Search Results

You are looking at 1 - 10 of 12 items for

  • Author or Editor: R. H. Clarke x
  • Refine by Access: All Content x
Clear All Modify Search
R. H. Clarke

Abstract

“Morning glory” is a local name given to a frequently occurring, near-dawn squall, accompanied by long, low, narrow cloud bands, on the south coast of the Gulf of Carpentaria.

A series of simple computation experiments lends strong support to the suggestion that the phenomenon arises through katabatic drainage from the highlands to the east; a shallow layer of cold air tends to accumulate at the foot of the hills and to form a hydraulic jump, which propagates westward. The computed phase of the jump in regard to time of day and geographical location agrees well with that of the morning glory, while other features show reasonable correspondence.

It is concluded on the basis of these experiments that the formation of such hydraulic jumps should be favored by slack pressure gradients, cloudless skies, a shallow inversion, steeper slopes, low latitudes, a low drag coefficient, and topographic funnelling. Since such conditions should not be uncommon in low latitudes, jump-like phenomena associated with orography and nocturnal cooling are to be expected here.

Full access
R. H. Clarke

Abstract

Serial pilot balloon flights, radiosonde and aircraft data in the vicinity of dry cold fronts and seabreeze fronts in summertime have made possible a clearer view of the meso-scale structure of such phenomena. In the velocity field, closed circulations and wave motions appear to be the rule, but the density variations associated with these features are inadequately known. Strong vertical currents frequently accompany the leading edges of these fronts. A tendency for increasing development of the vertical circulation with distance from the coast was found in the case of seabreeze fronts moving inland, strong circulations being found up to 135 mi from the coast. Cross-isentrope flow in the sections is deduced to indicate the effects of turbulent transfers, as well as frontal deformation. Frictional rather than geostrophic constraint is indicated for many fronts, especially those of the seabreeze variety. Strong circulations about the leading edge of the front are probably promoted by coastal effects and diurnal heating, deeper fronts being more productive of strong circulations than shallow ones. Evidence is presented for a connection between strong circulations of this type on the one hand, and frontal squalls, multiple structure and pressure jumps on the other.

Full access
R. H. Clarke and G. D. Hess

Abstract

Data from a recent experiment indicate that the height scale for wind in the planetary boundary layer is u */F, where u */F is the friction velocity and f the Coriolis parameter, rather than zi, the height of the convection-limiting inversion, or |L| the Obukhov length. None of the scales u */f, zi, |L| appears to be appropriate for temperature except in slight-to-moderate stability where again u */f appears to be the most suitable.

The data do not support either Businger's expression which gives the depth of the idealized thermal boundary layer, depending on surface heat flux and rate of temperature change, or Tennekes' expression for bulk temperature difference across the boundary layer in free convection, based on the ratio of −L to the roughness length z 0.

Full access
G. D. Hess and R. H. Clarke

Abstract

No abstract available.

Full access
R. H. Clarke, R. K. Smith, and D. G. Reid

Abstract

This paper presents the results of a field expedition mounted in late September/early October 1979 to investigate the structure and origin of the “morning glory” of the Gulf of Carpentaria in northern Australia. The morning glory is a line wind squall, accompanied by a pressure jump, and often by a long roll-cloud or series of such clouds. It frequently occurs in the early morning, especially in October, in the Gulf area.

A light aircraft, fitted with a temperature and humidity probe, was flown in two glories to determine their thermodynamic structure, and wind fields wore obtained principally by tracking pilot balloons using the double theodolite method. Data also were obtained from a network of surface stations, recording wind velocity and pressure, installed at locations across Cape York Peninsula, which is believed to be the area of genesis.

The morning glory is identified as an internal undular bore propagating on the nocturnal and/or maritime inversion. Its origin appears to lie frequently in the interaction of a deeply penetrating sea breeze front with a developing nocturnal inversion, but there is evidence also that on occasion it may result from the effect of a katabatic flow. The factors which appear to make the Gulf region particularly favorable for the common occurrence of this phenomenon are discussed.

Full access
C. R. Mechoso, R. Wood, R. Weller, C. S. Bretherton, A. D. Clarke, H. Coe, C. Fairall, J. T. Farrar, G. Feingold, R. Garreaud, C. Grados, J. McWilliams, S. P. de Szoeke, S. E. Yuter, and P. Zuidema

The present paper describes the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study (VOCALS), an international research program focused on the improved understanding and modeling of the southeastern Pacific (SEP) climate system on diurnal to interannual time scales. In the framework of the SEP climate, VOCALS has two fundamental objectives: 1) improved simulations by coupled atmosphere–ocean general circulation models (CGCMs), with an emphasis on reducing systematic errors in the region; and 2) improved estimates of the indirect effects of aerosols on low clouds and climate, with an emphasis on the more precise quantification of those effects. VOCALS major scientific activities are outlined, and selected achievements are highlighted. Activities described include monitoring in the region, a large international field campaign (the VOCALS Regional Experiment), and two model assessments. The program has already produced significant advances in the understanding of major issues in the SEP: the coastal circulation and the diurnal cycle, the ocean heat budget, factors controlling precipitation and formation of pockets of open cells in stratocumulus decks, aerosol impacts on clouds, and estimation of the first aerosol indirect effect. The paper concludes with a brief presentation on VOCALS contributions to community capacity building before a summary of scientific findings and remaining questions.

Full access
H.J. Freeland, F.M. Boland, J.A. Church, A.J. Clarke, A.M.G. Forbes, A. Huyer, R.L. Smith, R.O.R.Y. Thompson, and N.J. White

Abstract

The Australian Coastal Experiment (ACE) was conducted in the coastal waters of New South Wales from September 1983 to 1984. The data obtained allow a detailed examination of the dynamics of flow on the continental shelf and slope and in particular allow a description of coastal trapped wave modes propagating within the coastal waveguide.

The trapped-wave signal is contaminated by energy from the East Australia current eddies approaching the continental slope. However, the data do allow a clear separation of the first three coastal trapped wave modes over the range of frequencies appropriate to the weather forcing band. Through that frequency range the phase speed is computed and an empirical dispersion relation determined for each mode. The empirical dispersion relations compare well with the theoretical relations indicating that a large fraction of the variance in current velocities on the continental shelf can be accounted for by coastal trapped wave theory.

Wind forcing of trapped waves is also considered and evidence presented that in the ACE area the motions are dominated by the propagation of free waves through the arrays.

Full access
John H. Seinfeld, Gregory R. Carmichael, Richard Arimoto, William C. Conant, Frederick J. Brechtel, Timothy S. Bates, Thomas A. Cahill, Antony D. Clarke, Sarah J. Doherty, Piotr J. Flatau, Barry J. Huebert, Jiyoung Kim, Krzysztof M. Markowicz, Patricia K. Quinn, Lynn M. Russell, Philip B. Russell, Atsushi Shimizu, Yohei Shinozuka, Chul H. Song, Youhua Tang, Itsushi Uno, Andrew M. Vogelmann, Rodney J. Weber, Jung-Hun Woo, and Xiao Y. Zhang

Although continental-scale plumes of Asian dust and pollution reduce the amount of solar radiation reaching the earth's surface and perturb the chemistry of the atmosphere, our ability to quantify these effects has been limited by a lack of critical observations, particularly of layers above the surface. Comprehensive surface, airborne, shipboard, and satellite measurements of Asian aerosol chemical composition, size, optical properties, and radiative impacts were performed during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) study. Measurements within a massive Chinese dust storm at numerous widely spaced sampling locations revealed the highly complex structure of the atmosphere, in which layers of dust, urban pollution, and biomass- burning smoke may be transported long distances as distinct entities or mixed together. The data allow a first-time assessment of the regional climatic and atmospheric chemical effects of a continental-scale mixture of dust and pollution. Our results show that radiative flux reductions during such episodes are sufficient to cause regional climate change.

Full access
C. L. Reddington, K. S. Carslaw, P. Stier, N. Schutgens, H. Coe, D. Liu, J. Allan, J. Browse, K. J. Pringle, L. A. Lee, M. Yoshioka, J. S. Johnson, L. A. Regayre, D. V. Spracklen, G. W. Mann, A. Clarke, M. Hermann, S. Henning, H. Wex, T. B. Kristensen, W. R. Leaitch, U. Pöschl, D. Rose, M. O. Andreae, J. Schmale, Y. Kondo, N. Oshima, J. P. Schwarz, A. Nenes, B. Anderson, G. C. Roberts, J. R. Snider, C. Leck, P. K. Quinn, X. Chi, A. Ding, J. L. Jimenez, and Q. Zhang

Abstract

The largest uncertainty in the historical radiative forcing of climate is caused by changes in aerosol particles due to anthropogenic activity. Sophisticated aerosol microphysics processes have been included in many climate models in an effort to reduce the uncertainty. However, the models are very challenging to evaluate and constrain because they require extensive in situ measurements of the particle size distribution, number concentration, and chemical composition that are not available from global satellite observations. The Global Aerosol Synthesis and Science Project (GASSP) aims to improve the robustness of global aerosol models by combining new methodologies for quantifying model uncertainty, to create an extensive global dataset of aerosol in situ microphysical and chemical measurements, and to develop new ways to assess the uncertainty associated with comparing sparse point measurements with low-resolution models. GASSP has assembled over 45,000 hours of measurements from ships and aircraft as well as data from over 350 ground stations. The measurements have been harmonized into a standardized format that is easily used by modelers and nonspecialist users. Available measurements are extensive, but they are biased to polluted regions of the Northern Hemisphere, leaving large pristine regions and many continental areas poorly sampled. The aerosol radiative forcing uncertainty can be reduced using a rigorous model–data synthesis approach. Nevertheless, our research highlights significant remaining challenges because of the difficulty of constraining many interwoven model uncertainties simultaneously. Although the physical realism of global aerosol models still needs to be improved, the uncertainty in aerosol radiative forcing will be reduced most effectively by systematically and rigorously constraining the models using extensive syntheses of measurements.

Open access
W. P. Kustas, D.C. Goodrich, M.S. Moran, S. A. Amer, L. B. Bach, J. H. Blanford, A. Chehbouni, H. Claassen, W. E. Clements, P. C. Doraiswamy, P. Dubois, T. R. Clarke, C. S. T. Daughtry, D. I. Gellman, T. A. Grant, L. E. Hipps, A. R. Huete, K. S. Humes, T. J. Jackson, T. O. Keefer, W. D. Nichols, R. Parry, E. M. Perry, R. T. Pinker, P. J. Pinter Jr., J. Qi, A. C. Riggs, T. J. Schmugge, A. M. Shutko, D. I. Stannard, E. Swiatek, J. D. van Leeuwen, J. van Zyl, A. Vidal, J. Washburne, and M. A. Weltz

Arid and semiarid rangelands comprise a significant portion of the earth's land surface. Yet little is known about the effects of temporal and spatial changes in surface soil moisture on the hydrologic cycle, energy balance, and the feedbacks to the atmosphere via thermal forcing over such environments. Understanding this interrelationship is crucial for evaluating the role of the hydrologic cycle in surface–atmosphere interactions.

This study focuses on the utility of remote sensing to provide measurements of surface soil moisture, surface albedo, vegetation biomass, and temperature at different spatial and temporal scales. Remote-sensing measurements may provide the only practical means of estimating some of the more important factors controlling land surface processes over large areas. Consequently, the use of remotely sensed information in biophysical and geophysical models greatly enhances their ability to compute fluxes at catchment and regional scales on a routine basis. However, model calculations for different climates and ecosystems need verification. This requires that the remotely sensed data and model computations be evaluated with ground-truth data collected at the same areal scales.

The present study (MONSOON 90) attempts to address this issue for semiarid rangelands. The experimental plan included remotely sensed data in the visible, near-infrared, thermal, and microwave wavelengths from ground and aircraft platforms and, when available, from satellites. Collected concurrently were ground measurements of soil moisture and temperature, energy and water fluxes, and profile data in the atmospheric boundary layer in a hydrologically instrumented semiarid rangeland watershed. Field experiments were conducted in 1990 during the dry and wet or “monsoon season” for the southwestern United States. A detailed description of the field campaigns, including measurements and some preliminary results are given.

Full access