive resolution. Funding models that embrace the exploratory aspects of synthesis and provide adequate support to mentors and students over these long timescales would facilitate future hydrologic synthesis research. Copyright © 2011 John Wiley & Sons, Ltd.     

The influence of national temperature fluctuations on opinions about climate change in the U.S. since 1990

Public opinion in the United States about human-caused climate change has varied over the past 20 years, despite an increasing consensus about the issue in the expert community. Attitudes about climate change have been attributed to a number of factors including personal values, political ideology, the media environment and personal experience. Recent studies have found evidence that the temperature can influence one’s opinion about climate change and willingness to change behaviour and/or support climate policy. Although there is some evidence that individual cool or warm years have influenced large-scale opinion about climate change, the extent to which temperature can explain the past variability in public opinion and public discourse about climate change at the national level is not known. Here we isolate the relationship between opinion about climate change and temperature at the national scale, using data from opinion polls, a discourse analysis of opinion articles from five major daily newspapers, and a national air temperature database. The fraction of respondents to national polls who express “belief in” or “worry about” climate change is found to be significantly correlated to U.S. mean temperature anomalies over the previous 3–12 months. In addition, the fraction of editorial and opinion articles which “agree” with the expert consensus on climate change is also found to be significantly correlated to U.S. mean temperature anomalies at seasonal and annual scales. These results suggest that a fraction of the past variance in American views about climate change could potentially be explained by climate variability.     

Leading spiral arms,retrograde galaxy encounters and massive halos

We study, theoretically and with N-body simulations, the formation of spiral patterns in retrograde galaxy encounters. A one-armed leading spiral dominates in a disk if the tidal perturbation from the companion is large and the disk is surrounded by a massive halo. Otherwise, a trailing pattern forms. The leading arm is made up of particles in slightly elongated orbits whose turning points outline the arm. The arm rotates opposite to the disk rotation. We have found one spiral galaxy, NGC4622, with a leading arm near its nucleus. From the literature, we find that very few spirals, if any, in a sample of strongly perturbed galaxies have leading arms. A possible reason for this is that few spiral galaxies have a halo with larger mass than the disk within the visible disk.     

Formation of leading spiral arms in retrograde galaxy encounters

We study, theoretically and withN-body simulations, the formation of spiral structures in retrograde galaxy encounters. A one-armed leading spiral dominates in a disc if (i) the tidal perturbation from the retrograde companion is large enough, and (ii) the disc is surrounded by a massive halo. From the literature we find that very few spirals in a sample of galaxies with a large companion have leading spiral arms. A possible reason for this is that very few spiral galaxies have a halo with a larger mass than the disc mass.     

Domain of the Gods: an editorial essay

She [the earth] belongs to men as the sky belongs to God. –Plinius Secondus, Natural History Each year in the Fijian village of Matacawlevu on the island of the same name, the people hold a festival to celebrate the planting of crops. There is food, music and no shortage of kava. But the most important part of the festival is a church service where the local minister leads the village in prayers for good weather and a strong harvest. According to Fijian religious tradition, a mix of Methodism and animism typical of the South Pacific, proper prayer assures that the rains will come. In the event of a drought, people blame either each other for not being devout, or blame the minister for failing to properly deliver the people’s message. The tradition of the planting festival masks a sophisticated system of land management that has sustained indigenous people in Fiji and across many islands in Polynesia and Melanesia for centuries. In Matacawalevu, the Chief’s decision to sanction planting is based upon years of experience and the advice of a villager trained in agronomy. Crops are rotated and selected land is left fallow to maintain soil fertility. The village’s agricultural practices follow directly from a deeply held belief that people exert control over land. The weather, however, is up to God.