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  • Articles  (73)
  • Comets  (37)
  • Turbulence  (36)
  • Springer  (73)
  • Oxford University Press
  • 2020-2023
  • 1995-1999  (73)
  • Geosciences  (73)
  • 1
    Electronic Resource
    Electronic Resource
    An analysis of the characteristics of rough bed turbulent shear stresses in an open channel (1997)
    Springer
    Stochastic environmental research and risk assessment 11 (1997), S. 193-210 
    Details
    ISSN: 1436-3259
    Keywords: Turbulence ; sediment ; fluvial ; river ; bursting process ; statistics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Energy, Environment Protection, Nuclear Power Engineering , Geography , Geosciences
    Notes: Abstract Entrainment of sediment particles from channel beds into the channel flow is influenced by the characteristics of the flow turbulence which produces stochastic shear stress fluctuations at the bed. Recent studies of the structure of turbulent flow has recognized the importance of bursting processes as important mechanisms for the transfer of momentum into the laminar boundary layer. Of these processes, the sweep event has been recognized as the most important bursting event for entrainment of sediment particles as it imposes forces in the direction of the flow resulting in movement of particles by rolling, sliding and occasionally saltating. Similarly, the ejection event has been recognized as important for sediment transport since these events maintain the sediment particles in suspension. In this study, the characteristics of bursting processes and, in particular, the sweep event were investigated in a flume with a rough bed. The instantaneous velocity fluctuations of the flow were measured in two-dimensions using a small electromagnetic velocity meter and the turbulent shear stresses were determined from these velocity fluctuations. It was found that the shear stress applied to the sediment particles on the bed resulting from sweep events depends on the magnitude of the turbulent shear stress and its probability distribution. A statistical analysis of the experimental data was undertaken and it was found necessary to apply a Box-Cox transformation to transform the data into a normally distributed sample. This enabled determination of the mean shear stress, angle of action and standard error of estimate for sweep and ejection events. These instantaneous shear stresses were found to be greater than the mean flow shear stress and for the sweep event to be approximately 40 percent greater near the channel bed. Results from this analysis suggest that the critical shear stress determined from Shield's diagram is not sufficient to predict the initiation of motion due to its use of the temporal mean shear stress. It is suggested that initiation of particle motion, but not continuous motion, can occur earlier than suggested by Shield's diagram due to the higher shear stresses imposed on the particles by the stochastic shear stresses resulting from turbulence within the flow.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02427915
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  • 2
    Electronic Resource
    Electronic Resource
    On the existence of small comets and their interactions with planets (1996)
    Springer
    Earth, moon and planets 72 (1996), S. 243-249 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; small ; search program ; planets
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Arguments are presented for a substantial, unexplored population of comets with radii less than 1 km. Known examples confirm this population and extrapolation of any plausible size-distribution function indicates large numbers. However, their accurate numbers, orbital characteristics, and physical properties are unknown. Thus, even though the small comets may be the most frequent cometary bodies impacting the planets, a quantitative evaluation is not currently possible. We advocate an optimized, dedicated search program to characterize this population.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00117524
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  • 3
    Electronic Resource
    Electronic Resource
    Comet Hale-Bopp (1995 O1) (1996)
    Springer
    Earth, moon and planets 73 (1996), S. 15-22 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; named objects ; Comet Hale-Bopp (199501) ; photometry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Considerable interest has been raised by the discovery of Comet Hale-Bopp (1995 O1) and the possibility that it might become a very bright object in Spring 1997. The evidence to support either of the conflicting hypothesis (an intrinsically bright comet or a faint comet in a very large outburst) is too limited to reach solid conclusions and may remain so for some months yet. The pre-discovery observations encountered to date provide some limits to photometric models and suggest that the comet may be intrinsically bright, but do not yet permit a firm discrimination, even between extreme scenarios, due to the enormous extrapolation that must be made from the heliocentric distance at discovery, to that of perihelion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00058042
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  • 4
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    Monte Carlo Simulations of Oort Cloud Comets: the Influence of Mantle Blow-off on the Inclination Distribution of Jupiter Family (1997)
    Springer
    Earth, moon and planets 77 (1997), S. 239-244 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; Jupiter family ; simulations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We have developed an efficient Monte Carlo method by which we can evaluate the evolution of comets. There are many poorly known evolutional parameters, and we have investigated the influence of these parameters on the final populations and the inclination distributions of short-period comets. We compare the observed and calculated inclination distributions of different comet populations and obtain a good fit for the inclinations of the Jupiter family comets by assuming a mantle blow-off and a sudden brightening of the comet when its perihelion distance is lowered in a major jump.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006281618542
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  • 5
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    What is the Parent of CH in the Coma of Comet C/1995 O1 (Hale-Bopp)? (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 85-91 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; chemistry ; spectra
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The ratio of CH4/CO in comets is an important indicator of the region of their formation. However, it is difficult to measure the quantity of CH+. The Giotto Ion Mass Spectrometer experiment observed a quantity of CH+ which seemed inconsistent with the amount of CH+. Thus, it was proposed that a source of the CH+ was a distributed source in the dust. We tested this hypothesis by observing the CH emission strength in comet Hale-Bopp as a function of dustiness. We see no strong correlation between the dust and the gas. Thus, for Hale-Bopp,dust is unlikely to be a dominant source of CH.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006235715217
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  • 6
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    Detection Of Soft X-Ray Emission From Comet C/1995 O1 (Hale–Bopp) (1997)
    Springer
    Earth, moon and planets 77 (1997), S. 293-298 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; general-comets ; individual (Hale–Bopp 1995 O1
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We report the detection of soft X-rays from comet C/1995 O1 (Hale-Bopp) by the Low Energy Concentrator Spectrometer (LECS) on-board the X-ray satellite, BeppoSAX. The observations took place on 1996 September 10–11 approximately 1 day after a large dust outburst (Schulz et al., 1997–1999). After correcting for the comets motion, a 7σ enhancement was found centered (2.1 ± 1.3) x 105 km from the position of the nucleus, in the general solar direction. The total X-ray luminosity in the 0.1–2.0 keV energy band is 5 x 1016 erg s−1 which is at least a factor of ∼ 3 greater than measured by the Extreme Ultraviolet Explorer (EUVE)4 days later and suggests that the bulk of the emission measured by the LECS is related to the dust outburst. The extracted LECS spectrum is well fit by a thermal bremsstrahlung-like distribution of temperature of 0.29 ± 0.06 keV - consistent with that observed in other comets. We find no evidence for fluorescent carbon or oxygen emission and place 95% confidence limits of 1.0 x 1015 and 7.8 x 1015 erg s−1 to narrow line emission at 0.28 and 0.53 keV, respectively. We calculate that if such lines are present, they constitute at most 18% of the 0.1–2.0 keV continuum luminosity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006253028962
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  • 7
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    Collisional Quenching of OH Radio Emission from Comet Hale-Bopp (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 45-51 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; production rates ; OH
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Observations of comets in the 18-cm OH transitions offer a means to probe gas production, kinematics, and OH excitation in comets. We present initial results of OH observations of comet Hale-Bopp obtained with the NRAO 43 m antenna located in Greenbank, WV. Maps of the emission provide strong constraints on the amount of quenching of the inversion of the OH ground state Λ-doublet in the coma. Analysis of the total radio OH flux and maps of its radial brightness distribution indicate a quenched region on the order of ∼500,000 km during March and April 1997. This large value is generally consistent with previous observations of radio OH quenching in lower production rate comets when the high production rate of comet Hale-Bopp is considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006258908857
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  • 8
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    New Molecular Species in Comet C/1995 O1 (Hale-Bopp) Observed with the Caltech Ssubmillimeter Observatory (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 13-20 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; composition ; radio observations ; spectroscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present millimeter-wave observations of HNCO, HC3N, SO, NH2CHO, H13CN, and H3O+ in comet C/1995 O1 (Hale-Bopp)obtained in February–April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H2O is (4-13) × 10-4. HC3N, SO, and NH2CHO are detected for the first time in a comet. The fractional abundance of HC3N based on observations of three rotational lines is (1.9 ± 0.2) × 10-4. Four transitions of SO are detected and the derived fractional abundance, (2-8) ×10-3, is higher than the upper limits derived from UV observations of previous comets. Observations of NH2CHO imply a fractional abundance of (1-8) × 10-4. H3O is detected for the first time from the ground. The H13CN (3-2)transition is also detected and the derived HCN/H13CN abundance ratio is 90 ± 15, consistent with the terrestrial12C/13C ratio. In addition, a number of other molecular species are detected, including HNC, OCS, HCO+, CO+, and CN(the last two are first detections in a comet at radio wavelengths).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006281802554
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  • 9
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    The HNC/HCN Ratio in Comets (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 29-35 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; interstellar molecules
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4−3 rotational transitions of both species for heliocentric distances 0.93 〈 r 〈 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced insignificant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006202823878
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  • 10
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    Observations Of Scattered Light From Cometary Dust And Their Interpretation (1997)
    Springer
    Earth, moon and planets 79 (1997), S. 221-245 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; dust ; colour ; polarization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract This review begins with a discussion of the techniques needed for observations of scattered light from cometary dust. After an introduction into the basic concepts of the scattering process, observations of the phase curves of brightness, colour and polarization are covered. Images of colour and polarization are presented and the observed relation of colour and polarization in jets and shells is discussed. The interpretation of the measurements is based on the power law size distributions of dust grains observed from space. The power index must lie between 2 and 4 to provide the mass budget and visibility of the dust coma in accordance with the basic facts of cometary physics. Application of mechanical (radiation pressure) theory to cometary images allows us to derive related power law distributions for comets not explored by spacecraft. Grain scattering models are presented and compared with observations. A prediction is made of the spatial distribution of Stokes parameters U and V in the presence of aligned particles. Up to now such patterns have not been observed. Future work should include the exploration of comets at small and possibly very small phase angles and a detailed comparison of polarization and colour images of comets with thermal images and with models based on mechanical theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006249703639
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  • 11
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    Dust-Gas Interrelations In Comets: Observations And Theory (1997)
    Springer
    Earth, moon and planets 79 (1997), S. 275-306 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; Hale-Bopp ; dynamics ; photochemistry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The development of the expanding atmosphere from the evaporating cometary nucleus has traditionally focused on observing and modeling the separate development of two distinct components, gas and dust,which are coupled dynamically with one another at distances out to a few tens of cometary radii. In the last decade or so, however, direct evidence from observations and suggestions from theory suggest that the dusty-gas coma is a tightly coupled system where material is transferred between the solid and gaseous phase as an important integral part of the basic development of the coma. Comet Hale-Bopp (C/1995 O1) was discovered far from the sun and is the largest and most productive comet, in the sense of release of gas and dust in modern times. This has permitted observations to be made over an unprecedented range of heliocentric distance. This paper presents a review of a range of important issues regarding interrelations between dust and gas in comets, a description of the gas and dust environment for Hale-Bopp, and a summary of the preliminary results from Hale-Bopp which are relevant to these issues. Particular topics include dusty-gas models, dust fading and fragmentation, the role of dust and gas jets, the day/night distribution of gas and dust, and extended sources of gas.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006257922294
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  • 12
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    Dynamical evolution of comets and the problem of cometary fading (1996)
    Springer
    Earth, moon and planets 72 (1996), S. 35-40 
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    ISSN: 1573-0794
    Keywords: Comets ; dynamics ; fading problem
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Possibilities to explain the observed 1/a-distribution are discussed in the light of improved understanding of the dynamical evolution of long-period comets. It appears that the ‘fading problem’ applies both to single-injection and continuous-injection models. Although uncertainties due to nongravitational effects do not allow detailed results to be drawn from the observed 1/a-distribution at small perihelion distance q, that for q ≳ 1.5 AU shows that a constant fading probability cannot explain all the features of the observed distribution. Assuming that comets can reappear following a period of fading, values for the assumed constant fading and renewal probabilities, and the total cometary flux have been estimated for q 〉 1.5 AU.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00117500
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  • 13
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    Light curve behaviour in C/1995 O1 (Hale-Bopp) — II (1996)
    Springer
    Earth, moon and planets 75 (1996), S. 87-94 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; named objects ; C/1995 O1 (Hale-Bopp)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Comet Hale-Bopp is, without doubt, one of the most important objects for cometary science which has ever been observed. The light curve is well-observed from a heliocentric distance 7.2 AU and some information is available at distances as great as 17 AU. This allows the photometric evolution of the comet to be studied as different volatiles dominate the activity. Three different phases of activity are seen as the heliocentric distance reduces during 1996: the initial phase of very fast brightening, characterized by a r −5 law; a “standstill” in the light curve when the brightening law reduced to r −1, which coincides with the initiation of water sublimation at r ~ 4 AU; and a further phase of more rapid brightening with an r −3.5 law, similar to the mean for comets classed as “fairly new”, which initiated at r ~ 4 AU.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00054506
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  • 14
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    The Rotational Parameters Of Hale–Bopp (C/1995 O1) From Observations Of The Dust Jets At Pic Du Midi Observatory (1997)
    Springer
    Earth, moon and planets 77 (1997), S. 167-180 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; Hale–Bopp ; period ; rotation ; rotational state ; dust velocity ; dust size
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We observed comet Hale-Bopp (C/1995 O1) at Pic du Midi Observatory in 1997 from February 2.24 UT to March 31.89 UT with the 1.05-m telescope equipped with a CCD camera and broad- and narrow-band IHW filters. A total of 30000 images were acquired both during night- and day-time. The images were automatically reduced and all the images obtained within 10 min. were co-added to give a set of ∼1000 images used during the analysis. We can identify two jets on the images. The position angle of the brightest jet from February 2.24 UT to March 5.22 UT is measured using an automatic routine which searches for the averaged position angle of the maximum of brightness at a projected distance of 3200–6100 km from the optocenter. A preliminary model of nucleus rotation is used to fit the data and retrieve the rotational parameters of the nucleus. The best fit is found for a source located at a latitude of 64 ± 3°, a sidereal rotation period of 11.35 ± 0.04 h and a right ascension and declination of the North pole of 275 ± 10° and -57 ± 10°. This preliminary analysis shows no evidence for a precession. Grains with velocities of 450–600 m s−1 and radii 〈;∼ 1 μm dominate the optical scattering cross section in the jets.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006234813574
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  • 15
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    X-Ray Emission From Comet Hale–Bopp (1997)
    Springer
    Earth, moon and planets 77 (1997), S. 283-291 
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    ISSN: 1573-0794
    Keywords: Comets ; individual, X-rays ; solar system
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The discovery of X-ray emission from comets has created a number of questions about the physical mechanism producing the radiation. There are now a variety of explanations for the emission, from thermal bremsstrahlung of electrons off neutrals or dust, to charge exchange induced emission from solar wind ions, to scattering of solar X-rays from attogram dust, to reconnection of solar magnetic field lines. In an effort to understand this new phenomenon, we observed but failed to detect in the X-ray the very dusty and active comet C/Hale-Bopp 1995 O1 over a two year period, September 1996 to December 1997, using the ROSAT HRI imaging photometer at 0.1–2.0 keV and the ASCA SIS imaging spectrometer at 0.5–10.0 keV. The results of our Hale-Bopp non-detections, when combined with spectroscopic imaging 0.08–1.0 keV observations of the comet by EUVE and BeppoSAX, show that the emission has the same spectral shape and strong variability seen in other comets. Comparison of the ROSAT photometry of the comet to our ROSAT database of 8 comets strongly suggests that the overall X-ray faintness of the comet was due to an emission mechanism coupled to gas, and not dust, in the comet’s coma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006274812124
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  • 16
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    Gaseous Jets in Comet Hale-Bopp (1995 O1) (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 131-136 
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    ISSN: 1573-0794
    Keywords: Comets ; Hale-Bopp ; coma morphology ; narrowband imaging ; jets
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We report the identification of gas jets in comet Hale-Bopp in OH, NH, CN, C2 and C3. This is the first time OH and NH jets without an obvious optical dust jet counterpart have been identified in narrowband comet images. We also confirm the existence of CN jets as reported by Larson et al. (1997) and Mueller et al. (1998). Jet features can be seen in the March and April 1997 datasets, approximately a month before and after perihelion. Our results contribute to the understanding of both the chemical properties of the comet as well as the physical mechanisms necessary to produce these features.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006280908373
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  • 17
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    Evolution of Dust Shells and Jets in the Inner Coma of Comet C/1995 O1 (Hale-Bopp) (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 205-210 
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    ISSN: 1573-0794
    Keywords: Comets ; individual ; C/1995 O1 Hale-Bopp ; dust shells ; spiral jets
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Comet Hale-Bopp was observed with the 80 cm reflector + CCD at the Haute-Provence observatory (OHP) and with the 62 cm reflector + CCD at the Saint-Véran observatory (Queyras, France). The morphology of the shells was followed from their first appearence on 1997 Jan. 30, until their disappearance on May 9. These shells spread from the nucleus region with a velocity in agreement with a nuclear rotation period of about 11.33 hours. We report also a short and bright dust ejection on May 8. CN images show a long spiral jet in the tailward side invisible on continuum images. The circumnuclear structures have been followed at Saint-Véran from Apr. 5 to Apr. 11, 1997 with a high spatial resolution (200 km/pixel). We have followed the emergence of a recurrent linear polar jet. Measurements of its expansion show a constant acceleration of material with typical expansion velocity of 1 km/s. The CCD frames show the interconnection between spiral jets and the successive shells.
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    URL: http://dx.doi.org/10.1023/A:1006296932189
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  • 18
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    Spectrocam-10 Thermal Infrared Observations Of The Dust In Comet C/1995 O1 Hale-Bopp (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 265-270 
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    ISSN: 1573-0794
    Keywords: Comets ; infrared ; silicate feature
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We describe 5 to 18 μm broadband images and R ∼ 100 spectra of comet Hale-Bopp taken with SpectroCam-10 on the 5 m Hale telescope during six runs between 1996 June and 1997 April. Our data show the gradual warming of dust grains as the comet approached perihelion. In June, the 10 μm silicate emission feature was already stronger than observed in any other comet, and it increased to about 3 times the continuum level near perihelion. Spectral structure attributed to a crystalline olivine grain component remained relatively constant, but other features associated with pyroxenes appeared to vary with heliocentric distance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006217503964
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  • 19
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    Long-term Evolution of the Outgassing of Comet Hale-Bopp From Radio Observations (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 5-11 
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    ISSN: 1573-0794
    Keywords: Comets ; C/1995 O1 (Hale-Bopp) ; radio observations ; molecules
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract C/1995 O1 (Hale-Bopp) has been observed on a regular basis since August 1995 at millimetre and submillimetre wavelengths using IRAM, JCMT, CSO and SEST radio telescopes. The production rates of eight molecular species (CO, HCN, CH3OH, H2CO,H2S, CS, CH3CN,HNC) have been monitored as a function of heliocentric distance(rh from 7 AU pre-perihelion to 4 AU post-perihelion. As comet Hale-Bopp approached and receded from the Sun, these species displayed different behaviours. Far from the Sun, the most volatile species were found in general relatively more abundant in the coma. In comparison to other species, HNC, H2CO and CS showed a much steeper increase of the production rate with decreasing rh. Less than 1.5 AU from the Sun, the relative abundances were fairly stable and approached those found in other comets near 1 AU. The kinetic temperature of the coma, estimated from the relative intensities of the CH3OH and CO lines, increased with decreasing rh, from about10 K at 7 AU to 110 K around perihelion. The expansion velocity of the gaseous species, derived from the line shapes, also increased with a law close torh 3.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006229818484
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  • 20
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    Spectroscopy Of Comet Hale-Bopp In The Visible/Near Infrared: Modeling Of Dust Properties (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 305-311 
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    ISSN: 1573-0794
    Keywords: Comets ; C/1995 O1 (Hale-Bopp) ; Solar System
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Previous analysis of imaging spectroscopy data in the 0.4–1 μm spectral range of comet Hale-Bopp, have shown the presence of two regions on the sunward and antisunward sides of the nucleus exhibiting different continuum emission (Bellucci, 1998, hereafter paper I). In this work we present the modeling of the continuum emission in terms of size distribution and composition of the dust grains. The spectra are fitted by micron sized olvine grains. A mechanism to explain the spatial gradient is also proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006234007598
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  • 21
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    Aperture Polarimetry And Photometry Of Comet Hale-Bopp (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 347-352 
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    ISSN: 1573-0794
    Keywords: Comets ; comet Hale-Bopp ; polarization ; dust
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present results of polarimetric and photometric observations of bright comet C/1995 O1 (Hale-Bopp) obtained at the 0.7 m telescope of Kharkov University Observatory from June 18, 1996 to April 24, 1997. The IHW and HB comet filters were used. The C2 and C3 production rates for Hale-Bopp are more than one order of magnitude larger and the dust production rates are more than two orders of magnitude larger than the Halley ones at comparable distances. Hence, Hale-Bopp was one of the most dusty comets. The average UC-BC and BC-RC colours of the dust were −0.02 and 0.13 mag, respectively. The polarization of comet Hale-Bopp at small phase angles of 4.8–13.0° was in good agreement with the date for comet P1/Halley at the same phase angles in spite of the fact that the heliocentric distances of comments differed nearly twice. However, at intermediate phase angles of 34–49° the polarization of comet Hale-Bopp was significantly larger than the polarization of the other dusty comets. It is the first case of such a large difference found in the continuum polarization of comets. The wavelength dependence of polarization for Hale-Bopp was steeper than for other dusty comets. The observed degree of polarization for the anti-sunward side of the coma was permanently higher than that for the sunward shell side. The polarization phase dependence of Hale-Bopp is discussed and compared with the polarization curves for other dusty comets. The peculiar polarimetric properties of comet Hale-Bopp are most likely caused by an over-abundance of small or/and absorbing dust particles in the coma.
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    URL: http://dx.doi.org/10.1023/A:1006246310324
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    Infrared Observations Of Dust Emission From Comet Hale-Bopp (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 251-257 
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    ISSN: 1573-0794
    Keywords: Comets ; infrared ; radio ; rotation ; surfaces ; origin
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present infrared imaging and photometry of the bright, giant comet C/1995 O1 (Hale-Bopp). The comet was observed in an extended infrared and optical observing campaign in 1996–1997. The infrared morphology of the comet was observed to change from the 6 to 8 jet “porcupine” structure in 1996 to the “pinwheel” structure seen in 1997; this has implications for the position of the rotational angular momentum vector. Long term light curves taken at 11.3 μm indicate a dust production rate that varies with heliocentric distance as ∶ r−1.4. Short term light curves taken at perihelion indicate a rotational periodicity of 11.3 hours and a projected dust outflow speed of ∶ 0.4 km s−1. The spectral energy distribution of the dust on October 31, 1996 is well modeled by a mixture of 70% silicaceous and 30% carbonaceous non-porous grains, with a small particle dominated size distribution like that seen for comet P/Halley (McDonnell et al., 1991), an overall dust production rate of 2 × 105 kg s−1, a dust-to-gas ratio of ∶5, and an albedo of 39%.
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    URL: http://dx.doi.org/10.1023/A:1006261303055
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    C/1995 O1 (Hale-Bopp): Gas Production Curves And Their Interpretation (1997)
    Springer
    Earth, moon and planets 79 (1997), S. 55-77 
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    ISSN: 1573-0794
    Keywords: Comets ; cometary activity ; cometary nucleus ; sublimation processes ; numerical modelling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright comet over a large range of heliocentric distances rh. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion. CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate reached 1031 s−1 at perihelion. Gas production curves obtained for various species show several behaviours with rh. Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H2CO or HNC, they permit to constrain their origin in the coma. We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large rh) or free sublimation (typical example: H2O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a model with crystallization of amorphous ice close to the nuclear surface.
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    URL: http://dx.doi.org/10.1023/A:1006225030955
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    Summary of Conference (1997)
    Springer
    Earth, moon and planets 79 (1997), S. 309-319 
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    ISSN: 1573-0794
    Keywords: Comets ; comet Hale-Bopp
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The recent apparition of comet Hale-Bopp has provided the first opportunity to study a truely “Great Comet” with modern observational equipment, from ground and from space. An enormous amount of data was gathered and is still in the process of being analysed. When compared with elaborate and realistic theoretical considerations, important new insights are now being obtained into the complex physical and chemical processes in the cometary environment. This summary highlights some of the main achievements of this work, as they were presented at the first major international conference on comet Hale-Bopp in February 1998.
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    URL: http://dx.doi.org/10.1023/A:1006214223203
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    Horizontal, Two Point Coherence for Separations Greater Than the Measurement Height (1998)
    Springer
    Boundary layer meteorology 87 (1998), S. 459-480 
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    ISSN: 1573-1472
    Keywords: Coherence model ; Spatial coherence ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Wind speed measurements from the test site at Rutherford Appleton Laboratory have been evaluated with respect to the spatial coherence function. The experimental arrangement provides coherence information for separation distances of 62, 80 and 102 m. These are at least three times greater than the measurement heights of 18 m and 18.7 m. Based on these experimental data and data published in the literature, different theoretical formulations are compared and a new, but simple, model for longitudinal and lateral coherence is proposed. At large separations the turbulent wind field is not isotropic, theoretical models to describe the coherence function for such distances are not available. The new model we propose builds on the classical exponential approach. It takes into account the influence of turbulence intensity and models the angular dependence of horizontal coherence. It is found that, for constant turbulence intensity, the lateral coherence decay becomes independent of the mean wind speed.
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    URL: http://dx.doi.org/10.1023/A:1000997610233
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    Landscape Roughness Parameters for Sherwood Forest – Experimental Results (1998)
    Springer
    Boundary layer meteorology 89 (1998), S. 285-316 
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    ISSN: 1573-1472
    Keywords: Boundary layer ; Heterogeneous terrain ; Roughness length ; Surface fluxes ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The aim of this work is to present experimentally evaluated effective roughnesses (zoe) of a partly forested landscape. Although the ratio of boundary-layer height to obstacle size was only of the order of 50, there still seemed to exist a height range of 75–200 m where surface-layer similarity was approximately valid. Attempts were made to use conventional wind profile analysis to evaluate zoe, but the small height range and the large number of variables initially led to unacceptable uncertainties. Fixing the displacement height zd, rather than fitting it, reduced the data scatter to an acceptable level. The profile-derived roughness lengths zop obtained in this way were in good agreement with previous work, and with an alternative roughness length estimate zof for which flux-derived profile parameters u* and θ* were used. This implies that the profile-derived roughnesses were consistent with the measured surface-layer momentum flux. Comparison of both roughness estimates also yielded an improved estimate of the displacement height. Besides this, the authors tested a landscape roughness evaluation method which makes use of the gustiness parameter Tu = σu/U in the surface layer. The results obtained by this method were in fair agreement with the profile-derived data. In previous work, the gustiness method was advocated because it could be used at relatively low levels, perhaps even within the roughness sub-layer. At the present measuring site, this was not the case as the gustiness method was only valid in an approximate way, and for a limited height range.
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    URL: http://dx.doi.org/10.1023/A:1001743525309
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    Turbulent characteristics and bulk transfer coefficients over the desert in the HEIFE area (1996)
    Springer
    Boundary layer meteorology 77 (1996), S. 1-20 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Water vapor ; Turbulent flux ; Bulk Coefficient ; HEIFE ; Desert ; Similarity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Observations of surface-layer turbulence and turbulent fluxes were made over a desert in northwestern China as a part of HEIFE (HEIhe river Field Experiment). These show that the normalized variations of the vertical wind component and of the air temperature obey Monin-Obukhov similarity well, especially in free convective conditions. However, the variations of specific humidity do not obey Monin-Obukhov similarity. Mean bulk transfer coefficients of sensible heat and momentum flux are obtained as functions of stability over a wide stability range from the observed data of turbulent fluxes and mast profiles. However, the bulk transfer coefficient for water vapor could not be obtained because of the large scatter of the data. In free convective conditions, the sensible heat flux was found to be approximately proportional to the 1.4 power of temperature difference between the surface and 20m. The bulk transfer coefficient of sensible heat is also obtained as a function of the bulk Richardson number for practical convenience.
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    URL: http://dx.doi.org/10.1007/BF00121856
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    Optimization of Upstream Profiles in Modelled Flow Over Complex Terrain (1997)
    Springer
    Boundary layer meteorology 83 (1997), S. 285-309 
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    ISSN: 1573-1472
    Keywords: Adjoint ; Data assimilation ; Turbulence ; Complex terrain
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract This paper describes an adjoint method for data assimilation intoupstream boundary conditions of numerical modelsusing optimal control theory. Mathematical formalisms are given along with the numerical implementation of the schemein a column model of the atmospheric boundary layer. The optimized mean and turbulence profiles are used as an upstream solutionin a model of turbulent flow in complex terrain. To contrast thiswith other methods, two solutions for flow over an isolatedhill are calculated, one with an optimized upstream solution andone with a simple surface-layer formulation for the upstream solution.These two solutions are compared to observations and analytical theory. The adjoint optimization method is shown to producesolutions of flow in complex terrain that are substantively differentat the two solutions, with the optimized solution giving more accurate results.
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    URL: http://dx.doi.org/10.1023/A:1000229609412
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    Coherent eddies and temperature structure functions for three contrasting surfaces. Part II: Renewal model for sensible heat flux (1997)
    Springer
    Boundary layer meteorology 84 (1997), S. 125-147 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Canopies ; Temperature ramps ; Renewal models
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Sensible heat, latent heat, and other scalar fluxes cannot be measuredwithin short dense canopies, e.g., straw mulches, with standard approachessuch as eddy correlation, Bowen ratio-energy balance, aerodynamic, andvariance methods. However, recently developed surface renewal models, thatare based on the fact that most of the turbulent transfer within and abovecanopies is associated with large-scale coherent eddies, which are evidentas ramp patterns in scalar time series, offer a feasible solution. Wepresent a new air renewal model that calculates sensible heat flux atdifferent heights within and above a canopy from the average cubictemperature structure function, sampled at a moderate rate, and measuredaverage friction velocity. The model is calibrated and tested with datameasured above and within a Douglas-fir forest and above a straw mulch andbare soil. We show that the model describes half-hour variations ofsensible heat flux very well, both within the canopy and roughnesssublayers and in the inertial sublayer, for stable and unstable atmosphericconditions. The combined empirical coefficient that appears in the modelhas an apparently universal value of about 0.4 for all surfaces andheights, which makes application of the model particularly simple. Themodel is used to predict daytime and nighttime sensible heat flux profileswithin the straw mulch and within a small bare opening in the mulch.
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    Trajectory Curvature As A Selection Criterion For valid Lagrangian Stochastic Dispersion Models (1997)
    Springer
    Boundary layer meteorology 84 (1997), S. 411-425 
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    ISSN: 1573-1472
    Keywords: Trajectories ; Turbulence ; Dispersion ; Diffusion ; Numerical models
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Among well-mixed multi-dimensional Lagrangian stochastic (LS) dispersion models, we observe that those in poorest agreement with observations produce ‘spiralling trajectories,’ with an associated reduction in dispersion. We therefore investigate statistics of increments dθ ' to the orientation θ'= arctan(W'/U') of the Lagrangian velocity-fluctuation vector – as a possible means to distinguish the better LS models within the well-mixed class. ‘Zero-spin’ models, having 〈 dθ'〉 = 0, are found to provide best agreement with observations. It is not clear however, whether imposition of the zero-spin property selects (in conjunction with the well-mixed condition) a unique model.
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    URL: http://dx.doi.org/10.1023/A:1000474206967
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    Turbulent mixing of reactive gases in the convective boundary layer (1997)
    Springer
    Boundary layer meteorology 85 (1997), S. 197-222 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Chemistry ; Closure ; Convective boundary layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We study the interactions of chemistry and turbulent mixing of tracersin the convective boundary layer with a second-order closure model,including higher order chemistry terms. In order to limit the number of predictive equations we prescribe the profiles for ¯w¯Θ, ¯w¯θ ¯θ and the lengthscale l. However, for model validation we treat temperature and humidity asinert tracers, and compare the results with profiles observed during theAir Mass Transformation Experiment, and with similarity expressions for thesurface layer. We find good agreement of the mean profiles, but the (co-)variances are slightly underpredicted. Furthermore, the model usesdiagnostic equations expressing third moments of concentration in terms ofsecond moments and their vertical derivatives. They are compared withlarge-eddy model results, showing good agreement and, therefore, thesimplifications are justified. The model is applied to the transport of two gases subject to one bimolecular reaction. The importance of concentration correlations on themean transformation rate is studied. For two gases diffusing in oppositedirections we find for moderate and fast chemistry a 50% and90% decreased transformation rate due to the negatively correlatedconcentrations. These values are similar to large-eddy results of Schumannand Sykes et al. For two bottom-up tracers we find that the covariance ofboth reactive species is either positive or negative, increasing or reducingthe effective transformation rate depending on the Damköhler number (the ratio of the turbulent and the chemistry timescale). A significantdirect influence of chemistry on the flux divergence is found in bothcases. According to the model the effective transport to mid-levels of theboundary layer is increased when two reactive tracers diffuse in oppositedirections, and decreased in the case of two bottom-up tracers.
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    URL: http://dx.doi.org/10.1023/A:1000414710372
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    Analysis of Astex-Stratocumulus Observational Data Using a Mass-Flux Approach (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 63-87 
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    ISSN: 1573-1472
    Keywords: Stratocumulus ; Mass flux ; Lateral entrainment ; Turbulence ; Conditional sampling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A mass-flux approach is applied to observational data obtained in a convective boundary layer topped with stratocumulus clouds. The observational data were obtained from aircraft measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX). A conditional sampling method is used to calculate average updraft and downdraft values. The vertical fluxes calculated with the mass-flux approach are found to be proportional to the real (measured) fluxes, with a proportionality factor being about 0.6. This value is predicted by theory for two variables having a joint Gaussian distribution function; proportionality factor = 2π-1 ≈ 0.637. The horizontal fractional entrainment and detrainment rates calculated from the data (ε ≈ 1–2 × 10-2 m-1) are an order of magnitude higher than the rates obtained by large eddy simulations for cumulus convection (ε ≈ 2–3 × 10-3 m-1) and two orders of magnitude higher than those used in modelling cumulus convection with a mass-flux scheme in an operational weather forecast model (ε ≈ 3 × 10-4 m-1). A numerical mass-flux model for the thermodynamics was developed and showed that results are in good agreement when compared with measured profiles of the liquid water content.
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    Atmospheric Turbulence Characteristics Over a Temperature-Inhomogeneous Land Surface. Part II: The Effect of Small-Scale Inhomogeneities of Surface Temperature on Some Characteristics of the Atmospheric Surface Layer (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 103-124 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Surface temperature ; Surface inhomogeneity ; Source area
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Turbulence characteristics, vertical profiles of wind velocity u(z) and air temperature T(z), and also spatial variations in steppe surface radiation temperature Tr(x) are measured simultaneously. A marked effect of Tr(x) characteristics on the turbulence characteristics and T(z) profiles is observed in the lower part of the atmospheric surface layer. We suggest that variability in Tr(x) noticeably influences the surface-layer temperature field and leads to scatter in the values of the universal functions obtained by different authors; effects of Tr(x) are not accounted for in similarity theory. The introduction of the value of temperature zero-plane displacement dT in the calculation formulae (to determine temperature flux) noticeably improves the agreement between calculated and measured (by eddy-correlation method) results. The influence of footprint (or Source Area) on the obtained results leads to noticeable scatter in the data obtained from measurements of atmospheric turbulence.
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    Turbulence and Coherent Organizations in the Atmospheric Boundary Layer: A Radar-Aircraft Experimental Approach (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 147-179 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Convective boundary layer ; Clear air radar observations ; Coherent organizations ; Radar-aircraft joint observations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The scientific objective of the TRAC experiment (Turbulence Radar Aircraft Cells) was to investigate the respective roles played by small-scale turbulence and coherent structures in the vertical transfer within the atmospheric boundary layer (ABL). Field research held in June 1993 in France was based on coupled aircraft and Doppler radar measurements. The results discussed here are mainly focused on the evaluation of the performance of the radar in the 3D description of the clear air ABL, which was the technical goal of TRAC. During the experiment, the radar was able to provide continuous and coherent echo fields over a range of several tens of kilometres, extending up to about 3 km. Good agreement was obtained in the ABL between the radar-derived turbulent quantities and airborne measurements. As depicted by the reflectivity fields, coherent organizations were found to be a common feature of the eleven ABL cases analyzed. These organizations evolved during the day between a banded structure and a cellular pattern. A very weak correlation was found between the reflectivity field and the atmospheric parameters measured by the aircraft. However, in terms of characteristic scale, the reflectivity field appeared to be strongly related to the water vapour field. The inhomogeneity induced by the coherent circulations questions the representativity of one-dimensional sampling of these 3D fields and suggests the need to adapt the traditional statistical approach of the ABL.
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    URL: http://dx.doi.org/10.1023/A:1000613232592
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    Coherent Doppler Lidar Measurements of Wind Field Statistics (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 233-256 
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    ISSN: 1573-1472
    Keywords: Lidar ; Remote sensing ; Turbulence ; Velocity statistics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Coherent Doppler lidar measurements of wind statistics in the boundary layer are presented. The effects of the spatial averaging by the lidar pulse are removed using theoretical corrections and computer simulations. This permits unbiased estimates of velocity variance, spatial velocity structure functions, energy dissipation rate, and other point statistics of the velocity field.
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    On the Formulation of Lagrangian Stochastic Models of Scalar Dispersion Within Plant Canopies (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 333-344 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Dispersion model ; Non-uniqueness ; Plant canopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Lagrangian stochastic models, quadratic in velocity and satisfying the well-mixed condition for two-dimensional Gaussian turbulence, are used to make predictions of scalar dispersion within a model plant canopy. The non-uniqueness associated with satisfaction of the well-mixed condition is shown to be non-trivial (i.e. different models produce different predictions for scalar dispersion). The best agreement between measured and predicted mean concentrations of scalars is shown to be obtained with a small sub-class of ‘optimal’ models. This sub-class of ‘optimal’ models includes Thomson's model (J. Fluid Mech. 180, 529–556, 1987), the simplest model that satisfies the well-mixed condition for Gaussian turbulence, but does not include two other models identified recently as being in optimal agreement with the measured spread of tracers in a neutral boundary layer. It is therefore demonstrated that such models are not universal, i.e. applicable to a wide range of flows without readjustment of model parameters. Predictions for scalar dispersion in the model plant canopy are also obtained using the model of Flesch and Wilson (Boundary-Layer Meteorol. 61, 349–374, 1992). It is shown that, when used with a Gaussian velocity distribution or a maximum-missing-information velocity distribution, which accounts for the measured skewness and kurtosis of velocity statistics, the agreement between predictions obtained using the model of Flesch and Wilson and measurements is as good as that obtained using Thomson's model.
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    A Lagrangian Decorrelation Time Scale in the Convective Boundary Layer (1998)
    Springer
    Boundary layer meteorology 86 (1998), S. 525-534 
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    ISSN: 1573-1472
    Keywords: Lagrangian decorrelation time scales ; Structure function constant ; Turbulence ; Lagrangian spectrum constant
    Source: Springer Online Journal Archives 1860-2000
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    Notes: Abstract A new method for deriving the Lagrangian decorrelation time scales for inhomogeneous turbulence is described. The expression for the time scales here derived for the convective boundary layer is compared to those estimated by Hanna during the Phoenix experiment. Then the values of C0, the Lagrangian velocity structure function constant, and of Bi, the Lagrangian velocity spectrum constant, were evaluated from the Eulerian velocity spectra and from the Lagrangian time scales derived, under unstable conditions, from Taylor's statistical diffusion theory. The numerical coefficient of the lateral and vertical Lagrangian spectra in the inertial subrange was found equal to 0.21, in good agreement with previous experimental estimates.
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    The Use of State-of-the-Art Kites for Profiling the Lower Atmosphere (1998)
    Springer
    Boundary layer meteorology 87 (1998), S. 1-25 
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    ISSN: 1573-1472
    Keywords: Kites ; Atmospheric measurements ; Turbulence ; Profiling ; Ozone
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract This paper presents the capabilities and limitations of using state-of-the-art kites for atmospheric research. A brief historical review of the subject is first presented, followed by an outline of the current status of kite-borne measurement technology. The utility of the technique is then illustrated by presenting a series of recent measurements made using kite-borne technology. A summary of the advantages and limitations of kite-based measurements relative to other technologies is provided for reference.
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    URL: http://dx.doi.org/10.1023/A:1000812511429
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    Simulation of meteorological fields over a land-water-land terrain and comparison with observations (1999)
    Springer
    Boundary layer meteorology 91 (1999), S. 227-257 
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    ISSN: 1573-1472
    Keywords: Coastal boundary layer ; Initialisation ; Mesoscale model ; Sea model breeze ; Thermal internal boundary layer ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A numerical two-dimensional-mesoscale model with a level 1.5 closure scheme for turbulence is described. The model is used to simulate the boundary layer over coastal complex terrain. Meteorological data available from the Øresund land-sea-land terrain experiment are used to study the performance of the model. The model could simulate generally observed complexities in the mean wind and temperature fields. Internal boundary layers over the water and land surfaces were identified by the height of lowest value in the turbulence kinetic energy profile and this showed good agreement with radiosonde (RS) observations. Some disagreements with the data were also noticed, especially near the surface. The wind speed was over-predicted. Attempts were made to improve the model performance by adopting different schemes for model initialisation. Results showed that initialisation with an early model start time and observed wind profile near the inflow boundary improved the performance. The wind speed over-prediction could be further minimised by using a more realistic objective initialisation scheme. The problem centred around the proper estimation of the turbulent diffusion coefficient K through the closure scheme. Despite using the most popular empirical relationships in the level 1.5 closure scheme, these differences persisted. While this needs further investigation, the present model can be used to supply wind fields for practical purposes such as air pollution calculations.
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    The Effects of Probe-Induced Flow Distortion on Velocity Covariances: Field Observations (1999)
    Springer
    Boundary layer meteorology 91 (1999), S. 483-493 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Flow distortion ; Sonic anemometers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We carried out measurements to test a simple theory of the effect of probe-induced flow distortion on turbulence measurements. We used two three-component sonic anemometers mounted 1.8m apart at a height of 6.7 m. Behind one was a horizontal circular cylinder of radius 0.15 m and length 1.2 m, chosen to model two-dimensional probe-induced flow distortion in the limit where the scale of the turbulence is very large compared to the scale of the probe. The second sonic anemometer measured the undistorted flow. The measured flow-distortion effects on the Reynolds shearing stress and the variances of streamwise and vertical velocity agree well with the theory.
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    URL: http://dx.doi.org/10.1023/A:1001868030313
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    Turbulence in a Katabatic Flow (1999)
    Springer
    Boundary layer meteorology 92 (1999), S. 37-63 
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    ISSN: 1573-1472
    Keywords: Energy balance ; Glacier ; Katabatic flow ; Stable boundary layer ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Turbulence measurements performed in a stable boundary layer over the sloping ice surface of the Vatnajökull in Iceland are described. The boundary layer, in which katabatic forces are stronger than the large-scale forces, has a structure that closely resembles that of a stable boundary layer overlying a flat land surface, although there are some important differences. In order to compare the two situations the set-up of the instruments on an ice cap in Iceland was reproduced on a flat grass surface at Cabauw, the Netherlands. Wind speed and temperature gradients were calculated and combined with flux measurements made with a sonic anemometer in order to obtain the local stability functions φm and φh as a function of the local stability parameter z/L. Unlike the situation at Cabauw, where φm was linear as a function of z/L, in the katabatically forced boundary layer, the dependence of φm on stability was found to be non-linear and related to the height of the wind maximum. Thermal stratification and the depth of the stable boundary layer however seem to be rather similar under these two different forcing conditions. Furthermore, measurements on the ice were used to construct the energy balance. These showed good agreement between observed melt and components contributing to the energy balance: net radiation (supplying 55% of the energy), sensible heat flux (30%) and latent heat flux (15%). Local sources and sinks in the turbulent kinetic energy budget are summed and indicate a reasonable balance in near-neutral conditions but not in more stable situations. The standard deviation of the velocity fluctuations σu, σv, and σw, can be scaled satisfactorily with the local friction velocity u* and the standard deviation of the temperature fluctuation σθ with the local temperature scale θ*.
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    URL: http://dx.doi.org/10.1023/A:1001744822857
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    Large-Scale Motions in the Marine Atmospheric Surface Layer (1999)
    Springer
    Boundary layer meteorology 92 (1999), S. 165-183 
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    ISSN: 1573-1472
    Keywords: Coherent structures ; Turbulence ; Surface layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Multi-level turbulent wind data from the Risø Air-Sea Experiments (RASEX) were used to examine the structure of large-scale motions in the marine atmospheric surface layer. The quadrant technique was used to identify flux events (ejections/sweeps). Ejections, which appear to occur in groups, are seen to occur first at the upper level, moving successively to lower levels with small time delays. A strong correlation between events at different heights suggests that they may all be part of a single large structure. Cross-correlation between velocity signals was used to estimate orientation of the structure using Taylor's hypothesis. The inclination of this structure is shallow (≃ 15°) near the surface and increases with height. Spatial representations of the fluctuating wind vectors show a structure that is strikingly similar to conceptual models of transverse vortices and shear layers seen in laboratory flows and direct numerical simulation (DNS) of low Reynolds number flows. Spatial visualization of velocity fluctuations during other time periods and conditions clearly shows the existence of shear layers, transverse vortices, plumes, and downdrafts of various sizes and strengths. A quantitative analysis shows an increase in the frequency of shear related events with increasing wind speed.
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    URL: http://dx.doi.org/10.1023/A:1001837729368
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    Intercomparison of Three- and one-Dimensional Model Simulations and Aircraft Observations of Stratocumulus (1999)
    Springer
    Boundary layer meteorology 92 (1999), S. 453-487 
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    ISSN: 1573-1472
    Keywords: Closure models ; Drizzle ; Entrainment ; Large Eddy Simulation ; Observations ; Stratocumulus ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract As part of the EUropean Cloud REsolving Modelling (EUCREM) model intercomparison project we compared the properties and development of stratocumulus as revealed by actual observations and as derived from two types of models, namely three-dimensional Large Eddy Simulations (LES) and one-dimensional Single Column Models (SCMs). The turbulence, microphysical and radiation properties were obtained from observations made in solid stratocumulus during the third flight of the first 'Lagrangian' experiment of the Atlantic Stratocumulus Transition Experiment (ASTEX). The goal of the intercomparison was to study the turbulence and microphysical properties of a stratocumulus layer with specified initial and boundary conditions. The LES models predict an entrainment velocity which is significantly larger than estimated from observations. Because the observed value contains a large experimental uncertainty no definitive conclusions can be drawn from this. The LES modelled buoyancy flux agrees rather well with the observed values, which indicates that the intensity of the convection is modelled correctly. From LES it was concluded that the inclusion of drizzle had a small influence (about 10%) on the buoyancy flux. All SCMs predict a solid stratocumulus layer with the correct liquid water profile. However, the buoyancy flux profile is poorly represented in these models. From the comparison with observations it is clear that there is considerable uncertainty in the parametrization of drizzle in both SCM and LES.
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    URL: http://dx.doi.org/10.1023/A:1002006919256
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    Evolution of Magnetic Fields in Galaxies with Spiral Structure (1998)
    Springer
    Studia geophysica et geodaetica 42 (1998), S. 382-390 
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    ISSN: 1573-1626
    Keywords: Magnetohydrodynamics (MHD) ; Turbulence ; Magnetic fields ; Galaxies
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Abstract We present simulations of the 3D nonlinear induction equation in order to investigate the temporal evolution of large-scale magnetic fields in spiral galaxies. Our model includes differential rotation, ambipolar diffusion and, based on small-scale turbulence, eddy diffusivity and the tensorial α-effect with magnetic feedback. The nonaxisymmetric spiral pattern and – if considered – the vertical stratification of the galaxy are represented in its density and turbulence profile. Neglecting vertical stratification the lifetime and geometry of an initial magnetic field depend on the correlation time of interstellar turbulence τ corr . Short correlation times increase the lifetime of the initial magnetic field, but the field is rapidly wound up. Its pitch-angles develop to zero. The magnetic field has disappeared after at most 1 to 1.5 Gyr. A resonance like phenomenon is found by tuning the pattern velocity of the galactic spiral. The simulations then show an exceptional amplification of the magnetic field in the case that the pattern speed and a magnetic drift velocity have similar values. Considering a vertical stratification we achieve sufficiently long living grand-designed magnetic fields excited by dynamo action. The behaviour and geometry of the resulting field is again significantly influenced by the correlation time τ corr . Small values of τ corr lead to axisymmetric fields with small pitch-angles and field-concentration between the spiral arms. Increasing the correlation time the solutions show larger pitch-angles; and depending on very large correlation times the galactic dynamo rather generates fields clearly within the spiral arms and having a bisymmetric structure.
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    A Case Study of Rayleigh-Bénard Convection with Clouds (1998)
    Springer
    Boundary layer meteorology 88 (1998), S. 211-237 
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    ISSN: 1573-1472
    Keywords: Aircraft measurements ; Compositing ; Rayleigh-Bénard convection ; Spectral energetics ; Thermals ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A turbulence data set collected by the research aircraft Hercules and Falcon in the planetary boundary layer (PBL) over the North Sea during Rayleigh-Bénard convection (RBC) is analysed. Altogether nearly three hundred cell passages at different levels and in two different flight directions were sampled. The convective boundary-layer height (H) was about 1 km, and the RBC cells had a diameter D of roughly 2–3 km, resulting in an aspect ratio A = D/H ≈ 2–3. This value is also found in the case of RBC in laboratory-scale flows, whereas most of the recent PBL experimental work reports convection PBL rolls with A ≈ 3 and mesoscale cellular convection (MCC) with A ≈ 10–40 over the oceans. The large number of RBC cell passages made it possible to composite their average structure. Due to the more complex three-dimensional structure and the importance of thermals to the RBC dynamics, spectral, temporal and spatial decompositions and model calculations were necessary to illuminate structure, dynamics, energetics and organisation. The final impression is that the structure of RBC in the PBL is given by a honeycomb-like arrangement of short-lived mixed-layer thermals with more passive downward motions in between. The regularity of the Cu-cloud cover results partly from the more stationary flow in the cloud-free cell centres. On the other hand it is shown that active as well as inactive clouds contribute to the cloud cover. Thus, the PBL flow and the cloud cover are decoupled, at least temporarily and locally. Due to sparse observational and measured information about RBC occurrence and structure in the PBL, additional material was gathered, resulting in the impression that RBC is one additional realised mode of organised convection in the PBL, as has already been clarified for PBL rolls and MCC by recent investigations.
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    Evidence of Organized Large Eddies by Ground-Based Doppler Lidar, Sonic Anemometer and Sodar (1998)
    Springer
    Boundary layer meteorology 88 (1998), S. 343-361 
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    ISSN: 1573-1472
    Keywords: Heterodyne Doppler lidar ; Organized large eddies ; Planetary boundary laye ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.
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    URL: http://dx.doi.org/10.1023/A:1001167212584
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    An Investigation of Higher-Order Closure Models for a Forested Canopy (1998)
    Springer
    Boundary layer meteorology 89 (1998), S. 47-74 
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    ISSN: 1573-1472
    Keywords: Canopy closure model ; Pine forest ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Simultaneous triaxial sonic anemometer velocity measurements vertically arrayed at six levels within and above a uniform pine forest were used to examine two parameterization schemes for the triple-velocity correlation tensor employed in higher-order closure models. These parameterizations are the gradient-diffusion approximation typically used in second-order closure models, and the full budget for the triple-velocity correlation tensor typically employed in third-order closure models. Both second- and third-order closure models failed to reproduce the measured profiles of the triple-velocity correlation within and above the canopy. However, the Reynolds stress tensor profiles (including velocity variances) deviated greatly from the measurements only within the lower levels of the canopy. It is shown that the Reynolds stresses are most sensitive to the parameterization of the triple-velocity correlation in these lower canopy regions where local turbulent production is negligible and turbulence is mainly sustained by the flux transport term. The failure of the third-order closure model to reproduce the measured third moments in the upper layers of the canopy-top contradicts conclusions from a previous study over shorter vegetation but agrees with another study for a deciduous forest. Whether the third-order closure model failure is due to the zero-fourth-cumulant closure approximation is therefore considered. Comparisons between measured and predicted quadruple velocity correlations suggest that the zero-fourth-cumulant approximation is valid close to the canopy-atmosphere in agreement with recent experiments.
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    URL: http://dx.doi.org/10.1023/A:1001509106381
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    Characterisation and Simulation of the Multiscaling Properties of the Energy-Containing Scales of Horizontal Surface-Layer Winds (1999)
    Springer
    Boundary layer meteorology 90 (1999), S. 21-46 
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    ISSN: 1573-1472
    Keywords: Atmospheric surface layer ; Longitudinal velocity fluctuations ; Multifractals ; Spectra ; Statistical analysis ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The multiscaling statistics of atmospheric surface-layer winds at low wavenumbers above farmland and in the lee of a mountain range were examined using a hot-wire and lightweight cup anemometer. It was found that the horizontal velocity spectra could be broken into high and low-wavenumber regimes according to the parameters given by this analysis. The low-wavenumber end of the spectrum possessed a spectral slope parameter that varied between values of 0.8 and 1.35 at the farmland site during the period of the experiment, and the high-wavenumber end – corresponding to the inertial range – possessed a spectral slope slightly greater than -5/3. The larger values for this parameter for the low-wavenumber end appeared to coincide with unstable conditions. In the lee of the mountain range, the low-wavenumber spectral slope parameter was larger still, at 1.45. The low-wavenumber signals over farmland were much less intermittent than inertial-range signals, but in the lee of the mountain range the intermittency increased. From this analysis, it was shown that the statistical properties of the recorded wind signal could be reproduced using a bounded random multiplicative cascade. The model was successfully used to simulate the wind velocity field directly, rather than simulating the energy dissipation field. Since the spectral slope parameter for low wavenumbers appeared to be a function of atmospheric stability, the method presented is a simple way of generating wind signals characteristic of a variety of atmospheric conditions.
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    Observed Wind Profiles and Turbulence Fluxes over an ice Surface with Changing Surface Roughness (1999)
    Springer
    Boundary layer meteorology 92 (1999), S. 99-121 
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    ISSN: 1573-1472
    Keywords: Displacement height ; Ice surface roughness ; Katabatic flow ; Stable boundary layer ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Wind profile and eddy-correlation data obtained at two sites on a melting glacier surface in Iceland during the summer of 1996 are presented. Throughout the experiment the surface roughness increased rapidly from smooth to very rough, with the largest roughness element height obtained being about 1.7 m. In a layer close to the rough surface we find that the wind speed profiles were disturbed showing horizontal inhomogeneities as in a roughness sublayer. Its height was approximately two times the height of the main roughness elements (h) at both sites throughout the experiment. From the wind profiles and eddy-correlation data we calculated corrections for the displaced zero plane as a function of time and compared these with results obtained from a drag partitioning model. In general, the agreement was reasonable considering the ranges of uncertainty but the results indicate that the increasing horizontal anisotropy of the surface probably limits the use of the model. The values obtained for the roughness lengths are in good agreement with those calculated from a simple linear model, i.e., z0/h = 0.5λ with λ the frontal area index. Above the roughness sublayer the wind profiles, normalised standard deviations of wind speed, and the balance of the turbulence kinetic energy budget behaved as over an ideal homogeneous surface thereby confirming similarity of the flow.
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    Taylor’s hypothesis and two-point coherence measurements (1996)
    Springer
    Boundary layer meteorology 81 (1996), S. 399-410 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Coherence ; Taylor’s hypothesis ; Surface layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Experimentally obtained time coherence has traditionally been interpreted as streamwise one-dimensional spatial coherence through Taylor’s hypothesis. We calculate corrections to the highwavenumber part of the coherence to account for the errors caused by the deviation from Taylor’s hypothesis in high-intensity turbulent flows. The small-scale turbulence is assumed to be frozen and convected by a fluctuating convection velocity. Both Lumley’s two-term approximation and the Gaussian approximation are used in the calculations. In general, we find that the coherence for crossstream separations is significantly overestimated by the direct use of Taylor’s hypothesis, the error increasing with wavenumber; that for streamwise separations is underestimated. The analyses are compared with cross-stream coherence measurements in the atmospheric surface layer. Our results indicate that predictions from Lumley’s approximation yield better agreement with experimental data for cross-stream separations than those from the Gaussian model. Our study suggests that reliable measurement of two-point spatial coherence can be achieved only for scales not too small compared to the sensor separation.
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    URL: http://dx.doi.org/10.1007/BF02430337
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    AN EVALUATION OF SEVERAL TURBULENCE SCHEMES FOR THE PREDICTION OF MEAN AND TURBULENT FIELDS IN COMPLEX TERRAIN (1997)
    Springer
    Boundary layer meteorology 83 (1997), S. 43-73 
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    ISSN: 1573-1472
    Keywords: Model evaluation ; Mesoscale ; Turbulence ; Dispersion ; Convective boundary layer ; Complex terrain
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A prognostic three-dimensional mesoscale model has been developed andused in one- and two-dimensional modes to evaluate ten local turbulenceclosure schemes. The schemes ranged from first-order to the two-equationprognostic schemes. Predictions by the models were compared for aone-dimensional convective boundary layer using mixed layer scaling andmeasurements to interpret the results. Two-dimensional simulations were alsoperformed for a sea-breeze flow and for flow over a hill. The results showedthat for all of the models considered, minor differences were produced in themean meteorological fields and in the vertical scalar fluxes, but majordifferences were apparent in the velocity variances and dissipation rate.Predicted tracer concentrations were very sensitive to the turbulence modelformulation for dispersion from a point source in the convective boundarylayer, particularly for the prediction of maximum concentrations. Predictedtracer concentrations from a surface volume source for the two-dimensionalsimulations were similar for all models, although the degree of mixing in themorning growth period produced some differences. Generally, good results forthe mean meteorological fields can be obtained with first-order schemes, evenif they underpredict the magnitude of turbulence in the convective boundarylayer, and reasonable tracer concentrations can also be obtained with thesemodels provided near-source effects are not important. The two-equationprognostic models performed best for the prediction of turbulence in theconvective boundary layer.
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    Non-local mixing of momentum in the convective boundary layer (1997)
    Springer
    Boundary layer meteorology 84 (1997), S. 1-22 
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    ISSN: 1573-1472
    Keywords: Large-eddy simulation ; Turbulence ; Non-local closure ; Convective boundary layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Recently Frech and Mahrt proposed a closure scheme which includes alarge-scale stress term to represent the effects of non-local momentummixing in the convective boundary layer. Here large-eddy simulation (LES)datasets are used to evaluate the performance of this scheme across a rangeof stabilities between neutral and highly convective conditions, and as afunction of baroclinity. Generally the inclusion of the non-local term inthe closure model leads to results in better agreement with LES, althoughsome modifications to the model formulation are suggested.
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    A Three-Dimensional Correlation/Spectral Model for Turbulent Velocities in a Convective Boundary Layer (1997)
    Springer
    Boundary layer meteorology 85 (1997), S. 35-52 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Atmospheric convective boundary layer ; Correlation function
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A three-dimensional model for correlation functions and spectra in theatmospheric, convective boundary layer (CBL) is presented. The modelincludes vertical inhomogeneities introduced by eddy-blocking at the ground.By assuming the disturbance to the turbulent flow resulting from the groundblocking is irrotational, an equation is developed which allows one to writethe inhomogeneous, two-dimensional (2D) cross spectra for the blocked flowin terms of the 2D cross spectra for a homogeneous flow. VonKármán's energy spectrum then is used to determine thehomogeneous, 2D cross spectra. Although there are only two adjustableparameters in the model, the variance and a length scale, the model is shownto agree quite well with a diversity of previous results for the CBL.
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    Radiative Effects on Temperature in the Stable Surface Layer (1998)
    Springer
    Boundary layer meteorology 89 (1998), S. 141-159 
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    ISSN: 1573-1472
    Keywords: Radiation ; Stable conditions ; Turbulence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The interaction between longwave radiation and temperature fluctuations plays a role in the dissipation of temperature variance. This interaction is most easily described by spectral models of atmospheric turbulence and a spectral radiative dissipation function which gives the intensity of the damping at each radiative wavelength λ and wavenumber k. We have used a Corrsin–Pao closure for the spectral budgets of turbulent kinetic energy and temperature to study the coupling of radiation to turbulence. The spectral radiative dissipation function and a related integral have been fitted by analytical approximations with the correct asymptotic behavior. This resulted in a simple analytical formula for the dimensionless temperature spectrum as a function of Monin-Obukhov stability, and a new dimensionless parameter describing the relative importance of radiation in the temperature spectral budget. The radiative effects both on the temperature spectrum and on the dimensionless temperature variance can then be calculated. Based on typical values of the radiative dimensionless parameters for the surface layer, we conclude that radiative dissipation is probably negligible there.
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    Turbulent Flux Measurements Above and Below the Overstory of a Boreal Aspen Forest (1998)
    Springer
    Boundary layer meteorology 89 (1998), S. 109-140 
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    ISSN: 1573-1472
    Keywords: BOREAS ; Eddy correlation ; Eddy covariance ; Turbulence ; Understory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Turbulent flux measurements both above and beneath the canopy of a boreal aspen forest are described. Velocity skewness showed that, beneath the aspen canopy, turbulence was dominated by intermittent, downward penetrating gusts. Eulerian horizontal length scales calculated from integration of the autocorrelation function or spectral peaks were 9.0 and 1.4 times the mean aspen height of 21.5 m respectively. Above-canopy power spectral slopes for all velocity components followed the -2/3 power law, whereas beneath-canopy slopes were closer to -1 and showed a spectral short cut in the horizontal and vertical components. Cospectral patterns were similar both above and beneath the canopy. The Monin–Obukhov similarity function for the vertical wind velocity variance was a well-defined function of atmospheric stability, both above and beneath the canopy. Nocturnal flux underestimation and departures of this similarity function from that expected from Monin–Obukhov theory were a function of friction velocity. Energy balance closure greater than 80% was achieved at friction velocities greater than 0.30 and 0.10 m s-1, above and below the aspen canopy, respectively. Recalculating the latent heat flux using various averaging periods revealed a minimum of 15 min were required to capture 90% of the 30-min flux. Linear detrending reduced the flux at shorter averaging periods compared to block averaging. Lack of energy balance closure and erratic flux behaviour led to the recalculation of the latent and sensible heat fluxes using the ratio of net radiation to the sum of the energy balance terms.
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    Steps, Waves and Turbulence in the Stably Stratified Planetary Boundary Layer (1999)
    Springer
    Boundary layer meteorology 90 (1999), S. 397-421 
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    ISSN: 1573-1472
    Keywords: Gravity waves ; Planetary boundary layer ; Turbulence ; Wind structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The stably-stratified planetary boundary layer contains small-vertical-scale, step-like structures, waves on a multitude of scales, large horizontal eddies and small-scale turbulence, all of which constantly interact with, and modify, one another. Current knowledge of how the various components act in the vicinity of the step-like structures is surveyed. It is concluded that packets of internal waves are the main conduit for interaction within and across the boundary layer, and low-intensity critical-level absorption at the fringes of their spectrum probably maintains the step-like structures. Further investigation of the processes requires intensive observations of the four-dimensional structure of the region, but such an investigation will need a new generation of high-resolution sensing systems.
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    Nonlocal Turbulent Mixing Based on Convective Adjustment Concepts (Ntac) (1999)
    Springer
    Boundary layer meteorology 92 (1999), S. 263-291 
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    ISSN: 1573-1472
    Keywords: Convective adjustment ; Turbulence ; Mixing ; Nonlocal mixing ; Parameterization ; Numerical weather prediction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A nonlocal turbulent mixing parameterization is introduced in this study and denoted by the acronym NTAC, which stands for Nonlocal parameterization of Turbulent mixing using convective Adjustment Concepts. NTAC uses the average value of quantities in the turbulent domain in much the same way that local convective adjustment schemes use the average potential temperature. Averages are determined in the region with non-convective turbulence using information from the two end layers (denoted by TLA, Two Layer Average), while all layers contribute to the average in regions with convective turbulence (denoted by CLA, Convective Layer Average). The NTAC parameterization estimates the mixing percentage and uses this percentage as a mixing coefficient. These percentages are determined from a simplified turbulent kinetic energy equation. The scheme is versatile, conservative, and when programmed efficiently the proposed parameterization is a computationally acceptable nonlocal procedure that can be used in many existing numerical weather prediction forecast models. Numerical weather forecast model simulations using the NTAC parameterization and traditional K-theory are compared against radiosonde data. The accuracy of the proposed NTAC parameterization is found to be competitive with K theory. The greatest improvement of the NTAC over K-theory occurs during the daytime and early nighttime hours when (dry) convective activity is high. Also, areal cloud coverage is increased by the NTAC parameterization. Our findings show that the greatest nonlocal vertical mixing occurs between the layer nearest the earth's surface and the remaining layers making up the planetary boundary layer.
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    Multiresolution Flux Decomposition (1997)
    Springer
    Boundary layer meteorology 83 (1997), S. 117-137 
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    ISSN: 1573-1472
    Keywords: Reynolds averaging ; Turbulence ; Time series ; Cospectra ; Sampling error
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Geophysical variables are orthogonally decomposed by averaging timeseries using different averaging lengths, referred to as a (Haar)multiresolution decomposition. This simple and economic decomposition isassociated with cospectra that formally satisfy Reynolds averaging rules foreach averaging length. The multiresolution decomposition provides a naturalestimate of the random error in estimating a mean turbulent flux. The Fourierand multiresolution decompositions are compared using aircraft data fromBOREAS.
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    URL: http://dx.doi.org/10.1023/A:1000210427798
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    Coherent eddies and temperature structure functions for three contrasting surfaces. Part I: Ramp model with finite microfront time (1997)
    Springer
    Boundary layer meteorology 84 (1997), S. 99-124 
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    ISSN: 1573-1472
    Keywords: Turbulence ; Canopies ; Temperature ramps ; Structure functions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Air temperature time series within and above canopies reveal ramp patternsassociated with coherent eddies that are responsible for most of thevertical transport of sensible heat. Van Atta used a simple step-changeramp model to analyse the coherent part of air temperature structurefunctions. However, his ocean data, and our own measurements for aDouglas-fir forest, straw mulch, and bare soil, reveal that even withoutlinearization his model cannot account for the observed decrease of thecubic structure function for small time lag. We found that a ramp model inwhich the rapid change at the end of the ramp occurs in a finite microfronttime can describe this decrease very well, and predict at least relativemagnitudes of microfront times between different surfaces. Averagerecurrence time for ramps, determined by analysis of the cubic structurefunction with the new ramp model, agreed well with values determined usingthe Mexican Hat wavelet transform, except at lower levels within theforest. Ramp frequency above the forest and mulch scaled very well withwind speed at the canopy top divided by canopy height. Within the forest,ramp frequency did not vary systematically with height. This is inaccordance with the idea that large-scale canopy turbulence is mostlygenerated by instability of the mean canopy wind profile, similar to aplane mixing layer. The straw mulch and bare soil experiments uniquelyextend measurements of temperature structure functions and ramp frequencyto the smallest scales possible in the field.
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    URL: http://dx.doi.org/10.1023/A:1000338817250
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    The provenance and evolution of comets (1996)
    Springer
    Earth, moon and planets 72 (1996), S. 57-68 
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    ISSN: 1573-0794
    Keywords: Comets ; interstellar dust ; dynamics ; solar neutrino problem
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The process of comet formation through the hierarchical aggregation of originally submicron-sized interstellar grains to form micron-sized particles and then larger bodies in the protoplanetary disc, culminating in the formation of planetesimals in the disc extending from Jupiter to beyond Neptune, is briefly reviewed. The ‘planetesimal’ theory for the origin of comets implies the existence of distinct cometary reservoirs, with implications for the immediate provenance of observed comets (both long-period and short-period) and their evolution as a result of planetary perturbations and physical decay, for example splitting and sublimation. The principal mode of cometary decay and collisional interaction with the terrestrial planets is through the formation and evolution of streams of cometary debris and hitherto undiscovered ‘families’ of cometary asteroids. Recent dynamical results, in particular the sungrazing and sun-colliding end-state for short-period comet and asteroid orbits, are briefly discussed.
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    URL: http://dx.doi.org/10.1007/BF00117504
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    An analytical model of the Comet's collision with Jupiter (1996)
    Springer
    Earth, moon and planets 73 (1996), S. 23-32 
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    ISSN: 1573-0794
    Keywords: Comets ; Collision ; Jupiter
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract An important feature observed in the wake of the Jupiter-comet clash was the appearance of the ring structure axisymmetrically positioned around the center of the impact. The persistent expansion of the dark rings and its speed indicated an outward propagating gravity wave (Benka, 1995). We employ an analytical model of constant density, uniform finite depth and inviscid fluid layer to investigate the wave motion produced by the impact of Comet Shoemaker-Levy 9 on the Jovian atmosphere. The relevant thermal effects are neglected and an explosion resulting from the collision is then described by an initial impulsive pressure at the surface of the Jovian atmosphere. Under the assumption that all the kinetic energy of a comet fragment is completely converted into the energy of wave motions in the Jovian atmosphere, an analytical formula describing the relationship between the resulting wave motion in the atmosphere and the parameters of a comet fragment (the radius, density and speed) is derived. Results from the present simple analytical model give a qualitative agreement with observations regarding the distance and speed of the waves.
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    URL: http://dx.doi.org/10.1007/BF00058043
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    Hco+ in the Coma of Comet Hale–Bopp (1997)
    Springer
    Earth, moon and planets 77 (1997), S. 253-258 
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    ISSN: 1573-0794
    Keywords: Comets ; HCO+ ; chemistry ; plasmas
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Maps of comet C/1995 O1 (Hale-Bopp) in the millimeter-wave emission of the ion HCO+ revealed a local minimum near the nucleus position, with a maximum about 100,000 km in the antisolar direction. These observed features of the HCO+ emission require a low abundance of HCO+ due to enhanced destruction in the inner coma of the comet, within a region of low electron temperature (Te). To set constraints on the formation of HCO+ in the coma, as well as the location and magnitude of the transition to higher Te, the data are compared with the results of ion-molecule chemistry models.
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    URL: http://dx.doi.org/10.1023/A:1006285703521
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    Observations of the OH Radical in Comet C/1995 O1 (Hale-Bopp) with the Nançay Radio Telescope (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 37-43 
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    ISSN: 1573-0794
    Keywords: Comets ; OH radical ; radio astronomy ; C/1995 O1 (Hale-Bopp)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present OH 18-cm observations of comet Hale-Bopp (C/1995 O1) at the Nançay radio telescope. On nucleus and offset position observations allowed us to obtain both OH production rates and quenching radii. The maximum OH production rate was reached around perihelion, at about1031 s-1.
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    URL: http://dx.doi.org/10.1023/A:1006206924786
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    Sulfur Chemistry at Millimeter Wavelengths in C/Hale-Bopp (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 69-70 
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    ISSN: 1573-0794
    Keywords: Comets ; coma ; radio ; sulfur chemistry ; Hale-Bopp
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The recent availability of bright comets has given us an excellent opportunity to study cometary chemistry. Comet Hale-Bopp (1995 O1)gave us the particularly rare opportunity to study a bright and active comet for almost two years. Our program concentrated on millimeter-wave observations of sulfur-bearing molecules in an effort to understand the total sulfur budget of the comet. Using the National Radio Astronomy Observatory 12-m telescope on Kitt Peak we monitored both the long and short-term variations in H2S, CS, and OCS, as well as observing H2CS and SO. This was the first observation of H2CS in any comet (Figure 1). Additionally, we mapped CS with the BIMA interferometer. Variations in the line profiles and changes in line intensity as large as a factor of two were seen in day to day observations of both H2S and CS. An example for H2S is shown in Figure 2. This is the first time we can attempt to study the entire group of sulfur-bearing molecules. Models of the sulfur coma have thus far largely been based on observations of the daughter products CS and atomic sulfur made over the last 18 years using the International Ultraviolet Explorer (IUE) satellite, coupled with radio observations of CS and H2S in several recent comets. Four new sulfur-bearing species have been observed in comets Hale-Bopp and Hyaku take, three of them parent species. The high resolution maps in CS will also allow spatial information to be included in the sulfur model for the first time. C/Hale-Bopp is the first comet in which so many sulfur species have been observed. Analysis of the abundances of these species in comparison to the total atomic sulfur observed should reveal whether or not we can now account for all of the primary sulfur sources in comets. Perhaps the most interesting question that these observations raised was why C/Hale-Bopp appeared to contain so much more SO and SO2 (as observed by others) than any other comet. This spurred the discovery that the UV fluorescence models of these species were incorrect (S. J. Kim, this issue). Analysis of the data and modeling of the sulfur budget are still underway.
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    URL: http://dx.doi.org/10.1023/A:1006275412491
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    Comet Hale-Bopp: Evolution Of Jets And Shells During March 1997 (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 189-195 
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    ISSN: 1573-0794
    Keywords: Comets ; Hale-Bopp ; jets ; shells
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Hundreds of high quality CCD images obtained by the GAT group with the 33.5 cm reflector of the Sozzago Astronomical Observatory (SAS) provide a detailed insight into the temporal evolution of comet Hale-Bopp. The images cover the time period from May 1996 until May 1997 and allowed us to analyze various near nucleus phenomena. Among them are jet structures that were visible from August 1996 until May 1997 and shell structures that could be resolved from February 1997 to May 1997. We concentrate on the morphology of these phenomena during March 1997 and investigated them by radial masking techniques that reveal their precise shape and extent. In particular, polar coordinate representations could be used advantageously to derive numerical parameters of these features.
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    URL: http://dx.doi.org/10.1023/A:1006240714442
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    The Visual Light Curve Of C/1995 O1 (Hale-Bopp) From Discovery To Late 1997 (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 169-177 
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    ISSN: 1573-0794
    Keywords: Comets ; named objects ; C/1997 O1 (Hale-Bopp) ; photometry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present a light curve of C/1995 O1 (Hale-Bopp) compiledfrom more than 3000 visual observations of the comet made by members of the The Astronomer Group world-wide. These observations cover the period from discovery through to the end of 1997. The light curve shows that the rate of brightening of the comet varied widely at different times, with rapid rates of brightening at high heliocentric distance pre-perhelion and a comparably rapid post-perihelion fade. There is no evidence that the comet was suffering a large photometric outburst when first discovered, although a small outburst can be identified at perihelion. At least five difficult brightening regimes can be identified in the light curve between discovery and perihelion. From 2.5 AU to perihelion the rate of brightening with decreasing heliocentric distance was typical for “fairly” new comets(n ∼ 3.5, where “n” is the power law exponent of the heliocentric distance), although this was preceded by a period of very slow brightening with n ∼ 1 from r ∼ 4.0 AU to r ∼ 2.8 AU and followed by an initially more rapid brightening which appears to be related to the on-set of rapid water sublimation activity. We derive the light curve parameters at different stages of the comet's apparition.
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    URL: http://dx.doi.org/10.1023/A:1006228113533
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    Dust Morphology Of The Inner Coma Of C/1995 O1 (Hale-Bopp) (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 197-203 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; Hale-Bopp ; dust ; coma ; morphology ; rotation ; period
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Dust continuum imaging of comet C/1995 O1 (Hale-Bopp) was carried out with the Swedish Vacuum Solar Telescope (SVST)on La Palma in April, 1997. Images were reduced according to standard procedure, aligned, averaged, navigated and enhanced with azimuthal renormalization, rotational derivative, temporal derivative and unsharp masking processing. The rotational period of the nucleus was determined to 11.5 h and the mean projected dust outflow velocity to 0.41 km s−1. Shell envelopes in the sunward side of the coma were separated by a projected distance of ∼15 000–20 000 km and spiralling inwards towards smaller radii in the direction of local evening. Small scale inhomogeneities of size 1 000–2 000 km, interpreted as correlated with variations in dust emission activity, were seen at radii ≤20 000 km. Two overlapping shell systems with a relative lag angle of ∼55° were evident at the time. The north pole of the nucleus was directed towards the Earth. The dust emission pattern is very complex and may be due to several active areas. The shape of the incomplete spiral shell pattern indicates that the angle between the line-of-sight and the rotational axis of the nucleus was not large.
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    URL: http://dx.doi.org/10.1023/A:1006292715351
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    High-Dispersion Spectroscopy of the Sodium (NA) Emission within the Inner Coma of C/1995 O1 (Hale-Bopp) (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 93-98 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; sodium ; spectra
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Very-high spectral resolution observations of the neutral Na emission have enabled measurements of the velocity dispersions of the Na atoms within ∼40,000 km of the opto center of Hale-Bopp. Asymmetric Na D line profiles imply both an in situ or core Na source and a secondary Na source at locations within the inner coma. The core velocity distribution had a FWHM of 2 km s-1. The extended source FWHM increased with distance from the nucleus (up to ∼6 km s-1, but appeared smaller in the more dusty regions (∼2.5–3.0 km s-1) of the inner coma. The D2/D1 line strength ratio was consistent with an optically thin inner coma. Within 5,000 km of the opto center the continuum spatial intensity profiles decreased as ∼r-1 while the Na D emission decreased at less than r-1.
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    URL: http://dx.doi.org/10.1023/A:1006239832055
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    UV Photopolarimetric Imaging of C/1995 O1 (Hale-Bopp) with the Wide Field Imaging Survey Polarimeter (WISP) (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 161-167 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; dust ; polarization ; photochemistry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We report on the reduction and analysis of UVpolarimetric images of CI (λ1657 Å) and dust continuum (2696 Å emissions from C/1995 O1 (Hale-Bopp) taken using the Wide Field Imaging Survey Polarimeter (WISP) sounding rocket on 8 April, 1997. These observations represent the first imaging polarimetry of comets in the UV, and were performed in consort with ground based measurements of gas and dust polarization and distribution. The continuum results show 9% polarization across the image field with a polarization phase angle close to the 129° prediction. Comparison with ground based data implies minimal color dependence for Hale-Bopp in either the degree of polarization and in the position angle. The carbon polarimetry implies that most production occurs in the dense inner coma, and that it leaves that area in thermodynamic equilibrium. Its radial profile further constrains the carbon outflow speed to be sufficient to travel ≥5 × 106 km without photoionization.
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    URL: http://dx.doi.org/10.1023/A:1006297428846
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    Polarimetry Of Comet Hale-Bopp: Linear And Circular Polarization, Stellar Occultation (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 381-386 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; comet Hale-Bopp ; polarization ; grains
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Comet Hale-Bopp was observed with the 2.6-m and 1.25-m telescopes of the Crimean Astrophysical Observatory on March 9 and 11, 1997. We determined the linear and circular polarization in the coma. For the dust coma, the linear polarization varied from 7.8% to 12.4%. The degree of circular polarization was always negative and did not exceed 0.3% with an accuracy of ±0.04% on average. The passage of a bright star through the cometary coma was monitored with polarimetry and photometry. The wavelength dependence and spatial variations of optical thickness of dust are obtained from the stellar occultation.
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    URL: http://dx.doi.org/10.1023/A:1006266914867
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    Cometary Rotation: an Overview (1997)
    Springer
    Earth, moon and planets 79 (1997), S. 35-53 
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    ISSN: 1573-0794
    Keywords: Comets ; nucleus ; rotation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract I discuss our current understanding of the spin states of cometary nuclei. Cometary spin influences the temporal and spatial patterns of outgassing from the nucleus (through diurnal and seasonal effects) and is in turn influenced by outgassing-driven torques. The current challenge to cometary astronomers is to quantify the interactions between the spin, the outgassing, and the resultant torques on the nucleus, and to understand the role of rotation in determining the basic physical properties of the nucleus.
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    URL: http://dx.doi.org/10.1023/A:1006272914117
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    Dust Activity in Comet Hale-Bopp (1997)
    Springer
    Earth, moon and planets 79 (1997), S. 79-102 
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    ISSN: 1573-0794
    Keywords: Comets ; named objects ; C/1997 O1 (Hale-Bopp) ; dust emission
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Comet Hale-Bopp has shown an extremely elevated level of dust activity during its apparition. We examine the dust activity of the comet, particularly the dust jets which were observed so frequently between discovery and perihelion + 5 months. Other results from observations carried out during the observing campaign have shown that the dust emission was unusually strong, as witnessed by the considerable jet activity. It is also seen that the dust emission started at an unusually large heliocentric distance and that the dust to gas ratio was particularly large. This final factor is probably one of the key issues in understanding the activity of the comet close to perihelion. Periodicity in the jet activity, which appears to be related to the precession cycle of the comet, is seen clearly in 1995 and, more ambiguously, during 1996. A full understanding of the jet activity in 1995 requires a combination of nucleus topology, the changing solar aspect of the nucleus and precession. However, the stability of the jets seen in 1996 shows that the rotational state must be only minimally excited. A key issue which must be addressed is how typical comet Hale-Bopp's dust and dust activity are in relation to other comets? Whilst the gas activity is found to be absolutely typical (although much greater than for most comets), superheat values suggest that comet Hale-Bopp's dust was highly atypical in some senses, but only when the comet was close to perihelion. These abnormalities should not prevent comet Hale-Bopp from becoming a baseline reference for comparison with other comets.
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    URL: http://dx.doi.org/10.1023/A:1006277015025
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    Comparison of 1998 and 1999 Leonid Light Curve Morphology and Meteoroid Structure (1998)
    Springer
    Earth, moon and planets 82-83 (1998), S. 351-367 
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    ISSN: 1573-0794
    Keywords: Comets ; dust ; Leonids 1999 ; lightcurves ; meteoroids ; meteors
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Photometric low-light level video observations of 1999 Leonid storm meteors have been obtained from two airborne platforms during the Leonid multi-instrument aircraft campaign (Leonid MAC). The 1999 Leonid light curves tend to be skewed towards the end point of the trajectory, while the 1998 Leonid light curves were not. The variation in the light curves from 1998 and 1999 can be explained as an overall reduction in the mass distribution index, α from ∼ 1.95 in 1998 to ∼ 1.75 in 1999. We have interpreted this behaviour as being either indicative of a gradual loss of the "glue" that keeps the grains together, or the fact that the meteoroids sampled in 1998 had a different morphological structure to those sampled in 1999. The early fragmentation of a dustball meteoroid results in a light curve that peaks sooner than that predicted by classical single body ablation theory.
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    URL: http://dx.doi.org/10.1023/A:1017003019448
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