An analogy has been established between a plane mixing layer and the atmospheric flow near the top of a vegetation canopy. It is based on a common feature, a strong inflection in the mean velocity profile, responsible for hydrodynamical instabilities that set the pattern for the coherent eddies and determine the turbulence length scales. In an earlier study, this analogy was tested using a small data set from thirteen experiments, all in near-neutral conditions. It provided a good prediction of the streamwise spacing Λw of the dominant canopy eddies (evaluated from time series of vertical velocity) that appears to depend on a shear length scale Ls = U(h)/U'(h), where h is canopy height, U is mean velocity and U' the vertical gradient dU/dz. The present analysis utilizes an extensive data set of approximately 700 thirty-minute runs, from six experiments on two forest sites and a maize crop, with a large range of stability conditions. Λw was estimated for each run using the wavelet transform as an objective, automated detection method. First, the variations of Λw and Ls with atmospheric stability are discussed. Neutral and unstable values exhibit a large scatter whereas in stable conditions both variables decrease with increasing stability. It is subsequently found that Λw is directly related to Ls, in a way close to the neutral prediction Λw /h = 8.1Ls/h.The Strouhal number Str = Ls /Λw is then shown to vary with atmospheric stability, weakly in unstable conditions, more significantly in stable conditions. Altogether these results suggest that, to some extent, the plane mixing-layer analogy can be extended to non-neutral conditions. It is argued that the primary effect of atmospheric stability, at least in stable conditions, is to modify the shear length scale Ls through changes in U(h) and U'(h), which in turn determines the streamwise spacing of the active, coherent motions.

The control of coherent eddies in vegetation canopies: streamwise structure spacing, canopy shear scale and atmospheric stability.

The effects of topography on two measures of temperature, the daily minimum and the diurnal range, were investigated using the conventional network of climatological stations in the U.K. The effects of large bodies of water and buildings were minimized by restricting the analysis to data obtained from 145 inland rural sites.



Topographic influences were considered to be mainly due to the effects of ‘local’ and ‘large-scale’ shelter. Local shelter is associated with ‘height above the valley’ and is represented by the drop in altitude within 3 km, whereas large-scale shelter is associated with nocturnal drainage of cold air, and is represented by the average height of the terrain above the valley over a radius of 10 km. The height above the valley is the most important variable and its maximum effect on diurnal range occurs in the early autumn, when the difference in soil moisture deficit between valleys and summits reaches its maximum. Large-scale shelter is more important in winter than in summer, and for diurnal range, for rare rather than common events. This represents the increasing importance of nocturnal air drainage with the length of the night and suitable radiating conditions. For minimum temperatures in winter, however, the maximum impact of large-scale shelter occurs for events with a return-period around 2 years, and which are loosely associated with cold spells lasting a day or so. More extreme temperatures are usually attained at the end of cold spells lasting several days, when continued radiational cooling over level ground can produce temperatures as low as those attained through cold air drainage.

Relations between minimum temperature and topography in great britain

The growth rate and acceleration of landscape ecology in a scant fifteen years have been extraordinary. I believe two reasons explain this phenomenon. First, the field has opened a frontier of research scholarship and theory development focused on spatial pattern intertwined with processes and changes. Second, by focusing on the broad human scale and including all landscapes, it offers direct applications for solving vexing environmental and societal issues.

Horizontal Processes, Roads, Suburbs, Societal Objectives, and Landscape Ecology

Frost mapping using NOAA AVHRR data in the Pampean region, Argentina

Comparison of winter-nocturnal geostationary satellite infrared-surface temperature with shelter-height temperature in Florida

Surface temperature patterns of drained organic soil farmland in the Everglades Agricultural Area south of Lake Okeechobee, Florida, were studied using GOES-1 satellite infrared digital data for the winters of 1976-77 and 1977-78. Local thermal features were explained in terms of soil types, soil depths and climate modification caused by organic soil drainage. A cold-prone area was delineated in the Everglades Agricultural Area with satellite data. The winter nighttime surface temperatures of this area were more characteristic of north central Florida, approximately 370 km further north, than of surrounding areas of mineral soil or undrained organic soil.

Satellite-sensed winter nocturnal temperature patterns of the Everglades Agricultural Area

Moderating effects of Lake Apopka, Florida, on downwind surface temperatures were evaluated under cold-air advective conditions. Point temperature measurements north and south of the lake and data obtained from a thermal scanner flown at 1.6 km indicate that surface temperatures directly downwind may be higher than surrounding surface temperatures by as much as 5 C under conditions of moderate winds (about 4 m/s). No substantial temperature effects were observed with surface wind speed less than 1 m/s. Fluxes of sensible and latent heat from Lake Apopka were calculated from measurements of lake temperature, net radiation, relative humidity, and air temperature above the lake. Bulk transfer coefficients and the Bowen ratio were calculated and found to be in agreement with reported data for nonadvective conditions.

Observations of the convective plume of a lake under cold-air advective conditions

A steady-state, two-dimensional numerical model is used to simulate air temperatures and humidity downwind of a lake at night. Thermal effects of the lake were modelled for the case of moderate and low surface winds under the cold-air advective conditions that occur following the passage of a cold front. Surface temperatures were found to be in good agreement with observations. A comparison of model results with thermal imagery indicated the model successfully predicts the downwind distance for which thermal effects due to the lake are significant.

Simulating the moderating effect of a lake on downwind temperatures

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Satellite-sensed winter nocturnal temperature patterns of the Everglades Agricultural Area

Observations of the convective plume of a lake under cold-air advective conditions

Simulating the moderating effect of a lake on downwind temperatures