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J. A. Davies

Abstract

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P. J. Robinson
and
J. A. Davies

Abstract

A spectrophotometer was used to determine surface emissivity of water samples in a laboratory. The results confirm a previous field result that emissivity is independent of water composition and is approximately equal to 0.97. This value was reduced by about 3% in the presence of surface oil films but the effect of different types of oil was negligible.

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J. A. Davies
and
C. D. Allen

Abstract

Micrometeorological data collected over cropped surfaces at Simcoe in southern Ontario are used to evaluate various evaporation rates. Data collected in 1971 from two plots of perennial ryegrass, one of which was irrigated, show that daily potential evaporation (λPE), taken as the evaporation rate from the irrigated plot, is related to an “equilibrium” rate (λPES) by λPE = αλPES, where α = 1.27. This value of α is very similar to that reported by Priestley and Taylor. Previous data for other crops support this value. Data from elsewhere are included to show why the value should be a best estimate of α for air temperatures between 15 and 30C. It follows that at temperatures over 25C the surface net radiation should be a good estimate of λPE. Actual and equilibrium evaporation values for the dry grass plot in 1971 compared closely on moderately dry days, confirming previous results. An approach to calculating daily evaporation is presented which utilizes λPES and surface soil moisture.

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T. C. Uboegbulam
and
J. A. Davies

Abstract

Unsworth and Monteith's aerosol attenuation coefficient τ A was calculated with hourly cloudless sky data at Goose Bay, Montreal and Woodbridge (near Toronto) for the period 1968 to 1978. Turbidity was less on average (τ A < 0.11) than for British urban locations (0.25 < τ A < 0.55). Values of τ A were largest in summer and in southerly flow, increased with relative humidity and varied inversely with visibility. Turbidity decreased over the 1968–78 period at the three stations. The decrease was a feature of air flow from most directions. It coincides with decreases in air quality indices reported by Ricci et al. for Canada between 1971 and 1975.

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A. M. Davies
and
J. Lawrence

Abstract

This paper presents a very brief overview of the development of a three-dimensional hydrodynamic model involving a flow-dependent eddy viscosity and including enhancements of bottom friction due to wave-current interaction in shallow water. The main point of the paper is to examine the physical nature of the process. Consequently, references to published work are given for the background detail.

Calculations using both tidal and wind forcing show that tidal elevation amplitude and phase are significantly changed in shallow near-coastal regions due to enhanced frictional effects associated with wind-driven flow and wind wave turbulence.

An analysis of tidal current profiles, at the fundamental harmonic and higher harmonies, computed with tidal and wind forcing, shows that significant changes in tidal current profiles can occur due to coupling between the wind-induced current shear and a time-evolving viscosity. The importance of the nonlinearity produced by a surface wind-induced shear and a flow-dependent viscosity in influencing tidal current profiles is confirmed using a single point model in the vertical.

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J. N. Aldridge
and
A. M. Davies

Abstract

A high-resolution (0.5′ north-south by 1.0′ west-east, approximately a 1-km grid) hydrodynamic tidal model of the eastern Irish Sea is used to examine the sensitivity of M2 tidal currents to changes in open boundary data and bottom drag coefficients.

The model uses a finite-difference grid formulation in the horizontal, with an expansion technique in the vertical, giving a continuous current profile from sea surface to sea bed. A slip boundary condition is applied at the sea bed. Eddy viscosity in the model is a function of the flow field.

Comparison of observed and computed tidal elevation and currents show good agreement, although in general, currents are slightly higher than observed.

The effect on tidal current profile of changes in bottom friction coefficient is found to be influenced by the proximity of the open boundary to the region of interest. This suggests, and calculations confirm, that it is not just the local coefficient of bottom friction (i.e., that within the eastern Irish Sea) that influences current profiles in the region, but frictional effects throughout the whole Irish Sea. This illustrates the limitations of limited-area models where the solution can be dominated by the open boundary.

Numerical models are generally run with uniform values of bed friction coefficient, although it is known that in principle this coefficient should change according to the nature of the bed. The effect of spatially varying the bed friction as a function of bed composition is investigated and shown to have a relatively small effect on tidal currents but a significant effect on the bed stress derived from the model.

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J. E. Jones
and
A. M. Davies

Abstract

Although the problem of predicting storm surge elevations has received significant attention, the simulation of currents has suffered because of lack of current observations during surges. Current measurements made during surge conditions are presented here and are used in combination with three-dimensional models to understand processes producing storm currents in the Irish Sea. A coarse-grid (resolution of order 7 km) model of the west coast of Britain together with a fine-grid (of order 1 km) model of the eastern Irish Sea is used to examine the processes, namely, open boundary forcing of the west coast model and wind fields, that produced flows within the eastern Irish Sea during the storm surge of November 1977. Simulations of the surge show that the fine-grid model nested within the west coast model can reproduce observed coastal changes in surge elevation. However, an observed major inflow that was recorded by current meters in the region, prior to a storm surge elevation peak, is not represented, although subsequent inflows and outflows are reproduced. The flow fields in the west coast model giving rise to these currents are analyzed in detail. Also, computations are performed with idealized open boundary forcing and wind fields to understand their role in determining the circulation within the region. An analysis of computed flows shows that outflows from the eastern Irish Sea following major storm events are determined by sea surface elevation gradients in the region and topographic effects. Observed flows under these conditions are reproduced by the model. Inflows, however, are more difficult to compute and depend upon a delicate balance of northern and southern boundary forcing of the west coast model and wind fields over the region. The first observed inflow event, which was not reproduced in the model, was associated with a current from the south. A second inflow event that was reproduced arose from a combination of an inflow from north and south, and a third event was again reproduced in the model due to a current from the north. Without a more comprehensive observational dataset, it was not possible to determine the exact reason why the first inflow was not reproduced.

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J. A. Davies
,
M. Abdel-Wahab
, and
J. E. Howard

Abstract

Transmissivities are determined for different cloud types using nine years of hourly irradiance measurements under overcast skies at six Canadian stations. Values for individual stations and for pooled data using irradiances uncorrected for multiple reflections are similar to values for Blue Hill, Massachusetts but 1arger than values for Hamburg, West Germany. It is argued that transmissivities used in numerical models which utilize surface observations of cloud layer amounts and types should be determined from irradiances without correction for multiple reflections. This would ensure at least partial compensation for attenuation by undetected cloud above overcast. The superior performance of transmissivities calculated in this manner is demonstrated in numerical model calculations of irradiance. It is also shown that there is no need to replace Blue Hill transmissivities with either the new values for Canada or the values proposed by Atwater and Ball for such models. There is also no indication in the Canadian results that cloud transmissivity varies with cloud amount as suggested by Atwater and Ball. Regional and seasonal variations in the Canadian transmissivities have a negligible effect on calculated irradiance. Irradiance calculations can be simplified with little loss in accuracy using an average transmissivity for each cloud layer; 78, 42 and 32% for high, middle and low cloud, respectively.

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J. A. Davies
,
P. J. Robinson
, and
M. Nunez

Abstract

Field determinations of surface emissivity and temperature were carried out at the western end of Lake Ontario between July and November 1969. Emissivity was obtained from infrared thermometer measurements using an aluminum cone. Surface temperature was measured directly with floating thermistors and estimated from outgoing longwave radiation.

Emissivity was constant at 0.972 with no detectable variation with water turbidity and chemical content changes. Float and infrared thermometer temperatures were in good agreement at times when there were no sensor problems. Temperatures calculated from the radiation balance agreed well at night with the other methods but showed irregular fluctuations and systematic overestimation during daytime probably due to errors in determining outgoing longwave radiation. Comparison with float data over the experimental period showed temperature differences ΔT with a change of sign in September corresponding to a sign change in the difference between air and water surface temperatures. Both the sign and order of magnitude of ΔT may have been due to divergence of the upward longwave radiation flux.

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Michael J. Garay
,
Roger Davies
,
Clare Averill
, and
James A . Westphal

Stratocumulus clouds are common in the tropical and subtropical marine boundary layer, and understanding these clouds is important due to their significant impact on the earth's radiation budget. Observations show that the marine boundary layer contains complex, but poorly understood processes, which, from time to time, result in the observable self-organization of cloud structures at scales ranging from a few to a few thousand kilometers. Such shallow convective cloud features, typically observed as hexagonal cells, are known generally as mesoscale cellular convection (MCC). Actinoform clouds are rarer, but visually more striking forms of MCC, which possess a radial structure.

Because mesoscale cloud features are typically too large to be observed from the ground, observations of hexagonal cells historically date only to the beginning of satellite meteorology. Examples of actinoform clouds were shown in the venerable “Picture of the Month” series in Monthly Weather Review in the early 1960s, but these clouds were generally forgotten as research focused on hexagonal cells.

Recent high-resolution satellite images have, in a sense, “rediscovered” actinoform clouds, and they appear to be much more prevalent than had been previously suspected. We show a number of examples of actinoform clouds from a variety of locations worldwide. In addition, we have conducted a detailed case study of an actinoform cloud system using data from the Multiangle Imaging SpectroRadiometer (MISR) and the Geostationary Operational Environmental Satellite (GOES), including analysis of cloud heights, radiative properties, and the time-evolution of the cloud system. We also examine earlier theories regarding actinoform clouds in light of the new satellite data.

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