Multi-Century Tree-Ring Reconstructions of Colorado Streamflow for Water Resource Planning

Water resource management requires knowledge of the natural variability in streamflow over multiple time scales. Reconstructions of streamflow derived from moisture-sensitive trees extend, in both time and magnitude, the variability provided by relatively short gage records. In this study, we present a network of 14 annual streamflow reconstructions, 300–600 years long, for gages in the Upper Colorado and South Platte River basins in Colorado generated from new and existing tree-ring chronologies. Gages for the reconstruction were selected on the basis of their importance to two of the largest Colorado Front Range water providers, who provided the natural flow data for the calibration with tree-ring data. The reconstruction models explain 63–76% of the variance in the gage records and capture low flows particularly well. Analyses of the reconstructions indicate that the 20th century gage record does not fully represent the range of streamflow characteristics seen in the prior two to five centuries. Multi-year drought events more severe than the 1950s drought have occurred, notably in the 19th century, and the distribution of extreme low flow years is markedly uneven over the past three centuries. When the 14 reconstructions are grouped into Upper Colorado, northern South Platte, and southern South Platte regional flow reconstructions, the three time series show a high degree of coherence, but also time-varying divergences that may reflect the differential influence of climatic features operating in the western U.S. These reconstructions are currently being used by water managers to assess the reliability of water supply systems under a broader range of conditions than indicated by the gage records alone.     

The remarkable wide range spatial scaling of TRMM precipitation

The advent of space borne precipitation radar has opened up the possibility of studying the variability of global precipitation over huge ranges of scale while avoiding many of the calibration and sparse network problems which plague ground based rain gage and radar networks. We studied 1176 consecutive orbits of attenuation-corrected near surface reflectivity measurements from the TRMM satellite PR instrument. We find that for well-measured statistical moments (orders 0 < q < 2) corresponding to radar reflectivities with dBZ < 57 and probabilities > 10^− 6, that the residuals with respect to a pure scaling (power law) variability are remarkably low: ± 6.4% over the range 20,000 km down to 4.3 km. We argue that higher order moments are biased due to inadequately corrected attenuation effects. When a stochastic three — parameter universal multifractal cascade model is used to model both the reflectivity and the minimum detectable signal of the radar (which was about twice the mean), we find that we can explain the same statistics to within ± 4.6% over the same range. The effective outer scale of the variability was found to be 32,000 ± 2000 km. The fact that this is somewhat larger than the planetary scale (20,000 km) is a consequence of the residual variability of precipitation at the planetary scales. With the help of numerical simulations we were able to estimate the three fundamental parameters as α ≈ 1.5, C₁ = 0.63 ± 0.02 and H = 0.00 ± 0.01 (the multifractal index, the codimension of the mean and the nonconservation parameter respectively). There was no error estimate on α since although α = 1.5 was roughly the optimum value, this conclusion depended on assumptions about the instrument at both low and high reflectivities. The value H = 0 means that the reflectivity can be modeled as a pure multiplicative process, i.e. that the reflectivity is conserved from scale to scale. We show that by extending the model down to the inner “relaxation scale” where the turbulence and rain decouple (in light rain, typically about 40 cm), that even without an explicit threshold, the model gives quite reasonable predictions about the frequency of occurrence of perceptible precipitation rates.While our basic findings (the scaling, outer scale) are almost exactly as predicted twenty years ago on the basis on ground based radar and the theory of anisotropic (stratified) cascades, they are incompatible with classical turbulence approaches which require at least two isotropic turbulence regimes separated by a meso-scale “gap”. They are also incompatible with classical meteorological phenomenology which identifies morphology with mechanism and breaks up the observed range 4 km–20 000 km into several subranges each dominated by different mechanisms. Finally, since the model specifies the variability over huge ranges, it shows promise for resolving long standing problems in rain measurement from both (typically sparse) rain gage networks and radars.     

Generalised scale invariance and multiplicative processes in the atmosphere

Many geophysical fields show highly intermittent fractal structures spanning wide ranges of scale. However, few are isotropic: texture, stratification, as well as variable (scale dependent) orientation of structures is far more common. To deal with such fractals, we must generalise the idea of scale invariance beyond the familiar self-similar (or even self-affine) notions. Taking the atmosphere as our primary example (however, we also model galaxies), we outline the necessary formalism (generalised scale invariance), and show how it can be used to deal with the strongly intermittent structures which result from multiplicative (cascade type) processes concentrating matter or energy into smaller and smaller scales.We illustrate these ideas with rain data from blotting paper and radar, showing first how to directly estimate the elliptical dimension characterising the stratification, and second, how to determine universal scale-independent (invariant) codimension functions that characterise the distribution of the intense rain regions.     

Carbon to nitrogen (C:N) stoichiometry of the spring–summer phytoplankton bloom in the North Water Polynya (NOW)

The carbon to nitrogen (C:N) stoichiometry of phytoplankton production varied significantly during the spring–summer bloom in the North Water Polynya (NOW), from April through July 1998. The molar ratio of particulate organic carbon (POC) to nitrogen (PON) production by phytoplankton (ΔPOC:ΔPON) increased from 5.8 during April through early June to 8.9 in late June and July. The molar dissolved inorganic carbon (DIC) to nitrate+nitrite (NO₃) drawdown ratio (ΔDIC: ΔNO₃) increased from 6.7 in April and May, to 11.9 in June (no estimate for July because of ice melting). The discrepancy between ΔPOC:ΔPON and ΔDIC:ΔNO₃ was likely due to dissolved organic carbon (DOC) production. Increased ΔPOC:ΔPON of phytoplankton and surface water ΔDIC:ΔNO₃ throughout the phytoplankton blooms resulted from changes in physical properties of the upper water column, such as reduced thickness of the surface mixed layer that exposed phytoplankton to increased photosynthetically available radiation (PAR), accompanied by NO₃ depletion. This is expected to have significant effects on the cycling of carbon (C) and nitrogen (N) in pelagic ecosystems, as the increased C:N ratio of organic matter decreases its quality as substrate for grazers and microbial communities. Based on ΔPOC:ΔPON, the ratio of POC to chlorophyll a (Chl) production (ΔPOC:ΔChl) and the relationship between Chl yields and NO₃ depletion, we estimate that 71±17% and 46±20% of the depleted NO₃ went to PON production in the euphotic zone over the polynya from April to early June, and late June to July, respectively. The remaining NO₃ was likely channelled to dissolved organic nitrogen (DON) and heterotrophic bacteria, which were not returned to the dissolved inorganic nitrogen (DIN) pool through recycling during the course of the study. Hence, the autotrophic production of organic N and its recycling by the microbial food web were not coupled temporally.     

Trophic structure and pathways of biogenic carbon flow in the eastern North Water Polynya

In the eastern North Water, most of the estimated annual new and net production of carbon (C) occurred during the main diatom bloom in 1998. During the bloom, at least 30% of total and new phytoplankton production occurred as dissolved organic carbon (DOC) and was unavailable for short-term assimilation into the herbivorous food web or sinking export. Based on particle interceptor traps and ²³⁴Th deficits, 27% of the particulate primary production (PP) sank out of the upper 50 m, with only 7% and 1% of PP reaching the benthos at shallow (≈200 m) and deep (≈500 m) sites, respectively. Mass balance calculations and grazing estimates agree that ≈79% of PP was ingested by pelagic consumers between April and July. During this period, the vertical flux of biogenic silica (BioSi) at 50 m was equivalent to the total BioSi produced, indicating that all of the diatom production was removed from the euphotic zone as intact cells (direct sinking) or empty frustules (grazing or lysis). The estimated flux of empty frustules was consistent with rates of herbivory by the large, dominant copepods and appendicularians during incubations. Since the carbon demand of the dominant planktivorous bird, Alle alle, amounted to ≈2% of the biomass synthesized by its main prey, the large copepod Calanus hyperboreus, most of the secondary carbon production was available to pelagic carnivores. Stable isotopes indicated that the biomass of predatory amphipods, polar cod and marine mammals was derived from these herbivores, but corresponding carbon fluxes were not quantified. Our analysis shows that a large fraction of PP in the eastern North Water was ingested by consumers in the upper 50 m, leading to substantial carbon respiration and DOC accumulation in surface waters. An increasingly early and prolonged opening of the Artic Ocean is likely to promote the productivity of the herbivorous food web, but not the short-term efficiency of the particulate, biological CO₂ pump.     

The delineation of rain areas from visible and IR satellite data for GATE and mid‐latitudes

Abstract

Two‐dimensional pattern matching has been used to delineate raining areas of clouds from GATE and Montreal GOES visible and IR satellite data, with radar as ground truth. For the cases examined, the cloud cover was of the order of 4 times larger than the rain area, requiring skill to separate out low‐thick or high‐thin non‐precipitating clouds from cumulus systems, which is difficult using a single threshold. The more flexible approach described here has allowed useful rain maps to be generated for all the types of weather systems examined. The optimum boundary separating raining from non‐raining areas is relatively insensitive to diurnal and day‐to‐day variations, but is different for the tropical Atlantic and for Montreal.     

TREE-GROWTH RESPONSE TO ENSO EVENTS IN THE CENTRAL COLORADO FRONT RANGE

The influence of climate associated with El Niño/Southern Oscillation (ENSO) events on tree growth in the central Colorado Front Range is investigated through the analysis of two high altitude tree-ring chronologies. Dendrochronological techniques are used to determine if ENSO-related climatic effects are detectable in tree-ring width patterns in the central Colorado Front Range. The form of the tree-growth response is identified and the variability of the influence of these events on tree growth over time is investigated. Results indicate that tree growth in this area does respond to ENSO events, but the response varies with species and type of event. El Niño-influenced climate tends to result in larger tree rings the year of or year following the event, while La Niña-influenced climate tends to result in smaller rings the year after the event, reflecting spring moisture conditions. Trees have a more consistent response to La Niña events, but El Niño events seem to have a greater effect on extremes in growth. The relationship between the Southern Oscillation Index (SOI) and tree growth has varied over time, probably because of the fact that ENSO events, characterized by the SOI, vary in magnitude and amplitude. [Key words: ENSO, dendrochronology, Colorado Front Range.]     

Multifractal objective analysis: conditioning and interpolation

 We investigate various ways of statistically estimating multifractal fields from sparse data. First, the problem is set in the general framework of conditional expectations, and the effect of (multi) fractal sampling on the statistics of the measured process is investigated, showing how analytical expressions describing the statistical properties of the phenomenon should be modified by the sampling. Then, several techniques are introduced, our goal being to estimate the intensity of a field at resolution λ, given samples of the process collected by networks at higher resolutions Λ>λ. The general strategy underlying all the estimating techniques presented is to approximate the unknown field values at resolution λ by means of most likely estimates conditional to the available information at resolution Λ>λ. Finally, the procedures are tested on simulated lognormal multifractal fields sampled by means of fractal networks, and the propagation of the errors in a scaling framework is also discussed. These techniques are necessary for estimating geophysical processes in regions where no monitoring stations are present, a scenario often encountered in practice, and may also be of great help in studying natural hazards and risk assessment.     

Scaling vesicle distributions and volcanic eruptions

Models of coalescence-decompressive expansion of the later stages of bubble growth predict that for diverse types of volcanic products the vesicle number densities (n(V)) are of the scaling form where V is the volume of the vesicles and B₃ the 3-dimensional scaling (power law) exponent. We analyze cross sections of 9 pumice samples showing that over the range of bubble sizes from 10 m to 3 cm, they are well fit with B₃0.85. We show that to within experimental error, this exponent is the same as that reported in the literature for basaltic lavas, and other volcanic products. The importance of the scaling of vesicle distributions is highlighted by the observation that they are particularly effective at packing bubbles allowing very high vesicularities to be reached before the critical percolation threshold, a process which—for highly stressed magmas—would trigger explosion. In this way the scaling of the bubble distributions allows them to be key actors in determining the rheological properties and in eruption dynamics.Editorial responsibility: D. Dingwell