Incorporation of internal fluctuations in a meandering plume model of concentration fluctuations

A meandering plume model that explicitly incorporates the effects of small-scale structure in the instantaneous plume has been formulated. The model requires the specification of two physically based input parameters; namely, the meander ratio,M, which is dependent on the ratio of the meandering plume dispersion to the instantaneous relative plume dispersion and, a relative in-plume fluctuation measure,k, that is related inversely to the fluctuation intensity in relative coordinates. Simple analytical expressions for crosswind profiles of the higher moments (including the important shape parameters such as fluctuation intensity, skewness, and kurtosis) and for the concentration pdf have been derived from the model. The model has been tested against some field data sets, indicating that it can reproduce many key aspects of the observed behavior of concentration fluctuations, particularly with respect to modeling the change in shape of the concentration pdf in the crosswind direction.List of Symbols C Mean concentration in absolute coordinates - C _r Mean concentration in relative coordinates - C₀ Centerline mean concentration in absolute coordinates - C _r,0 Centerline mean concentration in relative coordinates - f Probability density function of concentration in absolute coordinates - f _c Probability density function of plume centroid position - f _r Probability density function of concentration in relative coordinates - i Absolute concentration fluctuation intensity (standard deviation to mean ratio) - i _r Relative concentration fluctuation intensity (standard deviation to mean ratio) - k Relative in-plume fluctuation measure:k=1/i _r ² - K Concentration fluctuation kurtosis - M Meander ratio of meandering plume variance to relative plume variance - S Concentration fluctuation skewness - x Downwind distance from source - y Crosswind distance from mean-plume centerline - z Vertical distance above ground - Instantaneous (random) concentration - Crosswind dispersion ofnth concentration moment about zero - _ny Mean-plume crosswind (absolute) dispersion - _y Plume centroid (meandering) dispersion in crosswind direction - _y,c Instantaneous plume crosswind (relative) dispersion - Normalized mean concentration in absolute coordinates:C/C ₀ - Particular value taken on by instantaneous concentration,      

A submillimetre survey for protostellar accretion discs using the JCMT–CSO interferometer

We present a submillimetre continuum survey for accretion discs around seven embedded protostars in the Perseus and Serpens molecular clouds. Observations were made at frequencies between 339 and 357 GHz using the James Clerk Maxwell Telescope–Caltech Submillimeter Observatory single-baseline interferometer on Mauna Kea, Hawaii. All the objects in our survey show compact dust emission on scales ≲1 arcsec, assumed to arise in a circumstellar accretion disc. We compare the properties of this compact component with evolutionary indicators, such as the ratio of compact to extended emission, and bolometric temperature. We find that discs of mass ∼0.01 M_⊙ have formed by the Class 0 stage, and that similar mass discs are observed in Class I and Class II sources. A trend is observed whereby the ratio of compact to extended emission in our sources increases from Class 0 to Class II sources. For three of the objects in the survey, NGC 1333 IRAS2:CR1 and SVS13 in Perseus, and FIRS1 in Serpens, the signal-to-noise ratio is sufficient to allow us to model the brightness distributions with elliptical Gaussian and power-law disc models. The Gaussian fits give semimajor half-power radii of approximately 90 to 140 au, at the assumed distance of 350 pc to the Perseus and Serpens clouds.     

Radar Detection of Near-Earth Asteroids 2062 Aten, 2101 Adonis, 3103 Eger, 4544 Xanthus, and 1992 QN

We describe Doppler-only radar observations of near-Earth asteroids 2062 Aten, 2101 Adonis, 3103 Eger, 4544 Xanthus, and 1992 QN that were obtained at Arecibo and Goldstone between 1984 and 1996. Estimates of the echo spectral bandwidths, radar cross sections, and circular polarization ratios of these objects constrain their pole-on breadths, radar albedos, surface roughnesses, taxonomic classes, rotation periods, and pole directions. Aten's bandwidth is consistent with its radiometrically determined diameter of 0.9 km. Adonis has a rotation periodP≤ 11 h and an effective diameter (the diameter of a sphere with the same projected area as the asteroid) between 0.3 and 0.8 km. The radar properties of Adonis suggest it is not a member of taxonomic classes C or M. The effective diameter of Xanthus is between 0.4 and 2.2 km with a rotation periodP≤ 20 h. Echoes from 1992 QN constrain the asteroid's pole-on breadth to be ≥0.6 km and probably exclude it from the C and M taxonomic classes. The strongest Eger echoes are asymmetric with bandwidths that set lower bounds of 1.5 and 2.3 km on the minimum and maximum breadths of the asteroid's pole-on silhouette. If Eger is modeled as a 1.5 × 2.3 km biaxial ellipsoid, then its effective diameter for an equatorial view is 1.5 km end-on and 1.9 km broadside or pole-on, implying a geometric albedo smaller than published values but still consistent with a classification as an E-type object. The near-unity circular polarization ratios of Adonis, Eger, and 1992 QN are among the highest values measured for any asteroid or comet and suggest extreme near-surface roughness at centimeter to meter scales.     

Bulk density optimization to determine subsurface hydraulic properties in Rocky Mountain catchments using the GEOtop model

Integrated watershed models can be used to calculate streamflow generation in snow‐dominated mountainous catchments. Parameterization of water flow is often complicated by the lack of information on subsurface hydraulic properties. In this study, bulk density optimization was used to determine hydraulic parameters for the upper and lower regolith in the GEOtop model. The methodology was tested in two small catchments in the Dry Creek Watershed in Idaho and the Libby Creek Watershed in Wyoming. Modelling efficiencies for profile‐average soil–water content for the two catchments were between 0.52 and 0.64. Modelling efficiencies for stream discharge (cumulative stream discharge) were 0.45 (0.91) and 0.54 (0.94) for the Idaho and Wyoming catchments, respectively. The calculated hydraulic properties suggest that lateral flow across the upper–lower regolith interface is an important driver of streamflow in both the Idaho and Wyoming watersheds. The overall calibration procedure is computationally efficient because only two bulk density values are optimized. The two‐parameter calibration procedure was complicated by uncertainty in hydraulic conductivity anisotropy. Different upper regolith hydraulic conductivity anisotropy factors had to be tested in order to describe streamflow in both catchments.     

Comparison of mid-Pliocene climate predictions produced by the HadAM3 and GCMAM3 General Circulation Models

The mid-Pliocene warm period (ca. 3 to 3.3 million years ago) has become an important interval of time for palaeoclimate modelling exercises, with a large number of studies published during the last decade. However, there has been no attempt to assess the degree of model dependency of the results obtained. Here we present an initial comparison of mid-Pliocene climatologies produced by the Goddard Institute for Space Studies and Hadley Centre for Climate Prediction and Research atmosphere-only General Circulation Models (GCMAM3 and HadAM3). Whilst both models are consistent in the simulation of broad-scale differences in mid-Pliocene surface air temperature and total precipitation rates, significant variation is noted on regional and local scales. There are also significant differences in the model predictions of total cloud cover. A terrestrial data/model comparison, facilitated by the BIOME 4 model and a new data set of Piacenzian Stage land cover [Salzmann, U., Haywood, A.M., Lunt, D.J., Valdes, P.J., Hill, D.J., (2008). A new global biome reconstruction and data model comparison for the Middle Pliocene. Global Ecology and Biogeography 17, 432-447, doi:10.1111/j.1466-8238.2007.00381.x] and combined with the use of Kappa statistics, indicates that HadAM3-based biome predictions provide a closer fit to proxy data in the mid to high-latitudes. However, GCMAM3-based biomes in the tropics provide the closest fit to proxy data.     

Refining rainfall projections for the Murray Darling Basin of south-east Australia—the effect of sampling model results based on performance

One of the aims of developing new climate projections is to better address the requirements of stakeholders—particularly those who require less uncertainty and/or probabilistic information to work with. Projections are continually updated over time as more, and newer, climate model simulations of the future become available but this can introduce problems when it comes to interpreting large samples with differing results. Regional projections of rainfall are characterised by a high level of uncertainty, partly because of different sensitivities of the different models. Some models can be demonstrated to perform relatively poorly when assessed by their ability to simulate present-day means and variability and here we show that the uncertainty in model projections can potentially be reduced when the projection from these models are either discounted or ignored entirely. When applied to the Murray Darling Basin of south east Australia, it is possible to demonstrate a clustering of the results from the better performing models. These indicate that the rainfall changes to be expected as a result of increased greenhouse gas concentrations into the future are more likely to be at the drier end of the full set of model results. This occurs because the better performing models indicate decreases in winter and spring which are significantly different to the changes indicated by the other models. These results suggest that there are compelling reasons for discounting, if not entirely dismissing, some model results based on their failure to satisfy some basic performance criteria.     

Teleconnections in a warmer climate: the pliocene perspective

Migrations toward altered sea surface temperature (SST) patterns in the Indo-Pacific region are present in the recent observational record and in future global warming projections. These SSTs are in the form of ??permanent?? El Ni?o-like (herein termed ??El Padre??) and Indian Ocean Dipole (IOD)-like patterns. The Early Pliocene Warm Period, which bears similarity to future warming projections, may have also exhibited these Indo-Pacific SST patterns, as suggested by regional terrestrial paleo-climatic data and general circulation model studies. The ability to corroborate this assessment with paleo-data reconstructions is an advantage of the warm Pliocene period that is not afforded by future warming scenarios. Thus, the Pliocene period provides us with a warm-climate perspective and test bed for understanding potential changes to future atmospheric interactions given these altered SST states. This study specifically assesses how atmospheric teleconnections from El Padre/IOD SST patterns are generated and propagate to create the regional climate signals of the Pliocene period, as these signals may be representative of future regional climatic changes as well. To do this, we construct a holistic diagnostic rubric that allows us to examine atmospheric teleconnections, both energetically and dynamically, as produced by a general circulation model. We incorporate KE??, a diagnostic adapted from the eddy kinetic energy generation field, to assess the available energy transferred to these teleconnections. Using this methodology, we found that relative to our Modern Control experiments, weaker atmospheric teleconnections prevail under warm Pliocene conditions, although pathways of propagation still appear directed toward the southwestern United States from our tropical Pacific sector forcing. Propagation directly emanating from the Indian Ocean forcing sector appears to be largely blocked, although indirect teleconnective pathways appear traversing the Asian continent toward the North Pacific. The changes in the atmospheric circulation of Indian Ocean region in response to the underlying specified SST forcing (and indicated by Pliocene paleo-data) may have a host of implications for energy transfer out of and into the region, including interactions with the Asian jet stream and changes to the seasonal monsoon cycle. These interactions warrant further study in both past and future warm climate scenarios.